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Thread dial/gear mismatch

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  • Thread dial/gear mismatch

    After some odd threading results on my new to me Toolmex/Polamco TUM-35D1 lathe I did some digging and noticed the thread dial has 20 divisions and the gear only has 16 teeth so it won’t land properly on the lines. The lead screw is 4 TPI and the manual indicates it should have 16 divisions and an older parts manual shows 16 teeth on the gear. The dial seems wrong but also very original. These have a unique painted thread dial and matching chart on the apron. The previous shop also said they never threaded with it and only had the standard full range English/Metric change gear so I cant image there was a different dial installed for the module / DP threads that could also be cut.

    Would be easy enough to replace the dial with a homemade job but it just seems very odd to me with all the design and effort in this factory dial to be wrong.

    anyone have any ideas??

  • #2
    Click image for larger version

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    This is a photo of a thread dial on a Toolmex lathe. Is this similar to yours? It looks like it's set up to handle metric threading. On South Bend lathes there was a similar arrangement as an option for metric threading where the dial ran through change gears depending on the thread being cut. I'm wondering if yours is like that.


    • #3
      For English threads, those measured in TPI, one revolution of the standard thread dial provides an indication of 4" of travel by the rotation of the lead screw. This allows all whole number threads to be synchronized as well as any threads with a 1/2, 1/4, or 3/4 fraction. This pretty much covers all threads that you will find in the US and older, inch based threads in the rest of the world. This means that for your 4 TPI lead screw, the 16 tooth gear (4 threads X 4 inches = 16) is the correct one.

      And yes, you are correct to say that the 20 divisions on the dial do not make synchronization of these threads easy. The standard number of divisions is either 8 or 16. These numbers of divisions will allow faster synchronization by using the common rules for these dials. But you can still use the 20 division dial by simply using only one of those 20 divisions. If you always use the same line, then your threads will be synchronized. And that includes all the fractional threads that I mentioned above.

      Personally I would make a new dial with either 8 or 16 divisions for this thread dial. It could be a new part or just an overlay for the dial that you already have. You could just glue it on. Then it will work as you expect.

      There may be a reason for that 20 division dial, perhaps it was for metric threads or some other special use. I do not know. An interchangeable 16 division one may have been lost sometime in the past history of that lathe.
      Paul A.
      SE Texas

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


      • #4
        The TUM35 threading dial is wrong in a couple of places.... I have a write up about it somewhere... The original was banded in colors, but the paint didn't hold up.

        Here's someone's response to my asking the same question a decade back

        "Your dial/chart doesn't need demystifying, it needs correction.

        You appear to have a wonderful thread dial and chart in concept, almost self demystifying, except they appear to have buggered it up. The big error is that they reversed the letters A - E on the chart. Then they put some threads where they don't belong.

        Ok, you have a 4TPI lead screw. Do you have 16 teeth on the threading dial gear? I.E. 4" lost travel per rotation of the dial? Scale C suggests so.

        move 3 from D to C (as currently marked) Move 6 from E to D (as currently marked). Then reletter the lines on the chart, from top to bottom E, D, C, B, A instead of A, B, C, D, E.

        For 4TPI/16tooth:
        A - Any TPI divisible by 4
        B - Any TPI divisible by 2 that doesn't qualify for A
        C - Any TPI divisible by 1 that doesn't qualify for A or B
        D - Any TPI divisible by 1/2 that doesn't qualify for A, B, or C
        E - Any TPI divisible by 1/4 that doesn't qualify for A, B, C, or D.

        If you like, number the marks on the C scale on the dial, starting with the one that goes all the way to the E ring, 1, 2, 3, 4 in the order that they cross the index mark when doing right hand threads.

        I normally would delete the pictures from the quoted text, but in this case I think it helps to include them."

        Now to find the pics...

        Last edited by lakeside53; 10-06-2020, 09:58 PM.


        • #5
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          • #6
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            • #7
              I think this came from the guy that wrote me the text aboveClick image for larger version  Name:	threadingdial8TPI.png Views:	2 Size:	8.4 KB ID:	1903585
              Last edited by lakeside53; 10-06-2020, 10:31 PM.


              • #8
                This is actually in color but Polamco sent me this in B&W. The colored bands and moveable dial pointer made it a "no brainer" .Click image for larger version  Name:	tum35 dialplate .jpg Views:	0 Size:	279.6 KB ID:	1903589
                Last edited by lakeside53; 10-06-2020, 11:07 PM.


                • #9
                  Thanks guys and especially lakeside for going through all the effort. My chart and dial are slightly different but of the same idea. They are in decent shape so it’s unfortunate they’re not right. I’ll have to take a closer look at the chart. I think mines accurate. A 20 tooth gear would make it all work but I’m not sure it’s worth the effort compared to a simple sheet metal dial.

                  I’ve emailed toolmex as well maybe they can send me a 16 division dial like lakeside shows.
                  Last edited by JCByrd24; 10-07-2020, 07:35 AM.


                  • #10
                    For now you can do what I did until it was figured out - a felt tip marker to make a line on a blank plate when the indicator was engaged. Sometimes painful to wait for it to come around each time but it works.


                    • #11
                      Problem solved. First time using super glue and painters tape work holding. Worked awesome.
                      You may only view thumbnails in this gallery. This gallery has 1 photos.


                      • #12
                        Originally posted by JCByrd24 View Post
                        Problem solved. First time using super glue and painters tape work holding. Worked awesome.
                        And now you have posted the original, did you notice the two inner bands on it (E & F) are for use with a 21 tooth gear? They have 7 and 3 divisions respectively.

                        So the dial is undoubtedly for a metric machine, where 20, 21 (and 22 - but very unusual, only for 5.5mm and 11mm pitch threads) will do all your threads. What is good about these three gears is that you can cut them all on a blank sized for 21 teeth (so the lower and upper ones are pitch-shifted) and do not need to alter the centre distance when changing them.


                        • #13
                          Interesting I hadn’t noticed the 3 and 7. While that makes sense this dial and associated labeling certainly look original and the machine is definitely imperial in both lead screw pitches, dial graduations, and labeling. It’ll do metric threading as well with no change gears, but lead screw must remain engaged. Being Polish they undoubtedly made metric lathes as well so possible it was a simple part swap at the factory but sounds like it happened commonly.
                          Last edited by JCByrd24; 10-27-2020, 08:58 PM.