Process capability

Process capability

Process capability is a tool

to measure ability of process to meet specification

A process is said to be highly capable when all the outputs from the process lie closer to the central value

We can also say that Process capability is a measure of dispersion of process output (like range, standard deviation ), 
means it measure how much output from process are closer/spread/disperse around its central value.

Process is said to be statistically controlled then all outputs of the process will lie between -3σ to +3σ . which means
 process capability of process would became 6σ

                        ∴ process capability =  6σ                where, σ = standard deviation

 Process capability index -  Cp

         

                    
 Where    Cp  - Capability index

              σ  - Standard deviation

Cp = 1 then process output lie within specification limit

Cp > 1 then process meet specification

Cp < 1 then Process  is not capable to meet specification

Process capability can be describe in terms with voice of customer , voice of process










Limitation

Limitation of process capability index is it does not measure process performance
 in terms of process is centered or not
 (ie Cp  not changed if process average is shifted)

To overcome that

When the process is not centered,  Cpk is given by following formula

• For process average shifted to lower side

• For process average shifted to upper side

 If Cpk = 1 then process is said to be marginally capable

Cpk > 1 then process is said to be capable

Cpk < 1 then process is said to be incapable

• Companies see that they achieve at least a Cp=Cpk value of 1.33 to be on the safer side

• Cp will be equal to Cpk when the process is centered
• Value of Cp does not change as a change occurs process in center
  
  
  

  EXAMPLE

  One can understand and calculate process capability easily by below example and format
  
  Process capability Example and format link 
  
  
  

  			WALL THICKNESS ANALYSIS REPORT (Std.Dev., CP, CPK Study)
  	Date
  	Product Name	Agri Pipe
  	Size	90 MM 4 Kg selfit
  	Sample Qty.	48 Nos
  	OD Min Req.		mm
  	OD Max Req.		mm
  	Thick Min Req.	2.10	mm
  	Thick max Req.	2.60	mm
  	Batch No
  	M/c No.
  Time	Sample No.	Wall Thickness at 4 pts.					Pt.1(marking side)
  		Pt.1	Pt.2	Pt.3	Pt.4
  7:30:00 AM	1	2.30	2.25	2.38	2.26
  	2	2.25	2.20	2.37	2.27
  8:30:00 AM	3	2.31	2.29	2.35	2.29	Pt.4	Pt,2
  	4	2.27	2.20	2.30	2.27
  9:30:00 AM	5	2.29	2.25	2.31	2.25
  	6	2.27	2.20	2.31	2.21
  10:30:00 AM	7	2.25	2.20	2.37	2.36		Pt.3
  	8	2.27	2.21	2.31	2.25	(Position of WT measurement)
  11:30:00 AM	9	2.24	2.22	2.35	2.26
  	10	2.20	2.17	2.31	2.28
  12:30:00 PM	11	2.22	2.19	2.33	2.26
  	12	2.22	2.32	2.30	2.25
  1:30:00 PM	13	2.18	2.30	2.34	2.28
  	14	2.20	2.20	2.37	2.27
  2:30:00 PM	15	2.22	2.25	2.35	2.25
  	16	2.27	2.23	2.30	2.22
  3:30:00 PM	17	2.19	2.15	2.38	2.31
  	18	2.20	2.17	2.34	2.30
  4:30:00 PM	19	2.21	2.23	2.30	2.32
  	20	2.22	2.16	2.37	2.31
  5:30:00 PM	21	2.23	2.16	2.38	2.34
  	22	2.20	2.17	2.39	2.32
  6:30:00 PM	23	2.23	2.14	2.34	2.30
  	24	2.21	2.16	2.35	2.32
  7:30:00 PM	25	2.22	2.16	2.36	2.31
  	26	2.23	2.19	2.35	2.28
  8:30:00 PM	27	2.20	2.19	2.32	2.32
  	28	2.22	2.15	2.33	2.27
  9:30:00 PM	29	2.24	2.16	2.36	2.31
  	30	2.20	2.16	2.31	2.34
  10:30:00 PM	31	2.18	2.19	2.34	2.30
  	32	2.21	2.17	2.33	2.29
  11:30:00 PM	33	2.15	2.22	2.29	2.33
  	34	2.10	2.16	2.28	2.31
  12:30:00 AM	35	2.16	2.21	2.32	2.28
  	36	2.18	2.25	2.26	2.31
  1:30:00 AM	37	2.15	2.19	2.29	2.30
  	38	2.17	2.19	2.30	2.34
  2:30:00 AM	39	2.17	2.16	2.34	2.30
  	40	2.13	2.14	2.31	2.29
  3:30:00 AM	41	2.20	2.18	2.26	2.31
  	42	2.12	2.20	2.30	2.34
  4:30:00 AM	43	2.18	2.21	2.37	2.32
  	44	2.15	2.22	2.35	2.31
  5:30:00 AM	45	2.21	2.26	2.28	2.34
  	46	2.19	2.21	2.32	2.32
  6:30:00 AM	47	2.14	2.24	2.27	2.29
  	48	2.20	2.23	2.28	2.30
  		Min range	2.10		Note : We took 48 pipes (6 Mtr.each) at an equal intervals in a span of 24 hrs. production run . Wall thickness measured on pipe spigot end.
  		Max range	2.60
  		Min	2.10
  		Max	2.39
  		Avg	2.26
  		Range	0.29		Guidelines :
  		Mod	2.31		CP <1 , Poor Process, not within control
  How to find standard deviation in MS Excel and google sheet		Std dev	0.07		CP = 1 , Process is Ok, require further tuning How to calculate Standard deviation in MS Excel & google sheet
  		Total Tole	0.50		CP > 1 , Process is good & well within control
  		CP	1.26		CP>1.50, Process is excellent & well within
  		CPK1	0.81		control
  		CPK2	1.71		Value of CPK1 & CPK2 tells about location of
  					process Curve (Centrally located ,
  					shifted to lower side or shifted to upper side)
  Remarks :How to calculate standard deviation in google sheet and MS Excel and manualy
  				Prepare By : ---------------------
  Analysis Result			Process is good & well within control

   
  
  
  
  
  
   Note : For achieving Cp =1, we have to maintain (Improve Process) standard deviation is 
  equal to 1/3 rd of specification limit  
  
  and for achieving Cp>1, we have to improve process, so that standard deviation would improve
  less than 1/3 rd of specification limit