I have need for a swing arm that is mountable on the shaft of a motor. This thing allows me to play with different mounting hole sizes, lengths, and thickness. Doing the math and calculating the exact size, weight, etc for the specified motor torque would be easier, but less fun.

In general, since I model using OpenScad, I find it useful to include the .scad file so that others can play with it without me having to generate more models.

1) Print the thing out
2) Slip it onto the shaft of your motor
3) Rejoice

You might have to play with the radius values to get the kind of fit you want for your specific shaft.

It could be improved by having a captured nut and holding screws, but that's for a future design.

The holes use $fn=32 for smoothness. You could play with that to suit your needs as well.