JP2016107273A - Crankshaft manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a crankshaft manufacturing method which can secure a dimensional precision, improve the yield of materials and suppress an increase in molding loads.SOLUTION: A crankshaft manufacturing method includes a first squeeze-molding step, a squeeze-second molding step, a rough molding step, and a finish molding step successively performed for a billet 60 to obtain a crankshaft. In the first rough molding step, second metal molds 13, 14 are provided with second burr stoppers in vicinities of center balance weight. In the second rough molding step, third metal molds 15, 16 are provided with third burr stoppers in vicinities of the axial end side weight part. In the finish molding step, burr are made extendable over the whole circumference.EFFECT: The first rough molding step enables the molding of a center side shape and the suppression of an increase in molding loads. The second rough molding step enables the molding of an axial end side shape and the suppression of an increase in molding loads while the center side shape is maintained.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a forging technique in which a workpiece is moved between a plurality of dies and a plurality of forging steps are sequentially performed.

  2. Description of the Related Art Conventionally, a crankshaft manufacturing technique in which a plurality of forging processes are sequentially performed to manufacture a crankshaft from a billet is known (see, for example, Patent Document 1 (FIG. 1)).

The technique of patent document 1 is demonstrated below based on drawing.
As shown in FIG. 11 (a), in the first process mold 100 for molding the crankshaft, the gap g10 of the burr molding part at the center is smaller than the gap g11 of the burr molding part at the shaft end. By using the first process mold 100 having a small gap g10 at the center, the shape of the center part of the crankshaft is formed.

  As shown in FIG. 11B, in the second process die 101, the gap g12 of the burr molding portion at the center is larger than the gap g13 of the burr molding portion at the shaft end. The shape of the shaft end portion of the crankshaft is formed by using the second process die 101 having a small shaft end portion gap g13.

  In the first process, the material flow in the axial direction is promoted by using the first process mold 100 having a small gap g10 at the center. When molding the shaft end portion, by using the second process mold 101 having a small gap g13 at the shaft end portion, material filling into the weight portion at the shaft end portion is promoted, and the yield of the material is improved. Moreover, since the gap at the center is large and the movement of the burr at the center is not restricted, an increase in molding load can be suppressed.

  However, since the gap in the central part is large, material flow from the shaft end part to the central part occurs, and the shape of the central part of the crankshaft formed in the first step is affected. Therefore, there is a need for a crankshaft manufacturing method that can ensure dimensional accuracy, improve material yield, and suppress an increase in molding load.

Japanese Patent Laid-Open No. 2-55636

  It is an object of the present invention to provide a method for manufacturing a crankshaft that can ensure dimensional accuracy, improve material yield, and suppress an increase in molding load.

  The invention according to claim 1 is a crankshaft manufacturing method for manufacturing a crankshaft having a weight portion at one end of a pin portion from a rod-shaped billet, and includes a crushing forming step, a first rough forming step, and a second rough forming step. A molding step and a finish molding step are sequentially performed on the billet. In the crushing molding step, the first metal is placed on a portion of the first mold corresponding to the vicinity of the center balance weight of the crankshaft with respect to the billet. Protruding portions provided in the mold are bitten to distribute the volume of the billet in the axial direction, and a first burr stopper is provided in the first mold near the journal portion adjacent to the central balance weight. The first intermediate is obtained by flowing the material in the direction. In the first rough forming step, a second burr stopper is provided in the second mold in the vicinity of the central balance weight. In addition, the second intermediate body is obtained by molding the central balance weight into a substantially crankshaft material shape with respect to the first intermediate body in the second mold, and in the second rough forming step, the shaft end side is obtained. In the vicinity of the weight portion, a third burr stop portion is provided in the third mold, and the burr in the vicinity of the center balance weight is expandable in the width direction with respect to the second intermediate body in the third mold, The third intermediate body is obtained by molding the end weight portion into a substantially crankshaft material shape. In the finish molding step, the fourth mold is finished into the crankshaft material shape with respect to the third intermediate body. In addition, the burr can be extended all around.

  According to the second aspect of the present invention, in the first rough forming step, the gap between the upper die and the lower die burr forming portion in the second die is set smaller than the other regions only in the vicinity of the center balance weight. In the molding process, the gap of the burr molding part in the vicinity of the central balance weight in the third mold is substantially equal to or less than the gap of the burr molding part in the vicinity of the central balance weight in the first rough molding, and in the vicinity of the weight part on the shaft end side. The gap of the burr molding part is set to be almost equal to or greater than the gap of the burr molding part near the center balance weight in the first rough molding process, and the gap of the burr molding part in the finish molding process is the same over the entire circumference of the burr molding part. It is characterized by.

