So when I was working through my apprenticeship tests, frequently I'd be given a print for a part to make with a lot of very tight tolerances.

Not just overall dimension, but relationship between two completely unrelated dimensions in such a way that retired multiple set ups. Then heat treated which causes the material to warp, and tolerance had to be maintained.

Now these weren't bullshit parts, sometimes the tolerances were bullshit but it was for a test so it was to be expected, but even if they worked, they could still fail if I missed a dimension.

And when I say missed a dimension, we could be talking about a part that's met every dimension but one, and it's only out by .0002 inches.

For reference, a piece of printer paper is .004 thick. So imagine cutting that in half, and then cutting one of the halves in half, and do that two more times, and you're approaching the thickness of the tolerance we're talking about here.

Anyway if I missed that one dimension by that much, the entire test was failed, even if the part worked just fine.

All of this is to say that 70% probably isn't good enough here.