Cpk analysis for +.000/-.004 problem

J

Joshua Sale

#1
I have an problem with a couple of people understanding if a Cpk value can be determined off of the following specifications:

Spec for hole: .165 +.000/-.004.

One person is telling me no Cpk value can be determined b/c there is no nominal spec and that this is a 1-sided specification limit.

My statistical engineer states that for Cpk analysis there is no nominal needed and the USL is .165 and the LSL is .161.

I tend to agree with the stats engineer.

Please point me in the right direction of a article or book that will state one or the other view point correctly. I need to clearly give a direction so that the arguement may end.

Thanks for your help.
 
Elsmar Forum Sponsor

Al Rosen

Staff member
Super Moderator
#2
Joshua Sale said:
I have an problem with a couple of people understanding if a Cpk value can be determined off of the following specifications:

Spec for hole: .165 +.000/-.004.

One person is telling me no Cpk value can be determined b/c there is no nominal spec and that this is a 1-sided specification limit.

My statistical engineer states that for Cpk analysis there is no nominal needed and the USL is .165 and the LSL is .161.

I tend to agree with the stats engineer.

Please point me in the right direction of a article or book that will state one or the other view point correctly. I need to clearly give a direction so that the arguement may end.

Thanks for your help.
This may help.
Calculating capability indices with one specification
 
A

Arvind

#3
Design unilateral tolerances need to be converted to bilateral tolerance for process

Unilateral tolerances are very common in engineering however these need to be converted to bilateral tolerance from process point of view.
In your case, even though design nominal is 0.165 +0/-0.04, you cannot take 0.165 as process nominal. If you do, then you run into risk bigger fraction or tail portion of population beyond specification.
For process people, the process specification needs to be 0.163+/- 0.02 which is conforming to randomly normally distributed output around the mean which coincides with target.

Arvind
 

Tim Folkerts

Super Moderator
#4
Joshua,

I would agree with your engineer.

Joshua Sale said:
One person is telling me no Cpk value can be determined b/c there is no nominal spec and that this is a 1-sided specification limit.
This is not a one-side spec. A true one-sided spec would include the idea of "at least" or "at most". For example "the resistance should be at least 10,000 Ohms," or " the moisture content should be no more that 0.5%". Al Rosen gave a nice link to a webpage discussing one-sided specs where one of the limits is zero.

Joshua Sale said:
My statistical engineer states that for Cpk analysis there is no nominal needed and the USL is .165 and the LSL is .161.
That is my interpretation, too, although I don't have a handy reference to appeal to. However, the equations themselves support this interpretation. Since the equations make no mention of a nominal or target value, all you need are the two spec limits from the customer, and the mean and st.dev. from the data.


Tim F

P.S. This points out one weakness of the whole idea of cpk. cpk measures the capability of staying within the spec limits, but in no way measures the capability of matching the desired nominal value.

Intuitively, a spec of .165 +0/-0.004 means you want the part as close to 0.165 as possible without going over, while 0.163 +0.002/-0.002 says you want the part as close to the middle as possible. If you can make parts with a tolerance of 3 sigma = 0.001, then in the first case you would want to set the nominal value somewhere near 0.164 to match the customers desire. However, making the parts closer to what the customer actually wanted would LOWER cpk from 2 to 1!
 
D

Darius

#5
I am not agree with the calculation of Cpk for one sided specs

http://elsmar.com/Forums/showthread.php?t=7047

or at least the statistic can obtain confusing conclusions, for example if

Quote: (Originally Posted by Joshua Sale) My statistical engineer states that for Cpk analysis there is no nominal needed and the USL is .165 and the LSL is .161.
If you use Cpk and the process is centered on 0.163, you would obtain the best results and it's typical that nearer the target (0.165) the better.
:bonk:
 
