Trouble tuning axis for first time, ferror doesn't go to zero after move

Hello Folks;

System:
Converted knee mill with  old SEM brushed servos running on amc servo drives in openloop mode.
Mesa 6i25 + 7i77
Loop closed with old prototrak  axis scale encoders, not from the motor encoder.

No matter how I try to tune PID & FF, many times the axis moves stop off about .004 sometimes much better.
I have deadband set to 0.
My max feed is 60.0 in/min.
Have tried various accels  now around 30 in/minsq
Most PID seem to have P around 50. No I or D
FF1 from .5 to 2.0 seem to get a reasonable error halscope.
 Just being more than  about a thou off isn't good enough for real milling.
Any info to help or graphs needed just let me know. I have played around with this ever so often for a long time.
But this time, I seriously want to finish this mill
regards,
Jon P.

I think that you are likely to need some I to pull in the last bit of error. Have you tried adding I (very small numbers, .00001 maybe as a starting point)
Are the servos operating in velocity mode or torque mode? (ie, is there a velocty loop closed by motor tachometers)

The scaling of I  is such that you need pretty large numbers
to make any visible difference

Hello Jpevner
What AMC drives do you have? 
First, you may need to re-tune your AMC drives. Software may still be available at AMC. I would set the drives to torque mode if possible.
After this is done then adjust the PID etc in Linuxcnc.
I converted a Denford Novaturn. My set up is Pathpilot, IBOB2, MESA 7i77, 6i25, Yaskawa SGDA 01AS servos in torque mode.  I first tuned the servos for my machine's inertia. After this was done then the PID settings.
Some of the work is located here: 
DENFORD:  NovaTurn Servo retrofit AMD Gizmosphere iBOB2 PCIe x1

Hope this helps,
Iron-Man