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These are the final steps to make a coupler that connects a LEGO wheel to a gearmotor. At this point, the metal coupler body has been machined on a lathe.
If you have a four-jaw chuck, you can drill the setscrew hole on a lathe. However, most beginners opt for the factory-supplied three-jaw chuck since it auto-centers on the workpiece. Since most readers don’t have the four-jaw lathe chuck, I decided to present instructions on how to drill the setscrew holes on a drill press or milling machine.
Lining up a drill over a center hole in a rod in a v-groove in a machining vise.
A small machinist vise usually includes a v-shaped groove for holding round material (or for holding square material like a diamond).
If you have a milling machine or a drill with an x-y table, a quick way to visually line up the drill is to place it in front of the motor-shaft hole on the coupler. Then, lift up the drill and move the y-axis until the drill is in the desired location over the coupler.
If you’re creating a bunch of similar couplers at the same time, you can simply remove the drilled coupler from the vise and put the next one in (without moving the mill or drill).
You'll notice in subsequent photographs that I chose to drill two setscrew holes in each coupler for added holding power. This is optional.
After drilling, use a tap to add threads to the holes. I used a #43 drill and a #4-40 tap to use #4-40 screws on my couplers.
There are two methods for securing the Lego cross axle in the wheel coupler: epoxy adhesive and press fit.
Try inserting the cross axle by hand. If the axle slides in and can be rotated in the coupler, then it needs to be glued in place using epoxy. This is the usual method of choice, simply because it doesn’t require special tooling.
Force-fit a Lego cross axle into a motor coupler by using an arbor press.
However, if the hole is too tight to insert the cross axle by hand, then the cross axle can be forced into place with an arbor press or the jaws of a vise. I suppose you could try banging it in with a hammer, but the advantage of an arbor press or a vise is that they exert steady pressure in a controlled direction rather than a sudden shocking force in a potentially angled direction.
Lately, I’ve been using the arbor press method of installing a cross axle rather than using epoxy, simply because the epoxy is messy and sloppy.
Damage to the sides of the Lego cross axle caused by sharp inside edges.
My first prototype didn’t have chamfered edges on the cross axle hole. When the arbor press forced the cross axle into the hole, the sharp excess material scrapped off curls of plastic. This reduces the diameter of the cross axle inside of the coupler, which may cause it to slip under heavy usage.
If I had used epoxy, the sharp edges would have wiped off the epoxy. So, don’t skip the step of using a larger diameter drill to chamfer the inside of the cross axle hole on the lathe.
A pair of finished brass couplers ready to install on a robot motor.
This completes the lathe creation of cross-axle-based couplers that join Lego wheels with standard DC gearhead motors. If you don’t have a lathe, my books describe other methods. But, I think the lathe produces the most consistently centered couplers in the least amount of time.
Since this article was written, I’ve come up some minor improvements to allow you to create shorter, more compact couplers.