JP2898572B2 - Screw rolling machine that prevents bending of the workpiece shaft - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thread rolling machine in which a workpiece material, in particular, a workpiece material comprising a shaft portion and a roll-formed portion, is prevented from bending a shaft.

[0002]

2. Description of the Related Art Conventionally, as a method of processing a thread of a male screw part, a method by cutting and a method by rolling by plastic working are known. However, a method based on rolling is mainly used because the threading speed is high, the thread precision is high when a high-precision rolling die is used, and the thread strength increases due to work hardening. In addition to a rolling machine, an NC automatic lathe is used as a device for forming an accurate screw.

[0003] For example, a thread rolling machine utilizing rolling is disclosed in Japanese Utility Model Publication No. Sho 46-22426, Sho 46-233.
10, a work material (material) to be threaded is sandwiched between a pair of thread-rolled round dies, and the screw-rolled round dies are rotated and driven to press the work material. A screw is formed by causing plastic deformation on the material surface. Therefore, the precision of the formed screw is
Generally approximates the accuracy of a thread-rolled round die that has been finished by grinding.

[0004] When mechanically examining the force acting on the work material at the time of thread rolling, the thread of the thread rolled round die applies a load from both sides of the work material. This is approximately considered to be a beam in which a concentrated load acts on each span of a continuous beam simply supported at equal intervals. Therefore,
If the thread of the thread rolling round die has an odd number of threads, that is, if the screw advances by an odd number of pitches when the thread rolling round die is rotated once, the thread crest of one thread rolling round die and the other thread The thread valleys of the thread-rolled round dies face each other, and a large bending moment is generated in the work material at the end of the thread-rolled round dies. Therefore, when the shaft diameter of the work material is small and the shaft is long,
The portion of the work material extending from the cut-up portion of the screw to the shaft portion is bent during thread rolling. As a result, when the shaft portion is rotated by being supported by a bearing or the like, a deflection occurs, and a predetermined accuracy cannot be exhibited as a screw. Therefore, conventionally, after the work material is removed from the thread rolling machine, the degree of the bending is measured to perform the work of correcting the bending. This work
Not only is it necessary to form threads but also to form worms with a thread rolling machine.

[0005] In the thread cutting of a work material in an NC lathe, one end of the work material is gripped by a chuck and driven to rotate.
The other end is rotatably supported by a center, and the blade is moved in the axial direction of the work material while being pressed against a cutting tool provided at the tip to cut a screw. Usually, the cutting amount of the cutting tool and the feed amount in the axial direction are numerically controlled, so that accurate thread cutting can be performed. However, when the diameter of the work material is small and long, the center part has a larger diameter than the end part because the central part is larger than the end part due to the pressing force of the cutting tool. is there. Therefore, it is difficult to machine a screw that requires high precision.

[0006]

As described above, in the conventional thread rolling machine, it is necessary to correct the bending of the shaft after the screw or worm is formed by rolling, which complicates the manufacturing operation, and consequently makes the manufacturing operation complicated. There was a problem that the manufacturing cost was high. Further, in the NC lathe, for example, there is a problem that it is difficult to perform screw processing requiring high precision, such as a lead screw used for reading and writing of a magnetic desk. The present invention has been made under the above-described background, and has as its object to provide a thread rolling machine that prevents bending of a shaft during thread rolling.

[0007]

According to a first aspect of the present invention, there is provided a thread rolling machine according to the first aspect of the present invention, wherein a work material is sandwiched between a pair of rolling round dies, and the rolling round dies are rotationally driven. In a thread rolling machine for forming a screw or a worm on the outer periphery of a work material by pressing the work material, a bend preventing means for preventing a bend of a shaft of the work material is provided.

[0008] The thread rolling machine according to the second aspect is the first aspect.
In the thread rolling machine of the above, a plurality of plain round dies provided on the front and / or rear side in the axial direction of the rolled round dies or both are pressed against the peripheral surface of the work material to prevent bending of the work material shaft. It is characterized by doing.

The thread rolling machine according to the third aspect is the second aspect of the invention.
Is constituted by a pair of opposed cylindrical plain round dies, and the plain round dies are provided coaxially with the rolled round dies with a bearing interposed therebetween.

[0010] The thread rolling machine according to the fourth aspect is the first aspect.
The bending preventing means according to the third aspect is characterized in that the plain round die starts pressing the work material in the finishing step of the rolled round die.

