dtmbiz
Aerospace
- Sep 23, 2008
- 292
Single or Compound Datum?
1) If I have a block with 2 flanges (mounting features) on either side of a cutout. And those flanges had mounting holes in them, I would/could use the single planar datum from two surface features and call them e.g. A-B to establish a datum plane.
2) If I have a large let’s say .250” thk. plate 48” x 24” with a .125” wide by .125” deep groove across the width of the plate, would this be a single datum feature with a groove? Would it be proper to call datum A as the planar datum or use either side of the groove for A-B?
3)If the groove did not cross the entire part surface but stopped an inch or two short of the part edge, then I would say it is a single surface in that scenario and would use just a single datum letter A to identify the entire surface
I have had this discussion before as described in the 1st example (the block) and the project engineer sees both flanges as a single datum, even after I showed him the A-B from the 1994 standard.
The large flat surface with a "small break" across the width of the part resulting in to distinct surfaces is the interesting thought for me. That is considering "large surface area" with a "small" feature that interrupts that surface.
-Basically is the size of the interrupting feature of any concern or.....
is it the strict technical definition of being 2 separate features requiring the A-B callout regardless of feature sizes involved?
Is there anywhere in the 1994 or 2009 standard that gives more detail on this question?
Comments?
1) If I have a block with 2 flanges (mounting features) on either side of a cutout. And those flanges had mounting holes in them, I would/could use the single planar datum from two surface features and call them e.g. A-B to establish a datum plane.
2) If I have a large let’s say .250” thk. plate 48” x 24” with a .125” wide by .125” deep groove across the width of the plate, would this be a single datum feature with a groove? Would it be proper to call datum A as the planar datum or use either side of the groove for A-B?
3)If the groove did not cross the entire part surface but stopped an inch or two short of the part edge, then I would say it is a single surface in that scenario and would use just a single datum letter A to identify the entire surface
I have had this discussion before as described in the 1st example (the block) and the project engineer sees both flanges as a single datum, even after I showed him the A-B from the 1994 standard.
The large flat surface with a "small break" across the width of the part resulting in to distinct surfaces is the interesting thought for me. That is considering "large surface area" with a "small" feature that interrupts that surface.
-Basically is the size of the interrupting feature of any concern or.....
is it the strict technical definition of being 2 separate features requiring the A-B callout regardless of feature sizes involved?
Is there anywhere in the 1994 or 2009 standard that gives more detail on this question?
Comments?
