Greetings,
I am having a problem with the accuracy when milling pockets. I have re-adjusted my leadscrews and also checked the X andY travel with a dial indicator to compare with the displayed measurements in Mach3 and all compares well.
I have used the backlash mode on and off in Mach but the round pockets are still consistently smaller than the programmed size. i.e about .5 mm on 35 mm Dia. (They are perfectly round though) I have checked the tool settings and the settings are correct..
I generate the G-code using LazyCam.
Any help would be much appreciated.

Post some code and maybe someone can check to see if its good, also state what size cutter you are using.
Hood

Thanks for the reply,
I am using a 3/16 endmill (4.76mm in the tool setting).
This is the code for a 35mm dia. pocket. comes out at 34.64mm dia.

(File Name = bearing hole Draft on Monday, March 15, 2010)
N10 (Default Mill Post)
N15  G91.1
N20 M5 M9
N25 M6 T1(TOOL Change oney)
N30 G43 H1
N35 G0  Z0.0000
N40 M3
N45  X15.1200  Y15.8600
N50  Z1.1000
N55 G1  Z-3.0000  F5.00
N60 G2  X15.8600  Y15.1200  I0.0000  J-0.7400
N65  X14.3800  I-0.7400  J0.0000
N70  X15.1200  Y15.8600  I0.7400  J0.0000
N75 G1  Y18.2400   
N80 G2  X18.2400  Y15.1200  I0.0000  J-3.1200
N85  X12.0000  I-3.1200  J0.0000
N90  X15.1200  Y18.2400  I3.1200  J0.0000
N95 G1  Y20.6200   
N100 G2  X20.6200  Y15.1200  I0.0000  J-5.5000
N105  X9.6200  I-5.5000  J0.0000
N110  X15.1200  Y20.6200  I5.5000  J0.0000
N115 G1  Y23.0000   
N120 G2  X23.0000  Y15.1200  I0.0000  J-7.8800
N125  X7.2400  I-7.8800  J0.0000
N130  X15.1200  Y23.0000  I7.8800  J0.0000
N135 G1  Y25.3800   
N140 G2  X25.3800  Y15.1200  I0.0000  J-10.2600
N145  X4.8600  I-10.2600  J0.0000
N150  X15.1200  Y25.3800  I10.2600  J0.0000
N155 G1  Y27.7600   
N160 G2  X27.7600  Y15.1200  I0.0000  J-12.6400
N165  X2.4800  I-12.6400  J0.0000
N170  X15.1200  Y27.7600  I12.6400  J0.0000
N175 G1  Y30.1400   
N180 G2  X30.1400  Y15.1200  I0.0000  J-15.0200
N185  X0.1000  I-15.0200  J0.0000
N190  X15.1200  Y30.1400  I15.0200  J0.0000
N195 G0  Z0.0000
N200  X30.2400  Y15.1200
N205  Z0.1000
N210 G2  Z0.0000  I-15.1200  J0.0000
N215 G0  Z0.0000
N220 M5
N225 M30

That should come out at 35mm according to the code so its not to blame. Is there any slackness in the Z axis that could cause it?

Hood

I did tighten the leadscrews both X and Y. Thanks very much for confirming rhe G-Code for me. That removes one variable.  I will investigate the mechanical side of things more.  I am also checking the cutters for size and any deflection.
Rob.

Rob,
If the code is correct then either the cutter is undersized or steps per unit are incorrect. At least it's cutting round. Maybe you are allowing for a finish in LazyCam?
RICH