On Sun, 29 Jul 2007 22:26:38 -0000,
wrote:

On Jul 29, 5:13 pm, F. George McDuffee gmcduf...@mcduffee-
associates.us wrote:
snip
=========
Much depends on what you are using it for and how durable it must
be [and under what conditions]. More info = better advice.

4 lead anything will be expensive.

Can you use a larger piece of allthread with a coarse pitch, and
a nut clamped in place? If back lash is excessive, you could use
two nuts with a spring between them clamped against the slide.
This will reduce backlash and allow adjustment for wear. Stronger
spring = less backlash but harder to turn.

Seehttp://www.use-enco.com/CGI/INSRIT?PMAKA=619-1523&PMPXNO=16719279&PAR...
if you have to have acme seehttp://www.use-enco.com/CGI/INLMK32?PARTPG=INSRAR2
if you have to have 4 tpi actionhttp://www.use-enco.com/CGI/INLMK32?PARTPG=INSRAR2

These are examples only and your local mill supply / fastener
shop will most likely have something suitable in stock.

7/16X20 and 1/2X20 (UNF) make a nice lead screw in that it is
easy to calibrate for movement, i.e. 1 turn = 0.050 inches.

Unka' George [George McDuffee]
============
Merchants have no country.
The mere spot they stand on
does not constitute so strong an attachment
as that from which they draw their gains.

Thomas Jefferson (1743-1826),
U.S. president. Letter, 17 March 1814.

This is for a rather ambitious robotics project. I need the lead
screw to lift a small platform about 4 inches, weighing no more than
10 or 15 pounds. I also need a lead screw mechanism to slide a
platform in a horizontal direction mounted on a rail or track. That
platform would also be about 10 pounds, and I need it to slide at
least 3 feet. Both of these would be connected to rotary motors.
According to my research, it seemed like a lead screw was the least
expensive screw mechanism available. If there is something less
expensive, I would much rather use that. Also I need a moderate
degree of precision so I can move the platforms in increments of a
millimeter at a time, and it must be able to hold its position and not
"drift" when the motor is not pushing it along.
=====
Sounds like a natural for allthread and some nuts.

Unless you have space/weight limitations use 1/2X20 or 7/16 X20.
Given your accuracy requirements, you can simply tap a hole to
the same thread size. 1/4X20 should be adequate is you can keep
the screws under tension.

These will wear out in fairly short order (a few thousand to 10s
of thousands of cycles depending on lube and nut material], but
should be adequate for your needs, i.e. proof of concept. Self
lubricating material such as nylon, delrin and HDPE with
relatively long threads [c. 1 inch] will last longer. A one inch
[2X] "depth of thread" in a polymer should eliminate any backlash
until wear become evident.

FWIW -- with a depth of thread engagement equal to the major
diameter, most screw materials will fail before the threads
strip. With a 1 inch depth of thread [2X diameter], a 7/16 or
1/2 screw *WILL* fail before the threads do until the wear
becomes totally excessive.

Allthread [or Redithread] typically comes in 2, 3, and 6 foot
lengths in the US. For example click on
http://www.barnhillbolt.com/

http://www.fastfixdirect.co.uk/code/...inCategoryID=4
http://www.use-enco.com/CGI/INLMK32?PARTPG=INSRAR2
http://www.use-enco.com/CGI/INLMK32?PARTPG=INSRAR2

FWIW -- lead screws are generally very high accuracy [like
micrometer threads] and are used on lathes to generate the
threads. Very much over-kill for what your are to do. Term you
need to search on is "linear actuator."
for some examples click on
http://www.trossenrobotics.com/store...Actuators.aspx
http://www.surpluscenter.com/sort.as...key word=EALD
http://www.surpluscenter.com/sort.as...key word=EALA

remember that you can trade speed for power and accuracy by
gearing the stepper motors down to the threaded rods. Small cog
[timing] belts are generally used.
click on
http://www.sdp-si.com/D265/D265Cat.htm
http://www.sdp-si.com/D785/D785Cat.htm

***************
Welcome to the world of engineering/design where the criteria are

*strong
*light
*cheap

and you get to pick any *TWO*
******************

Good luck on your project, and please feel free to ask if you
have more questions.

Where are you located? Your use of "millimeter at a time"
indicates you may not be a US resident.


Unka' George [George McDuffee]
============
Merchants have no country.
The mere spot they stand on
does not constitute so strong an attachment
as that from which they draw their gains.

Thomas Jefferson (1743-1826),
U.S. president. Letter, 17 March 1814.