(1) The losses remain hidden
Minor stops may be simple and easily corrected, but the losses they generate are surprisingly large if their rate of occurrence is high or if it is a long time before they are noticed. They are often not treated as a serious problem because people are unaware of the big losses they cause; however, in the case of automatic assembly machines and other automatic equipment, they rival changeover in accounting for the largest (20% – 40%) proportion of total losses. The first step in addressing them is therefore to measure the size of the losses.
Although it is important to find out how many minor stops occur each day or shift, either the mean time or the number of machine cycles between occurrences should also be monitored. In a high-speed machine, it is better to monitor the mean number of cycles between occurrences; often this is 100-200 before improvement but greater than 10,000 after.
(2) The causes are not addressed satisfactorily
Minor stops are not usually given the attention they deserve. They are only examined superficially and are typically dealt with by stopgap remedies that do not go the whole way and only address part of the problem, treating the symptoms but not the underlying causes.
(3) The phenomena are not observed closely enough
The most important thing to do when trying to find the causes of a minor stop is to observe it actually happening. Being in exactly the right place at exactly the right time is difficult, however; and even if a minor stop is observed, it often happens so quickly that it is hard to know what went on. This leads people to form conclusions from nothing more than the evidence left behind, with the result that any remedial action tends to be superficial and incomplete. To reduce minor stops effectively, it is essential to spend sufficient time on the shop floor patiently observing, analyzing and stratifying them before deciding what their causes are and how to eliminate them.
(4) Eliminating them is an essential precondition for unattended operation
When minor stops are rife, the following ill effects occur:
- Automated lines have to be staffed by more people than originally intended in order to achieve the required availability and output, because the original crew cannot cope with all the problems.
- It is impossible to achieve the required ratio of machines to operators (for example, despite it being theoretically possible for one operator to look after 15 machines, in practice 10 is the limit).
- Little is gained from running the line unattended during lunch breaks, because it stops producing after the first minor stop.
- The level of defectives and rework stays high.
Many production areas aspire to unattended operation, hoping to progress from attended operation (where materials still have to be supplied and removed manually) to partly unattended operation (where the line runs on its own during lunch breaks and other limited periods) and finally to fully unattended operation (where the line can run on its own all night). Various engineering and technical solutions are implemented at each stage, but the problems of minor stops and cutting-tool life often remain unsolved.
Many lines that have reached a high level of automation, with fully automated processing equipment, materials handling equipment and measurement systems, still suffer from frequent minor stops. As a result, the hopes that a high level of automation would permit the lines to be operated without anyone present during breaks or between shifts are not realized. Even when a line has a high Cp value (i.e. no quality problems) and never breaks down, little will be gained from unattended operation if a minor stop occurs right after the operators have left, or even half an hour or so later, because the line will then be inactive for the rest of the unattended period.
Nevertheless, although many companies experience this problem when they first attempt to run their lines unattended, they usually succeed in reaching their goal by carefully observing the minor stops and finding effective solutions.
The most important issue in achieving unattended operation is thus to look very closely at how to extend the mean time between minor stops. It is an issue that absolutely must be resolved, particularly if night-time unattended operation is the goal.