SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERINGLESSON TWO MACHINING ACCURACY Product quality is always a matter of prime importance in manufacturing. It should be given the highest priority in process planning. The quality of a machined part can be expressed in geometrical parameters (dimension, shape, coordination and surface finish, etc.), physical parameters (electrical conductivity, thermal conductivity, magnetic conductivity, etc.), chemical parameters (corrosion resistance. etc.) and mechanical parameters (strength, hardness, etc.). The requirements of these parameters of a part are determined by the designer, according to the condition under which the product is to function.(c) coordination 相互位置 surface finish 表面光洁度parameter 参数，参量SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING(c)The degree of coincidence of the real parameters obtained after a part is manufactured, with the parameters defined in part design, represents the manufacturing quality of the part. For the convenience of analysis, the degree of coincidence between the real macro-geometrical parameters (dimension, shape and coordination) of a part obtained after machining, and those specified in part design is defined as machining accuracy. The degree of coincidence between the real micro-geometrical parameters (surface finish) and the parameters of the physic-mechanical properties of part surfaces, obtained after machining a part, with those specified in part design is defined as surface quality. Other physical and chemical parameters are involved only in some special cases.SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERINGMachining accuracy is expressed quantitatively by the magnitude of machining error. Machining error is defined as the difference between the parameters of a machined part and that of a perfect (absolutely accurate) part specified in the part drawing. The machining error varies from part to part even in the same batch of production. Although machining errors always exist in machining processes, a part can be considered acceptable, provided the magnitude of machining errors do not exceed the tolerance limits. The tolerance represents the maximum allowable machining error. The required machining accuracy of a part is specified by the designer, while the machining accuracy requirements of the part in each machining stage are determined in accordance with the needs and accuracy of the process by the process planner.公 差SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERINGParameters of Machining Accuracy and Surface Quality Machining accuracy is associated with a number of macro-geometrical parameters and can be classified as follows: l. Machining accuracy of part surfaces. - Accuracy of dimensions of part surfaces (for example, accuracy of the diameters of cylindrical surfaces, accuracy of the diameters of spherical surfaces, and accuracy of the angles at the top of the cones, etc.). - Accuracy of shapes of part surfaces (for example, flatness of a plane, roundness, and cylindricity of a cylindrical surface, etc.).flatness 平直度,平面度roundness 圆度，球度cylindricity 圆柱度SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING2. Machining accuracy of relative positions between part surfaces.- Accuracy of dimensions between part surfaces (for example, the distance between two parallel planes or the centers of two holes, etc.).- Accuracy of positional relations between part surfaces (for example, the parallelism and perpendicularity of two planes or two axes, etc.). The surface quality of a part indicates the surface characteristics of the part. It includes either geometrical or physic-mechanical parameters, which are listed below:l) Surface roughness.2) Physic-mechanical state of surface layer.parallelism 平行度 perpendicularity 垂直度surface roughness 表面粗糙度SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERINGAmong the various parameters of machining accuracy, the shape accuracy of part surfaces relies on the cutting tools, and the relative motions between cutting tools and workpieces. The dimensional accuracy, including accuracy of dimensions of part surfaces themselves and dimensions between part surfaces, can be achieved through different method. Methods for Obtaining Required Dimensional AccuracyIn machining, different measures can be adopted to attain the required dimensional accuracy. There are basically two different methods: trial cutting (or grinding) and automatic dimension maintenance. Trial CuttingTrial cutting achieves the required dimensional accuracy in several trial cutting (or grinding) passes. A certain length of the surface to be machined is first cut and measured.(c) 试切法SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING(c) The cutting tool position with respect to the workpiece is then adjusted in the next cutting pass. The process repeats until the machined dimension falls into the specified tolerance limits.