一篇外国的经典论文

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Reduction and Compensation of Thermal Errors in Machine ToolsM. Weck (I), RWTH Aachen, Laboratory for Machine Tools and Production Engineering (WZL), Germany, Active Member P. McKeown (11, Cranfield University, Cranfield Precision Engineering Ltd, UK (retired), Honorary Member R. Bonse and U. Herbst, RWTH Aachen, Laboratory for Machine Tools and Production Engineering (WZL), Germany, Non-Members with Contributions from ClRP Members Noted in the Reference

AbstractThe main reasons for dimensional and geometric errors in workpieces produced on machine tools include low static stiffness of the machine structure, low dynamic performance of feed drives, tool wear and thermal deformations of the tool, machine and workpiece. This paper describes the latest research in analysing and reduction of thermally induced deformations in machine tools which lead to thermal drift displacements between tool and workpiece. A brief introduction to the problem is followed by an analysis of different heat sources and how they deformations. Attention is drawn to measures for reducing thermal drift as a mayor cause of errors in machine tools.

Keywords: Accuracy, Thermal Stress, Optimisation, Compensation

NomenclatureCi Ti t x, y, constant time constant time coordinates

Heat Sourc esHeat sources can be classified as internal and external. Internal ones are primarily the heat produced by running the machine and the process itself. External heat sources are mainly the changes in environment e.g. solar radiation, space heaters, lighting etc. E X T E R N A L H E A T SOURCES Variation of the ambient temperature causes temperature gradients both vertical and horizontal and these causes thermo-elastic deformations of the machine tool.

IntroductionInternal and external heat sources cause thermo-elastic deformations of machine tools and in the end result in geometric inaccuracies of the work-piece. Thermal effects can contribute more than 50% to the overall error. The necessity of reducing this error source has been recognised in the early Sixties, and research in this field was pioneered by Bryan et al.[l, 2. It is the responsibil1 ity of both the machine tool manufacturer and the user for diminishing thermally induced errors. Fiaure 1 gives an overview on the subject of thermal effects. Different heat sources combined with different mechanisms of heat transfer lead to a uniform temperature other than 20"C or to a non-uniform temperature distribution over the machine structure causing size and geometric errors in the measuring system, the machine structure and thus the workdece.

F J THERMAL ERR0

Figure 2: Influences on the temperature distribution in a machine hall[3] The amplitude of the temperature will vary with the geographical location, the season and the thermal character-

Figure 1: Diagram of thermal effects 11 2

Annals of the ClRP Vol. 44/2/1995

589

一篇外国的经典论文

istic of the machine shop. The upper part of shows the solar energy radiation over a 12

month period for Frankfurt and the temperature range over the same time. The lower part shows the typical variation of the temperature in a machine shop. A dynamical temperature range of 5"C during winter time and 15"C during summer time is not unusual.

m,then in a positive direction. After 12 hours the drift at thecutting edge is nearly zero again. Other work has demonstrated that changes of environmental temperature cause thermal drift even if the spindle is running. This and other internal heat sources cause machine temperatures significantly higher than the changes in the ambient temperature[6]. Equalising the time constants for components involved is effective in reducing thermal drift caused by external heat sources.Rob*m

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Figure 3: Horizontal and vertical temperature distribution in a machine shop[3] The temperature distribution within a machine shop depends on its location and height. Differences of temperature of more than 5"C at the same time can be seen, Eaure 3. The influence of the ambient temperature can be measured in an excellent way in climatic chambers[4, 51. This procedure is very time consuming and the costs of climatic chambers are very high. Fiaure 4 shows how a rapid ambient temperature change of 10"C causes distortion of a lathe[3].

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Figure 5: Reducing the sensivity of machine structural components to environment temperature affects (71 Fiaure 5 shows this effect for a large portal milling machine that was exposed to a poor environment[7]. Due to different wall thicknesses of the front and back of the column, the back wall warms up faster in the morning when the hall temperature rises and cools down more quickly in the afternoon than the front wall. This results in bending of the machine and thus in misalignments of two bore holes by 100 pm. Investigations revealed a larger thermal inertia of the front made of 150 mm steel plates than the 25 mm thick rear wall. By insulating the thin wall with 25 mm polystyrene the time constants can be made more equal. By this means the misalignments due to such workshop temperature changes were substantially reduced.INTERNAL

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Figure 4: Radial drift displacements of a lathe caused by rapid jumps of the ambient temperature in a climatic chamber[3] After the rapid cooling and heating of the chamber the ambient temperature is kept constant for approx. 12 hours. During the first three hours after the temperature rise the distance between tool and spindle reduces quickly by 40 pm followed by a slow increase during the following 8 hours. This result shows that the thermal driftbehaviour of a machine has large time-constants in reacting to ambient temperature changes. Re …… 此处隐藏：35781字，全部文档内容请下载后查看。喜欢就下载吧 ……