Confused about ups and downs

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  • January 28, 2015 at 6:59 pm #7321
    mristau
    Member

    As described here*, I’ve set my Z upper limit switch to max/homing and
    Z lower limit switch to min/disabled. Here are my settings:
    $4tr=2.540 (mm travel per revolution)
    $4mi=4 (microsteps, 1, 2, 4, 8)(default)
    $4po=1 (0=normal,1=REVERSE)(default)
    $zsn=0 (low, min z switch, 0=off)(default)
    $zsx=1 (high, max z switch, 1=homing)(default)
    $ztn=-95.000 (travel minimum)
    $ztm=0.000 (travel maximum)

    G28.2 Z0 homes exactly as expected: It moves UP…in the POSITIVE direction…toward zero 🙂

    However, I’m wanting to locate the top surface of material to be cut and find notes on “Automatic Z Homing When Milling PCBs”***
    The hookup makes sense but indicates that the result of issuing G28.2 Z0 (same as above) “will move the Z axis in the NEGATIVE direction.”
    How is this possible?

    *
    Homing Setup and Configuration (TinyG default settings)
    …Z is commonly zeroed at its highest point – making all plunges into the work negative values. This translates to the following settings:
    $zsn=0
    $zsx=1
    https://github.com/synthetos/TinyG/wiki/Homing-and-Limits-Setup-and-Troubleshooting#homing-setup-and-configuration

    **
    Automatic Z Homing When Milling PCBs
    “issue a G28.2 Z0 command[…]this will move the Z axis in the negative direction[…]it will be perfectly zeroed out”
    https://github.com/synthetos/TinyG/wiki/Automatic-Z-Homing-When-Milling-PCBs

    ***my Z axis
    TinyG, motor 4/Z, 1.8 degree, nema23, direct drive of 1/2″-10tpi ACME lead screw, 2.54mm per rev.

    January 30, 2015 at 12:27 am #7325
    mristau
    Member

    So I’ll answer my own question in case it’s helpfull to some other CNC nube.
    With the settings below I have (for my machine) both G28.2 and G38.2 commands working as they should. 🙂

    “G28.2z0” homes to my upper/max switch.
    Making no software setting changes, “g38.2 z-100 f1000” moves down to my alligator clip and touch plate (a Normally Open switch/sensor) that is connected to the TinyG Zmin and ground terminals.

    settings for my machine:
    (motor 4/Z, direct drive 1.8 degree, nema23, 2.8amp, 269 oz.in.)
    $4ma=2 (map to axis, 0=X,1=Y,2=Z)
    $4tr=2.540 (mm travel per revolution, 2.540)
    (1/2 dia., 10 tpi ACME right-hand lead screw)
    $4mi=4 (microsteps, 1,2,4,8)
    $4po=1 (polarity, 0=normal,1=reverse)
    $4pm=3 (power management)

    (Z axis)
    $ztm=95 (travel maximum)
    $ztn=.000 (travel minimum)
    $zvm=500.000 (velocity maximum, mm/min)
    $zfr=500.000 (feedrate maximum, mm/min)
    $zsv=200 (search velocity, mm/min)
    $zlv=100 (latch velocity, mm/min)
    $zlb=20 (latch backoff, mm)
    $zzb=5 (zero backoff, mm)

    February 4, 2015 at 7:32 am #7335
    cmcgrath5035
    Moderator

    It’s always good to keep in mind that up/down, left/right, positive/negative, max/min etc are relative to polarity setting, which is influenced by how you connect your stepper motors to tinyG. Here is a reference

    Stepper wiring varieties result in a varieties of setups.
    Setup step #1 is always, for your type of motot, to get it connected to tinyG so that tinyG, on command, can spin the motor ClockWise (CW) and CounterClockwise (CCW). Then depending on your machine, when a motor is installed on an axis, CW may be up, or may be down. you can reverse the “up movement” by either changing the motor polarity setting or by rewirng the motor leads connected to tinyG.

    Summary – what works perfectly for mristau with $4po=1 might require $4po=0 on another machine , depending on the wiring.

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