X and Y creep

I’m cutting aluminium at 1000mm/min and 0.1mm passes.
After a few passes the X and Y appear to offset by about 0.1mm and then continue to offset at each pass thereafter.
I only seems to experience this when cutting aluminium but that maybe because of the number of shallow passes. There’s not a lot of strain on the cutter - probably a lot less than when I cut MDF.
It seems to happen when the cutter retracts and moves back over to the work to do the next cut. It doesn’t seem to go back to exactly the same place each time.
Any ideas would be appreciated.

X-Carve 1000 using grblcontrol Candle.
I’ve just reset all the pots as described in RobertA_Rieke (thanks Robert - good videos), and retested. At each pass the X moves about 0.1.mm over from the pass before. The Y axis does too but to a lesser extent I think. I’ve also tested the belt tensions and the V wheel tensions and all are OK.

$$ < $0=10 (step pulse, usec)
$1=255 (step idle delay, msec)
$2=0 (step port invert mask:00000000)
$3=1 (dir port invert mask:00000001)
$4=0 (step enable invert, bool)
$5=0 (limit pins invert, bool)
$6=0 (probe pin invert, bool)
$10=3 (status report mask:00000011)
$11=0.020 (junction deviation, mm)
$12=0.002 (arc tolerance, mm)
$13=0 (report inches, bool)
$20=0 (soft limits, bool)
$21=0 (hard limits, bool)
$22=1 (homing cycle, bool)
$23=3 (homing dir invert mask:00000011)
$24=25.000 (homing feed, mm/min)
$25=750.000 (homing seek, mm/min)
$26=250 (homing debounce, msec)
$27=1.000 (homing pull-off, mm)
$30=12000. (rpm max)
$31=0. (rpm min)
$100=40.000 (x, step/mm)
$101=40.000 (y, step/mm)
$102=188.976 (z, step/mm)
$110=8000.000 (x max rate, mm/min)
$111=8000.000 (y max rate, mm/min)
$112=500.000 (z max rate, mm/min)
$120=500.000 (x accel, mm/sec^2)
$121=500.000 (y accel, mm/sec^2)
$122=50.000 (z accel, mm/sec^2)
$130=790.000 (x max travel, mm)
$131=790.000 (y max travel, mm)
$132=100.000 (z max travel, mm)

3mm 2 flute flat end mill for aluminium cutting 3 sides of a square so at each pass the cutter has to retract and move back to the start point.
Fusion360 2D contour with these settings

My best guess would be missing steps? However, 0.1mm is so small though… and being a consistent value per layer seems odd. Like more of a mechanical issue?
Maybe air cut some test prints and see if the shift happened with no load (that could rule out slipping steppers)

Have you double checked your pulley set screws? If they are slipping just a bit that could account for the consistent deviation.

I am not familiar with grblcontrol Candle.
Try generating some code for a simple shape, say a circle in 5 passes? Then look at the g code and confirm that it is going to the same points on each pass? (Make sure there is no kind of cumulative rounding error or something?)

How can I tell?

Thanks to RobertA_Rieke’s video on belt tensioning I think the problem is now solved.
Time will tell but I’m hopeful

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