Datum question

[hygear]

I am having a disagreement with a colleague of mine on the proper way to dimension a sheet metal part. I have included a crude sketch of the drawing since I can't share the actual drawing. Basically, there is a dispute on defining a reference datum other than A,B,C and whether or not the datum needs to be defined with an angularity tolerance and basic dimensions.

My argument: the hole position needs to be controlled in datum D so datum D must be defined and the holes must have a position tolerance that references D. If the hole position is not controlled properly, the parts will be difficult to assemble.

His argument: everything can be dimensioned and toleranced from A,B,& C and datum D is not necessary. If the parts are a little off, the will just bend the back on the assembly line. Also, datum D can be placed with regular +/- tolerances and the angularity tolerance is pointless.

What are your thoughts?

 

[hygear]

Seems my file did not upload properly the first time...

 

[pmarc]

Depending on a function, the holes can be positioned from A|B|C or from other datum reference frame including D.
Regadless of that the surface currently shown as datum feature D should be geometrically related to other features or other features related to D. In both cases an orientation tolerance should be used. For that I think profile of surface is much better choice since it is capable of controlling location of faces, while angularity is not.

 

[SeasonLee]

Fig 5-28 on ASME Y14.5M-1994 or
Fig 7-55 on ASME Y14.5-2009

Its a good reference for you.

SeasonLee

 

[hygear]

Thank you for the input! I took a look at figure 5-28 and I believe profile of surface would be the better way to go.