X Y axis don' work togheter

Can anyone help me to figure out what's going on.
I've made a stepper machine and running Linuxcnc 2.5 with Ubuntu 10.04
I manage to get the machine working using step configuration but I'problem holding sizes.
Moving X or Y axis on their own the position is correct not so moving both axis.
G2 and G3 result in oval holes.
Changing velocity and acceleration does not make any difference.
I'm pretty sure the machine is not loosing any steps because running the same G code several time it
ends up in the same place.
Running Linuxcnc on a Dell Dimension 4700 pc.

How big are the holes?
How much backlash do you have? I think that is usual cause of ovals.
are you using screws or something else? what kind of screws also.

Thanks for getting back so quickly.
Ball screw on all axis.The worst axis has a backlash of .o4mm.
The hole diameter is 50mm.The difference measuring across on a 45 is of 1mm.
I'attaching my ini file. may be I messed up there

Your X scale is different than Y and Z
SCALE = 251.968503937
is that right for X? X and Y are different lead screws? and Y and Z are the same?

Yes the x screw has a pitch of 6.35mm (1/4 inch.) while the y and z have 5mm pitch.
The X axis has a bigger screw and moves most of the weight.
What puzzles me is that I find difference even when i cut on a angle.
If I cut a square normal to the axis it checks out pretty good, cutting the same square rotated
45 I don't get a square any more.

Hmm. I'm not sure then. your ini looks good to me, but I am still learning.
Maybe your x and y arent perpindicular?

Hum! that is an idea, it was checked when the machine was assembled, may be something got loose.
I'm going to check tomorrow.
Thanks for the hint.

Are you using G61 or G64 P- Q- ?
Are the results the same?

Rick G

i would be looking at mechanical movement or flex in the gantry as the cutter is biting , etc

try placing a felt tip in the spindle and tracing the path it should be a image of the cut , if it is not then the chances are it's mechanical

I think Alan_3301 had the answer, to my surprise I found that X is not perpendicular to y; dial indicators don't tell me so.
I'm now fixing it and i will try again when done.
I'll let you know.
Thank very mach to Alan and everyone else that helped.



PS: Sometime you look far when what you'r looking for is under your nose.