Thread starter Similar threads Forum Replies Date
K Analysis variation among the lots during cpk calculation. Design and Development of Products and Processes 1
T Is Cpk Analysis only for Data that came from Normal Distribution? Capability, Accuracy and Stability - Processes, Machines, etc. 7
J Capability Analysis (Cpk) of torque driver process Capability, Accuracy and Stability - Processes, Machines, etc. 5
N Analysis of non-normal stratified data for cpk/ppk? Rupture test Capability, Accuracy and Stability - Processes, Machines, etc. 6
L Does Cpk analysis make sense on this study? Number of packaged parts per bundle Statistical Analysis Tools, Techniques and SPC 14
L SPC on Wave soldering machine - Cp and Cpk analysis - Soldering pot temperature Capability, Accuracy and Stability - Processes, Machines, etc. 11
J What causes Ppk to be greater than Cpk when a basic capability analysis is run? Capability, Accuracy and Stability - Processes, Machines, etc. 17
L Definition Cp Cpk Pp and Ppk Definitions - I'm doing SPC and Capability Analysis Definitions, Acronyms, Abbreviations and Interpretations Listed Alphabetically 7
W LTPD, AQL, Ppk and Cpk validation sampling plan table Inspection, Prints (Drawings), Testing, Sampling and Related Topics 0
M Cp and Cpk for straightness and parallelism Capability, Accuracy and Stability - Processes, Machines, etc. 8
Q Capability - CPk comparison values Capability, Accuracy and Stability - Processes, Machines, etc. 12
D Tolerance definition based on expected Cp/cpk Reliability Analysis - Predictions, Testing and Standards 14
S Recommendation for user friendly Gaga R&R and Cpk software Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 10
A How to set up Continuous CpK monitoring of an injection mold process Reliability Analysis - Predictions, Testing and Standards 7
A Interpretation with regards to Ppk > Cpk Capability, Accuracy and Stability - Processes, Machines, etc. 14
U Cpk Calculation - I analyse a double seam cans Manufacturing and Related Processes 15
Proud Liberal Cp / Cpk on position using multiple MMC bonuses Capability, Accuracy and Stability - Processes, Machines, etc. 2
I Can we use pin gauges to measure an accurate Cpk ? Capability, Accuracy and Stability - Processes, Machines, etc. 9
T Final process capability results - What I am supposed to present? Cp and CpK? APQP and PPAP 11
F CpK on a manufactured assembly? Reliability Analysis - Predictions, Testing and Standards 7
M How is the Cmk and Cpk calculated? Statistical Analysis Tools, Techniques and SPC 0
O Cpk for Unilateral Tolerance Capability, Accuracy and Stability - Processes, Machines, etc. 9
B How to calculate CPK with n = 1 Manufacturing and Related Processes 2
O CPK with a P value less than 0.005 Manufacturing and Related Processes 8
A Short Term vs. Long Term SPC Study - Where is Cp and Cpk Defined Statistical Analysis Tools, Techniques and SPC 8
A Cpk Formula seems off, need help!? Capability, Accuracy and Stability - Processes, Machines, etc. 11
D Cp/Cpk on Gages for MSA (vs. Cg/Cgk) Capability, Accuracy and Stability - Processes, Machines, etc. 21
E Questions regarding Cpk Calculation - Should we be using LSL/USL or LCL/UCL? Capability, Accuracy and Stability - Processes, Machines, etc. 5
B Maximum Material Condition with Bonus - Determine Cpk for this one kpc Measurement Uncertainty (MU) 2
Hami812 Cpk Setting Tolerance - Cart before the Horse? (Wifi Routers) Capability, Accuracy and Stability - Processes, Machines, etc. 3
T Calculating LCL, UCL, Cp, and Cpk in an Excel Spreadsheet Statistical Analysis Tools, Techniques and SPC 3
Proud Liberal Cpk on Position w/ MMC on Location and Size Capability, Accuracy and Stability - Processes, Machines, etc. 3
E SPC Production - Getting Cpk and Ppk Statistical Analysis Tools, Techniques and SPC 8
S Track Cpk trend Statistical Analysis Tools, Techniques and SPC 5
M Cp & Cpk about a diameter measured with min.,max. and average values. Capability, Accuracy and Stability - Processes, Machines, etc. 13
P Non-normal Data Cpk Statistical Analysis Tools, Techniques and SPC 5
alonFAI Cpk for Solder Paste Height Statistical Analysis Tools, Techniques and SPC 5
I As a part of validation, should i include the acceptance criteria of CPk index >1.33? Qualification and Validation (including 21 CFR Part 11) 4
T Cpk for 0 as Ideal Value Statistical Analysis Tools, Techniques and SPC 1
P Profile Tolerance and CPK Manufacturing and Related Processes 3
V When to use Cp Cpk and Pp Ppk Capability, Accuracy and Stability - Processes, Machines, etc. 4
M Calculating Cpk when sample size equals to 1 Capability, Accuracy and Stability - Processes, Machines, etc. 12
S CpK in Minitab when 0.0 is the lower spec limit Statistical Analysis Tools, Techniques and SPC 7
I Best way to Chart an On-Going CPK Requirement Capability, Accuracy and Stability - Processes, Machines, etc. 4
U Cpk for Contaminant Mass - Washing Parts Capability, Accuracy and Stability - Processes, Machines, etc. 6
G Help with Cpk procedure Capability, Accuracy and Stability - Processes, Machines, etc. 3
J Calculation of σ within subgroups for Cpk in Minitab Using Minitab Software 1
S Value of Ppk or Cpk when targeting thinner material Statistical Analysis Tools, Techniques and SPC 4
B How to validate Microsoft Excel Spreadsheet Calculations for the Cpk value Capability, Accuracy and Stability - Processes, Machines, etc. 9
D Calculating Cpk on Non-Normal Data Distribution Capability, Accuracy and Stability - Processes, Machines, etc. 10

Similar threads

Top Bottom