Axis of a square shaft

[CheckerHater]

I have a simple question which unfortutely doesn't have simple answer (or I couldn't find it).

Imagine a shaft having two sections - round and square
To simulate working conditions of the shaft I have to put square end into collet, rotate it and measure Run-out of the round end - something perfectly do-able in the real physical world.
Now to document this process I have to specify Run-out in relation to the axis ... of a square, and ASME Y14.5 is not very clear on that matter.

If you look into definition of the Coaxiality, it basically sais that everything can have an axis, as long as it has a cross-section, but I REALLY don't want to invoke Coaxiality on that part.

Any ideas?

 

[btrueblood]

Define the square faces as datum features, if necessary invoke symmetry across the centerline. Then apply position tolerance to the cylindrical feature relative to the datum(s). Kenat's reverse solution works the same way, but again, only position and/or symmetry, and/or using basic dimensions to locate the plane faces relative to centerline of cylinder will work to define the geometry within GD&T.

 

[MechNorth]

No, please don't take this thread to the cliff that is the use of symmetry or concentricity. PLEASE!?

Jim Sykes, P.Eng, GDTP-S
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