A process for gear-rolling a high accuracy gear

Abstract: According to the present invention, in a heating step, an outer circumferential portion of a workpiece (7) having a disk shape is heated (28) to high temperatures. In a hot rough-rolling step, the outer circumferential portion of the heated workpiece (7) is formed by use of a roller die (32,42) to generate a rolled-gear having teeth. In a warm finish-rolling step, the teeth of the rolled-gear are finish-rolled by use of the finishing roller die (33,43). The starting temperature T 1 of the hot rough-rolling step is set in the range of from 850 through 1100°C, the terminating temperature of the hot rough-rolling step T 2 is set in the range of from 500 through 700°C. The starting temperature T 3 of the warm finish-rolling step is set in the range of from 400 through 700°C, and the terminating temperature of the warm finish-rolling step T 4 is set in the range of from 200 through 650°C.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for gear-rolling a high accuracy gear made of iron based material. For instance, it is applicable to production of vehicle-flywheels of having teeth and gears used in driving systems. Description of the Related Art Generally, gears have been produced by way of a hob-cutting step and a shaving finish-step with respect to a disk-shaped workpiece. In this technique, when an outer diameter and a facewidth of the gear are increased, production efficiency is reduced and production costs are increased. Accordingly, there has been developed a gear-rolling process for generating gear-teeth by use of a rolling step. In accordance with this process, in a situation that a blank, a disk-shaped workpiece made of metal, is heated in high temperatures, a pair of rotating roller dies is squeezed into the outer circumferential portion of the blank; thus, the teeth can be generated in the outer circumferential portion of the workpiece. Such process is disclosed in SU-A-1 810 196. Moreover, conventionally, there has been developed a technique for cold-rolling the gear produced by means of a hob-cutting step to finish the gear. Although this gear-rolling process is advantageous in decreasing costs in comparison with the process using the aforementioned hob-cutting step and the shaving finish-step, this gear-rolling process is not sufficient in improving the accuracy of teeth. Also, according to the technique for cold-rolling the gear produced by means of a hob-cutting step to finish the gear, rectifying tooth-runout, cumulative pitch errors and so on all over the gear is substantially impossible. SUMMARY OF THE INVENTION The present invention has been developed in view of the aforementioned circumstances. It is therefore an object of the present invention to provide a process for gear-rolling a high accuracy gear made of iron based material which can acquire high accuracy teeth incapable of being produced in the conventional gear-rolling. This object is achieved by the features of claim 1. Preferably, the process for gear-rolling a high accuracy gear uses: a roller squeezing apparatus in which the roller die and the finishing roller die disposed coaxially and connected in series in the axial direction of the roller die, the warm finish-rolling step is continuously carried out immediately after the hot rough-rolling step rolling step without decreasing the temperature of the rolled-gear to a normal temperature range. _ The warm finish-rolling step can be carried out immediately after the hot rough-rolling step. Therefore, the rectified effect is ensured with respect to the rolled-gear to ensure the accuracy of rolled-gear. Accordingly, it is advantageous that as-rolled gear has high accuracy. The temperature of the hot rough-rolling step and the warm finish-rolling step is appropriate. In particular, the starting temperature of the warm finish-rolling step is appropriate. Therefore, the rectified effect in the warm finish-rolling step is ensured with respect to the rolled-gear to ensure accuracy of the rolled-gear advantageously. If the warm finish-rolling step is continuously carried out immediately after the hot rough-rolling step, the temperature neighboring the teeth of the rolled-gear produced by the hot rough-rolling step can be appropriately kept. Thus, on the basis of the lingering heat in the rolled-gear immediately after the hot rough-rolling step, the warm finish-rolling step is effectively carried out. Further, if the warm finish-rolling step is continuously carried out immediately after the hot rough- rolling step without the reset of rolled-gear, the axial aberration due to the reset of rolled-gear is avoided, and thereby accuracy of the rolled-gear is advantageously improved. From US 3,914,083 g a process for gear-rolling has become known, wherein the hot rough-rolling step is followed by a warm finish-rolling step. However, this known process is tailored to gears fax of thermo-plastic resins. BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the present invention and many of its advantages will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings and detailed specification, all of which forms a part of the disclosure: Figure 1 is a graph which schematically shows the relationship between temperature of the blank and time in one gear-rolling configuration; Figure 2 is a graph which schematically shows the relationship between temperature of the blank and time in another gear-rolling configuration; Figure 3 is a view which illustrates defects generated in the case where temperature is inadequate; Figure 4 is a graph which shows the relationship between starting temperature and die-life( the number of rolling); Figure 5 is a graph which shows the relationship between blank surface temperature and oxidation scale film thickness of the rolled-gear; Figure 6 is a constructive view which illustrates an engagement of a roller die and an engagement of a finishing roller die; Figure 7 is a graph which shows the relationship between starting temperature of the finish-rolling step and improved allowance in the tooth profile error, improved allowance in the tooth-groove runout, and improved allowance in the cumulative pitch error; Figure 8 is a profile view which shows the tooth profiles before the warm finish-rolling step; Figure 9 is a profile view which shows the tooth trace profiles before the warm finish-rolling step; Figure 10 is a profile view which shows the tooth profiles after the warm finish-rolling step; Figure 11 is a profile view which shows the tooth trace profiles after the warm finish-rolling step; Figure 12 is a profile view which shows the tooth-groove runout, the cumulative pitch error (R)(L) before the warm finish-rolling step; Figure 13 is a profile view which shows the tooth-groove runout, the cumulative pitch error (R)(L) after the warm finish-rolling step; Figure 14 is a plan view which schematically illustrates a whole apparatus of a Preferred Embodiment; Figure 15 is a front view which illustrates a major portion of the apparatus of a Preferred Embodiment; Figure 16 is a constructive view which illustrates a construction of a blank holding portion; Figure 17 is a constructive view which explains rigidity in a squeezing direction of the roller squeezing apparatus; Figure 18 is a constructive view which illustrates a phase-difference of roller dies; Figure 19 is a side view which illustrates the roller squeezing apparatus; Figure 20 is a constructive view which illustrates a situation wherein the forming teeth of the roller die and the forming teeth of the finishing roller die correspond with each other in a circumferential direction; Figure 