ISO 7870-3:2020 pdf download – Control charts —Part 3: Acceptance control charts.
The acceptance control chart is a useful tool for covering this wide range of approaches in a logical and simple manner. It distinguishes between the inherent variability components randomly occurring throughout the process and the additional location factors which contribute at less frequent intervals.
When shifts appear, the process may then stabilize at a new level until the next such event occurs. Between such disturbances, the process runs in control with respect to inherent variability.
An illustration of this situation is a process using large uniform batches of raw material. The within- batch variability could he considered to be the inherent variability. When a new batch of material is Introduced. Its deviation from the target may differ from that of the previous batch, The between-batch variation component enters the system at discrete intervals.
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 warn 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 judge the appropriate time to service the blanking die.
The acceptance control chart is based on the Shewhart control chart (i.e. X — R chart or X — s chart) hut 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 lithe Inherent variability of the process is comparatively large or a large traction of the total tolerance 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, Figure 1). there isa 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 but 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 acceptance 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 (I — a). This is called the acceptable process level APL. and it marks the outer boundary of the acceptable process zone located about the target value (see Figure 1).
Any process centred closer to the target value than the APL will have a risk smaller than a of not being accepted. So the closer 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 — fi). This undesirable process level is label led the rejectable process level (RPL). Any process located further away fruni the target value than the RPL will have a risk of acceptance smaller than fi.
The process levels lying between the APL and RPL would yield a product of borderline quality. That Is, process levels falling between the APL and RPL would represent quality which Is not so good that it would be a waste of time, or represent over-control, if the process were adjusted, and not so bad that the product could not be used if no shift in level were made. This region is often called the ‘indifference zone”. The width of this zone isa function of the requirements for a particular process and the risks one is willing to take in connection with it. The narrower the zone, i.e. the closer the API. and RPL are to each other, the larger the sample size will have to be. This approach will permit a realistic appraisal of the effectiveness of any acceptance control system, and will provide a descriptive method for showing just what any given control system is intended to do.