Profile error

[[Am...]]

Hi everyone, im getting a profile that is reading way out and there is just no way its correct.

The left edge there is Datum C, long side is Datum B and the curved surface is datum A. Ive tried this a couple different ways using curve for A and I still get around .500". The profile is to ABC, the C datum has a profile callout to AB and i get that in but the right side to abc keeps coming in way out. My form on all datums is about .0001". What is going on here?

[[Am...]]

I know datum a is ridiculous the diameter of the curve is over 45 inches, but seems to be working ok everywhere else ive used it.

[[Cl...]]

In your Base Alignment, I wonder if you might need to "balance" the part about
the Y axis? E.g., two points at the same (X+/X-) coordinates to planar rotate?

As the alignment stands currently, you could test that out by just probing a point
at X+ & X- and comparing the Z values.

[[Da...]]

Why not scan A as a surface and use the surface as a datum? No need to calculate a cylinder from insufficient data. You’ve had the right intuition there, A as a cylinder is ridiculous.

Besides: A as a cylinder and primary datum takes away four degrees of freedom, which is mighty powerful for an element based on such a small section. It would only leave two DOF for B and C, which would only be a translation and a rotation around the cylinder‘s axis (pretty much useless in this setup). A as a surface would only constrain three degrees of freedom and would be much more stable. B would provide a rotation and translation and I’d say that might be the designer's intention.

[[Ro...]]

Show a screenshot of the results, the arrows that show the form err.

[[Jo...]]

Very likely the result is correct. The origin of measurement is from the axis of datum feature A and since you have such a tiny piece of the overall cylinder, the axis is just not calculating well. There's a TON of uncertainty in this measurement.

[[Am...]]

Please sign in to view this quote.

oh I know, thats why i've tried a bunch of secondary alignments doing doing a best fit of several curves for Datum A has actually worked really well, the other side comes in within .010" while the other side is coming in at .500". This was a second program that I made to check Datum A in a free state and i threw these on there too, Its a lot better in the fixture with simulated Datums to constrain the part to but im covering like half the side so I wanted to do those in the free state.