1、Control chartsPart 8: Charting techniques for short runs and small mixed batchesBS ISO 78708:2017BSI Standards PublicationWB11885_BSI_StandardCovs_2013_AW.indd 1 15/05/2013 15:06National forewordThis British Standard is the UK implementation of ISO 78708:2017.The UK committee draws users attention t
2、o Figures 8-12. The figure titles suggest they are computer generated when they have actually been drawn by a drawing office. Computer generated figures are preferred, to avoid the time consuming and unnecessary checking of these figures and to more realistically show the outputs that users will enc
3、ounter. 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 include all the necessary provisions of a c
4、ontract. Users are responsible for its correct application.ISBN 978 0 580 89263 9ICS 03.120.30Compliance with a British Standard cannot confer immunity from legal obligations. This British Standard was published under the authority of the Standards Policy and Strategy Committee on 30 April 2017.Amen
5、dments/corrigenda issued since publicationDate Text affected BRITISH STANDARDBS ISO 78708:2017 ISO 2017Control charts Part : Charting techniques for short runs and small mixed batchesCartes de contrle Partie : Techniques de cartes pour petites sries et pour petits lots combinsINTERNATIONAL STANDARDI
6、SO7870First edition20170401Reference numberISO 7870:2017(E)BS ISO 78708:2017ISO 7870:2017(E)ii ISO 2017 All rights reservedCOPYRIGHT PROTECTED DOCUMENT ISO 2017, Published in SwitzerlandAll rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwi
7、se in any form or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below or ISOs member body in the country of the requester.ISO copyright officeCh.
8、de Blandonnet 8 CP 401CH-1214 Vernier, Geneva, SwitzerlandTel. +41 22 749 01 11Fax +41 22 749 09 47copyrightiso.orgwww.iso.orgBS ISO 78708:2017ISO 7870:2017(E)Foreword vIntroduction vi1 Scope . 12 Normative references 13 Terms, definitions and symbols 13.1 Terms and definitions . 13.2 Symbols . 14 H
9、ow to select the correct type of Shewhart control chart for continuous variables data 24.1 General . 24.2 How to select the correct type of Shewhart control chart for measured data generally . 24.3 How to select the Shewhart control chart when the characteristic does not have a constant aim or proce
10、ss spread . 35 How to prepare for short run, small mixed batch control charting 55.1 Focus on the process 55.2 Procedure for grouping similar processes 55.3 Typical applications 75.4 Preliminary process diagnosis 85.5 Procedure to establish the correct initial setup of a process characteristic 85.5.
11、1 Purpose 85.5.2 Scope and limitations. 85.5.3 Reasons for need of procedure 85.5.4 Method 95.5.5 Example 105.6 Procedure to pre-establish control limits for SPC charts for short run, small batch, processes 115.6.1 Purpose . 115.6.2 Scope of application .115.6.3 Reasons for need of procedure . 115.6
12、.4 Method . 115.6.5 Example 126 How to establish and apply short run, small mixed batch, control charts 156.1 General 156.2 Variable aim, individual and moving range chart 156.2.1 Purpose . 156.2.2 Scope of application .166.2.3 Method . 166.2.4 Example 166.3 Variable aim, moving mean and moving rang
13、e chart .186.3.1 Purpose . 186.3.2 Scope of application .186.3.3 Method . 186.3.4 Example 196.4 Universal, individual and moving range chart 206.4.1 Purpose . 206.4.2 Scope of application .206.4.3 Method . 206.4.4 Example 216.5 Universal, moving mean and moving range chart .226.5.1 Purpose . 226.5.2
14、 Scope of application .23 ISO 2017 All rights reserved iiiContents PageBS ISO 78708:2017ISO 7870:2017(E)6.5.3 Method . 236.5.4 Example 24Annex A (informative) Reproducible copies of control charts forms and normal probability worksheet .25Bibliography .31iv ISO 2017 All rights reservedBS ISO 78708:2
15、017ISO 7870:2017(E)ForewordISO (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 sub
16、ject for which a technical 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 (I
17、EC) on all matters of electrotechnical standardization.The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the different types of ISO documents should be
18、noted. This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).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 identifyi
19、ng any or all such patent rights. Details of any patent rights identified during the development of the document will be in the Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents).Any trade name used in this document is information given for the convenience
20、of users and does not constitute an endorsement.For an explanation on the meaning of ISO specific terms and expressions related to conformity assessment, as well as information about ISOs adherence to the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the foll
21、owing URL: www.iso.org/iso/foreword.html.The committee responsible for this document is ISO/TC 69, Applications of statistical methods, Subcommittee SC 4, Applications of statistical methods in product and process management.A list of parts in the ISO 7870 series can be found on the ISO website. ISO
22、 2017 All rights reserved vBS ISO 78708:2017ISO 7870:2017(E)IntroductionIt is generally recommended that at least 25 subgroups of data be collected, and plotted, before any constructive analysis can take place to form the basis for establishing standard traditional variables control charts. This rep
23、resents best practice for the application of standard statistical process control (SPC) charts to long production runs of a single product characteristic (for instance, a diameter) or a process parameter (for instance, temperature). However, it presents a problem in many potential applications of SP
24、C.In the business environment, there is an increasing need for versatility and flexibility in highly efficient systems. These support just-in-time inventories and create greater product variety, with smaller batches and shorter runs. The consequent ever-increasing resets, changeovers, die changes, a
25、nd so on, bring new challenges to the meaningful application of SPC. These occur at a critical time when the pressure for continual performance improvement has never been greater.Processes accommodate many part numbers, often of similar shape but different nominal sizes at best, and part configurati
26、ons having multiple characteristics with different specified nominal values, units of measure and tolerances. For example, a bolt maker with short runs of various size bolts (diameter and length), or a tube extruder with tubes of different size outside diameter, inside diameter and wall thickness. T
27、he customary approach is to put a different standard control chart on each characteristic of each part number. The consequences of this administratively cumbersome, product-focused, procedure would include the generation of large numbers of run charts each containing data too sparse to be useful, ei
