I'm looking for a very small screw drive to move a brass slug in and
out of a coil. (Read that as "Permeability-tuned oscillator" if you
know radio.) It'll be hand-operated, and maybe 10 or more turns.

How small? Right now I'm using a #6 brass screw, and some brass nuts
soldered to a plate and spaced to take out some of the backlash. The
backlash is acceptable, but spacing the nuts to take out the backlash
generates a fair amount of friction and makes it somewhat cumbersome
to turn the screw in and out by hand.

Is there such a thing as acme threaded rod with ball-nut as small as a
#6 screw? Is it, perchance, made in brass?

I suppose I could put a spring between two #6 nuts and see if this
reduces backlash acceptably while still allowing easy turning.

Any other ideas? Any cheapo no-backlash screw drives this small?

On Mon, 19 May 2008 13:50:09 -0700 (PDT), Tim Shoppa wrote:

I'm looking for a very small screw drive to move a brass slug in and
out of a coil. (Read that as "Permeability-tuned oscillator" if you
know radio.) It'll be hand-operated, and maybe 10 or more turns.

How small? Right now I'm using a #6 brass screw, and some brass nuts
soldered to a plate and spaced to take out some of the backlash. The
backlash is acceptable, but spacing the nuts to take out the backlash
generates a fair amount of friction and makes it somewhat cumbersome
to turn the screw in and out by hand.

Is there such a thing as acme threaded rod with ball-nut as small as a
#6 screw? Is it, perchance, made in brass?

I suppose I could put a spring between two #6 nuts and see if this
reduces backlash acceptably while still allowing easy turning.

Any other ideas? Any cheapo no-backlash screw drives this small?

Nylock.

Will work for a while, anyway.

Tim Shoppa wrote:

I'm looking for a very small screw drive to move a brass slug in and
out of a coil. (Read that as "Permeability-tuned oscillator" if you
know radio.) It'll be hand-operated, and maybe 10 or more turns.

How small? Right now I'm using a #6 brass screw, and some brass nuts
soldered to a plate and spaced to take out some of the backlash. The
backlash is acceptable, but spacing the nuts to take out the backlash
generates a fair amount of friction and makes it somewhat cumbersome
to turn the screw in and out by hand.

Is there such a thing as acme threaded rod with ball-nut as small as a
#6 screw? Is it, perchance, made in brass?

I suppose I could put a spring between two #6 nuts and see if this
reduces backlash acceptably while still allowing easy turning.

Any other ideas? Any cheapo no-backlash screw drives this small?


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

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.

On May 19, 4:50*pm, Tim Shoppa wrote:
I'm looking for a very small screw drive to move a brass slug in and
out of a coil. (Read that as "Permeability-tuned oscillator" if you
know radio.) It'll be hand-operated, and maybe 10 or more turns.
...
Any other ideas? Any cheapo no-backlash screw drives this small?

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.

Tim Shoppa wrote:
I'm looking for a very small screw drive to move a brass slug in and
out of a coil. (Read that as "Permeability-tuned oscillator" if you
know radio.) It'll be hand-operated, and maybe 10 or more turns.

How small? Right now I'm using a #6 brass screw, and some brass nuts
soldered to a plate and spaced to take out some of the backlash. The
backlash is acceptable, but spacing the nuts to take out the backlash
generates a fair amount of friction and makes it somewhat cumbersome
to turn the screw in and out by hand.

Is there such a thing as acme threaded rod with ball-nut as small as a
#6 screw? Is it, perchance, made in brass?

I suppose I could put a spring between two #6 nuts and see if this
reduces backlash acceptably while still allowing easy turning.

Any other ideas? Any cheapo no-backlash screw drives this small?

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.

--
Steve W.
Near Cooperstown, New York

Life is not like a box of chocolates
it's more like a jar of jalapenos-
what you do today could burn your ass tomorrow!

In article
,
Tim Shoppa wrote:

Any other ideas? Any cheapo no-backlash screw drives this small?

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.

--
Cats, coffee, chocolate...vices to live by

On May 20, 6:50 am, Tim Shoppa wrote:



Any other ideas? Any cheapo no-backlash screw drives this small?

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.

Keywords:
In article , Tim Shoppa wrote:
I'm looking for a very small screw drive to move a brass slug in and
out of a coil. (Read that as "Permeability-tuned oscillator" if you
know radio.) It'll be hand-operated, and maybe 10 or more turns.

How small? Right now I'm using a #6 brass screw, and some brass nuts
soldered to a plate and spaced to take out some of the backlash. The
backlash is acceptable, but spacing the nuts to take out the backlash
generates a fair amount of friction and makes it somewhat cumbersome
to turn the screw in and out by hand.

Is there such a thing as acme threaded rod with ball-nut as small as a
#6 screw? Is it, perchance, made in brass?

I suppose I could put a spring between two #6 nuts and see if this
reduces backlash acceptably while still allowing easy turning.

Any other ideas? Any cheapo no-backlash screw drives this small?

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

Jim Wilkins wrote:
On May 19, 4:50 pm, Tim Shoppa wrote:
I'm looking for a very small screw drive to move a brass slug in and
out of a coil. (Read that as "Permeability-tuned oscillator" if you
know radio.) It'll be hand-operated, and maybe 10 or more turns.
...
Any other ideas? Any cheapo no-backlash screw drives this small?

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.

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

On May 19, 6:02*pm, Jim Wilkins wrote:
You can salvage some nice motion-control mechanisms from old floppy
drives etc.

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.