Having one of these for fine-tuning yet another measuring tool, this so called differential screw achieves a relative pitch of 50 micron per turn (508 TPI, a "2 thou pitch" or 20 turns per mm). This is the very gadget that make micrometers work. Simple to make: Turn down 1/4" brass rod to 6 mm dia and (special) thread M6 x 0,75. Turn half the length down to 4 mm dia and (standard) thread M4 x 0,70. Tap out two moving parts to one size each, fit diff screw - add wheel with divisions to suit. Careful though - this mechanical advantage can easily strip the threads or break something. Less effort and less noisy threading with my cheap tapping fluid: Start with a small glass jar with app 10-15% of ATF, add bacon fat to top (from your stove, strained thru a piece of paper towel). The ATF adds lubing and keeps the stuff from getting rancid, add to tap/ die with a Q-tip. Also makes your workshop smell nice. Glass jar with metal lid keeps the mice from eating the stuff.