28、ther for control or improvement.In the same way that other functions have responded to the challenge, for instance, the introduction of lean methods and single minute exchange of die (SMED) in production, so the SPC facilitating function responds. This situation presents both a problem and an opport
29、unity.The problem arises because, in many organizations, production runs are often too small to generate enough data to apply standard control charts. This can occur in two ways. Firstly, there is the case where the batch, or lot, size itself is very small. Secondly, there is the situation where the
30、 run is very short; for instance, the high speed stamping operation that may run only for a short period. It is frequently not practicable, in either case, to generate enough subgroups to make the control chart meaningful.The opportunity arises because much current statistical process control is act
31、ually statistical product control, that is, SPC implementation is often product-focused rather than process-focused. Different products that are generated by a single or similar process are looked upon as dissimilar entities. Consequently, sources of process variation can be overlooked when analysin
32、g the product orientated control chart. Due to the sparseness of product information in short run, small batch situations, the focus has to be on the common element, the process. Short run SPC provides the means to transform a succession of short run productrelated jobs into a long term process. An
33、example is the “jobbing” shop that does not make many of the same part, but has a number of processes that are continually being employed. They turn many shafts, drill many holes, etc., continually. The grouping of drilling, turning, grinding processes and the like, or their corresponding facilities
34、 (for instance, machine tools) could make good candidates for the application of short run SPC.Some basic statistical concepts, terminology and symbols are introduced in this document; however, these are kept to a minimum. The language chosen is that of the workplace rather than that of the statisti
35、cian. The aim is to make this document readily comprehensible to the extensive range of prospective users and too facilitate widespread communication and understanding of the method.It is advisable that those who are not familiar with the control chart technique read both ISO 78701 and ISO 78702 bef
36、ore reading this document.vi ISO 2017 All rights reservedBS ISO 78708:2017Control charts Part : Charting techniques for short runs and small mixed batches1 ScopeThis document describes ways of applying regular variables control charts to short runs and small mixed batches where the sample size for m
37、onitoring is restricted to one. It provides a set of tools to facilitate the understanding of sources of variation in such processes so that the processes can be better managed.The charts described are process-focused rather than product-focused. The user can plot, monitor and control similar charac
38、teristics on different items, or different characteristics on an item, on a single control chart.NOTE 1 The terms short run and small batch size are not well defined. Here, short run and small batch size are taken to mean only a few items are manufactured before a different item is then produced.NOT
39、E 2 For situations where the subgroup size is larger than one, other standards apply.2 Normative referencesThere are no normative references in this document.3 Terms, definitions and symbols3.1 Terms and definitionsFor the purposes of this document, the terms and definitions given in ISO 3534-2 appl
40、y.ISO and IEC maintain terminological databases for use in standardization at the following addresses: IEC Electropedia: available at http:/www.electropedia.org/ ISO Online browsing platform: available at http:/www.iso.org/obp3.2 SymbolsCLcentre line of a control chartLCLLxCL, LxCLand LRCLare the lo
41、wer control limits for individuals, mean and range, respectivelyT target (aim) valuen subgroup sizeR the difference between the maximum and minimum of the valuesINTERNATIONAL STANDARD ISO 7870:2017(E) ISO 2017 All rights reserved 1BS ISO 78708:2017ISO 7870:2017(E)Rexpthe expected value of the range
42、of a particular characteristicRmovingmoving range, the difference between the maximum and minimum of the consecutive valuesS process standard deviations realized value of the process standard deviationu test statistic for setup acceptanceUCLUxCL, UxCLand URCLare the upper control limits for individu
43、als, mean and range, respectivelyX general value of a quality characteristic of the process meanx realized value of a quality characteristic of the process mean4 How to select the correct type of Shewhart control chart for continuous variables data4.1 GeneralThe business aim of statistical process c
44、ontrol (SPC) is to control and improve quality, increase productivity and reduce cost. The principal graphical tool of SPC is the control chart. There are three main classes of control charts: Shewhart, cumulative sum (cusum) and exponentially weighted moving average (EWMA).NOTE Cusum control charts
45、 are dealt with in ISO 7870-4 and EWMA in ISO 7870-6.The Shewhart control chart provides a graphical representation of a process showing plotted values of a representative statistic of a selected characteristic (for instance, the individual value, mean, range or standard deviation), a centre line, a
46、nd one or more control lines. The control line(s) and centre line are used as a basis for judging the stability of the process, namely, whether or not the process is in a state of statistical control. Control lines are derived from the actual performance of the process and are not to be confused wit
47、h specified limits or specified tolerances.Shewhart control charts provide a common language for communicating technical information on the performance of a process. Control charts are effective tools in understanding process behaviour. They distinguish between special and common cause variation. Wh
48、en no special cause is present, the process is said to be in a state of statistical control.When a process is in statistical control, its capability is predictable and can be assessed. Reducing common cause variation and improving process targeting can enhance process capability.Potentially, the con
49、trol chart has wide applicability throughout any organization.4.2 How to select the correct type of Shewhart control chart for measured data generallyThe procedure for selecting a Shewhart type measured data control chart is as follows.a) If the characteristic to be monitored is ongoing with a targeted constant aim and process spread, refer to ISO 78702.b) If the characteristics do not have a constant aim or process spread, and the sample size is limited to one, see 4.3.2 ISO 2017 All rights reservedBS ISO 78708:2017ISO 7870:201
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