1、BRITISH STANDARD CONFIRMED AUGUST 1985 BS 1936-1: 1952 Incorporating Amendment Nos. 1 and 2 Specification Undercuts & runouts for screw threads Part 1: Inch screw threads UDC 621.882.082BS1936-1:1952 This British Standard, having been approved by the Mechanical Engineering Industry Standards Committ
2、ee and endorsed by the Chairman of the Engineering Divisional Council, was published under the authority of the General Council on 31st December,1952 BSI 03-1999 ISBN 0 580 32663 2 Co-operating organizations The Mechanical Engineering Industry Standards Committee, under whose supervision this Britis
3、h Standard was prepared, consists of representatives from the following Government departments and scientific and industrial organizations: The Government departments marked with an asterisk in the above list, together with the following scientific and industrial organizations, were directly represe
4、nted on the committee entrusted with the preparation of this standard : Admiralty* Institute of Marine Engineers Air Ministry* Institute of Petroleum Associated Offices Technical Committee Institution of Civil Engineers Association of Consulting Engineers Institution of Gas Engineers (Incorporated)
5、Institution of Heating and Ventilating British Chemical Plant Manufacturers Engineers Association Institution of Mechanical Engineers* British Compressed Air Society Institution of Mechanical Engineers British Electrical and Allied Manufacturers (Automobile Division) Association* Institution of Prod
6、uction Engineers* British Electricity Authority and Area Boards Locomotive Manufacturers Association* British Engineers Association* Machine Tool Trades Association British Internal Combustion Engine Ministry of Fuel and Power Manufacturers Association Ministry of Labour and National Service British
7、 Iron and Steel Federation (Factory Department) British Railways, The Railway Executive* Ministry of Supply Crown Agents for the Colonies Ministry of Transport D.S.I.R. Mechanical Engineering Research Ministry of Works Organization* National Physical Laboratory* Engineering Equipment Users Associati
8、on* Office of the High Commissioner for India War Office Agricultural Engineers Association Radio Industry Council British Bolt, Nut, Screw and Rivet Conference Scientific Instrument Manufacturers Cut Thread Screwing Tool Manufacturers Association Association Society of Motor Manufacturers and Trade
9、rs Engineering Standards Co-ordinating Ltd. Committee Telecommunication Engineering and Ground Thread Tap Association Manufacturing Association National Association of Marine Enginebuilders Amendments issued since publication Amd. No. Date Comments 2248 August 1955 4923 October 1985 Indicated by a s
10、ideline in the marginBS1936-1:1952 BSI 03-1999 i Contents Page Co-operating organization Inside front cover Foreword ii Introductory notes 1 1 Scope 1 2 Form of undercut 1 3 Limits of tolerance on diameters of undercuts 2 4 Thread runouts on external threads 2 5 Thread runouts on internal threads: r
11、ecommended hole tapping practice 2 6 Designation of undercuts and thread runouts 2 Appendix Formulae for calculating widths of undercuts and thread runouts 9 Figure 1 Form A. Standard form of undercut for external or internal threads 7 Figure 2 Form B. Modified undercut profile with inner chamfer to
12、 centrejointing washer, etc., used generally on an external thread 7 Figure 3 Form B. Alternative form to Figure 2 for use on an external threadona cylindrical component of the same major diameter as the thread 7 Figure 4 Use of a counterbore or undercut on internal thread to clearrunouton external
13、thread, in place of an undercut on the latter. Thedimensions(W f , D, etc.) of the counterbore are the same as for an internalundercut 8 Figure 5 Diagram of a thread runout 8 Figure 6 Diagram of a blind tapped hole 8 Figure 7 Length of runout produced by various angles of lead and throat 10 Table 1
14、Nominal widths of machined undercuts and thread runouts (washoutthreads) from screwing tools 3 Table 2 Recommendations for minimum widths of undercuts and threadrunouts for various production requirements 4 Table 3 Limits of tolerance on diameters of undercuts 5 Table 4 Standard tools for undercuts
15、6 Table 5 Length of runout for various angles of throat or taper lead 9BS1936-1:1952 ii BSI 03-1999 Foreword In order to keep abreast of progress in the industries concerned, British Standards are subject to periodical review. Suggestions for improvements will be recorded and in due course brought t
16、o the notice of the committees charged with the revision of the standards to which they refer. A complete list of British Standards, numbering over4000, indexed and cross-indexed for reference, together with an abstract of each standard, will be found in the lnstitutions Yearbook. This standard make
