Method of manufacturing gear

Abstract: The invention discloses a gear having a high strength and a high dimension precision, and a method of making same. A cut material (1) is heated in an anti-oxidation atmosphere. Then, a form (4) including gear teeth (5) and having nearly completed shape is formed from the upset material (2) at one time by hot forging. The form (4) undergoes gear grinding (6), whereby a finished product (7) is obtained. The form (4) has a curved surface (R) previously formed at a corner along a tooth trace of a tooth flank.

Description: 1. Field of the Invention The invention relates to a method of manufacturing a gear according to the preamble of clailm 1, as for example known from EP-A-0 278 361. The invention in particular relates to a method of manufacturing a high-precision gear which is suitably available for a transmission for transmitting high torque for an automobile, an industrial machine or the like, for example. 2. Description of the Related Art In general, a gear for use in a transmission or the like has been heretofore manufactured as shown in Fig. 7. A cut material 1 is upset. Then, a blank 8 with no gear tooth, which is formed by using forging means, is cut (hobbed) in the circumferential surface thereof, whereby a form 4 with gear teeth is obtained. Then, the form 4 is gear-ground, whereby a finished product 7 is obtained. The cut gear thus formed has a flow line that is cut off by cutting, and thus the gear teeth decrease in strength. A solution such as a careful selection of the material or shot peening is consequently required. An attempt has been therefore made to simultaneously form the gear teeth together with the blank by using the forging means, i.e., to obtain the so-called form with gear teeth. This form with gear teeth is proceeding toward practical use. Forging means generally include hot forging and cold forging. Hot forgings are prone to change in dimension due to the wearing away of a die because the material or the die considerably changes in temperature. Cold forgings are susceptible to a variation in stress of the die or the material during pressurization. Thus, the gear teeth, which are formed simultaneously with a formation of the blank, vary widely. Moreover, the gear teeth are less precise. Therefore, such gear teeth must be cut so as to improve their precision. More particularly, the product requiring high precision is surface-finished by additional gear grinding. Accordingly, the gear requiring the high precision needs other processes such as the cutting and the gear grinding (polishing) in addition to forging. Recently, a streamlining has been increasingly demanded. In the field of the manufacturing of the above-mentioned gear, a further reduction of the process is desired for the manufacturing of the gear which has been hitherto able to be manufactured by at least the forging, the cutting, the gear grinding and so on. However, when the cutting is eliminated for this purpose, the gear grinding must compensate for the process equivalent to the eliminated cutting. Thus, the process is eliminated, but the time required to work considerably increases and consequently workability decreases. When one depth of cut is increased so as to make an attempt to reduce the working time, a problem arises, that is, the attempt causes damage to a grinding wheel. SUMMARY OF THE INVENTION It is an object of the invention to reduce processes and to streamline the manufacturing of a gear by increasing one depth of cut for gear grinding. A gear is formed by gear-grinding a form with gear teeth formed by forging means without cutting the form. A method of manufacturing a gear comprises the steps as defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow chart of a method of manufacturing a gear according to the invention; Figs. 2A and 2B are an enlarged view of gear teeth and a sectional end view taken on line A-A, respectively; Figs. 3A to 3C illustrate gear grinding; Figs. 4A and 4B illustrate the gear teeth of helical teeth; Figs. 5A to 5C are enlarged views of a modification of the gear tooth, and Fig. 5A is a sectional end view taken on line B-B; Fig. 6 is a flow chart of the simplest manufacturing method; and Fig. 7 is a flow chart of the method of manufacturing the gear of the related art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a method of manufacturing a forged gear according to the invention, which has precision maximized by adding other processes to the method, will be described with reference to the accompanying drawings. Fig. 1 illustrates a manufacturing process. A first process includes the following processes. In an upsetting process, a cylindrical cut material 1 whose volume is substantially the same as the volume of a finished product is used as a starting material, the cut material is heated in an anti-oxidation atmosphere, and the cut material is upset so as to have an appropriate volume distribution by hot forging. In a rough formation process, an upset material 2 is pressed so as to have a shape (including gear teeth) close to the shape of the finished product. In a finishing process, the precision of each portion of a rough form 3 having the shape close to the shape of the finished product is improved, whereby a form 4 having the nearly completed shape (including the gear teeth) is formed. In the above-mentioned hot forging, the gear teeth are formed simultaneously with the formation of a blank. Thus, a flow line of the gear teeth formed around the blank is continuous, and thus the gear teeth increase in strength compared with the gear teeth formed by machining. Therefore, a careful selection of the material, shot peening or the like is not required. Consequently, productivity becomes excellent and yield also improves. The above-mentioned form has gear teeth 5, each of which has a curved surface R having a curvature radius of about 0.2 mm at each corner along a tooth trace of a tooth flank, as illustrated in Figs. 2A and 2B. The technique of anti-oxidation normalizing is adopted to heat-treat the form having the nearly completed shape obtained by the above-mentioned hot forging. The heat-treated material undergoes surface treatment: the removal of scale on the material surface resulting from shot blasting; and cutting for giving a predetermined dimension to an end surface, an inner circumferential surface of a hole or the like except the portion having the gear teeth. In the above-mentioned shot blasting, during the