How to choose what is the main part when splitting or doing boolean functions?

gonzalo_chomon

Maybe it has been answered before but I was not able to find it on the searches.
This is the scenario: When trying to split a part from another how can be chosen what is the main part and what's the secondary one.
In example: Imagine a gearbox case (lets call it body) that you want to make a hatch, and you do it by splitting a part of the case with a surface. 
You get two parts that Onshape call both body by default but one of them is the case and the other one is the hatch.
The problem is that many times on assemblies the main part, the body is now the hatch, so all assembly is destroyed.

I know that the the instances can be replaced on the assemblies but it is still a pin in the ass... 

lana

There is no control over which pice  remains in assembly when using split. A possible better controlled approach is to use Extrude intersection. That would preserve identity of the target part. If you need the second part as well, you can make a copy before the extrude and make two extrudes: Intersection and subtraction.