Specificationfor
Unified screw threads --
Part3:Diametersbelow%in
Co-operatingorganizations
The Mechanical Engineering Industry Standards Committee under whosesupervision this British Standard was prepared consists of representatives from the following Government departments and scientific and industrialorganizations:
Institution of Heating and Ventilating EngineersInstitution of Mechanical Engineers*Institution of Mechanical Engineers (Automobile Division)Institution of Produetion Engineers* Lootive and Allied ManufacturersAssociation of Great Britain*London Transport Board* Machine Tool Trades Association*Ministry of Defenoe Army Department Ministry of Defence Navy Department*Ministry of Labour (H.M. FactoryMinistry of Power Inspectorate)Ministry of Technology - National Ministry of Public Building and WorksMinistry of Transport Engineering LaboratoryNational Coal BoardNational Physical Laboratory (Ministry of Technology)*Radio Industry Council* Royal Institute of British Architects
Associated Offices’Technicnal Committee Association of Consulting EngineersAssociation of Mining Electrical andBritish Chemical Plant Manufacturers Mechanical EngineersBritish Compresed Air Society AssociationBritish Electrical and Allied Manufacturers’British Gear Manufacturers’ Association Association*British Internal Combustion Engine Manufacturers" AssociationBritish Iron and Steel FederationBritish Mechanical Engineering Federation* British Railways Board*Crown Agents for Oversea Governments and AdministrationsEletricity Council The Generating Board and the Area Boards in England and WalesEngineering Equipment Users′ AssociationInstitute of Marine Engineers Gas CouncilInstitution of Gas Engineers Institution of Civil Engineers
The Government departments and scientific and industrial organizationsmarked with an asterisk in the above list together with the following were directly represented on the mittee entrusted with the preparation of thisstandard:
Scientifie Instrument Manufacturers’Screw Thread Tool Manufacturers AssociationAssociationSociety of British Aerospaoe Companies Ltd. Telemunication Engineering andManufacturing Assoeiation
Agricultural Engineers" AssociationElectronic Engineering Asociation British Bol Nut Serew and Rivet FederationFasteners and Turned Parts InstituteNational Asociation of Marine Engine Ministry of AviationBuilders
This British Standard havingMechanical Engineering been approved by theIndustry Standards Committee and endorsed by the Chairmanof the Engineering Divisional Council was published underthe authority of the General Council on11 August 1965
Amendmentsissued since publication
Amd. No. Date Comments8169 September19661344 January March 1979 Indicated by a sideline in the margin
e BSI 09-1999
The following BSI referenoes relate to the work on thisCommittee references MEB/1 standard:Draft for ment D 63/8464 MEE/1/13
ISBN 0 580 30476 0
Contents
PageCo-operating organizations Foreword Inside front cover i1 Seope 12 ISO/Unified thread form Standard thread series 14 Basic sizes 1 15 Screw thread classes Tolerances allowances and limits of size 16 7 Designation 3 18 Gauges 3Table 39 - Unified screw threads below 1/ in diameter Appendix Metric equivalents 13Table 40 Unified screw threads below 1/ in diameter standard standard series 3series. Limits of size 4Table 42 - Internal threads (Nuts). Class 2B Table 41 - External threads (Bolts). Class 2A 6Table 43 - External threads (Bolts). Class 3A 6 7Table 44 - Internal threads (Nuts). Class 3B 7Table 45 -External threads (Bolts). Class 2A Table 46 - Internal threads (Nuts). Class 2B 8 8Table 47 - External threads (Bolts). Class 3A 6Table 48 - Internal threads (Nuts). Class 3B Table 49 - External threads (Bolts). Classes 2A and 3A 10 6Table 50 - Internal threads (Nuts). Classes 2B and 3B 10Table 51 - Unified Coarse thread series UNC Table 52 - Unified Fine thread series UNF 10 11Table 53 - Unified Extra Fine thread series UNEF 11Table 54 - Thread data Unified thread from (24 to 80 t.p.i) 12
Foreword
BS 919 Screw gauge limits and tolerances - Part 1: Gauges for screw threads of Unified form.
This British Standard has been prepared under the authority of the MechanicalEngineering Industry Standards Committee to plete the British Standards for Unified screw threads covering a range of diameters from 0.060 in (No. 0)to 6 in and should therefore be read in conjunction with Parts 1 and 2 of BS 1580.
