1、raising standards worldwide NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW BSI Standards Publication BS ISO 7870-3:2012 Control charts Part 3: Acceptance control chartsBS ISO 7870-3:2012 BRITISH STANDARD National foreword This British Standard is the UK implementation of ISO
2、7870-3:2012. It supersedes BS 7783:1994 which is withdrawn. The UK participation in its preparation was entrusted to Technical Committee SS/4, Statistical Process Management. A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not
3、purport to include all the necessary provisions of a contract. Users are responsible for its correct application. The British Standards Institution 2012. Published by BSI Standards Limited 2012 ISBN 978 0 580 56116 0 ICS 03.120.30 Compliance with a British Standard cannot confer immunity from legal
4、obligations. This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 August 2012. Amendments issued since publication Date Text affectedBS ISO 7870-3:2012 ISO 2012 Control charts Part 3: Acceptance control charts Cartes de contrle Partie 3: Cartes
5、 de contrle pour acceptation INTERNATIONAL STANDARD ISO 7870-3 First edition 2012-03-01 Reference number ISO 7870-3:2012(E)BS ISO 7870-3:2012ISO 7870-3:2012(E) ii ISO 2012 All rights reserved COPYRIGHT PROTECTED DOCUMENT ISO 2012 All rights reserved. Unless otherwise specified, no part of this publi
6、cation may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISOs member body in the country of the requester. ISO copyright office Case postale 56 CH-1211 Genev
7、a 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in SwitzerlandBS ISO 7870-3:2012ISO 7870-3:2012(E) ISO 2012 All rights reserved iii Contents Page Foreword iv Introduction v 1 Scope 1 2 Normative references . 1 3 T erms and definitions . 1 4 Symbols
8、 and abbreviated terms . 2 4.1 Symbols . 2 4.2 Abbreviated terms . 3 5 Description of acceptance control chart practice 3 6 Acceptance control of a process 5 6.1 Plotting the chart . 5 6.2 Interpreting the chart 5 7 Specifications . 5 8 Calculation procedures . 6 8.1 Selection of pairs of elements .
9、 6 8.2 Frequency of sampling 8 9 Examples . 9 9.1 Example 1 . . . 9 9.2 Example 2 10 10 Factors for acceptance control limits 11 1 1 Modified acceptance control charts .12 Annex A (normative ) Nomographs for acceptance control chart design 14 Bibliography .20BS ISO 7870-3:2012ISO 7870-3:2012(E) Fore
10、word ISO (the International Organization for Standardization ) is a worldwide federation of national standards bodies (ISO member bodies ). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a tec
11、hnical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC ) on all matters
12、of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated
13、 to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible f
14、or identifying any or all such patent rights. ISO 7870-3 was prepared by Technical Committee ISO/TC 69, Applications of statistical methods, Subcommittee SC 4, Applications of statistical methods in process management. This first edition of ISO 7870-3 cancels and replaces ISO 7966:1993. ISO 7870 con
15、sists of the following parts, under the general title Control charts: Part 1: General guidelines Part 2: Shewhart control charts Part 3: Acceptance control charts Part 4: Cumulative sum charts Additional parts on specialized control charts and on the application of statistical process control (SPC )
16、 charts are planned. iv ISO 2012 All rights reservedBS ISO 7870-3:2012ISO 7870-3:2012(E) Introduction An acceptance control chart combines consideration of control implications with elements of acceptance sampling. It is an appropriate tool for helping to make decisions with respect to process accep
17、tance. The bases for the decisions may be defined in terms of a) whether or not a designated percentage of units of a product or service derived from that process will satisfy specification requirements; b) whether or not a process has shifted beyond some allowable zone of process level locations. A
18、 difference from most acceptance sampling approaches is the emphasis on process acceptability rather than on product disposition decisions. A difference from usual control chart approaches is that the concept of process acceptance is introduced in the process control. The process usually does not ne
19、ed to be in control about a single standard process level; as long as the within-subgroup variability remains in control and is much smaller than the tolerance spread, it can (for the purpose of acceptance ) run at any level or levels within a zone of process levels which would be acceptable in term
20、s of tolerance requirements. Thus, it is assumed that some assignable causes will create shifts in the process levels which are small enough in relation to requirements that it would be uneconomical to attempt to control them too tightly for the purpose of mere acceptance. The use of an acceptance c
21、ontrol chart does not, however, rule out the possibility of identifying and removing assignable causes for the purpose of continuing process improvement. A check on the inherent stability of the process is required. Therefore, variables are monitored using Shewhart- type range or sample standard dev
22、iation control charts to confirm that the variability inherent within rational subgroups remains in a steady state. Supplementary examinations of the distribution of the encountered process levels form an additional source of control information. A preliminary Shewhart control chart study should be
