Matthew Zenkar wrote:
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.
....

You haven't told us why the "professional gear designer" believes
an acme thread would be better. Whether that is so depends, of
course, on what your device has to do. Acme threads are used in
several places on lathes, for positioning things accurately and
solidly. You might be able to buy an assembly to do what you want;
eg search for compound slide on ebay to see items like
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7609388048
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=8923903725 and
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7607673342
(The first item has a 1/4-20 thread, not acme)

If the optics are lightweight, you might be better off adding a
DRO to your device and keeping the 3/8"-40 thread. See examples
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7606647083
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7606532048 and
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7606892197
and other items in same categories.
-jiw