CHAPTER 20 ■ COUPLER
Removing Extraneous Epoxy
After drying, sand or chip off extra epoxy as desired. Notice how easily the epoxy chips off the plastic LEGO
axle? Don’t worry! If the axle was very clean and you made notches with a file, the axle should hold quite
strongly in the coupler. If the axle falls out for some reason, a nice reverse “X” of epoxy remains in the
coupler. Try some hexane and cyanoacrylate adhesive to adhere the axle back in place within the epoxy
reverse “X”.
Fortunately, the axle doesn’t get tugged on very much as it gets rotated. Most of the motor force
transmits to the axle through the reversed epoxy “X”. As such, the epoxy’s shape and adherence to the tubing
are more important than adherence to the axle.
If you applied plenty of epoxy, it’s likely that some got forced down into the motor shaft’s side of the
coupler. Drill out the invading fragments with a rotary tool and a^1 / 8 -inch drill bit. As long as the chips come
out white, you haven’t hit the black plastic LEGO axle yet. The axle is much harder and resists drilling. If the
drill feels like it has bottomed out or black chips start coming out, you’ve drilled deeply enough. Some epoxy
sticking to some of the sides of the tube may still need attention from the drill.
Adding a Setscrew to the Coupler
The coupler is almost complete. Something needs to be added to hold the coupler on the motor shaft, yet
still allow removal.
A setscrew is a small screw that applies pressure against the flat portion of the motor shaft. Not only
does the screw prevent the shaft from sliding out of the coupler, but it also forms a “D” shape within the
coupler. The flat side of the “D” shape gives the flat portion of the motor shaft something to push against, so
that the shaft spins the coupler rather than the shaft just spinning within the coupler.
Marking a Hole for the Setscrew
Hold the coupler beside the motor shaft to determine a good location for the hole in the coupler. Aim for the
end of the flat portion of the shaft that’s nearest the motor. That tends to be around 6 mm or so from the end
of the coupler. Mark the spot with a fine-point felt-tip permanent marker (see Figure 20-18).
Figure 20-18. Marking desired location for the setscrew hole on a coupler using a fine-point permanent
marker