JPH07139542A - Screw rotor part and manufacture of screw - Google Patents

Abstract

PURPOSE:To provide a screw performance and yield of which can be improved, and manufacture thereof. CONSTITUTION:A blade part 2 made of aluminum alloy formed with helical projections is provided on the outer circumference of a shaft 1 (coefficient of linear expansion 17X10<-6>/ deg.C) made of stainless steel (SUS304) material of diameter 20-40mm. The blade part 2 is a clad composed of two layers, an inner layer material 2a (coefficient of linear expansion 16-17.5X10<-6>/ deg.C) made of powder alloy of low hot working adaptability, and an outer layer material 2b (coefficient of linear expansion 20X10<-6>/ deg.C) made of melt alloy of high hot working adaptability. When the thickness of the inner layer material 2a is set more than 3mm, it is easy to hold the strength due to shrinkage fiting. If the inner layer material 2a occupies from the center of projection height to the outside, the possibility generating a crack at hot rolling is increased. Considering the hot working adaptability at rolling, it is favorable that the inner layer material 2a is formed so as not to appeare on the surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a screw, a rotor part and a method of manufacturing a screw which are rotated at a high speed at a high temperature.

[0002]

2. Description of the Related Art For example, a supercharger of an automobile uses a rotor component in which a male rotor and a female rotor each having a protrusion are formed. In the screw that constitutes this rotor component, the male rotor and the female rotor need to mesh with each other with high accuracy, and resistance to high-speed rotation of tens of thousands of revolutions per minute is required. Therefore, even if it is thin (φ20 ~
(40 mm) Steel with such strength is used for the shaft part, and the blade part is made of a material that is relatively easy to form ridges.

When a screw rotates at high speed to compress air, the screw generates heat and its temperature rises. Since such a high speed rotation stop and operation cycle is repeated, it is exposed to a temperature change of 0 to 150 ° C. For this reason, if the shaft and the blade are fixed by a mechanical method such as a key or a method such as welding or crimping, a slight backlash is generated due to such temperature change and vibration due to high speed rotation as described above. Since they lack reliability, they are generally joined by shrink fitting. In this case, if the shaft and the blade do not have substantially equal linear expansion coefficients,
Since the shrink fit between the shaft part and the blade part may be loosened or misalignment may occur, the blade part satisfies the above conditions and has a linear expansion coefficient similar to that of steel to improve its performance. Is used, for example, an Al—Si alloy containing 15% or more of Si is used.

A screw using such a material is A
The eutectic point of 1-Si is 12% for Si, and if Si exceeds 12%, it is difficult to manufacture by the melting method (I / M method).
For example, it is manufactured by extruding a rapidly solidified Al-Si-Fe alloy by spray forming or powder metallurgy to process it into a rod shape, forming a ridge on the outer peripheral surface by cutting, and then shrink-fitting the shaft. .

[0005]

By the way, the shrink fitting obtains the fastening force by the stress when the material is elastically deformed. Therefore, in consideration of the temperature rise during use as described above, such fastening force can be obtained. It is necessary to use a material having a high degree of vibration for the blade portion. However, a material having high strength generally has a problem that it is difficult to form a ridge by cutting or rolling. In addition, the male rotor and the female rotor are not normally in contact with each other, but considering the possibility of contact for some reason and the plasticity due to rolling, the blade portion is required to have ductility.

Further, the conventional screw uses a high cost rapidly solidified alloy or powder metallurgy alloy material for all parts except the shaft, so that it is difficult to reduce the cost. Since such a material has poor hot workability, it is difficult to form a ridge by plastic working, and the ridge must be formed by cutting, and the yield is about 1/3. There is a problem of low. Also, the cutting tool has a short cutting life because of its large cutting resistance.

The present invention has been made in view of the above circumstances, and metal-bonds an inner layer material made of a metal material having low hot workability and an outer layer material made of a metal material having high hot workability. This outer layer material is rolled to form ridges, and the inner layer material is shrink-fitted with the shaft to improve the performance and yield while securing the fastening force for shrink-fitting with the shaft in the inner layer material. It is an object of the present invention to provide a screw, a rotor component and a screw manufacturing method capable of reducing the manufacturing cost.

