How to use features -> combine

[cecile]

Hi !

I have a part and I would like to create the "negative" of it (it's to make a mold in the end).
So I open the part, I add an extruded boss/base and then I want to do a "combine substract" so that I can substract my part from the new extruded boss/base. When I look at "insert -> features" the "combine" button is grey (= I can't use it).

What am I doing wrong ?
Thank you !

 

[cecile]

I found the solution : when I added my second part, I forgot to uncheck the "merge" box and therefore I had only one part and not 2.