Newbie understanding PP code for threading

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  1. #1
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    Default understanding PP code for threading

    I am trying to learn gCode technique by examining code produced by experts, including code produced by the PathPilot wizards.

    When PP's thread-milling wizard generates code for a 15-thread external thread, the gCode of a typical pass is
    Code:
    (Pass 3)
    G0 Z 1.0117
    G1 X 0.3641
    F 7.08 (Arc Feed, inches/minute)
    G2 X 0.3641 Y 0.0000 Z 0.2617 I -0.3641 J 0.0000 P 15   ; line A
    F 4.20 (Feed, inches/minute)
    G0 X 0.3869 Y 0.0000                                    ; line B
    G0 Z 1.0000                                             ; line C
    G0 X 0.3869 Y 0.0000                                    ; line D
    In Line A, I don't understand what function the J word serves. Since I, J, & K words are offsets, a J word of 0 doesn't seem to do anything. What am I missing?
    Lines B & C move the cutter clear, first horizontally and then vertically. Line D moves the cutter from where it is to ... where it already is (!). Does this line have some purpose that I've missed?

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  2. #2
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    Default Re: understanding PP code for threading (PARTIALLY SOLVED)

    In the terms of my original post, the functions of lines B & D are different.
    • Line B moves radially, to get the cutter clear of the threads. The thread in my example happened to consist of an integral number of full 360° turns, and the threading started & stopped at a 0° bearing from the center, so the B move was just a positive move in X. If the thread had ended with a fractional turn, the B move would have been less simple.
    • Line D moves to get back to a common clear position (X0, Y0), away from the thread at 0°.
    • If the thread didn't consist of an integral number of full turns, then before Line B the cutter could have been on the other side of the threads, so moving directly to (X0, Y0) couldn't be done safely.


    Last edited by Fenichel; 06-13-2017 at 10:59 AM. Reason: typo and added explanation


  3. #3

    Default Re: understanding PP code for threading

    In all circular moves (G2, G3) , I corresponds to X, and J corresponds to Y.
    Basically, I and J are asking "Where is the circle's center from the starting point?"
    Your starting point of the circular milling move is Y0. , So the distance from start-point to the center of your circle is also J0.
    Notice X start point is .3641, and I is NEGATIVE .3641 (Circle center is NEGATIVE .3641 in X from the starting point)



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    Default Re: understanding PP code for threading

    Thanks, but I knew how to read the code. My question was whether its J word served any purpose, inasmuch as the J number was 0. In other words, is there any difference between
    Code:
    G2 X 0.3641 Y 0.0000 Z 0.2617 I -0.3641 J 0.0000 P 15
    and
    Code:
    G2 X 0.3641 Y 0.0000 Z 0.2617 I -0.3641 P 15




  5. #5

    Default Re: understanding PP code for threading

    Run the program without it and find out. HAHA..
    Actually, I don't know.
    I assume it does need to be in there, just to tell the program where circle center is.
    But, as I said, you can experiment with no tool in the spindle, and see if it still interpolates the helix.



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    Default Re: understanding PP code for threading

    I've done a few experiments, all tending to show that a J number of 0 has no effect at all, so such a J word is superfluous. I haven't tested all of the various G codes that accept offset (I, J, K) words; there might be some in which the offsets are sticky, so that J 0 would usefully reset a cached value.



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understanding PP code for threading

understanding PP code for threading