Laser Cutter Tips and Tricks

April 11, 2016 by Kurt

Do designs in alignment, but then break up all the objects into “cut sheet” layers organized by the material they need to be cut from.
Do design in exact dimensions without kerf compensation, those tiny .002 changes are nearly invisible and will drive you crazy. “Did I already compensate that?” I only every apply the compensation to a copy of a single layer I’m going to cut and then save it out as a ToCut version of just that layer.
Design with groupings for easy editing, but collapse parts down into individual compound paths using the path finder when you are making your cut sheets. This helps offset paths do the right thing for kerf compensation. If everything comes out right, but you have a few holes that are way to large, you forgot to collapse them, and the kerf compensation made them bigger instead of smaller.
Control Cut order. If you’re cutting open paths or have closed paths that overlap the normal “inside before outside” cut ordering won’t apply if you care about cut ordering organize the parts into separate cut passes.
Look out for exactly duplicate paths hidden on top of one another. That causes double cut problems. Find them by zooming way out so the anti-aliasing makes the overlapping ones appear darker.
Avoid paths that are too close together. The thin sliver of material won’t be able to dissipate the heat as well as a wider section and may melt or burn.
Use tape to hold pieces in so the excess material forms it’s own parts organizer. You’ve already carefully laid these things out in an nice sheet, why not take advantage of that?
Use 3 ring binder sleeves to hold on to thin sheets of organized parts. If you’re using the tape and excess material trick above, if you limit yourself to 8.5″x11″ sheets of thin material you can orgnize a zillion sheets in one three ring binder. Just be careful not to pick it up upside-down.
Use pulse rate to control edge properties lower on wood for a less chared edge. Higher on plastic for smoother finish.
Remove uneven protective coatings. If cutting pastic with an uneven or bubbled protective sheet of plastic on top peal it away first to get more consistent cuts.
raster areas into blocks to minimize the raster span. When raterizing the laser goes back and forth over and over. The shorter the distance it has to go the quicker the raster progresses. Clumping your raster parts can really cut down on the amount of time spent waiting.
Pay attention to the grain orientation on wooden parts. It’s important visually and also important structurally. It’s not as important for plywood.
Use char artifacts to you advantage. When laser cutting wood the compressed air blast adds a charred “cut shadow” on the upper surface, you can orient parts to take advantage of this, by making that shading part of the design.
Do small test cuts with a particular laser and material before deciding on the settings for a long set of cuts. I usually use some 1/4″ circles in a scrap area of the design.
On wood go a bit slower than your test cuts indicated. Wood is not a uniform material go slower/use more power than the minimum you needed for your small test cuts so as not to leave a few tough areas uncut at the bottom that you’ll need to cut loose and touch up.
Push not-fully-cut wooden parts toward the bottom. If the wooden part very nearly cut though and you’re able to just push the parts free, pushing on the top of the part makes sure that any splinters that pull free at the uncut areas pull free from the scrap around the part. If you push the other direction the splinters come off your part, and ruin it.
Use a wood burning pencil to hide hand cut edges. If you have to cut some wood loose, trim splinters, or sand down some edge it will leave unburnt bright wood in the otherwise evenly burnt edge. You can shade these areas in with a wood burning pencil or cheap soldering iron so they mostly disappear.
Don’t make everything a rectangle. Part of the lasers power is awesome curve cutting. Don’t make everything a rectangle. Add some curves. Not everything has to be 90 deg or a strait line. Do not assume everything is a box.
Use the scrap around a cut piece to form a handy clamping jig. If you use complex shapes they can be hard to clamp for gluing. The scrap material around your part fits it like a glove and a few extra cut lines can make that come free and you can use it to hold your part in a clamp while gluing.
If your forming a box edge with mating fingers don’t use alternating squares. Work joins into your design. Make a nicer more intentional patterns.
Hide slot an tab connections if you can. Sometimes you need some tabs and slots to hold on a part, and you haven’t been able to make the tabs fun enough to work into your design, you can do other things to deal with them. You can hide them under glued on decoration or reinforcement pieces, you can position them under other attaching pieces. You can laser etch transitions to make them more interesting than just a weird rectangles. Jazz it up.
Use an alignment jig to force holes and pins to be plumb. Just because a pin joins two pieces don’t assume that the pin is plumb, the conical cutting of the laser means there’s nothing forcing it to be plumb. For 1/8″ pins I use a drill press to make a 1/8 plumb hole in a piece of wood, and put a stub of 1/8″ rod in that hole. Now if you stack two pices on the rod and down onto the surface they’ll be reasonablly plumb and you can glue them.
Use the protective paper on acrylic as a paint mask. If you laser etch into acrylic though the protective paper, the remaining paper makes an awesome paint mask. If you don’t need super alignment between the raster and vector cuts you can even do the raster pass first, paint, and then do the vector pass. That lets you paint the whole sheet willy nilly without having to worry about getting paint down in the cuts, or having to paint individual little pieces. Take care to use some extra power/slowness to fully etch though the paper and into the plastic. If you “only just barely” etch though you can have sticky residue from the paper still on the plastic and that is real pain to remove.
It takes more power/takes longer to cut/etch though plastic that has the paper on VS not. (and it’s different for paper/plastic protective layers)
If you’re cutting lots of small plastic parts consider pealing the protective paper away first. If you don’t need a super pristine surface finish free of any slight fogging near the cut lines it’s way easier to just peal the protective paper/plastic away before you cut. It’s a time sink peal it off a zillion tiny parts individually.
Use extra cut lines to separate and save unused areas. When you lay out parts for cutting, you often have some areas that are not used. If you add some extra cut lines to cut those ares into their own rectangles. It’ is much easier to re-use that material and it’s much more compact to store. Trying to hit some good areas in a big sheet full of holes is hard and a waste of your time.
Cut fancy paper weights. If you’re cutting very thin things like paper it will often just blow away while you’re cutting it because of all the air motion in the machine. If you take a wood or plastic sheet and cut out a part just inside/outside the lines you’re cutting in the paper the sheet can form a curve hugging paper weight for both the inside and outside of the cut. With those pieces in place while you cut the paper stays put.
Use tabs to hold paper shapes in place. Paper parts can blow away so put gaps in the cut edges to keep them in place. Position the tabs where it will be easy to trim them (avoid corners or curves) Cutting the gaps screws up the lasers inside/outside cut ordering so use explicit layers to control ordering. You can get away with a single tab if you put it “upwind” of the direction the compressed air on the laser will be blowing.
Make a paper clamp. If you want to cut a lot of sheets of paper all at once you can use the “fancy paper weight” idea but use 1/4-20 bolts and wing nuts to make a clamp to hold the paper together as a single chunk. (be on the lookout for fire though!) I did this to cut secret compartments out of books. It works but is messy and kind of a pain.
Use the mechanical properties of your material. Thin areas can be used to add springiness Keep in mind that sharp corners and nicks can concentrate stress.
Cut some extras. It’s always a trade off between cutting time and output, but if your project needs 5 of something consider cut 6 or 7 of them. If they’re tiny or easily broken cut more.
Look for the flash. If you want to know if the laser is compeletly cutthing though your material, look for the flash as the laser hits the honeycomb support below the material. If there are no flashes then you’re not getting all the way though. This is easier to see with acrylic, not so easy to see with wood, etc.
For 90 deg braces cut out the inside corners. If you are added braces to hold other pieces in alignment, and they’re just a tiny bit off the inside corner of the brace will push the brace away and spoil the glue bond. Removing the inside corner makes the glue up more forgiving and makes for stronger joints.
Use water to deal with bowed wood. Often with a sheet of plywood it will be somewhat cupped. Instead of trying to fight that with weights you can use a paper towel and water to moisten the concave side. In a couple of minutes the water will make that side of the wood expand and flatten out the wood. I’ve used this to great effect on 1/4″ plywood.
Use water to protect thin sections on multi pass cuts. In wood it’s easy to have thin sharp angled features char badly on the second pass. Between passes use a syringe to put a tiny amount of water into the groove at these points. The water will soak into the walls and keep them from charing as badly on the next pass.
Etch an alignment aid. Want to laser etch consistently at a specific location on an uneven shape? Etch an alignment sheet with the outline of the foot of the shape and use that to position the parts. If you etch the alignment sheet deep enough it can even positively position the part since it’s footprint will click into the etched depression. You can even use a flat bed scanner to figure out the footprint if it’s complicated.
Spiral cut to make deep pockets. Want to cut a deep pocket that doesn’t go all the way though? You can vector cut a tight spiral. You have to experiment with the specific material to get the right spacing. If you can try to etch from the center and spiral outward. Watch out for major heat build up. Fire risk.
Be careful about versioning. If you have a project witha lot of parts and your iteratively improving the parts take care not to mix versions of the parts. It’s better to pitch old versions of the parts then to try and tell which ones have modified slots that are 0.005″ narrower.
Check design orientation when cutting multiple passes in wood. If you have long features where the laser moves in the same direction as the forced air at the cutting head that air will blow back along the cut fanning any coals formed on the walls. This makes the charing much worse. Try to orient the wood so the air blows across the grooves not along them.
Slow rasterization for dark details. I used to always use max speed for rasterization, and adjust power to suit, but if you’d like to put dark details on wood it’s better to use lower power and slower speed to char dark details that are not deep.
Cut part way though to do kerf bending. If you cut a lot of parallel lines most of the way though the material it makes the material much easier to bend. If you are doing this in wood make sure to cut perpendicular to the grain so it doesn’t just split when you bend.

