Surface Profile vs. Form

[[To...]]

Sorry to re-hash this topic. We all know Calypso evaluates Form using Minimum Feature and Profile with no DRF using LSQ. And, we often hear "just use Flatness for a plane or Cylindricity for a cylinder or Roundness for a sphere, etc.", if you don't have Free Form. Currently, I am evaluating a sphere with .001" Profile with no DRF that is slightly out of tolerance but when I apply a Roundness with the same tolerance, it passes. I'm also seeing when I tie the Profile to A, B and C, that it is closer to being in tolerance than with no DRF.

To the best of my knowledge, Profile is considering the size of the sphere where Roundness is not. If so, the alternative use of Flatness, Cylindricity and Roundness would NOT be appropriate. Or, is Flatness the exception because a plane has no size? Is this correct?

What are some of the best practices that should be used to satisfy ASME Y14.5?

[[Ri...]]

I think that Flatness would be the exception, but if it's applied to a Cylindrical element, size is factored in.

I didn't realize that a lot of information was going around telling people to just evaluate a form characteristic if FF isn't available.

[[Aa...]]

So...for cylinders and spheres, why not use Form while constraining the diameter?

[[Br...]]

Surface profile can control all 4 types of variation in a feature; Form, Orientation, Size, and Location. But as you have pointed out, depending on the application, it may only control certain types. For instance. A Profile of surface tolerance applied to a planar feature with a fully constrained datum reference frame will control Form, Orientation and Location. Take the DRF away completely and it only controls form. You may have seen co-planar surfaces with a single profile tolerance and no DRF. The purpose here is to link the interrupted surfaces together to control their form. A flatness tolerance would only evaluate each surface individually. So in this case, If you measured all the surfaces together as a single plane feature and reported flatness, that would be the same thing.

Now, when we see features with a closed contour such as cylindrical or spherical surfaces, Profile now controls size. Cylindricity and Roundness are Form only tolerances. So in this case, they would not be an appropriate alternative to Profile.

Profile technically can be used as a refinement to size. In this case it would be controlling form, but this is very uncommon. The new 2018 standard has introduced a new modifier for composite profile tolerances called "Dynamic Profile". This unlocks the size from the profile and only controls the form.

[[An...]]

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The meaning of form:
Addition of min/max deviation.

The meaning of profile:
Duplication of max deviation

Profile with or without DRF evaluates always Least Squares.
Emphasizing the size not the form.

Roundness always evaluates Minimum.

Nominal size:
Ø20±0.2

Actual size:
19.96

Constrained Diameter:
20.0000
Roundness: 0.00500

Constrained Diameter:
20.1000
Roundness: 0.00500

Constrained Diameter:
20.2000
Roundness: 0.00500

The value of the constrained diameter
does not influence the result for roundness!

[[Ri...]]

I have the ultimate solution, make sure you have Curve & Free-Form 🙂