21 is a timing chart in the rolling step of a Preferred Embodiment; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A Preferred Embodiment of a process for gear-rolling a high accuracy gear according to the present invention will be hereinafter described with reference to the accompanied drawings. Configurations of the present invention will be hereinafter described. (1)Configurations According to the present invention, a continuous configuration shown in Figure 1 and a non-continuous configuration shown in Figure 2 can be employed. In the continuous configuration show in Figure 1, an iron-based workpiece, heated by means of high-frequency induction heating, is used. Thus, the warm finish-rolling step is continuously carried out immediately after the hot rough-rolling step by employing the lingering heat in the rolled-gear without decreasing the temperature to a normal temperature region. In the non-continuous configuration show in Figure 2, the rolled-gear is once cooled to a normal temperature immediately after the hot rough-rolling step. Thereafter, the rolled-gear is again heated to warm-temperatures by means of the high-frequency induction heating, and thereby the warm finish-rolling step is carried out with respect to the rolled gear. (2) Set Temperature Set temperature and its significance in the continuous configuration shown in Figure 1 and the non-continuous configuration shown in Figure 2 will be hereinafter described with the following (A) through (C): (A) As for the temperature for heating the outer circumferential portion of the blank, that is the workpiece, in the induction-heating step, the region being from 1 through 2 times as high as tooth-height, is heated from 900 through 1150°C. On the other hand, the temperature of the central portion of the workpiece is lower ( generally from 50 through 200°C) due to skin-effect of the induction heating. (B) The starting temperature T 1 in the hot rough-rolling step is set in the range of from 850 through 1100 °C. when the starting temperature T 1 is lower, as shown at the arrow (a) in Figure 3, rise-shortage to the tooth crest is easy to occur because the poor-fluidity in the plastic deformation. Further, as shown at the arrow (b) in Figure 3, burr-defects are easy to occur in the deddendum. Moreover, as can be seen from test-results of Figure 4, the lower the starting temperature for rolling is shifted, the higher the hardness of the blank corresponding to the workpiece becomes. Thus, die-life of the roller die is considerably decreased. From this point of view, the lower limit of the starting temperature T 1 in the hot rough-rolling is set at 850°C. When the starting temperature T 1 in the hot rough-rolling step is excessively high, an oxidation scale film comes to be thick on the surface of blank as can be understood from the test-results shown in Figure 5. From this point of view, the upper limit of the starting temperature T 1 in the hot rough-rolling is set at 1100°C.The terminating temperature T 2 in the hot rough-rolling step is set in the range of 500 through 700°C. When the terminating temperature T 2 is lower, the appropriate starting temperature of the warm finish-rolling step can not be obtained. Thus, the lower limit of the terminating temperature T 2 in the hot rough-rolling step is set at 500°C . When the terminating temperature T 2 is excessively higher, it is required that the starting temperature T 1 in the hot rough-rolling step must be set at the temperature region considerably exceeding 1100°C. Thus, the workpiece becomes undesirably and excessively hot. So, the terminating temperature T 2 in the hot rough-rolling step is set in the range of 500 through 700 °C.Further, since the temperature of the rolling portion in the blank passes the transformation point A 1 during rolling, the structure-refining effect can be expected on the basis of the thermomechanical treatment. (C) The starting temperature T 3 in the warm finish-rolling step is set in the range of 400 through 700 °C. Because rectified effect is small when the starting temperature T 3 is lower. In particular, not only rectifying the tooth-surface of the rolled-gear but also rectifying the tooth-groove runout and the accumulative error is hard in the range of below 400°c as can be understood from the test-results in Figure 7. Thus, the starting temperature T 3 is set at 400 °C. When the starting temperature T 3 in the warm finish-rolling is excessively high, a heat-contraction amount is larger resulting from temperature-factors during cooling, so the burnishing effect at the tooth-surface produced by finish-rolling step comes to disappear. From this point of view, the upper limit of the starting temperature T 3 in the warm finish-rolling is set at 700°C. The terminating temperature T 4 in the warm finish-rolling step is set in the range of 200 through 650 °C. When the terminating temperature T 4 is lower, the rectifying effect can not be obtained in the finish-rolling step. When the terminating temperature T 4 is higher, a heat-contraction amount resulting from temperature-factors increases during cooling, so the burnishing effect at the tooth-surface produced by finish-rolling comes to disappear. From this point of view, the terminating temperature T 4 in the warm finish-rolling is set in the range of 200 through 650°C. As can be understood from the aforementioned, each of the temperatures T 1 , T 2 , T 3 , T 4 is set within prescribed temperature band. On condition that this temperature band is kept, the aforementioned upper limit temperature can be decreased by 5, 10, or 15 °C and the aforementioned lower limit temperature can be increased by 5,10, or 15 °C depending on the rolling conditions in order that the temperature width may be narrowed because the appropriate temperatures for rolling vary sometimes depending on carbon content and the like of the workpiece. (3) Engagement configuration Figure 6 schematically shows an engagement configuration between the roller dies. As can be seen from Figure 6(A), the forming teeth 32c of the roller die 32 used in the hot rough-rolling step correspond to the teeth of the rolled-gear with die-symmetry. On the other hand, as can be seen from Figure 6(B), the forming teeth 33c of the finishing roller die 33 used in the warm finish-rolling step do not correspond to the teeth 78c of the rolled-gear with die-symmetry. Namely, as can be seen from Figure 6(B), although burnishing work is carried out on the teeth-surface 78d of the teeth 78c of rolled-gear, the burnishing work is not carried out on the tooth-crest 78e and tooth-flank 78f; because the tooth-crest 78c and tooth-flank 78f do not come into contact with the forming teeth 33c of the finishing roller die 33. (4) Experiments Figure 7 shows the relationship between the accuracy of the rolled-gear and the starting temperature T 3 of the warm finish-rolling step. The left side of the vertical axis in Figure 7 shows the improved allowance of tooth profile error, and the right side of the vertical axis in Figure 7 shows the improved allowance of tooth-groove runout and the improved allowance of accumulative pitch error. The improved allowance is exhibited as follows: [ (the dimension accuracy difference before and after finish-rolling step) / ( the dimension accuracy difference before finish-rolling step )] x 100%. The larger the improved allowance, the larger the rectified effect. The hatched mark in Figure 7 shows the tooth profile error, the circle mark shows the tooth-grove runout, and the half black-painted mark shows the accumulative pitch error. The tooth profile error, the teeth-groove runout, and