In the screw rolling machine according to the fifth aspect, a work material is sandwiched between a pair of opposed rolling flat dies, and the rolling material is pressed while being driven in parallel with the approaching direction. In a thread rolling machine for rolling a screw or a worm around an outer periphery of a work material, a bend preventing means for preventing a bend of a shaft of the work material is provided.

[0012] The thread rolling machine according to the sixth aspect is the fifth aspect of the invention.
Is characterized in that a plane flat die having a parallel plate shape is provided forward or rearward in the axial direction of the work material in a direction perpendicular to the moving direction of the rolling flat die, or both.

[0013] The thread rolling machine according to the seventh aspect is the fifth aspect.
Alternatively, in the screw rolling machine described in 6, the plain flat die is connected to a driving body of the flat rolling die with a roller interposed therebetween.

The thread rolling machine according to the eighth aspect is the fifth aspect of the invention.
In the thread rolling machines of Nos. 1 to 7, the plain flat die starts pressing the work material simultaneously with the start of the rolling by the rolling flat die.

The thread rolling machine according to the ninth aspect is the first aspect of the invention.
In the thread rolling machines of Nos. 1 to 8, a work material is provided with a press allowance for bending prevention of the shaft.

[0016]

In the thread rolling machine according to the first and fifth aspects, the bend preventing means of the work material acts during the screw or worm rolling. In the thread rolling machine according to the second or sixth aspect, the above-described bending is prevented by pressing a plane die against a surface of a predetermined portion of the work material. In the thread rolling machine according to the third and seventh aspects, the plain die rotates or moves at a linear speed substantially equal to the peripheral speed of the pressed portion of the work material. In the thread rolling machine according to the fourth aspect, the re-bending of the axis of the work material is performed after the start of the finishing work of the rolling, and the re-bending and the finishing work proceed simultaneously. In the thread rolling machine according to the eighth aspect, the re-bending of the shaft of the work material proceeds simultaneously with the start of the rolling.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. First Embodiment FIG. 1 shows a schematic configuration of a thread rolling machine embodying the present invention. In FIG. 1, a pair of thread rolling round dies 11, 13
Are fixed to the drive shafts 15 and 17, respectively. Drive shaft 1
5, 17 further include a pair of plain round dies 23,
25 and another pair of plain round dies 27 and 29 are arranged at positions on both sides of the thread rolling round dies 11 and 13.
The drive shafts 15 and 17 are rotatably supported by support frames 31 and 33 and are connected to rotary drive devices 19 and 21. Between the thread rolling round dies 11 and 13
The work material 10 is arranged, and the work material 10 is rotatably held on a rest (support plate) 35. The center axis of the work material 10 is substantially flush with or slightly lower than the center axis of the thread rolling round dies 11 and 13. The support frame 31 is fixed to a frame of a thread rolling machine main body (not shown), and the support frame 33 is connected to a piston cylinder mechanism 37 and is driven in the left-right direction in the figure. Hereinafter, the configuration of each unit will be described in detail.

FIG. 2 shows a schematic shape of an example of the work material 10 processed by the thread rolling machine of this embodiment. Work material 10
Is a lead screw used for reading and writing data on the magnetic desk device.
71, a first shaft portion 73, and a second shaft portion 75. For example, the following values are required for the dimensions and accuracy of the screw portion 71. The screw part 71 has a diameter of 3 mm and a length of 50 m.
m, the pitch is 1.4, and the total length is about 70 mm. The accuracy of the tolerance is, for example, a single pitch error of 5 μm, a cumulative pitch error of 10 μm, a random walk of 5 μm, an overpin diameter of ± 25 μm, and a coaxiality of 1 pitch on both ends of the threaded portion.
It is about 0 μm. Here, the single pitch error is the maximum allowable error in each pitch, and the cumulative pitch error is the allowable error in the total screw length. The random walk is the permissible error of the wobble per pitch of the screw portion, and the overpin diameter indicates the permissible error of the effective diameter of the screw. The coaxiality is an allowable error indicating the maximum deviation of the central axis.