The Unified serew thread system including the sizes covered in this standard has been remended by the International Organization for Standardization asthe international system of screw threads in inch units. The remendations are embodied in documents ISO R 68 for the basic and design thread profiles andISO R 263 for the diameter/pitch binations.
Although this Part 3 of BS 1580 refers to Parts 1 and 2 for the basic details of Unified screw threads including thread profile and tolerancing methods ete. sufficient information has been included herein to enable this part to be used independently of the other parts for general purposes. However since Part 3 isregarded as an extension to Parts 1 and 2 its table numbers have been made torun sequentially from these parts.
It will be noted that the range of diameters covered by this Part of BS 1580 fallswithin the range of B.A. serew thread sizes BS 93. The various pitches for the same diameter in the Unified system however offer a somewhat wider selectionof pitch/diameter binations for a specific application than in the B.A. system.The UNF series will be found to present the closest approximation to the B.A. pitch/diameter ratios.
A British Standard does not purport to inelude all the necessary provisions of acontract. Users of British Standards are responsible for their correct application.
Compliance with a British Standard does not of itself confer immunityfrom legal obligations.
Summary of pages
This document prises a front cover an inside front cover pages i and i pages 1 to 13 and a back cover.
This standard has been updated (see copyright date) and may have hadamendments incorporated. This will be indicated in the amendment table on the inside front cover.
1 Scope
This standard relates to Unified screw threads from Number 0 (0.060 in diameter) to Number 12 (0.216 indiameter) monly referred to as the “numbered sizes".
thread series and for the Number 12-32 UNEF extra-fine diameter pitch bination. It specifies the basic tolerances allowances and limiting sizes for the coarse (UNC) and the fine (UNF)
NOTEAttention is draw to the fset tbat with the different screw thred formsavailable there is the possibility of mismatch. It is tbe reponsibility of the designer of the end product to ensure that this possibility is reduced to a minimum.
2ISO/Unifiedthreadform
a) The basic form and design forms of the threads covered by this part are the same as those specified inPart 1 Clause 2.
Data relating to the various standard proportions of Unified threads from 24 to 80 threads per inch aregiven in Table 54. Partieulars of the symbols used in this table will be found in Appendix G of Parts 1and 2.
b) Screw threads for Aerospace purposes ohich are specified to ply with BS 1580.
i) External threads it is permitted for these to have the enlarged root radius specified for the UNJprofile in BS 40842. The minor diameter limits of size will acoordingly be modified to suit the enlarged root radius. The major and effective diameter limits of size are not modified and shall be in accordancewith those given in BS 1580 for the specified thread tolerance class.
i) Internal threads. Internal threads produced after 18 January 1974 shall conform with the minordiameter limits of size specified in BS 40842 The effective diameter limits of size are not modified andshall be in accordance with those given in BS 1580 for the specified thread tolerance class.
The major diameter is not affected as the dimensions are the same in BS 1580 and BS 40842).
3 Standard thread series
The standard diameter/pitch binations for Unified threads below 1/ in diameter are given in Table 39.Where possible the use of No. 1 No. 3 and No. 12 sizes should be avoided and preference should be givento the sizes in Column 1. Columns 1 and 2 list the first and second choices as detailed in ISO Remendation R 263.
4 Basic sizes
The basic sizes are given in Table 51 for the UNC series Table 52 for the UNF series and Table 53 for theNo. 12-32 UNEF thread.
5 Screw thread classes
the screw threads covered by this part are Classes 2A and 3A for external screw threads and Classes 2B For general information on the classes of Unified screw threads see BS 1580-1.3. The classes applicable toand 3B for internal screw threads. Classes 1A and 1B are not considered suitable for these sizes of Unified screw threads.
NOTEStainless stee bo nd ns Owing to the tendency forlose-fitingbolts and nuts f stainless stl tseize whentightenedtogether it is remended that stainless steel uncoated fasteners should not be made to Class 3 limits but rather to Class 2 limits.
6 Tolerances allowances and limits of size
a) Uncoated threads. The tolerances and allowances for Unified screw threads below / inch diameter of Classes 2A 2B 3A and 3B are based on the same formulae as those for the larger diameters (given inAppendix A of Parts 1 and 2) with the single exception of the tolerances for the minor diameters of Class 2B internal threads which are the same as those for Class 3B internal threads. These tolerancesare caleulated from the formula 0.05 a/β² 0.03p/D 0.002 except that they shall not exced 0.394p.
The tolerances and limits of size for finished uncoated threads of Classes 2A 2B 3A and 3B are givenin Table 40 to Table 50 inclusive.
