GEAR ROLLING METHOD USING CIRCULAR DIES

The present invention relates to a thread rolling method for a gear using cylindrical dies in an apparatus including a work having a cylindrical outer surface, a retainer element for retaining the work to be freely rotatable about an axis of the cylindrical outer surface, and a drive mechanism for thrusting the pair of cylindrical dies against the work, in which the pair of cylindrical dies are each rotatable about an axis parallel to the axis to face each other across the work and are synchronized with each other in rotating speed and thrusting amount, see e.g. US 2011/0302783 A1. A thread rolling gear according to the present invention is used for various kinds of industrial products typically represented by automobile parts. For example, techniques for developing hybrid vehicles and electric-powered vehicles have been rapidly advancing in the automobile industry. In particular, a demand for power-saving and small-sized electric actuators in all kinds will increase in future. Further, it is required that components of those actuators have good efficiency in power transmission and manufacture.

A number of gears are used in the actuators. Those gears have great influence on the functions and the manufacturing costs of the actuators. As a method of manufacturing the gears efficiently, a thread rolling method for thrusting cylindrical dies against a cylindrical solid work is generally known, for example. In such a method, the pair of cylindrical dies each having a profile of a desired gear that are provided to face each other across the work are allowed to thrust against the surface of the work, synchronized with each other in rotating speed and thrusting speed against the work, thereby to form the gear.

The thread rolling method using the dies uses linear rack dies or the cylindrical dies. In the thread rolling method using the rack dies, addendums formed in an end portion of the rack dies having a fixed length are brought into press contact with the work

and then fed with the dies being pressed against the work, thereby to allow the work to roll. In many cases, a tooth depth in the rack dies is different between an area in an early stage of the thread rolling process where the addendums begin to contact the work and an area in a finishing stage of the thread rolling process. Therefore, in the thread rolling using the rack dies, the profile of the gear to be formed is influenced by the length of the rack dies.

On the other hand, in the case of using the cylindrical dies, the cylindrical dies are rotated and concurrently thrust against the work. Therefore, the tooth profile is constant in any portion of the cylindrical dies, which provides a simple arrangement in the cylindrical dies. In addition, the rotating direction of the cylindrical dies can be switched over during the thread rolling process, which is advantageously applied to the thread rolling of large-sized worms and helical gears.

An example of the conventional thread rolling method using the cylindrical dies is disclosed in document JP 2010-075963 A. Such a method provides a thread rolling apparatus for helical gears, and worms and screws (mechanical elements having a lead in a flank), in which "slippage" can be reduced while simplifying the arrangement of the apparatus. In this, the "slippage" indicates a phenomenon in which the work moves along rotation axes of the cylindrical dies when the cylindrical dies are thrust against the work. This phenomenon occurs when the relative relationship between the addendums of the dies and tooth grooves formed in the surface of the work in advance is not proper. More particularly, the addendums of the dies are thrust against the tooth grooves of the work in a distorted manner to cause the dies and the work to follow advance/return motion of a screw, as a result of which the slippage occurs. When the slippage occurs, a portion of the surface of the work that should not be threaded is threaded, which makes quality maintenance difficult.

In the known technique described above, a biting tooth, a forming tooth, a finishing tooth and the like are provided in an outer circumference of each of the cylindrical dies provided to face each other across the work to form worms in the work during one rotation of each cylindrical die. A chamfered portion is provided in an axial one end of the outer circumference of each cylindrical die to allow the forming tooth or a clearance tooth to follow the worms in response to the axial movement of the work in forming the worms. The above prior art document describes that such an arrangement can eliminate the disadvantages caused by the slippage phenomenon without controlling the drive of the cylindrical dies by a controller.

On the other hand, since it is required to form the biting teeth, the forming teeth, the finishing teeth and the like in the outer circumference of the cylindrical die in the above conventional arrangement, a specially designed cylindrical die must be provided, which takes much time and effort in the manufacture and increases the manufacturing costs. In addition, since the conventional cylindrical die is driven for only one rotation against the work in the same manner as the rack die, the profile of the gear to be manufactured is limited. Thus, there is much room for improvement in forming the gear effectively in the conventional thread rolling method described above.

