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Author Topic: Old screw threads. How to match?  (Read 3261 times)

Todd

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Old screw threads. How to match?
« on: August 08, 2010, 08:52:31 PM »
Can anyone suggest a good way to deal with the problem posed by pre-modern screw threads?  I have two lock mechanisms needing repair and re-use --one English circa 1780-1800 and the other early/mid 18th Century European.  Both will need new screws (lockplate, frizzen), but I don't want to wreck the integrity of the locks by cutting in new threads.  Is there a good solution?

Todd
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Offline Dphariss

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Re: Old screw threads. How to match?
« Reply #1 on: August 08, 2010, 09:54:03 PM »
Quote from: Todd on August 08, 2010, 08:52:31 PM
Can anyone suggest a good way to deal with the problem posed by pre-modern screw threads?  I have two lock mechanisms needing repair and re-use --one English circa 1780-1800 and the other early/mid 18th Century European.  Both will need new screws (lockplate, frizzen), but I don't want to wreck the integrity of the locks by cutting in new threads.  Is there a good solution?

Todd

Find the thread pitch and diameter then look in places such as MSC to see if there is anything that is close.
In special thread taps and dies they have a LOT of options you may find a perfect match or something that is close enough to work.
Dan
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Offline davec2

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Re: Old screw threads. How to match?
« Reply #2 on: August 08, 2010, 11:07:27 PM »
The screws are easy, if you have a lathe or know someone who has one.  Just figure out the pitch and diameter, set up the lathe for that pitch, and single point the threads.
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Offline DutchGramps

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Re: Old screw threads. How to match?
« Reply #3 on: August 08, 2010, 11:43:01 PM »
Measure the pitch in the part by 'screwing - in'  a square piece of wood (preferably light coloured) . Then it is easy to measure the pitch with a thread gauge held on the imprint 8)
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Todd

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Re: Old screw threads. How to match?
« Reply #4 on: August 11, 2010, 06:04:54 AM »
Gentlemen,
Thank you.  This is helpful information and encouraging.  I'm glad to know that there's a way around this problem without tampering with the original threads.
Todd
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westbj2

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Re: Old screw threads. How to match?
« Reply #5 on: August 11, 2010, 01:53:05 PM »
As mentioned above by Dan and others, you need to determine the pitch and diameter first and with some luck find a mate with the correct head shape and other relationships.
Here is another alternative.  Recently made a bridal screw for a Purdey lock and obviously did not want to change the lockplate threads.  The problem screw was 48 TPI and about .005" smaller in diameter than our standard 4-48.  Made up a new 4-48 screw and with a forceps carefully immersed the screw body and threads in engravers acid.  The acid ate away the .005" oversize in just a few minutes and the screw worked perfectly.
Jim 
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