Steps on workpiece.. something not square?

[postalgbv]

Cutting some .125" aluminum, testing a bit and took several passes to pocket out a shape. Noticed these stair steps after I cleaned out all the chips. If you were looking down at the piece from above, it seems like the bit moves left and down each time it moves down in the Z (moves neg. Y and neg. X each step). You can't see it in the pictures, but the other sides of the pocket are clean, with no steps.. I assume because the final pass is cutting full height because of the offsetting that is going on.

I though maybe my piece was being held at an angle, but that wouldn't matter, it would still cut straight. The piece was screwed onto a table and definitely wasn't moving.

What is out of whack here, and how can I measure/adjust to fix it?

[todd71]

It could be backlash. Why so many passes for such a shallow depth?
With a good sharp carbide end mill and some coolent you should be able to do that in a couple of passes.
Another thing to consider is your tool path strategy. Are you cutting the cross pattern before or after the through hole? Climb milling or conventional?

[postalgbv]

I tested the gantry (Y direction) for backlash the other day and didn't see anything.. used a dial indicator, zero'd it on the face and moved the gantry away and back again over and over 1" and it was always .001 or less.

I could have definitely cut it with less cuts.. I did the chamfer in 2 passes and could have done one I think. I don't have good cooling yet, so I was being conservative, and then I noticed the steps on a previous cut so I did this one with many Z levels to try and diagnose it.

Oh and this piece was a scrap I was running tests on, hence the test cut goes through a previous cut.