Specifications

Cleco

Electric Tool Control TME-100 Series

62 PL12-1300 12/00 en04d141.fm, 30.01.2001

calculating the average or range for a sample, and plotting a point on the control chart incor-

rectly.

Special cause variation may be indicated by one of the following situations:

• A point outside a control limit.

• A shift, with several consecutive points inside a control limit but on the same side of the cen-

ter line.

• A trend of consecutive points that moves steadily toward a control limit.

Fig. 4-6: Indications of Special Cause Variation

Since we are controlling by evaluating only small sample sizes from the process, the criteria for

continuing production must be the control limits. For this reason, we do not place the specifica-

tion limits (which are recorded on the data sheet) on the control chart.

4.1.4 System Improvement

There are always two risks associated with statistical control. The first is the risk of accepting

bad product, and the second is the risk of rejecting good product. Both are undesirable, but our

primary concern is usually the first since it has a direct impact on customer satisfaction.

The six sigma spread of the process described in this procedure is in general use throughout

American industry. When properly applied, it assures us that at least 99.7% of all units produced

will meet specification. This relates to a risk of delivering three substandard units for each thou-

sand produced. While this provides much better quality than can be obtained with an uncon-

trolled process and 100% inspection, it may not be good enough in the future as competition

becomes stronger in the world market.

So it's important that we continue to improve the manufacturing system - to reduce the influence

of conditions that affect variations in the performance of people, machines, and materials (see

graph 4-7). There will be practical and economic limitations to this effort, but to the extent that

we can further reduce process variation we will increase the probability that all delivered product

meets the specification.

Fig. 4-7: Improving The System By Reducing Variation

Outside of Limit Shift Trend

Variation

Spec. Toler. Spec. Toler.