MSA (Measurement Systems Analysis) for gages used in final inspection

Z

z28tt

#1
Say you have a part with a tolerance of .010", being measured with a micrometer (.0001" resolution). The gage is only used for final inspection. You perform an Avg-Range MSA study, and see that the range between trials is either 0 or .0001", so the UCLr is very small (under .0001" and gage resolution) so anything other than zero shows up as out of control.

Folks have mentioned that using a larger sample size would result in a better %R&R, but I don't understand how. If the range across an individuals trials is consistent (0 or .0001"), is it just by adding more samples with a possible range of zero to drive down the Rbar? If Rbar goes down across all individuals, Rbarbar goes down lowering UCLr further. If the gage is strictly used for final inspection, is it OK to ignore the calculated UCLr?

The other question I'm wondering about is whether to use reference standards as the MSA parts vs actual parts. I realize it doesn't represent the "part" of the system since it's a standard, but it would seem to be a more accurate way of determining gage effectiveness w/o part variation muddying up the results (think tapered OD's, out of round, etc...). Seems that using parts would be the next step to see how GR&R compares between real parts vs standards...

I've probably read 40 or so threads here that have been very helpful already, and am absorbing the MSA 3rd Edition and formulas. Thanks for the insight!
 
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M

micsim

#2
You actually have to use the real parts to do an MSA and GRR with out them you will not include the part to part variations. The part to part variation, variation between operators, and gage readings make up the total variation of the system. The goal is to have a system that works with your real life situation, not a system that say work for one standard, one part, or say one almighty expert inspector.

Also, are you performing the MSA so the measurements are taken from the sample randomly and blindly by the operators? If the operators know what they got the first time or what the other operator measured, then you are introducing bias to the study.

Whether out of roundness or taper are a problem is another story that depends on the parts specifications. They could negatively impact your GRR and you may have to say mark a spot on the Diameter to measure to eliminate measurements take from different spots on the taper or different locations on the diameter.

I hope I helped a little. I am sure one of the experts will be along shortly to give you better advice

Good Luck
 

Miner

Forum Moderator
Staff member
Admin
#3
Say you have a part with a tolerance of .010", being measured with a micrometer (.0001" resolution). The gage is only used for final inspection. You perform an Avg-Range MSA study, and see that the range between trials is either 0 or .0001", so the UCLr is very small (under .0001" and gage resolution) so anything other than zero shows up as out of control.

Folks have mentioned that using a larger sample size would result in a better %R&R, but I don't understand how. If the range across an individuals trials is consistent (0 or .0001"), is it just by adding more samples with a possible range of zero to drive down the Rbar? If Rbar goes down across all individuals, Rbarbar goes down lowering UCLr further. If the gage is strictly used for final inspection, is it OK to ignore the calculated UCLr?
First question: Is a customer involved, or are you free to make an economically sound decision?

Here is the "By the book" comment: The first "test" that you perform on MSA results is to check the Range chart. If you do not have a minimum of 5 possible range values under the UCLR, the gage does not have sufficient resolution. and the other tests are invalid.

This is the practical, economic recommendation if no customers are involved to argue with you. You stated that your gage was used for inspection, not process control. The gage resolution is .0001 versus a tolerance of .01. This resolution is more than adequate for inspection. If all of your ranges are either 0 or .0001, you could safely ignore the range chart and move on to the other tests.

What was your %Tolerance (P/T Ratio)?

Final comment: If your gage is used for SPC or for statistical studies, I would not necessarilly make the same recommendation.
 
Z

z28tt

#4
First question: Is a customer involved, or are you free to make an economically sound decision?
There's always a customer involved! :) The customer's requirements are either to use their excel form (based on MSA 2nd Ed Avg-Range), or to a list of minimum requirements. I'm treating this as a fresh start, am ready to embrace MSA, and to make it as beneficial for us as possible (instead of doing just enough to satisfy customer reqs). <where's the fist pump icon?!!> :tg:

Here is the "By the book" comment: The first "test" that you perform on MSA results is to check the Range chart. If you do not have a minimum of 5 possible range values under the UCLR, the gage does not have sufficient resolution. and the other tests are invalid.

This is the practical, economic recommendation if no customers are involved to argue with you. You stated that your gage was used for inspection, not process control. The gage resolution is .0001 versus a tolerance of .01. This resolution is more than adequate for inspection. If all of your ranges are either 0 or .0001, you could safely ignore the range chart and move on to the other tests.
That's exactly where I'm at. If our machinists are holding parts consistently to a thousandth when there's wide open (i.e. .020") tolerance, I can't justify purchasing an expensive super-tool to measure process variation in between parts (when a simple caliper will do). We don't really run lots big enough to see tool wear (before the finish is junk), so it's basically a static process.

What was your %Tolerance (P/T Ratio)?
I'm doing a new MSA now (I don't trust our old data), and will have data in the next few days hopefully. Parts are about 20 lbs each and clumsy, so random sampling will get interesting. I think I'll have the operator sit at one table, and I just bring parts to him... Probably will follow 10 parts, 3 measurements, 3 individuals if our lot qty allows.

Final comment: If your gage is used for SPC or for statistical studies, I would not necessarilly make the same recommendation.
We do track some of the tighter features using a run chart that calculates CPK, but it's more of an easy way to log measurements than anything else. I've never seen the sawtooth profile of an insert wearing, operator changing it, and basically re-setting chart back to low run limit. I'd like to bring some more statistics-based decisions to our mfg flow, but I'm just getting my feet wet with MSA now and am focusing on final inspection (my domain). With small runs and open tolerances, it's tough for me to make a case on how better gaging for process control will save/make the company $.

We have 300 or so gages in our calibration system now, from calipers & mics to the CMM and about 10 shop guys with another few inspectors.

I'm betting the one gage we'll have trouble with is the Mueller Gage used for inspecting deep (in the part) ID grooves. It'd be a nice justification for a Zeiss CMM on the shop floor w/ a disc probe!
 
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