1、raising standards worldwideNO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAWBSI Standards PublicationBS ISO 7870-3:2012Control chartsPart 3: Acceptance control chartsBS ISO 7870-3:2012 BRITISH STANDARDNational forewordThis British Standard is the UK implementation of ISO 7870-3:
2、2012.It supersedes BS 7783:1994 which is withdrawn.The UK participation in its preparation was entrusted to Technical Committee SS/4, Statistical Process Management.A list of organizations represented on this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 purport to
3、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 2012ISBN 978 0 580 56116 0 ICS 03.120.30 Compliance with a British Standard cannot confer immunity from legal obligations.
4、This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 August 2012.Amendments issued since publicationDate Text affectedBS ISO 7870-3:2012 ISO 2012Control charts Part 3: Acceptance control chartsCartes de contrle Partie 3: Cartes de contrle pour
5、acceptationINTERNATIONAL STANDARDISO7870-3First edition2012-03-01Reference numberISO 7870-3:2012(E)BS ISO 7870-3:2012ISO 7870-3:2012(E)ii ISO 2012 All rights reservedCOPYRIGHT PROTECTED DOCUMENT ISO 2012All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or
6、 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 officeCase postale 56 CH-1211 Geneva 20Tel. + 41 22 749 01 11Fax
7、 + 41 22 749 09 47E-mail copyrightiso.orgWeb www.iso.orgPublished in SwitzerlandBS ISO 7870-3:2012ISO 7870-3:2012(E) ISO 2012 All rights reserved iiiContents PageForeword ivIntroduction v1 Scope 12 Normative references .13 Terms and definitions .14 Symbols and abbreviated terms .24.1 Symbols .24.2 A
8、bbreviated terms .35 Description of acceptance control chart practice 36 Acceptance control of a process 56.1 Plotting the chart .56.2 Interpreting the chart 57 Specifications .58 Calculation procedures .68.1 Selection of pairs of elements .68.2 Frequency of sampling 89 Examples .99.1 Example 1 . 99
9、.2 Example 2 1010 Factors for acceptance control limits 1111 Modified acceptance control charts .12Annex A (normative) Nomographs for acceptance control chart design 14Bibliography .20BS ISO 7870-3:2012ISO 7870-3:2012(E)ForewordISO (the International Organization for Standardization) is a worldwide
10、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 technical committee has been established has the right to be represented on that c
11、ommittee. 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 of electrotechnical standardization.International Standards are drafted in accor
12、dance 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 to the member bodies for voting. Publication as an International Standard require
13、s 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 for identifying any or all such patent rights.ISO 7870-3 was prepared by Technical C
14、ommittee 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 consists of the following parts, under the general title Control charts: Part 1: General
15、guidelines Part 2: Shewhart control charts Part 3: Acceptance control charts Part 4: Cumulative sum chartsAdditional parts on specialized control charts and on the application of statistical process control (SPC) charts are planned.iv ISO 2012 All rights reservedBS ISO 7870-3:2012ISO 7870-3:2012(E)I
16、ntroductionAn 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 acceptance. The bases for the decisions may be defined in terms ofa) whether or not a designated
17、 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 difference from most acceptance sampling approaches is the emphasis on process acceptability
18、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 need to be in control about a single standard process level; as long as the within-subgroup varia
19、bility 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 terms of tolerance requirements. Thus, it is assumed that some assignable causes will create shifts
20、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 control chart does not, however, rule out the possibility of identifying and removing assignable c
21、auses 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 deviation control charts to confirm that the variability inherent within rational subgroups remains in
22、 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 conducted to verify the validity of using an acceptance control chart. ISO 2012 All rights reserved
23、 vBS ISO 7870-3:2012BS ISO 7870-3:2012Control charts Part 3: Acceptance control charts1 ScopeThis part of ISO 7870 gives guidance on the uses of acceptance control charts and establishes general procedures for determining sample sizes, action limits and decision criteria. An acceptance control chart
24、 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 chart is typically used when the process variable under study is normally distributed; however, it can be
25、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 the determination of the sample size, the action limits and the decision criteria.2 Normative reference
26、sThe 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 undated references, the latest edition of the refferenced document (including any amendments) applies. ISO 3
27、534-1, Statistics Vocabulary and symbols Part 1: General statistical terms and terms used in probabilityISO 3534-2, Statistics Vocabulary and symbols Part 2: Applied statistics3 Terms and definitionsFor the purposes of document, the terms and definitions given in ISO 3534-1 and ISO 3534-2 apply.3.1a
28、cceptable processprocess 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 chart would be at the target value.NOTE 2 The acceptable process zone is shown in Figure 1. Information on the Stewha
29、rt 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)Figure 1 Two-sided specification limits: Upper and lower APL and RPL lines in relation to processes of acceptable, rejectable, and indifference (bor
30、derline) quality4 Symbols and abbreviated termsNOTE The ISO/IEC Directives makes it necessary to depart from common SPC usage 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 di
31、stinguish between abbreviated terms and symbols, abbreviated terms are given in Arial upright and symbols in Times New Roman 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
32、 acceptable process limit, APL, is valid but its symbol in equations becomes APL. The reason for this is to avoid misinterpretation of compound letters as an indication of multiplication. 4.1 SymbolsACLacceptance control limitsAPLacceptable process levelL lower specification limitn subgroup sample s
33、izep0acceptable proportion nonconforming itemsp1rejectable proportion nonconforming itemsPaprobability of acceptanceRPLrejectable process level or non-acceptable process zoneT target value, i.e. the optimum value of the characteristicU upper specification limitXaverage value of the variable X plotte
34、d on a control chartz variable that has a normal distribution with zero mean and unit standard deviationzp 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 c
35、entred at the RPL2 ISO 2012 All rights reservedBS ISO 7870-3:2012ISO 7870-3:2012(E) process meanwwithin-subgroup standard deviation corresponding to the inherent process variabilityXstandard deviation of the subgroup average corresponding to the inherent process variability: Xn=w/4.2 Abbreviated ter
36、msACL acceptance control limitsAPL acceptable process levelL lower specification limit (used as a subscript)OC operating characteristicRPL rejectable process level or non-acceptable process zoneU upper specification limit (used as a subscript)5 Description of acceptance control chart practiceIn the
37、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
38、 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 rarel
39、y 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
40、be minimized. Supporters of such an approach seek 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 ach
41、ieved, 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 contr
42、ol aspects but not product or process quality improvement programmes.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
43、 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 i
44、s a process using large uniform batches of raw material. The within-batch variability 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
45、 the system at discrete intervals.An example of this within- and between-batch 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
46、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. ISO 2012 All rights reserved 3BS ISO 7870-3:201
47、2ISO 7870-3:2012(E)The acceptance control chart is based on the Shewhart control 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
48、 if the inherent variability of the process is comparatively large or a large 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 th
49、e 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 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 acceptabl
