How to make a MSA for an Attribute Gauge if the Pp=Ppk=1.33

N

Nixverstehikus83

#1
Hello Everybody:bigwave:,

I have to introduce a method in the company for attribute MSA. I have read a lot about the different methods like:
- Bowker Method (VDA 5)
- Kappa Method (MSA 3th)
- Analytik Method (MSA 3th)
- signal detection (MSA 3th und VDA 5)

Did anyone of you really use one of the method for showing that all the attribute Gauges that are in use are OK.
For example: I take 50 parts from the produktion because I wannt to make the Kappa-test for the attribute gauge that is in use to check the parts. I started to check each of the part three times with the gauge my result was that each of the part ist good.

The (problem) ist that the production capability ist not like in MSA 3th edition ist wirtten Pp=0,5 the capability ist Pp>1,33.:deadhorse:

So I can?t show if the gauge is good or not because I don?t have the bad parts or the parts in the uncertainty range that I need for the test. How did you deal with this problem?

My Opinion is that such tests cost a lot of time/money but dosn?t really help to improve the quality. A lot of people maybe just do the tests because they have to do them not because they thing it will help to improve the quality.

Sorry for my bad english I am from a "non speaking english" country. Thanks in advance for your answers.
 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#2
Your conclusion about an attribute study for a highly capable process being mostly usless is correct. If you do not have defective parts there should be no need to produce them just for balance in teh study. In fact often deliberately made parts can be mis-leading depending on what the characteristic is and how it is made.

However, it is good to verify that the gage will not falsely reject good parts.
 
N

Nixverstehikus83

#3
Hello :bigwave:,
Thank you for your answer. I hope that the auditors accept the fact that we don?t make a attribute study for those processes how have a good capability. Becaus we don?t have the parts for the attribute study.

How do you think about attribute study for sight checking. In the most cases there are possibilities for do an attribute study because its easier to manupulate parts for the test. But at least the test only show that the worker know how to distinguish good parts from bad. The test doesn?t show whether he ist able to do it all the day?
 

Kales Veggie

People: The Vital Few
#4
Hello :bigwave:,
Thank you for your answer. I hope that the auditors accept the fact that we don?t make a attribute study for those processes how have a good capability. Becaus we don?t have the parts for the attribute study.

How do you think about attribute study for sight checking. In the most cases there are possibilities for do an attribute study because its easier to manupulate parts for the test. But at least the test only show that the worker know how to distinguish good parts from bad. The test doesn?t show whether he ist able to do it all the day?
I might have missed something.

If you have a process with a high capability (also for the "visual" characteristic), then why is it required to do visual inspection and an MSA study?

I suggest that you review your process FMEA and re-analyze Severity, Occurrence and Detection.
 
N

Nixverstehikus83

#5
Hello,
this should be more a fundamental question. I have do some sightseeing test with some workers. They checked 30 parts in 40 minutes. The 30 parts consist of 25 good parts and 5 bad parts. Not one of the 10 workers found 100 % of the bad parts. I?m just asking myself whether it makes sense to doing the R&R test for subjective testing. I ve read in some books that if the worker make sight checking for a longer time he made some mistakes because he can?t concentrate for a longer time. So why should i do a attribute R&R when at least the workers can only find maybe about 80% oft the parts.

Or to say it in another words. Attribute R&R for sight checking ist just a short-term analyse. It makes sure that the worker now the defects and are able to find them. But it doesn?t show wheter it works for a longer period.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#6
...So why should i do a attribute R&R when at least the workers can only find maybe about 80% oft the parts.

Or to say it in another words. Attribute R&R for sight checking ist just a short-term analyse. It makes sure that the worker now the defects and are able to find them. But it doesn?t show wheter it works for a longer period.
Well, like most things this isn't a "once and done" exercise. Gage R&R is intended to be part of a system of quality control.

Why do it in the first place?
The first R&R informs us as to what improvements if any should be made to the measurement system. Often we find the gage isn't capable for it's intended use and we either need to get another gage or in the case of visual inspection improve the inspectors or even accept that visual inspection isn't the best control and add other detection and preventive measures to control the quality.

What to do over the long run?
All gages wear and lose effectiveness over time. Operators of complex systems as well as visual inspectors lose 'focus' over time. This is where SPC comes in to warn us when quality changes. One of the first steps in responding to a signal of degrading quality (or in the case of 'improving' quality when no known improvement has been implemented) is to check the measurement system.
 
K

KnoKsuKaO

#7
This topic interested me and I would like to join the discussion

My an additional question on the topic:

Attribute MSA is performed eg 3x in a row and 3 operators, which monitors compliance with such controller.

This method must be applied to all workers? OR just selectively?

Because we apply a visual inspection performed approx. of 80%.
 
N

Nixverstehikus83

#8
Hello,
I can only give my own Opinion to this subject.
As the sight checking is a purely subjective test method it must be performed with each appraiser how use the this test method.
I think that a risk analysis can help in some cases as a excuse when you don?t do the attribute g&g. When your risk analysis show that there ist no risk why then lost your time on such attribute g&g. Upon reversion you don?t need sight checking if there is no risk:confused:
 
N

Nixverstehikus83

#9
Hello Bev D :bigwave:,
I didn?t understand what you meaning with SPC. I know what SPC is but if you use attribute gauge or make sight checking(with people) then there is no SPC. For attribute Gauge this is not a problem because there were calibrated. So the calibration makes sure that we have a stable measurement process. But what can you do with spc when people doing the sight check?
 
Last edited by a moderator:

Bev D

Heretical Statistician
Staff member
Super Moderator
#10
If you plot your defect rate from the visual inspection on an Attribute Control Chart (e.g. p-chart, c-chart or u-chart) you will detect trends and shifts in teh defect rate (should they occur). Typically a first response to an out of control signal is to check the measurement system. This applies for both continuous and categorical (attribute) data. So if your inspectors 'change' it will show up in the control chart. In this way you do not have to perform recurring MSAs on the inpsectors.

Of course this only works when the process is producing defects.

In the case of a high capability process the simple occurence of defects is the signal to check.
 
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