Awesome, that appears to have solved it (well -- mostly. I don't have enough clearance on the tool I'm currently using, but that's easily solved by changing inserts.)

I think the angle of the undercut is less than the insert -- I can change tools if needed. How do I allow undercut?

I have a MasterCAM file with a wireframe spline contour that needs to be turned on the inside of a part. However, MasterCAM all of the sudden "goes off track" at the end of the spline, and fails to cut it accurately. I think this is maybe a relief angle limitation? Any ideas on how to address this are appreciated. I've attached the file. Note that this operation is only machining part of a longer contour (as part of a repair), so we cannot extend the start or end of the contour. (Not that that helps, but just FYI.) Also note that this is not just the toolpath not rendering over the wireframe (due to where the tool origin is located). If that were the case, the backplot would render the tool in the correct location, which it does not: coords.mcam