Not sure if it will make much difference but what drawing standards do you work to and what thread form? Also male or female threads, or both.
For instance in the UK on male UN threads there was actually a spec (BS 1936 maybe) that addressed this to some extent, if I'm understanding your question correctly.
KENAT, probably the least qualified checker you'll ever meet...
From memory it essentially defines the tool to be used, based on thread size, which in turn gives you the dimensions for the actual cut-out. It's a bit backward from normal drawing practice but there you go.
As I recal from working with part 1 it gives some details on how to call it out on the drawing, but with more consideration I'm not sure it fully answers your question.
KENAT, probably the least qualified checker you'll ever meet...
I'm not aware of any standards but in my experience you call out the length and diameter, plus any corner radius if desired. The length should correspond to a standard cutting tool and the diameter should be below the root diameter. You should be generous with tolerances, and try to use the same relief for similar threads.
My experience with actual "thread reliefs" was with a Mil-spec defined by the FED-STD-H28. If this is what you are referring to, I would flag the geometry to a general note, referencing all thread reliefs to the Mil-spec.
If this is not the case, reference ASME Y14.6-2001. I believe it shows multiple methods of dimensioning and representing complete and/or incomplete threads which would include a thread relief.
I've always used the note "MINIMUM RELIEF REQUIRED TO FORM THREADS" and let the machinist worry about it. If it is critical see above post by Xplicator.
"Art without engineering is dreaming; Engineering without art is calculating."
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I know I'm opening up myself for criticism: If a thread relief is dimensioned, it must be measured. It has been my practice to state on the drawing "MINIMUM THREAD RELIEF" when good workmanship is good enough.
