Description Tool for Bonding and Compacting Motor end Cover and Casing Technical Field The invention relates to a tool for bonding and compacting a motor end cover and a casing, belonging to the technical field of a motor production tool. Technical Background of the Invention In the technical field of motor production, two end covers and the casing of some motors are fixed by bonding. In the prior art, during bonding, workers need to grasp the two ends by hands and wait for a certain period of time until adhesive is cured. On one hand, the abovementioned operation manner is low in working efficiency; and on the other hand, while the two ends of the motor end cover are grasped by hands, the end cover may relatively displace in a radial direction of the casing, thereby affecting the assembly quality of the motor. Summary of the Invention The purpose of invention is to provide a tool for bonding and compacting a motor end cover and a casing, which is conducive to improvement of both the production efficiency and the assembly quality of motors. The above technical purpose of the invention is mainly achieved by following technical solution: a tool for bonding and compacting a motor end cover and a casing, comprising a positioning sleeve, a compaction block and a compaction screw; the positioning sleeve is provided with a sleeve, a bearing base arranged below a lower end of the sleeve, and a cover plate arranged at an upper end of the sleeve and fixed with the sleeve, an opening is provided on a circumferential wall of the sleeve, the compaction screw is in thread connection with the cover plate and an inner end of the compaction screw passes through the cover plate to be located in the positioning sleeve, and the compaction block is connected with the inner end of the compaction screw. When in use, adhesive is respectively coated between the two end covers and the casing for connection, the motor coated with the adhesive is placed into the positioning sleeve through the opening on the circumferential wall, the compaction block is spirally propelled by the compaction screw to clamp the motor tightly, and after a period of time, the compaction block is loosened after the adhesive is cured to take the motor out, so that the bonding of the motor casing with the motor end covers is completed, the working efficiency is improved, and the stability of the assembly quality of the motor casing and the motor end covers is also improved. Preferably, an upper end of the compaction block is provided with a connection slot, an end part of the inner end of the compaction screw is provided with a connector matched with the connection slot. More preferably, the connection slot and the connector are of a mutually matched swallow-tail type structure or T-shaped structure. Therefore, the invention is conducive to improvement of both the production efficiency and the assembly quality of motors. Brief Description of the Drawings Fig. 1 is an exploded structure diagram when a motor is placed in a positioning sleeve in the invention; Fig. 2 is a three-dimensional structure diagram when a motor is placed in the invention; Fig. 3 is a plane structure diagram when a motor is placed in the invention; and, Fig. 4 is another exploded structure diagram when a motor is placed in a positioning sleeve in the invention; Detailed Description of the Embodiments The technical solutions of the invention will be further described in details as below by embodiments with reference to the drawings. Embodiment 1 Referring to Fig. 1, Fig. 2, Fig. 3 and Fig. 4, the positioning sleeve 1 is provided with a sleeve 2, a bearing base 3 arranged at a lower end of the sleeve 2, and a cover plate 4 arranged at an upper end of the sleeve 2 and fixed with the sleeve 2, the bearing base 3 and the sleeve 2 are integrally formed, two U-shaped slots 5 are provided on a circumference of the bearing base 3, an opening 7 is provided on a circumferential wall of the sleeve 2 to allow a motor 6 to pass through, and the cover plate 4 and the sleeve 2 are connected by a bolt 8. A compaction screw 9 is in thread connection with the cover plate 4 and an end part of an inner end 10 of the compaction screw 9 passes through the cover plate 4 to be located in the positioning sleeve 1, an end part of the inner end 10 of the compaction screw 9 is provided with a T-shaped connector 11, an upper end of a compaction block 12 is provided with a T-shaped connection slot 13 matched with the T-shaped connector 11, the compaction block 12 and the connector 11 are in fit connection at the inner end of the compaction screw 9 by the connection slot 13, a lower end of the compaction block 12 is provided with a straight slot 14 which extends in a radial direction and penetrates from a circumferential face to a center of the compaction block, and an open end of the straight slot 14 on the circumferential face of the compaction block corresponds to the opening 7 on the circumferential wall of the sleeve 2. When in use, adhesive is respectively coated between the two end covers and the casing for connection, the motor 6 coated with the adhesive is placed into the positioning sleeve 1 through the opening 7 on the circumferential wall, the compaction block 12 is spirally propelled by the compaction screw 9 to clamp the motor 6, and after a period of time, the compaction block is loosened to take the motor out after the adhesive is cured, so that the bonding of the motor casing with the motor end covers is completed.