Profile to one datum

[[Ca...]]

I'm sure this has been covered, but as the search function is set to ignore most input, I'll just ask.

I can't post pictures, so I'll do my best to explain it. I have a small, shallow rectangular pocket that's profiled to just one datum, it's top face, which we'll call datum A. Essentially, they want the pocket perpendicular to Datum A. When creating a profile characteristic, and entering Datum A as the primary, secondary and tertiary datums, it seems that I can get different findings based on how I set up my base alignment, which shouldn't be impacting anything, but looks to be influencing rotation. My preferred method of checking something like this is to create an alignment that uses Datum A as the spatial rotation, and Z origin, and create a geometry best fit alignment that references the Datum A alignment, and use that best fit to profile the pocket.

Questions time:

1. Does what I said make sense?
2. How can I keep the base alignment from being a contributor in a single datum FCF?
3. Is my best fit alignment strategy the best approach?

Thanks!

[[Ad...]]

Use all the points in the profile to create a geometry best fit alignment....then use the Evaluate Constraints and uncheck the appropriate boxes for example X,Y rotation and Z translation if it was a XY plane

Whenever I have done this I needed to create a second set of the points and probe them again to get accurate results, recall didn't cut it.

[[Cl...]]

My understanding is that you CANNOT use "Best-Fit" when there is a datum involved?

[[Ke...]]

Please sign in to view this quote.

How large (or small) is the Delta value of your results?
Did you test the repeatability of the measurements with any specific base alignment (without changing)?
Do you loop any of your alignments?
Can you post pics of just the alignments that you are using?

[[Ke...]]

Please sign in to view this quote.

But.... Adam's Geometry Best Fit Alignment "feels right"... any 2nd or 3rd opinions on this?

[[Ca...]]

Please sign in to view this quote.

The delta is about 0.0006/0.0007 (inches), which about half my total tolerance.
The alignments are not looped. In this case, I don't see the benefit. The rest of the part is in tolerance, with similar tolerances.

[[De...]]

Please sign in to view this quote.

A geometry best fit may be used even when there are Datum's called out, the only thing you need to remember is you can only best fit the translations or rotations that are NOT controlled by the Datum Reference Frame.

So lets say you had a Cylinder as your primary datum that controlled translation in X and Y and rotation about the X and Y axis, that would leave translation along Z and rotation around Z unconstrained, now lets say we have a secondary datum that is the plane at the end of the cylinder, it would control translation along the Z axis but leave rotation about Z unconstrained.

If the DRF only contained the cylinder as primary, then it would be acceptable to allow best fit along Z and around Z, in the case of the cylinder primary and plane secondary then it would only be acceptable to allow best fit of rotation around Z

So it really depends what your primary datum feature is and what it is constraining. The degrees of freedom it does not constrain are currently coming from the Base Alignment. The geometry best fit is being used to allow the actual values to be rotated / translated along/around the unconstrained axes to their best position relative to the nominal position of the feature rather than simply taking the default position relative to the base alignment.

[[Ad...]]

Please sign in to view this quote.

That is where the evaluation constraints come into play you will not allow change to the constrained degrees of freedom