accumulative error are defined on the basis of JIS-STANDARD. As understood from the test-results in Figure 7, when the starting temperature T 3 in the warm finish-rolling is more than 400°C, the improved allowance of the tooth profile error, the improved allowance of tooth-groove runout, and the improved allowance of the accumulative error are high. In particular, the betterment effect is larger in the improved allowance of tooth profile error. However, when the starting temperature T 3 of the warm finish-rolling step is less than 400°c, the rectified effect in accuracy is decreased. In this experiment, the target rolled-gear was the herical gear where the material of blank was carbon steel (JIS: S58C), a normal module of the target rolled-gear was set at 2.4, the number of the teeth was set at 10, and the helical angle was set at 30°. The number of specimens was 10(n=10). The blank holding portion was used as a flowing system capable of moving in the squeezing direction with the squeezing load applied from right and left. In the hot rough-rolling step, the starting temperature T 1 was set at 950 °C, the terminating temperature was set at 650°c. Each of the squeezing load of a pair of roller squeezing apparatus was set at 5 tonf, the time for squeezing operation was 3.5 seconds, and the time for sizing operation was 3.5 seconds. After the hot rough-rolling step was carried out on the basis of the aforementioned conditions, the warm finish-rolling step ( the starting temperature T 3 =600°C, the terminating temperature T 4 = 450°C ) was carried out on the basis of the configuration shown in Figure 2 to form the rolled-gear. The tooth profile was measured. Namely, Figure 8 shows the tooth profiles whose tooth (A) to (D) are disposed at intervals of 90° in the circumferential direction and which belong to the rolled-gear before the warm finish-rolling step. Also, Figure 9 shows the tooth trace profiles in the same tooth (A) to (D). The combination of (A) and (A) shows the tooth-surface being back to back with each other for the specific tooth. The combination of (B) and (B) shows the tooth-surface being back to back with each other in the specific other tooth. The combinations(C) and (C) and the combinations (D) and (D) are similar. In Figure 8, the values below the drawn profiles show the band width error (unit: microns) and the pressure angle error (unit: microns). In Figure 9, the values below the drawn profiles show the tooth trace error (unit: micron) and helix angle error (unit: micron). Figure 10 shows the tooth profiles after the warm finish--rolling step, and Figure 11 shows tooth trace profiles after the warm finish-rolling step. Also, Figures 10 and 11 show the band width error, the pressure angle error, the tooth trace error, and the helix angle error. As can be understood from a comparison between Figures 8 and 10, the betterment effect is seen in the band width error and the pressure angle error. Further, as can be understood from a comparison between Figures 9 and 11, the betterment effect is seen in the tooth trace error and the helix angle error. Moreover, Figure 12 shows the tooth-groove runout and the cumulative pitch error (R)(L) before the warm finish-rolling step. Figure 13 shows the tooth-groove runout and the cumulative pitch error (R)(L) after the warm finish-rolling step. Although the tooth-groove runout is 71 microns before the finish-rolling step, it is decreased to 24 microns after the finish-rolling step. Although the cumulative pitch error (R) is 113 microns before the finish-rolling step, it is decreased to 88 microns after the finish-rolling step. Although the cumulative pitch error (L) is 110 microns before the finish-rolling step, it is decreased to 80 microns after the finish-rolling step. According to the present invention, not only the induction heating but also other heating means, for instance heating means capable of heating the workpiece to high temperatures at a rapid speed, are used as the heating means. The apparatus according to a further embodiment will be hereinafter described with reference to Figures 14 through 21. Figure 14 is illustrates a plan view of the apparatus. Figure 15 illustrates a front view of the major portion of the apparatus. As can be seen from Figure 14, a blank holding portion 1, which operates as a workpiece holding portion, comprises a first blank holding portion 11 and a second blank holding portion 12 facing to each other. The first blank holding portion 11 includes a first blank holding shaft 11a having a large-diameter,'and the second blank holding portion 12 includes a second holding shaft 12a having a large-diameter. A first motor 21 operates as a blank rotating means for operating the blank, that is, the workpiece. When the first motor 21.drives, the first blank holding portion 11 rotates in a circumferential direction thereof ( i.e., the direction of the arrow "E1" in Figure 15). In Figure 14, there is disposed a second motor 22 for moving the first blank holding portion 11 to transfer the blank. When the second motor 22 drives, a ball screw shaft 24r rotates in a circumferential direction thereof, and thereby the first blank holding portion 11 and the blank 7 are transferred in directions of arrow "Y1" "Y2". In Figure 14, when a third motor 23 operates as a blank rotating means, the second blank holding portion 12 rotates by way of a torque transmitting variable clutch 26 (for instance, a powder clutch) in a circumferential direction thereof, namely, the same direction as the rotating direction of first blank holding portion 11. When a hydraulic cylinder 29 used for transferring the second blank holding portion 12, drives, the second blank holding portion 12 is transferred toward the first blank holding portion 11 in the direction of the arrow "Y3" by use of the ball-splined shaft 26f, and thereby the second blank holding portion 12 and the first blank holding portion 11 can hold the blank 5 forcibly. In Figure 14, on the other side of the first blank holding portion 11, there is disposed a high-frequency heating coil 28 which operates as a ring-shaped heating means for heating the blank 7 by means of induction-heating. A thermal sensor 28c, that is, a radiation pyrometer, detects situations of the heated blank. A roller squeezing apparatus 3 includes a first roller squeezing apparatus 31 and a second roller squeezing apparatus 41 constituting an opposing pair of holding the blank 7 in the radial direction of the blank 7. The first roller squeezing apparatus 31 comprises a first roller die 32 for working as a hot rolling tool, a first finishing roller die 33 for working as a warm rolling tool, a first connecting shaft 34, and a first housing 36. The first connecting shaft 34 connects the first roller die 32 and the first finishing roller die 33 in series along the axial direction and coaxitially. The first roller die 32 and the first finishing roller die 33 are rotatably held on the first housing 36. Further, the first roller squeezing apparatus 31 includes a fourth motor 24 and a first ball screw shaft 37. As can be seen from Figure 14, similarly, the second roller squeezing apparatus 41 comprises a second roughing roller die 42 for -working as a hot rolling tool, a second connecting shaft 44, and a second housing 46. The second connecting shaft 44 connects the second roller die 42 and the second finishing roller die 43 in series in the axial direction and coaxitially. The second roller die 42 and the second finishing roller die 43 are rotatably held on the second housing 46. Further, the second roller squeezing apparatus 41 includes a fifth motor 25 and a second