Need help, milling flange thats not get round..

[jotus]

Hi!

Have a problem on my job, milling a flange that are cut in half, so i milling a arc, and drill som holes. and then they put that pieces togheter and its going
to be a nice round hole there. but it aint,.. in one side the measure is 414 , (CORRECT MEASUREMENT) and in other axis it is 412.7 (WRONG)
but the measurement is good between all holes.

i have drawed a quick pic how i mean..

It milling with a 63mm milling tool with i think 12 plates.. the job is 35mm height, i put in some lines from prog.

we remove 4mm in radiell (ae)

G43 H2 Z-14 S.... M3
G0 X-175.5 Y50
G1 Y0 F...
G3 X175.5 R175.5
G1 Y10.
G0 Z10
X-175.5 Y50
Z-39
G1 Y0
G3 X-175.5 R175.5

We take out the Y0 at the "ears" and then move it down -3MM becuse we have 3mm from plasma cutting to remove too..
X0 is in the middle of the arc.

SORRY FOR BAD ENGLISH, I HOPE SOMEONE CAN GIVE ME SOME TIP!

Best regards.

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[warrenb]

A couple things could be going on. One is back lash in the machine. The other cause may be the part spread open with all that material removed and heat from the plasma. What i would do is drill the holes first. Then screw through the holes to a fixture plate. Then cut the outside. If it is still off then check for back lash and make sure you are actually measuring to the center of the arc correctly.