I have search and can not find what I am looking for or I am not looking in the right place. Oh yea I completed the mechanical build of my 36X53 machine.
Mach2
Gecko 320
Us-Digital E3 250 CPR
Bob Campbell breakout board
Reliant E-712 Servos
PS 60VDC 30A
Screw
X .75 OD .5 pitch
Y .5 OD .5 pitch
Z .5 OD .25 pitch

Problems:
When I power up the motors jump and free wheel at a constant pulse in one direction. If I add a load like my hand or the screw the motors still jump and free wheel but not as far and some times jump the opposite direction. If I put a lot of load on the motors they will jump back and forth. I have tried tuning the Gain and Damping per Gecko's instructions - this is how I was able to get them to bounce back and forth.
I think I may be setting up the motors wrong in Mach2 The pulleys are 1.75:1 with a .5 pitch screw and a 250 CPR encoder. I have X & Y set up at 7000 steps per unit. I am using inches.
Thanks for any and all help!

It sounds like you have either something miswired, or you have noise on the encoder lines. Are they running next to the power lines?

Also, I think your steps/unit should be 3500. 2 tpi x 1.75 x (250x4) = 3500

The encoder lines are shielded and the only time they run next to the power lines is ine the cable track.

3500 steps per unit thanks

They shouldn't be in the same cable track, if possible. Not sure if that's your problem, though. Are the shields grounded at one end only?