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  • Duel Lead Threads?

    Hi guys,
    I am inquiring about dual lead threads . Years ago I had a South Bend lathe that was capable of "splitting threads". Some shotgun chokes use duel lead threads. A well seasoned machinist called this "Dual Lead" therefore I am calling it such.

    As an example, the SB lathe might be set to turn 12 tpi. The chasing dial on my lathe had (to the best of my memory) even and odd numbers on the dial with un-numbered lines between each. To cut even number thread one had to make each cut on an even number (odd tpi on odd numbers). The first run is made starting on, say #2. Watching the dial (and here's where things get foggy), the next cut is begun on #4 or perhaps a non-numbered line just past the #4. The point is, by doing this, you are still cutting 12 tpi but the lead on each cut is 180 degrees out from the first cut and when finished, it would appear that you had threads on the work that was 24 tpi when in fact you had 12 tpi ..... it was two sets of threads the starting point being 180 degrees out for each thread.

    Sure enough, I could easily do this on the SB. I now have a new lathe (as in brand new) but it's not a SB (it's an import).

    Should all lathes be able to make a "Dual Lead Thread" or was this merely a unique feature on the model of SB I once owned?. Keeping in mind that one starting lead should be 180 degrees from the other, if this can be done on all models, how would it be done on a lathe having no numbers on the chasing dial?

    Harold
    Last edited by hwingo; 11-12-2007, 07:54 PM.
    For those having fought for it, Freedom has a flavor the protected will never know.
    Freedom is only one generation away from extinction.

  • #2
    Harold,

    There are a couple of other ways of doing this, the easiest is to cut the first start with the compound set to 90 deg to the crossslide and the backlash taken up, plunge cutting the thread.

    Once you've done that, move the compound half the pitch for twin start or a third of the pitch for 3 start etc and cut the next start. With this method you engage the half nuts at the same place or you can leave the half nuts engaged as you would cutting a metric pitch on an imperial leadscrew and run the saddle back to the start point after withdrawing the tool.

    Others will chime in with other ways I'm sure.

    Al

    Comment


    • #3
      Harold,
      If you don't want to follow Monsewer Dastardly's good idea of running at 90 degrees and want to infeed at 1/2 the thread angle as many have been taught, then you can cut one thread using the thread dial as per normal then disengage the gear train and carefully turn the chuck 1/2 a turn and re-engage the gears.

      You second pass, still using the same settings as before will now be between the original threads.

      .
      .

      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



      Comment


      • #4
        Great Heavens Above Al!

        It's not even 0500 and you're up answering questions. What's a youngster like you doing up at this hour? This is when all us old folks are suppose to be up and about. I'm on patrol walking perimeter defense keeping a vigilance on my new lathe. Can't have the enemy sneak through my lines of defense and peculate my "precious darling".

        oops! Another trooper is on patrol standing guard over this thread. Just got an incoming alert. Before I dive into you answer, let me see what the next answer is and get back to you.

        I can't believe you guys are up so early. It's still Zero Dark-Thirty at my AO.

        Harold
        For those having fought for it, Freedom has a flavor the protected will never know.
        Freedom is only one generation away from extinction.

        Comment


        • #5
          Me again,

          Just to clarify most multi-start threads I've cut have been square form therefore plunge cutting was easiest.

          When cutting the nut for vee form I don't infeed at 29 1/2 deg so the 90 deg method works too. Always move the compound in the direction that you've taken up the backlash in.

          Al

          PS: 1130 AM here in Blighty, sat shining my @rse at work.

          Comment


          • #6
            Sure is

            Originally posted by John Stevenson
            Harold,
            If you don't want to follow Monsewer Dastardly's good idea of running at 90 degrees and want to infeed at 1/2 the thread angle as many have been taught, then you can cut one thread using the thread dial as per normal then disengage the gear train and carefully turn the chuck 1/2 a turn and re-engage the gears.

            You second pass, still using the same settings as before will now be between the original threads.

            .
            Agree John.

            That is the most efficient and fool-proof way in the general case.

            But a couple of "traps" to be wary of.

            I presume that the gears to be dis-engaged will be the one mounted on the drive-train end of the spindle and the next. That being so, the method you suggest will work fine providing that gear has an even (and not an odd) number of teeth.

