KR102940521B1 - Manufacture method of Worm shaft For MDPS - Google Patents

Abstract

The present invention relates to a method for manufacturing a worm shaft for an MDPS (Motor Driven Power Steering System) used in an electric power steering system among the steering devices of an automobile, and simplifies the manufacturing process of the MDPS worm shaft by using a method of CNC machining after forging and heat-treating mechanical structural carbon steel (SM45C).
That is, the present invention relates to a method for manufacturing a worm shaft for MDPS by forging, heat treatment, and CNC, comprising: a cold forging step in which a rod-shaped member composed of structural carbon steel (SM45C) is cut to a predetermined length and then formed so that a shaft portion thicker than the diameter of the worm is connected to a head portion; a heat treatment step in which the cold-forged material is heat-treated; a first machining step in which the head portion is machined by CNC machining; a second machining step in which the worm is machined by CNC machining; a worm grinding step in which the gear tooth profile of the worm machined by the second machining step is ground; a joint grinding step in which bearing joint portions are ground on both sides of the base material; and a worm brushing step in which burrs or foreign substances remaining after machining are brushed off from the surface. The product is manufactured by sequentially performing a worm burnishing step in which the worm, cleaned by brushing in the worm brush processing step, is pressed with a rolling tool to process the surface of the worm to the desired surface roughness.

Description

Manufacture method of Worm shaft For MDPS Manufacture method of Worm shaft For MDPS

The present invention relates to a method for manufacturing a worm shaft for an MDPS (Motor Driven Power Steering System) used in an electric power steering system among the steering devices of an automobile. More specifically, the invention simplifies the manufacturing process of the MDPS worm shaft by using a method of CNC machining after heat-treating and forging machine structural carbon steel (SM45C), thereby enabling the manufacture of a high-quality MDPS worm shaft with excellent durability.

MDPS is a futuristic steering system that is a step advanced from conventional hydraulic power steering systems. While hydraulic power steering systems, designed to allow easy steering, represent the first generation of power steering systems, MDPS incorporates a drive motor, sensors, and an electronic control unit (ECU) into the power steering system to enable optimal steering performance according to the vehicle's driving conditions.

The components constituting the MDPS house the MDPS worm shaft, sleeve, output shaft, and input shaft inside the housing, and the MDPS worm shaft is configured to be connected to a worm wheel to transmit rotational force.

At this time, the worm shaft for MDPS is composed of a head part and a shaft part, a worm is formed in the shaft part, and a plurality of protrusions are provided in the head part.

In the case of the above-mentioned worm, the manufacturer requires that it be supplied as machine structural carbon steel (SM45C).

In the conventional technology for the above-mentioned worm shaft for MDPS, a rod-shaped member cut to a predetermined length was prepared, and then cutting was performed by turning to reduce the diameter to match the dimensions of each part. Additionally, the worm is machined in a separate process using gear cutting. Since free-cutting steel with good machinability was used instead of machine structural carbon steel (SM45C) due to the difficulty of machining, the current situation is that the machine structural carbon steel (SM45C) product required by the re-investigation cannot be supplied. Furthermore, using free-cutting steel results in high material costs and reduced economic efficiency, and the high wear rate of the gear cutter used as a tool leads to increased manufacturing costs.

Recently, the automotive industry has been striving to increase its market share by strengthening competitiveness through quality improvement and cost reduction under a global system of unlimited competition. Conventional manufacturing methods for MDPS worm shafts have often made it difficult to achieve sufficient competitiveness. Therefore, there is an urgent need for a new method of manufacturing MDPS worm shafts.

As prior art, a method for manufacturing a pinion shaft for an automobile steering system is proposed in Korean Published Patent Application No. (A) 10-2001-0035744 (May 7, 2001). This method comprises, for manufacturing a pinion shaft used in an automobile steering system, a primary heat treatment process in which a carbon steel material formed into a certain shape is heated and then slowly cooled to remove internal cracks and increase ductility; a bonding process in which the heat-treated material is placed in an alkaline aqueous solution to remove foreign substances from the surface of the material and increase lubricity; a cold forging process in which the material is pressure-formed in a press to meet product specifications; a secondary heat treatment process in which the cold-forged material is heated and cooled to improve mechanical properties; a shot peening process in which the material subjected to the secondary heat treatment is struck with a steel ball to increase the fatigue life of the material surface; and a pinion while the shot-peened material is pressed by a roller die. It is characterized by sequentially executing the forming process.