If you have other laser cutting tips put them in the comments!

Plexitube Owl Clock

April 4, 2016 by Kurt

For Christmas I designed and built six steam-punk owl clocks that have a novel faux Nixie Tube display. Nixie tubes are wonderful with their glass tube bodies and brightly glowing numerals, but think how wonderful a solid-state low-power any color “Nixie Tube” would be!

The basic idea is that you position an LED at the edge of a piece of acrylic that has a pattern laser-etched into its surface, and the LED lights up the pattern. Long ago I’d realized that a stack of 10 laser-etched digits with an LED to light up each digit might have the same visual charm as a Nixie Tube, and the Plexitube was born.

I’m pleased with the way the project turned out. I’ve put the software, circuit designs, and illustrator files up on github. I even made a short video of one of the clocks in action. The project has even gotten some attention from Hackaday and won this Reddit unconventional clock competition. Thanks a lot guys!

At the time it was unclear if light bleed between the layers would make the digits hard to read, so I sat on the Plexitube idea for a long time waiting for a project that could use that kind of display.

Then I went to work at a new company, and I had to spend an entire day listening to benefits summaries and corporate on-boarding. To pass the time, I started to do some sketches of an owl made from only two pieces of wood. I used my Swiss army knife to cut my co-workers’ name tags into owl prototypes. I really liked the design, and I thought it would be perfect for an owl-shaped clock with Plexitube eyes. Would this be this year’s Christmas Project?

I can’t really start a Christmas Project until after the Halloween Costume builds are done, but this project couldn’t really take shape until I’d at least tested the viability of the Plexitube. I figured if I just spent one evening laser-etching a digit stack and holding it over an LED, I could figure out if the displays were going to be an unreadable mess. If it failed, I could pull the plug on the owl idea before they got out of the paper prototype stage. So I spent one of my Wednesday nights making a prototype stack.

The prototype stack had a lot of issues. The back digits where hard to read, there was a lot of inter-digit light bleed. I really couldn’t say for sure whether this was going to work. That’s pretty much the worst-case scenario for a make-or-break prototype. Really, I should have just stopped then, but I couldn’t stop thinking about the nice owl shapes.

The Impossible Project

Halloween came and went and I needed to decide. I even went so far as to do some prototypes of an alternative project that was much simpler, but I could tell that the owls were the ones that wanted to be made. I’m usually pretty careful planning one of these projects, and never before had I taken on one that had so many BIG unknowns. I’d never worked with surface mount electronics, never bent wood to such small radii, never designed a PCB with a lot of exact physical layout constraints, and never used an in-circuit programmer. I decided to go for it, but I knew I’d have to get first-time-success-lucky on a number of things, and I told my coworkers that I did not expect to succeed.

Two months and counting

Time to make a lot of quick choices and order stuff from China. I chose the popular Neopixel surface mount LED as my light source. They could be any color and all 40 of them could be controlled using just one pin from my controller. I decided to go with the ATMEGA328p, which is the same chip in an Arduino UNO. I was building 6 clocks, so I’d need 240 LEDs. I ordered a reel of 500 from China, along with some surface mount capacitor and resistor assortments, photo resistors, and some DS3231 real-time clock modules with battery backup. Ordering things from China is very cheap, but it can take a few weeks for the stuff to show up, so I had to get that moving quickly.

Now ordering printed circuit boards from OSH Park takes something like 10 days, so I really needed to get that in the pipe. If the first set of boards were a complete blowout, I could still get a second order in, but I had to hurry.

The LEDs I was using are much fatter than that 1/16″ acrylic layers I was using, so each layer needed to have a sort of integrated light guide leg that would stick out and sit directly above the 3 tiny color chips inside the LED. The alignment had to be exact. So I measured an 10-layer alternating stack of acrylic and black card stock to get the exact spacing, and then I had enough info to begin laying out the PCB.

I used Eagle CAD and finally had to spring for the non-free version so I could work with more than one page of circuit. I had to use some tricks with the Grid to get all the LEDs laid out in the exact right locations. After a full weekend of fighting with Eagle, I had something I could send off to OSH Park. I also quickly ripped one eye segment out into another file and ordered 3 small “test boards,” so I wouldn’t have to do my first attempt at reflowing a surface mount board with one of my giant and owl boards.

Then it was back to working in Illustrator on the design for the acrylic digit layers, black card stock separators, etc. I customized the digits of a font to make them slimmer. All the digits obstruct one another, so I wanted something that was lovely, but was mostly empty space so other digits could shine though. I also did some tricks like make the zero (which is in the back) a bit bigger/bolder so it would still read well though all the other digits.

Secret Symbols

Now on a clock there are 4 digits, but not every digit needs all 10 numerals. Heck, the tens of hours digit only really needs a 1. That leaves 9 layers for other special non-digit things. I could, in theory, have put some extra digits in the back of the tens of minute digit since it only goes 0-5, but I decided if I was going to be showing other info in those lower digits, it would be better to have all the digits there. I also decided to keep the 2 in the hour-tens digit so I could support 24-hour time in the future, and show a full year like 2016, etc.

I wanted to show the phases of the moon, sunrise and sunset times, moon rise and moon set times, etc. I used 6 of my 8 free digits doing the arcs of the moon. Because all the layers are at different depths, I staggered them so the arcs actually make a 3D sphere shape with the middle most arcs at the front. That left me with 2 layers. I used one for a sun outline that could be used along with the arcs to show things like the equinoxes/solstices.

The Magic Eye

That left me with only one free layer, which I used to add the outline of an eye. I realized that the arcs could be used to animate a slit-pupil eye looking back and forth. Of course I needed that! My owls eyes have eyes?! Yes … yes they do.

Plexitube Assembly

Each eye element can display two digits, and they are laboriously assembled by stacking 20 digit layers and 20 paper masks on 4 brass pins in careful sequence. Thankfully, the digit layers were fairly easy to keep track of because they already had digits etched into them! Each acrylic layer has to have its protective plastic peeled away, and its etching scraped and dusted. Because the pins are press-fit, I had to carefully push them down on the pins without cracking the 1/16″ acrylic. The first stack took me 2 hours to assemble. My finger nails were frayed from all the plastic peeling and I had 11 more elements to make. What had I gotten myself into?!

First Reflow

Finally the owl boards came! I had also ordered a solder stencil from OSH Stencils. When making a board like this, the basic process is a bit like silk screen printing. You position the PCB under a stencil and squeegee solder paste though the stencil holes onto the board. Then you place all the itty-bitty components onto the board with tweezers. Finally you oh-so-carefully put the board in an oven and heat it up until the solder paste melts and the parts all snuggle down exactly into place via surface tension. A while ago, I had built a special computer-controlled toaster oven in order to do just this process, but I had never used it!

