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  • A question for John Stevenson regarding threading

    A newbie here could use some help attempting to understand threading.

    On 12-13-08, regarding the thread "Threading Video", you posted the following: "I find most useful in the shop is a thread table that gives the distance of the compound needed for the various standards 60 for US and metric, 55 for British, etc. instead of having to work out depth x 0.880456378373552974653 [ approx ]."

    If we are talking about 60 degree form V threads, I understand the depth of cut to be calculated as: depth of cut=(sin 60 degrees)(pitch) or = (sin 60 degrees)/#of threads per inch or =.8660/# threads per inch. No problem, simple trig.

    Here's where I get lost. Assuming the compound would be set around 60.5 degrees, as referenced from the direction of the cut, which equates to 29.5 degrees, as referenced from a perpendicular extended from the round stock to be threaded, compound travel would have to exceed cross slide travel to provide the same depth of thread cut. My understanding is that when using the compound to cut, we would calculate the cross slide travel required =(depth of cut/(sin 60.5 degrees) or =(depth of cut)/.870 or =(depth of cut)(1.149).

    Again, you stated, "depth x 0.880456378373552974653 [ approx ]". I do not understand how this was derived. I did find that the figure .880456378373552974653 [ approx ] is about the sin of 61.5 degrees. Do you use a greater relief angle? If 61.5 degrees was correct, should you not have expressed your formula for cross slide travel as =depth / 0.880456378373552974653 [ approx ]?? Typo??

    What am I missing? Once I resolve this issue, I plan to create an Excel spreadsheet for compound threading travel, such as you mentioned. That could be a very handy item, as I just found out.

    Many thanks.....

    Fred

  • #2
    Urrrrr---I think that Atlas Press Company has already published the tables for the proper depth of the Compound set at 29+ degrees, using the cross feed only for retracting the tool and re-setting to Zero..

    Comment


    • #3
      Groovin'

      Fred,

      this is one of those simple issues that gets "talked-up", over-hyped and endless references to Guru's long dead.

      A bit of an explanation and hopefully a simplification of the issues seems to be well in order.

      The underlying assumption is that the tool is perfectly formed as regards the sides being straight, the angle between the sides being very accurate, the radius or flat on the tool being very precise as to form and size, the axis of the tool being precisely at 90 degree to the job/lathe axis.

      It is quite probable that at least one of those variables, and probably more than one are not as exact as might be presumed.

      The ultimate aim, mostly, is to get a screw-cut thread to fit a known or mating part sufficiently well to do a particular job. Often this is a mating part or a known quality nut or screw/bolt that is in good condition.

      The purpose - or theory - of using the 29.5 degree off-set of the top/compound slide is the gradually shave of a 1/2 degree wedge from the "trailing" (back) face/flank of the thread - that's all. It doesn't always work but it does work often enough and many seem to believe in it, use it and get good-enough results.

      Get the basics right and then "improve" if you have to or want to.

      You are quite correct in that the distance to be moved by the top/compound slide is 1/Cos 29.5 x depth of thread = 1.1490 x depth of thread ~ 1.15 x depth of thread which is depth of thread + 15%.

      If possible (this applies most times) turn a small length (say 1/8" to 1/4" long) plain "spigot" (round section) the same diameter as you calculated root or minor diameter at the right/tail-stock end. If possible turn a recess just wider than the spindle of a micrometer and at the root/minor diameter at the head-stock end of the thread (this one is for "stopping" at when you disengage your half-nuts and/or stop the lathe if your half-nuts are to remain engaged (think: metric threads on and inch lathe - or vice versa).

      Now lock the cross-slide and forget about it. Use the top/compound slide to just scratch the job with the lathe running. Set the top/compound slide dial to zero.

      Start screw-cutting - slow speed, lots of cutting oil (drip feed or brush), and small cuts (say 10 thou - 0.010") at a time. When you get to the groove at the head-stock end, disengage the half-nuts and/or stop the lathe. Wind the apron back to the start point, engage the nuts, and take a bit more cut set on the top/compound slide. Keep going until you scratch the plain turned spigot and groove (which are at the minor/root diameter). Try the mating part or the nut/screw that you want to match/screw into/onto. Keep taking more smaller cuts until the threads are "just right" for you.

      If your lathe spindle speed is not low enough, you may find that you get a better result toward the end of the thread cutting if you turn the chuck and the lathe spindle by hand - or better yet, perhaps by a handle you can make to fit into the rear end of the lathe spindle (a sort of expanding collet with a handle on it.).

      A screw thread is no more than a spiral groove with a specified shape or form.

