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Sign convention for measurements between mate connectors
aeromorrison
Member Posts: 14 PRO
Greetings all,
I'm trying to understand the sign convention on X, Y, Z distance measurements in Onshape. I have two mate connectors in an assembly and I'm measuring the X, Y, Z distance between the two mate connectors. Nether mate connector's axes are aligned with the world coordinates and also not aligned with one another. Depending on the sequence of selection I get different signs (+ or -) on the measure dialog. I can't seem to find any rules in the documentation related to the assumptions of directionality and signs when selecting two different mate connectors for measurement. I'm trying to use process of elimination but it's quite confusing given several of the dimensions are quite small and it's difficult to see the offsets in the geometry window when hovering.
Any help would be appreciated.
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The first key thing to realize is that the x,y,z are in the orientation of the first mate connector you select (you can re-order the items in the mate by dragging the list).
The second important thing to realize is that the offsets are applied TO the first one you select. This can produce unexpected (but logical if you think through it) results if your first selection is an item that doesn't move (for example if you apply a +x offset it will try to move the first part in that direction relative to the second but ends up effectively moving the second part in -x if the first one is fixed).
The rules I have set for myself is to always pick the MC of the part that is "moving" relative to the one that is "fixed". The "fixed" part might be movable in the assembly so the other way to think about it is select the part your "mating" first and the part you are "mating to" second.
If you do this, the offsets should predictably follow the "first pick" mate connector.