17、s reference to the following British Standards: BS84, Screw threads of Whitworth form. BS93, B.A. screw threads. BS811, Cycle threads. BS949, Screwing taps. BS1580, Unified screw threads. BS1936, Undercuts and runouts for screw threads . BS1936-2, ISO metric screw threads. A British Standard does no
18、t 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 confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside f
19、ront cover, pagesi andii, pages1 to10 and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in. the amendment table on theinside front cover.BS1936-1:1952 BSI 03-1999 1 Introductory notes During the preparation of this
20、standard it was appreciated that in present practice two alternative methods are used for defining the width of an undercut, namely from the shoulder to the crest of the first adjacent thread and from the shoulder to the intersection of the cylindrical portion of the undercut with the chamfer of the
21、 first thread. The latter method has been adopted in this standard. This nominal width of undercut is also the nominal width and designation of the tool for producing it. These nominal widths are in accord with a preferred number series (the10 series) and allow the production of an adequate variety
22、of undercuts on threads of many pitches with a minimum number of standard tools. It cannot be too strongly emphasized that too narrow undercuts, or the demand for perfect or full threads up to shoulders or to the bottoms of blind holes, add greatly to manufacturing cost by necessitating the use of s
23、pecial taps and dies, a second screwing operation, or resort to processes such as thread milling or grinding. It is suggested that where recommendations for any of the standard forms of undercut appear at first sight to be inapplicable, an endeavour should be made to accommodate them by alterations
24、in design, e.g.by using through-tapped holes. A counterbore or undercut on the first few threads of an internal thread may readily be drilled when tapping a hole and is much simpler to produce than the equivalent undercut on the mating external thread. (See Figure 4.) BS1936-2 published in1970 speci
25、fies the requirements for undercuts and runouts for ISO metric screw threads. 1 Scope This British Standard relates to dimensions for profile and width of undercuts for external and internal threads of Unified, Whitworth, B.A. and Cycle thread forms and provides for a series of standard tools for pr
26、oducing the undercuts recommended. It makes recommendations concerning practice in regard to undercuts and relates also to allowances for thread runouts. It includes the following: a) Nominal widths of machined undercuts and thread runouts (washout threads) produced from screwing tools. b) Recommend
27、ations for minimum widths of undercut or runout for various production requirements. c) Limits of tolerance on diameters of undercuts. d) Standard tools for undercuts. e) Formulae for calculating widths of undercuts and runouts. (See Appendix.) 2 Form of undercut When an external thread projects fro
28、m a component of larger size or diameter, an undercut of reasonable width allows the thread to be cut without the end face of the die fouling the component. The type of undercut, Form A, recommended for use on external or internal threads is shown in Figure 1. Design requirements are usually for a r
29、easonable degree of control on the diameter at the root or crest of the undercut to avoid weakening the section at this point, and for an ample radius at the end of the undercut to avoid stress concentration. The production requirement is for an adequate width of undercut to allow the use of dies an
30、d taps with a reasonable throat angle or taper lead. Recommendations of widths in terms of pitch are given in Table 2. (See also Appendix.) The dimensions of this form of undercut and of suitable tools are given in Table 1, Table 3 and Table 4. The diameter of the undercut D mor D fmust be calculate
31、d as this will vary with the diameter, pitch and form of thread used. Figure 2, Form B, shows a similar undercut, except that the shape is arranged to provide a register or location for the insertion of a jointing washer, gasket, etc., between the external and the internal thread. From the design po
32、int of view this undercut may not be as satisfactory under fatigue conditions as the other type. Detail dimensions for tools for this type of undercut are not given as each pitch requires a special tool. As in the case of Figure 1, Form A, the diameter must be calculated. Figure 3, Form B, shows an
33、undercut of form similar to that shown in Figure 2 but used on an external thread at the end of a cylindrical component of the same diameter as the thread.BS1936-1:1952 2 BSI 03-1999 Figure 4 illustrates the application of an undercut to the first few threads of an internal thread to clear a runout
