Newbie question on precision thread

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.

Reply to
Voltes34
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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

Reply to
Harold and Susan Vordos

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

Reply to
Matthew Zenkar

...

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

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and
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(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

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?ViewItem&item=7606532048 and
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other items in same categories.

-jiw

Reply to
James Waldby

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.

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Reply to
Richard J Kinch

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.

Reply to
Don Foreman

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.

Reply to
azotic

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

Reply to
Karl Townsend

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

Reply to
daniel peterman

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

Reply to
stans4

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

Reply to
jim rozen

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

Reply to
Robin S.

$65.00 an hour ?? I charge $200 an hour and make lots of mistakes.

Bob Swinney

Reply to
Robert Swinney

...

Ahha. That makes you a consultant, not a machinist.

Karl

Reply to
Karl Townsend

I raise your 200, make lots of mistakes, *and* break stuff too.

Plus I require large quantities of coffee and donuts too.

:^)

Jim

Reply to
jim rozen

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

Reply to
ff

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

Reply to
Robert Swinney

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

Reply to
Scott

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

Reply to
jim rozen

In order to achieve a level of precision that doesn't change when you lock the assembly, the thread is but a part of the requirement. It would be important that mating surfaces are properly fitted so the locking mechanism doesn't cause movement, or at least not enough to be a bother. It's all a matter of how important a few thou, or even a few tenths, are to the successful operation of your device. As the level of precision increases, you can expect the price to go up exponentially.

You may have to revisit your choice of materials. Stainless running on stainless is a recipe for disaster, particularly when you must have little, or no clearance. Stainless galls very easily--often without obvious provocation. Heat treated 400 series (or the use of the precipitation hardening varieties) would likely exempt you from that scenario, but would require a ground thread-----which is likely to be mandatory anyway, considering your statement:

While the use of the extra fine thread may be in your best interest regards the level of precision you might achieve, you'll come to realize that working with such fine threads has its own problems. One of them is how easily they can be damaged, or how easily the smallest piece of contamination would cause the thread to seize. Also, the level of precision required would eliminate the chance you could make the parts with any degree of success on anything but a grinder, assuming the fits you require are close so you don't get (back) lash. As threads get finer and finer, the tolerance of all the features gets smaller and smaller.

I'd encourage you to examine a micrometer for measuring in inches. They are built with a 40 pitch thread, so each revolution of the spindle is only .025". You should look carefully at such a thread and come to terms with how fine it is before making a decision to go to a finer one, particularly in 304 stainless steel, which is very soft and easily damaged.

See above statement about material selection.

One of the things you might consider incorporating in your design is a Delrin AF nut. If you can see a way to incorporate the idea in your design, you can run an interference thread fit on the stainless. No backlash, no galling, and no lubrication required.

All threads (not tapered pipe threads) from class 3 and down have dimensioning such that parts are guaranteed to fit, even when the male thread is on top tolerance and the female thread is on bottom tolerance. The clearance is generated not only on the major and minor diameters, but on the pitch diameters, so you end up with some slop in order for the parts to fit together. The higher the class, the tighter the fit, but there's always clearance. You can get around that by using an adjustable split nut, or two nuts opposing one another that can be adjusted, or, as I suggested above, by using an interference fit and dissimilar materials. It would be unusual design to expect threads to not have lash-----which would be lost fairly quickly in the real world once the device was put to use. Wear is always a problem.

Can't help you there----I'm not an engineer, just a retired toolmaker.

Harold

Reply to
Harold and Susan Vordos

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