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  • Mystery Thread Size

    Hello All,

    I have a "third hand" device that consist of a cast base and a couple of arms with alligator clips to hold things while using both hands for something else (often for holding items while soldering). The base is nice, black wrinkle finish, but the threaded fasteners used (funky thumb screws) are begging for an upgrade. The issue I'm having is matching the necessary thread size. The major diameter measures 0.152" and the TPI appears to be 32. Too big for 6-32 and too small for 8-32 Metric? Doesn't appear to match that. BA thread? Not a match. So any idea what the heck this is? At this point I'm thinking of just re-tapping 8-32 but wanted to check first to see if I'm missing something obvious.
    Thanks,
    Stan
    Last edited by sjaffe; 04-04-2022, 12:55 PM.

  • #2
    M4x0.8
    21" Royersford Excelsior CamelBack Drillpress Restoration
    1943 Sidney 16x54 Lathe Restoration

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    • #3
      Yes, if you don't have a metric thread gauge, get one. I have yet to find a thread that was not either imperial or metric. Non-standard diameters, sure, but never bogus TPI or proper relations of a mm.
      "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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      • #4
        Since 5/32" is 0.156" (plus some micro-inches), and screw threads are normally slightly under the nominal diameter, it sounds like 5/32" B.S.W. If so, my guess is that your device is of British origin and fairly elderly. The Whitworth profile is 55°.

        It's the thread used in 'Meccano' construction sets, and continued there for decades after it had fallen out of use in other places. If, as I suspect, you are in the US, I suggest you go for 8-32 instead of chasing something obsolete.

        George

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        • #5
          Thanks for the replies. I have both SAE and metric thread gauges. 0.8 is 31.75 TPI. I compared 0.8 gauge and 32 TPI gauge with my fastener using a microscope. The 0.8 is just a slight bit off, the 32 TPI looks like a better match. I believe 0.8 translates to 31.75 TPI, hence why they would be close (but no cigar). I have not seen M4x0.8 listed (only 0.7 and 0.5). M4 major diameter specs show a minimum of 3.838 mm (0.151") so it could be an M4, but it seems unlikely that all the fasteners would be right at the minimum spec(?) So I'm not buying it's M4x0.8

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          • #6
            Thanks George. While I cannot confirm, this seems plausible and what I was suspecting (something oddball). I have no idea where this tool was made. It sounds like I might get away with just running an 8-32 tap through the existing tapped holes (8 size measured about 0.010" larger).

            Stan

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            • #7
              Originally posted by sjaffe View Post
              Thanks George. While I cannot confirm, this seems plausible and what I was suspecting (something oddball). I have no idea where this tool was made. It sounds like I might get away with just running an 8-32 tap through the existing tapped holes (8 size measured about 0.010" larger).

              Stan
              This is probably the best solution. 6-32 is a notoriously weak thread, M4x.8 would be even worse.
              It's all mind over matter.
              If you don't mind, it don't matter.

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              • #8
                That would work but it would sure result in a sloppy fit. And the distinct risk of peeling the crests off the screws if you try to tighten something a little too much. Any chance of replacing the screws at the same time without too much extra work so the threads are all a proper #8?

                I sometimes wonder about the missing in between sizes in some of the thread systems. For example Uncategorized Groups machine screws are out there. Not commonly but still available without too much fussing. So at one point in history were the other missing odd number sizes also out there? Like 1/8 and 3/16 screws used to be a size but they were replaced by number sizes at some point in the last century?

                Chilliwack BC, Canada

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                • #9
                  Originally posted by BCRider View Post
                  That would work but it would sure result in a sloppy fit. And the distinct risk of peeling the crests off the screws if you try to tighten something a little too much. Any chance of replacing the screws at the same time without too much extra work so the threads are all a proper #8?
                  BC,
                  As I understand Stan's original post, he's planning to make new 'upgraded' thumbscrews, so the issue of a sloppy fit wouldn't arise.

                  Stan,
                  Not oddball but historic!

                  George

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                  • #10
                    If were mine and there is enough material to tap it #8-32 thats what I would do. Then I would order up some of the plastic caps that work with socket-head cap screws. They press into place with an Arbor Press, A Drillpress or a Vise.

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                    • #11
                      Originally posted by sjaffe View Post
                      Thanks for the replies. I have both SAE and metric thread gauges. 0.8 is 31.75 TPI. I compared 0.8 gauge and 32 TPI gauge with my fastener using a microscope. The 0.8 is just a slight bit off, the 32 TPI looks like a better match. I believe 0.8 translates to 31.75 TPI, hence why they would be close (but no cigar). I have not seen M4x0.8 listed (only 0.7 and 0.5). M4 major diameter specs show a minimum of 3.838 mm (0.151") so it could be an M4, but it seems unlikely that all the fasteners would be right at the minimum spec(?) So I'm not buying it's M4x0.8
                      It is common for cheap threads to be undersized, hence why I suggested that. Measure a piece of 1/2" allthread, it's typically almost 20 thou under. I won't say it is for sure M4x.8, but that size exists on Google.
                      21" Royersford Excelsior CamelBack Drillpress Restoration
                      1943 Sidney 16x54 Lathe Restoration

                      Comment


                      • #12
                        #7-32 (with a nominal OD of 0.154") is listed as a standard screw size in my 1914 Machinery's handbook.

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                        • #13
                          Those devices are fairly common in electronic circles. Probably first made by a hobbyist and then picked up by the budget manufacturers. They were available in places like Radio Shack as well as local electronic supply houses. Probably others like DigiKey, Newark, Mouser, etc. I am sure they are still available. I have worked at places that had one but I found them to be of limited use.

                          I suspect they are mostly made in places like China and Taiwan. So a metric thread is a good bet. Of course, one which is at the limit of the tolerance range is no surprise.

                          Here's one on Amazon:

                          https://www.amazon.com/ProsKit-900-0...%2C5475&sr=8-4
                          Last edited by Paul Alciatore; 04-05-2022, 12:33 AM.
                          Paul A.
                          SE Texas

                          And if you look REAL close at an analog signal,
                          You will find that it has discrete steps.

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                          • #14
                            Originally posted by Paul Alciatore View Post

                            I suspect they are mostly made in places like China and Taiwan. So a metric thread is a good bet.
                            You might lose your money. I've just been working on a Chinese-made machine which uses Whitworth threads (as well as metric...)

                            George

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                            • #15
                              Most chinese import products to the US used to use US threads. More recently, the chinese have rebelled, and just use metric fasteners. They don't want to fool with the US stuff any more, take it, or leave it.

                              Originally posted by The Metal Butcher View Post

                              It is common for cheap threads to be undersized, hence why I suggested that. Measure a piece of 1/2" allthread, it's typically almost 20 thou under. I won't say it is for sure M4x.8, but that size exists on Google.
                              The threads are not necessarily undersized. They may be made to a looser class of fit, but the OD difference is due to the flat on the thread crest. Many are cold rolled threads, and you can see the "flat" which is formed as the two ridges of displaced material come together.

                              The percent thread may be lower, around 60 to 70%, but the thread size is correct.
                              CNC machines only go through the motions

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