  The invention according to claim 3 is characterized in that, in the first rough forming step, a burr stop is provided on the second mold in the vicinity of the shaft portion on the gear shaft side.

  In the invention according to claim 1, in the crushing molding process and the first rough molding process, since the material flows only to the burr stopper part is set to the central side, the material can be filled in the central part. it can. Further, if a burr stopper is provided over the entire circumference of the burr molding portion, the movement of the material is normally limited, so that the molding load increases. In this respect, in the crushing molding process and the first rough molding process of the present invention, since the material flows only to the burr-stopping portion on the center side, even if burr-stopping is provided over the entire circumference, Movement is not limited except at the center side, and an increase in molding load can be suppressed.

  In the second rough forming step, since the burr stop portion is provided only on the shaft end side, the shape portion on the shaft end side can be filled with the material. In addition, since the burr stopper is provided only on the shaft end side, an increase in the molding load can be suppressed. As described above, the burr stopper is provided in accordance with the portion to be filled with the material and optimal material distribution is performed, so that the material yield can be improved. In the finish molding step, the burr can be expanded without providing the burr stoppers over the entire circumference, so that the material flow from the shaft end side to the center side is suppressed. For this reason, the shape of the center side and the shaft end side can be maintained. That is, dimensional accuracy can be ensured, material yield can be improved, and an increase in molding load can be suppressed.

  In the invention according to claim 2, in the first rough forming step, the gap between the upper and lower burr forming portions of the second mold is set smaller than other regions only in the vicinity of the central balance weight. The shape portion can be filled with a material. Moreover, if the gap is reduced over the entire circumference, the movement of the material is limited, and thus the molding load increases. In this respect, in the first rough forming step of the present invention, the gap between the upper and lower burr forming portions of the second mold is set smaller than the other regions only in the vicinity of the central balance weight, so that the movement of the material is reduced. It is not limited partially, and an increase in molding load can be suppressed.

  In the second rough forming step, the gap in the vicinity of the central balance weight is substantially equal to or less than the gap in the vicinity of the central balance weight in the first rough forming, and the gap in the vicinity of the weight portion on the shaft end side is the center in the first rough forming step. It was set to be almost equal to or greater than the gap near the balance weight. For this reason, it is possible to fill the material on the shaft end side while maintaining the shape on the center side. Since the gap of the burr molding part in the finish molding process is the same over the entire circumference of the burr molding part, the material flow between the shaft end side and the center side is suppressed, and the once molded shape can be maintained.

  As described above, by appropriately providing the position of the burr stop portion and the burr thickness, the necessary portion is filled with the material and the dimensional accuracy can be ensured, and the process is appropriately distributed to suppress the increase of the load more than necessary. be able to. Furthermore, since the material can be optimally distributed in the axial direction and the width direction, the yield of the material can be improved and the productivity can be improved.

  In the invention according to claim 3, in the first rough forming step, the second mold is provided with a burr stopper in the vicinity of the shaft portion on the gear shaft side, so that the shaft portion on the gear shaft side is satisfactorily filled with material. Can do.

It is a figure explaining the whole forge-molding metal mold | die and conveying apparatus which concern on this invention. It is 2-2 arrow line view of FIG. It is a top view of a crankshaft. It is a figure explaining a crushing molding process. It is a figure explaining the state after a crushing molding process. It is a figure explaining a 1st rough forming process. It is a figure explaining the state after a 1st rough forming process. It is a figure explaining a 2nd rough forming process. It is a figure explaining the state after the 2nd rough forming process. It is a figure explaining a finish molding process. It is a figure explaining the basic principle of a prior art.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. The drawings are viewed in the direction of the reference numerals.

First, an embodiment of the present invention will be described with reference to the drawings.
As shown in FIG. 1, the forging apparatus 10 includes a first mold 11 and 12 for a crushing molding process, a second mold 13 and 14 for a first rough molding process, and a second mold for a second rough molding process. 3 molds 15 and 16, fourth molds 17 and 18 for a finish molding process, and a workpiece transfer device 31.

