"Loving" how customers state dimensions and tolerances like this . . .
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    Default "Loving" how customers state dimensions and tolerances like this . . .

    Yes, I'm being a crybaby this morning. Haven't had my coffee yet.

    DIM: ".2813 +.001 / +.0015". Therefore, .2813 means nothing to the part feature. .2823 is the LOW, .2828 is the HIGH. .28205 is median.

    We re-stated the DIM and tolerance differently on our reprint, and it got past our re-printing review because in the moment brain-translations did the math wrong, between two people.

    Our shop foreman has his own policy of reprinting, so that's not going away (stated because others here have mentioned in the past stop reprinting).

    I'm annoyed. Annoyed two of us MISSED this on our reprint (caught in time, job OK). But I'm also annoyed that a physical DIM statement on a blueprint baselines to nothing unless you math the whacky tolerance to find What the DIM is actually supposed to be. .2813 actually is not the DIM.

    Is there some esoteric Voodoo I'm noobishly ignorant of that suggests it is good practice to state a DIM and Tolerance like this?

    /pre coffee crying over

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    This sounds the same way metric holes are toleranced and I also hate it. D9 for example on a 20mm hole means 20.065 to 20.117.

    Table of Metric Hole Tolerances per. ISO 286 Chart Calculator | GD&T Tolerances - Engineers Edge

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    Sometime this is done as to the function of the part. It might clue you as to run the tolerance to the high or low side if your process is stable enough.

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    Quote Originally Posted by Philabuster View Post
    This sounds the same way metric holes are toleranced and I also hate it. D9 for example on a 20mm hole means 20.065 to 20.117.

    Table of Metric Hole Tolerances per. ISO 286 Chart Calculator | GD&T Tolerances - Engineers Edge
    Yep. Most metric tolerances for H and h (etc) are always +/+ or, -/- until you get into pretty loose sliding fits

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    Quote Originally Posted by Mike1974 View Post
    Yep. Most metric tolerances for H and h (etc) are always +/+ or, -/- until you get into pretty loose sliding fits
    OK, this at least tells me WHERE this comes from. At a guess, at some point on the Customer end the part in question, or a template blueprint, or the blueprinter, was in metric or used to metric nomenclature.

    Then at some point it was all converted to Inches and the convention carried forward.

    We do everything in Inches here.

    Thanks for the insight.

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    Quote Originally Posted by ttrager View Post
    OK, this at least tells me WHERE this comes from. At a guess, at some point on the Customer end the part in question, or a template blueprint, or the blueprinter, was in metric or used to metric nomenclature.

    Then at some point it was all converted to Inches and the convention carried forward.

    We do everything in Inches here.

    Thanks for the insight.
    Same with me....until you need a metric tapped hole.....
    For example: M20-2.5 x 2.50 terminology get's used allot.

    Note the "2.50" is the tap depth in inches.....

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    I think it stems from prints being made from the engineers perspective, with no thought of how it is made. In other words, they want the hole to be .2813, but it would still work if it was a thou over. On the manufacturing side, unless the guy making the parts is intimate with the end product, we don't care what "works," we care what the desired dimension is, and what tolerance is acceptable to cover variances in manufacturing and to make it economically feasible. If the part would fail if it had a +/- .005 tolerance, that's ok it needs to be tight. But if you spec the width of a shovel to be 8.0000 exact, sure it can be done, but not for long because Joe Contractor's not going to pay for it.

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    As I understand it, it's pretty common to do this with features where the nominal is a fraction of an inch. Such as .2500 +.0001 +.0003 for a dowel pin. I've dimensioned parts like this many times before. Your .2813 part is 9/32, so that checks out. I would never dimension a part as .2510 +.0015 +.0025 for example.

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    Any time I need to do a part with those types of tolerance I re-write them like this: 0.281³"/0.282⁸" - that makes it pretty hard to misunderstand.

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    Another reason the metric system is a thorn in the side of humankind. Two positive tolerances, and no negative tolerance.....

    I predict in 5 years we will have dimensions that have two positive tolerances and an emotional tolerance. If you fuck up the part but feel bad about it, it passes.

