Surface Datums

[JMorton99]

Hello,

When defining datums, does defining a surface datum constrain all 6 reference planes and axis? Would my part be fully constrained by a datum simulator or would I need to define one or two more planar surfaces?

What type of machine or simulator is used to define a surface datum? I want to have a visual grasp of how surface datums are physically defined so I have a better understanding when creating drawings. Thanks.

 

[JMorton99]

To add, the surface is curved and complex. Not a planar surface.

 

[axym]

JMorton99,

It depends what you mean by "surface". There is a figure in Y14.5-2009 (Figure 1-4, I think) that shows the datums and degrees of freedom constrained by various datum feature types. A "complex" surface can constrain all 6 degrees of freedom. The simulator would be a perfect inverse of the complex surface - the same shape, with the material on the other side.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

To clarify a tad -- I think Evan is referring to Fig. 4-3 on page 50 of that standard. The answer could be yes or no, depending on how you identify the datum feature on the print. That said, it's pretty darn rare to have a single datum constrain all 6 dof.

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

 

[axym]

JMorton99,

J-P is right, it's Figure 4-3 and not 1-4. Sorry, posting on a Friday afternoon. ;^) I also hadn't seen your clarification of a complex surface when I wrote the post.

I agree that it's rare to see a datum feature that constrains all 6 degrees of freedom on its own. It's probably best to avoid designing assemblies that way in the first place. The constraint will generally be incomplete and unpredictable - the surface will often rock on a perfect inverse simulator. Or, even worse, the part might be inspected with a CMM to create a stable "best fit" datum reference frame that isn't fully realistic.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca