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If a casting drawing has datums A through E on the stand alone casting drawing, what do you name the datums on the drawing for the machined version? Should the datums on the machined drawing start with A or F?

By the definitions found in ASME Y14.5M-1994 any drawing dimensions and referencing (datums) apply only on the drawing or at the drawing level upon which they appear (ref: Section 1.4(n)). Does this imply that, if we have a case where the casting is fully dimensioned with GD&T and uses letters A thru F for the various datums, then the machining drawing could also begin with letter A as well? This seems "OK" to me because each drawing stands on its own merit. By that I mean, if the part is being measured for example, each drawing will provide the inspection person with the information needed to properly understand the dimensions and references for that part at that specific level. The casting drawing is independent of the machining drawing.(?) Each drawing stands as its own set of requirements.(?) So what might be "Datum E" on the casting could very well become "Datum A" on the machining drawing. In the case of a "reside on" casting (one drawing for both casting and machining features) then repeating of Datum letters would not be acceptable. Typically, the machining drawing will use some cast references (datums) to start (locate) the first machined features.

So, again, the question: Should the datums on the separate machining drawing start with A or F if the stand-alone casting drawing used A thru E?

Hi Joey,

Yes, you are correct, if cast datum letters are referenced on the machining drawing, then you will use different datum letters for the machining drawing so there is no confusion. Eventhough, theoretically you could have the same datum letters for both drawings if the casting datums are not referenced on the machining drawing, in practice it is wise to use different datum letters for the machining drawing since most likely some of the datum features will be referenced. Most companies adhere to the practice of specifying different datum letters to avoid any confusion with cast datum features.

_________________
Best Regards,
Roy Cross
ASME GDTP Senior Level S-0488
Dimensional Engineering Mentor
Effective Training, Inc.
2118 S. Wayne Rd.
Westland, MI. 48186
royc@etinews.com
http://www.etinews.com