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Thread: Screwcutting

  1. #1

    Screwcutting

    I've done a trawl of previous posts about screwcutting and can find very little about the actual physical process. What, step by step, does it entail. I'm always intrigued by machining and lathes, not knowing anything about them and wishing I did. Why so many different threads?
    Anybody care to enlighten me?
    Cheers
    The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.George Bernard Shaw, Man and Superman (1903) "Maxims for Revolutionists"

    The road of excess leads to the palace of wisdom. William Blake

  2. #2
    There was a video on here of it being done but can't find the thread now....

  3. #3
    Check out Brock and Norris on YouTube should show all you need to know

  4. #4
    Quote Originally Posted by Markfox View Post
    Check out Brock and Norris on YouTube should show all you need to know
    Ah, I think it was them doing the video I was thinking of!

  5. #5
    I'm very rusty in that I haven't screwcut anything or turned a lathe on for about fifteen years or so but ultimately the process is the same whatever your turning. In very basic terms, and because I can't remember all the correct names/words for the process any more;

    You set the lathe cutting and travel speed for the type and diameter of metal being cut, zero the gauges using micrometers after initially measuring, and turn the material down to the outer diameter for the thread being cut and just shy of the length of thread required (I used to have a cutting booklet with all the diameters/pitches etc).

    Change tools to a chamfer (I think, it has been a long time) and cut a chamfer at the end where the thread will finish to the inner diameter of the thread (the end opposite to the crown), at the length required, this will be determined by the moderator and will be measured with a gauge.

    Change tools to the thread cutting tool with the correct pitch etc, the lathe is then turned down to a pretty slow speed and the thread cut and pitch settings engaged for the travel of the saddle (it might be wrong name or it is coming back to me!).

    You then cut the thread in several passes checking with a thread gauge and laterally the mod as you go as once material has been taken off it don't go back on. If you cut too fast or deep the finish is gash.

    The crown shouldn't require to be re-done but I'm not a gunsmith and they can probably knock holes in what I've written !

    The barrel may also be set up to cut on a running centre to alleviate vibration etc due to the length of the barrel and whether the action is attached.

    I may have missed something as it has been a long time but that is ultimately it. The reason for different threads is vast; metric, imperial, bsp, bsf, square etc etc.
    Last edited by Mick9abf; 03-10-2014 at 10:04.

  6. #6
    Not too far off.

  7. #7
    Quote Originally Posted by BRORA260 View Post
    Not too far off.
    Dont forget you should swing the compound slide round to 29 1/2 degrees if cutting unified or metric threads and advance the cut off the compound slide.
    This way the flanks are less prone to tearing then when the tool is advanced perpendicularly into the work piece and is the correct but slower way to generate precision threads.
    The depth of cut off the compound slide is the same as the pitch on all 60 degree froms.

    Ian.
    Last edited by Whitebeard; 03-11-2014 at 23:36.

  8. #8
    The reason for so many thread types, all relates to their uses, materials and applications. The earliest ones were individually cut by blacksmiths, and I think Whitworth was the first to specify threads per inch, angles of teeth etc so that bolts and nuts could be made in different places, and still fit
    Unified National Fine (UNF) and Unified National Coarse (UNC) are generally used by British or American Manufacturers, although these days Metric has largely taken over. These are typified by the diameter and number of threads per millimetre i.e. M8 x 1.0 or M10 x 0.8.
    It depends on the material, but generally finer threads have more "surface" area, and can therefore be tightened slightly more.
    But of course, I stand to be corrected........

  9. #9
    Ian is spot on. One thing to clarify why this process does not "tear" the threads. Because the compound-slide is used, the cutting tool is advanced along the flank of the thread so all of the cutting is done on one flank, the leading edge. If you try to plunge cut the threads the tool will chatter and cause the tearing.

    SS

  10. #10
    Quote Originally Posted by Whitebeard View Post
    Dont forget you should swing the compound slide round to 29 1/2 degrees if cutting unified or metric threads and advance the cut off the compound slide.
    This way the flanks are less prone to tearing then when the tool is advanced perpendicularly into the work piece and is the correct but slower way to generate precision threads.
    The depth of cut off the compound slide is the same as the pitch on all 60 degree froms.

    Ian.
    Huh, I didn't know that (and I've cut plenty of threads). Thanks, Ian!

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