above-described hot forging, both of the heating of the material and the normalizing take place in the anti-oxidation atmosphere, thereby preventing the decarburization of the surface and the occurrence of the scale and thus reducing a thickness of the scale to be removed. Thus, fine particles (having a particle diameter of about 0.5 mm, for example) can be used as shot particles. As a result, roughness of the finished surface is reduced, and thus a narrow margin for grinding is sufficient for the following grinding. Therefore, the material is little wasted and thus the yield is high. Then, carburizing takes place. Finally, gear grinding takes place, whereby a finished product 7 is obtained. After the surface treatment, the form having the nearly completed shape formed by the above-mentioned forging has the curved surface having a curvature radius of at least about 0.2 mm at each corner along the tooth trace of the tooth flank. When such a form is gear-ground, a contact of a grinding wheel 6 with the curved surface R of the gear tooth 5 is gradually increased as shown in Figs. 3A to 3C and thus the grinding wheel 6 is not abruptly shocked. Thus, even if the gear tooth 5 is deeply cut at one time without repeating many cuttings with a slight depth of cut, the grinding wheel is not damaged. When the depth of cut is about 0.2 mm, a smooth surface can be finished by a few reciprocating motions of the grinding wheel. Moreover, the depth of cut of about 0.2 mm satisfies a depth of carburizing required to ensure hardness. On the other hand, a cavity and a punch of a forging apparatus do not have an acute angle at the portion thereof corresponding to the corner along the tooth trace of the tooth flank of the form. This not only prevents a local temperature rise caused by friction during pressurization but also reduces wear and thus prevents damage to a die due to stress concentration. The gear tooth formed by the above-described forging includes not only a spur tooth but also a helical tooth. Also in the case of the helical tooth, the curved surface R is formed at each corner along the tooth trace of the tooth flank of the gear tooth 5 as shown in Figs. 4A and 4B. Moreover, many gears to be supplied as the product require chamfering at each corner along the tooth trace of the tooth flank. Thus, the trouble to cut a chamfer is taken. However, for such a gear, a chamfer H is previously formed by the forging so as to be continuously integrated with the curved surface R , as shown in Figs. 5A to 5C. Thus, similarly to the above-described embodiment, a wide margin for grinding can be provided for the gear grinding using the grinding wheel 6. Moreover, the chamfer H remains after the gear grinding, and thus the cutting for the chamfering is not needed. The apparatus, in which at least the die or an ejector is rotatable and removable while being rotated as disclosed in Japanese Patent No. 2832325, for example, is suitably used to form the helical tooth. Moreover, the gear to be formed is applied to various forms such as the gear having a boss on at least one side surface or the gear having a dog tooth formed integrally with the boss. In the above-mentioned embodiment, the method, which includes additional processes such as the shot blasting for removing the scale on the material surface and the cutting for giving a predetermined dimension to the end surface, the inner circumferential surface of the hole or the like except the portion having the gear teeth, has been described. However, these processes may be eliminated. Alternatively, only the hot forging, only cold forging or both of the hot forging and the cold forging can be used as the forging means. The simplest embodiment will be described with reference to Fig. 6. The same reference numerals and symbols as those of the above-described first embodiment are used. First, the cylindrical cut material 1 whose volume is substantially the same as the volume of the finished product is used as the starting material. The cut material is heated and upset in the anti-oxidation atmosphere. The form 4 having the nearly completed shape (including the gear teeth) is formed at one time by the hot forging. The form 4 undergoes the cutting of any portion other than the teeth and the carburizing. Then, a second process, namely, the gear grinding takes place, whereby the finished product is obtained. In this manner, two processes alone can complete the high-precision gear. Moreover, in this embodiment, the hot forging is adopted as the forging, whereby the product can be inexpensively manufactured and a large-sized product having a large module can be also manufactured. In any embodiment, the manufactured gear is formed by the forging. Therefore, the gear has the gear teeth having a continuous flow line and high strength. Moreover, these gear teeth have extremely high surface precision because they are gear-ground. According to the invention, the high-precision gear can be mass-produced by a small number of processes. More particularly, when the gear grinding is applied to the form with gear teeth having the corner on the curved surface, the grinding wheel starts the grinding while gradually increasing a contact surface with the tooth flank from a contact point of the curved surface. Thus, even if a wide margin for cutting is provided, the grinding wheel is little shocked at the time of the start of the grinding and therefore the grinding wheel is not damaged. Moreover, the hot forging is utilized, whereby the product can be inexpensively manufactured and the large-sized product having the large module can be also manufactured.

Claims: A method of manufacturing a spur gear or a helical gear, comprising the following steps: a hot forging step of forming a form (4) with gear teeth (5); and a gear grinding step (6) of grinding the gear teeth (5) of the form (4), characterized in that the form (4) is shaped to have a curved surface (R) on which a corner along a tooth trace of a tooth flank is formed, in that a contact surface of a grinding wheel (6) with the curve surface (R) is gradually increased, and in that the gear teeth (5) are deeply cut at one time by the grinding wheel (6). The method according to claim 1, wherein the gear is chamfered at each corner along the tooth trace of the tooth flank.

Inventors: SHIMOMURA MITSUHIKO, SHIMOMURA, MITSUHIKO
Applicants: O OKA CORP, O-OKA CORPORATION
Application year: 2000
Main classification: B23P15
IPC codes: B23P15, B21K1, F16H55, B23F19