Hi, Several years ago our company built a router with Mach 3 controls. The person who is most familiar with Mach 3 is no longer with the company. I have 2 issues that I would like to straighten out. 
1 is big. When I program a part to be 4" x 4" it comes out 4.010 x 4.010. Is there a way to permanently fix this? I know some code but not an expert. I've read that I could scale the output but that may create more problems.
2: We have air triggered hold down clamps that we use and move out of the cutters path with M code. One part we make we flip the part over. To keep the part square and oriented correctly we have 3 air cylinders that we manually pop up and locate the part and then let down for machining. I would like to add this to the program which would mean they would have to stay up when the program ends. I'm not sure how to add this feature to the program or to the Galil controller. I know there are some master builders and programs that read these posts, any help will be appreciated. Thanks.

I would suggest you request support by posting here:
 http://www.machsupport.com/forum/index.php/board,18.0.html
and see if someone is interested.

I don't support commercial facilities as I have given way to much of me for a companies success at my expense.
I think you'll understand the above.

RICH

Hi,
sounds to me like you should recalibrate your 'steps per unit' and the oversize problem should disappear.

Have a look on the Config/Motor Tuning page and have a look at the 'Steps per' in the lower left corner. Your description suggests that it is
0.25% to high. It maybe of course that you cant get it exact, the 'steps per' is an integer.

Craig

Thanks Craig,
I'll look at that.

Why are the dimensions off? Just adjusting your motor tuning based on a guess will likely make your problem worse. Test to determine the source of the error and go from there. If you do determine the steps per are off check your backlash and be sure to not include it in your steps per calculation.

http://www.machsupport.com/forum/index.php/topic,12513.msg81002.html#msg81002

http://www.machsupport.com/forum/index.php/topic,12512.msg81001.html#msg81001

Thanks guys. So I adjusted the "steps per unit" that didn't exactly fix the problem. When I set the steps to compensate for the extra .010, the parts were still comes out .002 - .005 big. There are 40000 steps so I adjusted that to 39900 which should be mathematically correct.  This was very disappointing. I have set up indictors and run the machine back to the exact machine positions and found about .002 TIR. which still doesn't explain why my parts are big. If that .002 is back lash I think I could live with it. If anyone has some other ideas I'm open to trying them.

How much do the dimensions change if you run a light slow finish pass?

I never settle on just the math to set steps per unit, only to get a starting point.  To many other physical machine factors that aren't accounted for that affect theoretical steps per unit calcs.
You really should calibrate your machine by commanding it to move a certain distance and then accurately measuring the actual distance moved.  Do this several times to get an average and then compute a new steps per unit based on the results.  Do this several times more until you are satisfied with the accuracy of each move.  Do each machine axis individually.  Record your steps per unit values for future backup use.