Dave,
It sounds like you are trying to cut a solid with an imported surface.
the deal is this when cutting with a surface:
1> the surface must be as big if not bigger than the block you are cutting from.
2 The surface must be free of imperfections that can usually accompany imported surface data.
3>The surface can not have any holes in it
4> the surface must not have any invalid geometry (faces) in it.
5> the surface must be one conituous surface (not broken into many faces in the Featuremanager)
in case number 1 above make sure that your block is not larger than the surface.
In case number 2 use your TOOLS/CHECK tool to analyze the surface to see if there are any imperfections. If there are any use the Diagnose tool to fix them. Right mouse on the surface in the feature manager, if you have more than one showing in the feature manager, any of them will do. Select "Diagnose" A property manager will come up and have two areas... one for gaps, and one for faces. Select diagnose on the gaps section first and if any come up in the results box select close all. In the faces then click "Diagnose" and wait for any invalid faces to appear.. if any do show up then click on repair and see if that fixes the problem
This should do the trick
Sometimes though the automatic repairs dont always work and you may have to use solidworks surfacing functionality to repair the surface yourself.
Hope that helps
Regards,
Jon
jgbena@yahoo.com