When developing homemade robot wheels, drilling a hole in the correct place for the motor shaft can be tricky. Even a slight error causes the wheel to wobble or at least rotate off center enough to reduce performance.
Tool and box for 1 1/2-inch center square
MicroMark sells a metal tool ($17.45 #82285) that helps locate the center of any small circle, square, or octagon.
Place the circle firmly against the inside of the tool. The tool has sides that are at a right angle (90 degrees).
(The picture above shows the tool under the circle. Actually, you'll need the circle to be underneath the tool -- flipped over -- in order to draw the line in the next step.)
Draw a line using the straight edge as a guide.
The straight edge bisects the right angle. In other words, the straight edge is 45 degrees from each side of the circle.
Rotate the circle 90 degrees or whatever and draw a second line.
The second line intersects the first line at the center of the circle no matter what angle is chosen.
The center of the circle is where the lines cross.
You don’t really need the tool. A ruler and a protractor will work just fine. The tool is just a little easier and precise.
Now if you could only drill straight.