Document US 2011/0302783 A1 discloses a form rolling method for an involute gear, which includes a work piece including a cylindrical outer peripheral surface having a predetermined radius, and a round die with an involute tooth profile including an addendum pitch corresponding to a pitch defined by dividing a length of an outer circumference of the work piece by number of teeth of the involute gear. The round die is pressed to the work piece while rotating when form rolling the involute gear. In particular, an outer diameter of the work is set so that the dimension obtained by dividing the initial outer circumference of the work by the number of teeth is equal to the addendum circular pitch of cylindrical dies.

JP 2003-340542 A has the object to provide a thread rolling material for a worm which enables a worm having a high-accuracy screw part to be manufactured only by thread rolling and can reduce a production cost of the worm. The thread rolling material for a worm is used for forming a screw part of a two-line screw by thread rolling using a die. The thread rolling material has a screw forming part at which a screw part is formed. A notch part for equalizing push-in resistance of the material to the die in forming threads in the circumferential direction of the screw forming part is formed in a taper part, which is the end of the screw forming part including a large-diameter part and a pair of taper parts.

Document JP S33010773 B1 discloses a device in which a work is driven to rotate. A tooth profile of a tool is negatively dislocated so as to achieve an outer diameter corresponding to an outer circumferential dimension of the work obtained by multiplying an addendum circular pitch of cylindrical dies by the number of teeth of the work.

JP 2005-193302 A pertains to the problem to provide a form-rolling method of a gear-shaped member. Vibration occurring in an early stage of form rolling is to be suppressed. A dividing process by which the number of teeth is divided at a prescribed pitch shall become possible. Furthermore, damage to the form-rolling device is to be reduced. By the form-rolling method of the gear-shaped member, the gear-shaped member is rolled by performing form rolling to the outer peripheral part of a blank with the working teeth of a roller die by using the roller die provided with a plurality of the working teeth for performing the form rolling to the blank on the outer peripheral part and forcing the roller die in the outer peripheral part of the blank at a prescribed speed while rotating the roller die. The forcing speed of the roller die in the early stage of the form rolling is set larger than the forcing speed of the roller die after the early stage of the form rolling and the amount of forcing for setting the larger forcing speed of the roller die in the early stage of the form rolling is taken as ≤ 40% of the whole amount of forcing of the roller die.

The object of the present invention is to achieve a thread rolling method for a gear using cylindrical dies capable of eliminating the above technical disadvantages and providing a proper tooth profile by the use of cylindrical dies in which no slippage occur during the processing.

The object is achieved by a method according to claim 1 Advantageous embodiments are carried out according to the dependent claims.

According to the present invention, a threaded rolling method for a gear using cylindrical dies includes the steps of placing a work having a cylindrical outer surface to be supported to a support to be freely rotatable about a rotation axis of the cylindrical outer surface, placing a pair of the cylindrical dies each rotatable about an axis parallel to the rotation axis to face each other across the work, thrusting the pair of cylindrical dies against the work, the cylindrical dies being synchronized with each other in rotating speed and thrusting amount via a drive mechanism, and determining an outer diameter of the work, when rotary movement of the cylindrical dies and the work rotated together is shifted from friction gear movement based on thrusting forces exerted from both the cylindrical dies and the work to gear movement based on the engagement between the work and the cylindrical dies at tooth grooves formed in the work by thrusting the cylindrical dies against the work, in which a dimension obtained by dividing an outer circumferential dimension of a dedendum circle of the tooth grooves by the number of teeth to be formed is equal to an addendum circular pitch of each of the cylindrical dies.

In the thread rolling process for the gear using the cylindrical dies, the surface of the work in the initial stage is a plain cylindrical surface. The cylindrical dies and the works are rotated together in the friction gear movement when they start to come into contact with each other as the cylindrical dies thrust against the work. Further, as the thrusting by the cylindrical dies advance, the tooth grooves are formed on the surface of the work. When the cylindrical dies and the work are brought into mesh-engagement securely with each other, they are rotated together in the gear movement. Although slippage occurs when the rotating speed of the work is different between the two modes of the movement in this matter, such slippage occurring in the early stage is gradually suppressed. Once the slippage has subsided, the tooth profile is formed stably in the subsequent process.

According to the present invention, the initial outer diameter of the work is determined with the tooth grooves being securely formed in the work such that the dimension obtained by dividing the outer circumferential dimension of the dedendum circle of the tooth grooves by the number of teeth to be formed is equal to the addendum circular pitch of each of the cylindrical dies. More particularly, the outer diameter of the work is determined, taking into account the depth of the tooth grooves formed in the work when the rotary movement of the work is shifted from the friction gear movement to the gear movement. In such a case, the precise division by the teeth is completed when the tooth grooves are formed. Hence, the cylindrical dies and the work are rotated in a proper relative phase to avoid any slippage in the thread rolling process, as a result of which an accurate gear can be achieved.

