Smallish no-backlash screw drives

Nylock.

Will work for a while, anyway.

Since there won't be much axial force required, how about threading a hole in a piece of nylon, slotting one side and using a clamp screw and nut to adjust the friction acting on your slug screw?

Jeff

You can salvage some nice motion-control mechanisms from old floppy drives etc.

How about a spring-loaded cord pulling on it?

Optical positioners use a micrometer screw working against springs.

Wow there's a blast from the past. I have an old Philco auto radio with the same tuning. It uses two tuning nuts that have 3 ball bearings inside them. The bearings ride on the threads and lower friction in the tuner. For a DIY system you might be able to use nuts made out of threaded ferrite. I have seen that used, They use two ferrite pieces with threads cut and then counter bored so only one full thread is left. Then they bond them with the threads at either end (looks like a hollow tube with raised threads inside each end. Low backlash and low friction from the ferrite.

UMHW plastic threaded with a "tap" made by grooving a screw is one cheap version, used somewhat commonly by the low-end home CNC machine builders.

Tim - email me, will send you a copy of the relevant drawing - was in our "Amateur Radio" magazine, one of those things that was blatantly obvious once you saw it....

Andrew VK3BFA.

Take a look at how various microwave & RF trimmer caps are done. There are several approaches out there. One is to make a cylindrical nut, put two fine opposing cuts staggered apart a small amount. You then crush the nut slightly so the threads have some tension. Another approach is to have a cylindrical nut, with two slots going in a little less than half way. A fine spring goes in the slots & picks up the threads on the screw. The spring tension keeps everything tight.

A lot depends on how often you are goignb to tune it. These schemes are really intended for vibration resistance and precision tuning with very little back & forth from repeated use.

Doug White

I was going to suggest the spring loading -- you beat me to it.

If you spring load, you'll find that whatever method you use to retain the slug radially while you let it move axially will have some backlash

-- you just can't win for losing. I suspect that a flexure would give you the best performance, but that decision is yours to make.

The more precision you can build into the brass screw the better it'll work -- the difference between "binding" and "backlash" is just about zero, so the closer you can make the screw the less backlash you'll have to allow while still not binding. I'd be doing this with the finest thread I could cut, which means the finest thread tap that McMaster or Small Parts carries, and a Really Sharp threading tool on my lathe.

Oh man... every 8" floppy drive I scrapped in the past decade had a very nice Acme-thread shaft and not a ball-nut, but at least a spring- loaded nut for head positioning. So that's where I'd seen smaller Acme threads before! In any event many of those drives ended up among some r.c.m regulars who like to melt down aluminum baseplates, at least they got some good from 'em!

Tim.