FIG. 3 shows a detailed cross section of the thread rolled round dies 11 and 13 and the plain round dies 23, 25, 27 and 29. In FIG. 3, thread rolling round dies 11 and 13 are round dies having the same shape, and a thread 53 for rolling is provided on the outer peripheral surface of a cylinder 51. This thread rolling round die 1
Reference numerals 1 and 13 are made of die steel, quenched and hardened, and finished by grinding. The thread rolling round dies 11 and 13 are driven to rotate in the same direction and at the same speed, approach each other, press the work material 10 on the rest 35, and plastically deform the work material 1.
The screw 71 is formed in the 0 thread rolling forming part 71. Rolling pressing force for plastically deforming a work material usually requires a large force. For example, this pressing force is a screw having a pitch of 2 mm and requires a force of 700 kg per 10 mm of the work material. In the thread rolling machine of this embodiment, the pressing force is generated by hydraulic pressure. The pressing force generating means may use a cam mechanism.

The plain round dies 23 and 25 have the same configuration. As shown in FIG. 3, the plain round dice 23 (2
5) is rotatably supported on a collar 61 via a bearing 65. The collar 61 is fixed to the drive shaft 15 by a key 55. Plain round die 23
The circumferential surface of (25) is a cylinder flat in the axial direction, made of die steel, and quenched and hardened on the surface. The outer diameter of the plain round die 23 (25) is the same as the thread rolled round die 1
At a predetermined time in the finishing process of thread rolling, first and 13
It is determined that the shaft portion 73 is brought into contact with the shaft portion 73 and pressurization is started. The plain round dice 27 (29) has substantially the same configuration as the plain round dice 23 (25), but differs in width and outer diameter.
That is, the dimensions of the width and outer diameter of the plain round die are determined in accordance with the length and diameter of the portion of the work material 10 that contacts the shaft. In the first embodiment, a plain round die 27 is used.
Since (29) is long, two ball bearings 69 are interposed between the collar and the collar 67. Instead,
A double ball bearing or roller bearing may be used. Plain round dies 23, 25, 27, 29
Since the bearings 65 and 69 are interposed therebetween, even if a rolling speed difference (circumferential speed difference) occurs due to a difference in diameter, the work material 1 is not affected.
No slippage occurs between zero and zero, thereby preventing scratches on the work material surface.

The rest 35 may be composed of, for example, a table on which the work material 10 is placed, and may have a flat or dish-like upper surface so that the height of the central axis of the work material 10 is substantially constant. . Further, instead of the rest 35, both ends of the work material 10 may be rotatably supported by a pair of centers that are slightly movable in the horizontal direction.

The rotary driving devices 19 and 21 are driving devices for rotating the driving shafts 15 and 17 in the same direction and at the same speed and synchronously. The rotation driving devices 19 and 21 are, for example,
As shown in FIG. 4, a drive gear 81, a driven gear 83, an idle gear 85, and a drive motor 87 with a reduction gear connected to the drive gear 81 may be used, and the drive motor 87 may be controlled by the controller 39. . Alternatively, two drive motors may be independently provided, and controlled by the controller 37 so as to rotate in the same direction and at the same speed in synchronization.

The piston cylinder mechanism 37 includes a piston 9
1, a hydraulic cylinder 93 and a control system 95 for controlling these. As shown in FIG. 1, the right end of the support frame 33 is connected to the piston 91. A spring 9 is mounted on one of the pistons 91 (left side in the figure).
7, which pushes the piston 91 rightward. A hydraulic port 99 is provided on the opposite side of the hydraulic cylinder 93. The hydraulic port 99 is connected to the outlet 10 of the direction switching valve 101.
1b and the hydraulic pipe 104
Connected by The directional control valve 101 is connected to a hydraulic pump 105, and the flow control valve 1
07, a pressure regulating valve 109 is provided. The hydraulic pump 105 is driven by a drive motor 111. The suction port side of the hydraulic pump 105 is connected to an oil tank 115 via an oil filter 113. Direction switching valve 10
The first outlet 101c and the outlet 103e of the on-off valve 103 are immersed in oil in an oil tank 115 via an oil tank 117.

The direction switching valve 101, the on-off valve 103, the flow regulating valve 107, the pressure regulating valve 109 and the drive motor 111 are controlled by a controller 39. The controller 39 uses C
It is composed of a digital computer having a PU (Central Processing Unit), a RAM, a ROM memory and the like. Further, the controller 39 is connected to the position sensor 119. The position sensor 119 is a sensor that detects the position of the support frame 33, and may be a mechanical, electric, or optical sensor.

