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  • Bastard thread

    I was asked to make some extensions for a plumber who is installing four baths in a house. The chrome overflow for the bath does not fit because the bath is to thick . I thought no problem ,just make an adapter out of delrin(acetal)

    The problem is I could not measure the pitch of the thread.The OD was exactly 39mm and the closest pitch I could measure was 18 tpi.
    But the chrome nut binds after about 7 full revolutions. It will work because it only has to tighten up about 4 revolutions to work but I wonder what thread this could be. Could it be a thread specific to the manufacturer like 1,4mm for example that manual lathes cant copy?
    And how do you measure a bastard thread ?

  • #2
    Measure as many threads with a caliper and divide by the total minus 1. You get enough precision this way
    Helder Ferreira
    Setubal, Portugal

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    • #3
      Originally posted by plunger View Post
      Could it be a thread specific to the manufacturer like 1,4mm for example that manual lathes cant copy?
      Anything is possible with custom hardware like these.
      But you can thread really odd pitches even on manual lathe after you work out all the possible gearing combinations.
      For example if your Maximat V13 came with inch, metric and module gears you can get lots of "unlisted" combinations that are normally no use but perfect for this kind of like purpose.
      My Kerry lathe would do following mm pitches:
      1,3846
      1,40625
      1,4000
      1,41111
      1,47321
      1,3750
      Location: Helsinki, Finland, Europe

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      • #4
        M4 standard pitch is 0,7 mm and on quite many lathes you can find lever position that doubles that to 1,4 mm. It is typically just unlisted as there is not much use for it.
        Location: Helsinki, Finland, Europe

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        • #5
          Hmmmm. You have a 39mm with 18 TPI threads... There's your problem, someone put some inches in your meters.

          But seriously, a manufacturer can use any thread pitch that they want. It's not uncommon for a manufacturer to use dies, taps or other tools that make progressively worse thread as they wear. Then they simply have someone on the assembly line test fit parts till they have some that match.

          Measuring the TPI is easy enough, in that you count how many threads are in the length of 1 inch, or some known fraction thereof. If you only have threads for 1/2 inch, then you count the threads and multiply by two. If you have 1/3 of an inch, then count those and multiply by 3.

          Dan
          At the end of the project, there is a profound difference between spare parts and left over parts.

          Location: SF East Bay.

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          • #6
            Those threads are specials, but as your 18 tpi screws on a fair way, the correct pitch is probably 19tpi which is used on smaller sizes of both NPT and BSP.

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            • #7
              I have encountered 19 tpi a few times, always to do with fittings , I had to change a gear to cut it.

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              • #8
                Is it possible the fittings are of European origin? If so, they will be metric thread, 40mm for 1 1/2".

                We have the same issue in UK and can buy an off-the-shelf adaptor from BSP (UK pipe thread) to 40mm: https://mcalpineplumbing.com/plastic...outlet-adaptor

                I have had a brief look and while everywhere tells me the thread is 40mm, I cannot easily find its pitch, so I am sorry for an incomplete answer.

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                • #9
                  My thread pitch guage tells me it is definitely not 19 tpi.I can turn 19 tpi on my lathe . But it is definitely an OD of 39mm. The standerd bath overflow is 40mm which is the one in white and it has a 1,5mm pitch

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                  • #10
                    Originally posted by plunger View Post
                    My thread pitch guage tells me it is definitely not 19 tpi.I can turn 19 tpi on my lathe . But it is definitely an OD of 39mm. The standerd bath overflow is 40mm which is the one in white and it has a 1,5mm pitch
                    18 TPI is .0555" lead, 1.5 mm is .0590" lead, the difference is sometimes difficult to see when using a lead gauge on a short length of thread.
                    That being said, most manufacturers are unlikely to make a part with a hybrid feature such as a thread with a metric major diameter and an inch lead although it is very simple to do.
                    If one desires to make parts with a proprietary feature there is no compelling reason to mix inches and metric dimensions when they could simply make it M39-1.59 mm lead.
                    Last edited by Bented; 09-01-2020, 07:16 PM.

                    Comment


                    • #11
                      Originally posted by plunger View Post
                      I was asked to make some extensions for a plumber who is installing four baths in a house. The chrome overflow for the bath does not fit because the bath is to thick . I thought no problem ,just make an adapter out of delrin(acetal)

                      The problem is I could not measure the pitch of the thread.The OD was exactly 39mm and the closest pitch I could measure was 18 tpi.
                      But the chrome nut binds after about 7 full revolutions. It will work because it only has to tighten up about 4 revolutions to work but I wonder what thread this could be. Could it be a thread specific to the manufacturer like 1,4mm for example that manual lathes cant copy?
                      And how do you measure a bastard thread ?
                      Open up the Machinery's handbook, in you edition look at the metric threads, measure the pitch not threads per inch, use the book to figure out the thread.
                      I have my original I purchased 45 years ago and it was used, Eighteen edition. Go to Iso Metric threads.

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                      • #12
                        Don't you just love bastard threads? I bought some toolholders for my S3 Dickson toolpost, cheap. They came with M10 square head screws in them (odd, because my other S3 holders all have M12 screws). The M10's were very very loose and clearly not the right size. M12's won't go in.

                        The holders still had the threaded piece for the height adjuster in them. This is a perfect fit in the tool holding holes. On measuring the OD of that thread, it's 10.8mm. M11 maybe? Not exactly the commonest size.

                        Even though Dickson are otherwise made to metric dimensions, could it be an imperial thread? 7/16" would have an OD of 11.11mm, so it doesn't seem to be that.

                        At the moment, unless I can think of another option or someone has a suggestion, they will be heading for the scrap bin. Most of the height adjuster mechanisms are missing too. Redrilling & tapping looks too difficult - they are hardened. For the effort involved, I'll just look for holders with more normal threads.

                        Ian
                        All of the gear, no idea...

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                        • #13
                          Yes they will be 11mm. Most store bought metric fasteners I've seen, threads will measure undersize by .15-.20mm.

                          Ed

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                          • #14
                            7/16 unf and M11x1.25 are damn close to each. Standard seat belt bolt size even in euro cars if you need one..
                            Location: Helsinki, Finland, Europe

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                            • #15
                              I was renovating our bathroom many years ago and bought a very expensive shower control (don’t remember the brand but think it was German made). The problem was that the pipe fittings to connect it wouldn’t thread into the body of the control. Long story short, it turns out the pipe thread was left hand and was supposed to come with the control but was missing. Couldn’t source those locally either as a separate item. I couldn’t wait for a replacement and, ironically, substituted a much less expensive unit instead.

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