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Re: [Phys-l] Sig figs

On 10/17/2010 04:38 AM, Michael Edmiston wrote:
* * * Slight aside... * * *

I mentioned in an earlier post that it is helpful to let the machinists know
what you are up to. If they are told that one piece fits inside another
piece, and you would like 0.002" clearance between the two, then they will
make it that way. If you just give the dimensions, and they are doing ±
0.001 work, you could end up with no clearance. Of course you can/should
specify the outer piece as "minus none" and "plus something" and vice-versa
on the inner piece. But I always found that machinists liked talk to you
about you are doing. They might not understand the physics, but they
understand the mechanical parts, and they can often make design suggestions
you haven't thought of. So I would visit the shop with sketches and
half-finished drawings, and ask for advice.

That's good practice. I don't think it is even slightly aside;
it's more important than most of the other stuff that's been
said in this thread.

It is on-topic in the sense that it demonstrates that thinking
in terms of sig figs will not achieve the desired results ...
not even close.

Of course this only works in an
environment where you can talk to the machinists face-to-face as opposed to
simply submitting finished drawings.

Indeed! To say the same thing another way, collaborating with the
other party works a lot better than merely contracting with the other
party. This applies to lots of things, not just shop work.


Concerning the round peg in the round hole: To a first approximation,
you don’t care about the diameter of the peg or the diameter of the hole,
provided the peg fits into the hole with the proper amount of clearance.
This is an example of _correlated_ uncertainties.
The amount of clearance is the proverbial small difference between large
numbers, which means that the relative uncertainty in the clearance will
be orders of magnitude larger than the relative uncertainty in the diameters.
For a one-of-a-kind apparatus you can customize one of the diameters to
give the desired clearance ... whereas in a mass-production situation
controlling the clearance might require very tight tolerances on both of
the diameters. At some point this becomes a no-win situation, and you’d
be better off using a tapered in in a tapered hole, or using a sellock
pin (aka spring pin).

For more on this, including other examples, see