[0008]

Means for Solving the Problems A screw according to a first aspect of the present invention is a screw in which a spiral protrusion is formed on an outer peripheral surface, and an inner layer material made of a metal material having low hot workability and a hot workability. It is characterized by comprising a clad in which an outer layer material made of a high metal material is metal-bonded.

The screw according to the second invention is characterized in that, in the first invention, the position of the metal bonding interface of the clad is located closer to the center than the center of the height of the ridge.

In the screw according to the third invention, in the first invention, the outer peripheral surface is covered with an outer layer material, and the axial cross-sectional shape of the metal bonding interface of the clad is a smooth polygonal shape corresponding to the ridge. It is characterized by doing.

A screw according to a fourth invention is characterized in that, in the first invention, a shaft made of steel is shrink-fitted in a central portion of the clad.

In the rotor component according to the fifth aspect of the present invention, the male rotor and the female rotor each having a spiral projection formed on the outer peripheral surface and further surface-finished are engaged with the convex portion and the concave portion,
In a rotor component rotated in opposite directions, a clad in which an inner layer material made of a metal material having low hot workability and an outer layer material made of a metal material having high hot workability are metal-bonded, and a central portion of the clad A male rotor and a female rotor each having a shaft made of steel shrink-fitted to are engaged with each other.

According to a sixth aspect of the present invention, there is provided a screw manufacturing method in which a round bar-shaped material to be rolled is manufactured, and the material to be rolled is spirally wound on the material to be rolled by an inclined rolling mill equipped with a plurality of rolling rolls having grooves formed on its outer peripheral surface. In the method of manufacturing a screw by forming a ridge, a material to be rolled by metal-bonding an inner layer material made of a metal material having low hot workability and an outer layer material made of a metal material having high hot workability A plurality of rolling rolls in which a barrel clad is formed, and the gap between the inlet side and the outlet side of the material to be rolled is gradually widened, and the depth is gradually increased to form an annular or spiral groove. Is characterized in that a ridge is formed on the outer peripheral surface of the material to be rolled.

[0014]

In the first aspect of the present invention, the outer layer material is made of a metal material having high hot workability, so that the ridges are accurately formed.

According to the second aspect of the invention, in addition to the function of the first aspect of the invention, the deformation amount of the inner layer material having low workability is suppressed when forming the ridge by plastic working. There is almost no occurrence of defects such as cracks in the inner layer material at the position corresponding to the tip of the line.

According to the third aspect of the invention, in addition to the function of the first aspect of the invention, the metal bonding interface of the clad is below the surface of the ridge. Excellent performance.

According to the fourth aspect of the invention, in addition to the function of the first aspect of the invention, since the clad on which the protrusion is formed is shrink-fitted with the shaft made of steel, the shaft and the protrusion are formed for high speed rotation. The fastening strength with the clad is high. This joint is a weld,
It is difficult to obtain sufficient bonding strength by methods such as crimping, and it is not possible to withstand temperature changes and rotational torque during high-speed rotation.

According to the fifth aspect of the invention, an inner layer material made of a metal material having low hot workability and an outer layer material made of a metal material having high hot workability are metal-bonded to each other, and a central portion of the clad. Since a male rotor and a female rotor that have a shaft made of steel that is shrink-fitted to the shaft are used, the shaft strength is high, the shaft and the clad on which the ridges are formed are securely fastened, and the accuracy is high. A ridge is formed. Therefore, when this rotor component is used, for example, in a compressor that compresses air, a good compression effect is obtained.

In the sixth aspect of the invention, since the outer layer material is made of a metal material having high hot workability, the ridge can be formed by plastic working with high accuracy and high efficiency. Also, in the finishing cutting process, the outer layer material can be easily cut. Further, since the rolling roll has a gradually widened interval from the entry side to the exit side of the material to be rolled and an annular or spiral groove whose depth is gradually deepened is formed, The ridge can be formed without crushing.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings showing the embodiments. FIG. 1 is a schematic view showing an axial cross section of a screw according to the present invention. FIG. 1 (a) shows a male rotor, and FIG. 1 (b) shows a female rotor. 1 in the figure is φ20-40
The shaft 1 is made of a stainless steel (SUS304) material having a diameter of 1 mm, and the outer periphery of the shaft 1 is provided with a blade portion 2 made of an Al alloy and having a spiral protrusion. The male rotor shown in FIG. 1 (a) is provided with a blade portion 2 having a tip diameter d ₂ = 80 mm and a root diameter d ₃ = 40 mm around a shaft 1 having a diameter d ₁ = 30 mm, and FIG. The female rotor shown in Fig. ₂ has a tip diameter d ₂ around the shaft 1 having a diameter d ₁ = 25 mm.
= 75 mm, and the tooth bottom diameter d ₃ = 35 mm. The blade portion 2 is an inner layer material 2a made of a metal material having low hot workability.
And an outer layer material 2b made of a metal material having high hot workability, which is a two-layer clad.

