Attempting to turn 5/16-18 Threads on 4140 Material. L20 Citizen Mitsubishi Controls
Turn down .312(.030 chamfer 45 degree), and Grooving clearance (Groove to .242) past the threads. I am using an Iscar 16ER18 turning insert, with a right hand holder that is at a 10 degree

Using the code below, I consistently get threads that fit in the go gage for pitch, not in the no go for pitch, but the back side appears to have what looks like nicks, and the first thread appears damaged.
Unsure how to control retract as wondering if machine is not retracting far enough to clear the part.

G96S700

G0X.355 Z-.225

G76 P020030 Q0005 R0005

G76 X.2405 Z.16 P0350 Q0010 R0 F.055


Any advice would be greatly appreciated.