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Old 1st March 2019, 05:22 PM   #1
JeanTate
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Engineering and Tolerances, some questions

In science - or at least the parts of science I am familiar with (e.g. astrophysics) - we have "uncertainties" or "error bars", and you cannot leave home without them (e.g. you have no hope of publishing a paper - on history aside perhaps - if you do not seriously address these).

I've always assumed that it's quite similar in engineering, whether civil, mechanical, EE, or any other kind. For example, a design document that's passed from one team/division/company to another absolutely must specify tolerances (though perhaps a different word?) for everything; so it's not just 1 cm x 10 cm, but 1.00.1 or 1.0000.002, say. The lack of these in such a document is, I would hope, grounds for refusing to accept it.

Is my assumption correct (more or less)?

I'd like to hear from those who have much more familiarity with engineering than I do.
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Old 1st March 2019, 05:47 PM   #2
Dr. Keith
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Originally Posted by JeanTate View Post
In science - or at least the parts of science I am familiar with (e.g. astrophysics) - we have "uncertainties" or "error bars", and you cannot leave home without them (e.g. you have no hope of publishing a paper - on history aside perhaps - if you do not seriously address these).

I've always assumed that it's quite similar in engineering, whether civil, mechanical, EE, or any other kind. For example, a design document that's passed from one team/division/company to another absolutely must specify tolerances (though perhaps a different word?) for everything; so it's not just 1 cm x 10 cm, but 1.00.1 or 1.0000.002, say. The lack of these in such a document is, I would hope, grounds for refusing to accept it.

Is my assumption correct (more or less)?

I'd like to hear from those who have much more familiarity with engineering than I do.
Sometimes the tolerances are implied or are standardized based on how the dimension is called out. When I was an engineer it was rare for each dimension to have a tolerance noted individually. That would only be done if the tolerance were outside the norm. Often there would be a general note in the legend of the drawings, but not always.

For instance, in laying out a reinforced concrete foundation the rebar will spec'ed at 16" OC, and everyone knows what that means. But the rebar around a critical area may have tolerance call outs if its placement is more critical than what an engineer would typically expect from a rebar crew.

That being said, it was decades ago and in civil engineering, where inches were typically the smallest increment of measure.
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Old 1st March 2019, 07:02 PM   #3
casebro
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Originally Posted by JeanTate View Post
In science - or at least the parts of science I am familiar with (e.g. astrophysics) - we have "uncertainties" or "error bars", and you cannot leave home without them (e.g. you have no hope of publishing a paper - on history aside perhaps - if you do not seriously address these).

I've always assumed that it's quite similar in engineering, whether civil, mechanical, EE, or any other kind. For example, a design document that's passed from one team/division/company to another absolutely must specify tolerances (though perhaps a different word?) for everything; so it's not just 1 cm x 10 cm, but 1.00.1 or 1.0000.002, say. The lack of these in such a document is, I would hope, grounds for refusing to accept it.

Is my assumption correct (more or less)?

I'd like to hear from those who have much more familiarity with engineering than I do.
The blueprints are a part of the contract. The machine shop needs to know the contract obligations, and what constitutes fulfillment. And they bid accordingly. Looser tolerances are faster to produce, so a cheaper bid is given.

Typical blueprint has a box with lines to fill in. The engineer specs the tolerance figure by the places past the decimal, thusly"

fractions: +.030, - .010
.0, +/- .020
.00, +/- .010
.000, +/- .001

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Old 1st March 2019, 07:37 PM   #4
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The second place I worked, if the dimension was to two decimals (inches), the tolerance was .06 inch, three decimals, .010 inch, unless otherwise specified. Any closely fitting parts needed to be specified. Most of the two-decimal ones were weldments, where it was pretty well impossible to do any better.
I forget what the default was at Boeing, but naturally tighter than that. Probably .010 for everything unless specified.
Have yourself a look at GD&T, which significantly changed how things were done about 30 years ago. I'd guess a lot of people STILL don't understand it very well. And I've forgotten most of what I knew about it, having moved away from detail design about 25 years ago.
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Old 2nd March 2019, 11:59 AM   #5
JeanTate
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Thanks for all the inputs!

So there's a quality aspect ("fitness for purpose") and a strong set of conventions, maybe even rules (sorta, by default, and unless specified otherwise, x carries with it y).

Any EE inputs?
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