Alignment Question. BF Line between Holes vs Autoalign Cylinders

Hello. I am programming a part using legacy. The top plane is the primary datum and the seconary/tertiary datums are two thru holes that are square to the top plane. So rotate to the line between the holes of course.

I have profile of a surface called out to these datums.

I tried two alignments:

- One alignment uses cylinders. I autoaligned to the top plane and the 2 cylinders.

- The other alignment I created a 2d best fit line between the cylinders. I autoaligned to the top plane, the best fit line, and one cylinder as my tertiary datum.

My profile of a surface looks crooked when I use the first alignment, and it looks much better using the second alignment. Can someone explain why my results would be different? Should I go with alignment #2? Please see screenshots.

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Matthew D. Hoedeman over 1 year ago

sounds to me like the 2 cylinders aren't 'coaxial' in reality. They may be to design, but not in the part.

In 30+ years, I've never used any of the 'autoalign' functions in the alignment, I pick what I want for what I want.

As for using the cylinders, the best-fit line is probably using the centroid of each cylinder while the other one may be using the entire cylinder, but I am not sure.

I would check the coaxaility of the cylinders.
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Matthew D. Hoedeman over 1 year ago

sounds to me like the 2 cylinders aren't 'coaxial' in reality. They may be to design, but not in the part.

In 30+ years, I've never used any of the 'autoalign' functions in the alignment, I pick what I want for what I want.

As for using the cylinders, the best-fit line is probably using the centroid of each cylinder while the other one may be using the entire cylinder, but I am not sure.

I would check the coaxaility of the cylinders.
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