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Re: [Phys-l] Sig figs

Well, yeah, I sort of used sig-figs, but I was doing it in a pejorative way... to point out the the typical machinist is not going to treat 1.0 and 1.000 much differently. I didn't give any explicit uncertainty because it depends on the shop or even on the particular machinist, or what positioning devices are present on the particular machine used.

When I submitted drawings to the machine shop when I was in graduate school (long time ago), most things came back at roughly +/- 0.0005" no matter whether my drawing showed one decimal place or three. (This is assuming it was not a large piece, say not having a dimension larger than 12 inches or so.) However, one machinist routinely did +/- 0.0001 work, and another routinely did +/- 0.001 work. They both worked at roughly the same speed. The person who routinely did the better work was 20 years older (45 versus 25) and was simply more experienced at working quickly as well as accurately.

My point was, if you specify 1.0" you certainly are not going to get 1.0 +/- 0.1 inch. I have never met machinist who would do that. If you specify 1.0" you are probably going to get +/- 0.001 inch, but you might get a little better (perhaps +/- 0.0005) or you might get a little worse (perhaps +/- 0.005). The professional machinists I have worked with would be embarrassed with +/- 0.005 work on small parts (several inches or less) even if you gave only one decimal place in your drawing.

* * * 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. 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.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
1 University Drive
Bluffton, OH 45817

419.358.3270 (office)

From: "Bernard Cleyet" <>
Sent: Sunday, October 17, 2010 3:34 AM
To: "Forum for Physics Educators" <>
Subject: Re: [Phys-l] Sig figs

I think the idea is if I specify I want an inch but write 0.999 is ok and he's tuning he'll stop there. This, of course is just before the accuracy available w/ position encoders.

bc doesn't remember what the UCSB machinist wanted (55 years ago)

p.s. I think you just used sig. figs. Don't let JD know.

On 2010, Oct 14, , at 19:56, Michael Edmiston wrote:

I don't know where your machinists came from, but here is what my experience
is, and I have worked with many machinists at many different places,

If you ask for 1.011" you will get 1.011".

If you ask for 1.0" you will get 1.000".

Same price

Forum for Physics Educators