According to the present invention, thrusting amount of the cylindrical dies against the work in shifting to the gear movement is preferably set to 0.1 mm to 0.3 mm.

In order to thrust the cylindrical dies against the work without slippage, it is preferable to form a wall that limits the addendum of the cylindrical die peripherally on the work. Generally, the work to undergo the thread rolling is metal and has predetermined plastic deformability. More particularly, when the cylindrical dies are thrust against the work by 0.1 mm to 0.3 mm, each addendum of the cylindrical dies is surrounded by the wall having such a height. As a result, a sufficient resistance is exerted for restraining the addendum. If the thrusting amount is small, the cylindrical dies may slip on the work, and the division by the teeth cannot be completed. On the other hand, if the thrusting amount is too large, the initial outer diameter of the work becomes large and the division of the dedendums would be difficult, though the addendum is advantageously restrained. If the outer diameter of the work becomes large, a volume of the base material of the work for contributing to the formation of the addendums is increased. Thus, the tooth profile becomes improper, or additional processing is required for setting the dedendum depth of the cylindrical die to a larger value in order to absorb the extra volume of the base material, which may mar the efficiency.

According to the present invention, the pair of cylindrical dies may be thrust against the work intermittently while the rotary movement is shifted from the friction gear movement to the gear movement.

With the above-described arrangement, the addendum of the die can be brought into contact with the surface of the work with an angle close to the normal direction by increasing the thrusting amount of the cylindrical dies intermittently while the rotary movement is shifted to the gear movement. Therefore, the occurrence of the slippage can be effectively restrained by reducing external force components applied from the teeth of the die to the work along the direction of the rotation axis or in a direction perpendicular to the direction of the rotation axis. In addition, since the thrusting operation of the cylindrical dies is performed intermittently in thrusting the addendums of the cylindrical dies against the surface of the work, the thrusting movement against all the tooth grooves can be uniform. In other words, the thrusting operation against all the tooth grooves is completed in a predetermined manner, and then additional thrusting operation is performed on each of the tooth grooves to form uniform tooth grooves over the entire circumference and complete the precise division. As a result, following thrusting operation is performed more reliably, which can achieve an accurate thread-rolling gear.

BRIEF DESCRIPTION OF DRAWINGS

• Fig. 1 is an illustrative view of a thread rolling apparatus;
• Fig. 2 is an illustrative view of a support structure for a work in the thread rolling apparatus;
• Fig. 3 shows dimensions of the work and a die before thread rolling operation is performed;
• Fig. 4 shows dimensions of the work and the die after the thread rolling operation is performed;
• Fig. 5 is an illustration for explaining how a diameter of the work is determined;
• Fig. 6 is a graphic representation showing relationships between thrusting amount of the dies and the rotating speed of the works;
• Fig. 7 is a graphic representation showing relationships between the thrusting amount of the dies and axial displacement of the works;
• Fig. 8 is a graphic representation showing relationships between the thrusting amount of the dies and the rotating speed of the works, in which the least slippage is exhibited;
• Fig. 9 is a graphic representation of an example, in which the axial displacement of the work relative to the thrusting amount of the dies is the least; and
• Fig. 10 is an illustration of a trajectory of addendums of the die thrust against the surface of the work.

[Outline of Thread Rolling Apparatus]

A thread rolling method for a gear using cylindrical dies (simply referred to as "dies" or "die" hereinafter) according to the present invention will be described hereinafter in reference to the accompanying drawings. Figs. 1 and 2 are schematic views of a thread rolling apparatus used in a current embodiment. A pair of dies 1 are thrust against a work 3 by hydraulic drive units 2, for example. Those dies 1 are rotated and driven by AC servomotors 4. The right and left dies 1 are synchronized with each other in rotating speed and thrusting amount by an unillustrated CNC device. As shown in Fig. 2, the work 3 is supported by opposite supports 5 aligned with a rotation axis X. The work 3 is freely driven by rotation of the dies 1 (rotated together). One of the supports 51 supporting the work 3 is fixed to a table 6. The other of the supports 52 is movable along the rotation axis X on the table 6. The support 52 pushes the work 3 along the rotation