How to select parts for a MSA Study

R

Rio BP Simbolon

#11
My Opinion. sometimes it's not easy to find 10 parts for MSA which represent their tolerance range, example at pierching process, which making hole with punch. Variation of its process depend on the punch.. and the diameter result is almost same with diameter punch, and we make punch diameter near to maximum spec....
In this condition we use sample which represent variation process.
 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#12
yes there are times such as tool wear where the initial process will set at a max (or min) position and the process variation at that level is relatively small.
In these cases we can't get an NDC calculation that has useful meaning. HOWEVER, we can plot the repeated measurements (either as vertical points for each part if using 3 readings or on a Youden plot if using only 2 repeated readings) vs the specifications and SEE teh amount of measuremetn error vs the tolerance spread and we can make a judgment as to the relative usefulness of the system.

Yes, it's subjective, but so is the 10%-20%-30% of the tolerance or some minimum NDC value. These 'acceptance limits' were subjectively set - they are not laws of physics.

We need to move beyond relying on 'following the rules' and checking some statistical output that says we met the goal or we didn't and begin to really think about our data and our processes.
 
U

unueco

#13
Re: How to select parts for a MSA study ?

Greetings!

I realize this discussion has been "dead" for quite a while. But it relates directly to some questions I have. So hopefully I can resurrect it.

Firstly....

Parts should be selected randomly such that they are representative of the full range and distribution of the process.
While this is the oft quoted advice concerning part selection, am I the only one who finds this to be a frustrating oxymoron? In the first place, if part selection is supposed to be "random" then how can I add a condition about the results of the selection? Secondly, how am I supposed to verify that the parts fit the criterion....oh, hey, I'll just measure them....but wait, I'm supposed to be verifying my measurement system ....so..???

Second.....
What is the actual effect of part variation on the results? I had a GRR that failed R&R at 78%. Just as an experiment, I added 0.02 to all the measurements from all the operators on the 10th part. The GRR results passed with an R&R of 3.08%. Why?

Assistance with understanding is greatly appreciated!

Thanks!!
 

Miner

Forum Moderator
Staff member
Admin
#14
Re: How to select parts for a MSA study ?

In the first place, if part selection is supposed to be "random" then how can I add a condition about the results of the selection? Secondly, how am I supposed to verify that the parts fit the criterion....oh, hey, I'll just measure them....but wait, I'm supposed to be verifying my measurement system ....so..???
Random as opposed to the frequent (and wrong) recommendation to select parts from the full range of the tolerance. The key is to understand how your process varies. Does it vary over each cycle of the machine, each batch of raw material, by shift? It it varies by batch of raw material, sample 1 part from 10 different batches, not 10 parts from a single batch.

Second.....
What is the actual effect of part variation on the results? I had a GRR that failed R&R at 78%. Just as an experiment, I added 0.02 to all the measurements from all the operators on the 10th part. The GRR results passed with an R&R of 3.08%. Why?
It depends on the metric that you use. If you are using %Tolerance, part selection has zero impact. If you are using %Study Variation, ndc, or %Contribution, the part variation is part of the calculation. Increased part variation will reduce the %Study Variation and %Contribution while increasing the ndc.
 

Golfman25

Trusted Information Resource
#15
Re: How to select parts for a MSA study ?

Random as opposed to the frequent (and wrong) recommendation to select parts from the full range of the tolerance. The key is to understand how your process varies. Does it vary over each cycle of the machine, each batch of raw material, by shift? It it varies by batch of raw material, sample 1 part from 10 different batches, not 10 parts from a single batch.
In my business that could take several years to collect enough parts (and even if we took a few from fewer batches, it could take a year). Now what?
 

Miner

Forum Moderator
Staff member
Admin
#16
If you have prior capability studies, use the variation from those studies to calculate % Process Variation.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#17
Re: How to select parts for a MSA study ?

In my business that could take several years to collect enough parts (and even if we took a few from fewer batches, it could take a year). Now what?
Be practical, take the parts from what you have. If there really is substantial variation between batches, use logic to determine if you need a quick check for every new batch that might be in a very different dimensional location that's when you did the first R&R. There is no need to wait.
 

Golfman25

Trusted Information Resource
#18
Re: How to select parts for a MSA study ?

Be practical, take the parts from what you have. If there really is substantial variation between batches, use logic to determine if you need a quick check for every new batch that might be in a very different dimensional location that's when you did the first R&R. There is no need to wait.
We try to be practical. But the MSA requirements seem to prevent this. Typically, the MSA requirement arises with a new part and a new. Then they tell us to use parts with variation. Well you'd need to mess up your perfectly good tool to do that, and sometimes it hard enough to get the thing going at first. Sometime is seems like an exercise in futility just to get the "right" number to "pass." There has to be an easier way.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#19
Maybe not an easier way, but the only ways: get a better relationship with your Supplier Quality people to invoke 'logic and reason' OR get new Customers. There are simply a few industries with a few Customers who have SQEs that are nothing more than 'check the box' people. Very frustrating yes, but in these cases I've found the only way is to find another job.
 

Golfman25

Trusted Information Resource
#20
Maybe not an easier way, but the only ways: get a better relationship with your Supplier Quality people to invoke 'logic and reason' OR get new Customers. There are simply a few industries with a few Customers who have SQEs that are nothing more than 'check the box' people. Very frustrating yes, but in these cases I've found the only way is to find another job.
It's not always the SQE people at the customer. Sometimes it's that third party auditor.

How about instead of a one size fits all mandate, use your brain and figure out the best way to accomplish your goal commensurate with the risks involved. But that would make too much sense.
 
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