X-Carve depth passes not lining up

I have had an 500x500 mm X-Carve for 8 months.
I have carved numerous parts with no problems.

Recently, I was carving multiple copies of an object in 1/4" acrylic with a 1/8" bit, standard settings.

On the ~5th copy, the depth passes no longer lined up.
I had not changed anything since my past successful carves.

Now when I machine an object (any object) each depth pass is slightly offset (a bit in X and Y)

When I carve a circle it produces a stack of slight offset cuts,
with a stepped slope on one edge and a vertical edge on the other side,
where the offset pass is cutting into the part.
On a 2" diameter circle, the first and last pass are 1/8" off producing an ellipse
at the top.

Similarly for square.

I suspect that the steppers are losing a step or two each pass,
so that when it starts the second pass it is not starting at the same location,
but I am not certain.

I have re-retightened the belts and checked all the V-wheels.
I have also adjusted the X& Y stepper pots, no improvement

Check your pulley set screws to be sure they are all still tight and the pulleys are not slipping on the motor shafts

Allen did the steppers that come with the x-carve have flats ground in them? If not, they should. And BLUE locktite should be applied to the grub screw. This is the most common, nefarious and hard to diagnose problem new x-carvers have it seems.

If your steppers do not have a flattened area on the shaft to receive your grub screw, they are easily added with a Dremel and a cut of wheel. Just a few passes back and forth to give you a nice flat area to keep that shaft from spinning inside the pulley.

The Nema 17 motors did not have a flat spot on them.

I purchased my Nema 23’s from another vendor since Inventables was sold out and those did have flat spots on the shaft.

I used a sharpie to draw a line across the end of the shaft and pulley so I can see if my pulleys ever slip.

Thank you for your suggestions
Unfortunately each depth pass is still offset from the last, leading to sloped walls.

I tried cutting at various locations on the wasteboard.
I tried cutting different plastics and mdf.
I tried tightening the stepper motor pulley set screws ( the X was loose).

No change.
The larger the depth pass, the larger the offset each pass,
when I do a shallower pass, the steps are smaller, the cut is 45 degrees to the Z axis.

I then cut a long vertical line in ¼” thick sintra:
3” tall x 0.126” wide with a 1/8” bit
(0.126” so bit is smaller than cut, needed for Easel)

The motion should be pure Y with no X.
However each depth pass is offset in X and in Y (see photo left),
By the end it is 1/4” off.

When the machine is cutting you can feel the Motor pulley making small steps even though there should be no X. If I turn down the pot on the X motor, the problem is still there.

Strange thing is:
If I completely unplug the X motor I get a perfectly straight Y line with vertical walls (see photo right)

Completely remove the x-axis wire from the shield. Then, use a new segment of wire and connect your x-axis to the shield. See if this problem remains.

Thank you for your help.

I believe I have identified the problem.

I think I am getting electrical interference from the Spindle motor,
which is causing the stepper motors to twitch and slowly drift each new pass up and to the right.

When in Y motion, you can feel the X motor twitch, even though it should be off.

To confirm this I removed the bit and clamped a sharpie marker to the spindle mount,
with the tip lower than the collet (see figure)

I disconnected the spindle and “carved” 4 thin passes of a 1/2 x2 rectangle
as shown in figure.

Spindle off: the passes all lined up.

Spindle on: each pass drifted up and to the right( ~1/8")

Spindle off again: the passes all lined up again.
(the line is thicker as the marker tip was squished, but the passes lined up)

I have an older 24DC motor, which some users reported issues with as well.

Holy EMF! Get rid of that spindle and get a router. In the meantime, you can put a dremel into that same mount to get you by - it works ten times better than the spindle anyway.

Are you using a Dewalt 611 router or the original 300 watt spindle?

If you are using the 300 watt spindle you may be getting electrical noise from it that is interfering with the X motor.