23、conducted to verify the validity of using an acceptance control chart. ISO 2012 All rights reserved vBS ISO 7870-3:2012BS ISO 7870-3:2012Control charts Part 3: Acceptance control charts 1 Scope This part of ISO 7870 gives guidance on the uses of acceptance control charts and establishes general proc
24、edures for determining sample sizes, action limits and decision criteria. An acceptance control chart should be used only when: a) the within subgroup variation is in-control and the variation is estimated efficiently; b) a high level of process capability has been achieved. An acceptance control ch
25、art is typically used when the process variable under study is normally distributed; however, it can be applied to a non-normal distribution. The examples provided in this part of ISO 7870 illustrate a variety of circumstances in which this technique has advantages; these examples provide details of
26、 the determination of the sample size, the action limits and the decision criteria. 2 Normative references The following standards, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For un
27、dated references, the latest edition of the refferenced document (including any amendments) applies. ISO 3534-1, Statistics Vocabulary and symbols Part 1: General statistical terms and terms used in probability ISO 3534-2, Statistics Vocabulary and symbols Part 2: Applied statistics 3 T erms and def
28、initio ns For the purposes of document, the terms and definitions given in ISO 3534-1 and ISO 3534-2 apply. 3.1 acceptable process process which is represented by a Shewhart control chart with a central line within the acceptable process zone NOTE 1 Ideally, the average value X of such a control cha
29、rt would be at the target value. NOTE 2 The acceptable process zone is shown in Figure 1. Information on the Stewhart control chart can be found in ISO 7870-2. INTERNATIONAL STANDARD ISO 7870-3:2012(E) ISO 2012 All rights reserved 1BS ISO 7870-3:2012ISO 7870-3:2012(E) F i g u r e 1 T w o - s i d e d
30、 s p e c i fi c a t i o n l i m i t s : U p p e r a n d l o w e r A P L a n d R P L l i n e s i n r e l a t i o n t o processes of acceptable, rejectable, and indifference (borderline) quality 4 Symbols and abbreviated terms NOTE The ISO/IEC Directives makes it necessary to depart from common SPC us
31、age in respect to the differentiation between abbreviated terms and symbols. An abbreviated term and its symbol can differ in appearance in two ways: by font and by layout. To distinguish between abbreviated terms and symbols, abbreviated terms are given in Arial upright and symbols in Times New Rom
32、an or Greek italics, as applicable. Whereas abbreviated terms can contain multiple letters, symbols consist only of a single letter. For example, the conventional abbreviation of acceptable process limit, APL, is valid but its symbol in equations becomes A PL . The reason for this is to avoid misint
33、erpretation of compound letters as an indication of multiplication. 4.1 Symbols A CL acceptance control limits A PL acceptable process level L lower specification limit n subgroup sample size p 0 acceptable proportion nonconforming items p 1 rejectable proportion nonconforming items P a probability
34、of acceptance R PL rejectable process level or non-acceptable process zone T target value, i.e. the optimum value of the characteristic U upper specification limit X average value of the variable X plotted on a control chart z variable that has a normal distribution with zero mean and unit standard
35、deviation z p normal deviate that is exceeded by 100p % of the deviate in a specified direction (similarly for z , z , etc.) risk of not accepting a process centred at the APL risk of not rejecting a process centred at the RPL 2 ISO 2012 All rights reservedBS ISO 7870-3:2012ISO 7870-3:2012(E) proces
36、s mean w within-subgroup standard deviation corresponding to the inherent process variability X standard deviation of the subgroup average corresponding to the inherent process variability: 4.2 Abbreviated terms ACL acceptance control limits APL acceptable process level L lower specification limit (
37、used as a subscript) OC operating characteristic RPL rejectable process level or non-acceptable process zone U upper specification limit (used as a subscript) 5 Description of acceptance control chart practice In the pursuit of an acceptable product or service, there often is room for some latitude
38、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 tol
39、erated. 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 differen
40、t 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 seek to improve the capability to mai
41、ntain 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 co
42、unterproductive 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. The accepta
43、nce 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 s
44、hifts 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 vari
45、ability could be 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-batc
46、h variation might 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
47、 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. ISO 2012 All rights reserved 3BS ISO 7870-3:2012ISO 7870-3:2012(E) The acceptance control chart is based on the Shewhart con
48、trol chart (i.e. X R chart or X 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 larg
49、e fraction 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 proces
