High value for Cp (i.e. low process sigma) - Is MSA required?

Q

Qaware

#1
Hello everyone

Cp is a measure of process variation. So if a process is in control and you have a high value for Cp (i.e. low process sigma), then you don't have to perform an MSA? At least not if you are interested in R&R as percent of total tolerance.

Is this correct or am I missing something... :confused:
 
Elsmar Forum Sponsor

Ron Rompen

Trusted Information Resource
#3
If nothing else, you would want to do your MSA to validate the measurement results that you are getting.

No point in having a Cp or Cpk of 5+, when your R&R is >75%.
 
Q

Qaware

#4
But in theory it's possible?

Ron Rompen said:
If nothing else, you would want to do your MSA to validate the measurement results that you are getting.

No point in having a Cp or Cpk of 5+, when your R&R is >75%.
I agree with you, but started thinking about this and did some calculations.
Let's say most of the process variation comes from the measurement system. In that case we can put

process sigma=sigma GRR.

Then Cp=(UTL-LTL)/6 sigma GRR

And GRR%TOL=6 sigma GRR/(UTL-LTL)

combining above gives GRR%TOL=1/Cp

I admit that you need to have a high Cp to say that the measurement system is good enough (Cp>10) for a 10% acceptance criteria, but this might be useful in some situations.
So with Cp>5 you have GRR%TOL<20%.

Please tell me if I made a mistake somewhere.
 
Last edited by a moderator:
Q

qualeety

#5
wrong assumption

Qaware said:
I agree with you, but started thinking about this and did some calculations.
Let's say most of the process variation comes from the measurement system. In that case we can put

process sigma=sigma GRR.

Then Cp=(UTL-LTL)/6 sigma GRR

And GRR%TOL=6 sigma GRR/(UTL-LTL)

combining above gives GRR%TOL=1/Cp

I admit that you need to have a high Cp to say that the measurement system is good enough (Cp>10) for a 10% acceptance criteria, but this might be useful in some situations.
So with Cp>5 you have GRR%TOL<20%.

Please tell me if I made a mistake somewhere.
since your measurment resolution should be 10% of the process limits can you explain how you can get process sigma = sigma grr? the remaining equations are moot point since your very first statement is improssible.

note: if the measuring resolution is greater than 10% of the process limits then you had an inadequate measuring system to start with.....no GRR need!!
 
Q

Qaware

#7
Not sure we are talking about the same thing here?

qualeety said:
since your measurment resolution should be 10% of the process limits can you explain how you can get process sigma = sigma grr? the remaining equations are moot point since your very first statement is improssible.

note: if the measuring resolution is greater than 10% of the process limits then you had an inadequate measuring system to start with.....no GRR need!!
Hi qualeety

By the above I make the assumption that you didn't read my first post in this thread? I am talking about the use of a gage for inspection, and not for process improvement. Big difference between R&R as % of total tolerance and as % of total variation. If a process is as capable as I stated above (Cp>5), it wouldn't be my top priority to redesign the measurement system in order to make further process improvement, centering is enough. :)

JSW05 said:
And where does this certainty come from, if not MSA?
Hi JSW05

Well, by this I mean that the probability, that the variation caused by the measurement system is larger than total process variation, is small. And therefore the worst case scenario would be that they are of equal size.
I would not bet my life that this assumption is correct though, two sources of variation could eliminate each other. :tg:
 

Statistical Steven

Statistician
Staff member
Super Moderator
#8
Cp versus Variation

It frustrates me when people confuse Cp with variation. Cp and Cpk are measures of how good your SPECIFICATIONS are versus the inherent variability of the process. You can make Cp any number you want by changing your specifications.

MSA and other variance components analysis determine how much of the variability is attributable to the different sources. Therefore, a MSA should be done regardless of Cp. You should also know how much of your process variability is attributed to the measurement system.

Theoretically, when your Cp and Cpk gets large enough, you tighten the specifications. When you Cp and Cpk are too low, you improve the variability.
 