I kept waiting for my test boards to show up, but eventually I gave up and decided to do my first reflow on one of my giant and indispensable owl boards. I populated the board, put it in the oven, and pushed the button. Then I held my breath as the oven went though its 3-minute cycle. Success! The board came out looking perfect! It looked like something totally commercial, not something I’d made with tweezers and a toaster oven. MAGIC! Surface mount is awesome. I’m never going back!

Ok, so the board looked like it was all soldered up, but would it work? I hooked the LED chain on the board up to an Arduino UNO (remember how those NeoPixels can be driven by just 1 LED?) and some quick code showed that all the LEDs were working! I blinked each one red-green-blue to test and they passed with flying colors! *phew*

Now for the real question: would the digit display actually look good or was the whole thing going be two months of work for an unreadable mess? I wrote some code to cycle though the numbers and positioned the eye module over the LEDs. It sort of worked, but it wasn’t great.There was quite a bit of cross lighting between digits, especially between digits 1 and 2.

I was extra sad I’d left that 2 in the hour tens digit. No one wants to look at a clock and not be able to tell if it’s 11:14 or 22:14. Something really needed to be changed. But what? I did, however, decide to get my second round of PCBs made. OSH Park sends out 3x boards, so if I wanted 6 by Christmas, I’d have to order the second round pronto.

I spent an evening designing a nice owl icon to put on the second round of boards, added two regulator bypass pins, and then sent the thing off. No time for anything fancy. It turns out the icon was a fail. Even though I followed OSH Park’s guide lines it did not render properly.

Eerie Is Good

One thing I did notice while I was running that test is that although the digits were not yet crystal clear, the whole thing had a very eerie feel to it. It was almost as if I couldn’t quite tell what the heck kind of technology I was looking at. It sure didn’t look like LEDs. The layers of acrylic have a wonder full “hall of mirrors” effect on the lit segments. I also noticed that the light bleeding off the tops/sides of the eye element were kind of hypnotic to watch go through their sequence. I had originally planned to cover the elements with a ridged copper cylinder segment, which would have looked steam punk, but not as strange/cool as that bleeding light. I decided to leave them uncovered.

Back To Body Work

Now that the boards and the eye element design were mostly settled, I had to get back to figuring out how to design the bent body of the owl. I did a second pass of designing the body in cardboard. I knew that trying to bend 1/8 plywood into a loop maybe 3 inches in diameter was going to be tricky and might be impossible. In a recent build, I’d used laser “kerf bending” to make it easier to bend curves in thicker plywood. Could I take that to an extreme? MicroKerf Bending™? I tried spacing the kerf cuts .1″ apart, cutting almost all the way though the plywood. The results where a satisfyingly bendy piece of wood. Still the owl has some very wide sections at the top of his eye arches and those were still quite stiff. Too much variation to be able to bend into a loop.It would have been nice to write some sort of script that would space the kerfs closer/wider based on the overall width of the piece, but I had to settle for just doing a few different spacings by hand.

The brows above the eye were delaminating when I’d apply the bend. That was bad. I fixed it by pre-sanding the upper layer thin in those areas and by putting a few little spring clamps on those edges during the bend. That solved the problem, but all these iterations on the body design were burning up time at a crazy rate. Would I make the Christmas deadline!? Here I’d been thinking I was saving time by having most of the owl form come from just one piece. So wrong.

Steaming and Clamping

I built a clamping jig out of a big board and carved away clearance for the owl feet. I’d steam an owl body in a pot on the stove and then bend it with my gloved hands whilst racing out to the garage to clamp the body around the face in my special jig. The first time I did this the face of the owl split in two! Yes, when the body is looped back on itself and clipped tail to feet the forces on the face are nice and balanced, but while you’re bending that around the forces on the face are actually pretty big. So I changed my clamping jig so it would support the face during the bend. Finally, it worked!

I had a lot of other little issues. I had to add some “foot prop” pieces so I could have a way of evening out feet after the main clamping was done. I thought I’d be able to adjust the owls stance by adjusting the tail angle, but a sort of scissoring action made it so any amount I pushed up the tail also pushed up the feet, canceling out the adjustment. So the owls look down a little bit more then I’d like. That’s okay though because they like to nest up high.

I wish I could have used 1/8″ mahogany plywood. Then I wouldn’t have had to deal with staining the outer body. However, that’s not something I can get locally so I was stuck with stain. I hate stain. I tired pre-staining, which gave nice crisp edges at the stain boundaries, but that made the gluing a lot weaker. I tried post staining with with pre-clear coating areas to try to reduce bleeding over, but it still bled some and required lots of awkward sanding to fix. If I had it to do again, I’d try to get hardwood ply. I did eventually get them all glued and stained.

The Secret Sauce

I’d been thinking about my eye element problems. How could I improve the contrast? I already had black card stock all over the place blocking as much light as I could. I realized that as tight as the fit was around the card stock, there were still micro gaps, and the light leaks I was seeing were probably coming right up from the LEDs and into the clear bottoms of the other segments. I decided to use a small brush to paint some black acrylic into those areas! EUREKA! That fixed the problem entirely and now the displays looked great.

Christmas Eve

It was Christmas Eve, and I was still clear-coating owl bodies. Applying wood finish on Christmas Eve is par for the course for me. The big issue was that I was still driving the displays from a separate Arduino and hadn’t fired up the on-board computer at all. I spent a while reading about fuse bits and fighting with boot loaders, and fiddling with Arduino IDE config files. Had I made some mistake with as-yet-untested parts of the PCB? Could I get lucky one last time on this “impossible” project? Yes! At last, I was able to program the chip on the clock board, get the real time clock synced with Greenwich Mean Time, and put enough code in the clock that it could actually display the time! The clock could go under the tree.

Software in the New Year

Early on I had decided to punt on all but the most basic software until after Christmas. I had pages of ideas on fancy software features, and I knew I’d have enough trouble just getting the things physically built by Christmas. This was going to be a Christmas gift with an eventual “firmware upgrade” gift chaser.

I knew there was going to be a lot of software, so I wrote a iPhone owl clock simulator to give me an easier platform to develop some of the code. I had been reading Jean Meesus’ Astronomical Algorithms, and I wanted to have the clock not only show the phases of the moon, but be able to display sunrise/set times and moon rise/set times.

I had included a light sensor on top of the clock so the clock could dim a bit when it was in the dark, but it could also tell if your hand waved over the clock and maybe then it would cycle though showing the sun going though sunrise colors while showing the sunrise time and the show the sunset time while cycle though the sunset colors.

Color Picking the Old Fashioned Way

I really wanted to use nice colors in the clock, but just choosing 3 numbers for Red/Green/Blue doesn’t tell you what those are going to look like in the final display. I needed a way to search for good colors. So I made a super simple color picker by hot gluing three potentiometers to a board. I used an Arduino UNO to read the three RGB values and display the color on a jury rigged LED + acrylic digit. Go Go gadget gaffer’s tape! The program would print the values out to the serial monitor. That way I could find just the right pink by fiddling with the knobs and then write down that RGB triple from the screen. I used that to make a whole pallet of colors I liked.

The Trouble With Doubles

The big problem was that most of the Astronomical Algorithms need double precision math. The solar system is big and you need the precision, but the Arduino tool chain doesn’t have real double precision math. I spent a while (stupidly) writing a math lib that did approximate double precision math using two single precision float values based on this paper targeted at GPU computation with floats. I did get the math working, but to my horror, I discovered that the code it produced to compute the equinoxes/solstices was so huge that it didn’t fit into the 32k of memory on the ATMeg328p. That was a huge waste of time. I did, however, use the code to generate the next 100 years of solstices and equinoxes and compress them down into just 100 bytes of Progmem, but the sunset/sunrise times were doomed. I went back to some still usable moon calculation code so the clock can show the current phase of the moon.

Then I wrote a lot of silly features. On the clock recipient’s birthday, it displays in their favorite colors. (And on their kids’ birthdays, their kid’s favorite color, etc)

I wrote all the code to set the time/date/gmt offset with nice auto repeat ramp ups and useful icons. (When you’re setting the month a sliver of moon shows, when you’re setting the day there’s a sun icon, etc) I even used the right arc of the circle to show AM and the left arc for PM like they were the first and second parts of a 24 hour clock face. Not that I expect people have to set the time/date much, but a clock needs to do that.

