Hey Mark, thanks for sharing. This is a great way to make a finely tuned adjustment. It checks all the right boxes to on simplicity, cost, executability (if that's a word) and real world results. There's multiple ways of connecting the different size and pitch bolts so just putting it out there with your well worded & explained details with the corresponding pics is a godsend for a lot of people. Great thinking if you figured it out, if not great research and great articulate post. Thanks again!

I've never heard of a differential screw, thanks for the introduction to the concept. I enjoyed reading about your prototyping process for this, the lathe and die method is an great solution.

Before I looked into this, I hadn't heard of them either, even though they are fairly common in precision machinery.

Could you continue the idea with a third stage to produce an even smaller movement?

The output of this type device is linear motion, not rotational, so I can't think of any easy way to add an additional stage to it.  You could select standard thread pitches that are closer- M3x0.5 and M2.5x0.45 would give 50 um per full turn of the screw.

If that's not fine enough control, you could couple a differential screw to a flexure based mechanism.

Thread inside another bolt, screw them together, and then turn the outside of that one as well.  Then we can have two levels of overkill.  :)  You got the Like purely for the nice execution of your idea sir.

Daren Schwenke that would jutst make two adjustments since you couldn't do all at the same time. You'd be better just doing one with a real close difference. A whole turn to .1mm is pretty fine since it's very easy to do a quarter or an eighth of a turn. Your hobbyist project and equipment isn't going to have or need tolerances like that. But I feel your whole "you ever seen the back of a 20 dollar bill, on weed?" Jon Stewart vibe. Sorry I replied who you replied to cause your comment didn't have a reply option for me.

Hi @Mark Rehorst lovely execution. For those without the ability to make from a single piece of threaded rod, may be worth mentioning some alternative options to create the threaded elements, such as epoxy gluing or clamping a section of M4 threaded rod to a section of M5. Perhaps using bolts with the heads back to back would help with fixing. 

I tried epoxy and wasn't successful, but it might work if done right.  The easiest thing to do would probably be to get an M4x0.7 coupling nut, drill half of it to out 5mm, then tap that side with M5x0.8 threads.  Screw together the parts with a drop of locktite on both screws.  The overall length would increase by the length of the coupling nut, but it might still have an acceptable length depending on your application.