In Cavity Mill or Z level operations, the toolpath is generated in the regions which are not part of Part Geometry defined in Workpiece. This was not the case in NX12 or older versions.
Solution
There has been an Enhancement from NX1847,
For the following fixed-axis operations, you can select a cut area that is not included in the part geometry:
This is helpful when you want to include temporary surfaces, such as capping faces or extensions, in only these operations. Adding the temporary surfaces to the part geometry would affect all operations with the same part geometry parent, changing the status of the other operations to Regenerate , and preventing them from machining features underneath the capping surfaces.
You must make sure that you select the correct cut area geometry, which specifies the set of faces that form the cut region. NX does not warn you when you select geometry from the wrong part. The operation will generate without errors, but give unwanted results.
For more information please refer to the documentation with link below:
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Notes