Many thanks for all the replies.

I get the feeling that I'm somewhat out of my league here. I do not
have a mechanical background, and I have been teaching myself what I
need to know for my project as I go along. I'm doing the best that I
can under the circumstances, and the help is appreciated.


On Wed, 12 Apr 2006 20:53:10 -0700, "Harold and Susan Vordos"
wrote:


"Matthew Zenkar" wrote in message
news
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.
snip

Thanks in advance for your response.

Best Regards,
Matthew

Acme threads are generally restricted to rather coarse pitches-----so you're
highly unlikely to be successful in your quest to use 1/2"-40.

Thanks. That is valuable information to have.

Standard 60 degree threads (the unified thread series) are generally
selected for your application, and are available in pretty much any
pitch/diameter combination you desire. *******, or non-standard, taps are
readily available from supply houses, but should you elect to go with
something that is considered unusual, or non-standard, you can have taps
made for a reasonable price. You should be able to buy a 1/2"-40 tap with
no trouble.
I had no idea that "non-standard" threads of many different sizes were
available. As I see it, 1/2-80 or even 1/2-100 thread would bring me
much closer to my goal in the assembly.

First, let me say this, I am not working on an off-the-shelf optical
mount. The components that New Focus makes give me an idea of what is
possible, however, the stock components do not appear suitable for my
application.

Assemblies of the type that I am working on exist in the commercial
marketplace, however, they are extremely poor in terms of precision
positioning and, once the precision adjustment is achieved, they are
poor at locking the adjustment in place. Once the adjustment is locked
in place, it is highly unlikely that a user would reposition the
assembly. There are cases where this might happen, however, the
majority of use cases would not require this. (On that note, to the
person who replied with the DRO suggestion, that option might come in
handy for these few use cases. Thank you for that suggestion.)

Also, let me say this. I have been working on this for a long time. I
have some pretty good 3-d modeling and FEA tools, so I have been
through quite a few configurations and extensive analysis of the
assembly. While I realize that the 3-d and FEA tools may not perfectly
reflect the real-world device, I have a good idea of how to accomplish
my goal, and I have a solid background in physics and math. I'm open
to suggestions. However, I may be reluctant to deviate much from my
current course especially if it is going to require a significant
redesign of the assembly.


The nature of your device may dictate the need for a ground thread.

The primary goal is to achieve precision positioning.

Threads can be generated by various means, from cutting with a die, a die
head, rolling, single point threading on a lathe, or grinding. I may
have even left out some other method. It might be helpful to know more
about your thread requirement before suggesting a system that would be in
keeping with your requirements. Are you looking for one, or many? How
long must the thread be? What material? Considering you are using the
thread for adjusting, or locating, you may even wish to consider threads
without lash, or clearance.

For now, I am looking to have perhaps three or four each of
internal/external threaded pieces to build three or four prototypes of
the assembly. After that, I do not know if I will need more. I am
hoping to go to market with it.

Currently, here is what I am looking at -

1/2-80 or 1/2-100 thread as follows: (I'd prefer the 1/2-100, however,
I don't know what can be done. I'm hoping for feedback as to what is
possible - even if that means someone out there has to tell me that
I'm insane - ;-) )

Assuming this is possible given the dimensions of the material, the
external thread will be six-inches long starting from one end of a
seven-inch piece of 304 stainless tubing, 0.5" OD, 0.37" ID

The internal thread will be 1/2-inch long in a four-inch disk of the
same material.

As to lash (what I am familiar with is backlash - or if one were to
reverse the direction of the screw while adjusting it, how far one
would have to turn the screw before movement in the opposite direction
would occur), again, I don't know what is possible and I do not know
how to specify this. However, if it were possible to make threads
without lash without incurring excessive expense, that would be ideal.
(The current products have significant lash and this is a drawback,
however, I don't see it as being the major problem with the current
products.) I suppose that I really need to know how difficult this is
to do and have some idea of how expensive it is to minimize lash.

I don't really understand the term clearance. What I think it means is
the distance between the external and internal threads. I would
greatly appreciate an explanation of it, and how it would affect the
thread. (That said, it seems to me that having minimal or zero
clearance between external and internal thread that are mating would
make for more precise movement than with threads that have significant
clearance. Would this not also make the threads more difficult to
adjust, i.e, would the friction between the mating threads be higher
such that the adjustment would be more difficult?)

As to the load that the assembly will carry, I expect that it will may
have to carry 250 pounds, however, the majority of cases fall in the
less than 130 pounds range.

Thanks once again for all the replies. The help is appreciated.

Best Regards,
Matthew


Harold