On the prior page, we drilled precise screw holes in a cylinder using a digital readout. The final purpose of those holes is to firmly attach a Lego wheel. To fully convert the raw cylinder into a coupler, screw holes must now be drilled in sides of the cylinder to tighten screws against the motor shaft.
Less rugged couplers use only a single screw. Professional couplers tend to use two screws at 90 degree angles. To better ensure against wheel loss, this coupler will have three screw holes spaced evenly at 120 degrees.
One of the most difficult aspects of drilling setscrew holes is to make sure they will exit in the center hole where the motor shaft will reside. An errant starting position or angle may miss the center hole completely or be offset enough to reduce contact with the motor shaft, thus eliminating or reducing the setscrew’s grip.
The digital readout can guide the x-y table back to 0, 0. That’s where the center hole was drilled. Now I simply need a trick to repeatedly position each cylinder in the same location as it resided when it was laid flat.
Start with a cylinder in the flat position. Place squared blocks against one side. Clamp those blocks in place or back them with strong magnets.
Magnets and steel block position when coupler centered in V block
Remove the cylinder and reinsert it on its side. Gently press the cylinder up against the block to align it in position and tighten the vise. A setscrew hole can now be drilled with a #43 drill.
Drilling setscrew through center of cylinder positioned by magnet fixture
I was hesitant to use magnets to secure the metal block. Magnets tend to be weak against slippage. However, the block is not holding the cylinder against any forces during machining; the vise takes the entire load. The block only needs to resist the gentle pressure I apply when inserting the cylinder in the vise.
The magnets held fine. If you’re really concerned, you can use a clamp or invent a sturdier fixture.
Loosen the vise, rotate the cylinder 120 degrees, tighten the vise, and drill. Repeat for the final hole.
The 120 degree spacing between setscrew holes does not need to be precise. However, I lined up the tops of the front screw holes to act a guide. The only critical concern is that the setscrew holes be drilled without intersecting where the front screw holes pass through the cylinder body.
After tapping all the holes with a 4-40 tap, the finished product is pictured below.
Motor coupler machined from CPVC plastic
These are the most precise, reliable, and smoothly-rotating couplers I’ve created to date. I am very pleased with the results.
There were several minor mistakes that do not detect from functionality, only aesthetics.
Minor mistakes in coupler
During drilling of the setscrews on the side, I accidently drilled a few holes on the wheel end of the coupler. The setscrews should only be drilled on the end of the coupler where the motor shaft is inserted.
The other error was that my wonky attempt at separating the cylinders from the rod mangled the edge of one of the cylinders to a depth that was not removable during facing.
Neither of these mistakes matter, as the coupler is partially hidden when installed on the robot.
If you have the right equipment (lathe, milling machine with DRO) and you want to securely attach a medium or large Lego wheel, then I highly recommend this style of motor coupling.
If the machining in this article is beyond you, there is nothing wrong with the beginner’s coupler presented in Robot Building for Beginners. In that book, you can make a coupler with telescoping tubing, a Dremel rotary tool, a hand tap, and some epoxy.