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Coarse Internal Threads

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  • Coarse Internal Threads

    In the hardback book, 'Projects 4', is an article by Theodore M. Clarke: "Single Point Cutting" of Coarse Internal Threads. Per the index on this website, it was originally published on pg 53 of HSM, Jan-Feb 1989. Mr Clarke's method involves a combination of compound feed and incrementally extending the tool bit further out from the boring bar to deepen the thread.
    He contends this will cut the deeper threads without exposing the cutting edge to the full flank of the thread, thereby less cutting forces and a better thread.
    Since my lathe spindle is 2.25 X 4tpi this holds special interest for me. However I have studied his description and drawing til I'm blue in the face and still can't reconcile his word description with his diagram. I've about decided his diagram must be drawn wrong.
    If someone else has access to that article and could render their opinion, I'd be grateful.
    Lynn (Huntsville, AL)

  • #2
    lynnl, I have read the article several times myself over past couple of years.
    IMHO, many people spend much time making a simple project difficult. This is an example, special tool holder, etc.
    If this were such an improvement, the machining world would have fallen on it, and immediately adopted it as a standard.
    To cut the threads for your backplate, use a good stout boring bar, a sharp cutting tool, plenty of cutting oil and follow standard practices. If you are concerned about heavy cutting forces, take light cuts, and take a couple passes without changing setting from time to time.
    Jim H.


    • #3
      I'll both agree and disagree with JC. He's right about the big boring bar. With a nearly 2" minor diameter, you have room in the hole for a really stout tool, and that makes everything easier.

      On the other hand, the width of the thread flank at full depth is 0.156", and that's a pretty wide chip. On my lathe, depending on the material, I'd have to take really light cuts, which would mean probably over 100 passes on a thread that size.

      I think I finally convinced myself that Clarke's setup would work, but it's awfully complicated. I decided that if I were ever faced with that situation, I would probably use a square nose grooving tool, set the compound parallel to the spindle axis, keep real careful track of where I am and use the cross slide and the compound together to hog out 60 or 80% of the material and then switch to a threading tool and follow JC's advice.

      Another possibility is a technique I saw for cutting multi-start threads. The guy put a collar on his boring bar and used spacers between the collar and the tool holder to adjust the tool location along the axis. Probably easier and less risky than my idea with the grooving tool.


      • #4
        UncleDunc, That's a clever idea about the collar. I like that. In one of the Old reprint books from Lindsey Pubs. "Running an Engine Lathe" by Fred Colvin, he describes a guy doing over 100 studs/day of one particular thread. The guy developed a method which started with a deep cut, then purposefully offset the second cut, then came back put the third cut between the two, then finished w/1 or 2 cleanup passes. Doing 100/day, day in/day out he became able to visually adjust the position.
        Regarding Clarke's diagram: It looks to me like his cut shows the outlines on the wrong flank of the thread.
        I have successfully threaded 2 or 3 devices for my spindle, but as you say it's been a tedious, nerve wracking undertaking to do it.
        Lynn (Huntsville, AL)


        • #5
          I just finished a job like this. I wanted to mount a 6 jaw Buck Tru-adjust chuck to my lathe. The lathe had a 2 3/8 X 8 threaded nose. I purchased a 2 1/8 X 8 back plate. Chucked it in the 4 jaw chuck and indicated as best I could. Bored out the threads to the minor Dia. of the 2 3/8 X 8 thread. Then bored out the thread recess in the back of the plate to .001 over the lathe noses register dia. Used a 3/4" boring bar and fed the 60 degree tool bit straight in. Yes, I had to take a lot of cuts but a good sharp bit will take good amount each time. When I got close to the thread depth I would unscrew the chuck with the back plate and try it on the spindle nose thread. Once it would thread on I took a skim cut off the register area then mounted the back plate and trued it by taking skim cuts on all the surfaces. Thread was a little loose but no worse than the chucks that came with the lathe. Once the back plate hits the register area it snugs right up tight and will wipe the bluing off all around the contact area were it butts up to the spindle register. Just have at it and do not fret.


          • #6
            I hog out larger threads with a triangular insert (60* "T" type) it can also be done with a "V" type insert (35* diamond). This greatly reduces the wear on the very expensive sharp-V threading inserts.

            When doing your backplate just do lighter cuts and concentrate on not crashing the tool into the workpiece - 4tpi comes up way too fast!

            When you get it finished and it fits right have a cool one, pat yourself on the back, and relax!