Can, must, May rule

[[Ma...]]

I've heard this rule several times but I can't find the definition of it anywhere and I was wondering if anyone has a PDF or something I can reference to use it.

[[Ky...]]

I'm afraid I can't find one, at least not one that I feel comfortable sending out since we don't make it. If you want a simple step by step breakdown, you go through this process for all 6 degrees of freedom possible in a rigid object (i.e. translation and rotation in X, Y and Z):
 

1. Can - Is the datum physically capable of constraining this degree of freedom? For instance, a plane with the axis in the Z direction can stop translation in the Z, but it CANNOT stop translation in either the X or Y (think of putting that plane on the granite of the CMM, you can't push it further in the Z, but you can slide it in the X and Y as much as you like). Likewise, a cylinder or circle in Z axis can stop translation in the X and Y, but not in the Z (think of putting a pin in the feature).

 

2. May - Is there a higher priority datum already constraining that degree of freedom? If two datums pass step 1, then the one that comes earliest in the datum structure of the callout gets priority (that's why its primary, secondary and then tertiary).

 

3. Must - If the datum fulfills the requirements in step 1 and 2, then this datum MUST control that degree of freedom.

 

Hopefully this helps, at least a little. I know it isn't the PDF you were looking for, but in terms of concise explanations, hopefully that gets the job done.

[[Ma...]]

I think on Calypso it's working like what is first it's taking whatever is possible. Exceptions are for points and circles - they should not give vectors - only position.

I like it in Zeiss Inspect, where you see result.
Also Inspect is much advanced about coordinate systems. It allows you to select multiple features to act like pattern and you can also select if you want from it only direction or point also.

So here is an example:

Primary: circle -> origin
Secondary: line -> rotational axis
Tertiary: plane -> stopping rotation

[[Ch...]]

can may must rule:

if a datum feature can (is physically able to) and may (is permitted to by the drawing) constrain a degree of freedom (DOF), then it must do so.

Most often, your primary datum will constrain 3 degrees of freedom, your secondary will constrain 2, and your tertiary will constrain 1.   But not always, it all depends on what your datum 'can may and must' constrain.  

[[Pa...]]

You won't find it in a standard.  It's a "rule of thumb" meant to simplify what IS in the standards.

[[Sh...]]

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Just reference

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 to anyone or any emails you have to address.  He'll set em straight! 🙃

[[Ra...]]

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I can vouch for Mark Foster. Best ASME class I ever took with him as the instructor.