1、BRITISH STANDARD BS 2856:1973 Precise conversion of inch and metric sizes on engineering drawings UDC 744:53.081.004.68BS2856:1973 This British Standard, having been approved by the Mechanical Engineering Industry Standards Committee, was published under the authority of the Executive Board on 8 Oct
2、ober 1973 BSI 08-1999 First published May 1957 First revision October 1973 The following BSI references relate to the work on this standard: Committee reference MEE/10 and MEE/10/1 Draft for approval 72/33231 ISBN 0 580 07752 7 Co-operating organizations The Mechanical Engineering Industry Standards
3、 Committee, under whose supervision this British Standard was prepared, consists of representatives from the following Government departments and scientific and industrial organizations: The Government departments and scientific and industrial organizations marked with an asterisk in the above list,
4、 together with the following, were directly represented on the committee entrusted with the preparation of this British Standard: Associated Offices Technical Committee Engineering Equipment Users Association* Association of Consulting Engineers Gas Council Association of Hydraulic Equipment Institu
5、tion of Civil Engineers Manufacturers Ltd. Institution of Gas Engineers Association of Mining Electrical and Institution of Heating and Ventilating Mechanical Engineers Engineers British Compressed Air Society Institution of Mechanical Engineers* British Electrical and Allied Manufacturers Instituti
6、on of Mechanical Engineers Association* (Automobile Division) British Gear Manufacturers Association* Institution of Plant Engineers British Internal Combustion Engine Institution of Production Engineers* Manufacturers Association London Transport Executive British Mechanical Engineering Confederati
7、on Machine Tool Trades Association British Pump Manufacturers Asociation Ministry of Defence British Steel Industry Ministry of Defence, Army Department* Crown Agents for Oversea Governments and National Coal Board Administrations National Physical Laboratory (Department of Department of Employment
8、Trade and Industry) Department of the Environment Process Plant Association Department of Trade and Industry* Railway Industry Association* Department of Trade and Industry National Royal Institute of British Architects Engineering Laboratory Telecommunication Engineering and Electricity Council, th
9、e Central Electricity Manufacturing Association Generating Board and the Area Boards in England and Wales* British Metalworking Plantmakers Institution of Engineering Designers Association Ministry of Defence, Navy Department British Railways Board Post Office Department of Education and Science Sci
10、entific Instrument Manufacturers Draughtsmens and Allied Technicians Association of Great Britain Association Society of British Aerospace Companies Drawing Office Material Manufacturers and Society of Motor Manufacturers and Traders Dealers Association Ltd. Electronic Engineering Association United
11、 Kingdom Atomic Energy Authority Imperial College of Science and Technology University of Manchster Institute of Science and Technology Amendments issued since publication Amd. No. Date of issue CommentsBS2856:1973 BSI 08-1999 i Contents Page Co-operating organizations Inside front cover Foreword ii
12、 Scope 1 Introduction 1 Basis of conversion procedure 2 Recommended conversion procedure 2 Rules for the conversion of inch and millimetre sizes 3 Case 1. Simple features having two limits of size 3 Case 2. Groups of simple positional features 4 Case 3. Datum dimensions 4 Case 4. Untoleranced dimens
13、ions (excluding positional and datum dimensions) 6 Appendix A Conversion tables 7 Figure 1 Examples of datum dimensions 5 Table 1 Conversion of inch sizes to millimetre sizes 2 Table 2 Conversion of millimetre sizes to inch sizes 2 Publications referred to 10BS2856:1973 ii BSI 08-1999 Foreword In th
14、e conversion of inch and metric sizes on engineering drawings, it is necessary to employ a standard procedure which will give the essential accuracy required for precise dimensional interchangeability. This standard, which was first published in1957, gives the conversion procedure recommended for th
15、is purpose; it is based on a paper prepared by Mr L. W. Nickols, of the Department of Trade and Industry: National Physical Laboratory, who had been associated with the late Mr J. E. Sears, CBE, in this work. Due to the widespread use of this standard since its first publication it has been decided
16、that the revision should be kept to a minimum. The changes which have been made are, first to correct certain inaccuracies which occurred in the text and secondly to align the document with the revision of BS308, “Engineering drawing practice”, published in1972. The conversion of sizes in work of a
