Calculating Cpk on a single sided (unilateral) tolerance

L

lee01

#1
Calculating Cpk on a single sided tolerance

Okay I have a diameter that is .250” +0.010” (note there is no minus), how do I calculate Cpk? And is it possible to calculate Cp as I think it is not based on the Cp equation (USL – LSL / 6 standard deviations). In this instance there is only size – plus, and not size plus and minus.
Also, is there an excel formula for the Cpk calculation (if poss)


Best Regards

Lee01
 
Last edited by a moderator:
Elsmar Forum Sponsor

Mike S.

Happy to be Alive
Trusted Information Resource
#2
The issue of Cp and Cpk has been debated a great deal recently. Do a search for more info.

If the spec.is .250" - .260", consider the possibility of using a spec. of .255' +/- .005" for your spec. in calculating Cpk.
 
R

Ravi Khare

#3
Lee,

In case you have a one sided tolerance (let's say upper tolerance only), the Cpk is evaluated as Cpk upper. Cp is not defined in such cases.

However from what you have written, it does look as though you really have a two sided tolerance. The tolerance is expressed on the plus side only as a convention (like in a hole basic system). Would it be acceptable for instance, if your diameter went below 0.250" ? It could even go to zero if you don't really specify a lower tolerance and the hole or the shaft under consideration would completely vanish.
A two sided Cp/Cpk formula would apply here.

One sided tolerances are typically found in parameters like surface roughness, breaking strength, and form tolerances like straightness, flatness, roundness and cylindricity.

Hope this helps.

Ravi
 

Darius

Quite Involved in Discussions
#4
As Mike said "The issue of Cp and Cpk has been debated a great deal recently. Do a search for more info."

In

http://Elsmar.com/Forums/showthread.php?t=6410

I posted, that Cpm, or Cpmk are in my opinion better capability index than Cp, Cpk for one sided spec.

The problem that I saw in Cpkl or Cpku for one sided spec is that they don't take the target in account, and "Quality is to the target with the minimum variation".
:smokin:
 
L

lee01

#5
Ravi,

This is a single sided tolerance as the size described is 0.250 + 0.010. This means that obviously it cannot be below the .250 size.

I have been told that there is an equation that takes into consideration the Skewness and Kurtosis to give a value, this value is then used to calculate Cpk.


Help please

Lee01
 
R

Ravi Khare

#6
Lee,

1. If the size cannot go above 0.260 (0.250 + 0.010), the Upper Spec Limit = 0.260

2. If the size cannot go below 0.250 (as referred by you above), the Lower Spec Limit = 0.250

Any parameter that has both, USL & LSL is to be treated as a two sided tolerance.

To determine Kurtosis and skewness, you will have to look at the histogram. Kurtosis describes the peakedness of the distribution. Skewness describes whether the distribution is lopsided.

If you have to determine the process capability using a Non Normal distribution there are methods to transform the data. You could look up Pearson and Johnson families of distributions for this.

Ravi
 
D

Dave Dunn

#7
Lee, I think the confusion you're running into is between a single-sided tolerance which has only one direction that it can go, such as speed (i.e. you can never go slower than 0), and a unilateral tolerance, or unevenly distributed tolerance in which you may want the result to be closer to one end than the other, but is still possible to go out of spec on both ends.
To say that the hole size cannot go below .250 is more of a technicality. It can go below .250, but it should not due to design requirements, thus it is a two-sided tolerance with uneven distribution of the tolerance limits.


lee01 said:
Ravi,

This is a single sided tolerance as the size described is 0.250 + 0.010. This means that obviously it cannot be below the .250 size.

I have been told that there is an equation that takes into consideration the Skewness and Kurtosis to give a value, this value is then used to calculate Cpk.


Help please

Lee01
 
T

Thomas_Crown

#9
Single sided gauges,

We have had these in the form of hole position using GD&T (a diametric gauge) Where it can deviate from its nominal in any 360 degree direction.

In a similar manner that perhaps a call centre will respond to a qaury, it can only answer it in from the time it gets the call, not before.

We use the Cp to monitor the process, and Cpu (Cp upper) for the process in relation to the desired tolerances

Thomas
 
S

santhosh_mech

#10
Calculating Cpk on a single sided tolerance

Okay I have a diameter that is .250” +0.010” (note there is no minus), how do I calculate Cpk? And is it possible to calculate Cp as I think it is not based on the Cp equation (USL – LSL / 6 standard deviations). In this instance there is only size – plus, and not size plus and minus.
Also, is there an excel formula for the Cpk calculation (if poss)


Best Regards

Lee01
Its very simple, conver the .250 diameter to 0.255" ± 0.005" and take SPC for that parameter, U can get the Cp and Cpk
 