ball screw shaft 47. The first housing 36 is capable of squeezing the blank 7 in the direction of the arrow "X1" and is capable of withdrawing from the blank 7 in the direction of the arrow "X2". The second housing 46 is capable of squeezing the blank 7 in the direction of the arrow "X1" and is capable of withdrawing from the blank 7 in the directions of the arrow "X2". As can be understood from Figure 14, the first housing 36 , having a "channel-shape" in a plan view, includes two first faced thick-wall portions 36a,36b facing each other, and a first connecting thick-wall portion 36c for connecting the first faced thick-wall portions 36a, 36b. Also, the second housing 46 , having a "channel-shape" in a plan view, includes two second faced thick-wall portions 46a,46b facing each other, and a second connecting thick-wall portion 46c for connecting the second faced thick-wall portions 46a, 46b. As can be understood from Figure 15, the first housing 36 and the second housing 46 are movable along the guiding portions 3b fixed on the base 3a for supporting themselves in the directions of the arrow "X1" "X2". Turning back to Figure 14, when the fourth motor 24 is driven, the driving force of the fourth motor 24 is reduced by use of the first speed reducer 24i and is transmitted to the first ball screw shaft 37. Then, the first ball screw shaft 37 is rotated in the circumferential direction, and the first housing 36 is transferred in the direction of the arrow "X1"; hence, the first roller die 32 and the first finishing roller die 33 which are held on the first housing 36 are transferred toward the blank 7 in the same direction. Also, when the fourth motor 24 is conversely rotated, the first ball screw shaft 37 is conversely rotated in the circumferential direction thereof, and thereby the first housing 36 is transferred in the direction of the arrow "X2". Accordingly, the first roller die 32 and the first finishing roller die 33 are transferred together in the same direction to be withdrawn from the blank 7. Hence, the fourth motor 24 and the first ball screw shaft 37 operate as squeezing and withdrawing means for squeezing the first roller die 32 and the first finishing roller die 33 toward the blank 7. Similarly, in Figure 14, when the fifth motor 25 is driven, the driving force of the fifth motor 25 is reduced by use of the second speed reducer 25i and is transmitted to the second ball screw shaft 47. Then, the second ball screw shaft 47 is rotated in the circumferential direction thereof, the second housing 46 is transferred in the direction of the arrow "X1"; hence, the second roller die 42 and the second finishing roller die 43 are transferred toward the blank 7 in the same direction. When the fifth motor 25 is conversely rotated, the second ball screw shaft 47 is conversely rotated in the circumferential direction, and thereby the second housing 46 is transferred in the direction of the arrow "X2". Accordingly, the second roller die 42 and the second finishing roller die 43 are transferred together in the same direction to be withdrawn from the blank 7. Hence, the fifth motor 25 and the second ball screw shaft 47 operate as squeezing means for squeezing the second roller die 42 and the second finishing roller die 43 toward the blank 7. The load working on the first housing 36 is detected by use of a first load cell 36r, and a transferred amount of the first housing 36 is detected by use of a first liner scale 36k. The load working on the second housing 46 is detected by use of a second load cell 46r, and a transferred amount of the second housing 46 is detected by use of a second liner scale 46k. Each of detected signals is inputted to a controller system. The aforementioned fourth motor 24 and fifth motor 25, constituting a servo-motor respectively, are controlled on the basis of squeezing synchronous command signals and withdrawing synchronous command signals from the controller system, and thereby operating the first ball screw shaft 37 and the second ball screw shaft 47 synchronously. Accordingly, the first roller die 32 and the second roller die 42 can be synchronously squeezed in the direction of the arrow "X1" and can be synchronously withdrawn in the direction of the arrow "X2". Also, in Figure 14, when the motor 5 constituting the servo-motor for rotating the dies is driven on the basis of driving command signals from the controller system, the first reducer 52 is worked by way of gears 50,51 for reducing speed. Then, the first connecting shaft 34, the first roller die 32, and the first finishing roller die 33 are rotated together by way of the rotating shaft 52e and the first constant speed universal joint 53, and thereby the rolling step is carried out. Moreover, the driving force of the motor 5 for rotating the first die is transmitted to a phase adjusting mechanism 55x, a second reducer 55, a rotating shaft 55e, and a second constant speed universal joint 56. Accordingly, the driving force of the motor 5 is transmitted to the second connecting shaft 44, the second roller die 42, and the second finishing roller die 43 ; therefore, they are rotated. The phase adjusting mechanism 55x is used for adjusting the circumferential phase of the forming teeth of the first roller die 32 to the circumferential phase of the forming teeth of the second roller die 42. The phase adjusting mechanism 55x has a function for canceling the phase-difference between the first roller die 32 and the second rolLer die 42. With the object of realizing this function, the phase adjusting mechanism 55x has a pair of disks 55y including a lot of engaging teeth extending in a radial direction and connecting means for connecting the disks 55y. Controlling the engagement between the engaging teeth of the disks 55y realizes that function. Now, the holding mechanism of the blank holding portion 1 will be described hereinafter. As shown in Figure 16, the first blank holding portion 11 includes a first holding shaft 11a, an operating shaft 14, a tightening body 15 having a sleeve-shape, a collet 16, a pressing body 17 having a ring-shape. The first holding shaft 11a, having high rigidity, includes a first conical surface 11c having a reducing outer diameter as it goes to an axial end. The operating shaft 14 is slidablely inserted in an inserting hole lid of the first holding shaft 11a. The tightening body 15 is disposed at the end of the first holding shaft 11a to be engaged with a flange 14c positioned at the axial end of the operating shaft 14. The collet 16 operates as a engaging claw capable of moving in the direction of the arrow "C1", namely, the radius outward direction. The pressing body 17 is held at the end surface of the first holding shaft 11a by use of bolts( not shown). In Figure 16, when the operating shaft 14 is operated in the direction of the arrow "D1", the tightening body 15 is moved in the same direction. Thus, the conical surface 15h of the tightening body 15 pushes a conical surface 16t of the collet 16 forcibly. As a result, the collet 16 is moved in the direction of the arrow "C1", and the collet 16 urges the inner wall surface 71 constituting the central hole of the blank 7 in the direction of the arrow "C1", and thereby the blank 7 is firmly held by use of the first blank holding portion 11. The second blank holding portion 12 comprises an inserting bore 18 formed at the axial end thereof and a ring-shaped pressing body 19 held with bolts (not shown) at the axial end. A guiding wall surface 18k with a slight inclination is formed at the inner surface of the inserting bore 18. When the first blank holding portion 11 and the second second