The clock counts down to midnight on New Year’s Eve and shows fireworks and a spinning golden globe at midnight. On JRR Tolkien’s birthday, the the eye of Sauron looks around and sometimes a golden ring spins and the digits are red and fade up and down like like a breathing beast. etc. I don’t want to give to many surprises away.

Needless to say, there are a year’s worth of Easter Eggs in the clock code, even on Easter.

Two Words “Optical Theremin”

When I was working on the auto-dimming feature, I wanted to have a direct readout of the light sensor, so I could fine tune a mass/spring system for the dimming rates. When I first hooked it up, I realized that with a much stiffer spring it was pretty neat to play with. You’d wave your hand over the clock and the digits would smoothly go up and down. It was kind of mesmerizing.

I was showing it to a friend, and when I started waving my hand around he said, “Wait, it has a Theremin too!?” and the Optical Theremin was born. I hooked that code up to the hue of an hsv conversion and now when you press the down button, you get one minute of “Optical Theremin” where waving your hands around changes the colors and numbers in a soothing and lovely display of Plexitube awesomeness.

It’s hard to stop writing code for the clock, but after the “Optical Theremin” feature, I started getting close to the memory limits and I decided to call it quits. It was time to give out festive springtime firmware upgrades! I had been trying super hard to have the code all done before the spring Daylight Savings Time change because people had clocks they couldn’t set! I only missed that deadline by a week. I blame double precision!

Clarence was starting to think that perhaps this hazing ritual was going a bit too far…

Cutting 3D Shapes on a Laser Cutter

September 1, 2015 by Kurt

When my local TechShop got a 120-Watt Epilog Fusion laser cutter, I knew it was time to try out something new. Being able to cut though fairly thick material made me wonder if it was possible to cut out 3D objects with the laser. Most laser cutters produce 2D output. They either cut and etch sheets of material in X & Y, or they cut and etch cylindrical objects (drinking glasses, etc) by turning them with a rotary axis which replaces the Y motion. In the past, I’ve sometimes used an indexing jig to turn the object and make XY cutting passes at various angles. What if I automated this rotation? Could I produce a 3D object by rotating the object and cutting out various 2D silhouette profiles? The process is limited buy the max cutting thickness of the laser. You can’t hack an object out of a spinning 2″ rod of material if the max you can cut though is 1/4″.

The new 120-Watt laser seemed like it might be powerful enough make this idea practical. I’d never seen anyone do this sort of work with a standard laser cutter. In industry, it’s common to add additional axes to cut at angles, but this XY plus rotational axis (A) was hard to google for. I liked the general shape of the project. At first, I could make a simple rotational jig and manually run a profile cuts though the laser just to see if there was any hope for the idea. If it seemed like it was possible, I could build a motorized A axis and use an optical sensor to sync it to the laser’s motion. From there, all sorts of interesting things could be done in the software to improve the output.

Hey we were featured on Hack-A-Day!

I find a partner in crime

I was pretty excited about the project, and I knew it was going to involve a fair amount of software and hardware. I wrote a super long email to my friend Lawrence pitching the idea. A few hours later he sent me an animated gif of a test model’s rotating silhouette. Clearly, he was in and already on the case! Woot! Lawrence and I have done a number of projects together, including the solar plotter project. He’s great to work with. I knew that with him on board, this project might really have some legs.
Right at this point, my TechShop announced that they were moving to a new location. The laser was going to be unavailable for a few weeks, so I decided to skip ahead and build a motorized A axis. This might’ve seemed overzealous. Why not do some tests with a manual jig first? Well, I was excited about the project and wanted to start building. As an added bonus, building the motorized version would get me to finally troubleshoot the PCB’s I’d designed for my motorized camera rig.

Figuring out what was wrong with that PCB would advance that project even if the laser project crashed and burned. So it wasn’t that big a risk to jump ahead, and I was VERY eager to get building. I ordered a cute little 3 jaw chuck so the motorized axis would be able to hold cylindrical stock of various sizes. The only down side to the chuck was that I’d have to machine a shaft with a very concentric 1mm thread. Not a big deal, but an additional hurdle for other folks wanting to build a rig like this.

I did write up that machining job, mostly so I’ll have something to refer to the next time I need to do single point threading.

The rig is direct drive using a stepper motor, a zero backlash coupling, and two 608 skateboard bearings. As Lawrence worked on a way to extract the silhouette profile curves and export them as .svg files, I worked on the most basic version of the device. I used an Arduino Nano and a little stepper motor controller all wired up a on protoboard with a button to advance 1/16th of a turn.
When TechShop reopened, I lasercut the rig to hold the motor, bearings, and chuck so I could glue it together. Lawrence had gotten his profile curve extraction code working, so we were finally ready to do a basic test of the idea.

Instead of having a fully automated cut-out in these initial tests, the idea was to export each of the profiles as their own print job. We could print them out manually pressing the advance 1/16 button between each file.

Here you can see our very hairy initial setup. The Arduino Nano is in red and the stepper driver in purple. Look Ma, no heat sink!

The First Night

Our initial test model was a chess knight. We started by chucking in a 3″ length of 1″ poplar dowel into the motorized chuck. We figured we could keep printing the same profile until we cut all the way though and then hit the advance button and move on to the next profile. One cut, 2 cuts, 3, 4, 5, 6, 7,8,9,10,11, finally it cut though. That was a lot of cuts. The wood was pretty charred. We knew the subsequent cuts would be quicker because of all the material removed by that first huge cut, so we continued on. Lawrence loaded, configured, and printed each of the profile cuts, and I opened up the laser, pressed the advance button, closed it back up, and fired off the next cut. Like a pair of bureaucratic button-pushing relay racers, we finally made it to the finish line. The results? The knight was pretty charred and battle scarred. His ears burnt entirely away, but he was recognizably a knight! We were jubilant.

The Second Night

One of the reasons the laser was taking so many passes was that the thick material had a lot of material far from the focal plane, and that meant more charring and less cutting from the laser. We realized that although we couldn’t move the model vertically, we could have the focal point above the center of the model, and then by rotating 180 deg and cutting the mirror image of the profile we could effectively cut the same profile with the laser focus at two different levels.

So I added a button to the rig that would rotate the model 180 deg. The first time we tried to cut the mirrored profile after 180 deg rotation, we discovered that the laser’s idea of the center of rotation was off from ours. We needed to measure and adjust for the offset. After compensating for that, offset the idea worked. We once again did the tag-team cutting process, this time with a somewhat more complicated sequence of rotations that we checked off on a list. By the end of the night, we’d managed to cut out a less charred and scared knight! We even had some ear nubs! We were getting better step by step.

Can you use acrylic rod?

We did try this system on acrylic. Acrylic cuts very cleanly with no charring, and we thought it might have great results, but the process really depends on chunks of material being able to fall away. However, when slowly cut, thick acrylic has the tendency to melt just enough to make the chunks stick and not fall away. We decided to focus on wood for now, with the idea of revisiting acrylic at a later date.

Process Improvements

What were the next big steps? We realized that with better path planning, we could cut thin layers off the side of the rod in multiple passes to minimize the thickness we needed to cut at any one time. This spiraling-in process would allow us to cut each outline only once. We also wanted to add some cut lines to the outside edge of the material so the cut chunks would drop away more easily. Lawrence worked on those things, while I worked on building some sort of laser sensor so we could automate the rotary axis motions. Sitting around with a checklists pushing buttons was not a viable way to be doing this with ever-more-complicated cut sequences.

I built a better two-button remote from some PVC, so we could at least not have to open the laser between passes. I also finally got around to troubleshooting the PCBs I’d had made for the camera motion rig. It turns out the Arduino Nano package I’d downloaded from the internet was for V2 of the Nano, but I had V3. For some unknown reason, they had reversed the order of the analog pins, which is a fatal change if you’re using most of the analog pins for digital IO. Once I had that fixed in software, the board was only 1 oops wire away from being fully functional, so in an evening I was able to go from hairy protoboard to svelte PCB.

I also made a centering jig that made it easy to put the rods into the chuck nicely centered. That way I could quickly chuck new rods without quite as much tapping and fiddling to get them to spin without a wobble. The next big improvement was a way to automatically advance the rig though its rotations as the laser went through its sequence. It would be so nice to be able to just hit “print” and have this system cut out a 3D model.