  The first molds 11 and 12 for the first crushing molding process include an upper mold 11 and a lower mold 12. The second molds 13 and 14 for the second crushing molding process include an upper mold 13 and a lower mold 14. The third molds 15 and 16 for the rough forming process include an upper mold 15 and a lower mold 16. The fourth molds 17 and 18 for the finish molding process include an upper mold 17 and a lower mold 18.

  The upper molds 11, 13, 15, and 17 are provided on the ram plate 21 and are moved up and down together with the ram plate 21 by lifting and lowering means (mold clamping means). The lower molds 12, 14, 16 and 18 are provided on the bolster 22. In the transfer device 31, an arm 33 is extended from the transfer device body 32, and a work holding unit 34 is provided at the tip of the arm 33. The work holding part 34 can rotate around the shaft 35 and can be conveyed while maintaining the posture of the work and placed on the lower mold 14 in the next process.

  In addition, the conveying apparatus 31 can convey a workpiece | work by rotating a predetermined angle, and can arrange | position to the lower mold | type 14 of a next process. For convenience, the conveying device 31 is illustrated only between the first molds 11 and 12 and the second molds 13 and 14, but the second molds 13 and 14, the third molds 15 and 16, and the fourth mold are illustrated. It is also installed between the molds 17 and 18. Furthermore, the conveyance device 31 is not limited to the configuration shown in the embodiment, and other configurations may be used as long as the workpiece can be conveyed to the next process die.

  As shown in FIG. 2, a first mold (lower mold) 12, a second mold (lower mold) 14, a third mold (lower mold) 16, and a fourth mold (lower mold) 18 are arranged in this order. Has been. The first mold 12, the second mold 14, the third mold 16, and the fourth mold 18 have molding surfaces 12a, 14a, 16a, and 18a so that the axes (longitudinal directions) of the crankshaft are arranged side by side. Is provided.

Next, the crankshaft will be described.
As shown in FIG. 3, the crankshaft 50 is provided at a journal portion 51 that is held by the engine and functions as a shaft, a crank arm 52 that extends from the journal portion 51 in a direction perpendicular to the axis, and a tip portion of the crank arm 52. And a pin portion 53 to which a connecting rod is connected.

  The crankshaft 50 further includes a central balance weight 54 on the axial center side that relieves inertial force due to the movement of the piston and the connecting rod, and a weight part 55 on the axial end side that relieves inertial force due to the movement of the piston and connecting rod. Prepare. The weight portion 55 on the shaft end side is provided at one end of the pin portion 53.

The operation of the forging apparatus 10 described above will be described.
As shown in FIGS. 1 and 4, a billet 60 serving as a starting material is set in the first mold (lower mold) 12 of the forging apparatus 10. The billet 60 is disposed close to the right side of the molding surface 12a of the lower mold 12 in the drawing. (Preparation process)

  The crankshaft 50 (see FIG. 3) has a shaft portion on the gear shaft side having a large volume at the right end of the drawing. For this reason, by arranging the billet 60 on the right side of the molding surface 12a in advance in the drawing, the right end portion of the crankshaft 50 can be formed mainly by material flow in the radial direction, and the material on the left side of the center portion is shown on the left side Can be made to flow. Moreover, the billet 60 can shorten the length of an axial direction by flowing a material to a longitudinal direction. The length of the billet 60 is shorter than the length in the longitudinal direction of the molding surfaces 11a, 12a of the first molds 11, 12.

Next, the crushing process will be described.
As shown in FIG. 4A, the first molds 11 and 12 are provided with molding surfaces 11a and 12a along the longitudinal direction of the crankshaft 50 (see FIG. 3). The first molds 11 and 12 are provided with protrusions 11b and 12b at portions where the vicinity of the central balance weight 54 of the crankshaft 50 is molded. The upper mold 11 of the first molds 11 and 12 is moved as indicated by the arrow (1), and the billet 60 is pressed.

  As shown in FIG. 4B, the protrusions 11b and 12b start to bite into the center side of the billet 60 as indicated by the arrow (2). As the protrusions 11b and 12b bite into the center side of the billet 60, the material on the shaft end side of the billet 60 is pushed and flows in the axial direction as indicated by the arrow (3). Thus, the volume distribution of the billet 60 in the axial direction is performed by the protrusions 11b and 12b.