blank holding portion approach each other relatively along the axial direction, as can be understood in Figure 16, the inserting bore 18 of the second holding shaft 12a of the second blank holding portion 12 is forcibly inserted into the tightening body 15. Accordingly, the tightening body 15 is restricted in the -radius direction. Thus, the force for restricting the blank 7 becomes highly rigid, and the first blank holding portion 11 and the second holding portion 12 hold the blank 7 securely. Therefore, the blank 7, which is held by use of the the first blank holding portion 11 and the second holding portion 12, can not fluctuate substantially in the directions of the arrow "X1,X2". So, this embodiment employs a non-flowing method which is sometimes called as a locking method. In this embodiment, the blank holding rigidity is set more rigid than 0.1 mm/tonf in the direction of the arrow "X1", namely, the squeezing direction. Concretely, from 0.01 through 0.085 mm/tonf, or from 0.07 through 0.08 mm/tonf. The aforementioned blank holding rigidity on the basis of the blank holding portion 1 is defined as follows: As shown as broken line in Figure 16, it is supposed that the first holding shaft 11a and the second holding shaft 12a are bent due to umbalanced force ▵ W' to generate the deflection ▵ Bs of the blank 7 in the directions of the arrow "X1,X2", that is, the squeezing direction. For comprehension of them, the broken line draws the deflection in an exaggerated manner. In the case where the blank holding rigidity is indicated as E B , E B is calculed as follows: E B = {▵ B S (mm) /▵ W' (tonf)} In order for the blank holding rigidity E B to be more rigid than 0.1mm/tonf, the following considerations (A)(B) are required: (A) A rickety movement, existing between the outer wall surface of the collet 16 and the inner wall surface 71 of the blank 7, is to become infinitesimal or a zero; and (B) The rigidity of the first holding shaft 11a of the first blank holding portion 11 and the second holding shaft 12a of the second blank holding portion 12 is more rigid in the squeezing direction (i.e., the directions of the arrow "X1,X2) than other directions. For realizing the abovementioned (A) and (B), the following (a) through (e) are important. (a) Increasing the diameter of the first holding shaft 11a and the second holding shaft 12a; (b) Thickening the first housing 36 and the second housing 46; (c) Increasing the number of reinforcing ribs for enlarging the rigidity of the housing 36,46; (d) Selecting the material having high-rigidity as a base metal for the housing 36,46; and (e) Setting the rickety movement of the sliding surface for transferring the housing 36,46 to zero by way of a locking mechanism such as a hydraulic pressure mechanism. The squeezing synchronous precision means an average deflection in a squeezed amount of the first roller die 32 and the second roller die 42 during the rolling step when both of the roller dies 32,42 are synchronously squeezed with respect to the blank 7. In this embodiment, the squeezing synchronous precision L between the first roller die 32 and the second roller die 42 is set higher than 0.03 mm in the direction of the arrow "X1", namely, the squeezing direction. Concretely, it is set in the range from 0.005 through 0.03 mm. In this embodiment, not only the squeezing synchronous precision between the first roller die 32 and the second roller die 42, but also the squeezing synchronous precision between the first finishing roller die 33 and the second finishing roller die 43 is in the aforementioned same range. The squeezing synchronous precision is expressed as follows: In Figure 15, the distance between the outer end of the first roller die 32 for contacting the blank 7 and the central axial line of the blank holding portion 1 is indicated as L LS (mm); the distance between the outer end of the second roller die 42 for contacting the blank 7 and the central axial line of the blank holding portion 1 is indicated as L RS (mm); and the affixed "S" in "L LS " and " L R " means the outer end of the roller die. When a squeezing synchronous precision as a moment value at a certain time is indicated as ΔL', ΔL' means an absolute value of the difference between the a squeezed amount of the first roller die 32 and a squeezed amount of the second roller die 42 for a finite time. In other words, ΔL'= | L LS - L RS | As the aforementioned Δ L' is a moment value, it varies from start to termination in the rolling step; therefore, the average value of the aforementioned moment values Δ L' is determined as the squeezing synchronous precision Δ L in the present invention. The aforementioned Δ L' is under the influence of the originally feeding precision on the basis of the roller squeezing apparatus 3 in the no-load condition and a bending amount of the roller squeezing apparatus 3 during the rolling step. In order to improve the squeezing synchronous precision L for obtaining high-rigidity like this embodiment, it is thought that an oil system using oil pressure is insufficient because of lack of feeding precision. The aforementioned squeezing synchronous precision having high precision is archived as follows: As shown in Figure 14, the ball-screw system having the accurate ball screw shafts 37, 47 is employed, and the servo-controlled system operating the ball screw shafts 37,47 synchronously by way of the motors 24,25 operating as servo-motor is employed. A combination of these systems shows that the feeding precision for transferring the first roller die 32 and the second roller die 42 in the squeezing direction is improved to be high, and the rigidity of the roller squeezing apparatus 3 is high. In this embodiment, the rigidity of the roller squeezing apparatus 3 is set in the region of more than 0.03 mm/tonf. Concretely, it is set to be in the range of from 0.033 through 0.01 mm/tonf. The rigidity of the roller squeezing apparatus 3 is defined as follows: As shown in Figure 17, L RSO (mm) indicates the distance from the central axial line of the blank holding portion 1 to the outer end of the roller die 42 under no-load. On the other hand, when load "F" applies to this apparatus, L RSK (mm) indicates the distance the central line of the blank holding portion 1 to the outer end of the roller die 42. Here, the rigidity of the roller squeezing apparatus 3 is indicated as E R , wherein E R is computed according to the following: E R (mm/tonf) = { (L RSK - L SRO )/F } In Figure 17, the deflection is exaggeratingly drawn by use of the broken line for comprehension purposes. In this embodiment, the phase-difference between the first roller die 32 and the second roller die 42 is controlled on the basis of the controller system. Therefore, the deflection ( = an average deflection during rolling ), existing between the rotating angle of the second roller die 42 and the rotating angle of the first roller die 32 with respect to one rotation of the first roller die 32, is suppressed within 0.1°. This deflection is preferably within 0.03°. This small phase-difference can be advantageously realized on condition that the motor 5 constituting the servo-motor for rotating the die is controlled by use of the controller system 9, the phase adjusting mechanism 55x is employed, the constant speed universal joints 53,33 having high precision are employed, and a back-lash removing mechanism (not shown) is employed. Taking that the number of the teeth of the rolled-gear as an odd number for example, the phase-difference of the dies will be hereinafter explained. As shown in Figure 18, "O L " indicates the central line of the first