This is probably the most accurate method posted:


http://www.machsupport.com/forum/index.php/topic,15902.msg107130.html#msg107130

Thanks Jeff,
Russ

Just a pure shot in the dark here…

I am not sure about the MarchantDice machines but some Chinese machines are not fitted with proper, preloaded, thrust races on the ball-screw at the stepper end. They rely on the stepper to take the load which, as we know, usually has a spring washer mounted bearing and thus it’s shaft can move axially under load.
This problem does not show itself as ‘backlash’ because the movement only occurs under reasonably high load – could this be the issue ??

Tweakie.

Hi Guys,

So we have done the backlash test as described the results are as below:

Y axis

we asked it to move 0.004mm (4 steps at 0.001mm)

on the fourth step the dial gauge moved about 0.001 does this mean there is a backlash of 0.003? is this enough to notice the sort of inaccuracy we re seeing in the part (about 0.5mm over 350mm).

X axis

we asked it to move 0.003 (3 steps at 0.001)

on the third step the dial gauge moved 0.001

Thanks

Luke.

Hi Luke,
  I'd say the backlash is OK. Especially if you are in MM and not inches. Resolution plays in but you should be good there.

Next thing would be to verify squareness. Indicate one edge of the most precision square you have true to x or y and clamp it to the table, then check along the other edge by manually jogging.
  Or, if you have a good straight edge, you can place 2 sheets of material on the mill (as large as practical) atop of one another, mill 2 adjacent sides untill they clean up. Un-clamp and flip the top piece (book-match) and check the other 2 milled edges with the straight edge. Done carefully, a good straight edge will display squareness to within the amount of error you are seeing.
A good (large) machinist would be best though.