Description Fixture for Lathing Motor Stator Technical Field of the Invention The invention relates to a fixture for lathing a motor stator. Background of the Invention Generally for the lathing of an end face of a motor stator in the prior art, the motor stator is sleeved on a positioning body fixed with the lathe through a central through hole of the motor stator and then clamped by the fitting of a gland nut and threads of the positioning body, to lath the end face. For the above positioning method for the motor stator, after each motor stator is lathed, the gland nut needs to be completely screwed from the positioning body to sleeve another motor stator on the positioning body, resulting in long time for replacement of another motor stator and low working efficiency or the like. Summary of the Invention The purpose of the invention is to provide a fixture for lathing a motor stator, with advantages of convenient replacement of another motor stator, time and labor saving and high working efficiency. The above technical purpose of the invention is mainly solved by the technical solution as follows. A fixture for lathing a motor stator comprises a positioning portion, a fixture body of a hollow structure is arranged on an upper part of the positioning portion, multiple slots arranged in an axial direction are provided on a circumferential wall of the fixture body, an outer part of the circumferential wall is of a cylindrical structure, an upper end of an inner part of the circumferential wall is provided with an open inner cone, a locking block is 1 fitted with the inner cone of the fixture body through an outer cone, and the locking block is in movable connection with the fixture body in an axial direction. Wherein, the slots are communicated with an upper end of the fixture body. Wherein, the cylindrical structure of the outer part of the circumferential wall of the fixture body is convenient for fitting with a central through hole of the motor stator, the motor stator is sleeved on the outer part of the circumferential wall of the fixture body through its central through hole, after the motor stator is sleeved, the locking block is moved towards the fixture body to make the outer cone of the locking block fit with the inner cone of the upper end of the inner part of the circumferential wall so as to expand the circumferential wall of the fixture body, so that the fixture body clamps the stator tightly. After the motor stator is lathed, the locking block is moved a certain distance away from the fixture body in the axial direction, the circumferential wall of the fixture body returns to the original size so that the lathed motor stator may be taken out smoothly, and then another motor stator to be lathed is placed in position for further lathing. Preferably, a counterbore is provided in a center of the locking block, a threaded end of a screw passes through the counterbore of the locking block to be connected with threads on a bottom of the hollow structure of the fixture body, and a head end of the screw comes into contact with a step of the counterbore. Preferably, a reset spring is provided between the locking block and the bottom of the hollow structure of the fixture body. Therefore, the invention has the advantages of convenient replacement of another motor stator, time and labor saving and high working efficiency and the like. Brief Description of the Drawings Fig. 1 is a structure diagram of a released state of a locking lock of the 2 invention; Fig. 2 is a structure diagram of a state of a motor stator locked by a fixture body of the invention; Fig. 3 is a front view of Fig. 2; and Fig. 4 is a view of Fig. 3 along line A-A. Detailed Description of the Invention The technical solutions of the invention will be further described in details as below by specific embodiments with reference to drawings. Embodiment 1 As shown in Fig. 1, Fig. 2, Fig. 3 and Fig. 4, a positioning portion 2 is arranged at a bottom of a fixture body 1 of a hollow structure and integrated with the bottom. Four slots 3 arranged in an axial direction and communicated with an upper end of the fixture body are arranged on a circumferential wall of the fixture body 1. An outer part of the circumferential wall is of a cylindrical structure, which is convenient for fitting with a central through hole of the motor stator; and an upper end of an inner part of the circumferential wall is provided with an open inner cone 4, and a locking block 5 is fitted with the inner cone 4 of the fixture body 1 through an outer cone 6. A counterbore 7 is provided in the center of the locking block 5, a threaded end of a screw 8 passes through the counterbore 7 of the locking block 5 to be connected with threads on the bottom of the hollow structure of the fixture body 1, and a head end of the screw 8 comes into contact with a step of the counterbore. A reset spring 9 is provided between the locking block 5 and the bottom of the hollow structure of the fixture body 1. A groove 10 is arranged on a lower end of the inner part of the circumferential wall of the fixture body 1 in a peripheral direction. A circular boss 11 is arranged on a lower end of the outer part of the circumferential wall of the fixture body 1 for supporting a lower end of the motor stator. As shown in Fig. 2, the motor stator 12 is sleeved on the outer part of the 3 circumferential wall of the fixture body through the central through hole of the motor stator, after the motor stator 12 is sleeved, the locking block is pushed by tightening the screw through a spanner to expand the slots, so that the fixture body clamps the motor stator tightly. As shown in Fig. 1, after the motor stator is lathed, the locking block is pushed by the spring as soon as the screw is slightly released, the sleeve returns to the original size, so that the motor stator 12 may be smoothly taken out. 4