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Thread: Standards on Inspection of GD&T parts

  1. #1
    Associate Engineer
    Join Date
    Oct 2021

    Standards on Inspection of GD&T parts

    I don't typically have to step-in to the role of a manufacturing engineer, but I got drag into one with the new job, because the QC/QA inspection was done haphazardly. The issue at hand was that the inspection of a cam surface profile and that of the reference datum flatness was done with way too few inspection points, and out of spec parts managed to snuck in.

    So, my question is... Is there any sort of standards that govern how the machine shop or the QC guys do their inspections per Y14.5? Like how many points of measurement they suppose to take to check if the surface is flat? I really hope they did more than 3. LOL It's mad to think that after all the work of doing the tolerance analysis and specifying the GD&T, someone can undermind all that by cheating on doing a low resolution inspection.

    I really don't want to spell out how to inspect the parts on the drawing, and seem to recall seeing some ASME standards on this some years ago, related to Y14.5, but can't remember what that may be. Anyone with more manufacturing and inspection experience know?
    Last edited by MZUNGU54; 10-05-2021 at 04:31 PM.

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    Well, I'd rather you get a premium membership and read my book Geometric Metrology, Dimensional Tolerances Inspection and Practices in Manufacturing Fundamentals --->

    But, I'll give you a short answer.

    The most correct answer is that all surface elements should be measured and verified to be in tolerance conformance to the specified geometric dimensions and tolerances. However, this is not practical in the real world. When using a CMM, the number of measurements points is a matter of confidence or repeatability of measurements actually taken. In general, parts can be categorized into three groups:

    1) Low (unknown) confidence in quality and repeatability.
    2) Medium confidence in manufacturing quality and repeatability.
    3) High confidence in manufacturing quality and repeatability.

    Ultimately, the inspector or quality engineer needs to decide where a particular part lies and make the minimum number of inspection steps to ensure tolerance conformance.

    General recommendations:

    For category 1, low or unknown quality and repeatability, the inspector should consider increments of measurements around 10 x of the specified geometric tolerance range. For example, for a flatness tolerance of .005”, increments equal to .005 x 10 = .050 could be made. This may seem like a lot of measurement points, and it is, however until an understanding of the repeatability of manufacturing as it relates to quality is made one must ensure dimensional and tolerance conformance.

    Category 2, measurement of not less than 50 x of the specified geometric tolerance range.

    Category 3, measurement of approximately 200 x of the specified geometric tolerance range.

    It is also recommended that quality organizations define a standard practice for all inspectors to follow in regards to number of required measurements points or data sets.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
    Join Date
    Oct 2021
    Thanks Kelly. Much Appreciate. That'll give me a starting point to get the incoming inspection point organized. I'll look into that book.

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