Manufacturing Process Capability. Statistical Process Control...A capable process delivers uniformity in product output, which results in low defect rates, the greater the process capability, the lower the actual % defective. In a manufacturing process where there are in-process and final inspection stages, the greater the manufacturing process capability, then the lesser the need for those same inspections, or where the inspections stages remain, then lower sample sizes are required. There are positive financial implications in addition to the quality improvements associated with good process capability. With improving capability, process engineers have more room to set product specifications, either aligning closer to customer requirements, or to align with the needs of the business. Clearly improving process capabilities will contribute to lowering the levels of rework, repair, scrap and waste.
When we consider process capability, it may be appropriate to reference the words of Dr. W.E. Deming – “The disease is variability…. You must have a thorough understanding of variation in your processes, then work to reduce the variation”.
Capable processes, are processes with low levels of variation.
Capability and stability. A process is capable if the output of the process (the product) is predictable, in that the output product will be within allowed product specifications.
A process is stable, if it is only influenced by common cause of variation. (Common causes of variation are those causes naturally inherent in the system, they affect everyone working in the system and affect all outcomes of the system).
You don’t actually need to know the process specifications to determine process stability, but you must know the specifications to determine process capability.
Process capability measure – Cpk
Cpk is a measure called the capability index. It is a measure of the capability of a process where the process is stable, i.e. we exclude any special, once off, unusual causes of variation, but do include routine common causes of variation.
The Cpk measure varies from 1 to 2 as follows:
Cpk < 1.00 Process is not capable
1< Cpk< 1.33 Poor process capability
1.33 <Cpk <1.67 Moderate process capability
1.67<Cpk<2 Process is well capable
Cpk = 2 for a 6δ process (i.e. a 6 sigma process)
If we perform a capability analysis and obtain a Cpk close to “1” then this indicates that the process is not capable of producing product to the required specifications on a routine basis. The net result will be high level of variation between units of output product and intensive levels of inspection will be necessary to control the output, with excessive rework, repair and scrap rates.
If on the other hand, we obtain a Cpk close to “2”, then this indicates that the process is well capable of producing output which is within the required product specifications and consequently, in-process and final inspection requirements will be lessened, reject and rework levels will be low.
Therefore, it is clear, that when considering manufacturing process capability, Cpk can be very usefully applied. A key advantage of the process capability measure, is that it is relatively simple to determine and can therefore be utilized as a very powerful force for driving improvement in a manufacturing process.
The formulae for calculating a manufacturing process potential and a manufacturing process capability are:
Process Potential Cp
(Upper Specification Limit – Lower Specification Limit) / 6 Standard Deviation
Process Capability Cpk
(Upper Specification Limit – Process Mean ) / 3 Standard Deviation
(Process Mean – Lower Specification Limit) / 3 Standard Deviation
Use whichever of the two options above that provide a positive result.
The Cp measure determines the capability of the process to produce output product within the required specifications.
The Cpk, as per the Cp measure, determines the capability of a process to produce the required product to specification, but the Cpk also gives an indication of how well centered the process is between the specification limits.
What can you do if a process is not capable?
The initial action may be to increase inspection levels to ensure that confidence with respect to the quality of output product is increased. Clearly, quality cannot be inspected into a product or process, therefore, the next steps will be to look at how to improve the capability of the process. Remember, this can also include reviewing the product specifications, as by widening the specifications, the capability can be increased. This can only be performed, if any proposed specification changes are acceptable per customer needs.
Then looking at the process itself, there will be a need to identify the sources of variation. Is it due to raw materials, human factors, environmental, equipment variation, test method variation? You may need to set-up an SPC control process, perform some form of experimental design, instigate staff retraining, commence a review of operating procedures, review equipment calibration or preventative maintenance programs, etc..
One of the first questions to ask, will be to determine if the process is centered. The measurements obtained from the Cpk calculations will give this indication. If the lack of capability is due to poor centering, then this may be a relatively quick fix.
The actions to be taken to address poor process capability will need to be carefully considered, however, ensuring that the concept of capability is understood within the manufacturing process, is a good first step in driving product and process improvement.
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