roller die 32, and "O R " indicates the central line of the second roller die 42. The " O L -O R " line connects both of the central lines. When one of the teeth-groove centers 32t in the first roller die 32 is always disposed on the "O L -O R " line during rolling, and when one of centers 42r of the forming teeth in the second roller die 42 is always disposed on the "O L -O R " line during rolling, the difference between both the dies comes to be 0°. Here, the phase-difference between both the dies 32,42 during rolling is influenced by the sum adding an initial phase-difference Δ to a speed dispersion Δ m in the rotating mechanism. The initial phase-difference Δ , existing between the first roller die 32 and the second roller die 42, will be hereinafter described: It is requested before rolling that the center 32t,42r in the roller die 32,42 must be ideally disposed on the "O L -O R " line. In spite of this request, when the center 42r of forming teeth in the second roller die 42 is shifted by Δ with respect to the "O L - O R " line before rolling, the angle Δ is defined as the initial phase-difference between the first roller die 32 and the second roller die 42. Moreover, when the first roller die 32 is rotated by rotational angle  L , it is ideally requested that the rotational angle  R of the second roller die 42 is equal to  L . However,  R is not equal to  L in a microscopic level because of the influence of rotational dispersion of the rotational mechanism. Thus, generally,  R =  L + Δm' Here, Δ'm is defined as a speed dispersion in the rotational mechanism. Δ'm is a moment value at a certain time, and varies slightly during rotating. So, in this embodiment, not a moment value but an average value from the starting of the rolling to the terminating of the rolling is defined as the aforementioned Δm. In the case where the number of the teeth of the rolled-gear is an even number, one of the teeth-groove of the forming teeth of the first roller die 32 is disposed to face with one of the teeth-groove of the forming teeth of the second roller die 42. In such circumstances, when one of the teeth-groove centers of the first roller die 32 and one of centers of the teeth-groove of the second roller die 42 are disposed on the "O L - O R " line, the phase-difference comes to be 0°. In Figure 14, the carbon steel based blank 7 (material; JIS-STANDARD S58C ), being kept in a normal temperature range, is held on the first blank holding portion 11 by chucking work. Next, the second motor 22 is driven to transfer the blank 7 in the direction of the arrow "Y1" and to dispose the blank 7 in the high-frequency heating coil 28. In this circumstances, the motor 21 is driven to rotate the blank 7 in the circumferential direction ( i.e., the direction of the arrow "E1" in Figure 15 ). While the blank 7 is rotated, the outer circumferential portion of the blank 7 is induction-heated by use of the high-frequency heating coil 28. The range heated up to 900°C in the blank 7 is from the outer circumferential of the blank 7 to a depth being approximately 1.3 times of the tooth height. The heating time is set in the neighborhood from some seconds through 30 seconds. As soon as the outer circumferential portion of the blank 7 is heated to the designated temperature range (more than 900°C), the rolling step is carried out. The time from the termination of the heating step to the start of the hot roughing-rolling step is set within 5 seconds. The reason is that the heat-transmission into the inside of the blank 7 is suppressed to reduce the increasing of the temperature in the middle portion of the blank 7 and for improving a temperature-distribution in the blank 7. After the heating, the ball screw shaft 24r is operated by use of the second motor 22, and the blank 7 is transferred in the direction of the arrow "Y1" to be disposed at a forming location "R1" in Figure 14. At this time, the second blank holding portion 12 is moved in the direction of the arrow "Y3"; thus, both of the second blank holding portion 12 and the first blank holding portion 11 hold the blank 7 forcibly as illustrated in Figure 16. The forcible force is secured to several [tonf] by use of the hydraulic cylinder 29. In this circumstances, the blank 7 is rotated in the circumferential direction thereof on the basis of the driving force of the third motor 23. At this time, since the first motor 21 is off, the blank 7 is rotated only by use of the third motor 23 . Moreover, the first roller die 32 and the second roller die 42 are rotated at a predetermined constant speed. On the basis of the squeezing synchronous command signals outputted from the controller system (not shown), the first roller die 32 and the second roller die 42 are synchronously squeezed to the outer circumferential portion of the blank 7 in the direction of the arrow "X1" ( squeezing speed: 6mm/sec). Thus, a rise of tooth is generated. After this rise, sizing is performed upon the outer circumferential portion of the blank 7 during 5 to 20 rotations of the blank 7, so that teeth are generated during the hot rough-rolling step. After that, on the basis of the withdrawing synchronous command signals outputted from the controller system , the first roller die 32 and the second roller die 42 are synchronously withdrawn from the outer circumferential portion of the blank 7 in the direction of the arrow "X2". After the hot rough-rolling step is terminated as described above, the cylinder 29 and the second motor 22 transfer the blank 7 further in the direction of the arrow "Y1" to dispose the blank 7 at the finish-forming location "R2" shown in Figure 14. In this circumstances, on the basis of the squeezing synchronous command signals outputted from the controller system, the first finishing roller die 33, being rotated with the first roller die 32, is transferred in the direction of the arrow "X1" to be squeezed toward the blank 7. And the second finishing roller die 43, being rotated with the second roller die 42, is transferred in the direction of the arrow "X1" to be squeezed to the blank 7 synchronously with the first roller die. Therefore, the teeth of the blank 7 are finish-rolled in the range of warm temperatures ( from the starting temperature 600°C through the terminating temperature 400°C). After that, the first finishing roller die 33 and the second finishing roller die 43 are transferred in the direction of the arrow "X2" and are withdrawn from the blank 7. In this embodiment, the squeezing synchronous precision between the first roller die 32 and the second roller die 42 is high. As can be seen in Figure 15, the distance between the central axis line of the blank 7 and the central axis line of the first roller die 32 is indicated as L L , and the distance between the central axis line of the blank 7 and the central axis line of the second roller die 42 is indicated as L R . Here, L L and L R correspond with each other with high precision. Thus, a teeth-groove runout in the rolled-gear can be decreased, and it is advantageous in producing the rolled-gear having high accuracy. Moreover, in this embodiment, as can be understood from Figure 15, a first emitting device 76 for emitting liquid-lubricant is equipped to face the portion passing a rolling area in the first roller die 32. Also, a second emitting device 77 for emitting liquid-lubricant containing graphite powder is equipped to face the portion passing a rolling area in the second roller die 42. Namely, the first emitting device 76 and the second emitting device 77 are respectively separately disposed at perpendicular positions. Accordingly, This apparatus is advantageous