Thread starter Similar threads Forum Replies Date
T Calculating LCL, UCL, Cp, and Cpk in an Excel Spreadsheet Statistical Analysis Tools, Techniques and SPC 3
M Calculating Cpk when sample size equals to 1 Capability, Accuracy and Stability - Processes, Machines, etc. 12
D Calculating Cpk on Non-Normal Data Distribution Capability, Accuracy and Stability - Processes, Machines, etc. 10
B Please share a template for calculating Cp Cpk Document Control Systems, Procedures, Forms and Templates 3
M Calculating Cp and Cpk - Is my calculation correct? Capability, Accuracy and Stability - Processes, Machines, etc. 7
M Calculating correct Cpk - Getting different answers Capability, Accuracy and Stability - Processes, Machines, etc. 3
K Calculating StdDev and Cpk on Multiple Populations Capability, Accuracy and Stability - Processes, Machines, etc. 2
D Calculating Cpk for Tubing Wall Thickness - Extrusion Process - How to approach? Statistical Analysis Tools, Techniques and SPC 16
D Use of an outlier in calculating Cpk/Ppk Capability, Accuracy and Stability - Processes, Machines, etc. 8
S Calculating Cpk/Ppk Simultaneously in SPC software Capability, Accuracy and Stability - Processes, Machines, etc. 6
C Calculating Cp and Cpk for Product with only LSL (Lower Specification Limit) Capability, Accuracy and Stability - Processes, Machines, etc. 28
L Calculating the RFT using the Cpk value Capability, Accuracy and Stability - Processes, Machines, etc. 3
A Calculating Cp and Cpk on a Non-Normal Distribution Capability, Accuracy and Stability - Processes, Machines, etc. 14
Marc Calculating Defect Rate (PPM) from a Cpk value Capability, Accuracy and Stability - Processes, Machines, etc. 8
M Cp & Cpk - How are you calculating the standard deviation? Capability, Accuracy and Stability - Processes, Machines, etc. 3
C Calculating your KPIs ISO 13485:2016 - Medical Device Quality Management Systems 7
Kuldeep Singh Calculating Heat Dissipation Manufacturing and Related Processes 1
Q Calculating Defect Rates ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 4
S Formula for Calculating NoGo Major Diameter for UN gages Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
P Calculating a weight for a machine for the CE label CE Marking (Conformité Européene) / CB Scheme 0
R Remote Support - Calculating Number of Employees IATF 16949 - Automotive Quality Systems Standard 2
G Calculating Tolerance of a Panel Meter with a 2 degrees of an Arc General Measurement Device and Calibration Topics 0
V Product Development - When to start calculating Process Capability Capability, Accuracy and Stability - Processes, Machines, etc. 3
J Calculating Reliability for Subsystems in Series Reliability Analysis - Predictions, Testing and Standards 15
Stijloor Calculating (3rd Party) Audit Days for Company with Seasonal Employees General Auditing Discussions 3
T Formulas for Calculating Coefficients for RTD / PRTD Calibration According to ITS-90 General Measurement Device and Calibration Topics 5
J Calculating part variation from historic data for GRR study Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 1
P Purpose of calculating Ta, Tb & R Square in Linearity Study Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 1
P Purpose of calculating Uncertainty value in calibration study Measurement Uncertainty (MU) 5
M Calculating MTBF for plug-in PCI cards Reliability Analysis - Predictions, Testing and Standards 2
J Calculating impact force, expressed in lbs, of a 5 lb weight dropped 48 inches Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
K Calculating Capability of a process when data is skewed in the upper range Capability, Accuracy and Stability - Processes, Machines, etc. 4
A Calculating Accuracy for a Multimeter General Measurement Device and Calibration Topics 1
M Calculating Coefficients for an RTD (Resistance Temperature Detector) Probe General Measurement Device and Calibration Topics 3
T Calculating Plating/Coating Weight for IMDS RoHS, REACH, ELV, IMDS and Restricted Substances 3
D Calculating Lower and Upper 3 Sigma Control Limits Statistical Analysis Tools, Techniques and SPC 2
M Calculating Adequate Receiving Inspection Sample Size Statistical Analysis Tools, Techniques and SPC 2
M Calculating Capability of Delivery Performance Capability, Accuracy and Stability - Processes, Machines, etc. 5
K Advice on Calculating Control Chart Control Limits Statistical Analysis Tools, Techniques and SPC 13
B Calculating Combined DPMO and Sigma Level for Two or More Different Work Areas Six Sigma 3
G Calculating the Fraction Defective Confidence Interval for a Lot Statistical Analysis Tools, Techniques and SPC 2
A Calculating Combined Measurement Uncertainty - VDA 5 Measurement Uncertainty (MU) 1
B Calculating Precision to Tolerance Ratio Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 9
D DPPM - Calculating the Defect Rate of a Software Test Statistical Analysis Tools, Techniques and SPC 9
L FMEA - Calculating RPN = S*O*D FMEA and Control Plans 3
I Shewhart Constants vs Central Limit Theorem in calculating Control Limits Statistical Analysis Tools, Techniques and SPC 18
S Is a Stable Process (within Control Limits) required for Calculating Pp, Ppk? Capability, Accuracy and Stability - Processes, Machines, etc. 6
M Calculating the IATF TS 16949 Recertification Audit Man Days - Clarification ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 6
J IEC60825 - Laser Safety - Calculating Maximum Permissible Exposure Correctly Other ISO and International Standards and European Regulations 1
P Calculating Process Capability from Data of attached spreadsheet Statistical Analysis Tools, Techniques and SPC 4

Similar threads

Top Bottom