Here’s the job I was working on a long time ago – a massive granite beam held in place with M20 tie-rods in an anchor plate. The clearance holes in the beam were Ø30mm, so lots of room.
So I did the drawing using classical tolerancing, which should have been fine. A ±0.01mm tolerance should be more than needed to clear in case the holes weren’t perfectly straight.
This is how the tie-rods looked in the manufactured plate – not straight and there’s no way this will fit in the granite beam. The shop apologized and remade the plates with straighter tapped holes and it worked. They only had to remake 2 plates. Of couse the schedule took a hit.
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 should be
- It was only two plates – what if it was 1,000? The vendor could rightly have decided not to remake them, a financial loss to the company.
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 ‘P’) 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.
See the red axis boundary indicators below that the PTZ represents. This ensures that the cap plate will fit over the rods.