17、very high accuracy has been simplified by the adoption of1inch =25.4 millimetres as an exact conversion factor by the USA and the UK. This standard, which is in agreement with ISO Recommendation R370, “Conversion of toleranced dimensions from inches into millimetres and vice versa”, has been prepare
18、d under the authority of the Mechanical Engineering Industry Standards Committee. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself
19、 confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pagesi andii, pages1 to10, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicat
20、ed in the amendment table on the inside front cover.BS2856:1973 BSI 08-1999 1 Scope This British Standard provides a procedure for conversion which will give the essential accuracy required for precise dimensional interchangeability. The standard lays down an optimum fineness of rounding the convert
21、ed size so that the conversion is made with the required precision and, at the same time, an excessive number of decimal places in the converted size is avoided. The procedure adopted is designed to ensure correct acceptance or rejection of a component, with respect to sizes, whether manufactured in
22、 one system of units or the other, provided that the methods of measurement used in the two systems are of comparable accuracy. The conversion factor used is:1inch=25.4millimetres. This conversion factor is exact. It results from the legal definition of the yard in terms of the metre adopted by the
23、USA in1959 and by the UK in the Weights and Measures Act1963. The values given in the tables inAppendix A are taken to a sufficient number of decimal places to cover the finest rounding required by the rules for conversion of inch and millimetre sizes. Introduction The inter-conversion of inch and m
24、etric sizes is of great importance to trade. Such conversions may be intended1) to provide equivalent nominal designations or2) to achieve functional interchangeability. 1) Equivalent nominal designations. Approximate corresponding values are usually sufficient. The degree of approximation depends o
25、n the purpose of the conversion. Thus, bar stock for machining may be defined as “1 in (25mm) round bar”; and a watering can may be specified as having a “2 gallon (9litre) capacity”. No rule can be given for this type of conversion. 2) Functional interchangeability. The requirement of functional in
26、terchangeability necessitates more accurate conversion than in1), the degree of accuracy again depending on the type of function which the article is called upon to perform. There are two categories to be considered: a) Where sufficient latitude is given to enable existing near sizes or combinations
27、 of sizes in either system of measurement to be used without affecting functioning. Thus, a shackle pin10mm in diameter may generally be used instead of a) in diameter shackle pin without affecting functioning; and a sheet metal cover of nominal thickness0.036in, formerly known as20SWG, can be speci
28、fied as1mm. In an example taken from BS1916-1, a hole and shaft combination selected from the inch tables to give a certain “fit” (e.g. H7 g6) is functionally interchangeable, as far as “fit” is concerned, with the corresponding combination selected from the metric tables of BS4500, even where the s
29、izes are not exactly equivalent in both systems of measurement. No definite rules can be laid down for this category. b) Where components manufactured to the two systems of units are required to be within certain precise limits of size in order to be dimensionally as well as functionally interchange
30、able. Such components are mainly met with in general engineering drawings when firms which have established manufacturing or assembly plants in other countries find it necessary to re-dimension their engineering drawings in the system of units used in those countries. The difficulty encountered for
31、such conversions of sizes arises when it is necessary to ensure that component parts acceptable when inspected to a drawing dimensioned in one system of units are not rejected when inspected to the corresponding drawing dimensioned in the other system of units. Such dimensional interchangeability ca
32、n, of course, be maintained by making the conversion to a very high degree of accuracy, i.e. by calculating the converted size to a large number of decimal places; but there is little justification in the use of a number of decimal places not warranted by the available data, and too many decimal pla
33、ces in the converted size become an embarrassment in practical use.BS2856:1973 2 BSI 08-1999 Basis of conversion procedure The converted sizes are rounded to an accuracy better than that to which it is possible or necessary to measure the original sizes in practice. Effective dimensional interchange
34、ability of components made respectively to the original and to the converted sizes is thereby secured, and no inspection discrepancies or functional troubles are likely to arise if due account is taken of the accuracy of the method of measurement used, which is assumed to be the same for the inch an