Blind to the Laser

My first inclination was to use some sort of IR photo transistor to watch the laser pulses and get a sense for when the laser was on versus off, and from there we could keep track of where we were in the sequence and when to advance. We could also in theory eventually use a sequence of laser flashes to communicate rotation sequence information to the rig. That way a single print job could handle everything. There was just one BIG problem with this idea, but thus far I was blind to it.

I built an ATTiny85-based pulse train detector and put it in the laser bed. No reaction. I tried some other random IR photodiodes/transistors I had around. Still no dice. I hooked up a scope and saw zero evidence of the detector seeing the laser at all! I thought maybe the pulses were just so short that the system couldn’t see them, but then I did some more research. It turned out that the IR emitted by a big CO2 laser is totally out of the range of cheap IR phototransistors. In fact, room temperature versions of such devices had only recently become available and they were $800 used on eBay! Not an option for us. It’s counter intuitive that something so powerful that you can be blinded by even diffuse refection of its light can be entirely undetectable by cheap electronics, but there it was. I’d taken the project up a bind alley.

Acorn Nut Job

My backup plan was to use a thermistor armored in a small acorn nut. The reaction time would be quite slow, but it was simple. The acorn nut would protect the thermistor, and we could make the cutting sequence include having the laser blast the thermistor whenever the rotation rig was supposed to go to the next position. Hopefully, it wouldn’t get too hot over time.

I used a dab of heat-sinking compound on the tip of the thermistor and a glob of epoxy putty to turn the delicate glass thermistor into an armored frankensensor. When I measured across the terminals to make sure I hadn’t shorted them out I noticed the thermistor value was drifting. I was seeing the temperature rise of the epoxy setting! So that was working.

This slow sensor did mean we were going to need a different way to load sequences into the rotation rig. There was no way a system like this was fast enough to communicate a sequence of angles. Luckily, I had another way of doing this. My motorized camera rig used Bluetooth LE to communicate wirelessly to an app on my phone. It’s only 9600 baud, but it works well and lets you have all a full-on user interface running on a device you already have in your pocket. Much better than my huge PVC-wired remote. Best of all, I had already written an app that scanned and connected to this kind of bluetooth device, so I was able to quickly hack that up into an app to control the rig. An iOS app isn’t really that open though, so I was kind of sad to be adding this particular step to our tool chain. I gave the app the ability to receive sequences via deep link.

The Biggest Problem

We had, however, discovered a huge problem with our plan, a problem that was going to take weeks to resolve. I had been told by someone at TechShop that if you turned off “Smart Vector Sorting” in the lasers print dialog, the laser would output the vectors from back to front. Perfect! We could output SVG, import into Illustrator, and print it out. Sadly, that turned out to not be true. In fact, the order in which the vectors are cut out is entirely out of your control. #%@$#! The vectors seem to be roughly y sorted and that’s that. God help you if you want to cut a zillion curves that are all in the same place. I exchanged emails with folks at Epilog with no real help. This is not exactly a big priority for folks using a laser in its normal 2D capacity. For a little while, we worked around this by having one super long cut vector, but that was never going to work for models with holes, etc.

We noticed a project called Ctrl-Cut on Git Hub that was a third party laser control program for the Epilog Legend 36EXT. Perhaps we could get it to work for the Fusion 120? I contacted Amir, and he was super helpful and generous with his time. I output various print files for him to look at, and he tried making a special cut of Ctrl-Cut for the Fusion 120 which would output the vectors in order. Progress was slow since I could only get on the laser one night a week. The printer files he was generating still had some issues and would crash the laser, which was pretty scary. Meanwhile, in the background, Lawrence was picking though the raw printer output and trying to get his path planning scripts to output the printers .prn files directly. Eventually, Lawrence was successful and he was able to generate .prn files, and we could even change the speed and power settings between cuts in the same file. Awesome! For this, I hereby award Lawrence an honorary knighthood, and a special shout out to Amir for his help and ctrl-cut examples.

Raster mode awesomeness

One of the shortcomings of the system of cutting out a bunch of profiles is that you can’t get details that never appear on the silhouette edge of the model. Laser cutters have a “raster” mode where they sweep back and forth quickly and etch an image into the surface. The power of the laser at any point is modulated by the color of an input image. Because this is used to etch a bass relief, I knew it could be used to carve in details which were missed by the profile-cutting passes.

There were details I wanted to be able to faithfully reproduce in the wood, like the eye of the horse and the curve in behind the jaw. I hand drew a few details and tried applying them to the side of one our burnt knights.

To our surprise, the raster pass not only etched in the details, it also blasted away the surface material charred by the slower vector cutting passes. We realized that we may well be able to have entirely non-charred output by simply leaving a thin layer of extra wood on the surface and removing it later with a raster pass.

I had a feeling we could use these raster adjustments to cut out a model very close to the shape of the original model. I really wanted to see that in action, so I learned a bit of Scene Kit and wrote a quick system that could draw all the laser cutting passes as geometry, and then fire rays though them to find the distance from the closest hull to the model. This distance controls the darkness of that pixel in the raster image, which in turn controls the amount of material removed by the laser. This was going to be great!

There was just one problem. On closer inspection of our knight model, I realized that it didn’t have much in the way of eye or nostril detail. They were just painted on in textures and not modeled into the surface at all! This fancy ray tracing system was going to be very useful with our not very detailed model. Not wanting to abandon the knight, I cast around for help. I don’t have much in the way of modeling chops, so I knew touching up our model was way beyond me. Having spent more that 8 years as an R&D developer at PDI Dreamworks, I had a few contacts who were up to the task. After asking around for help, Joshua West stepped up and entirely remodeled the knight for us! My Hero! So it was back to the races, armed with his slick new model. I used my rendering system to render out a Visual Hull To Model Offset Map or “Stripy Horse Picture.”

Now we were talking! You can see how the inner ear, eye, mouth, and nostril are all represented. You can see how the raster is compensating for the faceting of the round base and the deep inset under the jaw, etc. I’d had this image in my head for months, and finally I had it rendered out where other people could see it. Now to see what happens when we apply it to one of our scorched and faceted knights.

Meanwhile, Lawrence had started porting his path planning code to a web app that turned out to be both much faster and more convenient than the python scrip we had been using. We were about to try a number of firsts all in one night. Our first fully automated cutting of the model with the thermal-switch advancing the rotary rig. Our first use of the new speedier web app for path planning and our first (admittedly hand aligned) attempt at fine tuning the model with a automatically generated raster pass.

We had a few false starts: I had to add even more averaging of the thermal sensor to smooth out motor noise that was causing false triggers, and Lawrence had to add back in the “burn a line on the thermal sensor” path segments which had gotten left behind in the port. We managed to get the thing cut out and rastered up, and I’ll be damned if it didn’t look pretty darn good!

You can see the eye detail, nostril, the jaw line, even the inside of the ear came out! We were ecstatic. There it was, 3D model to wooden model in only a few minutes worth of laser time! The dark lines on the model are actually the places where the model wasn’t touched by the raster pass. Kind of the negative of the dark lines in my rendering. I think, with the addition of a small amount of protective material left on for the raster pass to remove, we should be able to have a mostly-not-burnt looking knight.

We were so excited that we cut out another one, and when that came out we decided to go for broke and cut out a long DNA-shaped helix model. It was our first attempt at a model other than the knight. It was looking pretty neat, but near the end the thermal sensor missed one of the rotation signals. The final pass cut the twisted ladder away, rung by rung, until there was nothing left. Oops.

If you’d like to see the code, and the PCB designs you can check them out on github.

How to Build a Halo Master Chef Costume

November 4, 2014 by Kurt

No, that’s not a typo. This year I built my son a Halo Master Chef costume for Halloween. I love making Halloween costumes. They’re usually a wonderful excuse to do some sewing and pattern design. When my son asked to be “A Halo Spartan,” I knew I was in for a different kind of build. People who build Halo armor (There are more than you think!) often use a program called Pepakura to decompose 3-D models into paper cutouts. There’s a free viewer, and the full program is only $40. A few years ago, I built my son a Boba Fett helmet using Pepakura, and I’d already developed a system for outputting the designs and cutting them out of card stock using a laser cutter. There was still a lot of tedious gluing, but I could cut all the pieces out and perforate the fold lines, which makes it quite a bit quicker to assemble. I started building this set of armor mostly because the author had done a good job of providing the files and a spreadsheet to help you scale the costume parts.

