Replicating screws from original locks.

[Matt Maier]

Hello everyone. Pictured is a screw from what I presume is a Moroccan snaphance lock I am studying. Having single pointed many threads in my career as a machinist, I notice just how different the thread form is from that of modern screws. It’s quite shallow for one, though I can’t tell if that is due to wear or a quirk of the manufacturing techniques used at the time. Also, the peaks and the roots are quite broad and rounded. As I understand it, small screws such as this were created via swaging the blank through a die, as the wrought iron used at the time was very soft and prone to galling if actually cut. Any pointers on how I could replicate these screws? I may need to replace them in the end and I want to avoid significantly altering the lockplate if at all possible.

 

[Many Klatch]

Some screws were hand filed to suit. In that case a custom thread cutter was handmade and hardened. That was used to make a cutter for the screws. Since the gun was a one off handmade, it didn't matter that the screws were not a standard. Looks like you will have to duplicate the process.

 

[hawkeye2]

If that thread is used in more than one place you could make a tap then use that to make a die. Get them close to final shape on a lathe and finish with files. A lot of work but better than rethreading an original lock plate.

Basically what Many Klatch posted while I was composing this.

 

[rich pierce]

Agree. That screw could be used to make the tap. File flats on 2 sides and add some taper. Case harden it. Drill and tap a hole in good steel about 0.100” thick.

The trick is finding the right drill that will allow the tap to swage, not cut, the threads.

 

[Matt Maier]

I had suspected as such. I’m operating under the assumption that gunsmiths were also toolmakers, and each basically made their own taps and dies for their own work, and as such, no set of tools could make threads interchangeable with another.

 

[Zonie]

Making the picture larger on my computer by holding the "Ctl" button down and hitting the + key several times shows me the threads were probably cut by hand with a very narrow, rectangular file, probably with no cutting teeth on the large faces, just on the edge. That left an almost square thread form.

If you held the screw pointed upright in a vise you could figure out what the drop is from one side of the thread to a point 180 degrees away on the other side. Then, knowing the diameter of the thread at the place you measured the drop at, you could figure out what the "lead angle" or "helix angle" of the thread is.

With this lead angle known you could make a jig to set the angle to guide the file so the helix could be recreated.

For a file, I would look for the thinnest ******* file with teeth on the sides that was available. Then, using a grinder and water pan to keep the file cool, I would grind down both of the large faces to remove the teeth and to get the thickness down to the same width as the screw thread grooves.

Sounds like a lot of work but it can be done.

 

[Rudyard]

You already have three good advices I if I may add my view will give much the same in my words. It seems an initial tap is' hand & eyed ' more or less the desired spiral then flats filed square so giving the tap once case hardened enough to tap into a soft iron' die' bored a size to tap a rude thread the irregularitys evening out this rude' die' is then hardened , clearance holes being bored or filed to allow the soft wrought iron of the desired die will swage rather than cut a new screw to form the same tap & die to reproduce threads of the same pitch & size . . This would appear the way native gun makers make the threads .Ime thinking 1960s West Africa. But read 16th & 17th & beyond European lock & gun makers did them .The' Outer Mongolian Fine' or' BSA' ' Bizantium standard acme ' to our eyes . There are still to be found pre Whitworth trowel shaped tapering of die plates where the screw would be turned through as wire held in a brace it being soft iron and progresivley swages the desired' screw '. The shank & head turned into a toothed die again held in the leg vice at arms height ditto the screw heads . Some I have swage out the top jaw screw (pin) of the regular muskets that are rarely any modern die size . Brownells would have kittens trying to match these die & taps !.
Regards Rudyard

 

[WRussell]

If you happen to know someone with a CNC milling machine, that's another way to make one-of-a-kind threads. Make a cutting point on the end of a rod of a size to fit a collet for the mill. Spinning this gives you the profile. Put your screw blank vertical on the mill table and program the mill to move in X and Y around in the right diameter circle, and Z to get the pitch. Of course it will work for threading holes, too.
It would be a fun job for someone with one of the table-top CNC mills available now.

- Bill

 

[Rifleman1776]

Find a steel casting company. They can scan, print a perfect copy with a 3-D printer then cast perfect replicas with the lost wax process.

 

[Matt Maier]

Find a steel casting company. They can scan, print a perfect copy with a 3-D printer then cast perfect replicas with the lost wax process.

The only issue with that approach is that the heads of these screws are quite worn almost to the point where I couldn’t get enough purchase with my screwdriver to get them out. Having them cast would leave me with that very same problem.

I did have an idea for making a tap though. If I can figure out what the modern equivalent in threads per inch is, I can single point a shallow groove then use that as a guide for a hand file to go the rest of the way.