1、Control charts Part 8: Charting techniques for short runs and small mixed batches BS ISO 78708:2017 BSI Standards Publication WB11885_BSI_StandardCovs_2013_AW.indd 1 15/05/2013 15:06National foreword This British Standard is the UK implementation of ISO 78708:2017. The UK committee draws users atten
2、tion to 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 wi
3、ll encounter. 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 provision
4、s of a contract. Users are responsible for its correct application. ISBN 978 0 580 89263 9 ICS 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
5、 2017. Amendments/corrigenda issued since publicationDate Text affected BRITISH STANDARD BS ISO 78708:2017 ISO 2017 Control charts Part : Charting techniques for short runs and small mixed batches Cartes de contrle Partie : Techniques de cartes pour petites sries et pour petits lots combins INTERNAT
6、IONAL STANDARD ISO 7870 First edition 20170401 Reference number ISO 7870:2017(E) BS ISO 78708:2017 ISO 7870:2017(E)ii ISO 2017 All rights reserved COPYRIGHT PROTECTED DOCUMENT ISO 2017, Published in Switzerland All rights reserved. Unless otherwise specified, no part of this publication may be repro
7、duced or utilized otherwise 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
8、. ISO copyright office Ch. de Blandonnet 8 CP 401 CH-1214 Vernier, Geneva, Switzerland Tel. +41 22 749 01 11 Fax +41 22 749 09 47 copyrightiso.org www.iso.org BS ISO 78708:2017 ISO 7870:2017(E)Foreword v Introduction vi 1 Scope . 1 2 Normative references 1 3 T erms, definitions and s ymbols 1 3.1 Te
9、rms and definitions . 1 3.2 Symbols . 1 4 How to select the correct type of Shewhart control chart for continuous variables data 2 4.1 General . 2 4.2 How to select the correct type of Shewhart control chart for measured data generally . 2 4.3 How to select the Shewhart control chart when the charac
10、teristic does not have a constant aim or process spread . 3 5 How to prepare for short run, small mixed batch control charting 5 5.1 Focus on the process 5 5.2 Procedure for grouping similar processes 5 5.3 Typical applications 7 5.4 Preliminary process diagnosis 8 5.5 Procedure to establish the cor
11、rect initial setup of a process characteristic 8 5.5.1 Purpose 8 5.5.2 Scope and limitations. 8 5.5.3 Reasons for need of procedure 8 5.5.4 Method 9 5.5.5 Example 10 5.6 Procedure to pre-establish control limits for SPC charts for short run, small batch, processes 11 5.6.1 Purpose .11 5.6.2 Scope of
12、 application .11 5.6.3 Reasons for need of procedure .11 5.6.4 Method .11 5.6.5 Example 12 6 How to establish and apply short run, small mixed batch, control charts 15 6.1 General 15 6.2 Variable aim, individual and moving range chart 15 6.2.1 Purpose .15 6.2.2 Scope of application .16 6.2.3 Method
13、.16 6.2.4 Example 16 6.3 Variable aim, moving mean and moving range chart .18 6.3.1 Purpose .18 6.3.2 Scope of application .18 6.3.3 Method .18 6.3.4 Example 19 6.4 Universal, individual and moving range chart 20 6.4.1 Purpose .20 6.4.2 Scope of application .20 6.4.3 Method .20 6.4.4 Example 21 6.5
14、Universal, moving mean and moving range chart .22 6.5.1 Purpose .22 6.5.2 Scope of application .23 ISO 2017 All rights reserved iii Contents Page BS ISO 78708:2017 ISO 7870:2017(E)6.5.3 Method .23 6.5.4 Example 24 Annex A (informative) Reproducible copies of control charts forms and normal probabili
15、ty worksheet .25 Bibliography .31 iv ISO 2017 All rights reserved BS ISO 78708:2017 ISO 7870:2017(E) Foreword 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
16、 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 committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the w
17、ork. ISO collaborates closely with the International Electrotechnical Commission (IEC) 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 di
18、fferent approval criteria needed for the different types of ISO documents should be 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
19、 may be the subject of patent rights. ISO shall not be held responsible for identifying 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/pa
20、tents). Any trade name used in this document is information given for the convenience 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 Tr
21、ade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following 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 proce
22、ss management. A list of parts in the ISO 7870 series can be found on the ISO website. ISO 2017 All rights reserved v BS ISO 78708:2017 ISO 7870:2017(E) Introduction It is generally recommended that at least 25 subgroups of data be collected, and plotted, before any constructive analysis can take pl
23、ace to form the basis for establishing standard traditional variables control charts. This represents 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 (f
24、or instance, temperature). However, it presents a problem in many potential applications of SPC. 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 small
25、er batches and shorter runs. The consequent ever-increasing resets, changeovers, die changes, and 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 man
