1、BRITISH STANDARD BS958:1968 Specification for Spirit levels For use in precision engineeringBS958:1968 This British Standard, havingbeen approved by theMechanical Engineering Industry Standards Committee and endorsed by the Chairman ofthe Engineering Divisional Council, was published under theauthor
2、ity of the General Council on 22 March1968 BSI 07-1999 First published July1941 First revision March1968 The following BSI references relate to the work on this standard: Committee reference MEE/59 Draft for comment 66/22974 ISBN 580 00094 X Co-operating organizations The Mechanical Engineering Indu
3、stry Standards Committee, under whose supervision this British Standard was prepared, consists of representatives from the following Government departments and scientific and industrial organizations: Associated Offices Technical Committee Association of Consulting Engineers Association of Mining El
4、ectrical and Mechanical Engineers Board of Trade British Chemical Plant Manufacturers Association British Compressed Air Society British Electrical and Allied Manufacturers Association British Gear Manufacturers Association British Internal Combustion Engine Manufacturers Association British Iron an
5、d Steel Federation British Mechanical Engineering Federation British Pump Manufacturers Association Crown Agents for Oversea Governments and Administrations Electricity Council, the Central Electricity Generating Board and the Area Boards in England and Wales Engineering Equipment Users Association
6、Gas Council Institute of Marine Engineers Institution of Civil Engineers Institution of Gas Engineers Institution of Heating and Ventilating Engineers Institution of Mechanical Engineers Institution of Mechanical Engineers (Automobile Division) Institution of Production Engineers Locomotive and Alli
7、ed Manufacturers Association of Great Britain London Transport Board Machine Tool Trades Association Ministry of Defence Ministry of Defence, Army Department Ministry of Labour (H.M. Factory Inspectorate) Ministry of Power Ministry of Public Building and Works Ministry of Technology National Enginee
8、ring Laboratory Ministry of Transport National Coal Board National Physical Laboratory (Ministry of Technology) Radio Industry Council Royal Institute of British Architects Amendments issued since publication Amd. No. Date CommentsBS958:1968 BSI 07-1999 i Contents Page Co-operating organizations Ins
9、ide front cover Foreword ii 1 Scope 1 2 Sensitivities 2 3 Nomenclature and definitions 2 4 General features of design 3 5 Recommended scale and indication of sensitivity 3 6 Adjustments 3 7 Finish and accuracy of working surface 4 8 Accuracy of performance 4 9 Case 4 10 Protection against climatic c
10、onditions 4 11 Marking and manufacture 5 Appendix A Choice of sensitivity of a level 6 Appendix B Testing of precision levels 6 Figure 1 Type1 level 1 Figure 2 Type2 level 1 Figure 3 Type3 level 2 Figure 4 Scale divisions 3 Table 1 Accuracy of performance 4BS958:1968 ii BSI 07-1999 Foreword This sta
11、ndard makes reference to the following British Standards: BS1133, Packaging Code Section6: Temporary Prevention of corrosion of metal surfaces (during transportation and storage) Section19: Use of desiccants in packaging. BS3509, Spirit level vials. Since this standard was first published in1941, sp
12、irit levels have become generally accepted as tools for use in precision engineering. Recognition of the fact that they provide a quick and accurate means of checking surfaces for flatness, alignment etc. is evidenced by their inclusion in the equipment specified in the British Standard and in the I
13、SO Recommendation for methods of testing the accuracy of machine tools. They are also referred to in recommendations for the testing of other precision measuring equipment such as cast iron surface plates and tables and straightedges. One of the main purposes of the present revision is to take accou
14、nt of the publication of BS3509, “Spirit level vials”. The requirements for accuracy of performance are based on that standard and relevant definitions are also included to help ensure that those requirements are correctly interpreted. Other new features include reference to a smaller level than any
15、 included in the previous standard, recommendations for general dimensions and the use of millimetres only for all linear quantities. 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. C
16、ompliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pagesi andii, pages1 to6 and a back cover. This standard has been updated (see copyright date) and may have had amendments incor
17、porated. This will be indicated in the amendment table on the inside front cover.BS958:1968 BSI 07-1999 1 1 Scope 1.1 This British Standard specifies requirements for spirit levels for use in precision engineering. It provides for three types of levels as follows: Type 1. A spirit level with an unre
