How did Dietzgen make this thumbscrew?

I have a Dietzgen compass with a broken fastening screw. The body is German silver and the screw is steel. It's M1.8 x 0.35 by measurement. The knob is 8.64 mm diameter. The area around the screw appears to be clearance; I can't see any female threads. The face of the knob was turned after the screw was installed.

I tried heating the knob with a hot air gun and tapping on the remaining screw against a bench block, but it didn't move at all. Scared to tap harder as I'd like not to wreck it. Do you think it's just a press fit, or maybe the knob is threaded deeper inside? I could run a drill through it but I don't see how that will help me.

Obviously I could remake the entire thing if I had to, but it wouldn't match precisely. This is a restoration, not just a functional repair, where a socket head cap screw would work as well.

Hard to tell from the photos but I wonder if the screw had threads at both ends with a smooth shank in between. Possibly they threaded one end into the knob and then swaged the knob to lock it in. That would explain why no female threads are visible. Could also be straight knurled and pressed in.

Conrad:

As one of the old-time engineers who still makes drawings 'on the board' rather than using CAD, I can appreciate your wanting to keep the Dietzgen compass as original as possible. Dietzgen was once a real "engineering supply" firm, offering anything from basic drafting supplies to surveying instruments. A lot of the drawing instruments used to be made under their label in Germany. That being said, I will offer my own 'two cents' as to how the thumb screw was likely made:

Dietzgen (or their subcontractor) used a 'poor man's press fit' which let them get by without holding the closer tolerances that an interference fit would require. Back in the day when the drafting instruments were made, things like "Loctite" were unknown. From the appearances in your photo, the stud was made separate from the head of the thumb screw, and there is a slight gap surrounding the stud where it enters the boss on the head of the thumbscrew.

I doubt that the stud was threaded into a tapping in the head of the thumbscrew, as it would then have been necessary to lock the threaded connection. Otherwise, use of the instruments might have unscrewed the head from the stud. Without Loctite in those days, the option would have been to "sweat' the threads together with solder to lock them permanently together.

Given the fact this was a small part and likely being produced in large numbers, I doubt that any soldering/sweating was done to secure the threaded stud into the head of the thumbscrew. Rather:

-the knurled head of the screw was machined as one part, with a hole drilled into the 'boss'. The diameter of this hole was 1,8 mm or thereabouts. The knurled head of the thumb screw was made in an automatic screw machine in one set of operations. After being parted off, it was held by the boss in a collet to allow the decorative facing of the head of the thumb screw to be done. This eliminated any burr or center tit from the parting-off operation.

-the threaded shank of the screw was machined as a separate part, call it a stud. The stud was machined from 1,8 mm diameter stock. One portion of the stud being 'fine diamond' knurled. The remaining portion of the stud got threaded. Easy enough to do in an automatic screw machine.

-the fine diamond knurled portion of the stud was then pressed into the 1,8 mm drilling in the boss on the head of the thumb screw.

This was a quick way of fastening or locking the threaded stud into the head of the thumbscrew. No close tolerance machining, no tapping of the head of the thumbscrew, no sweat/soldering.

Heating with the hot air gun is not going to break any fit between the thumbscrew head and stud. You will likely have to drill out the stud. A new stud can then be machined, and you have the luxury of setting it using something like Loctite 'cylindrical parts fixing' compound (603, I think, a 'green Loctite' which I use and find to be quite effective on little parts like this one). Getting centered to drill out the stud is a matter of what machine tools and tooling you have at hand. You can open the hole a bit more and ream it to any convenient size, then size the unthreaded end of your new stud to fit.

Think I would soak it for a week in WD or the like the and the vise the stick-out and turn as if threaded Failing I would grind it flat, dermal a center spot, and drill it for in Loctite in screw.

patented-antiques.com antique surveying instruments & tools

Guess I would use a catalog SS screw to Loctite into the old head.

No doubt they made it as difficult as possible. FWIW, the steel visible on the face appears to be larger than 1.8 mm, so there was a head or screw thread or something.

As an aside, these drafting sets are common as can be on eBay, and you can find a couple Dietzgen catalog pdfs online, but finding anything about dates or other info is surprisingly difficult. This set is a "Master-Pro" #1214 and I've yet to find it in a catalog. Guessing it's 1940s. Missing a piece and still needs cleaning up, but a pretty good set with the big beam compass. (Joe & Mich, thanks! I was typing at the same time.)

Clockmaker says it was probably riveted and then turned sort of flat and smooth. Chuck it up in a small lathe and cut a groove near the outer edge of the steel center part. Work carefully with a graver and it should come apart.

