Thank you for taking the time. I really appreciate it.

This is a cross section of a differential drive in a pan/roll wrist on the next version of the arm.

When the J & K motors turn together, A pans around I. When they turn opposite, A rolls around H. The yellow parts are 3d printed. the green are laser cut. the red are bearings. The grey I have to buy or already own. I is a smooth 8mm bolt. H is a 1/4" hex bolt that does not roll.

The big picture is that I need to measure the angle of pan and roll, instantaneously. Backing up to hit a switch and counting from there is not good enough.

My current plan is to measure rotation of D and C to calculate pan/roll. I have 12-bit hall effect rotary encoder. A small magnet turns 1mm away from a chip, the chip picks up the magnetic field and calculates the angle. For this to work one magnet needs to be centered on the axis of rotation of C and another needs to be centered on the axis of rotation of D.

Both halves are symmetrical, so consider one half of the problem: C needs to somehow go through B1, over I, and hold the magnet in place. It can't permanently block access to I and it needs to turn with low friction. How do I do that? B1, tapered bearing (L?), C, bushing (M?), I, M, C, L, B1?

To further complicate my life, the budget is very small. I don't have access to a break press, a milling machine, a lathe, or any kind of metal cutting. I can get most any kind of bearing and open timing belt. Endless timing belts are way out.

Also posted to: https://www.reddit.com/r/AskEngineers/comments/3edcos/long_history_for_short_mechanical_engineering/

...and Hey! If you're reading this, please leave a message. I work alone in a vacuum. Even a simple "hello" would mean a lot to me. Thanks.