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    yeah this type of thing erks me.
    put on the drawing what you actually want. no deciphering or code breaking involved. like the KISS principle

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    Quote Originally Posted by ttrager View Post
    Yes, I'm being a crybaby this morning. Haven't had my coffee yet.

    DIM: ".2813 +.001 / +.0015". Therefore, .2813 means nothing to the part feature. .2823 is the LOW, .2828 is the HIGH. .28205 is median.

    We re-stated the DIM and tolerance differently on our reprint, and it got past our re-printing review because in the moment brain-translations did the math wrong, between two people.

    Our shop foreman has his own policy of reprinting, so that's not going away (stated because others here have mentioned in the past stop reprinting).

    I'm annoyed. Annoyed two of us MISSED this on our reprint (caught in time, job OK). But I'm also annoyed that a physical DIM statement on a blueprint baselines to nothing unless you math the whacky tolerance to find What the DIM is actually supposed to be. .2813 actually is not the DIM.

    Is there some esoteric Voodoo I'm noobishly ignorant of that suggests it is good practice to state a DIM and Tolerance like this?

    /pre coffee crying over
    It's to the point now where any new job I get I take the first day to comb through the print and model to make sure they match up. We have a couple of customers where a solid 80% of all prints are one way "toleranced" and the model is to the high, low, or straight up out of tolerance like you're experiencing.

    It's a total pain in the ass but I've been burned on it enough times at this point to know better. We've even got one big customer that explicitly states on their prints something to the tune of this print might be fucked up and we aren't responsible for it.

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    Quote Originally Posted by ttrager View Post
    Y
    Is there some esoteric Voodoo I'm noobishly ignorant of that suggests it is good practice to state a DIM and Tolerance like this?
    Yes. Like the folks who made the print, just ignore the fact that machining processes obey gaussian statistics, and then it all makes sense. Also forget anything you might know, about gaussian statistics. Then this crap makes total sense.

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    Quote Originally Posted by ttrager View Post
    Yes, I'm being a crybaby this morning. Haven't had my coffee yet......

    Is there some esoteric Voodoo I'm noobishly ignorant of that suggests it is good practice to state a DIM and Tolerance like this?

    /pre coffee crying over
    Yes, ISO & ANSI standard for “Preferred Metric Limits and Fits,” ANSI B4.2-1978. It's in Machinery's Handbook as well. Nominal dimension doesn't mean it's the target to machine to.

    Sorry you missed it but the print isn't wrong.

    I always have a dilemma deciding to leave these as is, use pure limit dimensions (.2823/2828) at the risk of losing the information of the intended nominal size or use the fit class (H7/s6 for example) which runs the risk of having the machinist have NO idea of what that means or using the wrong values when looking it up.

    Cheers

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    This is a problem?
    Bob

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    Unfortunately, the print describes a functional part, its not to make the machinist job easier.

    I've had a lot of prints that left me scratching my head.. Why???? would they dimension the
    print like that, that doesn't even make sense... And then on occasion I got to do the assembly,
    and then it all clicks and makes sense...

    I got bit on one the other day.. AND I shipped it.. The print had some ± tolerances, and a
    +.030 -0.. And one dimension that just had "dim +.020". Since it didn't have a -.000, I didn't
    even give it a second look and assumed ±.020 since there was only one # after the dimension.

    I was even writing my measured dims right on the print.. Right next to the dimension. I
    never saw that it was only +.. Quick easy fix. I was minus one or two.

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    The purpose of a print is to communicate the part requirements to the machinists and inspection. Anything that makes it more difficult to machine and inspect is suboptimal.

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    Quote Originally Posted by mhajicek View Post
    The purpose of a print is to communicate the part requirements to the machinists and inspection. Anything that makes it more difficult to machine and inspect is suboptimal.
    This is the way I view it too. If you're sending a print to a shop, why make it any harder for the shop to perform its job well?

    In cases Like this, the folks writing the standards should have been made to work in a machine shop for a year first.

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    Same here. Anything on the print that is making a guy sit there and think about it or do math unnecessarily is not a real good idea. That's adding time to the job for no good reason.

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    Just use the spec MIL-TFP-41. You'll never have another issue.


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