Another example is the method for manufacturing a worm shaft for MDPS disclosed in Korean Published Patent Application No. (A) 10-2010-0077393 (July 8, 2010), which was previously filed by the inventor.

The aforementioned prior art relates to a method for manufacturing a worm shaft for MDPS used in an electric power steering system of an automobile, wherein the MDPS worm shaft is composed of a head portion and a shaft portion, and a worm is formed in a portion of the shaft portion; the method comprises a first forging step in which a smooth rod-shaped member made of carbon steel is formed to continuously connect the head portion and the shaft portion while varying in diameter; a second rolling step in which the head portion is formed concavely to form a plurality of protrusions; a rolling step in which a worm is machined in the portion where the worm is to be formed; and a CNC finishing step in which a center is set on the same axis of both ends of the rolled product and a finishing operation is performed on the entire product including the worm formed by the rolling in a CNC. This allows for reducing raw materials and lowering manufacturing costs, and enables the provision of a high-quality MDPS worm shaft by finally machining the worm with respect to the center axis of the MDPS worm shaft.

However, in the former case, since the rolling process is carried out using a die after performing heat treatment and shot peening processes, the shaft is physically deformed during the rolling process, so the straightness and roundness of the shaft cannot be guaranteed; in the latter case, since the worm is cold-rolled, there were problems such as poor rollability and severe surface irregularities after rolling, which increased the amount of machining; and in addition, in the case of finished worm shafts for MDPS, since the finished product is provided after heat treatment following finishing, it has a significant impact on straightness and roundness, and consequently, problems such as operating noise are not completely resolved.

Accordingly, the inventors have developed a method for manufacturing a worm shaft for MDPS according to Korean Registered Patent Publication No. (B1) 10-1633499 (June 27, 2016) in order to solve the above-mentioned problems.

In the case of the aforementioned prior art, the worm formed on the worm shaft for MDPS, which is provided as machine structural carbon steel (SM45C), can be provided by rolling, thereby meeting the requirements of the manufacturer, and has effects such as reducing raw materials, drastically reducing the amount of machining, and improving productivity by shortening processing time.

However, there is a need to improve this, as there are problems such as excessive wear on the rolling rollers used to roll the worm and difficulty in rolling to the required depth, which requires separate post-processing to grind the worm part to provide the required dimensions.

KR 1020010035744 A 2001.05.07. KR 1020100077393 A 2010.07.08. KR 101633499 B1 2016.06.27.

Accordingly, the inventors of the present invention have researched and developed the present invention to provide a novel manufacturing technology for a worm shaft for MDPS that combines the advantages of forging and machining, in order to eliminate the problem of providing a product with reduced durability when manufactured by gear cutting using conventional free-cutting steel, and the problem of excessive wear of the rolling roller and the manufacturing process of machining the worm after rolling when manufactured by the rolling method.

In other words, the present invention aims to provide a method for manufacturing a worm shaft for MDPS that simplifies the manufacturing process and provides a high-quality worm shaft for MDPS using durable carbon steel for machine structures, by cutting the carbon steel for machine structures to a certain length, forging and heat-treating the head portion, then machining the head portion and shaft portion using CNC, and finally grinding the worm tooth profile.

As a means of solving the problem, the present invention,

A method for manufacturing a worm shaft for MDPS by forging, heat treatment, and CNC; comprising: a cold forging step in which a rod-shaped member composed of structural carbon steel (SM45C) is cut to a predetermined length and then formed so that a shaft portion thicker than the diameter of the worm is connected to a head portion; a heat treatment step in which the cold-forged material is heat-treated; a first machining step in which the head portion is machined by CNC machining; a second machining step in which the worm is machined by CNC machining; a worm grinding step in which the gear tooth profile of the worm machined by the second machining step is ground; a joint grinding step in which bearing joint portions are ground on both sides of the base material; and a worm brushing step in which burrs or foreign substances remaining after machining are removed from the surface with a brush; The method is characterized by sequentially performing a worm burnishing step in which the worm, cleaned by brushing in the worm brush processing step, is processed on the surface of the worm using a rolling tool to achieve a desired surface roughness.

