[garylb]

over the last couple of days i have noticed a problem with the accuracy of my machine when boring. I set the boring bar in the offsets and machined a 1 1/8" bore but when i machined a larger bore of 3" it was about .008" undersize. Cant figure out whats causing this. Any ideas?

[mfgbydesign]

1. Height of tip relative to centerline
2. straightness of X axis
3. linear accuracy of X
4. tool clearance angle (rubbing on smaller bore?)
5. shake (movement in X when moving Z) - is the bore at the same Z?
6. Lost motion (approach bore from same distance, direction, and feedrate)
7. tool wear?
8. ask yourself what else is different (material, hardness, SFPM, FPR, environmental conditions, etc.)

[garylb]

thanks for the reply! The boring bar was slightly off center line and also was at the wrong angle slightly so this could possibly have been the cause. I will test it out as soon as i get a chance

[mfgbydesign]

A couple more thoughts:
remember any error you have is doubled on diameter. So you are actually looking for a .004" error.
Depth of cut should be consistent. From experience 10% of existing error is left after a finish operation. On a tight tolerance bore I try to take the same amount in a semi-finish pass as on a finish pass. Even then, I might take a shallow cut, check, then finish. Or leave extra stock on the face for the test cut if that first 1/16" or so needed for gaging MUST be within tolerance.

Don't overlook the approach move. As a machine tool applications engineer I learned about the profound effects of shake, which is a movement in X when you move Z (or any other combination of axis). You can see this with an indicator on a test bar. Set it to zero in X then when you reverse Z you will witness movement. Might indicate you need a gib adjustment.

[garylb]

thanks for all the advice. I have just tested the accuracy on an od. I took a cut off a 4" dia and then the same size cut from a 5/8 dia bar and it measures up ok so i think this proves the machine is ok and it was just the setting up of the boring bar which caused me the issues.