244 and the position difference correction rail 245 are equipped with a magnetic material in this invention if the motor 254 that drives the sandstone disk 251 is a built in one the main shaft of the motor will serve as the stator and should not rotate to attain this purpose a main shaft control device 244 should be added fig9 a shows above main shaft control device 244 which comprises small bearings 2442 vertical shaft 2443 small spring 2444 screw 2445 and main shaft hole 2446 the small bearings 2442 are mounted on a vertical shaft 2443 and the main shaft control device 244 is adjusted with screw 2445 and spring 2444 to adapt to the rail 241 the main shaft control device 244 may also employ any other structure fig9 b shows an adjustable main shaft control device 244 u2032 different to fig9 a the sides of the main shaft control device 244 u2032 are smooth iron sheets 2447 which are adjustable to adapt to the rails in this invention the main shaft control device 244 is designed as a sliding block the outer diameter of which is identical to the inner diameter of the small rails 241 thus the main shaft control device 244 can slide in the small rails 241 and keep the main shaft of the motor 245 stationary.