Detail of GB/T 17421.1-1998

Introduction of GB/T 17421.1-1998

1 Scope The aim of this standard is to standardize methods of testing the accuracy of machine tools, operating either under no-load or under finishing conditions, by means of geometric and machining tests. The methods may also be applied to other types of industrial machines where geometric and machining tests are concerned. This standard covers power-driven machines, not portable by hand while working, which can be used for machining metal, wood, etc. by removal of chips or swarf or by plastic deformation. This standard relates only to the testing of geometric accuracy. In particular, it deals neither with the operational testing of the machine tool (vibrations, stick-slip motion of components, etc.) nor with the checking of characteristics (speeds, feeds), as these checks should normally be carried out before testing of the accuracy of the machine tool. When a measurement method not described in this standard can be shown to offer equivalent or better facilities for measuring the attributes to be studied, such a method may be used. 2 General Considerations 2.1 Definitions relating to geometric tests A distinction should be made between geometric definitions and those designated in this standard as metrological definitions. Geometric definitions are abstract and relate only to imaginary lines and surfaces. From this it follows that geometric definitions sometimes cannot be applied in practice. They take no account of the realities of construction or the practicality of geometric verification. Metrological definitions are real, as they take account of real lines and surfaces accessible to measurement. They cover in a single result all micro- and macro-geometric deviations. They allow a result to be reached covering all causes of error, without distinguishing among them. Such distinction should be left to the manufacturers. Nevertheless, in some cases, geometric definitions [e.g. definitions of run-out (out-of-true running), periodic axial slip, etc.] have been retained in this standard, in order to eliminate any confusion and to clarify the language used. However, when describing test methods, measuring instruments and tolerances, metrological definitions are taken as a basis. 2.2 Test methods and use of measuring instruments During the testing of a machine tool, if the methods of measurement only allow verification that the tolerances are not exceeded (e.g. limit gauges) or if the actual deviation can only be determined by high precision measurements for which a great amount of time would be required, it is sufficient, instead of measuring, to ensure that the limits of tolerance are not exceeded. It should be emphasized that inaccuracies of measurement due to the instruments, as well as to the methods used, are to be taken into consideration during the tests. The measuring instrument should not cause any error of measurement exceeding a given fraction of the tolerance to be verified. Since the accuracy of the devices used varies from one laboratory to another, a calibration sheet should be available for each instrument. Machines under test and instrumentation should be protected from draughts and from disturbing light or heat radiation (sunlight, electric lamps too close, etc.), and the temperature of the measuring instruments should be stabilized before measuring. The machine itself shall be suitably protected from the effects of external temperature variation. A given measurement should preferably be repeated, the result of the test being obtained by taking the average of the measurements. However, the various measurements should not show too great deviations from one another. If they do, the cause should be sought either in the method or the measuring instrument, or in the machine tool itself. For more precise indications, see Annex A (Informative). 2.3 Tolerances 2.3.1 Tolerances on measurements when testing machine tools Tolerances, which limit deviations to values which are not to be exceeded, relate to the sizes, forms, positions and movements which are essential to the accuracy of working and to the mounting of tools, important components and accessories. There are also tolerances which apply only to test pieces. 2.3.1.1 Units of measurement and measuring ranges When establishing tolerances, it is necessary to indicate: a) the unit of measurement used; b) the reference base and the value of the tolerance and its location to the reference base; c) the range over which measurement is made. The tolerance and the measuring range shall be expressed in the same unit system. Tolerances, particularly tolerances on sizes, shall be indicated only when it is impossible to define them by simple reference to corresponding standard for the components of the machine. Those relating to angles shall be expressed either in units of angle (degree, minute, second) or as tangent (millimetres per millimetres). When the tolerance is known for a given range, the tolerance for another range comparable to the first one shall be determined by means of the law of proportionality. For ranges greatly different from the reference range, the law of proportionality cannot be applied: tolerances shall be wider for small ranges and narrower for large ranges than those which would result from the application of this law. 2.3.1.2 Rules concerning tolerances Tolerances include inaccuracies inherent in the measuring instruments and test methods used. Inaccuracies of measurement should consequently be taken into account in the permitted tolerances (see 2.2). Example Tolerance of run-out: x mm; Inaccuracy of instruments, errors of measurement: y mm; Maximum permissible difference in the readings during the test: (x - y) mm. Errors due to inaccuracies arising from comparative laboratory measurements, inaccuracies of form of machine parts used as reference surfaces, including surfaces masked by styli or by support points of measuring instruments, should be considered. The actual deviation should be the arithmetical mean of several readings taken, due to the above causes of error. Lines or surfaces chosen as reference basis should be directly related to the machine tool (e.g. line between centres of a lathe, spindle of a boring machine, slideways of a planing machine, etc.). The direction of the tolerance shall be defined according to the rules given in 2.3.2.4. 2.3.2 Subdivisions of tolerances 2.3.2.1 Tolerances applicable to test pieces and to individual components of machine tools It should be noted that the rules for indicating geometric tolerances on drawings given in the corresponding standard apply to the geometric accuracy of individual parts. These rules should be adhered to on manufacturing drawings. 2.3.2.1.1 Tolerances of dimension The tolerances of dimension indicated in this standard relate exclusively to the dimensions of test pieces for machining tests and to the fitting dimensions of cutting tools and of measuring instruments which may be mounted on the machine tool (spindle taper, turret bores). They constitute the limits of permissible deviations from nominal dimensions. They shall be expressed in units of length (e.g. deviations of bearings and bore diameters, for the setting up and the centring of tools). Deviations should be indicated numerically or by the symbols given in the corresponding standard.

Contents of GB/T 17421.1-1998

Foreword i ISO Foreword iii 1 Scope 2 General Considerations 3 Preliminary Operations 4 Machining Tests 5 Geometric Tests 6 Special Tests Annex A (Informative) Instruments and Equipment for Testing Machine Tools Annex B (Informative) Relevant Standards