roblem 2 1) A process is used for fabricating gasket for auto engines. The dimen

Question

roblem 2 1) A process is used for fabricating gasket for auto engines. The dimensions (thickness) could not meet the customer specifications. Initially engineers did a capability study to "prove" that the process needed radical change, i.e., new equipment. They selected what they thought was the best operator and the best equipment and took careful measurement on one shift products >70). Then, they set out to use these data to calculate mean and standard deviation (s)of the process for capability analysis. After they did this they found the C-0.477 and Cpk-0.45, So they thought they proved the process was not capable. Please discuss the inherent dangers in their approach and discuss the relationship between process capability and process control

Explanation / Answer

First of all we need to understand, Process control and Process capability.

Process Capability- Process capability shows that whether the output or product of the process is capable of satisfying the customer specification or requirement. Whether the product is in correct measurement. This is usually measured using either the Capability Ratio (Cp) or the Capability Index (Cpk). If  value of either Cp or Cpk is 1.0, it implies that the process output is equal with the specification or requirement.

While calculating Cp or Cpk for a process, these three conditions must be satisfied. One, the output of the process must be a continuous variable. Two, the output of the process must be normally distributed. And three, the process output must be in a state of statistical control, i.e. process control.

Process control - Process control gives the idea of whether the output of the process is in a state of statistical control. When can we say a process is in stastical control?

A process is in statistical control when all special causes of variation have been removed and only common cause variation remains. Common cause? Special cause? There is variation in the output of all processes, and this variations may be due to common causes or special causes. Common causes are inherent in the process, while special causes are specific to a human or an event. And it is to note that, if a variation is present in the output of the process, whether by common cause or special cause, may or may not leads to defects.

If a process exhibits special cause variation, it is said to be out of statistical control. Which implies that the output is not stable or predictable. If a process exhibits common cause variation, it is said to be in a state of statistical control. This implies that the output is stable and predictable.

Again, whilst a process is in control, that does not imply that the output of the process is normally distributed. We determine whether or not a process is in control by using either a run chart or control charts, and preferred method would be control charts.

To know a process to be in capable, it first has to be in stastical control, which is defined by process control.

Now, in the present case, 1. the process has to be, first, distributed normally.

2. The output of the process has to continuous variable, over several run.

3. For the process to be in stastical control, we must run the process with different operator or different equipment, to find the common and specific cause of the variation. Carefully analysing the cause and eleminate the special cause variation in the process.

4. A process is said to be capable if nearly 100% of the output from the process is within the specifications or requirement as per customer satisfaction. Sometime a process can be in control, yet it may fail to meet specification requirements. In this situation, you would need to take steps to 'improve or redesign the process'.

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