Minibot Motor Fastener Size?

[Tom Line]

I'm looking for the motor fastener size for the tetrix motors - specifically the fasteners that attach directly to the motor, and the long fasteners that hold the transmission on. Unfortunately I don't have access to the thing for another day at least, and I'm trying to get some cad work done.

[Bill_B]

There are two threaded holes on the gearbox face. They are M3xP0.5 despite what a drawing I saw once that claimed they were M2. Not sure about the penetration allowed into the gearbox housing before interference with the gears will happen. Also not sure about the three screws holding the gearbox on. Pretty good guess is that they are metric. Might even be M3 too. Just take one out and try it in the mounting hole. LMK-OK?

[LH Machinist]

We ordered M2 screws and of course they didn't fit. The M2.5 or the M3 screw charts don't match the minor diameter of the threaded holes. I tried a 4-40 screw and it’s a good fit...imagine that!

[Bill_B]

Originally Posted by LH Machinist

We ordered M2 screws and of course they didn't fit. The M2.5 or the M3 screw charts don't match the minor diameter of the threaded holes. I tried a 4-40 screw and it’s a good fit...imagine that!

I tried a 4-40 in my motor, 'cause that's what're in my parts cabinet. I didn't have the same luck you mention. I fetched some M2s based on the drawing, i.e. a trip downtown. I didn't have metric charts for reference. Another trip for M3s gives me some hardware that fits the holes I have OK.

Given the drawing discrepancy and both our empirical observations, perhaps there are different gearboxes involved. The Tetrix motor mounting clamp allows eccentric rotation of the output shaft. This aids and simplifies gear and axle placement in what may be called an advanced hobbyist robotics kit. I'm not a Tetrix expert, but it doesn't look like there is a hole placement in this system that will allow use of those holes. None of the standard Tetrix hardware appears to fit those holes either. (I haven't tried any of the set screws) I can imagine Tetrix, Inc. placing an order for gearmotors without specifying those mounting holes one way or another. It might result in the supplier sending out mixed batches. Some might have metric and some might have SAE mounting holes, theoretically.

Meanwhile, readers are free to experiment with their in-hand hardware. Please note your results and let us know what you find.