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  • Threading problem, threads to long

    Good day
    Im looking for a solution on a problem with threading on a lathe.
    The description is in this images. Long story short, the treads always are a little to long

    The treads are for a rifle barrel that need so have 18tpi on a 1050 diameter shank. seen on the right photograph
    The lathe is a chines made with a 10tpi lead screw.

    I'm hoping someone here knows what the fault is?



    Click image for larger version

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  • #2
    Put your thread pitch gauge on the lathe leadscrew and see how close to 10 tpi it actually is.
    On a China lathe, assume nothing.

    ---Doozer
    DZER

    Comment


    • #3
      10TPI is very close to 2.5mm pitch but if that was the problem it would be cutting short not long.

      I suggest that you check the tooth counts on all of the gears in the train.

      What model lathe is it? If you can, put up a photo of the threading plate and the end train.
      Peter - novice home machinist, modern motorcycle enthusiast.

      Denford Viceroy 280 Synchro (11 x 24)
      Herbert 0V adapted to R8 by 'Sir John'.
      Monarch 10EE 1942

      Comment


      • #4
        Originally posted by Doozer View Post
        Put your thread pitch gauge on the lathe leadscrew and see how close to 10 tpi it actually is.
        On a China lathe, assume nothing.

        ---Doozer
        Better yet put a 1" travel dial indicator against the carriage. Turn the leadscrew by hand 8 or 9 turns. The indicator should read .800 or .900. If not the problem is the leadscrew. If the leadscrew is OK check the gears to see if the marked tooth count matches the actual count.

        Comment


        • #5
          I like the idea of the dial indicator or some other fairly precision measuring device to check the travel of the carriage.

          While I was at it I'd also pull out a rule graduated in 1/10's and offer it up to the lead screw threads and along a 10" length check for any runout. I'm assuming Acme or square cut threading on the rod. So like up the end with one of the edges of the top flat and compare the teeth over a 10" long run. It may turn out that they fudged things and went with a 2.5mm pitch. In that case over 10 inches you'll see 0.4mm of runout. That's not really an amount you'd see easily over the length of a thread gauge. But when reflected through the gearing might be enough and in the direction to produce this thread variation.

          I'm also thinking that your barrel might not in fact be 18TPI. After all the 1.05" major diameter you measured suggests a 1 1/16 thread. It might be something weird for the thread as well. You didn't mention the make of the rifle. Perhaps do a web search on that model and include "barrel thread" and see if the actual number is something more odd.

          I ran into this custom issue when making up a barrel seating sleeve for my cowboy action 1897. The thread pitch on one side was a pretty common pitch. The other side needed 30TPI which is not common at all. Luckily in my case I was able to swap a couple of gears around under the head cover to alter the ratio to get me the 30 TPI I needed.
          Chilliwack BC, Canada

          Comment


          • #6
            RC Rider The action is a Borden Rimrock they all have 18 tpi. in one of the images in the first post Im holding a thread gauge against the cut threads from the lathe. I have other barrels from 3 different gunsmith all are 18tpi. I will check for the run out. I have counted the threads on the lead screw over 2 inches and they come out 20 tpi over 2 inchClick image for larger version

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            • #7
              Originally posted by Peter. View Post
              10TPI is very close to 2.5mm pitch but if that was the problem it would be cutting short not long.

              I suggest that you check the tooth counts on all of the gears in the train.

              What model lathe is it? If you can, put up a photo of the threading plate and the end train.
              This is a image of the plate, Im using 50-85-60-60 I have tried to cut 16tpi , same result, a little to long

              Click image for larger version

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              Comment


              • #8
                If you are unsure of the machines threading accuracy simply measure the machine lead before threading.
                Use an indicator for this, 18 TPI is .055555 lead per revolution of the spindle.

                Measure the lead before threading (-:

                Comment


                • #9
                  If the leadscrew is correct, then the gearing must be wrong. Check all the gears against what the manual says they should be. Possibly one of the gears has the wrong number of teeth.

                  Also, can you determine the exact error amount?

