I cut a 3" square and inside that square I cut a 1" circle. The x dimension on the square is 3.003 The y dimension on the square is 3.02. The x dimension on the circle is .985. The y dimension on the circle is .970.  So if I push up the steps per the circle will get bigger but so will the square. I have tried backlash but it didn't seem to help in the circle. I am using g41 in the program. But until I get Y matching X it won't help. I was hoping to get to + - .005.  I have tightened everything on the machine I can think of ( this is a router made from wood and alum. from build your cnc) and played with the settings in smooth stepper.   + - .010 I could accept or is this the best I am going to get.

Thanks for the replies
Scott

here is the program

Spitter, steps per will be a hard number and will not change unless you change hardware.  It will be a set number and no matter how good or bad it runs steps per will have to be what they are determined by your hardware.  Backlash can vary depending on flex and other less than perfect hardware elements.  Accel and velocity settings can wreak havic on a flexiable machine.

1st, do the math and set your steps per.  This link may help. 

http://www.machsupport.com/forum/index.php/topic,12512.0.html

2nd, check that your steps per is correct.  If not, detune your motors untill you can get consistent solid results that your not looseing steps.

3rd, check you backlash. This link should help with that.

http://www.machsupport.com/forum/index.php/topic,12513.0.html
Brett

Those machines are not very rigid. But your going awfully slow, with a very shallow depth of cut, so unless the tool is really dull, flex may not be much of an issue (in this case).

Is this machine chain drive? That could be where the issue lies. Try cutting in a different area on the machine, and see if the results are the same.

yes I purposely made it go slow and shallow so there would be as little tool deflection as possible. The machine uses 5 start 10tpi lead screw with plastic nuts. I have cut all over the machine and it's pretty consistent. I will detune the motors but I thought 100 ipm and 40 accel was low. I will check backlash also. thanks for the suggestions.

Scott

I run at 150ipm and 10 accel.
Try reducing the accel, as that could be causing the flexing.