Further, by pressing the billet 60, as shown in FIGS. 5A and 5B, a first intermediate body 61 whose center side is crushed is obtained.
As shown in FIG. 5 (b), the gap between the upper die 11 and the burr molding portion 12c of the lower die 12 in the first molds 11 and 12 is set smaller than other regions only in the vicinity of the central balance weight 54. Yes. A burr 62 is formed in a part of the burr molding portion 12c.

  As shown in FIG. 5C, the gap of the burr forming portion 12c in the vicinity of the central balance weight 54 is L1. For example, L1 = 12.0 mm. Further, in the vicinity of the journal portion 51 adjacent to the central balance weight 54, the first dies 11 and 12 are provided with a first burr stopper 12d.

  As shown in FIG. 5D, the gap of the burr forming portion 12c near the weight portion 55 on the shaft end side is L2. For example, L2 = 20.0 mm. Further, in the vicinity of the weight portion 55 on the shaft end side, the material does not flow to the burr stop portion, and the burr stop effect is not exhibited. For this reason, in the crushing molding process, the material can be satisfactorily filled in the central shape portion. Note that the values of the gaps L1 and L2 are not limited to the values in the above-described embodiment. If the gaps L1 and L2 are set only in the vicinity of the central balance weight 54 and smaller than other regions, L1 = 10 mm and L2 = 18 mm. Other values are acceptable.

Next, the first rough forming step will be described.
As shown in FIG. 6A, the second molds 13 and 14 are provided with molding surfaces 13a and 14a along the longitudinal direction of the crankshaft 50 (see FIG. 3). The upper mold 13 of the second molds 13 and 14 is moved as indicated by the arrow (4), and the first intermediate body 61 is pressed.

As shown in FIG. 6B, the protrusions 13b and 14b of the second molds 13 and 14 bite into the center side and the shaft end side of the first intermediate body 61 as indicated by arrows (5).
As shown in FIG. 6C, the second mold 13, 14 is provided with a second burr stopper 14 d in the vicinity of the central balance weight 54. For this reason, the burr (material) 72 that has flowed to the burr molding portion 14c on the center side is stopped by the second burr stopper 14d. For this reason, the material is filled on the center side, and the material flow is promoted from the center side to the shaft end side as shown by an arrow (7) as shown in FIG. 6 (d). The volume distribution in the axial direction of the first intermediate body 61 is performed by the second burr stopper 14d, and the central shape portion is filled with the material.

  Further, by pressing the first intermediate body 61, as shown in FIGS. 7A and 7B, a second intermediate body 71 whose center side and shaft end side are crushed is obtained. As for the 2nd intermediate body 71, the center balance weight 54 part is shape | molded by the substantially crankshaft raw material shape.

  As shown in FIG. 7B, the gap between the burr molding portion 14c of the upper mold 13 and the lower mold 14 in the second molds 13 and 14 is set smaller than the other areas only in the vicinity of the central balance weight 54. Yes.

  As shown in FIG. 7C, the gap of the burr forming portion 14c in the vicinity of the central balance weight 54 is L3. For example, L3 = 4.0 mm. As described above, the second burrs 14 d are provided in the second molds 13 and 14 in the vicinity of the central balance weight 54.

  As shown in FIG. 7D, the gap of the burr forming portion 14c near the weight portion 55 on the shaft end side is L4. For example, L4 = 12.0 mm. Further, in the vicinity of the weight portion 55 on the shaft end side, the material does not flow to the burr stop portion, and the burr stop effect is not exhibited. For this reason, in the first rough forming step, the central portion can be satisfactorily filled with the material. Note that the values of the gaps L3 and L4 are not limited to the values in the above-described embodiment. If the gaps L3 and L4 are set only in the vicinity of the central balance weight 54 and smaller than the other regions, L3 = 4.5 mm, L4 Other values such as = 14 mm may be used.

  Further, in the first rough forming step, burr stops were provided on the second molds 13 and 14 in the vicinity of the shaft portion of the first intermediate body 61 on the gear shaft side. For this reason, the shaft portion on the gear shaft side can be satisfactorily filled with the material.

Next, the second rough forming step will be described.
As shown in FIG. 8A, the third molds 15 and 16 are provided with molding surfaces 15a and 16a along the longitudinal direction of the crankshaft 50 (see FIG. 3). The upper mold 15 of the third molds 15 and 16 is moved as indicated by the arrow (8), and the second intermediate 71 is pressed.

