GDT Position - Rotation Translation

[[Em...]]

I have a sketch of the position diameter and how I defined it in Calypso. Can someone visually explain to me when rotation and translation are activated? Based on the explanation from the Calypso Advanced example on page 67, the current explanations seem a bit contradictory. I’m not sure whether rotation and translation are even activated if we have ABC defined... Thanks for your help.

[[Ma...]]

If you have only A, which would be plane where your holes are, then you can use rotation and translation.

Once you have defined also B, which controls direction, then you can only translate ( depending on base dimensions )

On fully defined ABC you can not have any of bestfit, unless you have a (M) or (L) modifier on B or C.

[[Em...]]

If I look at it theoretically, I agree with you. But in my case, when I have ABC and use a gauge controller, I can barely keep it centered on B, and the center slightly shifts, to better align with the four holes, even if I don’t have M.

Can you imagine all the factors I want to include in this CZ and gauge... and what I’m trying to say?

[[Ky...]]

The short answer is that when those checkmarks are on, the results are allowed to best fit to give you better answers. In the example you have above, you are allowing it to rotated around the axis of the primary datum (Y in this case) and the resulting best fit alignment is rotated -0.808 degrees from the datum reference frame you have specified.

 

As Martin mentioned, as the datum reference frame is fully constrained, you probably don't want it turned on as it changes the datum reference frame. To figure out what you would want on and off, you will need to apply the "Can, May, Must" rules in GD&T.

 

Additionally, note that this particular method only works in two dimensions (in the plane of the primary datum usually) and can only work if all the normal vectors of the features in question are in the same direction.

 

Hope that clears it up.

[[Ri...]]

The old 2d Bore Pattern Best Fit can be confusing. The majority of the time it is used when you are unconstrained translationaly and/or rotationaly. In those cases, it is pretty straightforward, you activate it by turning on the Translation or Rotation - that means it will best fit the feature(s) for those constraints. Where it does get confusing is if you activate the View Tolerance so that you can utilize MMB/LMB on the Datum, in those cases you are supposed to activate Translation or Rotation for which constraint the datum with the MMB/LMB is controlling. Page 53 of the Cookbook shows an example of this. 

There are other use cases where you could utilize the 2d Bore Pattern Best Fit for Composite Tolerances where say both the Upper and Lower Tier have a fully defined DRF (ABC). Page 55 of the Cook Book shows some examples of this as well. 

I just say that if you can, update to Calypso 2025 and utilize the new GD&T engine and forget about dotting your Is and crossing your Ts on which setting should/shouldn't be activated. 🙂