ISO 7870-3 pdf download – Control charts —Part 3: Acceptance control charts

ISO 7870-3 pdf download – Control charts —Part 3: Acceptance control charts.
5 Description of acceptance control chart practice
In the pursuit of an acceptable product or service, there often is room for some latitude in the ability to centre a process around its target level. The contribution to overall variation of such location factors Is additional to the inherent random variability of Individual elements around a given process level. In most cases, some shifts in process level must be expected and can be tolerated. these shifts usually result from an assignable cause that cannot be eliminated because of engineering or economic considerations. They often enter the system at Infrequent or Irregular Intervals, but can rarely be treated as random components of variance.
There are several seemingly different approaches to treating these location factors contributing variation beyond that of inherent variability. At one extreme is the approach in which all variability that results in deviations from the target value must be minimized. Supporters of such an approach seck to improve the capability to maintain a process within tighter tolerance limits so that there Is greater potential for process or product quality improvement.
At the other extreme Is the approach that If a high level of process capability has been achieved, It Is not only uneconomic and wasteful of resources, but it can also be counterproductive to try to improve the capability of the process. This often is the result of the Introduction of pressures which encourage “tampering” with the process (over-control) by people qualified to work on control aspects but not product or process quality improvement programmes.
An example of this within- and between-batch variation can very well occur in a situation where a blanking die Is blanking a machine part The purpose of the chart Is to determine when the die has worn to a point where it must be repaired or reworked. The rate of wear is dependent upon the hardness of the successive batches of material and is therefore not readily predictable. It will be seen that the use of an acceptance control chart makes It possible to udge the approprbte time to service the blanking die.
The acceptance control chart is based on the Shewhart control chart (i.e. , — R chart or , — s chart) but Is set up so that the process mean can shift outside of control limits of the Shewhart control chart If the specifications are sufficiently wide, or be confined to narrower limits if the inherent variability of the process is comparatively large or a large fraction of the total tolera nce spread.
What is required is protection against a process that has shifted so far from the target value that it will yield some predetermined undesirable percentage of items falling outside the specification limits, or exhibits an excessive degree of process level shift.
When a chart of the average value of data sets from a process is plotted, in sequence of the production, one notices a continual variation In average values. In a central zone (acceptable process, Fiure 1). there is a product that is indisputably acceptable. Data in the outer zones (Figure 1) represent a process that is producing product that is indisputably not acceptable.
Between the inner and the outer zones are zones where the product is acceptable hut there is an indication that the process should be watched and, as the outer zone is approached, corrective action may be taken. These criteria are the basic concepts for the acceptant-c control chart. The description In this document is designed to provide practices for the establishment of appropriate action lines for one- and two-sided specification situations.
Since it is impossible to have a single dividing line that can sharply distinguish a good from an unsatisfactory quality level, one must define a process level that represents a process that should be accepted almost always (1 – a). This is called the acceptable process level (APL), and it marks the outer l boundary of the acceptable process zone located about the target value (see hgure 1).
Any process centred doser to the target value than the APL will have a risk smaller than a of not being accepted. So the doser the process is to the target, the smaller the likelihood that a satisfactory process will not be accepted.
It is also necessary to define the process level that represents processes that should almost never be accepted (1 — U). This undesirable process level is labelled the reectable process level (RPL). Any process located further away from the target value than the RPL will have a risk of acceptance smaller than B.