in uniformalizing spraying timing and spraying time of lublicant , and is advantageous in uniformalizing a sprayed amount of lublicant with respect to the first roller die 32 and second roller die 42. So, this apparatus is advantageous in uniformalizing the lubricated property and the temperature distribution, and is advantageous in producing the rolled-gear with a high degree of accuracy. Figure 19 shows the first roller squeezing apparatus 31. As can be seen from Figure 19, in the first roller squeezing apparatus 31, a keyway 34h is formed at the first connecting shaft 34, rotatably held on the first housing 36, along the axial direction. Further, a mating keyway 32i is formed at the inner circumferential portion of the fitting hole of the first roller die 32, and a mating keyway 33i is formed at the inner circumferential portion of the fitting hole of the first finishing roller die 33. A key 34m is engaged with the mating keyways 32i, 33i and a keyway 34h formed at the first connecting shaft 34, thereby the dies 32, 33 are integrated with respect to the circumferential direction. Accordingly, as car. be understood from Figure 20, when the center of one of the forming teeth 32c in the roller die 32 is adjusted to the plumb-line "PL", the others of forming teeth 32c are disposed at intervals of 1 angle degrees. Also, when the center of one of the forming-teeth 33c of the finishing roller die 33 is adjusted to the plumb-line "PL", the others of the forming teeth 33c are disposed at intervals of 1 angle degrees. In other words, the circumferential phase of the forming teeth 32c of the roller die 32 agrees with the circumferential phase of the forming teeth 33c of the finishing roller die 33. Therefore, the aforementioned key and keyways operate as the forming teeth phase adjusting means. The total number of the teeth in the finishing roller die 33 is as many as those of the roller dies 32. The total number of the teeth in the finishing roller die 43 is as many as those of the roller die 42. Here, Figure 20 shows only part of the forming teeth 32c, 33c. The second roller squeezing apparatus 41 has the similar construction to the first roller squeezing apparatus 31; therefore, as can be understood from Figure 20, the aforementioned key and keyways adjust the circumferential phase of the forming die 42c of the second roller die 42 to the circumferential phase of the forming teeth 43c of the second finishing roller die 43. (Timing Chart) Figure 21 shows an example of timing charts for the rolling step carried out by use of the embodiment apparatus. The horizontal axis in Figure 21 shows the passed time when the starting time for the hot rough-rolling step is set at "0". The lower part of vertical axis in Figure 21 shows advance and delay in the blank-rotation when a target rotational speed of the aforementioned blank 7 is set at N B . The upper part of the vertical axis shows a ratio of horsepower(h.p.) in the torque transmitting variable clutch 26. This ratio means the ratio at which the driving force of the third motor 23 is transmitted to the second blank holding portion 12. From time-a' in Figure 21, the roller dies 32, 42 are begun to be fed in the squeezing direction. From time-a , being immediately after time-a', through time-e, the hot rough-rolling step is carried out with respect to the blank 7. From time-e, the roller dies 32,42 are withdrawn from the rolled-gear 78 in the direction of the arrow "X2". From immediately after time-e', the finishing roller dies 33, 43 are begun to be fed in the squeezing direction ( i.e., the direction of the arrow "X1" ). At time-f, the forming teeth 33c,43c of the finishing roller dies 33,43 begin to engage with the teeth of rough rolled-gear 78. In this embodiment, a target rotational speed N B is set as follows: [[N B =N R x [ Z RH / Z B ]] The rotational speed of the roller dies 42(32) is indicated as N R , the number of the teeth in the roller dies 42(32) is indicated as Z RH , the number of the teeth in the rolled-gear 78 made from blank 7 is indicated as Z B , Here, the number of the teeth of finishing roller dies 33, 43 is set at that of the roughing roller dies 32, 42, namely, Z RH . As can be seen from Figure 21, the blank 7 is basically rotated at the target rotational speed N B except specified periods. Thus, the controller system, which operates as an engagement controlling means, controls the second holding shaft 12a of the second blank holding portion 12 in order to control the blank 7 without advance and delay with respect to the target rotational speed N B . Also, the roller dies 32, 42, 33, 43 are controlled on the basis of the controller system 9 to rotate at a rotational speed "N R . However, as shown from time-b to time-c in Figure 21 the rotational speed of the blank 7 is gradually increased when progressing with the hot rough-rolling step. For example, the rotational speed of the blank 7 is increased by +0.3% with respect to the target rotational speed N B . This reason will be described hereinafter: The engagement, which is between the teeth of the rolled-gear 78 and the forming teeth 32c,42c of the roller dies 32,42, is enhanced with the teeth of the rolled- gear 78 generated, so that the rotational speed of the rolled- gear 78 is increased under the influence of the rotational driving force of the roller dies 32,42. Accordingly, as shown as Δ T1 in Figure 21, the controller system controls the transmitting torque variable clutch 26 from time-b through time-d to decrease the rate of the transmitted horsepower in the range of less than 50% and to decrease the transmitting of the driving force from the third motor 23. Thus, the rotational speed of the blank 7( i.e., the rolled-gear 78 ) returns again to the target rotational speed N B . Therefore, the rotational speed of the blank 7 returns to the target rotational speed N B at time-d where the teeth are fitted to be a nearly steady state with the sizing operation progressing. In this example, since the hot rough-rolling step is terminated at time-e, the roller dies 32,42 are withdrawn from the rolled-gear 78 at time-e. Also, at time-e, the controller system controls the transmitting torque variable clutch 26 in such a manner that the transmitting horsepower efficiency is returned to 100%; Hence, the rotational speed of the blank 7 ( i.e., the rolled-gear 78) is kept at the target rotational speed N B . Also, the finishing roller dies 33,43 begin to engage with the rolled-gear 78 at time-f. The rotational speed of the blank 7 ( i.e., the rough rolled-gear 78) is kept at the target rotational speed N B . Besides, as mentioned above, because of the key 34m and the keyways 32i,33i, the forming teeth 32c of the first roughing roller die 32 and the forming teeth 33c of the first finishing roller die 33 agree with each other in the circumferential phase. Similarly, the teeth 42c of the second roughing roller die 42 and the forming teeth 43c of the second finishing roller die 43 agree with in the circumferential phase. Further, the roller die 32, 33,42,43 are controlled to be rotated usually at the constant rotational speed N R on the basis of the controller system. In this embodiment including the aforementioned construction, when the warm finish-rolling step is started, the relationship which exists between the teeth 78c of the rough-rolled-gear 78 and the forming teeth of the roller die 32 and which exists immediately after the termination of the hot rough-rolling step, is stably kept not to vary. Therefore, the teeth 33c, 43c of the finishing roller dies 33,43 can be smoothly engaged with the teeth 78c of the rolled-gear 78.

