1、BRITISH STANDARD BS6808-3: 1989 Incorporating Amendment No.1 Coordinate measuring machines Part 3: Code of practice ICS 17.040.30BS6808-3:1989 This British Standard, having been prepared under the directionof the Advanced Manufacturing Technology Standards Policy Committee, waspublished under the au
2、thorityof the Board of BSI andcomes into effect on 31 July1989 BSI 04-1999 The following BSI references relate to the work on this standard: Committee reference AMT/8 Draft for comment 87/97377 DC ISBN 0 580 17365 8 Committees responsible for this British Standard The preparation of this British Sta
3、ndard was entrusted by the Advanced Manufacturing Technology Standards Policy Committee (AMT/-) to Technical Committee AMT/8, upon which the following bodies were represented: Advanced Manufacturing Technology Research Institute British Telecommunications plc Cranfield Institute of Technology Depart
4、ment of Trade and Industry (National Engineering Laboratory) Department of Trade and Industry (National Physical Laboratory) Gauge and Tool Makers Association Institution of Production Engineers Ministry of Defence University of Manchester Institute of Science and Technology Amendments issued since
5、publication Amd. No. Date of issue Comments 9829 April 1998 Indicated by a sideline in the marginBS6808-3:1989 BSI 04-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2 Definitions 1 3 General 1 4 Environmental conditions 1 5 Choice of mechanical reference artefac
6、ts 3 6 Performance verification tests 9 7 Probing techniques 13 Figure 1 Step gauges 4 Figure 2 Typical support arrangement for minimum deflection at45 inclination 4 Figure 3 Support for gauge blocks up to1000mm arranged with measuring lines symmetrically disposed about a common centre 5 Figure 4 Ad
7、justable support system with a common end plane 5 Figure 5 Space frames 6 Figure 6 Ball-plate 6 Figure 7 A multi-purpose commercially-available artefact 7 Figure 8 Example of a ball-ended rod with magnetic cups for kinematic location 7 Figure 9 Extendable bar system 8 Figure 10 Machine checking gaug
8、e 9 Figure 11 The four cross-diagonal configurations 10 Figure 12 In-plane diagonal configuration 10 Figure 13 Suggested additional measuring line 11 Figure 14 Position of measuring points for mathematically determining alignment of test devices 11 Figure 15 Errors when aligning gauge blocks 12 Publ
9、ications referred to Inside back coverBS6808-3:1989 ii BSI 04-1999 Foreword This Part of BS6808 has been prepared by the Advanced Manufacturing Technology Assembly (AMT/-/8) at the request of manufacturers and users of coordinate measuring machines (CMMs). BS6808-2:1987, which covered the performanc
10、e assessment of coordinate measuring machines, has been superseded by BS EN ISO10360-2:1996. Amendment No.1 to BS6808-3:1989 deletes all reference to BS6808-2:1987, which has been withdrawn. This Part of BS6808 provides the user and supplier with more information about further testing recommendation
11、s for variable environmental conditions. A glossary of terms is given in BS6808-1. 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 itsel
12、f confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pagesi andii, pages1 to14, 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 indica
13、ted in the amendment table on the inside front cover.BS 6808-3:1989 BSI 04-1999 1 1 Scope This Part of BS6808 gives recommendations to the user and supplier on the more detailed testing recommended if the environmental conditions give rise to problems. This standard is not intended for coordinate me
14、asuring machines that have working volumes, any dimension of which is significantly greater than the dimension of the mechanical reference artefact, or for those having more than three non-redundant axes. 2 Definitions For the purposes of this Part of BS6808, the definitions given in BS6808-1 apply.