One of the big issues with Pepakura builds is that you really want the scale to be correct. After gluing 200 pieces together is not the best time to figure out that your scale is off. I think if I were doing a Pepakura deconstruction, I’d provide two versions of the model. A super simple rough draft with only about 10 pieces that would have the basic shape and let you adjust the size, and then a fully detailed version for the complete build. I didn’t really have time to do that. Lots of folks spend a year building their armor, and I was only going to have a couple of weeks. So I downloaded the files, fiddled with the scales, laid out the parts for 24″x14″ card stock that could fit in the laser, and exported the vectors into Illustrator. The vectors already have styles to show mountain VS valley folds, but you have to adjust the spacing to work better on the laser. You don’t want the card stock to get too weak at the fold lines, so no long dashes. I used .4pt dots and 2.5pt spaces for valley folds and 1/2.83/.25/2.83 pt for mountain folds. You can use Select -> Same -> Appearance to select all the mountain/valley/cut lines and adjust them all at once.

When I laser cut the parts, I used 2′ x 2′ card stock purchased from Michael’s for $.75 a sheet. I cut each big sheet in half to make two. Because I’m not printing on the paper, I can’t include any of the edge alignment numbers. I had to keep all the parts organized in some way, so I used blue masking tape to hold the parts in their original cut positions, and just kept all the pages in a folder made from one of the card stock sheets. Then I had to use Pepakura to look up where each piece was in the cut sheets. It was a bit like doing a puzzle. The torso has 200 parts on 6 sheets, and it was important that none of them went wandering off. Blue masking tape is very forgiving and can be pulled off without damaging the card stock. Don’t use normal masking tape, or you’ll be tearing your parts and then your hair out along with some gnashing of teeth.

I used Elmer’s white glue for the gluing. It’s great for this task because it has a long enough working time to let you get things into position and holds well. The one down side is you sometimes have to hold things in position for 30 seconds or so in order to have it start to hold. I modified a few clothes pins by sawing off the tips, making them into more nimble paper-pinching helpers.

The clothes pins were super useful for holding paper edges together long enough for the glue to set. Because I was using pre-perforated folds, I mostly didn’t have to crease the fold lines, and the few that I did crease I could do freehand with a bone folder following the connect-the-dots style along the fold lines. I only did this on a handful of folds. All the others could just be done freehand thanks to the perforations.

The laser cutting took about 2 minutes for cut lines and 6 minutes for the dotted lines. The cutter is stupid about the short segments in those perforated lines, so it actually cuts them significantly more slowly than the normal cut lines. Laser time was maybe 10 mins per page, including setup and taping after cutting. The torso was 6 sheets, so it took almost a full hour to cut, but I can only imagine the amount of time saved. How long did the gluing take? I spent a weekend gluing up two biceps and a forearm. I had started a new audio book and listened to it while I was working, so I unintentionally timed my gluing. It took 13 hours 45 minutes for those three pieces. They have about 45 segments each.

When I started, I told my son I wasn’t making any leg pieces or the helmet. I didn’t have enough time to build a full suit and the helmet and legs seemed like the pieces he wouldn’t be able to wear for trick-or-treating anyway. Partway though the build, he told me he actually wanted to be not just any Halo Spartan, but the Master Chief. I told him I couldn’t swap armor types at that late date, but I could paint it in the Master Chief color scheme. We were joking that maybe instead of a Master Chief he could go as a Halo Master Chef, and he could have a chef’s hat and apron instead of a helmet and leggings. Pioneer loved the idea, and that’s how the Halo Master Chef project was born. After rejecting ideas like the “Gravity Ladle,” we finally decided that the rolling pin was the funniest of the weapon options.

Once the paper shell was done, I decided to back the paper with hot glue. Many people use fiberglass for strength, but that seemed way too slow, messy, and toxic for something he was going to outgrow in a few months. Hot glue is deeply wonderful for doing this sort of thing. Just squirt it in with the gun and let gravity pull it into an even coating. Keep drizzling more hot glue at the front of the downward sliding wave of glue, and it just works.

When you have thick glue, it can take several minutes to fully cool and set. This long setting time along with the need to re-enforce all sides of various openings forces you to glue it in stages. You need to let each stage cool before working on the next side. Tilt things so the glue moves the right way and dams up in the places you want. You can also use things like a straight piece of wood to position edges so they cool flat instead of bulging, etc. It’s very quick, especially compared to fiberglass. I was able to hot glue all the arm pieces in a single evening. After that, the parts are quite tough. There was one casualty of this process. After pushing several pounds of glue though my trusty (but cheap) hot glue gun, my impatient squeezing finally drove the heating element right out of the front of the gun. I guess it’s time to try a slightly more upscale gun.

The torso is kind of a strange shape, and it was really hard to tell if it was going to be the right scale. I knew I’d have to modify the neck hole because it simply was too narrow for my son’s neck. I did some freehand paper design with card stock to make the neck hole bigger. After that had been re-enforced with hot glue, I had to hold my breath and cut open the entire torso with a hobby knife. I still didn’t know if my son would really be able to get into the torso. I even bought a longer chef’s apron in case he couldn’t get the torso piece on. I was worried I’d have to hack some bigger arm holes or perform other violence to make it work. Thankfully, we were able to get it on him. *phew*

Getting the torso armor off and on was a bit of a squeeze, but it worked. The only down side was that part of the front panel has to flex a bit to be able to open the back, which puts some wrinkle marks on the most visible part of the suit. I hot glued a magnet in the upper corner of the suit to hold that in alignment, and I also added two Velcro straps. I think it would have been nicer to have pairs of rare earth magnets all along the seam, so it all would get held in perfect alignment, but those magnets would need to have some sort of holding ring to give them enough surface area for the glue. I didn’t have time to figure that out or order special magnets, so Velcro was an easy on-hand solution.

I painted all the parts with primer then applied coats of army green. I masked the neck and arm areas of the torso and painted them black. After that, it was time to hand paint a bunch of dirt and grunge and a dry brush silver paint on various high points and projections to make it look as if the original paint had scraped off to show the metal underneath.

I laser cut some blue masking tape so I could spray paint the identifying numbers on the torso. I used 051 which is a nod to my namesake Kurt (Ambrose). I also put a black laser cut UNSC Eagle on. I finished Pioneer’s torso detailing the night before Halloween, and it was all ready to wear to school. I was surprised how well the suit held up. Paint was scraped off all the way down to the paper on some of the joints where the arm pieces rubbed against the torso, but in general it still looked good after a day and a night of candy-fueled revelry. We added a black sock as padding around the neck, but that was pretty much the only game-day alteration. Thankfully, the whole rig is still pretty light. I thought he would be less mobile with that rig on, but it wasn’t that bad. He could even ride in the car and use the seat belt without any problem.

I’m going to mark this one down as a success. I have a video of the Master Chef looking tough on Halloween Morning.

Angels and Devils: Making A Smoking Man Candle Holder

January 31, 2014 by Kurt

Every year is build a big Christmas project. Usually I get started on those projects right after Halloween, but this year I started way late. At Thanksgiving, I was sitting out on my sister’s porch and finally managed to sketch something that I liked enough to build. The problem with such a late start was that I kept thinking, “I need to keep it simple, and depend more on excellent design rather than absurd complexity.” This would have been great if I hadn’t eventually let the “keep it simple” part fall to the floor.

In Germany, they have Räuchermänner (smoking men), which are little wooden figurines that have a small compartment to hold an incense cone. The burning incense smoke comes out of figure’s mouth. I’ve always wanted to make one. I considered traditional figures like a hunter, and wackier ones like a dragon, but eventually I decided it might be fun to make a devil with smoke coming out of his mouth. A lot of German Christmas decorations have angels on them, so I thought a candle-holding angel and a smoking devil would make a nice contrasting pair. I started some sketches and paper cut-outs, and I finally started designing the thing in earnest on Dec. 5th. That was an epically late start.