Q

Qaware

#9
Statistical Steven said:
It frustrates me when people confuse Cp with variation. Cp and Cpk are measures of how good your SPECIFICATIONS are versus the inherent variability of the process. You can make Cp any number you want by changing your specifications.
No one is confusing Cp with variation. Here is the full story. I work as a manufacturing engineer in the aerospace industry. The parts we manufacture are complex and tolerances are small. We are not in the position to change any specifications. So to me it's of little interest if specs are good or not, my job is to see to it that process variation is small enough i.e. Cp and Cpk are high. In other words low Cp, to me means that variation is to big and nothing else. Also the consequenses of failing to meet specs could result in accidents. This is why we want to make sure our inspection process is ok. Since we have a lot of different gages, reducing the need for MSA for processes with small variation would free resources to improve other processes, with low values for Cp and Cpk.
 
Last edited by a moderator:

Bev D

Heretical Statistician
Staff member
Super Moderator
#10
well I'll wade in a here a bit.

IF you have a controlled process and your process variation (Pp & Ppk) is small compared to to your tolerances, THEN the measurement error cannot be very large. Remember that the OBSERVED variation is the square root of the sum of the actual variation and the measurement error, hence a small observed variatoin (relative to the tolerances as stated above) = a small enough measurement error.

As for 2 sources of variation eliminating each other it does happen but not on every measurement: for the observed variation to be small compared to teh tolerances, the measruement error and the actual variation would have to be almost always opposite of each other for the OBSERVED variation to be small....

Now if it were me, there are several different scenarios for performing an MSA. If I had a stabel process and was fairly confident in my ability to detect any negative trending (SPC, etc.) and catastrohic shifts were unlikely AND the severity of the defect escaping was very low OR catastrohic shifts were catchable via some poke yoke device THEN I wouldn't perform an MSA. Otherwise if the severity was high enough and catastrophic shifts were possible I would do the MSA to determine if the gage would detect parts at the spec limit and then I would also most likely guardband.

Certainly I wouldn't put this process on the top of my problem solving priority list if there wre others with worse process performance.

As for teh comment concernign reducing tolerances for very good processes - *I* would think twice about it. If there were evidence that the specs were too loose (field failures that werein spec) I would change them - regardless of the process variation actually. If there were no evidence of 'too loose' specs and their was engineering logic or data supporting their validity then I woudl not change the limits. To me tolerances and processes performance are - and should be - separate things.

There are some who feel that if your performance gets better, you should automatically decrease your tolerance - but if decreasing the tolerance has no value add to the performance (and to the customer - why do it? If we automatically decreased the tolerances on well perfromign processes, we get in the endless loop of having to now improve our Performance to get the desired Ppk - which has tightened up over the years - then we tighten our tolerances then we improve our performance and so on as we fly in ever decreasing concentric circles until we fly up our own...self.
 
