Newbie question on precision thread

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?
Also, does anyone have any recommendations as to where I might get this done (preferably by a professional shop)?
Thanks in advance for your response.
Best Regards, Matthew
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| 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? | | Also, does anyone have any recommendations as to where I might get | this done (preferably by a professional shop)? | | Thanks in advance for your response. | | Best Regards, | Matthew
Given the same amount of tolerance, the finer the thread pitch the more precise the movement. The diameter of the rod is really not a critical factor.
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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.
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. Bastard, 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.
The nature of your device may dictate the need for a ground thread. 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.
Harold
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Matthew Zenkar wrote:

...
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?ViewItem&itemv09388048 http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item 23903725 and http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&itemv07673342 (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?ViewItem&itemv06647083 http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&itemv06532048 and http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&itemv06892197 and other items in same categories. -jiw
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Matthew Zenkar writes:

Absolutely. Instrumentation is full of it. A common example is filter threads on cameras.
You seem to have settled a lot of design questions prematurely, as if you hadn't considered aspects like backlash and thread pitch inaccuracy.

I design and built a lot of finely threaded parts for optical applications.
http://www.truetex.com /
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On Wed, 12 Apr 2006 23:27:31 -0500, Matthew Zenkar

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.
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wrote:

Correct micrometers have V-threads and the threads for decimal micrometer will be 40tpi and the tumbler has 25 equal divisions which gives you a .001 resalution, i purchased a 1/2 X 40tpi die from msc a few years back just to clean up micrometer threads when they get gummed up from use in a coolent rich enviroment. I also have some 1/2 X 80tpi taps and dies left over from a prototype i built back in the early 90's that came from msc.
MSC or Field Tool can supply those special thread taps and dies.
Best Regards Tom.
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Haven't seen your design, so this may not work. Years ago, I had to me able to adjust a position within 0.005". We started with a micrometer, machined off the C frame to a mount and then bonded the plunger to the device that had to move. Cheap quick and accurate.
Karl
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I have some precision positioni assemblies that were, oddly enough procured from a screen printing table. These tables were used in the manuf. of printed circuit boards. The very fine threaded rod threads thru a nylon ball that is captured between two aluminum plates with concave divits. This allows the assembly to swivel and still be precise. A trained operator can make minor adjustments as small as ..001" maybe less as temp stabilizes. would like to sell these for $30 plus shipping
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One place to start with this sort of thing is a design textbook on kinematics, you might find your design changing. Acme threads are used for heavy-duty load moving, generally not for precision work. For short-distance precision manual movements of 1-2", micrometer heads are readily available. A lot depends on what you define "precision" as and how fine you want your adjustments to be. Are they to be powered? You can get fine metric threaded rod from a number of sources although if you insist on large diameter and fine threads, you're going to have to look for somebody to thread up something custom. Or see if you can find some surplus optical table X-Y positioners and use those screws and nuts or maybe the whole assembly. Edmund Optics has a lot of stuff like that, just got their new catalog.
Stan
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Do not re-invent the wheel. There are a plethora of companies out there (Thorlabs, Ealing, Newport, etc) that manfacture all kinds of optical tranlators, in varying sorts of load capacities.
If you read those catalogs you will get some idea of how this is best done.
Unless you specify a load rating and required precision for your particular application, any suggestions about what kind of threads to use will be sketchy at best.
Jim
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And certainly cheaper than paying someone $65+/hour to build an unproven design. I know New Focus used to sell their precision fine-pitch screws with matching brass nuts, thumbwheels and ball-bearing points as a package. Not sure how expensive, but definitely cheaper than a job shop.
Regards,
Robin
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$65.00 an hour ?? I charge $200 an hour and make lots of mistakes.
Bob Swinney
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...

...
Ahha. That makes you a consultant, not a machinist. <Grin>
Karl
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I raise your 200, make lots of mistakes, *and* break stuff too.
Plus I require large quantities of coffee and donuts too.
:^)
Jim
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Robert Swinney says...

Yeah but Jim, you gotta public job! *Poor* craftsmen have to forego some of the perks, don't you know. 200 bux an hour is cheap labor compared to the defense contractor workers that only make $50 an hour but each project takes inordinately long because of the ridiculously tight, usu. unnecessary, tight tolerances.
Bob (ducking incoming) Swinney
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Get a New Focus catalog for some design ideas if you want to go that route.
A micrometer head can be purchased for a lot less than it would cost to have one made.
Fred
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On Sat, 15 Apr 2006 18:55:25 -0500, Matthew Zenkar wrote:

Have you considered a differantial screw? Basicly a left hand pitch working with a right hand pitch. By selecting the differance you can come up with some very fine feeds and with a couple of tricks one can reduce backlash to near zero.
Scott
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By all means then, obtain and read a catalog from an optical house supplier. Like the McMaster Carr catalog, they provide excellent tutorials. You can learn a great deal about nearly any optical fundamentals that way, including the basics of translator design and the specification of exactly what detailed requirements you will need.
Jim
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jim rozen wrote:

Edmond Scientific specializes in optics. I bought my first 6 inch reflecting telescope mirror kit from them many years ago.(1957) They are located in New Jersey.
John
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