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Thread: Hole Pattern with GD&T

  1. #1
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    Welcome Hole Pattern with GD&T

    Hello Guys,

    I'm new in GD&T topic so I would like to ask a bit help from the group!
    I have a task what I should solve. I have a rectangular hole pattern where I have two position tolerance. I'm a bit uncertain about the position tolerance with datums. I have no MMC or LMC in this case.

    I have attached a picture about the task. Could somebody help me and could explain how can I define and calculate this pattern position?

    gd&t.png

    Thanks in advance for your help,
    Best Regards!

  2. #2
    Administrator Kelly Bramble's Avatar
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    The upper horizontal Feature Control Frame (FCF) specifies that relative to the datum reference frame (coordinate system) established by datum A (primary) and datum B (secondary) that each of the four as built holes axis independently from start to finish must fall within diametric tolerance boundaries of dia. 0.2 mm located at the basic dimensions. The position tolerance is applied at Regardless of Feature size (RFS) - no modifier.

    The lower FCF specifies that there are four diametric tolerance boundaries equal to dia. 0.05 mm applied at RFS that are located at the basic dimensions from each other as a pattern. This pattern of four tolerance boundaries can move left, right, up and down but must move as a pattern. Each of the as-built hole features from start to finish must fall with their perspective tolerance cylinders.

    Both upper and lower Feature Control frames apply simultaneously.

    The holes are to be as manufactured at dia. 2 mm not to exceed a tolerance specified by H7 - probably ~ per. ISO 286. I have not seen the whole engineering drawing so I'm guessing.

    The above applies to ASME, ANSI and ISO dimensioning and Tolerancing standards.
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  3. #3
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    Thanks for your help. The situation is clear with one exception. This is the upper horizontal frame with A-B datums.
    If I thinking well the A-B base are symmetry axes of the part (because of the datum is on the dimension).
    I don't understand exactly how can I interpret this situation.
    The pattern can translate up/down-left/right but can't rotate and the distance between the holes should be 8 mm always?

    My task with this FCF is the calculation of hole pattern and decide it's in the tolerance or not.

  4. #4
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by gray_beard View Post
    Thanks for your help. The situation is clear with one exception. This is the upper horizontal frame with A-B datums.
    If I thinking well the A-B base are symmetry axes of the part (because of the datum is on the dimension).
    I don't understand exactly how can I interpret this situation.
    The pattern can translate up/down-left/right but can't rotate and the distance between the holes should be 8 mm always?
    My task with this FCF is the calculation of hole pattern and decide it's in the tolerance or not.
    For the upper FCF - Datums A and B are actually Datum planes not axis and establish an origin about the center of the part. Datum A and B would be established most-correctly by collapsing two parallel surfaces onto the perspective surfaces. In theory one will get three areas of contact on one of the collapsing planes and one area on the opposite. From those two parallel planes (surface blocks) a center plane is established and used to measure distances. Datum B is established separately but the two collapsing datum B planes must be held perpendicular to the already established datum plane A to define that center origin. So, the origin is a cross down the center of the part where 0,0 is in the middle both vertically (datum plane A) and horizontally (datum plane B). Measurements are taken or established from that center origin. Remember that the four tolerance boundaries related in the upper FCF are at a fixed location (basic dimensions +/- 4 mm) and the hole features derived axes are compared the fixed location tolerance cylinders.

    Lower FCF is a pattern of four tolerance cylinders located at the basic dimension from each other. Each tolerance cylinder is dia. 0.05. Since there are not any restraining datums the pattern is not located by distance or orientation (pattern can rotate) on the block. To measure one would establish one of the holes as an origin or zero and then by orienting the part appropriately measure the shortest distance from that zero hole to the other three. A little trig may be needed.

    Each hole axis (when not using a CMM) can be established by inserting the largest gage pin that does not wiggle or bind into the hole. From that gage pin a center hole axis is established and measured from or to the other holes.
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  5. #5
    Administrator Kelly Bramble's Avatar
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    You should consider referencing one or both of my GD&T books. You can get a hard print or with a premium membership access these on this website. The premium membership gets you more stuff than you can use or read in a lifetime.


    See:
    Print version

    ASME Y14.5-2009 Geometric Boundaries II

    Geometric Metrology (Dimensional Inspection)


    Premium Membership access


    ASME Y14.5-2009 Geometric Boundaries II


    Geometric Metrology (Dimensional Inspection)
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  6. #6
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    Really thanks for your help! I think I will try to registry for the membership in the future.

    Otherwise based on your explanation and many reading I have made calculations about this problem.

    gd&t.png

    1., With the Pythagorean theorem the difference: Tolerance [0.05]

    Measured position Difference Tolerance (0.05)
    X Y
    4.050 3.990 0.0510 Out of tolerance
    3.930 -4.050 0.0860 Out of tolerance
    -4.050 -3.980 0.0539 Out of tolerance
    -3.980 4.020 0.0283 In tolerance

    2., True position - Location of a feature: Tolerance [0.2|A|B]

    Based on this picture:

    True-Position-Formula.jpg

    1 0.10198 In tolerance
    2 0.17205 In tolerance
    3 0.10770 In tolerance
    4 0.05657 In tolerance

    What do you think, can it be right?

  7. #7
    Administrator Kelly Bramble's Avatar
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    Looks like you're relating the lower FCF to the datum origin A|B as opposed to just the pattern.

    So, no your calculations are wrong..

    You might see:

    https://www.engineersedge.com/calcul...sition_pop.htm
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  8. #8
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    What did you think about the wrong relating?

    I have the same result with the calculator what you linked (otherwise the calculator is so cool):

    gd&t_calculator_result.PNG

    Based on this calculator and my computation the first position:

    ------------------------------------------------------------------------------------------------
    Designed position Measured position Difference Tolerance (0.2|A|B)
    X Y X Y
    1 4.000 4.000 4.050 3.990 0.10198 In tolerance
    ------------------------------------------------------------------------------------------------
    Designed position Measured position Difference Tolerance (0.05)
    X Y X Y
    1 4.000 4.000 4.050 3.990 0.0510 Out of tolerance
    ------------------------------------------------------------------------------------------------
    Measured position Summarize
    X Y
    1 4.050 3.990 The measured point does not meet the two tolerances
    ------------------------------------------------------------------------------------------------

  9. #9
    Administrator Kelly Bramble's Avatar
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    How are you establishing your origin of measurement for the lower FCF specification?
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  10. #10
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    In the drawing the lower FCF is without datum, so I have used the theoretical position of the holes one by one. After it with the Pythagorean theorem I have calculated the distance. With this method I get the same result like the calculated radius offset in calculator.

  11. #11
    Administrator Kelly Bramble's Avatar
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    theoretical position of the holes one by one
    That's roughly what I thought, which is very wrong...

    I don't mind helping with short and to the point dimensioning and tolerancing questions however I'm not going to do extensive training/mentoring on this forum platform. You should seek out either self paced or formal training on measurement metrology methods using open setup as well as CMM's as required. Also, you should do the same with ASME Y14.5 GD&T and if applicable ISO 1101 G&T.

    I wish you the very best..
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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