Contacting State on Imaginary Rack of Crossed Helical Gears

 11  (3) Here means thermal expansion coefficient of grating ruler. T  means difference value between temperature of 0 T and 1 T . The prediction model of zero thermal drift error ero z  in X direction of the X axis grating measuring system could be expressed as following equation. T L zero   01   (4) The above results show that there exists the zero thermal drift error in the measurement direction of the grating measuring system due to the change of environmental temperature. The effect of error must be considered in the subsequent building process of comprehensive error correction model of CNC. 6.2 The indication thermal error prediction model of grating measuring system (a) The relation between the coordinates of any point in the X direction within the range of [0, 01 L ] which was measured by the X axis grating measuring system before and after the thermal deformation could be expressed as following equation.