Servo motors are a nice option for making things move. They’re readily available and easy to control using either a Raspberry Pi or Arduino. Unfortunately, they connect to whatever you’re making with a proprietary spline. This has previously limited me to off-the-shelf parts from a company like Servo City. Admittedly, there are plenty of options to choose from, but I consistently find myself trying to create something strange: a partial gear or an in-between pitch diameter. It got me wondering if I could cut my own gears on a laser cutter. Good news – it’s relatively easy.
Step 1: Finding the Spline
I was perusing another instructable created by friend and fellow AiR, Palo Salvagione, and discovered he was 3D printing parts matching the spline for a standard sized Hitec servo. This is the same servo I use in many of my projects, so I shamelessly stole the spline profile he was using in his String Fountain and added it to my own geometry.
Step 2: Measuring the Laser Kerf
Whenever you cut material, the material that’s removed is called the “kerf”. To accurately cut the spline and gear teeth, you need to offset the cut line by half the kerf to allow for the material lost to cutting. So what’s the kerf for a laser cutter? The laser I’m using has a sticker claiming it’s 0.01 inch but a test quickly showed that was not true for my setup. I suspect it changes depending on the material, its thickness, and the mood of your particular laser. To find the exact kerf for cutting 1/8 inch thick material, I drew a 1 inch square in Adobe Illustrator and sent it to the laser. Measuring both parts showed me that 0.01 inches of material was removed. There are two vertical cuts for the side of the square, so the laser kerf is running about half that, or 0.005 inches. If I offset my line by half the kerf, or 0.0025 inches (I rounded down to 0.002), all should be good.
Step 3: Cutting a Test Spline
To check my assumptions, I cut several test splines at various offsets: 0.002 (assumed offset), 0.001 (slightly tighter), and 0.003 (slightly looser). After cutting this test piece I tried to press the part onto a servo spline. What’d ya know - the spline offset 0.002 was a perfect fit.
Step 4: Cutting Gears
Using CAD (I like Autodesk Inventor), I designed some new gears that included this spline. Exporting the face of the gear to a DWG file gave me something I could again open in Adobe Illustrator. With my 2D profile open in Illustrator I offset the lines. The only potentially confusing part here is the direction of offset: Lines on the outside of the part (gear teeth on my specific part) are offset away from the part (a positive offset). Lines on the inside of the part (the spline on my part) get offset toward the center of the part (a negative offset). Take a minute to think about your final part and where material will be removed. If you accidentally get set the offset in the wrong direction it won’t go well for your parts.
I've included all the files I used to make these gears. Go make something move!