As shown in FIG. 8B, the second intermediate body 71 is shaped to approximate the shape of the molding surfaces 15a and 16a by being pressed.
As shown in FIG. 8C, no burr stopper is provided in the vicinity of the central balance weight 54. For this reason, the burr 82 in the vicinity of the central balance weight 54 can be expanded in the width direction. As shown in FIGS. 8C and 8E, the burr 82 of the second intermediate 71 has an arrow ( It flows from the burr molding part 16c to the outside as in 9). As a result, the molding load can be reduced.

  As shown in FIG. 8D, a third burr stopper 16d is provided in the third molds 15 and 16 in the vicinity of the weight portion 55 on the shaft end side. Therefore, the burr (material) 82 that has flowed to the burr molding part 16c near the weight part 55 on the shaft end side is stopped by the third burr stopper 16d. Therefore, the material is filled on the shaft end side, and the material flow is promoted toward the shaft end side as shown by an arrow (10) as shown in FIG. 8 (e).

  Further, by pressing the second intermediate body 71, as shown in FIGS. 9A and 9B, a third intermediate body 81 formed into a substantially crankshaft material shape is obtained. In the third intermediate body 81, the weight portion 55 on the shaft end side is also formed in a substantially crankshaft material shape.

  As shown in FIG. 9B, for the burr molding part 16c of the upper mold 15 and the lower mold 16 in the third molds 15, 16, the gap of the burr molding part 16c in the vicinity of the central balance weight 54 is the first. It is set to be substantially equal to or less than the gap of the burr forming portion 14c in the vicinity of the central balance weight 54 in the second molds 13 and 14 (see FIG. 7) in the rough forming step.

  Further, the gap of the burr molding part 16c in the vicinity of the weight part 55 on the shaft end side of the third molds 15 and 16 is the burr molding part in the vicinity of the central balance weight 54 in the second molds 13 and 14 in the first rough molding process. It is set to be approximately equal to or greater than the gap of 14c.

  As shown in FIG. 9C, the gap of the burr molding portion 16c near the center balance weight 54 is L5. For example, L5 = 4.0 mm. As described above, the third metal molds 15 and 16 are not provided with a burr stopper in the vicinity of the central balance weight 54.

  As shown in FIG. 9D, the gap of the burr forming portion 16c near the weight portion 55 on the shaft end side is L6. For example, L6 = 4.5 mm. As described above, the third burr stopper 16d is provided in the vicinity of the weight portion 55 on the shaft end side. For this reason, in the second rough forming step, the shape portion on the shaft end side can be satisfactorily filled with the material. Note that the values of the gaps L5 and L6 are not limited to the values in the above-described embodiment, and may be other values.

Next, the finish molding process will be described.
As shown in FIG. 10A, the fourth molds 17 and 18 are provided with molding surfaces 17 a and 18 a along the longitudinal direction of the crankshaft 50. The upper mold 17 of the fourth molds 17 and 18 is moved, and the third intermediate 81 is pressed.

As shown in FIG. 10 (b), the third intermediate body 81 becomes a crankshaft material shape by being pressed.
As shown in FIG. 10C, the burr 56 can be extended over the entire circumference of the crankshaft 50. For this reason, as shown in FIG. 10D and FIG. 10E, the burr 56 of the crankshaft 50 material shape flows from the burr molding portion 18c to the outside.

  Further, the gap between the burr molding part 18c of the upper mold 17 and the lower mold 18 in the fourth molds 17, 18 is set to be the same over the entire circumference of the burr molding part 18c. Specifically, as shown in FIG. 10D, the gap of the burr forming portion 18c in the vicinity of the central balance weight 54 is L7. For example, L7 is 3.5 mm to 4.0 mm.

  As shown in FIG. 10E, the gap of the burr molding portion 18c in the vicinity of the weight portion 55 on the shaft end side is L8. For example, L8 = 3.5 mm to 4.0 mm. The burr 56 flows to the outside of the burr molding part 18c, and the molding load can be reduced. Further, since the gap of the burr molding portion 18c is set to be the same over the entire circumference, the material flow between the shaft end side and the center side is suppressed, and the once molded shape can be maintained.

  Table 1 below shows the gaps in the burr formed part and the presence or absence of the burr stop part in the crushing forming process, the first rough forming process, the second rough forming process, and the finish forming process. In Table 1, the case where there is a burr stop portion is indicated by ○, and the case where there is no burr stop portion is indicated by ×. However, the burr stopper on the shaft end side in the crushing molding process and the first rough molding process means that the material does not flow to the burr stopper and does not exhibit the effect.