35、d metric measurements. The accuracy of inspection of a dimension, whether by gauging or measurement, is not entirely dependent on the basic accuracy of the means of inspection employed but is affected to a considerable extent by such factors as the nominal size, the quality of surface finish and the
36、 errors of geometric form of the feature being measured. The conversion procedure is based on the assumption that the highest practicable accuracy of measurement of a dimension is closely related to the stated or implied tolerance on the dimension, as this tolerance is related to the surface finish
37、and to errors of geometric form, and is itself dependent on the nominal size of the dimension. The required dimensional interchangeability can be obtained by rounding the converted size to the nearest5% of the tolerance on the component. The maximum difference in size between the original and the co
38、nverted limits will then be not more than2.5% of the tolerance. Recommended conversion procedure The recommended procedure is to follow in turn a short sequence of rules covering successive steps in the conversion. These rules are set out on later pages. By following them no difficulty should arise
39、in converting sizes expressed in inch units into their metric equivalents, or vice versa, to the required accuracy. Associated with these rules areTable 1 andTable 2 showing the recommended fineness of rounding the converted sizes corresponding to a particular tolerance. The tolerances considered ha
40、ve been grouped into different ranges as shown in the tables. In doing so, it has been necessary to depart from the strict basis given above in order to obtain a reasonably simplified procedure; but the grouping has been so arranged that no additional loss of accuracy is introduced. The case of a si
41、mple feature is dealt with first, and the recommended procedure is then applied to the more difficult case of a group of simple features which have to conform to a definite positional relationship with one another, e.g. the bolt holes in a flange are simple features having sizes which must lie betwe
42、en specified limits, and, in addition, the position of each hole in relation to the others in the group is specified. A recommended procedure for the conversion of datum dimensions is also given. Dimensions of this type, though not carrying tolerances, are often related to some requirements involvin
43、g tolerances. Procedure for the conversion of untoleranced dimensions, including dimensions not covered by a general tolerance note on the drawing and dimensions given as maximum or minimum values, is also given. Tables of conversion from inches to millimetres, and vice versa, are included to facili
44、tate the work (seeAppendix A). Table 1 Conversion of inch sizes to millimetre sizes Table 2 Conversion of millimetre sizes to inch sizes InTable 1 andTable 2, the maximum error in conversion, expressed as a percentage of the values in Columns1 and2, lies within2.5% and0.25%. 1 2 3 Original tolerance
45、 in inches Fineness of rounding. Derived millimetre sizes to be rounded to the nearest: From Up to but not including in in mm 0.000 01 0.0001 0.00001 0.0001 0.001 0.0001 0.001 0.01 0.001 0.01 0.1 0.01 0.1 1 0.1 1 2 3 Original tolerance in millimetres Fineness of rounding. Derived inch sizes to be ro
46、unded to the nearest: From Up to but not including mm mm in 0.0005 0.005 0.000001 0.005 0.05 0.00001 0.05 0.5 0.0001 0.5 5 0.001 5 50 0.01BS2856:1973 BSI 08-1999 3 Rules for the conversion of inch and millimetre sizes Case 1. Simple features having two limits of size RULE 1. From the tolerance on th
47、e feature, by means ofTable 1 orTable 2, determine the fineness of rounding the converted value and accordingly the number of decimal places to be retained therein. When the tolerance is not given directly on the drawing it should be derived by subtracting the minimum limit of size from the maximum
48、limit of size. RULE 2. Convert the two limits of size in one system of units into the equivalent limits of size in the other system of units, using the conversion factor1 in =25.4mm and retaining temporarily two more decimal places than the final number of places determined by Rule1. (SeeAppendix A)
49、. If the two limits of size are not directly stated on the drawing they should be derived. Thus, for a dimension stated as1.600in the limits of size are1.599 in and1.598 in. Conversion to the required accuracy can be carried out, with the help of the tables inAppendix A, by simple addition and subtraction of the component parts of a size. Thus, for a size of15.344mm converted accurately to seven decimal places: From these converted values the requirements of Rules1 and2 can be met. RULE 3. Round the converted limits of size obtained by Rule2 to the fineness
