The present invention relates to a method for manufacturing a small-sized motor in which an end portion of a shaft of a rotor is machined into a noncircular shape.
A small-sized motor is used in various fields in order to, for example, adjust an angle of or to retract a motor-driven mirror of a car, or to drive a mechanism of a toy. In such an application, in order to establish mechanical coupling between a small-sized motor and a gear, for example, of a device to be driven, the following techniques have been used conventionally. That is, an end portion of a round shaft of the small-sized motor is machined into a noncircular shape (for example, a D shape), while a center hole of the gear for receiving the shaft is formed in a shape (for example, a D shape) corresponding to the noncircular shape of the shaft end, thereby establishing relative-rotation-prevention means. Alternatively, an inner bore of a cylindrical body for receiving the noncircular shaft end portion is formed in a circular shape, while, for example, a screw is screwed into the body from outside and perpendicular to the shaft such that an end of the screw is pressed against the noncircular portion (a flat portion of the D shape) of the shaft end, thereby establishing relative-rotation-prevention means. Conventionally, an end portion of a shaft of a small-sized motor is ground or cut into a noncircular shape before the shaft is assembled in the motor.
FIG. 5 shows a conventional shaft before assembly having a D cut 12 formed thereon so as to prevent relative rotation. In FIG. 5, reference numeral 11 denotes a shaft, and reference numeral 16 denotes an edge portion of the D cut 12. Herein, the term xe2x80x9cD cutxe2x80x9d refers to grinding or cutting of an end portion of a round shaft into a noncircular shape, typically, a D shape.
According to a conventional practice for forming a D cut on a shaft before assembly through grinding or cutting, about 100 shafts, for example, are fixedly arranged in a rest jig and are ground by use of a rotating grinding wheel. However, this prior art involves the following drawbacks.
About 100 shafts are typically ground simultaneously in order to form a D cut thereon. Therefore, grinding shafts in a smaller lot in order to fulfill an order results in a higher machining cost.
The amount of production of motors equipped with a shaft bearing a D cut must be grasped at the stage of manufacturing shafts. If customer demand should vary, the stock of shafts might increase beyond a scheduled level.
When a laminated core is to be fitted to a shaft bearing a D cut, the laminated core is press-fitted to the shaft from an end opposite the D cut (a shaft end not bearing the D cut) in order to avoid the edge portion 16 of the D cut scraping a bore wall portion of the laminated core. Therefore, shafts to be fed to a press-fitting machine for press-fitting laminated cores to the corresponding shafts must be oriented such that shaft ends on which the D cut is formed face the same direction.
When a thrust bush produced from brass is press-fitted to a shaft in order to axially position a rotor, the edge portion 16 of the D cut 12 scrapes a bore wall portion of the thrust bush, and scrapings adhere to the edge portion 16.
An object of the present invention is to provide a method for manufacturing a small-sized motor in which a D cut is formed on a shaft, which method enables determination of whether or not the D cut is to be formed on the shaft, at a later stage of a motor assembly process, thereby requiring no need to control the stock of shafts bearing the D cut.
Another object of the present invention is to eliminate the need to orient shafts in the same direction in preparation for press-fitting of laminated cores of rotors to the corresponding shafts.
Still another object of the present invention is to prevent generation of scrapings when a thrust bush is press-fitted to a shaft.
A method of the present invention for manufacturing a small-sized motor comprises a step of cutting or grinding a shaft end portion of a rotor into a noncircular shape over a predetermined length for establishing a mechanical coupling with a device to be driven, and is characterized in that the cutting or grinding work is performed after a laminated core 7, a winding 6, a commutator 5, and a strap bush 8 are assembled on a shaft 11 to thereby assemble a rotor.
A method of the present invention for manufacturing a small-sized motor is characterized in that a shaft end portion of a rotor is cut or ground into a noncircular shape for establishing a mechanical coupling with a device to be driven, after a casing 1, an end bell to be fitted to an opening of the casing 1, and a rotor are assembled into the small-sized motor.
Thus, the present invention does not involve stock control of shafts bearing a D cut, need to orient the shafts such that their D-cut ends face the same direction when laminated cores of rotors are to be press-fitted to the corresponding shafts, or generation of scrapings when thrust bushes are press-fitted to the corresponding shafts.
The shaft end portion can be cut by combined use of a punch 14 and a die 13 having a hole formed therein for receiving the shaft end portion.
The punch 14 to be used in combination with the die 13 may be stepless or may have a plurality of steps and can cut the shaft end portion in a single stroke.