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GRR on 100% dimensionals?

Discussion in 'Gage R&R and MSA - Measurement Systems Analysis' started by Ben Chapman, Sep 26, 2016.

  1. Ben Chapman

    Ben Chapman New Member

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    Hello,

    I have been asked the following question and am looking for answers. Since our 100% dimensional measurements (performed on the CMM) are on the control plan, do we need to perform a GRR for these? Thanks!
     
  2. Eric Twiname

    Eric Twiname Well-Known Member

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    FWIW...I would.

    Surely you can "bucket" or categorize a number of the dimensions into a single GRR...say, lengths between 0.5mm and 3mm, O.D's between 2-6 inches, etc.
    This might make it a bit easier to justify (and easier to do...)
     
  3. Bev D

    Bev D Moderator Staff Member

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    are you - or someone - thinking that because you measure each part (instead of a sample of parts) that a GRR is not needed? If so, the sample size of the inspection is not relevant to the applicability of a GRR

    Or are you asking about using about using the 'family' of similar characteristics for the GRR so that you don't have to perform a GRR on each individual characteristic?

    If so, families of characteristics can be utilized, however, you need to be cautious about how you group them. CMM's are sensitive to the type of characteristic (geometry), the access/touch off path (Spatial), the fixturing method, any within part dimensional differences that can be confounded with other factors, the type of material and the resolution as well as the size of the feature...I have seen two very similar parts (same shape and features just a different size) have completely different GRRs due to some of the factors I listed.
     
    Atul Khandekar likes this.
  4. ncwalker

    ncwalker Well-Known Member

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    Here's what customers typically want:

    1) Most often: Gage R&Rs on anything that measures a key characteristic. CMMs included. Your annual CMM calibration is NOT a Gage R&R. If you have customers that accept this instead of a Gage R&R, they are wrong and you are lucky.
    2) Not as often: Gage R&Rs on any gage mentioned in your control plan. Why? They don't always have the time to review them all.
    3) Very rarely: Gage R&Rs on anything used to measure the part, in the control plan or not.

    Other points:
    1) Regardless of what your customer wants, you should convince yourself your gages work. (Unless you are making the business decision to roll the dice and go with uncertified measurement systems in the hopes nobody notices).
    2) Bev is correct when she states that you can go with a family strategy on a CMM. For example, do the diameter with the tightest tolerance. If that passes, it is reasonably safe to assume all the other diameters will. With the warning that you have to be mindful of probe hits, etc. If your really tightly controlled diameter has 37 probe hits and everything else has 4, you may want to do one with 4 hits. At the end of the day, there is no cut and dried formula for figuring out what you should check. ALSO - you're measuring the parts and the results are in a computer anyway. So doing more Gage R&Rs isn't any more work once the data is electronic if you're clever.
     

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