Claims: A process for gear-rolling a high accuracy gear from a workpiece made of iron-based material using: a roller die for generating teeth and a finishing roller die for finishing the teeth; and the process comprising the steps of: a heating step of heating an outer circumferential portion of a workpiece having a disk shape to high temperatures; a hot rough-rolling step of hot-rolling said outer circumferential portion of said heated workpiece by use of said roller die to generate teeth at said outer circumferential portion of said workpiece so that a rolled-gear is formed; a warm finish-rolling step of warm-rolling said teeth of said rolled-gear by use of said finishing roller die; wherein a starting temperature T 1 of said hot rough-rolling step is set in the range of from 850 through 1100°C, a terminating temperature of said hot rough-rolling step T 2 is set in the range of from 500 through 700°C, a starting temperature T 3 of said warm finish-rolling step is set in the range of from 400 through 700°C, and a terminating temperature of said warm finish-rolling step T 4 is set in the range of from 200 through 650°C. A process according to claim 1 using: a roller squeezing apparatus (31, 41) in which said roller die (32, 42) and said finishing roller die (33, 43) disposed coaxially and connected in series in the axial direction of said roller die, wherein said roller die (32, 42) having a lot of forming teeth (32c, 42c) arranged in a circumferential direction for said hot rough-rolling step, said finishing roller die (33, 43) having a lot of forming teeth (33c, 43c) arranged in a circumferential direction for said warm finish-rolling step. A process according to claim 1 or 2 using: a roller squeezing apparatus (31, 41) in which said roller die (32, 42) and said finishing roller die (33, 43) disposed coaxially and connected in series in the axial direction of said roller die, wherein said warm finish-rolling step is continuously carried out immediately after said hot rough-rolling step without decreasing the temperature of said rolled-gear (7) to a normal temperature range. A process according to claim 1 or 2, comprising the steps of: a cooling step of cooling said workpiece (7) to a normal temperature range after said hot rough-rolling step; and a second heating step of heating said outer circumferential portion of said workpiece (7) more than said starting temperature of said warm finish-rolling after said cooling step and before said warm finish-rolling step. A process according to one of the claims 1 to 4, wherein said heating step is carried out by use of induction heating to heat said outer circumferential portion of said workpiece, and each of said temperatures T 1 , T 2 , T 3 and T 4 means the temperature of said outer circumferential portion of said workpiece. A process according to one of the claims 1 to 5, wherein said starting temperature T 1 is aimed at 950°C, said terminating temperature T 2 is aimed at 650°C, said starting temperature T 3 is aimed at 600°C and said terminating temperature T 4 is aimed at 450°C. A process according to one of the claims 1 to 6 using: a workpiece holding portion (11, 12) for holding said workpiece (7) and including a first workpiece holding portion (11) and a second workpiece holding portion (12) facing each other along the axial direction of said workpiece (7), the process comprising the step of; a holding step for holding said workpiece (7) by use of at least one of said first workpiece holding portion (11) and said second workpiece holding portion (12) before said heating step, wherein said heating step is carried out with said workpiece (7) being held by use of at least one of said first workpiece holding portion 811) and said second workpiece holding portion (12). A process according to claim 7, wherein workpiece holding rigidity of said workpiece holding portion 811, 12) is set more rigid than 0.1 mm/tonf. A process according to one of the claims 1 to 8 using: a roller squeezing apparatus comprising a first roller squeezing apparatus (31) and a second roller squeezing apparatus (41) facing each other in the radius direction of said workpiece (7) so as to form said outer circumferential portion of said workpiece to generate said teeth, wherein squeezing synchronous precision between said first roller squeezing apparatus (31) and said second roller squeezing apparatus (41) is set in the range of from 0.005 mm through 0.03 mm. A process according to one of the claims 1 to 9 using: a roller squeezing apparatus comprising a first roller squeezing apparatus (31) and a second roller squeezing apparatus (41) facing each other in the radius direction of said workpiece so as to form said outer circumferential portion of said workpiece, said first roller squeezing apparatus (31) comprising said roller die (32) and said finishing roller die 833) connected coaxially and in series in the axial direction of said roller die (32), and said second roller squeezing apparatus (41) comprising said roller die 842) and said finishing roller die (43) connected coaxially and in series in the axial direction of said roller die (42).

Inventors: DANNO ATSUSHI, FUJIWARA YASUYUKI, MIYAMOTO NORITAKA, ONISHI MASAZUMI, SAWAMURA MASATOSHI, TANAKA TOSHIAKI, DANNO, ATSUSHI, FUJIWARA, YASUYUKI, MIYAMOTO, NORITAKA, ONISHI, MASAZUMI, SAWAMURA, MASATOSHI, TANAKA, TOSHIAKI, Danno, Atsushi, c/o Kabushiki Kaisha Toyota, Fujiwara, Yasuyuki, c/o Toyota Jidosha, Miyamoto, Noritaka, c/o Toyota Jidosha, Onishi, Masazumi, c/o Toyota Jidosha, Sawamura, Masatoshi, c/o Kabushiki Kaisha Toyota, Tanaka, Toshiaki, c/o Kabushiki Kaisha Toyota
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1996
Main classification: B21H5
IPC codes: B21H5