The invention relates to a hex nut assembly machine for a screw part of a numerical control tool grinding machine, comprising a bottom plate and an upper platen welded on the bottom plate, a motor ismounted at the bottom of the upper platen, and the motor is connected with a disk. A slide rail is arranged on the bottom plate, and a transmission frame is slidably mounted above the slide rail alonga front and rear direction, the left part of the transmission frame is provided with a transverse slot, an eccentric position on the bottom surface of the disk is provided with a convex block horizontally slidingly matched with the transverse slot, the right side of the transmission frame is welded with a single-sided rack, the single-sided rack is engaged with a thick gear, the thick gear is meshed with a thin gear, the thin gear is connected with a round shaft through a key, the left end of the round shaft is welded with a screw, the right side of the upper end of the upper platen is fixedly provided with a screw seat matched with the thread of the screw, and the left side of the screw is provided with a clamping device. According to the invention, on the basis of the function of clamping hex nuts, the hex nut assembly machine can conveniently automatically complete screwing of a large batch of nuts one by one into a screw component of a grinding machine through left and right reciprocating precession of the clamping device controlled by gear transmission, and has high assembly efficiency.