Surface Datums
Surface Datums
(OP)
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.
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.





RE: Surface Datums
RE: Surface Datums
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.
www.axymetrix.ca
RE: Surface Datums
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Surface Datums
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.
www.axymetrix.ca