I need to build a new part that its face will be the negative of the assembly face so they will mate
pls advise what is the way to do it on sw2013
You can create a new part in the assembly (insert>new part). You can then use indent (cut) or cavity on this new part to transfer the features you need from the assembly to it.
So, why couldn't you make an offset surface work?
One trick I use is to offset a surface in the original file that I call a memory surface. I do that so that later changes to the original file (provided they are below the memory surface in the feature tree) don't change the surface. That sometimes helps.
You need to be editing your new part in the assembly, with the original part visible. With the assembly open, left or right click on the part you want to edit and pick the second icon, the one that looks like an assembly icon with a blue part. As you float your mouse over the icon it should say "Edit Part".
In order to be able to see the part that you want to pick the surface or surfaces from, it helps to go to Tools/Options/Display/Selection and go to the box for "Assembly transparency for in context edit". Click on the downard pointing triangle and pick "Opaque assembly" or "Maintain assembly transparency". Now you should be able to pick the Offset Surface feature and then pick the memory surface (in the Surface Bodies in the feature tree) or the faces (in the graphics window).
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