radial pocket is not really round !

[Calico]

hi,
I think my X or Y has a problem.
when I cut circle pocket I have no problem, may be is not repeat cut.
but twhen I cut using Radial Pocket, is not really round, it's like loosing step.
I use step motor btw.
using mach 3
been using this machine like 2 years, now I have a problem ..

when I use Dial Indicator, like moving the x0 and x 50 and x0 and x50 repeat..
I found the x is not really coming to zero.

how to check which is the problem ?
the step motor or other ?

thank you
ruddy

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