In this embodiment, the minimum thickness of the inner layer material 2a is 3 mm.
However, a thickness that can maintain the strength due to shrink fitting in an environment where the temperature changes drastically and high-speed rotation is performed may be appropriately selected. Also, outside the center of the ridge height (tooth direction)
When the inner layer material 2a occupies up to, the possibility of cracking during hot rolling increases. Therefore, the position of the metal bonding interface between the inner layer material 2a and the outer layer material 2b is located closer to the center than the center of the height of the ridge. Further, taking into consideration hot workability during rolling, the axial cross-sectional shape of the metal-bonded portion of the clad forms a smooth polygonal shape corresponding to the protrusion as shown in FIG. 1, and the inner layer material 2a does not appear on the surface. It is desirable to set the degree.

Next, the composition of the inner layer material 2a and the outer layer material 2b will be described. The composition of the inner layer material 2a is such that Si is 10
~ 30%, containing at least one of Fe, Mn, and Ni, the total is 3 to 10%, Cu is 0.5 to 6%, and Mg is 0.2 to 3
%, The balance is Al and inevitable impurities.

Fe, Mn, and Ni are added to the Al alloy in an amount of 200 to 4
It has the effect of suppressing the decrease in strength after heating at 00 ° C. A preferable content range is 3 to 10% in total. If it is less than 3%, the effect is small, and if it exceeds 10%, the machinability deteriorates. Cu,
Mg coexists and forms an Al ₂ CuMg phase by aging and precipitating the alloy to enhance the strength at room temperature. The preferred content ranges are 0.5-6% and 0.2-3, respectively.
%, The effect is small when the content is less than the lower limit, and the machinability and corrosion resistance of the alloy are impaired when the content exceeds the upper limit.

The composition of the outer layer material 2b is, in wt%, Si.
11 to 14%, Fe 1% or less, Cu 0.5 to 1.3%, M
g is 0.8 to 1.3%, Ni is 0.5 to 2%, and the rest is Al and inevitable impurities. With such a composition, it is possible to form the linear expansion coefficients of the outer layer material 2b and the inner layer material 2a with no great difference.

The linear expansion coefficient of the shaft 1 made of stainless steel is
15 to 17 × 10 ⁻⁶ / ° C., and the coefficient of linear expansion of the inner layer material 2a must be approximately equal to this in order to perform shrink fitting with the shaft 1, 16 to 17.5 × 10 ⁻⁶ / P / M (Powder M
etallurgy: powder metallurgy) alloy (Al-17% Si alloy) is used. The outer layer material 2b is not so different from the linear expansion coefficient of the inner layer material 2a (20 × 10 ⁻⁶ / ° C.) I / M (Ingot Metall)
urgy: Melt casting) An alloy (JIS4032; Al-12% Si alloy) is used. Further, although the male rotor and the female rotor are not normally in contact with each other, an alloy having elongation is desired in consideration of the possibility of contacting them for some reason, but this JIS4032 alloy has sufficient elongation.

Next, a method of manufacturing the screw according to the present invention will be described. First, a round bar-shaped clad billet composed of two layers of the inner layer material 2a and the outer layer material 2b is manufactured. As this method, four methods are shown below. FIG. 2A is a schematic cross-sectional view in the axial direction showing a method for manufacturing a billet by the P / M method. According to this method, the powder 4a for the inner layer material 2a is sealed in a hollow billet or extruded tube 3 made of an I / M alloy (JIS4032). Then, deaeration is performed from the deaeration pipe 5 to obtain a round rod-shaped clad billet.

