You have a process for ensuring equipment can consistently manufacture within the design tolerance limits
Statistical Process Control (SPC) was an initiative championed by leading Japanese vehicle manufactures to ensure that all their suppliers used equipment that was able to manufacture within design tolerances (process capable). This dramatically improved the quality and consistence of vehicle components, transforming their reliability in service
Key Issues
·         SPC is used to monitor and control manufacturing processes within required tolerance bands, reducing variation and making processes more predictable
·         SPC charts plot the change in two variables know as the ‘average’, and the ‘range’ over the production cycle, if these variables move outside established limits, the process is not capable and a capability study is needed to measure variations by statically sampling parameter changes affecting the process
·         Process capability (Cp) is influenced by a number of factors: the operator, environment, machine, measurement, materials and methods of operation
·         There are two types of variability, inherent variability that is due to the design, construction or nature of its operation and assignable variability that can be attributed to a specific cause, tool wear, operators, and such
·         The process capability is expressed as a capability index Cp, which is the ratio of the tolerance over the natural process variation (+/- 3 sigma)
o     If Cp is less than 1.3 the process not capable
o     If Cp lies between 1.3 and 2.5 the process is considered capable
·         A classical process capability study involves measuring a key parameter for a sample of over 40 components without making any adjustments to the equipment and plotting the spread of tolerances observed due to variation in order to calculate the Cp
·         If the natural distribution plotted from this is not symmetrical between the upper and lower control limits and the process capability Cp value is influenced by either of these limits, the process capability calculation is expressed as a Cpk value
·         Other techniques, such as Peter Shain’s “Precontrol”, can be used for evaluating the capability of a process using a lower number of samples
·         Establishing the capability of the measuring system is also important because it can introduce considerable errors, which can usually be more easily eliminated
·         Process capability has to be established and consistently maintained for parameters that represent the key characteristics for a particular product
·         The mandated process capability index Cpk has to be reached by a series of process changes until the standard can be achieved consistently
·         SPC and process capability is at the core of good quality; no manager can justify allowing people to work on equipment that is not capable to produce items consistently within the specified design tolerances
Factors for Success      
Factors to  Avoid
1.     All of your processes are capable
2.     All process Capability levels are checked on a routine basis
Making excuses for not knowing Cp’s
Who does this apply to?
Senior managers, module leaders/supervisors, suppliers
Realisable Benefits
Improved quality, greater product reliability, a step change in overall performance


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