Perpendicularity which involves a cylinder

I need to dimension a perp between a plane and a cylinder. When dimensioning the perp, I get two very different results when I switch back and forth between PLANE to/from CYL.

1. Why are the results so different? (we are talking 0.0003 vs 0.0238).
2. Is there a best practice for dimensioning perps? To have the plane back to the CYL (having the cyl be the datum), -OR- the CYL back to the plane (having the plane be the datum)?

p.s. Yes I know to follow the drawing, I just need to explain to engineering what the CMM sees, and have a peace of mind.

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E30wassabi over 1 year ago

Thanks for your input.
Its interesting, it is as if pcdimis has a hard time seeing the vector of the cylinder axis when the cylinder is too short.
So really there's nothing the CMM operator/programmer can do about it.
I still have a hard time wrapping my head around why it changes so much for the to/from - even on parts that have a more balanced ratio of plane:cylinder features. That picture above helps a bit though.
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Malgorzata over 1 year ago in reply to E30wassabi

Study and use your imagination what JAFMAN posted, this picture explains everything very precisely - imagine a long plane and a short cylinder.
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Malgorzata over 1 year ago in reply to E30wassabi

Study and use your imagination what JAFMAN posted, this picture explains everything very precisely - imagine a long plane and a short cylinder.
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