I’m working on an indoor robot that needs to survive bumps and collisions. The weakness of my usual Lego-wheel-to-commercial-gearmotor coupler is that the wheel slowly works its way off the Lego cross axle when driving. Obviously, Lego parts are designed to be easy for a child to assemble and remove, so I don’t fault Lego for the cross-axle design.
Wheels eventually come loose on Lego couplers with only a cross axle
Pictured below are some of the larger Lego wheels that I might want for the new robot. Although I could permanently attach the selected wheels, I’d still like the flexibility of swapping out different wheels for experimental purposes.
Lego wheels that include hubs with peg holes
One of the common features of these wheels is that they have at least three peg holes arranged around the center.
Lego hub with three peg holes
Looking back on previous robots, I was pleased with the coupler design that I used on StreamHawk. Specifically, rather than relying on the friction of the cross axle, three screws fasten the wheel to a larger coupler. So, I reused that design element on this coupler, as demonstrated in the photograph below.
Lego wheel securely attached to escap gearmotor
The coupler itself attaches to the gearhead motor shaft with a setscrew. I’ve had the setscrew loosen before, so this coupler has three setscrews spaced evenly around the perimeter. This coupler is made from plastic, as opposed to metal, because plastic tends to bounce back after machining. That means the setscrew hole and threads will be a tiny bit undersized, which will hold the setscrew more firmly against vibration.
Escap gearhead motor with coupler
The holes for the screws that attach to the hub are drilled on a milling machine with digital readout (DRO). The DRO is fantastic for positioning the drill with high precision and accuracy. The readout displays the position of the workpiece in horizontal (x) and vertical (y) coordinates in either millimeters or inches.
Digital readout (DRO) is critical for accurate machining
Aside: The text display pictured in the DRO above is a 4x20 vacuum florescent tube that is very easy to read and hooks up with an industry-standard 14-pin LCD interface. Only the top two lines are used, because the original display was a 2x20 OLED. Unfortunately, OLED displays slowly die when exposed to moisture, such as in my basement.
To create this coupler, the first issue is figuring out how to convert the Lego peg holes positions to an x-y coordinate design for drilling on the milling machine (or drill press). That’s going to require some geometry and trigonometry.
The next step will be cutting the precise diameter and thickness cylinders from a raw-stock plastic rod on a lathe. Then, as mentioned previously, the holes will be drilled on a milling machine. Finally, the threads will be cut into the holes by hand with a 4-40 tap.
All of this means that this article is about math and machining. Therefore, only loyal Robot Room readers will click the Next Page link.