      Take it slowly and don't get bogged down in the math or "super precision". Just don't invent or put unnecessary obstacles in your own way.

      Once you get that first thread done nicely it will be a huge confidence booster and achievement.

      You can try some of the other "ways" and "tricks" from others later when you have the first steps under your belt.

      Just hasten slowly and take your time.

      Comment


      • #4
        I think John was actually after a table that would show what the compound dial would read at the end of a successful thread cut, I presume for common threads he would turn in the UK. If a person used the cross slide only then it would move the needed depth of cut, but the compound needs to move a but further as we all know. The confusion comes with the aplomb and drama John included with his wish that ran on to 12 decimal points.

        Comment


        • #5
          I got lambasted for a similar table that I worked out and posted on another forum, never went back.

          It was never meant to be a absolute end for many of the reasons OT mentioned, but, it does get you towards that end quickly and therefore I find them useful.

          Ken

          Comment


          • #6
            Oldtiffe:

            Thanks for responding and your excellent written description of the threading process. You're absolutely correct about using a groove cut at minimum root diameter to help determine proper thread cutting depth (a technique I have used). But in some situations a groove is not available (threading to a shoulder) and the mating part has yet to be made or is not available for some other reason. Using the same methodology you outlined, measurement of compound travel would be essential in determining proper cutting depth to produce such a part. Again, required compound travel could be easily calculated as a simple trig exercise, based on depth of cut.

            John, however, has come up with some numbers for this process I don't understand. John being (from what I gather from many posts) a very experienced person, may have information (the formula I questioned) that might improve my results in this area, hence my question. Once resolved, I'll complete my 'cheatsheet" for # of threads/compound travel for my personal convenience.

            Am I making too much out of this?? Maybe, but I want to make sure there is not something I'm overlooking in expanding my knowledge base. There is a wealth of information on this board (you included) and I want to make sure I 'mine every nugget'.

            Al Messer: Any idea where those tables were published?

            Cheers

            Fred

            Comment


            • #7
              Originally posted by oldtiffie
              Now lock the cross-slide and forget about it. Use the top/compound slide to just scratch the job with the lathe running. Set the top/compound slide dial to zero.

              Start screw-cutting - slow speed, lots of cutting oil (drip feed or brush), and small cuts (say 10 thou - 0.010") at a time. When you get to the groove at the head-stock end, disengage the half-nuts and/or stop the lathe.
              Missed it on first read but right here there's an omission - you need to back the cutter out so it doesn't rake the just cut threads when you rewind the carriage.

              Wind the apron back to the start point, engage the nuts, and take a bit more cut set on the top/compound slide. Keep going until you scratch the plain turned spigot and groove (which are at the minor/root diameter).

              Comment


              • #8
                If we assume a correctly-formed toolbit with a flat of the correct width at its tip, calculating the along-flank infeed of the compound slide to cut a correctly formed screwthread on a workpiece sized right on the theoretical Major Diameter is very straight-forward for three common threadforms:

                For the Unified and ISO Metric threadform, the flat at the tip of the toolbit should be 1/4 x Pitch for an external thread, 1/8 x Pitch for an internal thread, and the along-flank infeed should be 5/8 x Pitch.

                For a Sellers (aka American National or U S Standard) threadform, the flat at the tip of the toolbit should be 1/8 x Pitch for both external and internal threads, and the along-flank infeed should be 3/4 x Pitch.

                If you insist on using the "number of threads per inch" (N) instead of Pitch, which is 1 / N inch, when cutting a Unified or Sellers threadform:

                For the Unified threadform, the flat at the tip of the toolbit should be 0.250 / N inch for an external thread, 0.125 / N inch for an internal thread, and the along-flank infeed is 0.625 / N inch.

                For the Sellers threadform, the flat at the tip of the toolbit should be 0.125 / N inch for both external and internal threads, and the along-flank infeed is 0.750 / N inch.

                If you're going to insist on setting the compound to travel a half-degree off the flank of the screwthread, you can use the same numbers because the difference is "in the noise" for screwthreads finer than 4 TPI.

                Having said all of that, oldtiffie offers some excellent real-world tips.

                John

                Comment


                • #9
                  I just spent the day learning to cut threads on a lathe I've had for several years. I never needed to do any threading until this week or at least that's my excuse. I really like Oldtiffies suggestion of turning a short spigot with the diameter of the root/minor diameter of the thread. I realize that this idea is not the answer forever but it sure helps a rookie like me.
                  CYa,
                  Pat
                  WANNABE

                  Comment


                  • #10
                    Assistance appreciated

                    Originally Posted by oldtiffie
                    Now lock the cross-slide and forget about it. Use the top/compound slide to just scratch the job with the lathe running. Set the top/compound slide dial to zero.

