Is the current implementation of MateConnector the primitive we need?

Some points:

• A plane (ePlane since we are talking about features) and a Mate Connector carry the same information (whatever their individual implementations). A location and orientation in space.
• cPlanes don't give users control of the origin (arguably not important) nor direct specification of location and orientation.
  
• Mate connectors must be attached to parts.You can't use them like cPlanes, can't build sketches off them, build cPlanes off them, etc
  

Currently datum axis and coordinate systems are missing from the feature tree. These are documentation & organization features, consolidating geometry info into a nicely think-about-able unit; essential in complex models with lots of new team members touching things. I still feel like the UX guys do not eat enough of their own dog food to understand this. However, my point here is that when datum coordinate systems are added then these will overlap in information with cPlanes and Mate Connectors. It's reasonable to predict that mate connectors would get co-opted to fill the role of datum coordinate systems to keep the UI clean.

Thinking about it, what would work better than the current situation:

1. A coordinate-system (I like 'frame' better) feature replaces mate connectors in the part studio. Similar behavior as now except that it does not need to be attached to a part.
2. cPlanes can be attached to coordinate systems by the user.
3. Parts each have a list of coordinate systems that they export to assemblies. Each coordinate system exported by a part appears in the assembly tab as a mate connector attached to that part. One coordinate system exported by multiple parts shows up in the assembly multiple times.
   

ilya_baran said:

My thought on where this needs to evolve is that when a custom feature asks for a mate connector, the Part Studio UI should allow creating the mate connector right while editing that feature.  That way, the feature is effectively asking for a coordinate system and the feature user can specify this coordinate system on the fly relative to any existing geometry.

Creo's ability to pause mid-feature-creation to insert that pesky plane, curve, or point, that you forgot you needed, is really powerful. A similar capability in Onshape would eliminate the need for something special for mate connectors.

It would be enough to have a pause button on the feature creation dialog that just accepts the current feature (regardless whether it builds correctly) and rolls back the tree to just before the feature.

I do mechanism design. At the moment I'm working on uploading a kinematic tree into Onshape for visualization and design refinement. Each rigid body (a collection of components and parts) in the mechanism has it's own frame (coordinate system) used for design and reference. This frame is most relevant in analysis of the mechanism but since we are doing all the analysis relative to the frames it's natural to think in terms of them when doing the physical design.

I could build a set of cPlanes at the frame origin of each rigid body, or I could put a coordinate system there. cPlanes are nice because one can build sketches off them, but they are not nice because they have a poor visual presentation and add too much noise to the display. Mate connectors are nice because they have an origin that is visible to the user. No ambiguity about where the point of interest is. But they have to be attached to parts.

If there was a coordinate system feature from which the user could build construction planes only when and where he needed, then  that would work.

This is my current use case. Does that help?