Selecting edges in assemblies?

Hi,

I have used SolidWorks for a number of years, but I am having an odd and annoying problem that I cant find the solution for easily.

Doubtless, some of the users here might have the solution.

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Here I have a large grey block in an Assembly. Its just going to be a machined nylon piece for a Jig.

The 'rocket' shaped parts are the components that I wish to locate into the jig, and I want to make a machined profile that will hold these in a cavity. With say, a 0.1mm offset edge so that they fit in the jig nicely, without 'jiggling' around.

Problem is, and I have had this before, that when I try to use AutoDimension to select the black SketchLine that I have drawn, the damn system wont select the outline edge of the component part that is in the Assembly.

The 'rocket' shaped component part is a SubAssembly comprising of two acrylic parts. They are transparent, but usually selecting ALT allows transparent faces to be selected. When I try to select the edge of the rocket shape, and dimension to the sketch line, the system wont select the edge of the component.

I keep coming up the this problem, when working with assemblies, and I have never been able to work out why this happens. Can anyone help??

Cheers, Bid

SolidworksAssemblies