In addition, the second machining step of cutting the worm on the shaft portion is characterized by fixing the cutting bit to the tool holder and enabling the work to be performed by moving the tool holder left and right and forward and backward through CNC precise numerical control.

According to the method for manufacturing a worm shaft for MDPS provided in the present invention, the following effects are achieved.

- A worm shaft for MDPS is provided by combining forging and machining using heat-treatable machine structural carbon steel (SM45C), and has excellent durability.

Productivity can be improved as the product is completed through forging and CNC machining.

- Since the head and worm machining can be performed simultaneously in the CNC, straightness and roundness can be provided precisely.

FIG. 1 is an external perspective view of a worm shaft for an MDPS completed according to the present invention.
FIG. 2 is a front view of the cold-forged state of the material in the present invention.
FIG. 3 is a front view of the material in the present invention in a state after the first processing.
FIG. 4 is a front view of the material in the present invention after the second processing.
FIG. 5 is a block diagram showing the manufacturing process sequence of a worm shaft for MDPS provided by the present invention.

A preferred embodiment of the method for manufacturing a worm shaft for MDPS provided by the present invention will be described below with reference to the attached drawings.

FIG. 1 is an external perspective view of a worm shaft for MDPS completed according to the present invention, FIG. 2 to 4 are front views of the material in the present invention in a cold-forged state, a first processed state, and a second processed state, and FIG. 5 is a block diagram showing the manufacturing process sequence of a worm shaft for MDPS provided by the present invention.

The worm shaft (1) for MDPS provided in the present invention is used as a component of the MDPS that constitutes the steering device of an automobile and is divided into a head part (2) and a shaft part (3). A worm (4) is formed in a part of the shaft part (3) and connected to a worm wheel (not shown) so as to transmit rotational force.

For such MDPS worm shafts (1), it is preferable to manufacture them using durable mechanical structural carbon steel (SM45C).

However, in the case of machine structural carbon steel (SM45C), it is difficult to machine the worm (4), so most companies manufacture it using free-cutting steel. In the case of manufacturing a worm shaft (1) for MDPS using free-cutting steel, cutting of the head part (2) is possible, but because it is difficult to machine the worm (4), forming is done by gear cutting using gear cutting, and thus, the machine structural carbon steel (SM45C) product required by MDPS is not being provided.

In addition, some technology is applied to manufacture the machine structural carbon steel (SM45C) by forging and rolling, but this has problems due to tool wear caused by rapid wear of the rolling roller when forging the worm (4), and since the worm (4) cannot be rolled to the required depth, it is finished by grinding, which increases the grinding time.

Accordingly, the present invention is characterized by providing a high-quality product that can minimize noise or vibration generated during use by improving the machinability of the worm shaft (1) for MDPS and improving the straightness and roundness of the finished product by solving the aforementioned problems through forging and CNC machining of the machine structural carbon steel (SM45C) product required by MDPS.

The manufacturing process of the worm shaft (1) for MDPS provided in the present invention is broadly performed by sequentially carrying out 1) a cold forging step (S10), 2) a heat treatment step (S20), 3) a first processing step (S30), 4) a second processing step (S40), 5) a worm grinding step (S50), 6) a joint grinding step (S60), 7) a worm brush processing step (S70), and 8) a worm burnishing step (S80) so that a high-precision worm shaft (1) for MDPS can be provided as a product of machine structural carbon steel (SM45C). Each step is described in more detail below.

1) Cold forging step (S10)

A smooth rod-shaped member is prepared and cut to an appropriate length, and a forged product is formed into a head part (2) and a shaft part (3) with different diameters through a cold forging step (S10).

That is, a rod-shaped member made of structural carbon steel (SM45C) is cut to a predetermined length and then formed so that the head part (2) and the shaft part (3) are connected. At this time, the shaft part (3) is formed to be slightly thicker than the diameter of the worm (4).

