Re-Drilling Old Union Tool Protractor: #5-40 to #6-32 or #8-32

[racookpe1978]

I have two old (pre-1975!) Union Tools, Orange MA Protractors. They are similar to the "modern" Starrett combination square/centerfinder/protractor, but I like the Union Tool's "antique" ability to swing the protractor arm 360 degrees for measurements. And, it's fun to still use old tooling - if they is still accurate.

Both protractors have "lost" the small thumbscrews that clamp the dial from rotating. Neither is "museum quality," but both are useable and still have an accurate level bubble.

By measurement, the screw holes are 0.101 minor diameter, 0.119 major diameter, and a matching screw that fits the hole fits a 40 TPI thread gage. This could be a Nbr 5-40 tapped hole, but I cannot find other nbr 5-40 screws, much less 4x nbr 5-40 male thumbscrews, to use the tapped holes as-is. (I would prefer not to try to cut the threads "free-hand on my little lathe.)

I have room on the protractor head to drill out the nbr 5 threads, and re-tap the holes for either nbr 6-32 or nbr 8-32 brass or stainless thumbscrews.

Any recommendation either way? Or am I just thinking this too hard?

 

[imcjoek]

How are you measuring this small thread?

Are you certain it's not the (much) more common #4-40?

 

[racookpe1978]

Physical measurement of a screw thread that fits: Micrometer across the major dia of the threads.

Also: A 4-40 slides into the current hole without engaging the threads. No nbr 6 screw (fine nor coarse threads) can go in at all.

 

[gmarken]

Why just not drill and tap to the next standard thread, one size up?

 

[racookpe1978]

Well - That is what I'm asking really.

Re-Drill and re-tap from a nbr 4 to a nbr 6 thread would absolutely give the new thumbscrew threads enough "bite" to work.

But, the measured diameters of major and minor threads on the existing screw holes are very close to what I'd expect from a nbr 5 - but the different ref's also have different values for nbr 5 threads since they are very rare nowdays.

So that drives my question: Can I get enough strength in a nbr 6 thread for a thumbscrew, or should I jump all the way to a nbr 8?

 

[gmarken]

As it is only a locking screw for a protractor, a #6 screw is more than adequate. Use a socket head screw and press on a knurled plastic cap like these

http://www.mcmaster.com/#socket-head-cap-screw-knobs/=b55z6h.

 

[KENAT]

You could always see if any metric screws can fill the gaps between " ones.

Posting guidelines faq731-376

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(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[ornerynorsk]

I have a problem with denoting "artifacts" that are pre-1975 as being old. Are you sure these are made of metal, or are they fashioned from stone, by any chance? Just kidding, good luck with your project.