26、y part numbers, often of similar shape but different nominal sizes at best, and part configurations 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 tu
27、be extruder with tubes of different size outside diameter, inside diameter and wall thickness. The 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 includ
28、e the generation of large numbers of run charts each containing data too sparse to be useful, either 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
29、, so the SPC facilitating function responds. This situation presents both a problem and an opportunity. 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 c
30、ase where the batch, or lot, size itself is very small. Secondly, there is the situation where the 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 contr
31、ol chart meaningful. The opportunity arises because much current statistical process control is actually 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
32、 as dissimilar entities. Consequently, sources of process variation can be overlooked when analysing 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
33、 the means to transform a succession of short run product related jobs into a long term process. An 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. T
34、he grouping of drilling, turning, grinding processes and the like, or their corresponding facilities (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
35、 are kept to a minimum. The language chosen is that of the workplace rather than that of the statistician. 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
36、that those who are not familiar with the control chart technique read both ISO 78701 and ISO 78702 before reading this document.vi ISO 2017 All rights reserved BS ISO 78708:2017 Control charts Part : Charting techniques for short runs and small mixed batches 1 Scope This document describes ways of a
37、pplying regular variables control charts to short runs and small mixed batches where the sample size for monitoring 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 descr
38、ibed are process-focused rather than product-focused. The user can plot, monitor and control similar characteristics 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
39、 batch size are taken to mean only a few items are manufactured before a different item is then produced. NOTE 2 For situations where the subgroup size is larger than one, other standards apply. 2 Normative references There are no normative references in this document. 3 T erms, d efinitions and s y
40、mbols 3.1 T erms and definiti ons For the purposes of this document, the terms and definitions given in ISO 3534-2 apply. ISO and IEC maintain terminological databases for use in standardization at the following addresses: IEC Electropedia: available at http:/ /www.electropedia.org/ ISO Online brows
41、ing platform: available at http:/ /www.iso.org/obp 3.2 Symbols C L centre line of a control chartL , and are the lower control limits for individuals, mean and range, respec tively T target (aim) value n subgroup size R the difference between the maximum and minimum of the values INTERNATIONAL ST AN
42、DARD ISO 7870:2017(E) ISO 2017 All rights reserved 1 BS ISO 78708:2017 ISO 7870:2017(E) R exp the expected value of the range of a particular characteristic R moving moving range, the difference between the maximum and minimum of the consecutive values S process standard deviation s realized value o
43、f the process standard deviation u test statistic for set up acceptanceU , and are the upper control limits for individuals, mean and range, respec tivelyX general value of a quality characteristic of the process meanx realized value of a quality characteristic of the process mean 4 How to select th
44、e correct type of Shewhart control chart for continuous variables data 4.1 General The business aim of statistical process control (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
45、 control charts: Shewhart, cumulative sum (cusum) and exponentially weighted moving average (EWMA). NOTE Cusum control charts 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 st
46、atistic of a selected characteristic (for instance, the individual value, mean, range or standard deviation), a centre line, and 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 sta
47、te of statistical control. Control lines are derived from the actual performance of the process and are not to be confused with specified limits or specified tolerances. Shewhart control charts provide a common language for communicating technical information on the performance of a process. Control
48、 charts are effective tools in understanding process behaviour. They distinguish between special and common cause variation. When 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
49、be assessed. Reducing common cause variation and improving process targeting can enhance process capability. Potentially, the control chart has wide applicability throughout any organization. 4.2 How to select the correct type of Shewhart control chart for measured data generally The 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)