15、 3 General The CMM under test should have validated software capable of determining from a series of data points the orientation in space of a best-fit plane. This ability will enable a much closer scrutiny of the measurement capability of the CMM to be made. 4 Environmental conditions 4.1 Vibration
16、 analysis 4.1.1 Principle. The vibration survey establishes the vertical and horizontal vibration environment at the interface between the CMM and its foundations and support system provided by the user, in order to allocate responsibility for faults between user and supplier. 4.1.2 Procedure. Make
17、the vibration survey using a directionally sensitive, low frequency(0.5Hzto100Hz) transducers capable of discriminating a peak-to-peak motion amplitude of0.25 4m. A band-pass filter of corresponding frequency range having variable high and low cut-off frequencies should be used to condition the tran
18、sducer signals prior to recording on an oscilloscope, a chart recorder or in some other way agreed between the user and the supplier. In the case of a contacting analogue probe, vibration between the mechanical probe and the workpiece can result from forces external to the CMM, or from forces genera
19、ted within the CMM itself. Therefore, a test should first be made to see if the CMM is the vibration source, prior to conducting a full measurement and analysis of vibration. If the CMM is motorized, the power to all motors should be removed and the CMM tested to determine if the vibration is still
20、present. If the CMM uses air bearings, the air pressure to the bearings should be varied sufficiently to establish whether air-bearing instability is causing vibration. If the CMM is not the source of the vibration, a complete vibration analysis at the CMM-to-support interface should be performed an
21、d the frequency spectrum analysed to determine if frequencies and amplitudes are present that exceed the CMM suppliers defined limits. Measurements should be made by transducers mounted on the CMM support in the direction of the vertical axis and the two mutually perpendicular horizontal directions
22、that approximately correspond to the principal axes of the CMM. Care should be taken that the survey is undertaken during a period representative of the CMM operating conditions. 4.1.3 Analysis. If any of the vibration parameters measured during the survey (see4.1.2) exceed the suppliers specificati
23、on, the user should correct the problem in order to conform to the specification or else accept a performance derating to be negotiated between supplier and user. If the vibration parameters are within the suppliers specification, it should be the sole responsibility of the supplier to correct the p
24、erformance of the CMM in order to meet the specification. 4.2 Air supply 4.2.1 Procedures are described for testing the air supply if there is evidence of excessive pressure fluctuation, inadequate supply pressure at the specified flow rate, excessive CMM thermal drift, or bearing contamination. Res
25、ponsibility for correction of faults is allocated between the user and supplier according to the analysis of the test results. 4.2.2 Test equipment. For the tests on the air supply, it is recommended that an air pressure gauge, air flow gauge, and a temperature measuring system should be used. The a
26、ir pressure gauge should be calibrated to cover the range between the minimum and maximum air supply pressure specified by the CMM supplier. The accuracy of the air pressure gauge should be 5% of the permissible air supply pressure fluctuation as specified by the supplier.BS6808-3:1989 BSI 04-1999 2
27、 The air flow gauge should have an accuracy of 20% of the maximum flow rate specified by the supplier. The temperature measuring system should be calibrated to an accuracy of 0.1 C. NOTEAn accuracy of 0.1 C is acceptable in most cases but would be dependent upon the specification of the CMM. 4.2.3 M
28、ethods 4.2.3.1 Pressure fluctuation. Mount an air pressure gauge in the supply line upstream of the CMM air filter. Observe the pressure under the condition(s) that resulted in evidence of excessive pressure fluctuation. 4.2.3.2 Supply pressure and flow. Using the gauge arrangement as described in4.