                    Start screw-cutting - slow speed, lots of cutting oil (drip feed or brush), and small cuts (say 10 thou - 0.010") at a time. When you get to the groove at the head-stock end,
                    disengage the half-nuts and/or stop the lathe.
                    Originally posted by dp
                    Missed it on first read but right here there's an omission - you need to back the cutter out so it doesn't rake the just cut threads when you rewind the carriage.
                    Originally Posted by oldtiffie
                    Wind the apron back to the start point, engage the nuts, and take a bit more cut set on the top/compound slide. Keep going until you scratch the plain turned spigot and groove (which are at the minor/root diameter).
                    I am pleased that you picked that up Dennis.

                    I guess that you might not believe me if I said that I intended to take people on a ride on a magic Persian Carpet, which like all real Persian carpets (so I am told) has a deliberate defect as the only thing perfect is Allah. Just take care not to fall through the bloody hole though.

                    Hmmm. Yep. I thought you may say "Bull-$hit" as would have I, as Bull$hit it surely was as I left out that bit as I was hurrying (other things to do) and was counting on a good proof-reader - and you filled the bill nicely.

                    If some others had been picking vindictively, I'd have torn their arSe or their throats out as I was genuinely trying to help some who may have needed or appreciated it as I did when I was similarly helped a long time ago. All that the people who I was truly blessed with who mentored me wanted was for me to get it right and pass it on as they had done as I promised to do and as I tried to do. That was easy enough as it seemed to be the right thing to do. I hope I can fulfill their hopes and my obligation to them.

                    Thanks for the pointer and "pick-up" - appreciated - truly.

                    Comment


                    • #11
                      John's post was his usual somewhat tongue in cheek poke at a other posters who get hung up on multi decimal point accuracy, and not to be taken too literally.

                      The Atlas lathe manual has the table Al Messer mentions, and it is a probably the best manual on lathe work for the beginner, or even the experienced lathe hand.

                      Since most threads are cut with the compound set over something between 29 and 30 degrees, advancing the compound results in the cutting tool moving in a lesser distance. This distance can be calculated with standard trig, but since the degree markings on the compound are less than accurate and the setting is by eye, the procedure is less than precise. Dividing 0.750 by the TPI will give an infeed of the compound that is close enough to satisfy most requirements of threading a common 60 degree thread.
                      Jim H.

                      Comment


                      • #12
                        Dividing 0.750 by the TPI will give an infeed of the compound that is close enough to satisfy most requirements of threading a common 60 degree thread.
                        Excellent posting! Not an absolute, but one of those 'rules of thumb' that are so valuable in getting newbies started. You've probably helped 20-30 newbies that are following this thread (and scratching their heads), but are too timid to ask questions for fear of being labeled. The mechanical process of threading is frequently addressed, but the amount of cut is not. If you can figure out the threading charts, do some trig or find the 'right' formula, your OK. Somewhat daunting for many newcomers, but not for you "old salts".

                        Thanks for sharing, Jim.

                        Comment


                        • #13
                          And don't forget the Depth of Thread (DOT) program on Mavin Klotz's board. For any TPI and compound angle it will give you the various numbers for sharp threads, flat crest, flat root, etc. I ran calculations for most thread numbers I'm likely to encounter and printed them out for easy reference in my shop notebook.
                          .
                          "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

                          Comment


                          • #14
                            Originally posted by oldtiffie
                            I am pleased that you picked that up Dennis.

                            I guess that you might not believe me if I said that I intended to take people on a ride on a magic Persian Carpet, which like all real Persian carpets (so I am told) has a deliberate defect as the only thing perfect is Allah. Just take care not to fall through the bloody hole though.
                            Heh - you been standing downwind of those fires, Tiffie?

                            Actually, I just bought this kit http://www.hemingwaykits.com/acatalo...ol_Holder.html as a quick project while waiting for spring to return so was particularly wired to see what you suggested for backing out. I like Evan's threading stop for the cross slide but the pictures were missing last I looked at his link.

                            Comment


                            • #15
                              Originally posted by TGTool
                              And don't forget the Depth of Thread (DOT) program on Mavin Klotz's board. For any TPI and compound angle it will give you the various numbers for sharp threads, flat crest, flat root, etc. I ran calculations for most thread numbers I'm likely to encounter and printed them out for easy reference in my shop notebook.
                              That is pretty much what the Sudspumpmeister was looking for. It would be a nice table to have in my shop for 1/8" through 3/4" as those are my most common threading jobs though I normally use a die below 1/4".

                              Comment

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