I still had some 1/4″ mahogany plywood left over from the Egyptian labyrinth project, so I decided to use that for the devil. This turned out to be a big mistake. The devil wing design had very thin spars that would have been trivial to cut out of 1/8″ plywood. If I had used 1/8″, the laser could’ve been cutting quickly enough that I wouldn’t have problems with the wood heating and catching fire. Since I had committed to 1/4″, I had to develop an entirely new technique where I cut the devils in three passes, using a syringe to put water into the cuts at each wing tip and pointy corners that would otherwise smolder during the subsequent laser passes.

This worked, and it gave the devil an interesting burned look, but having to develop this technique burned a lot of my laser time. Christmas was fast approaching, and every minute on the laser was precious. I found myself sprinting back and forth to the bathroom with syringes of water. Not good. I also discovered that if you’re cutting multiple passes in wood, it’s best to orient the cuts so they are perpendicular to the direction of the compressed air blast at the cutting head. Otherwise the compressed air can blow along the cut and fan any sort of smoldering wood you may have left in your wake.

I really do not like multi-pass cutting. The wooden bases for the project were thicker than the devil wood, and I was able to cut them very, very cleanly in a single pass. It’s oak, and the tiny holes you see are vessels that form in the spring. You can see how they’re not blocked by sawdust and the radial rays are clearly visible radiating out from the center of the tree like spokes on a wheel. I cut this at 300 pulses per second, and you can see the tiny grooves left by the pulses. There is no charring or need to sand the edges. It’s lovely, but I was careful not to have any thin sections or sharp points in the outline of the base.

The devil was hard to cut, and he turned out to be somewhat tricky to assemble as well. I had to hand miter the top edge of the curved side pieces. I also had to glue a paper smoke dam into his neck since the top of the devil’s body had to be airtight to prevent leakage of incense smoke from his neck. The first two devils that I assembled still leaked a thin stream of smoke from that seam. In later versions, I suspended the assembled devils upside down and dabbed white glue down into the peak with a long stick to make sure the seam was sealed.

Another way that the devil was much harder to assemble than the angel was the slanted back. The angle forced me to hand sand a bevel on the base. The curved sides made it tricky to attach the front since I needed to push the sides out to get them into the curved groove on the front face, but without disturbing the anchor pieces, and without knocking the bottom plate out of whack. So it was an exercise in white glue octopus wrestling. If I applied too much clamping force, the slanted back would cause the base to come squirting out and whole process would begin again.

By the sixth devil, I was quite adept at this, but the first few had some unenviable gaps, which thankfully were not visible from the outside. I like the way the devil’s beards came out. They’re the hidden shape of a swooshing bat. I did some hand wood burning on the horn segments to make them a bit more interesting. The biggest disappointment with the devils was that when I finally applied the clear coat, the contrast between their faces and the facial hair dropped unexpectedly, and by then it was far too late to switch to black walnut or something else.

I like the way their gnarled little fists came out. Complete with thumbs! They’re made from three stacked segments glued together and then glued to the devil’s front. Here you can see the pieces ready for some dabs of glue.

The first two devils I cut had some burning on the inside corners of the belly door. This was because I hadn’t realized those corners also needed some water injection. Later devils didn’t have that problem, but to cover the first two I hastily designed some little feet I could glue on over that area. This design change is why you don’t see any feet in my original sketches.

I also had to punt on having a tiny flame theme around the incense holder because there simply wasn’t enough room, so I switched to a simple ring. The copper pan that the incense sits in was cut from a 1/2″ copper tube cap. I used these both for the incense cup and the candle holder.

I chucked a piece of 1/2″ copper pipe in the lathe and used that to hold the end caps so I could cut them off at two different depths with a parting tool. The only annoyance was that removing the remaining ring of metal from the 1/2″ was hard to do. The parting operation squeezed the copper rings so they were hanging onto that pipe for dear life, and I had to pry them loose with a giant flat-head screwdriver and a lot of elbow grease. Good thing I only had to do that twelve times.

Once the feet were on and the door was done, the only thing left to glue on the devil was the head of the pitchfork. I always glued that on last because it’s cut from thin sheet, and there’s no way to orient the grain of the wood to make them strong along their whole length, so they’re quite fragile between the tines where the grain cuts directly across those narrow sections.

The angels were comparatively simple to assemble. I carefully positioned them on the maple board so the maple’s figure would form the folds of her skirt and sleeves. I used different wood for her face, neck, and hands. I used a layering effect with the hair to try and keep her head from looking too much like 2D extrusion. I was going to try layering the area with her ear back one layer to make it even more 3D, but that would have required some more hair fragments, and a bit of iterating on the laser to get the ear size just right. Eventually I punted on that plan. So her head is a bit more of an extrusion than I would have liked. Oh, well.

I did have to have a slightly tricky clamping rig to glue on the top wing mounting bracket. I used a laser-cut scrap to match the shape of the bracket and make the clamping possible. Then I used some clothes pins to keep the scrap aligned while I tightened up the other clamps.

The only materials disaster I had with the angels was that the 1/8″ plywood I used to make the wing spars was defective, and some of the spars had their topmost layer of wood just fall off. I had to re-cut a bunch of them. I’d never had that happen before. I used clothes pins to provide even clamping force when gluing the spar to the feather veneer.

I had originally thought about using white paper angel wing feathers and black paper wing membranes for the devils, but when I was shopping for the figured maple board I used for the angels, I found some lovely dark figured veneer. I realized it would look SO much better than black paper on the devil. I already had the veneer I needed for the angels, so after I’d sprung for the devil veneer, I switched the angel over and never looked back. I’m glad I did.

I did end up having to sand a slight bevel onto the very bottom point of the wings to keep them from clonking into the angels bustle, but other than that they were really easy to put together. The wings are glued to the top bracket, but they are just slid into the bottom bracket. That keeps them from getting pried off by differential wood expansion of the body and wings.

The only serious annoyance I had with the angels were their faces. A face is so important, and I’d really sweated the design. I ended up trying to keep it super simple. Just a few lines. However, as I was assembling the angels, I realized that from a lot of angles the laser-etched faces were very hard to see.

Faceless angels are a bit creepy, so I knew I had to do something. For the first two, I used black acrylic and a fine paint brush to darken the lines after the clear coat had been applied. That was a pretty ticklish operation to do on otherwise finished pieces. It was no fun at all. After that, I simply highlighted the face details with a mechanical pencil. That was nice and easy, but when I used a brushed on clear coat the graphite ran and gave their expressions a somewhat haunted look. I spray coated the last two angels, and their faces came out the best. Nice clear details from any angle.

One of the design details I’m proud of is that I wanted a little cleavage V in the front aligned with her hands, but I knew that would look weird on the back. So I positioned the wing mounting bracket so it neatly trims that off, making it look more like a normal dress back.

I wanted to make halos for the angels. I went wandering through the hardware store to see what kind of rings or loops I could find. I purchased a few different kinds, but eventually settled on some straight knurled brass lock nuts. As the giant monolith of Christmas rolled inexorably toward me across my calendar, I decided to punt on the halos. No one would miss them.

However, during the Christmas break, I had a change of heart (and a bit more time), so I decided to make the halos even though that meant giving out a few “halo retrofit” kits. The alignment holes for the halos were in the original design, so adding them was just an insertion and a dab of E6000. Simple pimple. To make the halos, I used a boring bar on the lathe to machine the threads out. No self-respecting angel goes out in public with a halo that looks like it screws on. I then had to drill a tiny hole part way through the halo to mount a piece of piano wire.

Luckily, I already had the tiny center drill for the job, and I set up a depth stop on the drill press. Then it was just a matter of clamping and drilling the six golden rings. Four calling birds, Three french hens, Two turtle doves… Wait where was I?

I used a bit of JB weld to glue lengths of piano wire into the holes. I glued each wire to the halo at a jaunty angle so the angel would not look like she was balancing a book on her head at Angel Finishing School.

The piano wire feeds down through holes in both the upper and lower brackets on the back of the angel, and a dab of E6000 cements the wire to the bottom bracket in a slightly springy but tenacious way. Then it was just a matter of mounting each angel on a base, and adding a candle cup made from another 1/2″ copper pipe cap.