The control system 95 controls as follows. To move the piston 91 to the left in the figure, the second outlet 101c of the switching valve 101 is opened, and the on-off valve 103 is closed. The flow control valve 107 and the pressure control valve 109 are set to predetermined values, and the drive motor 111 is driven. After the operation is stabilized, the second outlet 101c of the switching valve 101 is closed, and the first
Open the exit 101b. The pressure oil flows in from the hydraulic port 99 via the first outlet 101b, increases the pressure in the oil chamber on the right side of the cylinder 93, and moves the piston 91 to the left. The movement of the piston 91 is transmitted to the support frame 33, and the amount of movement is detected by the position sensor 119. When the piston 91 is moved to the right, the switching valve 101
The second outlet 101c is opened, and the on-off valve 103 is opened. The pressure oil in the oil chamber of the cylinder 93 flows from the hydraulic port 99 to the on-off valve 10.
3. The oil flows into the oil tank 115 via the oil tank 117, and the oil pressure in the oil chamber decreases. On the other hand, the pressure oil from the pump 105 returns directly from the second outlet 101c of the switching valve 101 to the oil tank 115 via the oil tank 117. Accordingly, the piston 91 moves rightward by the force of the spring 97.

FIG. 5 is a graph showing a rolling process.
The horizontal axis indicates time t, and the vertical axis indicates the moving distance L of the thread-rolled round die. The distance (L1, L3) corresponds to the height of the thread, the distance (L1, L2) is the advance distance in the rough thread rolling process, and the distance (L2, L3) is the advance distance in the finish rolling. Usually, the distances (L2, L3) are the distances (L1, L
It is desirable to make it about 20% of 3).

The operation of the thread rolling machine according to the first embodiment will be described below. First, the work material 10 is thread-rolled round die 1
It is arranged on the rest 35 between 1 and 13. Next, while rotating the rotation driving devices 19 and 21 in the same direction and at the same speed,
The hydraulic pump 105 is driven to move the piston 91 to the left in the drawing. At time t1, the thread rolling round dies 11, 1
3 presses the work material 10 and starts thread rolling. The piston 91 is moved to the left until time t2 while the rotation speed of the thread rolling round dies 11 and 13 and the moving distance and speed of the thread rolling round die 13 are controlled by the controller 39 to perform thread rolling. . From time t1 to t2, a coarse screw is rolled into the threaded portion 71 of the work material 10 in a coarse screw rolling process.
At this time, as described above, the cut-up portion 71 of the screw portion 71
a and 71b generate a large bending moment, and the shaft portions 73a and 75 are thin, so that
The shaft of the workpiece 10 bends from to the shaft portion 73a and from the cut-up portion 71b to the shaft portion 75.

At time t2, the thread-rolled round dies 11, 1
The controller 39 is controlled so that the moving speed of the control unit 3 becomes slow.
The finishing process is from time t2 to t3. During this finishing step, at time tp, the plain round dies 23, 2
5, 27 and 29 come into full contact with the shaft portions 73 and 75 of the work material and pressurize them. By this pressure, the work material 1
The axis bending that occurred at zero is fixed. At this time, the plain round dies 23, 25, 27, 29
Since it is rotatably supported by 5, 69, it rotates at a rotation speed different from the rotation speed of the drive shafts 15, 17.
The rotation speed is a speed at which the peripheral speed substantially coincides with the peripheral speed of the contact portion of the work material 10.

At time t3, the controller 39 is controlled to switch the direction of the direction switching valve 101 to c, and at the same time
Open 3. The high-pressure oil in the cylinder 93 flows from the hydraulic port 99 to the oil tank 117, and the pressure in the cylinder 93 becomes low, and the piston 91 is moved rightward by the spring 97. At time t4, the thread rolling round dies 11, 13 and the plain round dies 23, 25, 27, 29 are completely separated from the work material 10.

Since the thread rolling machine of the first embodiment is constructed and operates as described above, the finish rolling is performed in a state where the shaft of the work material is not bent and the shaft is not bent. For this reason, the accuracy of the whole screw as a result of the rolling is high, and the shaft portion is also formed to a predetermined size. Further, since no slip occurs between the plain round die and the shaft portion, an effect that the shaft portion is not damaged can be obtained.