FIG. 2B shows another method for manufacturing a billet by the P / M method. This method is a powder for the inner layer material 2a
4a and CIP; placed in (C old I sostatic P ress cold isostatic compression) sealed cans 6 (6063), JI the space around
The powder 4b of S4032 alloy is put in and deaeration is carried out from the deaeration pipe 5 in the same manner to obtain a round bar-shaped clad billet. In this case, unlike the case shown in FIG. 2 (a), the canister 6 is required.

Further, there is a method using spray forming. In this method, a billet and an overspray powder are produced by Osprey, and a clad billet is obtained in the same manner as in the above two methods. As another method by spray forming, as shown in FIG. 2 (c), first, a bar 7 to be the inner layer material 2a is formed by the spray forming method, and this bar 7
Then, JIS 4032 alloy powder 4b is sprayed again to form a clad billet. After that, this clad billet is heated and extruded in the same manner as in the prior art to prepare an extruded rod, and those with a skin (encapsulated can 6) are externally cut and cut into a predetermined length (about 2 m) to obtain a rolled material. To do.

Next, a ridge is formed on this rolled material. 3, 4, and 5 are schematic diagrams showing a state in which a screw is manufactured by an inclined rolling mill provided with three rolling rolls having spiral grooves. 3 is a front view seen from the upstream side in the transfer direction of the material to be rolled, FIG. 4 is a perspective view seen from the direction A in FIG. 3, and FIG. 5 is a top view seen from the direction B in FIGS. 3 and 4. It is a figure.

In the figure, 11, 11 and 11 are three cone type rolling rolls constituting the inclined rolling mill.
Rolls from the smaller diameter side of 11, 11, 11 (hereinafter referred to as the inlet side)
A metal material that is a solid material with a circular cross section between 11, 11, and 11
14 is transferred by a transfer means (not shown).

FIG. 5 shows the metal material 14 and one upper rolling roll 11. The rolling roll 11 is in the shape of a cone having a half cone angle α, and on its outer peripheral surface, the intervals are gradually widened from the small diameter side to the large diameter side, and the depth is gradually increased. Is provided with a spiral groove. This groove is formed at an angle θ (hereinafter referred to as a spiral lead angle) with respect to a plane orthogonal to the axis of the rolling roll 11, but as described above, the groove interval is gradually changed, so that it depends on the position. The spiral lead angle θ is different. A rolling roll having a spiral groove in the left-hand screw direction is used when forming a screw in the right-hand screw direction, and a rolling roll having a spiral groove in the right-hand screw direction is used when forming a screw in the left-hand screw direction.

FIG. 6 is a schematic view for explaining the set angle of the rolling roll 11. As shown in FIG. 6A, the inclination angle β is an angle formed by the axis line and the pass line when viewed from the direction in which the rolling roll 11 and the metal material 14 overlap each other. Crossing angle γ
Is the angle formed by the axis and the pass line on the same plane where the axis and the pass line exist, as shown in FIG. 6 (b). In this way, each rolling roll 11 is installed with an inclination angle β and a crossing angle γ (positive). Depending on the spiral lead angle θ and the twist angle ω of the screw 15 to be manufactured (the angle formed by the ridge and the plane orthogonal to the axis of the screw 15),
The inclination angle β is set assuming the relationship of ω≈β + θ. The positions, intervals, widths and depths of the grooves are different for each rolling roll. Also in each rolling roll, the interval is gradually widened from the inlet side of the metal material 14 to the outlet side (large diameter side) according to the amount of extension of the metal material 14, and the groove depth is also the required ridge on the outlet side. The depth is gradually increased so that the height can be obtained.

In order to manufacture the screw 15 having the right-hand thread as shown in FIG. 4 in the cross type inclined rolling machine configured as described above, the rolling roll 1 having the spiral groove of the left-hand thread is formed.
As shown in FIG. 3 and FIG. 5, the small-diameter side of the rolling roll 11 is rotated in the direction of rotation of the surface of the metal material 14 facing the rolling roll 11.
The large diameter side of 11 is inclined in the reverse rotation direction by an inclination angle β. The phase of the spiral groove of rolling rolls 11, 11, 11 is 120 °.
They are shifted one by one.

