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Thread: How to Interpret Geometric Tolerance and Dimensional Tolerance

  1. #1
    Associate Engineer
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    Apr 2019
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    How to Interpret Geometric Tolerance and Dimensional Tolerance

    Hi,
    I'm confused with the Geometric Tolerance and Dimensional Tolerance. I have read many articles but it had not solved my doubt. I'm confused on how to put tolerance on a shaft and hole with Geometric Tolerance and Surface finish of 0.2 micrometer.

    This is My Issue:

    I have to design sliding fit of 15.988+/-0.005 (Shaft) and 16.006 +/-0.007 (Hole) dimesional tolerence that gives a clearance of 0.0298 and allowance of 0.006. Which provide me a Sliding fit.

    When i add geometric tolerance to this with cylindrical(0.002), circularity(0.002) and parallelism(0.0025)

    with

    Surface Finish of 0.2 micrometer

    MMC of shaft will be -> 15.993(MMC)+cylindrical(0.002)/circularity(0.002)+parallelism(0.0025)+Surface Finish(0.0002)= 15.9977mm

    OR

    MMC of shaft will be -> 15.993(MMC)+cylindrical(0.001)/circularity(0.001)+parallelism(0.00125)+Surface Finish(0.0001)= 15.99535mm

    OR

    MMC of shaft will be -> 15.993(MMC)

    Similerly for LMC.

    I'm confused on how to calculate whole.

    Please help me with this doubt.

    Please see the attachment for more clarification.
    Attached Images Attached Images
    Last edited by adirajck; 04-20-2019 at 01:43 AM.

  2. #2
    Administrator Kelly Bramble's Avatar
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    Regardless of which dimensioning and tolerancing standard you are referring to (ASME or ISO) the surface variations allowed by the surface finish specification will always be contained within the geometric (geometrical) and limit of size tolerance variations specified.

    Therefore, the 0,0002 surface variations are ignored from a size tolerance or boundary calculation. Tolerance specifications, limits of size and geometric (geometrical) have hierarchical relationships.

    To answer your question in regards to the outer boundary calculations limits of size + geometric (geometrical) tolerance variations...

    What dimensioning and tolerancing standard are you referring to? Is Rule #1 for ASME invoked also called the envelope principle or in the case of ISO and the Envelope Requirement is not specified?

    ASME or ISO standard and what year for clarity?
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
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    Apr 2019
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    Thank You for Your Valuable Reply.

    Sorry, i didn't mention, I am following ISO Standards. For fits i am referring to ANSI B4.2.

    I've not been in Precision Product Design for long. Many naive doubts are stuck in my head.

    So, Basically, Geometric Tolerance should be within the Limits of Size. Which leads to my 3rd assumption [MMC of shaft will be -> 15.993(MMC)].

  4. #4
    Project Engineer
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    May 2015
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    You have a .002 cylindricity and a roundness call out. That seems like double dimensioning to me.

    Could you just call out a .005 cylindricity and a surface finish on the basic shaft diameter and similarly .007 cylindricity plus surface finish on the hole and be happy?

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