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# GRR and Set-Up Process

## GRR and Set-Up Process

(OP)
Hello All:

I am working on the set-up of a machining process in order to meet 0.30000" inner diameter with +/- 0.000080" tolerance. The plan is to measure the inner diameter with CMM. The following tasks were performed.

(1) Performed a repeatability study using a ring gage on the CMM machine. The "certified" size of the ring gage is 0.30000". CMM measured 0.300040" on average, which is 0.000040" oversize.

(2) GRR was performed on the CMM machine to measure the inner diameter. The % GRR tolerance is <30%.

Questions:

(1) Knowing that the CMM measured 0.000040" oversize, do I need to use a smaller machining bore so that the expected inner diameter is 0.000040" below my target (nominal of 0.30000")?

(2) In relation to # 1 above, since %GRR is <30%, which is acceptable, does it matter if the CMM measured 0.000040" based on the ring gage repeatability study.

### RE: GRR and Set-Up Process

Hello weisusu

1)For perform a GR&R.The test samples diameter should distribute equality over the whole Tolerance.And It is better to have 10 samples.So in your case,may be you need 10 samples size form 0.299992,.......to 0.30007 which are distribute equality between 0.30000(+/-0.00008)

2)%GR&R <30% means it is acceptable Measurement System but not Adequate. if %GR&R can <10% means that is a Adequate Measurement System.

### RE: GRR and Set-Up Process

Curious.

What item only gives 0.000080" tolerance?

### RE: GRR and Set-Up Process

Exactly how did you do the gr&r?  Did you calculate the gr&r using .00016 as your total tolerance?  Did you do it using the exact method and parts that you will be measuring in process?  I guess the reason I ask is that tolerance is really tight and the average cmm probably would struggle passing a gr&r with that tolerance.  What kind of cmm do you have?

As far as the ring gage, the ring gage will have some tolerance depending on what class it is, not too mention your cmm will have some measurement uncertainty.  Is the ring gage a plus tolerance?

Need more info but if I were to guess I would venture that the tolerance exceeds the capabilities of your measuring equipment.

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