In the crushing molding process and the first rough molding process, the gap on the center side is small, and a burr stopper is provided on the center side. For this reason, while shape of the center side is shape | molded previously, material can be made to flow to the shaft end side.
In the second rough forming step, the gap between the center side and the shaft end side is equal and set smaller than that in the first rough forming step. Further, a burr stopper is provided on the shaft end side. For this reason, while shape | molding the shape of a shaft end side later, a material flow from a shaft end side to a center side can be suppressed, and the shape of the center side shape | molded previously can be maintained.

  In the finish molding process, the gap between the center side and the shaft end side is the same, and no burr stopper is provided over the entire circumference of the burr molded part. For this reason, finish molding can be performed while maintaining the shape of the center side and the shaft end side. In addition, since any of the crushing molding process, the first rough molding process, the second rough molding process, and the finish molding process is not in a state in which the burr stopper is formed over the entire circumference, the molding load can be reduced. .

  Note that the application of the crankshaft manufacturing method described above is not limited to the shape of the crankshaft of the embodiment, and may be a crankshaft for three cylinders or a crankshaft of another shape.

  The crankshaft manufacturing method of the present invention is suitable for forging a crankshaft in which a workpiece is moved between a plurality of dies and a plurality of forging steps are sequentially performed.

  DESCRIPTION OF SYMBOLS 10 ... Forging molding apparatus, 11 ... Upper mold | die (1st metal mold | die), 12 ... Lower mold | die (1st metal mold | die), 11b, 12b ... Protrusion part, 12c, 14c, 16c, 18c ... Burr molding part, 12d ... 1st 1 deburring part, 13 ... upper mold (second mold), 14 lower mold (second mold), 14d second deburring part, 15 upper mold (third mold), 16 ... Lower die (third die), 16d ... third burr stopper, 17 ... upper die (fourth die), 18 ... lower die (fourth die), 50 ... crankshaft, 51 ... journal part, 53 ... Pin part, 54 ... Center balance weight, 55 ... Weight part on the shaft end side, 60 ... Billet, 61 ... First intermediate, 71 ... Second intermediate, 81 ... Third intermediate.

Claims (3)

A crankshaft manufacturing method for manufacturing a crankshaft having a weight portion at one end of a pin portion from a rod-shaped billet,
A crushing molding process, a first rough molding process, a second rough molding process, and a finish molding process are sequentially performed on the billet,
In the crushing molding step, a protrusion provided on the first die is bitten into a portion corresponding to the vicinity of the center balance weight of the crankshaft with respect to the billet in the first die, thereby causing the billet to move in the axial direction of the billet. While performing volume allocation,
In the vicinity of the journal portion adjacent to the central balance weight, the first mold is provided with a first burr stopper, and the first intermediate is obtained by flowing the material in the axial direction.
In the first rough forming step, in the vicinity of the central balance weight, a second burr stopper is provided on the second mold,
With respect to the first intermediate body in the second mold, the second intermediate body is obtained by forming the central balance weight into a substantially crankshaft material shape,
In the second rough forming step, in the vicinity of the weight part on the shaft end side, a third burr stopper is provided in the third mold,
With the third mold, the burr near the center balance weight can be expanded in the width direction with respect to the second intermediate,
A third intermediate is obtained by molding the weight part on the shaft end side into a substantially crankshaft material shape,
In the finish molding step, the fourth mold is finished into the crankshaft material shape for the third intermediate,
A method of manufacturing a crankshaft characterized in that the burr can be extended all around. In the first rough molding step, the gap between the upper mold and the lower mold burr molding portion in the second mold is set only in the vicinity of the central balance weight and smaller than the other areas,
In the second rough molding step, the gap of the burr molding part near the center balance weight in the third die is substantially equal to or less than the gap of the burr molding part near the center balance weight in the first rough molding,
The gap of the burr molding part in the vicinity of the weight part on the shaft end side is substantially equal to or greater than the gap of the burr molding part in the vicinity of the central balance weight in the first rough molding step,
2. The method for manufacturing a crankshaft according to claim 1, wherein a gap of the burr molding portion in the finish molding step is made uniform over the entire circumference of the burr molding portion.   The crankshaft manufacturing method according to claim 1 or 2, wherein, in the first rough forming step, a burr stop is provided on the second die in the vicinity of the shaft portion on the gear shaft side.

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