the following detailed description is of the best presently contemplated modes of carrying out the invention . this description is not to be taken in a limiting sense , but is made merely for the purpose of illustrating general principles of embodiments of the invention . the scope of the invention is best defined by the appended claims . referring to fig4 , a wafer grinder of the present invention includes a rotary worktable 1 , an air pressure spindle 2 , a housing 3 and an adjustment module 4 . the rotary worktable 1 includes a spindle 16 positioned at a center of the air pressure spindle 2 and a housing 3 . the rotary worktable 1 includes a rotary worktable body 19 , which has a vacuum nozzle 17 and a pump hose 18 . the adjustment module 4 is positioned at perimeter of the air pressure spindle 2 and the housing 3 and is equally separated by 120 degrees , as shown in fig6 . the rotary worktable 1 includes a vacuum disk 11 made of porous material 15 . as shown in fig4 , a support plate 12 is defined below the vacuum disk 11 . further referring to fig7 , an adjusting screw 121 is provided in the support plate 12 so as to adjust the vacuum disk 11 to hold a wafer of 4 inches , 6 inches or 8 inches . the rotary worktable 1 also includes a coupling 13 made of rubber . the coupling 13 connects with a timing plate belt pulley 14 , so motor power is delivered by the timing plate belt pulley 14 . the air pressure spindle 2 includes an air inlet 21 and an air outlet 22 , as shown in fig5 . an air inlet hose 311 and an air outlet hose 321 are used to deliver static pressure air to the air pressure spindle 2 . as shown in fig4 , the adjustment module 4 includes a piezoelectric actuator 41 to adjust a tilt angle of the rotary worktable 1 . a pre - compression bolt 42 , a hexagonal socket screw 43 , a disc spring 44 and a locking screw 45 generate a pre - compressive force on the piezoelectric actuator 41 . the result is an improved rigidity of the rotary worktable 1 . a displacement meter is positioned within the adjustment module 4 so as to measure the displacement of the rotary worktable 1 . the displacement meter and the piezoelectric actuator 41 are placed in the same geometrical position . fig8 illustrates a relationship between the displacement of the piezoelectric actuator 41 and corresponding input signal . most of the relationship is linear . an auxiliary groove 33 is integrally formed with a holding body 34 to facilitate machining the airflow hose ( the air inlet hose 311 and the air outlet hose 321 ). as shown in fig4 , the wafer grinder according to present invention includes a housing module having a holding body 34 and being fixedly positioned on the wafer grinder . the rotary worktable module includes a worktable body 19 and spindle 16 that are rotatably positioned on the holding body 34 of wafer grinder . the rotary worktable module also has a wafer holding sub - module . the air pressure spindle module is positioned on the holding body 34 of the wafer grinder and has an air channel ( the air inlet hose 311 and the air outlet hose 321 ). the air channel is used to direct air with a certain pressure to the housing module and the rotary worktable module to act as an air cushion spindle to support the worktable body 19 and the spindle 16 . the adjustment module is positioned on the holding body 34 and has a piezoelectric actuator 41 and a displacement meter . the longitudinal rotation between the spindle 16 and the worktable body has an air padding to offset the lateral force during the grinding process . the description of the present invention is as follows . the holding body 34 includes the auxiliary groove 33 to facilitate drilling of the airflow grooves . the base structure of the wafer grinder is usually a conventional civil structure for setting up a machine to ensure stability . the wafer holding sub - module is provided on the worktable body 19 and also includes the vacuum nozzle 17 and the pump hose 18 so that a wafer is held steadily by a vacuum . in general , the spindle 16 is connected to the worktable body 19 to rotate the worktable body 19 so that it is more convenient for the configuration of all components . the rotary worktable module further includes an adjustment sub - module positioned within the worktable body 19 . the adjustment sub - module includes the adjusting screw 121 to block longitudinally the vacuum nozzle 17 to adjust to wafers of different sizes . to make measurements accurate , the piezoelectric actuator 41 and the displacement meter are in the same geometrical position . to make displacement uniform , three pairs of piezoelectric actuator 41 and displacement meter are positioned at the bottom of the worktable body 19 by the same separation to adjust a tilt angle of the worktable body 19 . to make movement accurate , the spindle 16 is driven by a flexible belt - like structure to prevent shock from being transmitted to the spindle 16 . to make design of the grinder easy , the spindle 16 further includes the coupling 13 and the timing plate belt pulley 14 . the coupling 13 connects with a timing plate belt pulley 14 , so motor power is delivered by the timing plate belt pulley 14 . to keep rigidity of the wafer grinder , the disc spring 44 generates a pre - compressive force on the piezoelectric actuator 41 . the advantages of the present invention can be summarized as follows : 1 . the present invention provides an air pressure worktable that can adjust a machining angle with greater precision . in particular , the spindle 16 is subjected to a lateral cutting force so that cutting precision and lifetime of grinder can be extended . the air pressure spindle 2 helps to rotate the rotary worktable 1 with precision . the air pressure spindle 2 experiences the displacement of the piezoelectric actuator 41 and generates the tilt angle . 2 . the present invention provides the capability of measuring tilt of a rotary worktable 1 because the piezoelectric actuator 41 and a displacement meter are in the same geometrical position . the disc spring 44 and a locking screw 45 produce a pre - compressive force on the piezoelectric actuator 41 so that rigidity of the rotary worktable 1 is increased and displacement is controlled with much higher precision . 3 . the present invention provides a rotary worktable 1 to hold wafers of 4 , 6 or 8 inches . four adjusting screws 121 are used to adjust a groove within the support plate 12 and make changes to the holding surface . while the invention has been described with reference to the preferred embodiments , the description is not intended to be construed in a limiting sense . it is therefore contemplated that the appended claims will cover any such modifications or embodiments as may fall within the scope of the invention defined by the following claims and their equivalents .