Z moving twice distance - Was working - now it's double



  • What in the world have I done to cause this?

    G1 Z20 puts me at 40mm from the build plate.

    Prints were great, now everything is printing at 0.4mm layer height when it should be at 0.2. My 2cm cube is 2x2x4.

    It homes just fine, and correctly reports Z0.

    Using a standard 4 start 8mm trap screw and 1.8 steppers. 800 steps/mm is what I have been using for 3 weeks, I shouldn't have to set that to 400 with the exact same hardware....

    I have no idea what I could have done to change this. I even swapped my board out with another duet card with the same result.

    Thanks!

    ; Drives
    M569 P0 S0                                         ; physical drive 0 goes backwards X
    M569 P1 S0                                         ; physical drive 1 goes backwards Y
    M569 P2 S1                                         ; physical drive 2 goes forwards Z
    M569 P3 S1                                         ; physical drive 3 goes forwards E
    M584 X0 Y1 Z2 E3                                   ; set drive mapping
    M350 X16 Y16 Z16 E16 I1                            ; configure microstepping with interpolation
    M92 X100.00 Y100.00 Z800.00 E401.51                ; set steps per mm
    
    M566 X600.00 Y600.00 Z12.00 E400.00                ; set maximum instantaneous speed changes (mm/min)	X900.00 Y700.00 Z12.00 E1100.00
    M203 X7200.00 Y7200.00 Z250.00 E5000.00            ; set maximum speeds (mm/min)
    M201 X1000.00 Y1000.00 Z20.00 E2000.00                ; set accelerations (mm/s^2)	X1100.00 Y1000.00 Z20.00 E1000.00
    M906 X1428 Y1428 Z1428 E850 I40                    ; set motor currents (mA) and motor idle factor in per cent
    M84 S180                                           ; Set idle timeout
    


  • @RedMachine About the only thing I can think of which would explain that behaviour is if the steps per mm or microstepping that is set in config.g, is being overridden elsewhere. Has an errant M350 or M92 crept into a config-override.g file or one of your homing files?



  • I don't have an override at all.
    I just confirmed that it happens even if i raise Z before I home.

    homeall.g

    G91				; relative mode
    G1 S2 Z10 F200			; hop Z up 10
    G1 S1 X-299 Y-299 F3000		; move up to 240mm in the -X and -Y directions until the homing switches are triggered
    G1 S2 X4 Y4 F600		; move slowly 6mm in +X and +Y directions
    G1 S1 X-10 Y-10			; move up to 10mm in the -X and -Y directions until the homing switches are triggered
    G90				; back to absolute mode
    
    G29 S2				; disable mesh compensation
    G1 X143.9 Y147.3 F2000		; put probe at center of bed
    G30				; home z
    G1 X-10 Y240 Z20		; move tool out of way
    G4 P1000			; dwell a sec
    G29 S1				; enable compensation
    


  • @RedMachine Just a quick test: does Z scale properly if you set the steps/mm to 400 in your config.g?



  • @infiniteloop
    Yes, I just changed Z to 400 steps/mm it and it does appear to be correct with just a quick caliper check.

    Am I wrong in thinking that a 8mm trap and 1.8 steppers at x16 should be 800 steps / mm?



  • @RedMachine said in Z moving twice distance - Was working - now it's double:

    Am I wrong in thinking that a 8mm trap and 1.8 steppers at x16 should be 800 steps / mm?

    Counter-question: how could your printer ever work with 800 steps - you said he did that happily for three weeks? The only reason I can imagine is that you had a second M92 (with Z400.00) in your config before you took some spring-cleaning action 😎



  • @RedMachine said in Z moving twice distance - Was working - now it's double:

    @infiniteloop
    Yes, I just changed Z to 400 steps/mm it and it does appear to be correct with just a quick caliper check.

    Am I wrong in thinking that a 8mm trap and 1.8 steppers at x16 should be 800 steps / mm?

    It depends on the lead of the screw which is how far a nut will travel in one revolution. This is a function of the pitch and the number of thread (starts). So if the pitch is 1mm and it's a 4 start thread, then the lead is 4mm. A 1.8 degree stepper has 200 full steps per revolution. So 200 full steps will move a nut 4mm and from that we can say that 50 full steps gives 1mm of linear movement. 50 full steps X 16 microsteps = 800 which is what you have. But if the pitch is 2mm then the lead is 8 mm giving 25 full steps per mm = 400 microsteps at 16X.



  • For posterity: Z_steps_per_mm = (motor_steps_per_rev * driver_microstep) / screw_lead

    Does not take any gear ratio into account.



  • @Phaedrux I'm so glad you used the term "lead" rather than"pitch" (it's a pet hate of mine)🙂



  • @deckingman It's the least I could do.



  • Which does indeed = 400. So I either, suffering from some brain problems, changed it to 800 and assumed it was like that all along or did have something overriding it that I inadvertently changed. Not sure which is the better scenario.

    Either way, thanks for your patience.

    tr8x8



  • @RedMachine No worries - you got it fixed which is the main thing.


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