Description An Auxiliary device for riveting end cover and shell of motor Technical Field The invention relates to a motor production auxiliary tooling, in particular to an auxiliary device for riveting motor end covers with a housing. Background of the Invention During the production process of some small motors, motor end covers and motor housings adopt the direct riveting way for connection and fixation; as the motors are smaller in volume, the structures where the motor end covers are riveted and connected with the motor housings are finer, thereby being difficult to utilize a punching machine during the normal production for punching; however, if the punching machine against the type of the motors is specially manufactured, the practice is not in line with the market demands obviously. For example, the description of an invention patent which was announced and authorized by China's State Intellectual Property Office on September 26, 2007 and in the authorization and announcement number of CN200953520Y disclosed a device for riveting motor end covers with a housing, the device was designed against the small motor with only one end cover, the device had great limitations during the actual use, a certain protection for the motor housing was absent, and the motor housing was possible to deform and even scrap due to pressure during the riveting work. Summary of the Invention The problem to be solved by the invention is to provide an auxiliary device for riveting motor end covers with a housing, which is simple in structure and convenient to manufacture and mount, and can press rivet sheets on the motor housing onto the motor end covers, further avoid damaging the motor end covers and the housing, realize strong practicality, show the excellent operation performances during the actual application and obviously improve the production efficiency and the product quality. In order to solve the technical problem, the invention adopts the following technical scheme: the auxiliary device for riveting the motor end covers with the housing comprises a lower seat and an upper seat, wherein a lower die for positioning the lower end of a motor is arranged on the lower seat, an upper die connected with the upper seat is arranged above the lower die, the upper die corresponds to the upper end of the motor, a riveting pressure head I corresponding to the part riveted with the upper end of the housing of the motor is arranged on the upper die, and the auxiliary device is characterized in that a riveting pressure head II corresponding to the part riveted with the lower end of the housing of the motor is further arranged on the lower die, and a cushioning device is arranged in the upper die and the lower die. Further, the cushioning device comprises an upper positioning block arranged in the upper die and a lower positioning block arranged in the lower die, a stepped hole I 1 with a step I is arranged in the upper die, a step III corresponding to the step I is arranged on the upper positioning block, the upper positioning block moves up and down in the stepped hole I and uses the step I for position limitation, a stepped hole II with a step II is arranged in the lower die, a step IV corresponding to the step II is arranged on the lower positioning block, the lower positioning block moves up and down in the stepped hole II and uses the step II for position limitation, an elastic part I for pushing the upper positioning block downwards is arranged above the upper positioning block, and an elastic part II for pushing the lower positioning block upwards is arranged below the lower positioning block. Further, the elastic part I is an elastic metal sheet or a spring. Further, the elastic part II is an elastic metal sheet or a spring. Further, a guide column is arranged on the lower seat, a guide sleeve which is matched with the guide column and sheathed on the guide column is arranged on the upper seat, and a spring for pushing the guide sleeve upwards is further arranged on the guide column between the guide sleeve and the lower seat. A press is pressed on the upper seat during working, after the motor housing is well riveted with the end covers, the press is lifted and the upper seat restores automatically. Further, guide curved surfaces for transition to the inner sides are arranged at the head parts of the riveting pressure head I and the riveting pressure head II. Then the rivet sheets on the motor housing are automatically pressed to the riveting parts of the motor end covers along the guide curved surfaces. After the technical scheme is adopted, the invention has the following advantages: the auxiliary device can simultaneously rivet the upper end cover and the lower end cover of the motor with the housing by clamping once; furthermore, the cushioning device is used for protecting the motor end covers, thereby avoiding damaging the motor end covers and the housing and being strong in practicality; during the actual application, the excellent operation performances are shown, and the production efficiency and the product quality are obviously improved. Brief Description of the Drawings In combination of the following figures, the invention is further described. Figure 1 is a schematic diagram of structure of one embodiment of the invention; Figure 2 is a bottom view of upper die; Figure 3 is a top view of lower die. Detailed Description of the Invention According to a schematic diagram of the structure of one embodiment of the invention as shown in Figures 1-3, an auxiliary device for riveting motor end covers with a housing comprises a lower seat 1 and an upper seat 2, wherein a guide column 11 is arranged on the lower seat 1, a guide sleeve 21 which is matched with the guide column 11 and sheathed on the guide column 11 is arranged on the upper seat 2, and a spring 12 for pushing the guide sleeve 21 upwards is further arranged on the guide column 11 between the guide sleeve 21 and the lower seat 1; a lower die 3 for positioning the lower end 5 of a motor is arranged on the lower seat 1, an upper die 4 2 connected with the upper seat 2 is arranged above the lower die 3, the upper die 4 corresponds to the upper end 6 of the motor, a riveting pressure head I 8 corresponding to the part 61 riveted with the upper end of the housing of the motor is arranged on the upper die 3, a riveting pressure head II 7 corresponding to the part 51 riveted with the lower end of the housing of the motor is further arranged on the lower die 3, a guide curved surface 81 for transition to the inner side is arranged at the head part of the riveting pressure head I 8, and a guide curved surface 71 for transition to the inner side is arranged at the head part of the riveting pressure head II 7; a cushioning device is arranged in the upper die 4 and the lower die 3, the cushioning device comprises an upper positioning block 41 arranged in the upper die 4 and a lower positioning block 31 arranged in the lower die 3, a stepped hole I 42 with a step I 421 is arranged in the upper die 4, a step III 411corresponding to the step I 421 is arranged on the upper positioning block 41, the upper positioning block 41 moves up and down in the stepped hole I 421 and uses the step I 421 for position limitation, a stepped hole II 32 with a step II 321 is arranged in the lower die 3, a step IV 311 corresponding to the step II 321 is arranged on the lower positioning block 3, the lower positioning block 31 moves up and down in the stepped hole II 32 and uses the step II 321 for position limitation, an elastic part I 43 for pushing the upper positioning block 4 downwards is arranged above the upper positioning block 4, and an elastic part II 33 for pushing the lower positioning block upwards is arranged below the lower positioning block 3. In the embodiment, the elastic part I is an elastic metal sheet or a spring; and the elastic part II is an elastic metal sheet or a spring. The auxiliary device is pressed on the upper seat through a press, thereby being capable of simultaneously riveting the upper end cover and the lower end cover of the motor with the housing by clamping once; furthermore, the cushioning device is used for protecting the motor end covers, thereby avoiding damaging the motor end covers and the housing and being strong in practicality; during the actual application, the excellent operation performances are shown, and the production efficiency and the product quality are obviously improved. 3