18、lieved flat base of steel hardened and lapped. This type of level is usually made with base lengths from about100mm to200mm. The advantage of the lapped base is that the base length of the level can be varied at will by wringing two slip gauges on to the base at the desired distance apart. Type 2. A
19、 spirit level, mounted in a body usually of cast iron or steel, having a base formed with flat bearing surfaces at the two ends, i.e.the middle portion is relieved. The bearing surfaces may either be plain or contain a longitudinal120 V-groove for use on cylindrical surfaces. In the latter case a sh
20、ort cross-level or small circular level is provided. This type of level is usually made with base lengths from about250mm to500mm. Type 3. A square block level, usually of cast iron, and about200mm square. The four bearing surfaces are flat and may have the middle portions relieved; alternatively, t
21、he base and one adjacent surface may each contain a longitundinal120 V-groove for use on cylindrical surfaces. In the latter case a short cross-level is provided. 1.2 Whilst this standard deals specifically with spirit levels, it is recognized that levels operating on other principles perform a simi
22、lar function and it is recommended that such instruments should conform to the same general requirements for accuracy and performance where applicable. Figure 1 Type1 level Figure 2 Type2 levelBS958:1968 2 BSI 07-1999 2 Sensitivities The sensitivities (see3.2.1) recommended for general precision eng
23、ineering purposes are as follows: 3 Nomenclature and definitions 3.1 For the purposes of this British Standard, the nomenclature given in Figure 1 to Figure 3 has been adopted. 3.2 For the purposes of this British Standard, the following definitions apply: 3.2.1 sensitivity that angle of tilt which
24、causes the bubble in the vial to move through one scale division 3.2.2 range the total angle of tilt through which a vial can be used 3.2.3 roll error the change in reading when the instrument is rotated about the axis of the vial 3.2.4 repeatability the measure of the ability of the level to repeat
25、 its readings under all ordinary methods of operation 3.2.5 settling time the time taken for a bubble to return to its steady position after the level has been disturbed 3.2.6 flatness tolerance the maximum permissible distance between two imaginary parallel planes within which the surface under con
26、sideration can just be enclosed Figure 3 Type3 level Type of level Recommended sensitivity (Value of1 scale division) Type1 or 0.0025mm in100mm i.e.approximately5 seconds of arc a 0.005 mm in100mm i.e.approximately10 seconds of arc a Types2 and3 or 0.005mm in100mm i.e.approximately10 seconds of arc
27、a 0.010mm in100mm i.e.approximately20 seconds of arc a a 5 seconds of arc=0.002425mm per100mmBS958:1968 BSI 07-1999 3 3.2.7 squareness tolerance the maximum permissible distance separating two imaginary parallel planes which just enclose one of the surfaces under consideration and which are perpendi
28、cular to the other surface. Squareness tolerance is expressed over the total length of one of the surfaces 3.2.8 parallelism tolerance the maximum permissible variation in distance between the surfaces under consideration 4 General features of design 4.1 To minimize the differences in thermal expans
29、ion between the glass vial and its mount, the glass should have a co-efficient of thermal expansion approximating to that of the mount. To allow for any residual difference in expansion, the glass vial should preferably be mounted so as to permit slight relative longitudinal movement. 4.2 It is reco
30、mmended that the dimensions of the glass vial should be such that both ends of the bubble can always be read against the scale over a temperature range of from10 C to40 C. To achieve this, it is recommended that the alteration in the length of the bubble for a temperature change of1 C between 10 C a
31、nd40 C should not exceed1% of its length at20 C. 4.3 It is recommended that the glass vials of levels be protected by a permanent transparent cover. 4.4 It is recommended that the bodies of Types1 and2 levels should be provided with a convenient heat insulating finger hold. 4.5 All unmachined surfac
32、es on the level should be painted with a high-grade enamel or should have an equivalent protective finish. 4.6 It is recommended as general guidance to manufacturers that the weight of the more sensitive levels should be kept to the minimum consistent with stability to avoid undue deflection of the
33、surfaces on which the level is supported when in use. 4.7 Whilst no general dimensions are specified, it is recommended that the following be adopted for new designs: 5 Recommended scale and indication of sensitivity 5.1 It is recommended that scales on all levels be divided into2.5mm divisions, the
