Design challenges with laser cutting mother of pearl and laser engraving a wooden remote
Step 4/4 in the “Everything you need to build your own Turn Touch smart remote” series
This is part of the full guide on how to make your own Turn Touch from scratch. This is the story of the design challenges faced when trying to make a seamless remote and how to overcome them. If you follow this guide, using the accompanying open-source design files, then you will be able to build your own Turn Touch that you can use to control your smart devices and apps on your phone and computer.
If you want to get your own, Turn Touch is on Kickstarter.
I set out to make the most beautiful remote I could. And after observing what many of the most beautiful handheld objects I’ve seen had in common, it became clear that Turn Touch deserves an inlay.
The source code for the inlay paths and laser cutter settings is available on Github.
The choice of mother of pearl came naturally. Wood and shell are both natural materials that are sustainably grown and can be beautiful to behold. The trouble with mother of pearl is that it is an exceptionally brittle material. Great care must be taken in both cutting the shell and then adhered it to the wood.
There are two main considerations to make when thinking about cutting the mother of pearl to fit. First is that the remote needs to be engraved to perfectly fit the mother of pearl. Second is that the mother of pearl itself needs to be cut to fit the engraved wood with no margin between the two.
Engraving the wood
Let’s begin with how I started the engraving process. The wood is considered the source of truth and will be engraved with no compensation for the mother of pearl. The shell must compensate for the wood.
Originally I used the ShopBot CNC machine to engrave the logo into the wood using a 1/32" flat end mill. There were a few issues with using the CNC to engrave:
- Every inside corner turns into a 1/64" corner radius due to the limits of axial router cutters.
- The depth of the cut (height) kept changing due to machine tolerance.
- This involves a tool change, which is costly and time consuming.
Worst of all, due to differences between machines, the laser cut mother of pearl would have slightly different dimensions from the CNC engraved wood in both the x and y axes, resulting in a not-quite-perfect fit, leaving a variable sized gap between the shell and the wood.
The fix is to use the same machine for both engraving and cutting so that the same machine tolerances are applied, resulting in a uniform offset between wood and shell.
Using the laser cutter to both raster cut and vector cut
Engraving on a laser cutter is very different than engraving on a CNC machine. Whereas a CNC will route a tool bit in a pocket, stepping over itself with a minimal amount as to be efficient as possible, a laser cutter will rasterize the pocket shape and work in a way not unlike a 3D printer: line by line.
Before we begin working with the laser cutter, we take simple safety steps to ensure our health. The first few times I worked on the laser cutter, after about an hour my throat started to hurt. Probably due to all the acrylic being massacred by other people sharing the pool of laser cutters.
Wear gloves and wear a dust mask. Your lungs, skin, hair, and fingers will thank you.
Here’s what we’re going for. We need to accurately engrave into the wood. This means that the cut needs to be centered and oriented. If we were to just place the remote bottom side up and shove it in the corner of the work bed, the engraving would come out askew.
Here we can see the laser cutter do its magic. To achieve this registration and orientation of the remote in the laser cutter, we need to do the same thing we did for the CNC machine: we make a fixture.
A fixture to ensure accurate centering
Lucky for us, this fixture is easy to make. I just took a ¼" piece of plywood and ran the laser cutter around the outline of the remote at full power to cut through. I had to adjust my offsets a bit to fit the remote, but it only took a couple tries.
The remote is sitting inside this hole. Because the laser cutter made the hole, we can just center the logo engraving (or custom inlay engraving) in the hole to ensure that we are correctly registered. The plywood fixture itself is pushed up into the top right of the work bed so that it remains in a constant location.
Finding the right power settings for laser cutting wood
The toughest thing about using a laser cutter to engrave wood is optimizing power settings. We want to perform the work as quickly as possible which ostensibly means using the highest power and fastest speed available. But we need to watch out, as too much power makes the wood burn.
There are three variables we can change: power, speed, and throughput. Power and speed are self-explanatory. Throughput is the density of the cut. Low throughput means that the laser cut spends longer making a higher resolution rasterized cut. So low throughput takes 3 minutes while high throughput takes only 15 seconds.
We want to start with establishing a minimum throughput that gives us the minimum acceptable quality of the cut we want but at the fastest speed possible. So we start with a low throughput to establish high quality and then move up in throughput until we reach a cut that’s too sparse.
Take a look below at this mahogany wood blank to see what happens when wood burns, even subtly. Starting on the right-hand side you can see how the laser cutter’s exhaust system pulls the smoke up and over the wood, leaving a burn scar above the cut.
