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Thread: Dimensioning of hole from tolerances

  1. #1
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    Dimensioning of hole from tolerances

    Hi
    I hope you guys can help.

    I have two parts that should be bolted together. The tolerances of one of the parts are quite big and the holes for this part are fixed to 6. I need to determine the size of the holes of the other part in order to for them to fit together. I have attached a photo showing the tolerances.

    I am in doubt how to calculate the size of the hole, is this the correct way?
    D=211.5-210+3.5+3.5+6 = 1.5+2*3.5+6=14.5

    Thank you in advance
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    Last edited by Kelly Bramble; 07-11-2016 at 10:37 AM.

  2. #2
    Administrator Kelly Bramble's Avatar
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    The general equation is:

    Holemmc - Shaftmmc = Tolhole + Tolshaft + C

    Where:

    Holemmc = Hole at MMC size
    Shaftmmc = Shaft at MMC size
    Tolhole = Location tolerance of hole
    Tolshaft = Location tolerance of shaft
    C = Mating Clearance at MMC (if required)

    See: Mechanical Tolerance design and Calculators

    and specifically:

    Fixed Fastener Condition Tolerance Calculator
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  3. #3
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    Thank you for your reply!

    Okay, the shaft is M6 bolt, so I guess the tolerances are relative low for that, and the unknown hole size is drilled, so I'm going with +/- 0.1 mm. The 6 hole is a bushing with a thread, but I don't need to take account of that, right?
    However, I am unsure about the clearance. The distances between the holes for the two parts differ, so should the clearance be the tolerance of the distance plus the difference in lengths? For instance for the left side
    C = Toldist + Deltadist = 3.5 + (211.5-210) = 3.5 + 1.5 = 5

    So the equation becomes:

    Hole
    mmc = Tolhole + Tolshaft + C + Shaftmmc

    Holemmc = 0.1 + 0.1+ 5 + 6 = 9.7

    Is that correct?

  4. #4
    Administrator Kelly Bramble's Avatar
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    The equation is useful if using GD&T with datum structures or a fixed origin reference for all features considered. The physical distance between the holes is meaningless in the calculations and is only a consideration when determining manufacture-able tolerances.

    "C" or clearance is considered when all features are at MMC and the location of the features are at maximum allowed. This would ensure a clearance is present when an extreme condition of the as-manufactured tolerance assembly. Typically C is zero for a line-to-line fit.

    What GD&T standard are you using?
    Last edited by Kelly Bramble; 07-11-2016 at 09:29 AM.
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  5. #5
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    Hmm okay, this may be posted in the wrong subforum then. I am just trying to figure out how big the hole should in order for the parts to fit together with four bolts.

    If I were using any GD&T standard, I guess it would be one of the ISO standards.

  6. #6
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by tdog View Post
    Hmm okay, this may be posted in the wrong subforum then. I am just trying to figure out how big the hole should in order for the parts to fit together with four bolts.

    If I were using any GD&T standard, I guess it would be one of the ISO standards.
    You're in the right sub forum... Based on your questions and responses you need mechanical tolerancing design and analysis training..

    Respectfully...
    Last edited by Kelly Bramble; 07-11-2016 at 10:37 AM.
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  7. #7
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    Quote Originally Posted by Kelly Bramble View Post
    You're in the right sub forum... Based on your questions and responses you need mechanical tolerancing design and analysis training..

    Respectfully...
    No offense taken. I'm a student, so yes, I do need mechanical tolerancing design and analysis training seeing as I have had none so far.

    Can you help me out with my question though?

  8. #8
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by tdog View Post
    Can you help me out with my question though?
    I already did - the answer is there...

    This is a engineering discussion forum and my contributions will be limited to short and to the point answers. I simply don't have the time nor desire to fully train, design, calculate, engineer, engage in meetings, pick materials or stop what I'm doing for days to solve every bodies design challenge in this format. I have make a living...

    I've been delivering GD&T training as well as Design for Manufacturability training in industry for 16 of my 34 year career. Much of that training also includes mechanical design practices and method.

    It is interesting that your education has yet to cover these skill sets yet you are attempting to use them.. But then again - this is why I have customers..
    Last edited by Kelly Bramble; 07-12-2016 at 08:22 AM.
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  9. #9
    Administrator Kelly Bramble's Avatar
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    One more hint....

    Design to function and let manufacturing comply with the end item functional requirements.. See:

    Standard Tolerance Limits Fits ANSI B4.1
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  10. #10
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    Having read the material you supplied I am still unsure as how to take the misalignment between the two parts into account.

  11. #11
    If you want help employing GD&T on your drawings then we need a more complete drawing to start with. You need the distance from the datums to the holes to be called out with basic dimensions. Also please confirm if the bottom holes are tapped (fixed fastener condition) or if the two plates are attached with bolts & nuts (floating fastener condition). Once this information is provided it is relatively easy to calculate the size tolerances and positional tolerances for all the holes. Of course, you will have to have a datum scheme for both parts to do this. Also please provide the exact type of fastener to be used (needed to determine the LMC size of the clearance hole).

    floating fastener: H = F + T

    H = clearance hole MMC diameter
    F = Fastener MMC diameter (nominal size)
    T = clearance hole location tolerance at MMC (used on each hole the fastener passes through)

    fixed fastener: H = F + t1 + t2

    t1 = threaded hole location tolerance at MMC
    t2 = clearance hole location tolerance at MMC

    Note that if it is a fixed fastener condition then the tapped holes will need to use a projected tolerance zone.
    Last edited by Andrew T; 07-13-2016 at 09:07 AM.

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