Bothe, Davis R. (1989, ASQC) International Quality Institute, Inc., Northville, MI
Up until now, job shops have had great difficulty in applying the powerful methods of statistical process control (SPC) to their processes because of limited lot sizes. These small production runs cause problems when attempting to use traditional Shewhart charts because there is never enough data to calculate control limits. And even when there is enough data, if a different part number is run on the equipment, a new chart has to be started. Since most job shops have hundreds (if not thousands) of part numbers, a mountain of paperwork is created. The operator must waste valuable production time to search for the proper chart, thereby decreasing his efficiency. The recent development of a control chart designed especially for the short production runs of job shops is an answer to all these problems. The Short Run X-bar and R chart allows an operator to plot multiple part numbers on the same chart by the use of a special data transformation that scales the data from different part numbers to one, common distribution. This standardizing of the data allows the operator to draw one common set of control limits on the chart which apply to all part numbers plotted, thus eliminating the need for hundreds of separate charts. Control limits can be determined sooner, and since all the process data is now plotted on one chart, any time-related changes in the process can be more easily detected since they are not spread over several different charts.
Since the data transformation standardizes all measurement data, the Short Run X-bar and R chart permits (in addition to the plotting of multiple part numbers) the plotting of different part characteristics on the same chart. This allows a chart to follow the material for a particular lot through the plant, with the data from all operations plotted on it.
Statistical process control (SPC),Short runs,Automobile industry