How to solve if the CNC lathe Turning Sizes are getting smaller?

Problem: The CNC lathe is machining aluminum parts, spindle speed is 3000rpm, max. cutting depth is 1mm, feeding rate is 200mm/min., the finish parts sizes often smaller than last part.

Measure the finished parts and found that it was occasionally problem, and each error is 0.02mm smaller. (from http://www.CNCmakers.com)

Diagnose: normally there are several causes:

1-      The CNC program is correct or not? For example: is there a G50 command at the beginning of the program?

2-      The U V W feedback cables or the output cables of AC servo motors’ encoders in poor contact or not?

3-      The double interval of the CNC controller is set correct or not?

In this case we found the encoder cable of X axis was scratched, change it and the CNC lathe turning sizes back to normal.