Starting from right to left:
- High power, 45 sec: notice the burn scars above the logo. That’s caused by the exhaust fan of the laser cutter pulling the burn up. You can reverse the direction of your cut so that you cut from bottom up, “erasing” the burn scar by burning it, and then possibly using a light power setting by the time the laser reaches the top quarter and going over it multiple times. But that’s quite a commitment to shave maybe 15 seconds off, and even then you still might end up with a burn scar.
- Medium power, 45 sec: Same time as before but less power means less burn scar. We need to adjust the density (throughput) of the cut now.
- Low/high throughput, 180 sec/15 sec: This photo clearly shows the difference between low throughput (high density) burning on the right and a high throughput skipping every 5th line on the left.
- Slow speed, high power, 45 sec: I’ve established a desired throughput and now I’m experimenting here with changing the speed but maintaining high enough power. The cut is reasonable but it still burns due to the speed. If we turn up the laser to max speed but turn down the power output, we get a nice cut at a fast speed, but it’s just not deep enough. To fix that we just need to run the cut twice.
- Medium throughput, medium power, high speed, double run, 35 sec ⨉ 2: Here we get to a much more reasonable cut. I’m using 20% power with 100% speed and a 5/7 throughput, but it’s still not deep enough.
- Medium throughput, medium power, slower speed, double run, 60 sec ⨉ 2: And now we’ve got it. Moving down to a slower speed but at only a medium power we get the cut we want at a reasonable time of 120 seconds.
YMMV on your particular laser system, but this is a good way of narrowing down values until hitting local maximas.
Problem at the edges
When a laser cutter cuts through material, the beam diffuses and creates a sloped edge. You can see how this edge can be a problem below.
When you try to inlay the shell later, these slopes will cause the edges of the shell to sit higher than the rest of the shell. It would be very easy to crack the shell if you press it in, since it can’t handle the stress of resting on the sloped edge.
One way to reduce the sloped edge is to adjust the focus of the laser. Even if you successfuly set the height of the focus, it still might be off by ±0.005", which is half the height of the shell itself. So it makes sense, once you establish power, speed, and throughout settings, to also attempt a few cuts above and below your laser cutter’s established focal point, in 0.0025" increments, and see if the slope goes away with the adjustment.
Alternatively, since the slopes are only at the edges, you can perform additional cuts with the laser, feathering the inner edges, offset by enough distance so that the diffusion of the laser doesn’t show up outside the engraving.
This is what I do and it only takes a couple extra seconds to make a few cuts at the edges on one side. Sloped edges do not necessarily happen on every edge, since the grain of the wood can naturally reinforce an edge or make it harder for the laser to make a clean cut. So in my case I only had to feather one edge of the engraving to get the shell to fit perfectly. And because the cuts are inside the engraving, it doesn’t show and the shell fits in at the proper depth.
Vector cutting the mother of pearl shell
This is the good stuff. Mother of pearl is such a delicate material that it only needs a light power setting. The only variables I messed around with were power and speed, since vector cutting has no throughput component like the rasterized engraving in the wood above.
Let’s cut the inside edges of the design first. Just as you don’t want to paint yourself into a corner, you want to laser from the inside out. Otherwise your newly free-moving material may not stay in place for those inner cuts.
The Universal laser cutter I used allows you to set colors that applies an order to each cut. I also used a lower power setting for the outside edge cut. Since the entire square is easier to separate that the inner cut, a faster speed cut keeps the mother of pearl marginally connected so that the individual pieces don’t go flying as they get cut.
I could have run the laser at a higher speed (at a higher power to compensate), but I kept it at a low speed to ensure a quality cut. If the wood engraving isn’t perfect, it matters less than if the shiny shell has rough corners that become a lot more noticeable.
Adhering the wood and shell together
Once you’ve got the two cut materials, the wood and the shell, you’ll need to get them to stick together. Since there’s no forces acting upon the inlay other than to reinforce it down into the engraving, we don’t need a messy epoxy or expensive shell glue to hold it in.
I tried many glues and settled on squeezable Extra Time Control super glue gel. The gel makes application easier, instead of being thin and runny. However, using off-the-shelf super glue (cyanoacrylate) without time control gave me only 5 seconds between application and drying. So placing the shell into the engraving was a one-shot deal, with the shell becoming permanently affixed to the wrong place if you didn’t nail its position the first time.
Time control or not, it’s still super glue and it adheres within 15 seconds. But because it has time control, any excess glue easily wipes off the wood with a paper towel.
That’s it! That’s the last step. We now have a complete Turn Touch ready to ship to backers.
If you want to be one of those backers, head to Kickstarter and get your own Turn Touch.