[FanucIssues]

I could not think of how to word my issue...We are cutting cast iron, and trying to hold a 12 micron tolerance. We have done this for years, and have done it well, but the best guys we have have not been able to figure it out...If the size of the parts is too big, you can offset it down to where it needs to be, but we need to maintain a zero dimension over a little distance. we have a taper adjustment in the program, and can alter the finish point on X, which works, up until you get around 10 microns of differance between start point, and finish point. I have over adjusted the program, checked backlash, indacated from this to that, that to this, that, to that....I do not see much movement on my indicator....I am at my whits end.

We previously scrapped the gib, (side to side gib). But, I wonder what kind of wear pattern should there be...Even through out the gib, or two "patches" on both ends.
New ball screw, and bearings which worked for a while. Nothing seems to be rubbing on the part, this is a finish cut with a +/- 0.08mm inprocess rough cut leaving 0.8mm of material for the finish, so the stock is controlled, and not rough casting....
Any suggestions/questions would be wonderful. Thanks: John

[underthetire]

Think you need to start at the basics. Squareness check on all axis, headstock, etc. Check repeatability, both with and without the turret indexing, machine level, ballscrew run-out and stretch amount. There are a lot of variables to affect final part.

[littlerob]

You said you were doing this well for years, 5 tenths is wiggle room, I would guess something happened without you knowing about it. (CRASH).

So I agree with underthetire you should start with basics.

Robert

[Green Button]

If you machine lot of cast iron, check also:
-if gibs are worn
-if guideways are worn
-if bearings are worn (both ball screw ends & spindle)

Put a measuring clock to you tool and give a toolpost a good shake by hands.
If any movements in clock, y can start to look cause of a trouble.
Do a same thing to work piece. Use a piece of wood to lift carefully work piece.

Also i think that your headstock in not straight. Like others posted:
squareness check to all axis

[DouglasR]

This sounds rather obvious, but I think that perhaps tooling/workholding could be a problem.

I have a mint LB15 that is accurate as anything, and I experienced a tolerance problem on a shaft job. Turned out that the profile turning tool had a bad set screw and the insert wasn't anchored properly.