Here’s the thing in this case, and many others – there are some serious disadvantages with classical dimensioning.
- There’s no way to say how straight the holes need to be. What should I have said...... "pretty darned straight?" Anyway, on a very shallow granite beam, this would have been just fine.
- It was only two plates – what if it was 1,000? The vendor could rightly have decided not to remake them, a huge financial loss.
So I redid the drawings using GD&T, as below. This is a 3-D system that ensures there only one way to measure and verify the part. In this case, it allowed me the following benefits:
- I could loosen up the unnecessary tolerance to 0.030" from 0.010", saving manufacturing cost
- I could tighten up the tolerance where needed, by specifying a PTZ (Projected Tolerance Zone ⓟ) on the axis of the tapped hole.
- This saves money and time, as the requirement is unambiguous and the part will be delivered correctly the first time. Well, lets just say if not....... it won't be my fault!