Same part - two sizes?

[steinmini]

Made an "A-arm" for a double wishbone front suspension part). Created it from separate parts and saved as an assembly. Since there is an upper and lower "A-arm" and the difference is the position of the mounting supports for the suspension shock, I also saved the same assembly as a part. I used the **same** part to create a new assembly, the upper and lower arm, adding the supports for the suspension shock. Again, saved as two different parts, differing only in the mounts positions. Once I placed the rod ends to the ends of the arms, (same sub-assembly), I've made a drawing. Even though the upper and lower arms are actually originating from the same part, I got a difference in their lengths. In real life that is not really measurable and far from being a problem, but the values are 290.721mm and 290.726mm respectively. That was just enough to spoil all the fun when I tried to apply the "parallel" mate after both ends were fixed with concentric mates... How can that be? Same part, every other distance is identical, yet, somehow I created two parts from the same assembly differing in just enough to get measurable difference which ruins a whole day work...?

 

[kellnerp]

I seem to remember from somewhere that the representation of a part in an assembly has to go through some math transforms depending on where it is in the model. There might be some roundoff.

You didn't use mirror assembly by any chance did you?

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[steinmini]

I often use a mirror, not for creating upper and lower arm, though. I made a construction geometry and mated the components along lines, axes and planes. All was dimensioned up to full definition. I read about problems with mirroring being somewhat risky if perfect symmetry is to be achieved, and I avoid it in assemblies, use it in sketches to make slotted holes and that sort of things.