First, the metal material 14 to be rolled is heated to a predetermined temperature (400 to 530 ° C., preferably 430 to 4) in a heating furnace (not shown).
After heating to 80 ° C), it is transferred to this inclined rolling mill and introduced between rolling rolls 11, 11, 11. And the rolling roll 11,
When 11 and 11 are rotated in the direction of the arrow shown in the figure (counterclockwise when viewed from the front side), the metal material 14 introduced from the small diameter side of the rolling rolls 11, 11 and 11 rotates clockwise around the axis. Rotate in the direction. At this time, when the ridge formed by one rolling roll 11 separates from this rolling roll 11, this ridge is guided to the groove of the next rolling roll 11. In this way, the metal material is simultaneously bitten into the three rolling rolls 11, 11, 11.
14 is rolled at three places in the circumferential direction. As a result, a ridge in the right-hand screw direction is gradually formed on the metal material 14, and rolling is performed in a spiral shape, and the metal material 14 is sent to the large diameter side of the rolling rolls 11, 11, 11.

By the above-mentioned rolling, the inside of the rolled portion of the metal material 14 is stretched in the axial direction, whereby the interval between the ridges is widened.
The ridges formed on the metal material 14 are designed so that the grooves of the next rolling roll having the groove portions corresponding to the intervals of the ridges come into contact with the ridges with the widened intervals. Is not crushed during rolling. Moreover, since the groove is gradually deepened, the ridge is gradually raised to manufacture the screw 15 as a product. Then, a hole having a diameter of 20 to 40 mm is provided in the axial direction of the screw 15 with a punching tool such as a drilling machine, and a predetermined hole size is obtained by, for example, reaming. The shaft 1 that is 10 to 20 μm larger than this is shrink-fitted in this hole (shrink-fitting temperature is 2
50 ~ 350 ℃) to make rotor parts.

In the rotor component according to the present invention, since the shaft portion is made of steel having high physical strength, it can sufficiently withstand high speed rotation. Since the blade part uses a metal material with high hot workability, it has high-precision ridges, and the linear expansion coefficient between the shaft part and the blade part is similar, so when using it at high temperatures. The performance of is also good. When the ridges are formed by rolling as described above, the yield can be significantly improved as compared with the case of cutting. That is, in the present invention, since the cutting portion is only a slightly finished portion of the outer layer material and the punched portion of the inner hole,
The yield was about 1/3 in the past, but in the present invention,
70-80%. Moreover, since the cutting amount of powder alloy (quenching solidified alloy), which is relatively expensive, is almost eliminated, it is possible to significantly reduce the cost.

Although the clad has two layers in this embodiment, it may have three or more layers. Although three rolling rolls 11, 11, 11 are used in FIG. 3, four rolling rolls may be used.
However, when the number is two, the rotary forging effect (so-called Mannesmann effect) peculiar to the inclined rolling acts, and for example, cracks easily occur at the center of the metal material starting from inclusions and porosity. Therefore, when rolling down a solid metal material by inclined rolling, the number of rolling rolls is required to be 3 or more. If the number of rolling rolls is 3 or more, the rotary forging effect as described above is significantly suppressed. I know that

On the contrary, when the number of rolling rolls increases, the adjacent rolling rolls come into contact with each other so that the set gap cannot be reduced, and if the number of rolling rolls is 5 or more, the structure of the rolling mill becomes complicated and Geometrically, the diameter of the rolling roll cannot be increased relative to the diameter of the material, and the axial diameter of the rolling roll cannot be increased, so that the rigidity and axial strength of the mill are reduced. From the above, the practical number of rolling rolls is three or four.

The lead length is short (twist angle ω is small).
In the case of manufacturing a screw, the applicant of the present invention is Japanese Patent Publication No. 3-64217
If it is manufactured by the manufacturing method using a rolling roll having an annular groove, which is proposed in Japanese Patent Publication, it is possible to form a highly accurate ridge as in the case described above.

[0040]

As described above, in the screw according to the first aspect of the present invention, the outer layer material is made of a metal material having high hot workability, so that the ridges are accurately formed.

The screw according to the second aspect of the present invention can prevent defects such as cracks from occurring at the tip of the ridge.

In the screw according to the third aspect of the present invention, the metal bonding interface of the clad is below the surface of the ridge, so that the processing at the time of forming the ridge is accurately performed, and good performance is obtained.

