Process Standard deviations and equivalence of Cp & Cpk

A

anrappa

#1
Hello Ladies & Gentlemen

By principle of natural tolerancing, the product tolerances are to be so selected that they lie within +/- 3SIGMA of the mfg process

My questions are :

1. Is a table / database giving the standard deviations of various processes(specifically machining processes)available.If yes, could somebody give me the reference/links.

If no, do we have to arrive at the standard deviation of various processes only thro analysis of past data.Could sombody clarify.

2. It is said that cpk = 1.67 implies cp = 2.00. How is this arrived at.

Regards::confused:
 
Elsmar Forum Sponsor
D

Dave Strouse

#2
Don't believe it exists

Thoughts on your questions-

(1) I don't think you will be successfull in finding any tables for process standard deviations. There are so many factors, I can't imagine they would be tabulated.

How are you going to use it anyway? I believe you will need to develop this data yourself.

(2) WHO says.
I don't understand what in the world is meant.
Cp is (USL-LSL)/6s.
Cpk is closest or minimum of (USL- xbar/3s, Xbar-LSL/3s). The relationship of Cp to Cpk is Cpk =Cp(1-k) where k (|m-u|/(USL-LSL)/2 and m = (USL+LSL)/2 or the midpoint. But 0 to 1 is the range for k. So Cpk can be almost any value in relation to Cp.
The statement as I read it says that a Cpk of 1.67 always has Cp of 2. One counter example should disprove this.
Consider where LSL is 0 , USL is 10 sigma . Midpoint of spec is at 5 sigma. Cp is 10/6 or 1.67. Since a distribution could be centered on the mean i.e. k=0, we also have Cpk of 1.67.
Let's approach it another way. When will cp of 2 equal cpk 1.67?
From the formula above 1.67 = 2(1-k) or k =0.165.
So only at the k=0.165 value will the statement be true. Don't see how it implies anything remarkable.
 

Atul Khandekar

Quite Involved in Discussions
#3
anrappa,

There are no 'standard' values for process std. deviations for manufacturing processes. In any case if you set the tolerance limits 'within' your +/- 3 Sigma limits what would be the process capability? Tolerance represents Customer's voice and process spread is the voice of the process. You will have to study your individual process variation and review tolerances only when it is just not possible for the process to produce within tolerance. I will quote from Grant:

"..specification limit is, of course, the service need of the part. This is not primarily a statistical matter....It often happens that the service needs can be judged more accurately with the aid of statistical methods....There is no use specifying desired tolerances on any quality characteristic without some prospect that these can be met...."

You could determine the variaton by conducting experiments and probably tell the designers what you can achieve. If you best effort does not meet the service requirement, I suppose you'll have to look for a better process.

Cpk=1.67 DOES NOT automatically imply Cp=2. This should be apparent from the formulas used to compute these indices as well as from a simple diagram that explains Cp/Cpk. Look at :
http://www.symphonytech.com/articles/processcapability.htm

Cp is (Spec width/process width).This does not take into account the centering of the precess. Cpk does that. (As Dave has already explained)

I think you are confusing this with something else -like a six-sigma process has Cp=2 etc?

-Atul.
 
#4
That's it..

Yep that's it
Cp is (Spec width/process width).This does not take into account the centering of the precess. Cpk does that. (As Dave has already explained)
So, simply put you can have a process that is centered way outside tolerance limits and still get a good Cp value, because the spread is good. The Cpk value, on the other hand would be very poor indeed.