[Second Embodiment] FIG. 6 shows a main part of a thread rolling machine using a worm rolled round die according to the present invention. The thread rolling machine of the second embodiment is almost the same as the thread rolling machine of the first embodiment, and the description of the same parts will be omitted. 6, worm rolled round dies 201 and 203 are fixed to drive shafts 205 and 207 by keys (see FIG. 3). Further, the plain round dies 209 and 211 are rotatably supported with bearings interposed between them and the drive shafts 205 and 207 (see FIG. 3). The drive shafts 205 and 207 are rotatably supported by support frames 213 and 215. Also,
One end of the drive shaft is connected to the rotary drive devices 217, 219. The support frame 213 is fixed to a frame (not shown) of the thread rolling machine main body.
Is connected to a piston cylinder mechanism (see FIG. 1).

A work material 220 is rotatably supported by centers 221 and 223 between the worm rolled round dies 201 and 203. The centers 221 and 223 are supported so as to be movable in the left and right directions in the figure. Therefore, the axis of the work material 220 can move in the left-right direction in the drawing on the same plane as the center axis of the worm rolled round dies 201 and 203.

FIG. 7 shows an example of the work material 220. The work material 220 is used, for example, as a wiper worm for an automobile, and includes a worm portion (warm roll-formed portion) 225 and a shaft portion 227. For example, the wiper worm is required to have the following dimensions and accuracy. The diameter of the worm portion is 8 mm, the pitch is 0.93 mm with a single worm, and the pressure angle is about 10 degrees. The required accuracy is, for example, an outer diameter roundness of 12 μm, an outer diameter deflection for both centers of 50 μm, a worm effective diameter tolerance of ± 25 μm, and a worm tip boss deflection of about 50 μm. When processed with a conventional worm rolling machine that does not use plain round dies, the accuracy is 15 μm in outer diameter roundness.
m, outer diameter deflection is 50-100 μm, worm effective diameter tolerance is ± 25 μm, worm tip boss deflection is 100-300
μm, which caused poor precision due to bending. However, in the thread rolling machine having a plain round die according to the present invention, the finishing process of the rolling is performed in a state where the shaft portion is not bent, so that the accuracy falls within the above required accuracy.

As described above, in addition to the improved accuracy,
The bending of the shaft is improved, and the same effect as the thread rolling machine of the first embodiment can be obtained in the thread rolling machine using the worm rolled round die of the second embodiment.

Third Embodiment FIG. 8 shows a main part of a thread rolling machine using a thread rolling flat die according to a third embodiment of the present invention. The driving device and the like not shown in FIG. 8 have a configuration similar to that of the first embodiment, and a description thereof will be omitted. In FIG. 8, a thread rolling flat die 301 is fixed on a frame 307. Further, on both sides (up and down) of the flat rolled die 301, plain flat dies 303 and 305 are movable on the frame 307 in the moving direction of the flat rolled die (directions of arrows A and B shown in FIG. 8). It is provided in. Also, the frame 30
7 is fixedly connected to the driving member 309,
Reference numeral 09 is connected to a driving device (not shown), and is driven in the moving direction of the screw rolling flat die.

A thread rolling flat die and a plain flat die are fixed to the frame 325 in exactly the same manner with the work material 310 interposed therebetween.
27 is fixedly connected. The driving member 327 is driven in the directions of arrows A and B by a driving device (not shown) in the same manner as the driving member 309. Frame 307, 325
Can be adjusted in the front-rear direction in the figure (in a direction in which the distances are closer to each other). The work material 310 includes the centers 321 and 3
23 rotatably supported. Work material 31
0 is the same as that of the first embodiment.
This is a lead screw including a shaft 313 and a second shaft 315. Of course, a work material for another purpose may be used.

FIG. 9 shows details of the thread rolling flat die 301 and the plain flat dies 303 and 305. As shown in FIG. 9, the thread rolling flat die 301 is in contact with, for example, four flat portions, that is, the thread portion 311, and starts a biting portion 301 a that starts rolling of a coarse screw, a coarse screw forming portion 301 b, It comprises a finishing part 301c and a relief part 301d. The plane flat die 303 (305) is, as shown in FIG.
It is composed of a bending correction unit 303 (305) configured in a parallel plate shape, and is movably supported by a number of rollers 331. The height of the bending correction unit 303 (305) is determined so that a pressing margin provided in advance on the work material 310 is pressed so that the shaft has a predetermined size.