34、on an external thread, or to enable a jointing washer, etc. to be located adequately. This method is preferable to that shown in Figure 2. Data given for the normal internal undercut apply also to this type of recess, the width being that appropriate for the runout on the external thread. In many ca
35、ses it will be satisfactory to use an ordinary drill point, without having recourse to special tool shapes. 3 Limits of tolerance on diameters of undercuts Two grades of tolerance are provided, as shown in Table 3. Grade1 is suitable for general engineering purposes. Grade2 is a finer tolerance, sui
36、table for precision work and work in thin wall section. 4 Thread runouts on external threads The normal type of thread runout (or washout thread 1) ) left by an ordinary screwing die is shown in Figure 5. The length of this runout is the same as the width of the appropriate undercut, if used, and is
37、 measured from the end of full thread. 2)The imperfection of the last few threads is due to the necessary throat angle on the screwing die. (SeeAppendix.) Recommended practice in regard to the minimum length of thread runouts is given in Table 2. 5 Thread runouts on internal threads: recommended hol
38、e tapping practice Figure 6 illustrates a normal blind tapped hole (through-tapping is always preferred from the production point of view). The dimension “A” is determined by the engagement of the bolt, stud, etc., plus a nominal additional clearance of0.05 in. to0.1 in. The dimension “B” is the min
39、imum number of washout threads for easy tapping, plus a clearance for swarf, etc. Within limits the larger the runout “B” the easier will be the tapping operation. (SeeAppendix.) Recommended practice in regard to the minimum length of thread runouts is given in Table 2. 6 Designation of undercuts an
40、d thread runouts It is recommended that reference on drawings and specifications to undercuts and thread runouts specified in this standard should quote the form of undercut required, followed by the width of undercut in terms of pitch. The designation should appear as in the following example: BS19
41、36-1 A 2p Specification of the diameter of the undercut, which must be computed, will normally be made on the drawing, and the tolerance specified or quoted as “Grade 1” or “Grade 2.” 1) A washout thread is that part of the thread which is not fully formed at the root. 2) The length to the end of fu
42、ll thread is the distance from a specified datum plane normal to the axis to the parallel plane which passes through the point on the root diameter helix at which the thread ceases to be fully formed at the root. For a fuller explanation of its importance with regard to mating screw threads, see Dim
43、ensional Analysis of Engineering Designs, Vol.1, Part1.BS1936-1:1952 BSI 03-1999 3 Table 1 Nominal widths of machined undercuts and thread runouts (washoutthreads) from screwing tools 1 2 3 4 5 6 7 Threads of Whitworth,Cycle and Unified form B.A.threads Approx. pitch p Width W of standard tool to pr
44、ovide an undercut of width W (Figure 1) approximately equal to: Thread runouts (washout threads) as machine tapped. aapprox. 1.5 p 2 p 3 p 4 p 6 p t.p.i. No. in. in. in. in. in. in. 72 10 0.014 0.02 0.032 0.04 0.06 0.08 64, 60, 56 8 0.017 0.02 0.032 0.05 0.08 0.1 48 7, 6 0.021 0.032 0.04 0.063 0.10
45、0.12 40 4, 5 0.025 0.04 0.063 0.08 0.1 0.15 36 3 0.028 0.04 0.063 0.08 0.12 0.15 32 2 0.031 0.05 0.063 0.1 0.15 0.2 28 1 0.036 0.05 0.08 0.125 0.15 0.25 26 0 0.038 0.063 0.08 0.125 0.15 0.25 24 0.042 0.063 0.1 0.125 0.2 0.25 22 0.045 0.08 0.1 0.16 0.2 0.3 20 0.050 0.08 0.1 0.16 0.2 0.3 19, 18 0.056
46、0.08 0.125 0.16 0.25 0.35 16 0.062 0.1 0.125 0.2 0.25 0.4 14 0.071 0.1 0.16 0.2 0.3 0.45 13 0.077 0.125 0.16 0.25 0.3 0.45 12 0.083 0.125 0.16 0.25 0.35 0.5 11, 10 0.100 0.16 0.2 0.315 0.4 0.6 9 0.111 0.16 0.2 0.315 0.45 0.65 8 0.125 0.2 0.25 0.4 0.5 0.75 7 0.143 0.2 0.315 0.4 0.6 0.85 6 0.167 0.25
47、0.4 0.5 0.7 1 5 0.200 0.315 0.4 0.63 0.8 1.2 4, 4 0.250 0.4 0.5 0.8 1 1.5 a These dimensions have been rounded off for convenience.BS1936-1:1952 4 BSI 03-1999 Table 2 Recommendations for minimum widths of undercuts and thread runouts for various production requirements Width of undercut or runout in
48、 terms of pitch (approx.) External threads Internal threads 1.5 p Normal machined undercut or runout on screws of less than in. diameter in soft or free-cutting materials. Very special work on larger screws. Would require special bottoming tap. 2 p Normal machined undercut or runout on soft or free
49、cutting materials. Special work on normal or hard materials. Would require second operation with standard a (BS 949) bottoming tap. 3 p Normal machined undercuts on production parts. 4 p Holes tapped with care in soft materials. 6 p Normal production tapped holes. a BS949, “Screwing taps.” BS1936-1:1952 BSI 03-1999 5 Table 3 Limits of tolerance on diameters of undercuts 1 2 3 Nominal undercut diameter D mor D f Tolerance Grade 1 a(suitable for general engineering work) Grade 2 a(suitabl
