Making screws to fit existing threaded holes

NigelW

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Jan 2, 2015
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With my clock club's reopening I am back on my long term restoration of a c.1710 bracket/table clock. Next problem: how to make new screws to fit old holes? I want to intervene as little as possible with the existing plates so rethreading holes is not an option.

Needless to say none of the threads is standard. I have started by screwing in some pegwood to get an impression of the threads which I then compare against a thread gauge to get the number of teeth per inch (one bit of wood got stuck, which was not a good start). As a next step I am making some trial threads in brass rod to test in the holes (steel could damage the existing threads) but have ordered some plastic rod which might be even better? The first hole I reckoned was 1/8 diameter and 32 tpi. An existing old screw from elsewhere in the clock fitted.

I set my screw cutting lathe up for 32 tpi and made a dummy screw in brass (turning the lathe by hand). It didn't go fully in so I carefully checked the the pitch again; on closer inspection it appears to be somewhere between 30 and 32. I don't have a 31 (or 62) tooth change wheel but by fiddling about with my existing set of wheels I reckon I can get something close to 31 tpi so will try again.

I don't have much experience of screw cutting in a lathe. One feature of the old screw is the highly rounded profile of the peaks and troughs of the thread - like a Whitworth but even more so. I understand true Whitworths are tricky to make for that reason and are often finished with a chaser to get the right profile, but if my pitches are non standard this would not be an option.

Any ideas or suggestions would be most helpful.
 

DeanT

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Mar 22, 2009
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Do any of the other screws currently in the clock also fit in this hole? Often the same size screws are used in multiple places. The last clock for which I made screws there were numerous holes which fitted the same screw and I copied one of the originals. You might be lucky....

And Jerry Kieffer is a master...(different spelling)...if he doesn't know no one will.
 

Jerry Kieffer

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May 31, 2005
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With my clock club's reopening I am back on my long term restoration of a c.1710 bracket/table clock. Next problem: how to make new screws to fit old holes? I want to intervene as little as possible with the existing plates so rethreading holes is not an option.

Needless to say none of the threads is standard. I have started by screwing in some pegwood to get an impression of the threads which I then compare against a thread gauge to get the number of teeth per inch (one bit of wood got stuck, which was not a good start). As a next step I am making some trial threads in brass rod to test in the holes (steel could damage the existing threads) but have ordered some plastic rod which might be even better? The first hole I reckoned was 1/8 diameter and 32 tpi. An existing old screw from elsewhere in the clock fitted.

I set my screw cutting lathe up for 32 tpi and made a dummy screw in brass (turning the lathe by hand). It didn't go fully in so I carefully checked the the pitch again; on closer inspection it appears to be somewhere between 30 and 32. I don't have a 31 (or 62) tooth change wheel but by fiddling about with my existing set of wheels I reckon I can get something close to 31 tpi so will try again.

I don't have much experience of screw cutting in a lathe. One feature of the old screw is the highly rounded profile of the peaks and troughs of the thread - like a Whitworth but even more so. I understand true Whitworths are tricky to make for that reason and are often finished with a chaser to get the right profile, but if my pitches are non standard this would not be an option.

Any ideas or suggestions would be most helpful.
First, the kind words of confidence by others is much appreciated, but now if I were only up to the task.

I have been down this road many times over the years and in some cases no two threads are the same pitch even in the same movement. Under these conditions, it is occasionally practical to adjust a thread ever so slightly with special pitch commercial taps and make new screws with a matching die. An example of available special threads can be seen in the following link

However, duplicating any existing thread is also not an issue, but will of course require far greater effort.

First, nothing much will happen without the exact screw OD. thread shape and exact thread pitch. If you have an original screw, then of course you have that information.
If not, then you can determine the information as follows.

(1) first, I use a "Q" tip or whatever to grease the hole thread with auto bearing grease.

(2) If the hole is not a blind hole, one end must be sealed with a plug or laid on something that will seal it.

(3) Next, insert a piece of square stock or flat screwdriver blade in the hole.

(4) Then heat and pour low temp metal such as Bismuth in the hole until full.


Once cool, the bismuth can be screwed out of the hole giving the exact information for duplication. While thread gages may or may not match your thread, its a simple matter to measure the pitch. In these cases, I compare several threads from thread tip to tip to the end of a gage pin under optics per attached photo. Then do the math.

From this point, a special gear can be machined for your threading train to single point cut the thread. Since I see your from the UK, your in luck in that it is a land of many model engineers. You should have no problem finding a model engineer with a CNC lathe with thread cutting program.
I am sure many would be happy to help.

At least in the past, BHI exhibited at the larger model engineering shows and may be able make recommendation of someone who is CNC equipped.

Jerry Kieffer

DSCN5064.JPG
 

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