Can the order of datum features change throughout a drawing?

[BasicDim]

thread1103-287623

Hello,

Can a FCF that is ABC in one view for a particular feature be a different order (ie, BCA or CBA) for another feature?

 

[3DDave]

Yes.

But this also means understanding the relationship between the features that use different orders becomes complicated.

As usual everything depends on the exact geometry, the exact tolerances, the exact use, the relationship with all mating parts and functions.

 

[Burunduk]

It can, but usually it shouldn't.If you have some kind of multi-functional part that can be used in several applications, and in each application it mounts a bit differently on the same features — so that A,B,C correspond to one application and C,B,A to another — and there are specific features on the part dedicated to each application, you would want to reference A,B,C for some and C,B,A for others.

 

[BasicDim]

If the datum reference order does need to be changed, can C ever precede A or B when datum C is defined as perpendicular to A and B?

 

[Burunduk]

The common way of qualifying datum features, "datum feature A is flat within x.xx, datum feature B is perpendicular to A within x.xx, datum feature C is perpendicular to A,B within x.xx," is driven by their order of precedence in the feature control frames that reference them. If that order changes from one control to another, then I would suggest qualifying them by a different method. It's possible to specify an adequate profile tolerance to control the three of them as a group. 3X preceding a datumless profile tolerance, according to the ASME Y14.5 standard, would create a pattern—a rigid structure of tolerance zones that will hold the three datum features within limited variation relative to their true form and true (basic) orientation relative to each other. 

 

[Belanger]

If the datum reference order does need to be changed, can C ever precede A or B when datum C is defined as perpendicular to A and B?

Yes, that's certainly possible; the letter of the alphabet is merely a name for the datum. However, since you mentioned that the datum reference order does not need to be changed anywhere on the drawing, most people would say it's best to make A primary and B secondary, if only to avoid confusion downstream from people who might not be well versed in the topic.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[looslib]

What I was taught was to think of the FCF datum callouts as how you would fixture the part for inspection. Place the object on the first datum with 3 points of contact, slide it against the second datum with 2 points of contact and finalize the location with the third datum and a single contact point. You should try to minimize multiple FCF callouts, as each change requires new fixturing for inspection. A lot of this has been minimized with the newer measuring machines, but I still think along these lines when I look at parts and GD&T callouts.

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Ben Loosli