12 Apr 04 14:28
I agree with you about avoiding non-associative notes on CAD drawings and the USUAL dimensioning procedure, but your example doesn't fully solve the problem. Bodie is concerned with cluttering his drawing (a sign of a good drafter).
Following your example, nothing prevents the dimension from being incorrect if one of the holes is clocked other than 60 degrees after the drawing is created. The only way to avoid this would be to dimension each angle, thus you would have 6 separate degree dimensions defining your hole pattern. You would also have to dimension each hole separately, in case someone decided that one of the holes should be of a different size. Of course, in trying to create a fully associative document, you end up with a cluttered, hard to decipher drawing. In creating a concise, easy to interpret drawing, some shortcuts are necessary and desirable. It is not a requirement of ASME Y14.5-1994 to fully define each and every feature and location separately, or it wouldn't allow for "X" to denote the number of places.
I have not been "careful to draw everything to scale" since I was on the drawing board. That is part of the function of the software. I do model everything to scale, but that is the only way to create a valid model.
There are ways to call out the bolt circle in a note that are associative. I suppress the leader line when dimensioning bolt circles, add the “EQ SP ON” and "BC" notation and move the dimension below the hole callout, resulting in an associative "note" type of callout that you recommend against. Do you add separate diameter and depth dimensions for counterbored holes (I do, using the same method as for BC)? How do you ensure that a tapped hole is UNC and not UNF? How about the angle of a countersunk hole? Having to show all of these details separately can result in a five page drawing vs a one page drawing.
You should never assume that your drawing is correct after updating your model. Granted, if care is taken in your dimensioning, mistakes should be minimal, if any.
I'm sorry to rant about this, but I feel that some shortcuts are necessary in complex drawings. The intent is to create an accurate, CONCISE definition of a part, something easily understood by the fabricator of that part.