Hi All,

I've been milling a bunch of circular pockets and am finding them all to be
roughly .010" undersized in diameter (that's a lot!).

It doesn't matter whether it's a 2" diameter circle or a 0.5" diameter.

I'm using Mach3 and have compensated for backlash. In fact, the milled
circles are the same diameter vertically, horizontally and diagonally, which
I don't think would be possible if there were a backlash issue.

I've also used Mach3's axis calibration to verify that all axis are dead on
(ie. a move of 3.000" is exactly 3.000").

I've measured my .250" endmill and found it to be .247" in diameter. Rather
than setup the tool compensation table, I simply told my CAM program I was
using a .247" endmill.

The resulting g-code for the finish pass of a 2" diameter circle centered at x1, y1 is as follows:

G20 G40 G49 G54 G80 G90 G91.1

(LOAD .247 ENDMILL FINISH, PRESS Cycle Start)
T2 M6
M0

S2000 M3
G90 G54 X1.8765 Y1.
M8
G1 Z-.1 F1.
G17 G2 X1.8765 Y1. I-.8765 J0. F3.
G0 Z.1
M9
M5

Both my CAM program and Mach3 are using incremental mode for I and J.

I'm totally stumped, so any suggestions would be appreciated.

Thanks,
Ken

Have you trammed the spindle in?  Is something flexing? 

Brett

Hi Brett,

The mill is trammed but I now believe that this is a mechanical error. I have a number of tests to run and will report back when I figure out what's happening.

Thanks,
Ken