Thread starter Similar threads Forum Replies Date
Y Poor Gage R&R Results - High R&R result and poor ndc value Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 5
D High order value suppliers - Analysis of Supplier Data 8.4d ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 6
D Using a Value Stream Map as High Level Process Map Process Maps, Process Mapping and Turtle Diagrams 5
D High level understanding of EUDAMED EU Medical Device Regulations 3
S Complexity Rating - CB adding another audit day for "high complexity" AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 6
D Supplier Quality level category help - high level ISO 13485:2016 - Medical Device Quality Management Systems 6
R DFMEA/PFMEA mitigation of high severity (9-10) in low volume products IATF 16949 - Automotive Quality Systems Standard 1
T How to justify this High %Tolerance Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 3
J Production Line Test Brasil - High Voltage Dielectric Strenght Test Other Medical Device Regulations World-Wide 5
S High voltage testing - ISO 17025 - 7.2.2 Validation of methods and 7.3 Sampling ISO 17025 related Discussions 3
R What is meant by Use of COMPONENTS WITH HIGH-INTEGRITY CHARACTERISTICS in ME EQUIPMENT (clause no 4.9) IEC 60601 - Medical Electrical Equipment Safety Standards Series 3
D Performance of high shear mixer (or rapid mixing granulator Qualification and Validation (including 21 CFR Part 11) 4
Q % Study variation low, % tolerance high - GR&R Interpretation help Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 3
N Heavy tare to simulate high load in a routine sensitivity test of a floor scale? General Measurement Device and Calibration Topics 3
B HIGH QA Software - Auto ballooning software Quality Assurance and Compliance Software Tools and Solutions 2
F 5520A High Performance Multi-Product Calibrators General Measurement Device and Calibration Topics 0
N Use part of high risk device for establishing low risk device EU Medical Device Regulations 0
M Informational EU – Candidate List of substances of very high concern for Authorisation Medical Device and FDA Regulations and Standards News 0
M Informational From RAPS – Another Notified Body Bows Out Ahead of EU MDR: ‘Investment Too High’ Medical Device and FDA Regulations and Standards News 2
M Sampling Plan for Alumin High Pressure Die Castings Manufacturing and Related Processes 0
M Informational EU – Draft Functional specifications for the European Database on Medical Devices (Eudamed) – First release (High(1)) to be audited Medical Device and FDA Regulations and Standards News 0
I DOE: High variance and small effects in Minitab Using Minitab Software 1
S High warpage and negative shrinking in PPO + PS blend Design and Development of Products and Processes 0
Sidney Vianna High number of certificate suspensions in the IAQG OASIS database AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 14
Paul Simpson Informational The role of Annex SL - High Level Structure of ISO MSS's - Revision Update February 2019 ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 26
E High level structure - Planning and operation control Occupational Health & Safety Management Standards 2
M Process Capability in a High Precision Environment Capability, Accuracy and Stability - Processes, Machines, etc. 7
S Opinions on the "Best" CMM for High Accuracy Machining General Measurement Device and Calibration Topics 9
O Is the Quality Objective for Company-wide Training as >90% too high? Training - Internal, External, Online and Distance Learning 4
N Definition Needed: High Level Acronym Buzzword Definitions, Acronyms, Abbreviations and Interpretations Listed Alphabetically 9
N Timing for Closing High FMEA RPN Items FMEA and Control Plans 4
M ISO 13485 and the High Level Structure (HLS) ISO 13485:2016 - Medical Device Quality Management Systems 0
P Is the next revision of ISO 15378 following the High Level Structure? Other ISO and International Standards and European Regulations 5
P Accounting for Variability on High Side of Specification Reliability Analysis - Predictions, Testing and Standards 1
J Uncertainty budget for IR thermometry at high T - 1000 - 1500?C Measurement Uncertainty (MU) 1
G Stable, Predictable, Control - High Volume Mfg Statistical Analysis Tools, Techniques and SPC 9
N Microsoft Word - How to make high-light go away after text is put in Excel .xls Spreadsheet Templates and Tools 5
J Meeting Feasibility Requirement - High number of part quotes IATF 16949 - Automotive Quality Systems Standard 6
C Is there a standard for HALT (High Accelerated Life Testing)? Reliability Analysis - Predictions, Testing and Standards 15
Hershal This week in the High Desert Coffee Break and Water Cooler Discussions 14
B Determing Machine Capability in a High Mix, Low Volume Sheet Metal Shop Manufacturing and Related Processes 4
Y How to cut off Extremely High and Low Data Statistical Analysis Tools, Techniques and SPC 2
C Suggestions on Sample Plans for High Volume/Low Cost/Low Risk Components Inspection, Prints (Drawings), Testing, Sampling and Related Topics 2
F High Level Disinfection Risk - Neonatal CPAP Device EU Medical Device Regulations 1
M Viscometer MSA / GR&R failure - High Variability Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 11
2 High-tech medical equipment to reduce medication errors Medical Information Technology, Medical Software and Health Informatics 5
M Differences between High, Medium, Low Risk Suppliers Supplier Quality Assurance and other Supplier Issues 5
K Probability of Occurrence Ranking for Low Volume Manufacturing vs High Volume FMEA and Control Plans 11
M Quality issues in a manufacturing plant - High-pressured solvent spray cans Quality Manager and Management Related Issues 2
T Mercury is covered under RoHS but is not a substance of very high concern (SVHC) ? RoHS, REACH, ELV, IMDS and Restricted Substances 2

Similar threads

Top Bottom