ARGH I Have a PROBLEM 
I've been chasing tiny errors in the engraving line spacing for what seems ages - convinced I got the wrapping program set wrong, or something in my original drawing like too many or too few lines, so in desperation I decided to to a manual test:
a/ I skimmed off one of the 'doughnuts' to 48.5 mm o/d and mounted it up on the 4th axis
b/ I sent the 4th axis to -10 degrees, then to 0 degrees to eliminate any back lash
c/ I then manually drove the engraver into contact and scribed a line then raised it again.
d/ I then drove the 4th axis to 360 degrees then manually drove the engraver into contact and scribed a line then raised it again.
e/ I then drove the 4th axis to 720 degrees then manually drove the engraver into contact and scribed a line then raised it again.
f/ I then drove the 4th axis to 1080 degrees then manually drove the engraver into contact and scribed a line then raised it again.
Now all those lines that I scribed SHOULD fall on the same place and over lay each other BUT THEY DON'T

As you can see there is an increasing cumulative error. To try and quantify it the gap between lines is of the order of 0.833 mm on a circumference of 152.367 (Pi x 48.5) so of the order of 0.833 x 100 / 152.367 or 1/2 % which is far too large to tolerate !!!
Back lash has been eliminated by travelling in the same direction all the time so what the heck could cause that
