Process Capability Analysis | Cp & Cpk Calculation Excel Sheet with Example

Last updated on November 16th, 2024 at 08:39 am

Process Capability Analysis | Cp & Cpk Calculation Excel Sheet with Example

Process Capability Analysis: – The Process Capability (Cp) and Process Capability Index (Cpk) are the important tools, which give an Idea about the Process Capability of a Stable Process. Here we will discuss on Calculation of Cp and Cpk with Examples. We are offering here Process Capability Excel Template / Format for you, hence click on the below links to Download the Excel Format.

DOWNLOAD (Cp & Cpk Excel Template / Format-Sample copy)

Process Capability (Cp):

  • Process Capability (Cp) is a statistical measurement of a processā€™s ability to produce parts within specified limits on a consistent basis
  • It gives us an idea about the width of the Bell curve.
  • The Process Capability for a stable process is typically defined as ((USL-LSL)/ (6 x Standard Deviation)).
Cpk-Process Capability Index :
  • It shows how closely a process is able to produce the output to its overall specifications.
  • More Value of Cpk means more process capable.
  • The Process Capability Index for a stable process is typically defined as the minimum of CPU or CPL.
Process Capability Analysis:

Industrial Example:

As per the Quality Assurance Plan, The shift engineers of Core Shop have started collecting the readings of the scratch hardness of Core. Given below are the details of Product Characteristics;

Specification of Scratch hardness is 70Ā±10.

The Upper Specification Limit is 80.

The Lower Specification Limit is 60.

Tolerance is 20.

Scratch hardness readings Table:
Table-1
Sl.No. 1 2 3 4 5 6 7 8 9 10
SG 1 72.00 71.00 72.00 71.00 72.00 71.00 73.00 71.00 72.00 73.00
SG2 71.00 72.00 72.00 72.00 72.00 72.00 72.00 73.00 73.00 71.00
SG 3 72.00 72.00 71.00 71.00 71.00 73.00 72.00 72.00 71.00 73.00
SG4 70.00 70.00 70.00 70.00 71.00 70.00 71.00 70.00 71.00 70.00
SG 5 72.00 72.00 72.00 72.00 72.00 72.00 72.00 71.00 72.00 71.00
Table-1 [Scratch hardness readings Table]
Table-2
Sl.No. 11 12 13 14 15 16 17 18 19 20
SG1 71.00 72.00 71.00 71.00 72.00 73.00 71.00 72.00 73.00 71.00
SG2 72.00 73.00 73.00 72.00 71.00 72.00 71.00 73.00 71.00 70.00
SG3 72.00 71.00 73.00 72.00 72.00 72.00 71.00 71.00 71.00 70.00
SG4 71.00 70.00 71.00 70.00 70.00 71.00 70.00 71.00 71.00 70.00
SG5 70.00 70.00 71.00 71.00 72.00 71.00 72.00 71.00 71.00 72.00
Table-2 [Scratch hardness readings Table]

In the above two tables (Table-1 &2), we have taken the 100 readings i.e. (20 times X 5 readings at a time).

Range=Maximum Value-Minimum Value

Average of Range=2.15

Value of d2=2.326 (For Subgroup size 5)

USL = 80, LSL = 60.

Standard Deviation:

 = Average of Range/d2

 2.15/2.326

=0.92

Process Capability (Cp):

 = ((USL-LSL)/ (6 x Standard Deviation))

(80-60)/ (6 x 0.92)

20/5.52

= 3.61

Process Capability Index (Cpk):

CPU:

= ((USL-Average of Mean)/3 x Standard Deviation)

(80-71.43)/ (3 x 0.92)

8.57/ 2.76

= 3.10

CPL:

= ((Average of Mean-LSL)/3 x Standard Deviation)

(71.43-60)/ 2.76

10.4211.43/2.76

=4.14

Cpk= 3.10 (minimum of CPU or CPL).

After doing the Process Capability Analysis on Scratch hardness readings, we got the below result value:

Characteristics: Scratch Hardness
Cp (Process Capability) = 3.61
Cpk (Process Capability Index) = 3.10
[ Cp & CpK ]
Process Capability Analysis with Manufacturing Example

The process engineer has collected the 100 nos laddle temperature reading and the same is mentioned in the below table.

Laddle Temperature Specification= 600 Ā± 15Ā°C

USL = 615

LSL = 585

Table-1
 12345678910
S1605599610605603604600609605601
S2603601612599601598603610603598
S3604598609610612609605612604603
S4600603605598599610598609600610
S5602602607609605612599605609603
Max.605603612610612612605612609610
Min.600598605598599598598605600598
Range55712131477912
Average of Range9.85         
Mean602.8600.6608.6604.2604606.6601609604.2603
Average of Mean603.92         
Table-2
 11121314151617181920
S1599601602604598598609598600598
S2610598602603603603605603603610
S3598603607598610607612607605598
S4609610609603603598604598607602
S5600603605607598610603610598603
Max.610610609607610610612610607610
Min.598598602598598598603598598598
Range1212791212912912
Mean603.2603605603602.4603.2606.6603.2602.6602.2

d2=2.326

Standard Deviation = Average of Range / d2 = 4.23

Cp = (USL-LSL)/6*Standard Deviation = 1.2

CPU = ((USL-Average of Mean)/3 x Standard Deviation) = 0.872

CPL = ((Average of Mean-LSL)/3 x Standard Deviation) = 1.489

CpK = 0.872(minimum of CPU or CPL).

Note: Download the Cp & Cpk excel template or format and deploy it in manufacturing process. downloading links are provided at top of this Article.
FAQ:
What is the difference between Cp & Cpk?

Ans.: Cp & CpK are termed as process capability and process capability index. In both cases, we would like to verify whether the process can meet the customer’s requirements or not. Generally, it is used when the process is under stable & statically control.

What is the formula of Cp & Cpk?

Cp= ((USL-LSL)/ (6 x Standard Deviation)) , where USL=Upper Specification Limit & LSL=Lower Specification Limit.

Cpk= Minimum of CPU or CPL, where CPU= ((USL-Average of Mean)/3 x Standard Deviation) & CPL= ((Average of Mean-LSL)/3 x Standard Deviation)

What are the good values of Cpk?

Generally, the customers provide the Cpk value to their supplier to maintain it in their manufacturing process. but for your knowledge, a Cpk value of 2 or greater than 2 is an excellent one.

What is cpk?

The cpk is the process capability index which shows how closely a process is able to produce the output to its overall specifications.

What is the IATF 16949 requirement of Statistical Concepts or SPC?

Application of statistical concepts in the IATF 16949 standard has been mentioned in Clause no-9.1.1.3, both Control chart (variable and Attribute) and process capability are the mandatory requirements. The application of statistical concepts shall be understood and used by the employees involved. We have published a separate article on Control Charts for our readers and you can Download Control Chart Excel Template / Format.

More on TECHIEQUALITY

Add a Comment

Your email address will not be published. Required fields are marked *

error: Content is protected !!