            There is a "trap" with lathes with a geared head like mine. There is a 4:1 reduction within the gear-box between the lathe spindle and the first drive gear in the external drive train. Rotating the lathe spindle 1/2 turn will result in the first drive-train gear only moving (1/2 x 1/4 = )1/8 turn so that gear needs to be a multiple of 2, 4 or 8 (I think).

            Fortunately I have a 6-lobe "Dog" clutch on my lead-screw so I can disengage and engage the lead-screw in 6 different but equal divisions without disengaging my gear-train. Which in my set-up is very handy.

            Al's (DickDastardly40) method has a lot of things going for it as he can leave his gears alone and use the cross-slide in conjunction with the compound/top slide to get a similar effect as the "offset the compound by half the thread included angle" method and still us the same settings to cut his multiple-start (or any) thread/s.

            Because of my lathe set-up (my lathe is "metric" with a 3mm lead-screw) I agree with and support the logic with Al's method as I am able to avoid the multiple disengage-engage of the gear train. This is particularly the case if I am cutting "inch" threads no matter how many "starts" there are.

            So - I favour and use the method as described by Al.

            Others preferences and circumstances may well differ from mine - that's fine - but whatever works works.

            Comment


            • #7
              Sorry, misspost.
              Last edited by joegib; 11-12-2007, 09:15 AM.

              Comment


              • #8
                I believe some of the methods above may depend on the lathe or on the exact TPI of the thread. They may work in some cases and not others. Setting the compound to 90 deg and using it to offset the thread is good in all cases but could be less accurate for small threads as the offset could be just a few thousanths.

                Another method is to mount the work in a manner that allows it to be rotated 180 degrees in the chuck after cutting the first start. Of course, it must be perfectly aligned horizontally for both starts. This would work for any number of starts by rotating 120, 90, etc. degrees.

                As for your SB trick using the divisions on the threading dial, that only works for some TPIs, even on the SB. But it would work on any lathe that has the multiple divisions on the thread dial. The trick here is that the TPI would normally REQUIRE starting on an exact number or on an even number or even on a number as opposed to an inbetween (un-numbered) division. And the rules would be different with different threads. For instance, I have an 8 TPI lead screw on my SB. If I am cutting a 8 TPI thread, I can engage on any division and it will still cut the original path. I could never cut a multiple start thread of 8 TPI with this lead screw. On the other hand, a 13 TPI thread would require that I start on the same number each time. So if I started on even numbers for the first start then starting on odd numbers for the second start would product a two start thread. And I could make a four start, 13 TPI thread by using the half divisions: even, then odd, then on a half division following an even number, and finally on a half division following an odd number. But I couldn't make a three start thread this way. There are several different combinations for different TPIs. I could NOT cut multiple start metric threads with this method using my 8 TPI lead screw and a transposing gear.
                Paul A.
                SE Texas

                Make it fit.
                You can't win and there IS a penalty for trying!

                Comment


                • #9
                  OK Guys,

                  I am sooooo confused. You guys are far above my head.

                  What information about my lathe should I provide so you guys can help me through this? I have a manual for the lathe (though it's not the best manual in the world) and maybe it would provide me information that I can convey to you so you can get me on the right path.

                  I will be cutting in TPI most all the time and rarely in metric (though I will have a need for metric from time to time). However, dual lead threads will be cut in "inches" using the V design.

                  Harold
                  Last edited by hwingo; 11-12-2007, 07:55 PM.
                  For those having fought for it, Freedom has a flavor the protected will never know.
                  Freedom is only one generation away from extinction.

                  Comment


                  • #10
                    duel?

                    I challenge you to one. Suggest spelling books at 20 paces.

                    Comment


                    • #11
                      Harold, another method is to thread between centers using a driving dog. For a two start leave the four jaw chuck on and index the dog off of opposite chuck jaws, or a three start with a three jaw chuck. You could also use a face plate or dog plate and lay your indexing out on them by drilling holes or using clamps in existing holes.
                      James

                      Comment


                      • #12
                        Originally posted by tdmidget
                        I challenge you to one. Suggest spelling books at 20 paces.
                        Yes, I made a mistake .... several times. I have made the necessary corrections to put your mind at ease. Now, would you like to make a significant "on topic" contribution so I may learn more?

                        Harold
                        For those having fought for it, Freedom has a flavor the protected will never know.
                        Freedom is only one generation away from extinction.