/Claes
 
Thread starter Similar threads Forum Replies Date
D Using "Particle Size Standard" templates as gauges - How to avoid giving a gauge # while using for process control? General Measurement Device and Calibration Topics 2
T Where in the ISO 9001 standard it asks for process based auditing? Process Audits and Layered Process Audits 6
M Process Records requirements for TS16949 standard IATF 16949 - Automotive Quality Systems Standard 6
A Standard Root Causes in a process orientated engineering company Problem Solving, Root Cause Fault and Failure Analysis 17
V Standard for Metal Stamping Deburring Process APQP and PPAP 5
M Change Control Process - Deviation from Standard for Parts Document Control Systems, Procedures, Forms and Templates 1
H Is there a Standard on the Reuse of Resin in Extrusion Process? ISO 13485:2016 - Medical Device Quality Management Systems 1
K Process Interaction Model in ISO 9001 Standard Document Control Systems, Procedures, Forms and Templates 6
J Process Control/Standard Deviation Question on a One Sided Specification Capability, Accuracy and Stability - Processes, Machines, etc. 6
Z Reduced Sampling based on Process Capability (AS9100 Standard) Inspection, Prints (Drawings), Testing, Sampling and Related Topics 3
B Declining Standard of QMS Certification process ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 0
L Standard for process control level depending on gravity (severity)? FMEA and Control Plans 7
D Industry Standard or rules or best practices pertaining to Automotive Process Flows? APQP and PPAP 3
Sidney Vianna New ISO/IEC standard: quality of IT system / software engineering life cycle process Other ISO and International Standards and European Regulations 1
5 Process based Internal Audits, instead of auditing to the elements of the standard Internal Auditing 21
H Design of a Process - Standard Work with no Layout Design and Development of Products and Processes 7
J Does GM require an Inspection Standard in the APQP process? APQP and PPAP 3
C Standard for TS 16949 process audits? Process Audits and Layered Process Audits 7
L Standard Deviation - Process Output is Stable and Normally Distributed Statistical Analysis Tools, Techniques and SPC 6
I ISO 13485 Practical Internal Audit Checklist - Standard vs. Process Based Internal Auditing 26
V Planning Internal Process Audits to the new standard - ISO 9001:2000 Process Audits and Layered Process Audits 7
Marc SPICE: ISO Standard for Software Process Assessment Software Quality Assurance 1
J Process FMEA Template with examples - Cold and Hot Forged components FMEA and Control Plans 1
D Questions regarding process validation ISO 13485:2016 - Medical Device Quality Management Systems 5
D Question regarding ECO process, specifically for Life Science products and defining form fit and function ISO 13485:2016 - Medical Device Quality Management Systems 1
P Managing How PPAP Requirements are Communicated in the Manufacturing Process and to Suppliers APQP and PPAP 4
I Control Plan (Product/Process specification/ Tolerance) acceptance FMEA and Control Plans 27
L Process changes and biocompatibility (ISO 10993-1) Other Medical Device Related Standards 1
A Validation of Forced Aeration Process ISO 13485:2016 - Medical Device Quality Management Systems 3
R Error Proofing Label process Manufacturing and Related Processes 4
B New Release of CQI-12 Coatings Process (Revision 3- Release date- 7/2020) Customer and Company Specific Requirements 2
D Layered Process Audits - FCA 9.2.2 - Exemption Clause? IATF 16949 - Automotive Quality Systems Standard 4
B ISO 11607-2 "Critical Parameter" vs. "Process Parameter" Other Medical Device Related Standards 3
Q Example Process orientation to the process leadership, management, goals (tasks of the top management) ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 10
S Process Inspection -Sub assembly process inspection sheet Lean in Manufacturing and Service Industries 3
T ISO 13485 8.3 - Non-Conforming Materials - on-line rework or part of process? ISO 13485:2016 - Medical Device Quality Management Systems 11
T Outsourced process in ISO 45001 Occupational Health & Safety Management Standards 2
L Manufacturing Process Audit Help IATF 16949 - Automotive Quality Systems Standard 6
T Formal Q Self Assessment - Problem with assigning Product and Process Customer and Company Specific Requirements 1
K Problems with process equipment ISO 13485:2016 - Medical Device Quality Management Systems 4
W Need for current design or process control FMEA and Control Plans 2
M Hiring Decisions - How much effort do you put into the hiring process these days? (7/2020) Misc. Quality Assurance and Business Systems Related Topics 6
hockeyhead Existing process never included in AS9100 scope of certification AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 10
A What is the difference between Design Process, Process Design and Design Control? 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 2
V Process, component or full product re-qualification: leaded to unleaded solder Qualification and Validation (including 21 CFR Part 11) 8
C Refreshing an old and boring topic - Job descriptions and Roles vs Process Documentation ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 38
R Where does IATF 16949 address Process mapping? IATF 16949 - Automotive Quality Systems Standard 3
C OEE and In-process Inspection Manufacturing and Related Processes 2
S Effective nonconforming process for AS9100 8.7 AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 6
T ISO 13485 - Process validation at critical suppliers ISO 13485:2016 - Medical Device Quality Management Systems 7

Similar threads

Top Bottom