Sounds like a bug. What version of NX are you using? If you are using an old version it is possible it works fine in another version. Can you upload a prt file?
Have you changed something in "Cut Levels"?
Make sure if problem still exists if you reset Cut Levels by changing "Range Type" option to "Automatic"
...and I agree that it would be the easiest if you upload PRT file
Production: NX 12.0.1
Development: C#, Tcl/Tk, CSE
If you use one range only, you will not have a level on each floor face that is perpendicular to the tool axis.
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As Marek and Stefan have said, you need to set your cut levels to Automatic so NX sees the range s (floors) properly. To do this go to cut levels and set the range type to automatic. You can go to the list and delete the last range if you want because it will save some time when you generate the operation. You will notice the range type now says user definied if you deleted the last cut range and this is normal. If you use single for range type then NX is not looking at the floors.Also, in case you missed it, the tool is not cutting the floor at Z-6.8mm because it cannot fit. I would have uploaded a prt file for you but I am working with versions 11 and 12 so you would not be able to open it in version 10.