If you e.g. import a vice as clamp, the complete model will be set to ignore (thickness set nr. 17). I did not find a way to get the surfaces out of this group till now. Therefore I import my vices as model and put them into a thickness set which I set to collision with separate radial and axial thicknesses.
You can copy the model (the clamp) and select "paste as new model" from the contextmenu of "Models". This will deliver a new model which can be used for machining or creating your block. But this may be obstructive in futher machining directions.
If there is no other way, you can draw a userdefined boundary from which you can sefine your block.
