Detecting and Correcting Manufacturing Problems

Robert F. Hart, Ph. D.
Marilyn K. Hart, Ph.D.

In assembly line manufacturing operations, problems of poor quality are frequently transient in nature. For this reason, improvement can only be made by continuously monitoring the process and responding promptly by taking corrective action when the need arises. An effective mechanism for monitoring the process will be described below, but it must be kept in mind that the best monitoring methods become only an idle exercise unless corrective action is taken when needed.

A primary roadblock to process improvement is lack of timely information on manufacturing problems. All too often, management is totally unable to answer the questions:

The prerequisite to process improvement is the ability to define opportunities to improve, sometimes called "problems." The first line of defense for management is timely, accurate information on manufacturing problems. As defects arise, they should first be tallied in real time. To do this effectively requires the development and use of tally sheets appropriate to the process. This frequently calls for the use of a number of different tally sheet forms, each one tailored to a specific area on the shop floor. At regular intervals, no less often than the end of each shift, the data on one tally sheet is summarized and transcribed to a single column of a "spreadsheet," which may appropriately be called a Percent Defective (or Defects per Unit) Summary Chart, discussed in the three references listed at the end of the article. The problem addressed here was a problem of dirt in a plastic injection molding process. Figure 1 is an example of a tally sheet and Figure 2 is a Percent Defective Summary Chart.

Percent Defective Tally Sheet
P/N: X42378	Name: Bottom Plate	Machine #: 12
Date: 2/11	Shift: 1	Operator: J.S.
Defect	Tally	Totals
Dirt	1 1 1 1 1  1 1	7
Burns
Short
Blisters
Poor Trim	1 1 1 1 1
Misc.
Total Number Rejected		12
Total Number Run		574
Percent Rejected		2.1%

Figure 1. Tally Sheet for Defectives, Shift 1, February 11

Figure 2. Percent Defective Summary Chart (for best viewing, you may want to print this article)

As seen in Figure 1, the tally sheet shows the accumulation during the shift of the defects of several types. In addition, the total number of defects, the total number run and the percent defective for that shift are shown on the tally sheet. On each shift, one person must be assigned the task of completing the tally sheet and posting the results onto the percent defective summary chart. Since this information is not completely available until the end of the shift, it means that the person responsible for posting the data might (start and) end his work period 30 minutes later than the regular shift.

The results from the tally sheet of Figure 1 are transcribed into the first column of the Percent Defective Summary Chart of Figure 2, and the percent defective is recorded on the graph next to the plotted point. As an alternative, the shift number may be used as the plotting symbol. Whatever procedure is used, when multiple shifts are plotted on the same chart, a distinctive symbol must be used for each. This is an application of the general rule that, when unlike things are plotted on the same chart, classifications must be clearly noted on the chart.

As shown in Figure 2, the Percent Defective Summary Chart is not a Shewhart control chart. It is simply a tabular summary of manufacturing problems which is updated religiously at the end of each shift, supplemented by a graph which shows the overall fraction defective (or number of defects per unit) for that shift. There is neither a centerline nor control limits shown on the graph; the emphasis here is upon the overview of process performance rather than upon potentially tedious statistical calculations.

Two important results are immediately clear from the Percent Defective Summary Chart in Figure 2.

The percent defective summary chart is intended to be a problem identification tool, not a problem solving tool. However, often the clear identification of the problem leads immediately to the solution. Such was the case in the real example shown here. Attention was given to shift-to-shift differences in the plastic injection processing techniques. Note that shift 2 has consistently more defects, primarily from "dirt." After learning this, the company started looking into the dirt problem and discovered that dirt was really a problem on all shifts. Shifts 1 and 3 had large, powerful men working the shift and they were strong enough to buff the dirt out. Shift 2 had a slight woman working, so the dirt could not be buffed out. Obviously, the solution was not to get a strong man to work Shift 2, but to eliminate the dirt problem to begin with. Upon further examination it was found that the reason the dirt was occurring was a void in the hopper that was collecting the pellets as they melted, so some pellets were getting stuck there, burning, charring, and then flaking off as dirt. The solution was to fill up that void and the dirt problem was eliminated. After that, defects ran less than 1%, with more improvements continually being made as they became more visible.

Of all the techniques used to maintain and improve processes, the percent defective summary chart is the most fundamental, simplest and most useful. It will tell when to go upstream for more in-depth detective work (run charts and control charts) to discover and remove the root causes for the symptoms which have been detected.

References

M. Hart, R. Hart, and R. Stanula. "Percent Defective Summary Chart: Solving a Real Shop Problem," Journal of Quality Engineering. 14(4) 2002, pp. 547-551.
M. Hart "Quality Tools for Improvement." Production and Inventory Management Journal, 33(1) 1992, pp. 59-63.
M. Hart and R. Hart. Quantitative Methods for Quality and Productivity Improvement. American Society for Quality Control Quality Press, Milwaukee, Wisconsin, 1989.

For more information, contact Drs. Robert and Marilyn Hart at robthart@aol.com or (541) 412-0425.

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