34、 style of graduation recommended being that shown in Figure 4. 5.2 All graduation marks shall be straight with sharp edges, at right angles to the longitudinal axis of the vial and of a uniform thickness equal to0.25mm 20 %. 5.3 Levels shall be permanently marked with the nominal sensitivity. It is
35、usual to append the scale interval itself to this figure, viz.20 seconds of arc/2.5mm,10 seconds of arc/2.5mm, etc. (or20 seconds/2.5mm,10 seconds/2.5mm. etc.). Alternatively, this angle may be expressed as the equivalent length of arc at the end of a250mm radius or base length. The connection betwe
36、en these two methods of expressing sensitivity is: 1 second of arc=0.0012mm in250mm. 6 Adjustments Each level shall incorporate a means of setting the vial level. This adjustment should be such that, when the level is placed on a horizontal surface, it should be possible to adjust the vial so that t
37、he two ends of the bubble give equal readings on the scale to within one-tenth of a division. The adjustment should be provided with an efficient locking device and, as far as possible, should be proof against tampering. At the standard reference temperature of20 C the length of the bubble should be
38、 equal to the distance between the centre lines of the scales to within one-half of a division. Base length 100 150 200 300 450mm Approximate width and height 25 35 40 50 60mm Figure 4 Scale divisionsBS958:1968 4 BSI 07-1999 7 Finish and accuracy of working surfaces 7.1 Type1 levels. The hardened ba
39、se shall be lapped to provide a wringing surface and shall be flat overall to within0.001mm. Any departure from flatness shall be in the nature of a concavity and not a convexity. In order to maintain this degree of flatness, the body of this type of level shall be stabilized. NOTEA suitable stabili
40、zing process is as follows: The hardened body of the level, before final grinding, is heated and maintained at a temperature of150 C for a period of510 h and allowed to cool slowly in the furnace. 7.2 Type2 levels. The base, including the120 V-groove where provided, shall be finished by scraping, gr
41、inding or lapping. The bearing surfaces of the base and the sides of the V-groove shall be flat and co-planar to within0.003mm for levels up to300mm long and0.004mm for larger levels. The V-groove when provided shall be parallel to the base to within the equivalent of half of a scale division. (0.00
42、12mm per100mm for5 second levels, 0.0025mm per100mm for10 second levels and0.005mm per100mm for20 second levels). 7.3 Type3 levels. The four surfaces of the square, including the V-groove when provided, shall be finished by scraping, grinding or lapping and each surface and each side of the V-groove
43、s shall be flat and co-planar to within 0.003mm. Adjacent working surfaces of the square shall be square to each other and opposite working surfaces shall be parallel to each other to within the equivalent of half of a scale division. Each V-groove shall be parallel to its own flat (upper) surface t
44、o within the same amount. 8 Accuracy of performance 8.1 Calibration. When the level is tested on a tilting-table and readings are taken at each division at both ends of the bubble, the average sensitivity shall not depart from the nominal sensitivity by more than the amount shown in column2 of Table
45、 1. In addition, the maximum deviation of the value of any one division from the average value shall not exceed the amount shown in column3 of Table 1. 8.2 Repeatability. Identical settings of the level tilt shall be made and the departure of any one reading from the mean of six readings shall not e
46、xceed the tolerance given in column4 of Table 1. 8.3 Roll error. Roll error shall not exceed the tolerance given in column5 of Table 1. 8.4 Settling time. After the level has been disturbed to make the bubble move approximately one scale division, it shall come to within one-tenth of a scale divisio
47、n of its steady position within10 seconds. 9 Case Each level shall be supplied in a durable protective case or box and a key for adjusting the level shall be provided. 10 Protection against climatic conditions During storage and transit, all working surfaces of the levels shall be protected against
48、climatic conditions by being covered with a suitable corrosion preventative preparation 1) . Table 1 Accuracy of performance 1) Temporary (easily removable) corrosion preventatives are dealt with fully in BS1133, “Packaging Code”, Section6, “Temporary prevention of corrosion of metal surfaces”. Guid
49、ance on sealed packs with desiccants is given in Section19, “Use of desiccants in packaging”. 1 2 3 4 5 6 Sensitivity per division Permissible departure of average from nominal sensitivity per division Maximum deviation of the value of any one scale division from average value Repeatability Permissible roll error over 10 from normal position Settling time per100mm Seconds of arc Division Seconds of arc Seconds of arc Seconds of time 0.0025 (5 seconds) 0.005 (10 seconds) 0.010 (20 seconds) 1 2 2/10 2/10 2/10 1 2 2
