I'm trying to determine how to calibrate my slaved A axis in Mach 3.

I have a CNC Router, 48 x 48" bed, Mach 3, Gecko G540.

The Y axis has two identical motors running identical screws (as close as I can determine), one on each side of the bed. 

When I ran a 10 minute long carving program recently, the A axis ended up not in the same place as it started as measured from fixed points (ended closer to zero). Consequently, the carving is drooping slightly as it is being made. 

The A axis is slaved to the Y axis in Mach 3. 

I have tried a variety of motor tuning methods (manual and the auto version on the Settings page) with no luck.  The steps per setting is Y=4004 and A=3996.  The calculated value should be 4000.

There is no “clunking” like the sound of losing steps during the run from any of the motors. 

The motor is running 155F typically (IR thermometer) and is quiet, at rest.  The couplings are tight.  With the system off, the motor turns easily and both sides of the gantry roll freely.

I have attached digital calipers to both ends of the ganty and using the "set steps per unit" button determined that a one inch move is slightly different at either end.  My goal is to get them to move the same distance, perhaps not exactly one inch.  I have tried the A axis button but it doesn't really apply in this situation, I think.

Am I missing something here or is this about as good as it will get?     Thanks!

Bill

Only experience I have of slaving an axis is the dual spindle on my coil winder but I was under the impression that when you slaved an axis it got the same steps per unit as the master axis, therefore it had to be exactly the same setup, could be wrong about that though?
Hood

When I ran a 10 minute long carving program recently, the A axis ended up not in the same place as it started as measured from fixed points (ended closer to zero).

You're losing steps. That's the only way to end up in a different position.

Set the A steps/unit the same as the Y. What you probably need to do is lower the acceleration.

You could try increasing the pulse width, though.

What are the specs on your computer? You may need to look at what is going on in your computer and see if that is affecting A. There have been comments that 3 Axis will work fine, but that a fourth axis requires just a little more computer, or you need to look seriously at the optimization of the operating system (are you running XP or 2000) and other details like that.

I run a slaved axis on a 1Ghz PIII with 256MB of RAM, and it works fine. I don't think it's the CPU usage, but it could require optimization as you say.