Description A Worm Cavity Machining Auxiliary Device for A Worm Gear -Worm Case Technical Field The invention relates to tooling in the field of mechanical processing, in particular to a worm cavity machining auxiliary device for a worm gear -worm case. Background of the Invention Worm reduction box is a power transmission mechanism, which transmits motion and power depending on spatially-intersected worm gear and worm, the angle between the two axes can be an arbitrary value, commonly 90 degree, and thus the box structure of such reducer differs from that of other gearboxes. The worm reduction box belongs to special-shaped boxes and is internally provided with a worm gear cavity and a worm cavity, and the two cavities generally cross at 90 degree. In machining process, special-shaped boxes are the parts difficult to machine and needs surfaces machining and holes machining in different directions, and its machining surfaces have very high requirements of position accuracy for spatial positions, and therefore, during machining a worm gear-worm case, especially a worm gear cavity, a common clamp has poor positioning effect, and a special tooling clamp is needed so as to obtain a stable and reliable positioning. Summary of the Invention The object of the invention is to provide a worm cavity machining auxiliary device for a worm gear-worm case, which has simple structure and convenient operation, provides stable and reliable positioning for the machining of a worm cavity of a warm gear-worm case, and facilitates the machining of the worm cavity. In order to realize the above object, the invention employs the following technical scheme: a worm cavity machining auxiliary device for a worm gear-worm case is characterized by comprising a fixing seat and a clamping mechanism arranged on the fixing seat, wherein the clamping mechanism comprises a support part, a clamping part and a positioning spindle connecting the support part and the clamping part, the clamping part is movably connected with the end part of the positioning spindle and capable of axially moving along the positioning spindle, and the support part and the clamping part correspondingly press the cavity body of a worm gear cavity of a worm gear-worm case to realize the machining positioning of a worm cavity. Furthermore, the clamping part comprises a pressure plate corresponding to the support part and a screw bolt penetrate-mounted on the pressure plate, and the screw bolt is movably screwed on the upper end of the spindle. Furthermore, the support part is the upper end surface of the fixing seat. Furthermore, a positioning mechanism for radially positioning the worm gear cavity is disposed at both ends of the positioning spindle. Furthermore, the positioning mechanism comprises a first positioning plate and a second positioning plate which are respectively arranged at both ends of the 1 positioning spindle, the first positioning plate is butted against a positioning step at the open end of the worm gear cavity, and the second positioning plate is butted against a support step of the output shaft hole of the worm gear cavity. Furthermore, one end of the fixing seat is connected with a machining device via a connection mechanism. Furthermore, the connection mechanism comprises a mounting plate connected with one end of the fixing seat and a connecting spindle disposed on the side surface of the mounting plate. Furthermore, the connecting spindle is spatially perpendicular to the positioning spindle. The invention realizes the machining positioning of a worm cavity by fixing the cavity body of a worm gear cavity of a worm gear-worm case by the clamping mechanism, has simple structure and convenient operation, and provides reliable positioning for the machining of the worm cavity of the worm gear-worm case; the connecting spindle is spatially perpendicular to the positioning spindle, and the worm gear cavity is also spatially perpendicular to the worm cavity, namely that the in machining the worm cavity is spatially parallel with the connecting spindle, which coincides with the cutter feeding direction of the machining device and thus facilitates the machining of the worm cavity. Brief Description of the Drawings The invention will be further described by combining the following drawings: FIG.1 is a structural diagram of the worm gear-worm case; FIG2 is a structural diagram of the worm gear cavity of the warm gear-worm case; and FIG3 is a structural diagram of the invention. Detailed Description of the Invention Referring to FIG. 1 to FIG.3, the invention provides a worm cavity machining auxiliary device for a worm gear-worm case, which comprises a fixing seat 1 and a clamping mechanism arranged on the fixed seat 1, wherein the clamping mechanism comprises a support part, a clamping part and a positioning spindle 2 connecting the support part and the clamping part, the clamping part is movably connected with the end part of the positioning spindle 2 and capable of axially moving along the positioning spindle 2, the support part and the clamping part correspondingly press the cavity body of a worm gear cavity 6 of a worm gear-worm case to realize the machining positioning of a worm cavity 7, the preferred structure is as follows: The clamping part comprises a pressure plate 3 corresponding to the support part and a screw bolt 31 penetrate-mounted on the pressure 3, and the screw bolt 31 is movably screwed on the upper end of the spindle 2; the support part is the upper end surface 11 of the fixing seat 1; a positioning mechanism for radially positioning the worm gear cavity 6 is disposed at both ends of the positioning spindle 2 to improve the machining accuracy, the positioning mechanism comprises a first positioning plate 21 and a second positioning plate 22 which are respectively arranged at both ends of the 2 positioning spindle 2, the first positioning plate 21 is butted against a positioning step 61 at the open end of the worm gear cavity 6, and the second positioning plate 22 is butted against a support step 62 of the output shaft hole of the worm gear cavity 6. One end of the fixing seat 1 is connected with a machining device via a connection mechanism, the connection mechanism comprises a mounting plate 4 connected with one end of the fixing seat and a connecting spindle 5 disposed on the side surface of the mounting plate 4, and the connecting spindle 5 is generally connected with a cutter head of the machining device. In mounting, the pressure plate 3 is detached, the worm gear cavity 6 of the worm gear-worm case is inserted corresponding to the positioning spindle 2, and then the mounting plate 3 is mounted and screwed tightly by the screw bolt 31, such that the worm gear cavity 6 of the worm gear-worm case is fixed on the device. The connecting spindle 5 is spatially perpendicular to the positioning spindle 2, and the worm gear cavity 6 is also spatially perpendicular to the worm cavity 7, namely that in machining the worm cavity 7 is spatially parallel with the connecting spindle 5, which coincides with the cutter feeding direction of the machining device and thus facilitates the machining of the worm cavity 7. In addition to the above preferred embodiment, the invention has other embodiments. Various changes and modifications made by those of ordinary skill in the art according to the invention are seen to fall within the scope of the invention as defined by the appended claims without departing from the spirit of the invention. 3