Gage R&R for Uni-Micrometers on a rod or bar OD (Outside Diameter)

C

chasf

I have been attempting to get good results for a Gage R&R but without success. I am using Mitutoyo Uni-Micrometer digital resolution .00005. These are micrometers that can use either a flat anvil or a pin. I am measuring the OD of a .2181 (.2195 / .2165) rod or bar to complicate this and the reason for using this type of gage is this rod is bent to a 4.890 radius and also shot peened. I have to measure on the inside of the radius to confirm the diameter has remained in tolerance after we bend it to the specified radius. I have marked a reference point so that the inspectors will measure in the same are of the rod and have given instructions to use the friction thimble to tighten the gage.
I have performed this R&R twice the first time we had a 90.95 then with more instruction we had 73.68. This product is for a medical device implant not automotive but I would like to get the results to less than 30%. That could be a tall order. I could provide a copy of the R&R and would appreciate any comments or advice you might have.
 

Miner

Forum Moderator
Leader
Admin
Re: Gage R&R for Uni-Micrometers on an OD

If you could attach the results, we can better assist you.

With the bending and shot peening, it is very possible that you have that much within-part variation (variation in form).
 
D

David DeLong

Re: Gage R&R for Uni-Micrometers on an OD

Are you referencing your R & R study to the process or specification width?

If you are referencing the values to the process, then you must make sure that the samples used for the measurements are taking the full range of the process otherwise there will be an extremely high R & R value.

The higher your Cpk value, the tougher it is to achieve the 30% gauge value. I alway love to use the specification width but the latest AIAG MSA standard states that the readings should be relative to the process unless the specification width is smaller than the process width.

You have a hand held gauge, the readings will not be taken in the same spot every time even if you mark the spot on the part.

Using the thimble at the end is always difficult too. Should each Operator take 3 clicks or 2 clicks? How fast are the clicks taken? Tool and Die Makers and other tradesmen never use the thimble but use their "feel". Again, this will contribute to the high R & R value.

The third difficulty using this hand held gauge is the method of taking a reading. Readings must be taken perpendicular to the axis at the centre of the rod. If one uses a flat anvil, you will achieve the rod centre but you may read a higher value since you are on an inside radius. If you use a ball micrometer, the could read the actual size but would lose the centres.

I don't think that you will achieve the 30% using this instrument in this situation. The only suggestion would be to make a fixture of some sort and then use a digital indicator with a spherical contact and, maybe, a setting block.
 
C

chasf

Re: Gage R&R for Uni-Micrometers on an OD

Here is a copy of the better of two R&Rs. I would say that the samples used reflect the normal range of variation.
 

Attachments

  • Copy of 461 study 2.xls
    209.5 KB · Views: 229
M

martin elliott

Re: Gage R&R for Uni-Micrometers on an OD

Dave has it right in that the part to part variation Average 0.00026 - Absolute 0.00080 is not very significant to 0.003 tolerence.

Therefore the error in method is magnified i.e. you make the parts too good for the measurement system. I guess your CPK is very high in this case.

Since you are not automotive, are you tied to use GR&R and not able to use P/T Ratio, which in this case is 17.52%.
 
D

David DeLong

Re: Gage R&R for Uni-Micrometers on an OD

I agree with Martin, use the R & R value as a percentage of tolerance range and everyone will smile.
 
C

chasf

Re: Gage R&R for Uni-Micrometers on an OD

I am new to gage R&R and capability studies so basically are you both saying that if more of the part tolerance range was being used the R&R would come out better? Thank you by the way for your much needed help.
 

Kales Veggie

People: The Vital Few
Re: Gage R&R for Uni-Micrometers on an OD

If you are using the gage for "acceptance" of good product, your micrometer will suffice (at 17%).

If you use this micrometer for process control, the process may be overcorrected, since the micrometer can not discriminate the process variation (at 90%). One solution is using a relative measurement method with a master ring or pin and measuring the deviation from the master.
 
Top Bottom