The meaning of a size dimension - Page 2
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  1. #21
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    Quote Originally Posted by daredo222 View Post
    So you put a piece of thick wall tube in a lathe. You turn the outside, lets say it's 8" dia. and it's round and parallel. Using a ball end mic. to measure with you bore the i.d. until the radial thickness is .5003. Cut a piece out of that tube, at any point it is .5003 thick. Does it get past inspection?
    No because we are talking about flat plate

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    Quote Originally Posted by daredo222 View Post
    So you put a piece of thick wall tube in a lathe. You turn the outside, lets say it's 8" dia. and it's round and parallel. Using a ball end mic. to measure with you bore the i.d. until the radial thickness is .5003. Cut a piece out of that tube, at any point it is .5003 thick. Does it get past inspection?
    Cut a 5 degree slice worth of OD angle out of that and maybe.
    Cut 1 degree and probably.
    Cut a 0.1 degree pie slice and yes.
    Actually a 180 degree slice is still inside which is why you need to spec more than just a thickness measurement.
    Bob

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    Wonder if the op is gonna come back...last activity was when the thread was made.

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    Quote Originally Posted by plastikdreams View Post
    Wonder if the op is gonna come back...last activity was when the thread was made.
    Probably not. Either someone was pulling his leg or he was pulling ours. With "members" like that the first thing I do is look at their profile.

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    Quote Originally Posted by Gordon B. Clarke View Post
    Probably not. Either someone was pulling his leg or he was pulling ours. With "members" like that the first thing I do is look at their profile.
    So do I but some get upset about it.

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    Thickness may not be less than .5000, nor greater than .5006 at any point regardless of how it's measured.
    ASME rule 1 (comes in various flavors, but all standards cover this) states that, unless otherwise specified, all features of size, also control form.

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    If the customer checks it with a micrometer it would check less than the high and more than the low..
    If the customer checks it an a surface plate with an indicator and two stacks of JoBlocks it would it would check less than the high and more than the low.

    Checking method can not be assumed..

    *Yes I should have said at or less than the high and at or more than the low.. my bad. Yes that might give another bunch of millionths to make the job easier, but getting too close and the customer may not be able to measure to 6 - 12 or so millionths and reject the job due to his lacking measuring ability or handling with hot hands.

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  12. #28
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    Quote Originally Posted by Gordon B. Clarke View Post
    Probably not. Either someone was pulling his leg or he was pulling ours. With "members" like that the first thing I do is look at their profile.
    Chaotic Bliss was correct. Since 2009 the ASME std contains a rule that flatness is implied at MMC of a thickness tolerance. So a bar that is .5000-.5006 must be perfectly flat at .5006. Flatness varies as thickness decreases. That's the rule.

    If you don't like it, you need to add "Independency" (a circle with the letter I inside it) adjacent to the dimension.

    The OP was correct in his belief that the size tolerance implies form at MMC.

    Edit:
    What makes me crazy about this is that my company doesn't use it. We are still using the 1994 ASME std where a thickness tolerance is just thickness. So the casual observer won't be able to tell unless they look pretty hard (at the drawing border or engineering notes). So engineers using the new std better yell about it or they aren't going to get the parts they thought they were specifying.
    Last edited by AdamC; 06-19-2019 at 04:13 PM.


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