Drill a small hole 0.8 to 1.2 mm or so through the threaded stud but not through the body
Then use the hydraulic method
Bit of clay in the hole Then a fitting pin
Tap on the pin Ad clay till it is out
This should work if it is not threaded

Peter

Peter from Holland said:

Drill a small hole 0.8 to 1.2 mm or so through the threaded stud but not through the body
Then use the hydraulic method
Bit of clay in the hole Then a fitting pin
Tap on the pin Ad clay till it is out
This should work if it is not threaded

Peter

Click to expand...

Methinks snot a blind hole tho...

I think they are a press fit, some in my collection have 4 or 5 sides milled onto the shafts and are then pressed and the ends machined.

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Thanks for that link. I always forget about groups.io, though I'm a member of several.

I am not familiiar with the function of that screw. Is it a set-screw, the point of which presses on something? If so, the shaft-to-head joint must only resist torque, so a knurl of some kind is likely, and it should be possible to just press the threaded stud out in either direction

If the collar under the knurled head bears on a surface and the threaded portion is in tension, then a mere knurl might not hold it, and the steel stud may actually have a head (cylindrical or conical) that bottoms in a counterbore or countersink in the German silver button. Below the head of the steel screw could be a straight knurl to resist torque, then the threads.

The latter is how I guess it was made, because I see no clearance between the steel and the German silver on the top, which I would expect to see if the steel was a free fit made to interfere by knurling.

I do not think the steel is upset into the knob, because of the facing marks on the end. And I do not think the assembly was faced after assembly because the toolmarks on the steel and the German silver are different.

I would try to press the steel out, supporting the "top" of the head and pressing on the broken surface of the screw.

Post picture of what the stump looks like!

Someone here may know if there is a chemical that will dissolve away the steel without harming the German silver...

You could drill it out to match the minor diameter of the threads. If it IS threaded, you'd be able to see the root of the thread showing in the hole, then pick out the remnants of the screw from the knob and Loctite in a new one.

I bet that its a press fit, might even be knurled. Seen that on old indicator stands. You will have to drill them out. Start small and Good Luck.

magneticanomaly said:

If the collar under the knurled head bears on a surface and the threaded portion is in tension, then a mere knurl might not hold it, and the steel stud may actually have a head (cylindrical or conical) that bottoms in a counterbore or countersink in the German silver button. Below the head of the steel screw could be a straight knurl to resist torque, then the threads.

Click to expand...

This method gets my vote.

Microscopic examination of the face of the knob could be informative.

Lacking someone who knows, gonna be real hard to tell how they made it without taking that stud out. I'd have at it with a teeny drill, then a slightly larger one. Explore.

Duplicate the stud, put it back in, face it off.

Almost looks like that shaft goes all the way through the knurled part - the center at the opposite end sure looks like a different color than the telltale yellowish slightly tarnished silver. I'm going with either interference fit or solder - my bet is on interference fit - with all that surface area it wouldn't take much to hold it securely. Although for production purposes solder might be faster - drop hundreds of them on a tray and pop it in the oven. Tough call without cutting in and finding out. I'd just stick it in a collet and drill it out then install the new one with an interference fit (or solder it).

eKretz said:

....the center at the opposite end sure looks like a different color than the telltale yellowish slightly tarnished silver....

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Good call. This is a strong hint at the manufacture method.

No forward progress yet, as I'm waiting for my M1.8x.35 die to come. I thought I'd just steal the threaded part of a screw but it turns out M1.8 screws are rarer than hen's teeth. My plan is to drill it out from the top side and thread the center of the remaining steel part. I'm guessing it's a head because this is a clamp screw for the compass lead and point; I don't trust a simple Loctite and press fit not to pull out. Though I could try to bore out the steel part from the top, there's less chance of mucking it up by just drilling and threading. If I turn the new screw level with the top, it should be close enough to an invisible repair.

Do you have a watchmakers staking set?
It would be easy to give the center steel part a few taps with the right sized punch and see if it moves.

Oh. Check the diameter of the steel part on the exterior of the thumbscrew, vs the major dia of the thread itself. Bets the diameter visible on the face of the
part will be larger than the major diameter of the screw thread.

I opened up the spare parts kit for my compass/divider set here at work, and the spare screw has an end that tapers larger. This is how the screw locks into the
body of the instrument itself, the thumbnut is then used at the threaded end to clamp points or lead into the instrument leg. But if your part was assembled by
drilling a hole through the nut, threading that, and then reaming a taper on the far end, the screw (with a larger tapered end) would be fixed hard into the part
by simply threading it in from the backside. The taper on mine looks to be nearly self-holding.