SPC (Statistical process control) is a type of quality control where parts are statistically evaluated. This method is often used in the automotive and aerospace industry to statistically approve a production process of parts with crucial dimensions and tolerances. SPC is a way to move towards prevention-based quality control instead of detection based quality control. This means that the implementation of SPC has a lot of benefits when it comes to the reduction of scrap, the increase of productivity and the overall improvement of quality throughout our production processes.
Ppk (process performance) and Cpk (process capability) are the two main indicators within the SPC method that tell the operator and final inspector if a production process is either capable or not capable of delivering the desired quality requested by the customer. During a production controlled by SPC, the Ppk and Cpk are calculated from a number of samples entered into a SPC software package. If the value of the Cpk and Ppk are above the minimum value (often 1.33 or 1.67) requested by the customer, the production is deemed to be capable of delivering the desired quality. The meaning of of the value of the Ppk and Cpk matches a percentage of statistical defects within a production, often expressed in defective parts per million. Check the table for more information.
0,33
68,27%
317.300
0,67
95,45%
45.500
0,79
98,00%
20.000
1,00
99,73%
2.700
1,04
99,82%
1.800
1,06
99,86%
1.400
1,08
99,88%
1.200
1,10
99,90%
1.000
1,12
99,92%
800
1,18
99,96%
400
1,24
99,98%
200
1,30
99,99%
100
1,33
99,9937%
63
1.42
99,9980%
20
1.47
99,9990%
10
1,50
99,99966%
3,4
1,58
99,9998%
2
1,63
99,9999%
1
1,67
99,99994%
0,6
1,73
99,99998%
0,2
1,77
99,99999%
0,1
1,87
99,999998%
0,02
1,91
99,999999%
0,01
2,00
99,9999998%
0,002