The thread rolling machine of the third embodiment is configured as described above, and its operation is the same as that of the thread rolling machine of the first embodiment. That is, the same steps as shown in FIG. 5 are performed. First, the work material 310 is arranged as shown in FIG. Next, the driving members 309 and 327 are synchronously driven toward each other at the same speed toward the work material. At time t1, the biting unit 3
01a comes into contact with the screw portion 311 from both sides, and coarse thread rolling is started. At the same time, the shaft parts 313 and 315 are pressed by the bending correction units 303 and 305, thereby preventing the bending of the shaft. At time t2, the rolling of the rough screw is completed, and a finishing process is started. In the finishing step, the finishing part 301c performs finishing of the screw part 311. After the finishing step, the work material 310 is separated from the thread-rolled flat die by the relief portion 301d of the rolled flat die.

The thread rolling machine according to the third embodiment is constructed and operates as described above. In principle, however, the rotary motion of the thread rolling machine according to the first embodiment is simply changed to a linear motion. Since there is no,
Similar effects can be obtained.

Although the embodiment of the present invention has been described in detail with reference to the drawings, the specific structure is not limited to this embodiment, and there are design changes and the like within the scope of the present invention. Is also included in the present invention. For example, in each of the above embodiments, the plain die is formed integrally with the rolling die, but may be formed separately and controlled separately.
Also, one set of plane dice need not be composed of two dice, but may be composed of, for example, three or more dice. Further, the shape of the plane die is not limited to a combination of a cylindrical shape and a plate shape, but may be a combination of a cylindrical shape and a plate shape, or a combination of a cylindrical shape and a bay shape.

[0041]

As described above, according to the structure of the present invention, according to the first and fifth aspects of the present invention, there is obtained an effect that it is not necessary to correct the bending of the shaft after rolling. Can be According to the third and seventh aspects of the present invention, in addition to the above-mentioned effects, there is an effect that the shaft portion is not damaged by slippage. Further, in the inventions according to claims 4 and 8, in addition to the above effects, there is an effect that the accuracy of the rolling process is improved. According to the ninth aspect of the present invention, there is an effect that the dimensions of the shaft portion can be made more precise.

[Brief description of the drawings]

FIG. 1 is a view showing a schematic configuration of a thread rolling machine using a thread rolling round die of a first embodiment.

FIG. 2 is a view showing a shape of a work material of the first embodiment.

FIG. 3 is a diagram showing a cross section of a plain round die of the first embodiment.

FIG. 4 is a diagram illustrating an example of a driving device according to the first embodiment.

FIG. 5 is a graph showing a rolling process of the first embodiment.

FIG. 6 is a view showing a main part of a thread rolling machine using a worm rolled round die according to a second embodiment.

FIG. 7 is a view showing a shape of a work material according to a second embodiment.

FIG. 8 is a view showing a main part of a thread rolling machine using a thread rolling flat die of a third embodiment.

FIG. 9 is a diagram showing details of a plane flat die of the third embodiment.

[Explanation of symbols]

11, 13, 201, 203 Rolled round dies 10, 220, 310 Work material 19, 21 Rotary drive unit 23, 25, 27, 29, 209, 211 Plain round dies (bending prevention means) 37 Piston cylinder mechanism 65, 69 Bearing 95 Control system 301 Rolled flat die 303, 305 Plain flat die (bending prevention means) 331 Roller

Continuation of front page (56) References JP-A-7-112230 (JP, A) JP-B-55-26942 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) B21H 1 / 00-9/02

Claims (3)

(57) [Claims] 1. A thread rolling machine for forming a screw or a worm on the outer periphery of a work material by pressing a work material between a pair of rolled round dies while rotating and driving the rolled round die. In the above, as a bend preventing means for preventing bending of the axis of the work material, a cylindrical plain round die facing the front and / or both sides in the axial direction of the rolled round die is coaxial with the rolled round die. A thread rolling machine provided with a bearing interposed therebetween. 2. The thread rolling machine according to claim 1, wherein the bend preventing means starts the pressing of the work material by the plain round die in the finishing step of the rolled round die. 3. A work material is sandwiched between a pair of opposed flat rolling dies, and pressed while driving the flat rolling dies in parallel to the approaching direction, and a screw or worm is rolled around the outer periphery of the work material. In a thread rolling machine to be formed, as a bending preventing means for preventing bending of the shaft of the work material, the work material is axially forward or rearward in a direction perpendicular to the moving direction of the rolling flat die, or both. A flat rolling die having a parallel plate shape, and a roller being interposed between the flat rolling die and a driving body of the rolling flat die, and connected to each other.