Here’s the job I was working on a while ago – a massive yet incredibly precise 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, right?
I did the drawing using classical tolerancing, which should have been fine. A ±0.01" 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 – GAAAK - not straight and there’s no way this will fit in the granite beam. (It was tapped by hand!!!!). The shop apologized and remade the plates with straighter tapped holes and it worked. They only had to remake the two plates. Of course 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 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!
See the red axis boundary indicators (below) that the PTZ represents? This ensures that the cap plate will fit over the rods, but still allows the straightness of the hole to vary. Less rejects, less cost, less-stringent manufacturing, more accepted pieces.
Ta-daaaa.
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