Cpk calculation for short runs - Manufacturing FFC (flexible flat cable)

R

rikkyyee

#1
#1
Anybody can advise

In our production we are manufacturing FFC (flexible flat cable) The product life cycle for a model is very short due a lot that contain 2000-4000pcs can be finished within 1 hour. everyday a prodcution line (total of 4) are schedule to run minimun 5 to 6 models, each model may only run a day only and may need to wait a week or a month later to run again. Hence the production is almost running dilferent model from day to day.

due to customer request only Cpk data the management are also only concern about it.

our current method are randomly select a model/line in a week and get 30 sample/lot to calculate the Cpk data. and we will use it as a reference for that week Cpk reference for all model.

this method not working very well because the Cpk data keep fluctuating from week to week.

pls give some advise
 
Elsmar Forum Sponsor
A

Al Dyer

#2
#2
Cpk values will fluctuate no matter the sample size. Try increasing your sample size and see of the fluctuation between the samples is consistant.

I think the main concern would be the trends in the Cpk values. Possibly set a target Cpk and react when the trend indicates you are running close to or below the target.

Also consider the variables when reviewing Cpk:

Time
Shift
Operator
Setup
Maintenance
Material

etc......
 
D

Dave W 2005

#3
#3
Rikkyyee

Al is right,

There are 'confidence limits' for cpk. The larger the number of samples, the more confident you can be that the cpk value is actually correct (as you have captured more variation).
30 is actually quite a small number. For example, to guarantee that your cpk is actually 1.33 or above, you need to have calculated a cpk of 1.62 for your batch of 30. With 100 samples, this number drops to 1.47.
You should not be surprised if your calculated cpk values vary with 30 samples.

The other question is: are you confident that the same process variation affects all of your product groups in the same way?

Dave
 
R

Ravi Khare

#4
#4
Thomas Pyzdek has described methods of Short Run SPC in his Six Sigma Handbook...just may prove useful.

He advises to focus on the process than the small/ short batch that gets over before you can start looking for samples. The process focus could consolidate together batches of different products passing through the same process.
 
You must log in or register to reply here.
Thread starter Similar threads Forum Replies Date
T Short Run - Low Volume Cpk Calculation Capability, Accuracy and Stability - Processes, Machines, etc. 10
K Analysis variation among the lots during cpk calculation. Design and Development of Products and Processes 1
U Cpk Calculation - I analyse a double seam cans Manufacturing and Related Processes 15
E Questions regarding Cpk Calculation - Should we be using LSL/USL or LCL/UCL? Capability, Accuracy and Stability - Processes, Machines, etc. 5
J Calculation of σ within subgroups for Cpk in Minitab Using Minitab Software 1
J Calculation of the Mean - Student Question about Cpk calculation Capability, Accuracy and Stability - Processes, Machines, etc. 6
K In Process Data Collection and Cpk Calculation Manufacturing and Related Processes 13
M Calculating Cp and Cpk - Is my calculation correct? Capability, Accuracy and Stability - Processes, Machines, etc. 7
K Problem with Cpk calculation of dimension with nominal at the top limit. Statistical Analysis Tools, Techniques and SPC 6
E Asymmetric Tolerance - How does an Asymmetric Tolerance affect the Cpk/Cp calculation Using Minitab Software 3
B Why 3 sigma in Cpk calculation? Six Sigma 8
T Cp and Cpk calculation for Non-normal data? Statistical Analysis Tools, Techniques and SPC 11
J Methods used to calculate Cpk value - Need help for Cpk Calculation Statistical Analysis Tools, Techniques and SPC 8
A Is the normal Cpk calculation applicable for data with positive & negative values? Capability, Accuracy and Stability - Processes, Machines, etc. 4
R Is Cpk calculation of dBA (noise level) valid? Capability, Accuracy and Stability - Processes, Machines, etc. 9
G Cpk Calculation - Tolerance to maintain 1.33 & 1.67 Cpk respectively Capability, Accuracy and Stability - Processes, Machines, etc. 17
M Calculation of Cpk and Ppk for an unstable process Statistical Analysis Tools, Techniques and SPC 7
J Calculation/math to find 1.33 Cpk Capability, Accuracy and Stability - Processes, Machines, etc. 15
O Customer is telling me my Cpk calculation is incorrect Capability, Accuracy and Stability - Processes, Machines, etc. 13
C Software Calculation Errors for Cp and Cpk With Unilateral Tolerances Capability, Accuracy and Stability - Processes, Machines, etc. 4
R Cp, Cpk calculation for np chart - No USL or LSL - SMT Production Capability, Accuracy and Stability - Processes, Machines, etc. 16
D Increased positional tol. due to departure from MMC (Bonus)and calculation of Ppk/Cpk Capability, Accuracy and Stability - Processes, Machines, etc. 9
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
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
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

Similar threads

Top Bottom