True Position/Datum Shift

[[Me...]]

Calypso will not allow me to add Datum Shift to Datum -B- for my True Position callout for Datum -C-

Datum -A- is a face, Datum -B- is an OD that is about a half inch long. The picture shown below is a detail view of -C- which has a .001 TP callout to A & B.

In my Characteristic, I have measured -C- as a Cylinder with 3 circles and scanned. I scanned -A- and have -B- measured as a Cylinder with 2 circles.

Capture.PNG

[[Ri...]]

This response was taken from another thread, as this topic gets brought up a lot.

"Calypso can only apply 'Datum Shift' in 2D with cylindrical features. Here are a few things to keep in mind:

1) Modifiers will only be activated when the datum feature(s) are cylindrical elements and reside in the same plane as the considered feature.

2) The shape of the tolerance zone must be cylindrical.

3) Using a "Point" as a datum feature does not work in the position characteristic. If you're getting a result, either a bestfit has occurred or the base alignment is being used for the evaluation."

[[Me...]]

I've also noticed that it seems to constrain Cylindrical Datums to a face for some reason? Is that because it is recommended to change the Environmental settings to the ISO 1549 tangential setting for form datums?

[[Aa...]]

Yes, that's what the ISO 5459 setting does--every datum feature that's added to a DRF while it's turned on will automatically set so that its orientation is constrained to preceding datums.

You can right-click the datum feature in the Datum Reference area of the characteristic dialog to open the Evaluation Parameters dialog for that datum feature reference. There's a button "Form Datum as per ISO 5459" that will fill in the constraints for that specific datum reference according to 5459, regardless of whether the 5459 setting in Calypso is turned on.

Yes, this topic is brought up a lot, because it's a glaring omission, and not explained in the help.
This limitation persists because eliminating it would require a complete change in how Calypso calculates DRFs. (It would require simultaneous evaluation, rather than sequential evaluation.)

[[Me...]]

I'm getting everything out of tolerance when it constrains...I do not believe we have to follow the ISO standard in this regard because all of our prints are ASME...

[[Aa...]]

Yeah...the requirement that datum features be constrained to preceding datum features is also included in ASME Y14.5.

[[Me...]]

Oh poop. Well...I just made alternate alignments with the same features and the results are better.

[[Cl...]]

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Trying to understand what you are stating here. Are you saying that datum's used in secondary alignments should be constrained? Constrained to what exactly. Do you mean the base alignment? If so can you give an example. I don't remember anything on this subject matter being taught in the advanced class?

[[De...]]

Clarke,

If you turn on 'Form Datum per ISO 5049', the constraints will be created automatically within the DRF of the characteristics when they are created.

So lets assume your Primary Datum is Datum A which is a plane, Secondary Datum is Datum B which is a cylinder.

You set primary datum feature to plane A, if you look inside the DRF at the feature used you have the regular evaluation settings with nothing different from normal.

Now you set the secondary datum feature to cylinder B, when you look at the evaluation settings you will see that cylinder B is now constrained to plane A.

Having 'Form Datum per ISO 5049' turned on causes Calypso to add the constraint automatically for the secondary and tertiary datum features.

The constraints on the datum features is meant to properly align the part as you would on a surface plate. So prior to evaluating cylinder B, the datum reference frame is first aligned to the primary datum based on the evaluation settings that are defined for the constraint, only after the secondary datum is aligned to the primary datum is the secondary alignment evaluated.

The constraints can be added by hand whether the 'Form Datum per ISO 5049' option is turned on or not and to the best of my knowledge will be handled the same way assuming they are setup correctly.

The goal of all of this is simply to follow the concept to aligning to the datum reference frame in sequential order each new Datum being aligned to the previous one.

[[Cl...]]

Thanks Derek. Good simple explanation. I get it now.