# Cp and Cpk for straightness and parallelism

#### Michal

##### Registered
Hi

I am going through dimensional reports for 32 plastic samples. My task is to calculate cp and cpk for all dimensions but i am not sure how to calculate it for straightness (0.15) and parallelism (0.30).
I understand that 0 is nominal and 0.15 or 0.30 is upper limit but tere is no lower limit - 0 is perfectly straight or parallel. Is there a cp and cpk formula for unilateral tolerances ? How to work out ptocess capability. Minimum cp and cpk >1.33 .

#### Jim Wynne

Staff member
First, have a look at these Cove search results. The subject has been discussed extensively here. Cpk at its best is of dubious value, but it has no value at all unless the samples have been taken in rational subgroups from a running process and chronological order is maintained and the process is normally distributed and statistically stable. It's not something that can be derived from measurement results of a bunch of samples of unknown provenance with regard to the requirements stated above.

#### Michal

##### Registered
Sampes are colected in even intervals during Operational Qualification. Samles are numberd and than send to met lab for measuring. Process is statistically stable and normaly distributed. I only don’t know how to calculate cp and cpk or is it not posible for unilateral tolerances

#### Michal

##### Registered
If straightness is tolerance zone is 0.15 than 0 will be nominal, 0.15 is upper limit and there is no lower limit ? And when all results are 0.001 - 0.008 it is perfectness but cpk values are low . Can cp and cpk be calculated atcall for unilateral tolerances ?

#### Jim Wynne

Staff member
Sampes are colected in even intervals during Operational Qualification. Samles are numberd and than send to met lab for measuring. Process is statistically stable and normaly distributed. I only don’t know how to calculate cp and cpk or is it not posible for unilateral tolerances
Did you collect samples in subgroups, or just single samples? Was there any charting (x-bar/R, e.g.) done to establish that the process is stable and normally distributed? If not, how did you make that determination? Did you look at any of the threads in the search results I provided?

#### Michal

##### Registered
Did you collect samples in subgroups, or just single samples? Was there any charting (x-bar/R, e.g.) done to establish that the process is stable and normally distributed? If not, how did you make that determination? Did you look at any of the threads in the search results I provided?
Samples are collected in subgroups top half x 4 cavities x 32 samples and bottom half x 4 cavities x 32 . So we have 32 samples for each cavity x 2 diferrent pieces. Yes charters were done before. All measured results are within tolerance too.

#### Johnnymo62

##### Haste Makes Waste
Your excel file does not take into account subgroups.

#### Jim Wynne

Staff member
Samples are collected in subgroups top half x 4 cavities x 32 samples and bottom half x 4 cavities x 32 . So we have 32 samples for each cavity x 2 diferrent pieces. Yes charters were done before. All measured results are within tolerance too.
To say the least, you're sending mixed messages here, as what you've posted isn't consistent with the dimensional report form you posted. Assuming that you're just concerned about capability calculation for unilateral tolerances (and not whether what you're doing makes sense), have you looked through the search results I posted?

LTPD, AQL, Ppk and Cpk validation sampling plan table Inspection, Prints (Drawings), Testing, Sampling and Related Topics 0
Capability - CPk comparison values Capability, Accuracy and Stability - Processes, Machines, etc. 12
Tolerance definition based on expected Cp/cpk Reliability Analysis - Predictions, Testing and Standards 14
Analysis variation among the lots during cpk calculation. Design and Development of Products and Processes 1
Recommendation for user friendly Gaga R&R and Cpk software Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 10
How to set up Continuous CpK monitoring of an injection mold process Reliability Analysis - Predictions, Testing and Standards 7
Interpretation with regards to Ppk > Cpk Capability, Accuracy and Stability - Processes, Machines, etc. 14
Cpk Calculation - I analyse a double seam cans Manufacturing and Related Processes 15
Cp / Cpk on position using multiple MMC bonuses Capability, Accuracy and Stability - Processes, Machines, etc. 2
Can we use pin gauges to measure an accurate Cpk ? Capability, Accuracy and Stability - Processes, Machines, etc. 9
Final process capability results - What I am supposed to present? Cp and CpK? APQP and PPAP 11
CpK on a manufactured assembly? Reliability Analysis - Predictions, Testing and Standards 7
How is the Cmk and Cpk calculated? Statistical Analysis Tools, Techniques and SPC 0
Cpk for Unilateral Tolerance Capability, Accuracy and Stability - Processes, Machines, etc. 9
How to calculate CPK with n = 1 Manufacturing and Related Processes 2
CPK with a P value less than 0.005 Manufacturing and Related Processes 8
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
Questions regarding Cpk Calculation - Should we be using LSL/USL or LCL/UCL? Capability, Accuracy and Stability - Processes, Machines, etc. 5
Maximum Material Condition with Bonus - Determine Cpk for this one kpc Measurement Uncertainty (MU) 2
Cpk Setting Tolerance - Cart before the Horse? (Wifi Routers) Capability, Accuracy and Stability - Processes, Machines, etc. 3
Calculating LCL, UCL, Cp, and Cpk in an Excel Spreadsheet Statistical Analysis Tools, Techniques and SPC 3
Cpk on Position w/ MMC on Location and Size Capability, Accuracy and Stability - Processes, Machines, etc. 3
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
Cpk for Solder Paste Height Statistical Analysis Tools, Techniques and SPC 5
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
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
CpK in Minitab when 0.0 is the lower spec limit Statistical Analysis Tools, Techniques and SPC 7
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
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
Value of Ppk or Cpk when targeting thinner material Statistical Analysis Tools, Techniques and SPC 4
How to validate Microsoft Excel Spreadsheet Calculations for the Cpk value Capability, Accuracy and Stability - Processes, Machines, etc. 9
Calculating Cpk on Non-Normal Data Distribution Capability, Accuracy and Stability - Processes, Machines, etc. 10
Cpk relation to Reliability Capability, Accuracy and Stability - Processes, Machines, etc. 4
B Process Capability - Changing Limits to Improve Cpk or find Root Causes? Capability, Accuracy and Stability - Processes, Machines, etc. 6
Cpk vs Ppk to look at the Capability by Fixture Capability, Accuracy and Stability - Processes, Machines, etc. 4
Convincing Management Cpk without Stability is Meaningless Capability, Accuracy and Stability - Processes, Machines, etc. 7
C Hardness Testing - Low Cpk's using Minitab Misc. Quality Assurance and Business Systems Related Topics 10
L Cpk after Data Transformation - How to Transform LCL /UCL to get Capability Report Capability, Accuracy and Stability - Processes, Machines, etc. 8
Customer requires CpK 2 and PpK 1.5 Capability, Accuracy and Stability - Processes, Machines, etc. 2
G Ongoing Cpk and Global/Historical Data Statistical Analysis Tools, Techniques and SPC 7