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Thread: Overplunging

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  1. #1
    Member
    Join Date
    Aug 2019
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    1

    Overplunging

    Greetings:

    I was using a Gerber CNC mill, the spindle is a Makita router, the CNC is handled by Mach3 and the GCode obtained from Vectric Cut 3d, with a 1mm stepp per sweep, the machine had its Zaxis stepper changed recently.
    As it was rough pathing a block of MDF and little over half of the work done, the z axis failed to raise properly, and seemed to actually dive deeper than programmed (about 3 mm instead of 1) each time it had to descend.
    I am new to this job but the people here say the machine has done this before.
    I would appreciate any guidance towards finding out how to fix this.
    Thank you

  2. #2
    Member
    Join Date
    Jun 2010
    Posts
    3997

    Re: Overplunging

    Many possibilities. I would try reducing the Z axis speed and especially the permitted Z axis acceleration.

    Cheers
    Roger

  3. #3
    Member
    Join Date
    May 2020
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    1

    Re: Overplunging

    You can try reducing the Z axis speed.

  4. #4
    Member
    Join Date
    Jul 2018
    Posts
    1528

    Re: Overplunging

    Hi Dig - Firstly determine that the Z axis is moving the correct amount ie its calibrated. Then check the Z axis is mechanically in correct order, nothing loose or tight or wrong. (or do this first). If both these are correct then look at your tooling. Is the tool being used suitable for plunging? Does it have bottom clearance eg if the bottom of the tool fills up with sawdust it can't plunge or it has a very flat bottom, is this why you are cutting in 1mm levels? Not much of a cut?

    If the plunge stalls the Z motor CAM thinks its lower then it is, then next cut it cuts deeper. If you can program a ramp vs a plunge this helps or a spiral start hole. "The failed to raise" is a clue as the machine thinks its deeper then it actually is, this indicates the Z stalled and lost steps in the plunge. Photo of end of tool will help...Cheers Peter

    If it looses steps on the plunge it may stay high, I have to think hard about the logic... if MDF level is 0 and its meant to go to -1 then it stalls so z0 now is z0=-1 now its told to go to -2 so it goes to -1+-2=-3 so theres your 3mm dive.... I think Peter

  5. #5
    Registered
    Join Date
    May 2005
    Posts
    1545

    Re: Overplunging

    On a machine without positional feedback the control runs blind.
    If the g-code is (not running incremental in this example) G01 Z-1 F100 but the z axis stalls at Z0 the machine 'believes' it's plunged to Z-1.
    If the next line is G01 Z1 F100 the machine only 'knows' it's commanded to move 2 units positive on Z axis. If it makes this move successfully the machine will be at Z2 instead of the expected Z1.
    If the machine is running deeper than programmed due to lost steps the first place to look is the retract moves.
    Anyone who says "It only goes together one way" has no imagination.

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