In general workshops, the ambient temperature varies greatly from season to season, and the temperature difference between day and night is also great. In addition, at the same time, the temperature at different locations of workshops is different, and the temperature at different space heights is also different.
The linear expansion coefficient a of steel and cast iron is 1.2X10-5/C and 1.1X10-5/C respectively. For the casting with a length or diameter of 100mm, when the temperature rises by 1C, the elongation or expansion will be 1.1pm. In addition to directly affecting the elongation (expansion) and shrinkage of the workpiece, temperature changes have an impact on the accuracy of machine tools and measuring accuracy.
Influence of temperature on machining accuracy and measuring accuracy
Due to the different materials of various parts and components on the machine tool, the linear expansion coefficient is also different, which causes thermal deformation when the temperature changes, and destroys the original geometric accuracy of the machine tool. Similarly, due to different materials of workpiece and measuring tool (instrument) and different linear expansion coefficient, measurement error is caused. It is internationally recognized that 20C is the standard for temperature measurement. Shanghai CNC machining center pointed out that the measurement error caused by ambient temperature difference can be calculated by the following formula:
OL=L [am (Tm-20) - a (Te - 20) -... where: L - measured linear quantity (length or diameter, etc.); 0L linear measurement error; Cm, Tm - linear expansion coefficient and temperature of measuring tool ac, Te - linear expansion coefficient and temperature of workpiece If T=T.=T, there is OL=L [an (Tm - 20) - a (T. - 21 ---. 2 It can be seen from ② that to make △ L → 0, Tm=T.=20C or am=ae is generally less than 3x10-6/C for common materials am ae. Therefore, when processing 100mm long workpieces and measuring at standard temperature, it is required that:
When the measurement accuracy is 10pm, the allowable temperature fluctuation is 33C; The measurement accuracy is 1 μ Um, the allowable temperature fluctuation is 3C; The measurement accuracy is 0. l μ When m, the allowable temperature fluctuation is 0.3C
The weight and size of each part of the measured parts are different, so the heat capacity is different. To reach the same temperature, different measured parts need different cooling times. Therefore, to reduce the error caused by the temperature difference between the workpiece and the measuring tool during measurement, the workpiece should be placed for different times after entering the thermostatic chamber, so that it can reach the same temperature before measurement.
It can be seen from the thermal deformation law that large thermal deformation usually occurs within a period of time (2-6 hours) after the machine tool is started. Before reaching the thermal balance (the temperature reaches a stable value), the temperature rises with time, and its thermal deformation will change with the increase of temperature, which has a large impact on the machining accuracy. When the thermal balance is reached, the thermal deformation tends to be stable gradually. Therefore, precision machining should be carried out after the thermal balance.