CAM: What affects tool path precision?

[Kasey7]

What would cause NX to give me a .0002 taper on a horizontal path?
I’ve checked my model. The sketch curves creating the diameter are constrained horizontally, and the diameter at each end is Ø.6275. However, in my posted code the tool path starts at Ø0.6276 and ends at Ø0.6274.

Here’s the operation in question:

N60(FINISH_.6725)
G00T0404
(DNMG331-PP IC908)
(DDJNR163D)
G50S1000
G96M03S400
M08
Z0.1
X1.
Z-1.806
X0.6726
G01X0.6724Z-2.7346F.005
G00X1.
Z0.1
X12.
Z2.
M09
M00

The model is properly aligned with the MCS.

 

[Kasey7]

Well, I figured it out with the help of a friend. I'll answer my own question in case anyone else has a similar problem.

The taper was happening because Allow Undercut was toggled ON. (Path Settings > Cutting Parameters > Strategy tab > Cutting).
Why the taper on a horizontal path? I have no idea. In any event, toggling Allow Undercut OFF got rid of the taper for me.

One problem solved, but now my code was giving the path .0001 undersized. The coded path was X0.6724, but the diameter of the part was actually Ø0.6725. That was corrected by changing the Linear Motion Resolution of the post in Post Builder to .00005 (from .0001).

 

[Tingsryd]

I was having a similar problem with a fixed contour operation that was not quite what I expected. My intol/outtol was set to .0002 but the tool movements were not as accurate as I needed. GTAC told me the default modeling tolerance of .001 was the cause of the issue. I was surprised that a tolerance had to be set in modeling to affect manufacturing accuracy but it did the job. My tool path positioning immediately showed improvement.

nx7.5
CNC Programmer

 

[shags72]

It also could be caused by the post tolerancing.