The devil is mounted using T-shaped pins that pass through his floor plate and into matching holes in the base. The angel is simpler. She just has two pins on her bustle that align with slots in the board.

I managed to have two sets completely done by Christmas Day, and I finished up the other four sets a bit into the new year. Overall I’m very happy with the way this year’s Big Christmas Project turned out, especially considering the late start. I’m marking it down as a success.

Making a Pop-Up Card

May 21, 2013 by Kurt

When I was growing up, I did a some bike touring with my dad. My uncle joined us on some of those trips, including one summer the three of us spent biking around the British Isles. We slept at youth hostels and biked around England, Ireland, and Scotland. Sometimes when I lagged behind, my uncle came circling back and said something like “Oh no! A honey dipper truck turned over in the road. I had to turn back.” to jokingly explain why he was coming back to ride with me and keep me company. I really appreciated that.

When I heard he was seriously ill, I booked a flight to go visit him. With my flight 4 days away, I thought I would make something small to give him. I figured this might be the perfect chance to try my hand at making a pop-up card. I remembered one time in Scotland we zoomed down an hill and suddenly come across a lot of sheep in the road. None of us crashed, but there were several close calls. I guess a big dose of adrenaline helps make a lasting impression.

Making the card was going to be a race against the clock. If I could design and laser cut all the parts after work on Wednesday night (the night I have free to go to TechShop and use the laser cutter), I could spend some time Thursday night gluing the whole contraption together to have it ready to take on the plane with me Saturday morning.

The first thing I did was a quick sketch of the idea in my notebook. I put in a mountain, some foothills, a tree, my uncle on his bike, and a sheep. I figured that would be enough layers to give the final project some depth without having to design too much stuff. I also did a quick diagram of the various depths of the things and the road my uncle would be biking on.

I made a few notes about what colors of paper I’d need, and then I headed to the store. When I got there, the woman at the counter told me I had five minutes before they closed! Oh no! I rushed into the paper section and started grabbing sheets of colored card stock. In my mad rush to get paper, I ended up buying nearly $20 in paper sheets. It turned out to be a good thing that I didn’t spend too much time choosing paper because my laser reservation was at 10pm and still needed some design time before that. I snagged a quick dinner at Clark’s and headed back to work.

I went back to my desk and fired up Illustrator. I started with the bike wheels. I knew that with the laser I could cut all those impossibly delicate spokes without even breaking a sweat, something I’d never try to do with an razor knife. I also designed the spacers and hinge flaps that would anchor the parts and allow them to fold.

I used dotted lines to perforate the sheets where the paper needed to fold. At 9:30pm, I headed over to TechShop so I’d be there for my laser reservation, but I took my laptop along since I still wasn’t done with the design. I knew I could cut single sheets of paper at the machine’s maximum speed, but I had to do some test cuts to determine what power and pulse rate to use. I was able to get some settings that didn’t burn the edges of the paper much. With a low enough pulse rate you can even get the paper to be sort of microperfed, but not fully cut, in in some spots. This can be handy because the compressed air and blowers clearing the air in the laser can make your tiny pieces of paper get sucked into the exhaust system as soon as they are cut free.

I spent the first hour of my reservation frantically finishing the design. The second hour I worked on the laser cutting, swapping sheets of paper into the machine to cut. I finished just at the stroke of midnight. I took my pile of laser-cut paper parts home tucked into my notebook for safe keeping.

At this point, I needed to make some decisions about glue. In the past, I’ve used 3M’s Super 77 spray glue for paper, which works well, but doesn’t give you much chance to position parts once they’re in contact. So that was out. I’d used decent quality glue sticks, but those were far too blunt an instrument for something like my super tiny bike tires. Strike two. I’ve used white glue for some fairly stiff card stock, and although it is very strong and has a decent working time, it causes wrinkling if you use it on a larger area. I had to figure out something else.

I did some reading online, and the next day at lunch, I bought two kinds of glue: Zip Dry and Tombo Mono Multi. Both where supposed to work well with paper. I ended up being glad I got both.

Zip Dry is like super-refined rubber cement. It even smells like rubber cement. It has enough working time to fine tune the part positions. And cleanup is super easy. You can rub excess away cleanly and all you’re left is a pile of rubbery crumbs that can be brushed away.

The Mono Multi is white but supposedly dries clear. It gave me enough working time to position parts and set up fairly quickly with a strong bond. The big down side is that it’s far worse in terms of cleanup. Excess glue has to be skimmed away immediately or you get an ugly, sticky patch which can not be cleaned away without damaging the paper. It doesn’t much matter that it’s “clear” at that point.

An overnight test of the two glues showed that Zip Dry is not as strong as Tombo Mono Multi. When I peeled apart the pieces of paper I’d glued, the Zip Dry parted at the glue line while the Tombo Mono Multi delaminated the paper.

This turned out to be just the right combination of glues. The Zip Dry worked for things like delicate tires on spoked wheels, and the Mono Multi held together the flaps and hinge pieces that needed extra strength.

Here is my uncle on the bike fully assembled. I also completed the sheep and glued up the tree and the mountains. It was time to start assembling the card proper. My main concern when laying out the card was that the objects must be at least as far from the front edge as they are tall so nothing would poke out from inside the card when it was folded up.

I also wanted to add some struts behind each object, forming a parallelogram between back of the object and floor of the card. This parallelogram is what lets the object fold down flat and pop up when the card is opened. I know real popups use other kinds of tricks to make things unfold, but I didn’t have time to do any research into it. Just the basics!

One thing I did do right was to pass the bike’s white hinge tabs down through small slits I made in the road surface, and then gluing the tabs to the under side of the road. The white tabs from the wheels would have stuck out like soar thumbs if they were visible. It made the bike look like it was really standing up on the road surface. I wish I’d also done that for the sheep’s feet, the tree trunk, where the struts glued to the foothills, etc. That would have looked a lot nicer.

I started assembling the final card. My layers were mountains, foothills, tree, bike, and sheep. Each object had tabs at the bottom and struts that push/pull the object when the card opens and closes. The object and the sky on back of the card had to be parallel, and the struts had to be parallel with the road surface bottom of the card.

I glued on the mountains, then the foot hills. I discovered that the strut for the sheep was impractically long. I thought it would bow when pushing, so I improvised a little vertical leg half-way along to help it stay even.

The exact positioning was somewhat improvised since I had had to rush the design and wasn’t able to spare much time for the actual mechanism. If I’d had another 30 minutes, I could have laid out slits in the road/background that would have hidden the various glue tabs and made the positioning exact. I would definitely do that next time.

The loosy goosey by-hand approach made me use a somewhat tricky system where I would glue the object down, glue the strut to the back of the object, and then put a dab of glue on the other end of the strut and fold the card shut. This method forced the tab to be glued in the correct position and ensures that the card can close fully.

However, if I put too much glue and there was glue squeeze out, the card would glue itself shut. Not good! I carefully worked through gluing the mountains. Fine. The foot hills. Great. The sheep. No problem. Then I put the last two dabs of glue on the bike and the tree. The card was almost finished!

Disaster! On the very last action of making the card, I accidentally folded the struts up instead of down when I was closing up the card. The two struts glued themselves to the sky! NO! The Tombo Mono Multi sets up fast, and I wasn’t able to detach the two sheets without tearing some ugly rents in the sky. Oops.

At that point, I was too exhausted to quickly design some clouds, birds, or UFO’s to cover the mistakes. Thankfully, I still had some more of the blue- lined paper, and I was able to hand cut a patch. The horizontal lines do a good job of hiding the fix, and for the most part no one can see it unless I point it out. *Phew!* The card was done.

I shot a quick video of the card opening and closing. There’s also some footage of the bike being laser cut.

Sadly, my uncle died before I could give him the card. I don’t regret making the card though. It was nice to have spent that time thinking about him and our trips together, certainly much better than spending that time waiting and worrying. I did give the card to his son. Now I’m the only one who remembers that wonderful summer. I wish I were a better keeper of the past.

Retro Tech Journal

Tales of Creation and Recreation

Tag Archives: Laser Cutter