Newbie question on precision thread

Matthew Zenkar · 2006-04-13T04:27:31+00:00

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 precisionpositioning assembly for optics. I contacted a professional geardesigner, and I was told by this person that the design would beimproved if I used an acme thread for the threaded rod that is in theassembly.My initial design called for a 3/8"-40 thread as this is the fineststandard thread size I could find to fit a 3/8" diameter rod. However,looking in Machinery's Handbook, I found that the finest standard acmethread is 1/4-16. I also figure that I would be able to use a 20 pitchwith another minor adjustment in the assembly.Is it common practice to have custom threading made for those caseswhere standard threads do not meet the needs of a project? For mycase, I would like to have acme threaded parts where the thread sizeis 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 threadsmade in those particular sizes, what about having what Machinery'sHandbook calls "unified screw threads" cut to a non-standard size, say1/2-40 - is this done for cases where standard threads do not meet theneeds of a project?Also, does anyone have any recommendations as to where I might getthis done (preferably by a professional shop)?Thanks in advance for your response.Best Regards,Matthew

Apr 13, 2006 22 Replies

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steamer 20 years ago

--Not sure if Google would be the place to search it, but many moons ago there was an article in the Amateur Scientist column of Scientific American about constructing a "ruling engine", a device for making optical gratings. It used a principle called a "differential screw" to get reeeeeally fine divisions using fairly simple components. You might want to see if you can find a copy of the article.

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Ned Simmons 20 years ago

Take this to heart. It's much easier to position something to within .0002 than to lock it there. Several times I've had to design and build both manual and automatic mechanisms to do this and it's never as easy as you might think.

Read, not skim, the section "Screw Thread Systems" in Machinery's Handbook. It's many pages of dry material, but worth the effort if you need to understand screw threads.

Ned Simmons

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john 20 years ago

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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