                  If the thread pitch is slightly too large, then the leadscrew is turning too fast. One of the driven gears may be too small, OR one of the driving gears may be too big (too many teeth).
                  2730

                  Keep eye on ball.
                  Hashim Khan

                  Everything not impossible is compulsory

                  Comment


                  • #10
                    It might be your compound angle. The compound on American made machines is set on 29 1/2*_30*.
                    On some Asian made lathes the degrees start in a different place.
                    You'll need to find the correct counpound angle for Asian lathes.

                    Hal

                    Comment


                    • #11
                      Originally posted by Hal View Post
                      It might be your compound angle.

                      Hal
                      The compound slide angle is unimportant, it does not effect the lead. The lead is controlled by the gearing between the spindle and lead screw.

                      I most always thread on manual lathes from the cross slide straight in.

                      Modern lathes often give one a nearly unlimited choice of approach angle. Some very high end machines will alternate between cutting on the leading edge and the trailing edge of the tool.

                      A 7/8-13 thread X 3/4" long, the plunge/approach angle is whatever you choose, I find that it makes little or no difference.



                      Comment


                      • #12
                        Not trying to NitPic , as I am not familiar with that Lathe or it's construction.
                        But I have run into similar issues with foreign made machines where "Metric" is their base measurement system
                        and they either make a mistake OR actually try to fudge the parts to fit Imperial requirements. And this was with Large Industrial Machines.

                        So what Jerry Tiers said is right on the money !
                        But you need a fine tooth comb as they say- as the error is so slight, take nothing for granted !
                        For example , Even thread gauges can be wrong ( I have one that I can show, but that is another thread, but not here, too confusing ! )
                        The suggestion of a 2.5mm pitch to look for is a good one . I blew up your picture to show you
                        The red lines are centered on your caliper POINTS . note the slight difference in the tooth crests.
                        "If" it was a 2.5 thread the difference is only .030" over two inches and with a .100 pitch ( 10 TPI ), that is less than 1/3 of tooth spacing

                        ‚Äč
                        I suggest you try a 6 inch span as that would give you almost one whole tooth difference between 2.5 mm and 1- TPI

                        Just as " We Americans" try to figure out how to cut Metric Threads on our Imperial machines by using weird gear tooth counts and always having minute error,
                        do not accept that your foreign maker did not do that, they do in some cases.

                        When you take the headstock gear box cover off, do three things,
                        1. count the teeth on each gear
                        2. figure the reduction ratio with the gear on your machine and looking at the manual
                        3. Look at the gears themselves for tooth counts ( stamp or cast)
                        Rich

                        Edit
                        Another consideration is if you have a QC box . For your tests, try to set it up so you have direct drive ( 1:1 ) from the leadscrew to the last reduction gear shaft . The QC Box itself is also subject to checking.
                        Last edited by Rich Carlstedt; 09-27-2021, 10:48 PM.
                        Green Bay, WI

                        Comment


                        • #13
                          I have seen this situation a few times in the past where a Chinese or other lathe seems to have inbuilt thread pitch errors. However more than once we have found that the change diagrams are the problem.

                          On this machine just what does 'H' mean?


                          Meanwhile, have a look at this https://www.precisionmatthews.com/wp...v5-2021-09.pdf

                          that seems to be the same machine. Have a look at page 15....
                          Click image for larger version

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                          Attached Files
                          Last edited by The Artful Bodger; 09-27-2021, 11:55 PM.

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                          • #14
                            Whe you start the threading , do a scratch cut, then check against pitch guage.. what is it telling you ? If it dont match , cutting further wont fix it.
                            should always do a scratch and check for first thread..

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                            • #15
                              Poppycock! The angle of the compound has absolutely nothing to do with the pitch.

                              NOTHING!



                              Originally posted by Hal View Post
                              It might be your compound angle. The compound on American made machines is set on 29 1/2*_30*.
                              On some Asian made lathes the degrees start in a different place.
                              You'll need to find the correct counpound angle for Asian lathes.

                              Hal
                              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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