機(jī)械工程專業(yè)英語English In Mechanical Engineering,Prepared by Xiaomin Dong From College of Mechanical Engineering Chongqing University,40Precision of Machine Tools,Modern machine tools are marvels of near perfection. Consider a lathe producing round pieces to a tolerance of 50 m. That is expected performance; yet it means that the machine maintains a relationship between a whirling workpiece and gliding tool within 25 m on a side and does it over and over again. That dimension is less than half the thickness of a human hair. To turn to a tolerance of 25 m on a side, the tool first must be adjusted to within 12.5 m of exact size. Such a minute amount can scarcely be seen, and the machine has to magnify it to reveal it to the crude vision of the operator. Then as metal is slashed away with tremendous force at varying depth, the machine must guide the tool along a firm course and hold the workpiece steady so that neither deviates from the other by more than 12.5 m and this is not close work. Tolerances smaller than 2.5 m are held in some operations.,40Precision of Machine Tools,Errors in the work done by a machine tool are caused by (a) inaccuracy in its construction, (b) deflection, (c) wear, (d) thermal expansion, (e) dirt, and (f) human laxity. Good design and operation of a machine tool depend upon observing principles that keep errors small. Evidences of these principles are found in the structural members, the bearings and guideways for straight and rotary motions, the measuring devices, and the standards of construction of all good machine tools. Design for control of errors is especially important for automated machine tools, such as for numerical control, than for simpler manual machines, where the operator can compensate for errors. Structural Members An evident feature of all major machine tool members is rigidity. The components of standard machine tools made in lots are most often cast, but some, and especially special units, are built up by welding steel plates and sections together.,40Precision of Machine Tools,One purpose of rigidity in machine tool design is to keep deflections small and maintain true alignments. The principles of mechanics show that deflection of a machine member is inversely proportional to the cube of its depth in the direction of an applied load. For that reason, a machine tool section is deep in the direction of the main force and heavily ribbed and wide to take other forces and moments. Machine tool components are designed not only to be rigid, but to have the most rigidity with the least possible weight. One obvious reason is that a machine has to be moved, at least from where it is made to where it is used. A second reason is that rigidity with small mass avoids vibration, and raises machine productivity. A third reason is that moving components are more easily and thus accurately controlled with less inertia, of particular importance in automatic control. For a simple case, any object with a single degree of freedom has a natural frequency in cycles per second of W n= k/m .The factor m is the mass of the structure, and k is proportional to the stiffness of the object. Thus the stiffer the member and the less its mass, the higher is its natural frequency. A high natural frequency is desirable because it is less excited by the usual disturbances.,40Precision of Machine Tools,Guideways the preparation of guideways in machine tools requires significant effort. The requirements of guideways are high accuracy, low surface roughness, low value of frictional force and low wear rate. The types of guideways may be classified as guideways with sliding friction also termed slideways and guideways with rolling friction also termed as antifriction guideways. There are a number of slideways conventionally used n machine tools. They are V-slideways, flat slideways, and dove tails. Anti-friction guideways involve intermediate rolling members ( balls or rollers) between the sliding members, thus reducing the friction. Spindles The members that revolve workpieces or cutters on machine tools are called spindles. Most machine tool spindles today run in antifriction bearings. Roller bearings, usually tapered, are used for heavy service at low and moderate speeds. Ball bearings run freely, stand up at high speeds, and are favored for light service. For the highest precision, ball bearings are preloaded and selected in matched sets to minimize runout. Pressurized gas journal bearings of high rigidity with low friction are used for high spindle speeds.,40Precision of Machine Tools,Thermal Distortion Machine tool members get hot during operation, and the temperature rise is not uniform as a rule. This means that some members expand more than others, and alignments and settings change. This is most noticeable during the first two hours of operation of a machine tool, and usually conditions become stable after equilibria of temperatures are reached. A turret lathe running at 1 000 rpm can be taken as an example. During the first hour the temperature rose about 17 in the headstock, and the spindle moved 50 m upward and 180 m sideward. During the second hour, spindle movements were 10 m up and 2.5 m to the side. Room temperature change also has an affect; a day to night temperature difference of 4.5 is reported to have caused a large vertical tracer lathe to drift by 80 m. A skilled operator can do much to compensate for temperature changes, but the problem is more difficult with automatic systems.,40Precision of Machine Tools,Heat sources that cause distortion may be in the environment, the fabricating or cutting process and in the machine elements, such as bearings, motors, etc. An obvious remedy is air conditioning the room, or chilling coolant and oil, and flooding the work area. Machines for precision operations may be turned on well before starting time in a plant and kept running during breaks and lunch periods. A heat source, such as a motor, may be moved from inside to outside of a machine. Good machine design takes heat distortion into account, in such ways as providing adequately for heat dissipation, heater elements to maintain working temperature, and use of low expansion alloys.,