29、2.3.1, observe the pressure under the condition(s) that resulted in evidence of inadequate pressure. If the pressure is inadequate, mount an air flow gauge in the supply line upstream of the CMM air filter. Observe flow under the condition(s) that resulted in evidence of inadequate pressure. 4.2.3.3
30、 Air supply temperature. Mount a temperature measuring system pickup in the air line downstream of the CMM air filter or, if this is impractical, on a metallic part of the supply line as close to the inlet point on the CMM as possible. If the pickup is mounted on the line, the line and pickup should
31、 be insulated from ambient air to ensure that the temperature of the air supply is being measured. Measure the temperature under the condition(s) that resulted in evidence of excessive CMM thermal drift. 4.2.3.4 Contamination. Examine surfaces near air exhaust points for water, oil, or solid particu
32、lates, at a frequency dependent upon the frequency of use of the CMM but normally at least once per week. 4.2.4 Test analysis 4.2.4.1 Pressure fluctuation. If the supply line fluctuation exceeds the suppliers specification, it should be the responsibility of the user to correct the problem by, for e
33、xample, installing an accumulator. 4.2.4.2 Supply pressures and flow. If the flow rate exceeds the suppliers specification, it should be the responsibility of the supplier to reduce the flow required by the CMM, provided it can be demonstrated by the user that no significant leaks exist in the suppl
34、y. If the flow rate meets the suppliers specification but the line pressure does not, it should be the responsibility of the user to increase the supply pressure, for example by removing other devices using air from the line or by increasing its diameter. 4.2.4.3 Air supply temperature. If the tempe
35、rature of the air supply line does not meet the suppliers specification, it should be the responsibility of the user to correct the temperature, for example by installing a heat exchanger to enable the suppliers specification to be met. 4.2.4.4 Contamination. If contamination is present, it should b
36、e the responsibility of the user to change the air filter cartridge, clean the CMM air system using procedures recommended by the supplier, and correct the supply contamination problem. Two methods of correction are preferred: a) reduction of supply contamination; or b) decrease of the intervals bet
37、ween servicing the filter. NOTEExternal sources of dust, for example from air-conditioning and from ineffective cleaning procedures should be carefully controlled. Unexpectedly large variations in the values obtained from a series of measurements could arise from a contamination problem. 4.3 Externa
38、l temperature influences Temporal and spatial temperature changes within the working volume of the CMM will affect the accuracy capabilities of the CMM and whenever possible such variations should be minimized. Among the influencing factors are heat radiation from: sunlight, neighbouring machines, l
39、ighting and, where applicable, the proximity of the operator. The deviation of the CMM and/or the workpiece from a reference temperature will also produce measurement errors even when the temperature remains constant with time, unless the effective thermal coefficient of expansion and the temperatur
40、e of the CMM are the same as the expansion coefficient and temperature of the workpiece. It is recommended that the CMM and workpiece should be fitted with temperature sensors. The effects of temperature should be corrected by calculation based upon the known thermal parameters of the CMM and workpi
41、ece. It is recommended that such computations are based on the assumptions that the temperature is steady with time and is uniform in space. Careless handling of mechanical reference artefacts can cause significant errors. For example, if a400mm length bar is grasped with unprotected hands for about
42、10s, its length could be in error by more than15 4m even after15min.BS 6808-3:1989 BSI 04-1999 3 4.4 Other machine influences 4.4.1 Workpiece. Unless the CMM is specially designed and constructed to completely decouple the static and dynamic yield behaviour of the CMM from its measurement system, th
43、is will usually lead to the deformation of the guides and hence, measuring errors. To be able to keep such deviations within acceptable limits, it is recommended that the supplier specifies maximum values for the permissible deadweight of a workpiece and, where applicable, limiting values for the di
44、stribution of loads applied to the measuring volume of the CMM. The properties of the workpiece to be tested do have an influence on the measured coordinate values obtained. Surface quality, shape, hardness, elastic and, where applicable, plastic yield to gravity and the probing force, have effects
45、on the measured values. Workpieces may in some circumstances become deformed by the applied probing forces because of their shape and/or the material from which they are made. It is possible in many cases to correct for this effect. When measuring soft workpieces, for example, wax models and plastic
46、s, the probe sphere may become impressed in the workpiece due to the probing force and thus lead to error. 4.4.2 Definition of the measuring task. Poor definition of the measuring task on a technical drawing or in the test procedure should also be regarded as a potential source of error. With incorr
47、ect or ambivalent indication of dimensions or reference points, or the selection of unsuitable reference elements or the incorrect choice of definition for the fitting procedure, undesired deviations will occur in the determination of the result of a measurement. 5 Choice of mechanical reference art
48、efacts 5.1 Performance verification and periodic reverification 5.1.1 General. The choice of a mechanical reference artefact with which to undertake the verification and periodic reverification of CMM performance should be carefully considered. Several of the criteria are subjective and some may be
49、offset, to some extent, by the availability of a suitable artefact at the time of the test. Ideally, the initial verification and the periodic reverification should be performed using the same artefact, or failing that, an artefact of the same general type. In5.1.2 to5.1.4 some of the aspects that the user should consider are discussed for the types of artefact that possess the following essential features required for verification of CMM performance: a) mechanical perfection of the length-defining features; b) certified accuracy; c
