Let's say you want to make this window mullion using the moulding toolpath. The top of the mullion has a 3/16" cove on each side of the little rails, and those are where we want to use the moulding toolpath.
Here are the vectors that define the shapes for this mullion.
The parts where this issue is a problem are the middle cutouts. These areas are where you'd like to have the moulding toolpath cut on the inside of the vector to give nice sharp corners. Now, you might say "why not just pick the innermost vector and generate the moulding toolpath from that. Then it would be on the outside of the vector". The toolpath would look something like this:
Notice that the resulting toolpath is rounded on the top and bottom and is not giving me the sharp inside corners that I want. The better vector to use would be the next one out from the one in the last example - the one that defines the top of the profile, not the bottom. Unfortunately, this is where you need the moulding vector to go inside rather than outside the vector.
The answer is to break this vector into two separate vectors, neither of which is a closed vector. Use the Node Editing tool to cut the vector in two places. It's important that these two cuts be in the middle of a straight or gently curved section. If you make the cuts on a corner, you may get distortion in the final toolpaths. Now if you create a moulding toolpath for each of the resulting open vectors, and right-click to reverse the rail if necessary, you get a toolpath that looks like this (note the sharp corner):
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Repeat as required for the rest of the model.