In the screw according to the fourth aspect of the present invention, since the clad on which the ridge is formed is shrink-fitted with the shaft made of steel,
The fastening strength between the shaft and the clad on which the ridge is formed is high with respect to high-speed rotation. In this joining, it is difficult to obtain sufficient joining strength by a method such as welding or crimping, and it is not possible to withstand temperature changes and rotational torque during high speed rotation.

The screw according to the fifth aspect of the invention comprises a clad in which an inner layer material made of a metal material having a low hot workability and an outer layer material made of a metal material having a high hot workability are metal-bonded, and a central portion of the clad. Since a male rotor and a female rotor that have a shaft made of steel that is shrink-fitted to the shaft are used, the shaft strength is high, the shaft and the clad on which the ridges are formed are securely fastened, and the accuracy is high. A ridge is formed.
Therefore, when this rotor component is used, for example, in a compressor that compresses air, a good compression effect is obtained.

In the screw manufacturing method according to the sixth aspect of the present invention, since the outer layer material is made of a metal material having high hot workability, the ridges can be formed accurately. Further, since the high strength steel is shrink-fitted to the shaft, resistance to high speed rotation can be obtained. In addition, the rolling roll is gradually widened from the entrance side to the exit side, and an annular or spiral groove whose depth is gradually increased is formed. A line can be formed. In this way, by using the metal material of the clad, which is the outer layer material, having high hot workability for forming the ridges, and by shrink-fitting the metal material, which is the inner layer material, having low hot workability, with the shaft, the performance and yield can be improved. The present invention has excellent effects such as the improvement of

[Brief description of drawings]

FIG. 1 is a schematic view showing an axial cross section of a screw according to the present invention.

FIG. 2 is an explanatory view showing a method for producing a clad billet.

FIG. 3 is a front view seen from the upstream side in the transfer direction of the material to be rolled.

FIG. 4 is a perspective view seen from the direction A in FIG.

5 is a top view seen from the direction B in FIGS. 3 and 4. FIG.

FIG. 6 is a schematic diagram for explaining a set angle of a rolling roll.

[Explanation of symbols]

 1 axis 2 blade part 2a inner layer material 2b outer layer material 11 rolling roll 14 metal material 15 screw

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Inventor Akio Kikuchi 5-11-3 Shimbashi, Minato-ku, Tokyo Sumitomo Light Metal Industries, Ltd. (72) Inventor Makoto Otani 5-11-3 Shimbashi, Minato-ku, Tokyo No. Sumitomo Light Metal Industry Co., Ltd.

Claims (6)

[Claims] 1. A screw having a spiral projection formed on an outer peripheral surface, wherein an inner layer material made of a metal material having low hot workability and an outer layer material made of a metal material having high hot workability are metal-bonded to each other. A screw characterized by comprising a clad. 2. The position of the metal bonding interface of the clad is
The screw according to claim 1, wherein the screw is located closer to the center than the center of the height of the protrusion. 3. The outer peripheral surface is covered with an outer layer material, and the axial cross-sectional shape of the metal bonding interface of the clad has a smooth polygonal shape corresponding to the ridge. The screw described in 1. 4. The screw according to claim 1, wherein a shaft made of steel is shrink-fitted in the center of the clad. 5. A rotor component for rotating a male rotor and a female rotor, each having a spiral projection formed on the outer peripheral surface and having a surface finish, in the opposite rotational directions by engaging the convex portion and the concave portion with each other. An inner layer material made of a metal material having a low hot workability and a clad in which an outer layer material made of a metal material having a high hot workability is metal-bonded, and a shaft made of steel shrink-fitted to the center of the clad. A rotor component characterized in that a male rotor and a female rotor are engaged with each other. 6. A round bar-shaped material to be rolled is produced, and a spiral ridge is formed on the material to be rolled by an inclined rolling mill equipped with a plurality of rolling rolls having grooves formed on the outer peripheral surface thereof to form a screw. In the method of manufacturing, the inner layer material made of a metal material having a low hot workability and the outer layer material made of a metal material having a high hot workability are metal-bonded to each other to produce a clad as a material to be rolled,
The material to be rolled is a plurality of rolling rolls having an annular or spiral groove in which the interval is gradually widened from the entrance side to the exit side of the material to be rolled and the depth thereof is gradually deepened. A method for manufacturing a screw, characterized in that a ridge is formed on an outer peripheral surface of the screw.