Datum Ref Frame vs Base Alignment

[[Da...]]

Hi all,

I'm trying to figure out why Calypso is showing difference results between the base alignment and a datum reference frame which is setup to be the same as the base alignment. All evaluation methods for the relevant features are the same (outer tangential). From the look of things both alignments should end up with the same result, but instead I am getting something wildly different. What gives?

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[[Ri...]]

Is it because of ISO5459 settings and/or OTE settings?

By default the Base Alignment will not be constrained per ISO5459, and if you have OTE turned on for Datums that will not apply for the Base Alignment unless you manually set it.

[[Da...]]

I have it set to use ISO5459 and outer tangential by default.

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[[Te...]]

The form datums you turn on in extra settings only applies to the GD&T characteristics when you break them out to the individual datum features. EVERY Alignment system in Calypso by default uses LSQ and the "intersection" of the datum features.

[[Da...]]

There is a good reason why the ISO 5459 by default does not apply to the base alignment.

The base alignment has a special task and that task is to locate the part in the volume of the machine. The other thing is that all other elements following the base alignment are calculated based upon the base alignment. For this to work properly, the base alignment must be stable and that means it must be made of stable elements.

The problem is, that if you make an alignment and use ISO 5459 and OTE elements, in order to calculate the alignment correctly, you must provide the correct nominals (coordinates and vectors). But as long as the base alignment is not calculated, the nominals are based on the manual alignment (or the startup alignment if you use any). See the problem there? It's the old chicken and egg dilemma.

There is no single mathematical step to calculate the base alignment with the mathematical basis changing after calculation. It would require multiple iterative steps, and I'm not sure if that's what's happening in the background.

That is the reason why you should never use ISO 5459 for the base alignment and also you should never "recycle" elements that you use for the base alignment for other alignments.

And since "serious" ISO 5459 alignments with OTE elements only make sense when you scan with high point densities, you also shouldn't use elements with only a few points for them.

[[Ri...]]

You shouldn't use the word never around here... 😎 😎 😎 😎

[[Aa...]]

The features in the BA might be evaluated differently from the way the features in the DRF. Right-click on the plane icon for all features in both to see the evaluation settings. You can simply compare the two.

[[Ri...]]

I tell everyone I train or come in contact with regarding Calypso and Base alignments.
1. Never use Base Alignments for measurements.
2. Base Alignments are information to the CMM that there is a product in a specific area of the volumetric area.
3. Create a CMM Alignment then use that for features or a features relevant alignment per the B/P.

[[Er...]]

I've used CAD evaluation on my datums before to see where things are going sideways. I'd throw some flatness specs on the planes to see what they look like. The part may be twisted like a pretzel to get 2mm of change. With sheet metal I sometimes have to "choose wisely" where I take points. It's always nice when the customer defines them.