                        Comment


                        • #13
                          Originally posted by J. Randall
                          Harold, another method is to thread between centers using a driving dog. For a two start leave the four jaw chuck on and index the dog off of opposite chuck jaws, or a three start with a three jaw chuck. You could also use a face plate or dog plate and lay your indexing out on them by drilling holes or using clamps in existing holes.
                          James
                          Hi James,

                          Thanks for the reply. I got confused beginning with the first reply. I can see a great importance in doing it the way Al has suggested. If doing it as Al has suggested, this means that dual lead threads can be done on most any lathe using Al's method. That which confuses me is how Al's method starts a second thread 180 degrees from the first pass.

                          Also, I made another mistake (which is not at all uncommon). I had stated that my chasing dial had no numbers and this was incorrect. I have numbers 1, 3, 5, and 7 on the dial with only one line between each number. So help me out on this ............. To cut "even number threads" must I start on a number ....... just any number? If so, does this mean that to cut odd numbers I must start on a non-numbered line ...... any line? Finally, what's the significance of odd numbers on the chase dial? Does each number have a special significance?

                          Thanks,

                          Harold
                          For those having fought for it, Freedom has a flavor the protected will never know.
                          Freedom is only one generation away from extinction.

                          Comment


                          • #14
                            Their are a number of ways depending on the lathe because some are different. But my method is this .
                            1.even threads any line any time . 2,4.8,12,16,

                            2. odd threads same number or line every time.9.13, so on.
                            South Bend made a lathe with a 6 pitch screw which was really wearied.
                            Every Mans Work Is A Portrait of Him Self
                            http://sites.google.com/site/machinistsite/TWO-BUDDIES
                            http://s178.photobucket.com/user/lan...?sort=3&page=1

                            Comment


                            • #15
                              Some help on how to think about threading:

                              Perhaps the best way to think about it is that a threading dial is actually a linear measure. It shows the distance that the thread has traversed as the lead screw rotates. Thus, using my SB for an example, the lead screw is 8 TPI and the SB threading dial uses a 32 tooth gear or 8 X 4 teeth. When the lead screw has rotated 8 times, the carriage and thus the thread has traversed one inch. And that's 1/4 of the 32 teeth so one complete turn of the dial represents 4 inches of travel. The numbers, 1 to 4, on the dial represent inches. The un-numbered divisions between the numbers represent half inch increments.

                              With the above in mind, what you need to ask yourself is how far must the thread you are cutting traverse to come back in sync with the lead screw TPI. There is an opportunity for this every rotation of the lead screw but not all threads will match at every rotation. Obviously, an 8 TPI thread on the work will match on every rotation. As will any whole multiple of 8 TPI (16 TPI, 24 TPI, 32 TPI, etc).

                              Even numbered threads like 10 or 12 TPI will synchronize after a half inch and can be started on any mark on the dial since they represent half inches. Odd numbered threads like 7 or 11 TPI will synchronize after one inch so they can be started on any numbered mark. Threads with a 1/2 fraction will not synchronize until a full two inches have been traversed and can only be started on even or odd numbers to match the first start. Finally, threads with a 1/4 or 3/4 fraction must be started on the same number as the first start.

                              Since the threading dial only represents four inches, it's ability to synchronize threads ends with fourth fraction threads. Also, note that it can not synchronize a thread with any other fraction like thirds or fifths: only halves and fourths.

                              That's how a thread dial works. Metric dials are more complicated and often (always) have multiple gears for different pitches. This is because the fractions are more complicated, like fifths and tenths. With different gears on the dial, it can represent different distances and handle different fractions in the equation.

                              When you use a dial for multiple starts, you are actually starting out of sync with the first start. If you want to cut a two start thread, the TPI must have the inbetween divisions on the dial that would not work for a one start thread. Thus, the multiples of 8 TPI would not allow this as any mark on the dial would sync them. The other even TPI threads would allow it, but you would have to stare half way between the marks (that's half way between a numbered mark and an un-numbered one). And you could only make multiple starts that are a multiple of 2 (2, 4, 8, etc).

                              The important thing to think about is the synchronizing distance.

                              All of the above numbers are for an 8 TPI lead screw. Other lead screw pitches will change things, but the rules are the same. The things that you must know about a lathe to figure this out are the pitch of the lead screw, the number of teeth on the thread dial gear, the number of divisions (numbered and un-numbered on the thread dial, and the pitch of the thread being cut. That's all.
                              Paul A.
                              SE Texas

                              Make it fit.
                              You can't win and there IS a penalty for trying!

                              Comment

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