On Wed, 12 Apr 2006 23:27:31 -0500, Matthew Zenkar
wrote:

Please pardon me if this is not the correct forum for this message.

I am working on a design that I think is best described as a precision
positioning assembly for optics. I contacted a professional gear
designer, and I was told by this person that the design would be
improved if I used an acme thread for the threaded rod that is in the
assembly.

My initial design called for a 3/8"-40 thread as this is the finest
standard thread size I could find to fit a 3/8" diameter rod. However,
looking in Machinery's Handbook, I found that the finest standard acme
thread is 1/4-16. I also figure that I would be able to use a 20 pitch
with another minor adjustment in the assembly.

Is it common practice to have custom threading made for those cases
where standard threads do not meet the needs of a project? For my
case, I would like to have acme threaded parts where the thread size
is 3/8 or 1/2-20, more preferably, 3/8 or 1/2-40.

Also, if it is not heard of to have acme threads custom acme threads
made in those particular sizes, what about having what Machinery's
Handbook calls "unified screw threads" cut to a non-standard size, say
1/2-40 - is this done for cases where standard threads do not meet the
needs of a project?

Acme threads have better load-bearing properties than V threads due to
greater cross-sectional area, but it appears that your application is
more about positioning than about load. I think micrometers have
V-threads, not Acme threads.