As mentioned above, in the past, expensive free-cutting steel with excellent machinability was used in consideration of worm (4) forming. However, free-cutting steel contains lead components with excellent machinability, which has an adverse effect on the health of workers and is regulated due to environmental pollution issues. Although free-cutting steel with added bismuth compounds has been developed and is being used, it is expensive, making it difficult to apply to the worm shaft (1) for MDPS.

2) Heat treatment step (S20)

The heat treatment step (S20) is a process that relieves stress generated during the forging process in the case of a forged product formed by separating the head part (2) and the shaft part (3) in which the worm (4) is formed as described above, and provides the required strength and toughness.

The heat treatment step (S20) is a process of improving durability by enhancing hardness and toughness through tempering after quenching. The quenching temperature is heated to 900 degrees or higher and then rapidly cooled, and the tempering is performed by heating to approximately 600 degrees and then rapidly cooling to ensure that the stiffness and toughness meet the requirements.

3) First processing step (S30)

The first processing step (S30) is a step of cutting the head part (2) while the shaft part (3) processed by the CNC is clamped.

As described above, a bearing coupling part (2a) is formed on the head part (2) that has been ground as described above, and a coupling groove is formed on the head part (2).

4) Second processing step (S40)

The second processing step (S40) is a step of cutting the worm (4) on the shaft part (3) while the head part (2) processed by the CNC is clamped. At this time, the worm (4) is formed in a spiral shape, so precise numerical control of the CNC is required.

It also includes cutting the side bearing joint (3a) of the shaft part (3).

Ideally, the cutting bit is fixed to the tool holder, and the work can be performed by moving the tool holder left and right and forward and backward through precise CNC numerical control.

In the first and second processing steps above, centers are machined axially on both sides of the base material.

5) Worm grinding step (S50)

The worm grinding step (S50) is a process of precisely grinding the surface of the worm (4) operated in the third processing step (S50) to meet the required dimensions.

6) Joint grinding step (S60)

The joint grinding step (S60) is a process of precisely grinding the bearing joint (2a)(3a) that was roughed in the second and third processing steps (S40)(S50) to the final dimensions.

7) Warm brush processing step (S70)

The worm brush processing step (S70) is a process of grinding the outer diameter of the worm (4) and the bearing coupling part (2a) (3a) and finally removing any remaining burrs or foreign matter from the surface of the head part (2) and shaft part (3) after processing.

This can be achieved by methods such as contacting a brush with the surface of a rotating MDPS worm shaft (1).

8) Warm burnishing step (S80)

The worm burnishing step (S80) is a process for improving the surface roughness of the worm.

Preferably, the worm, which has been cleaned by brushing in the worm brush processing step (S70), is processed on the surface of the worm using a rolling tool to achieve a desired surface roughness, thereby finally processing the worm.

The manufacturing method of the worm shaft (1) for MDPS according to the present invention, using heat-treatable machine structural carbon steel (SM45C), provides a worm shaft (1) for MDPS by combining forging and machining, which has excellent durability. Since the product is completed through forging and CNC machining, although the material cost increases, the product can be produced at a lower cost compared to the tool cost incurred due to wear of the expensive rolling roller in the rolling method, thereby reducing production costs. Furthermore, it can improve productivity by simultaneously machining the head part (2) and the worm (4) of the shaft in the CNC, and is effective in providing precise straightness and roundness, thereby providing a high-quality worm shaft (1) product for MDPS.

Although specific embodiments are described in the detailed description of the present invention, it is understood that various modifications are possible within the scope of the invention. Therefore, the scope of protection of the present invention should not be limited to the described embodiments, but should be determined by the claims set forth below as well as equivalents.

1: Worm shaft for MDPS 2: Head section
2a, 3a: Bearing connection part 3: Shaft part
4:Worm
S10: Cold forging step S20: Heat treatment step
S30: 1st processing step S40: 2nd processing step
S50: Worm grinding step S60: Joint grinding step
S70: Warm brush processing step S80: Warm burnishing step

Claims (2)

In a method for manufacturing a worm shaft (1) for MDPS by forging, heat treatment, and CNC;
A cold forging step (S10) in which a rod-shaped member made of machine structural carbon steel (SM45C) is cut to a predetermined length and then formed so that a shaft portion (3) thicker than the diameter of the head portion (2) and the worm (4) are connected;
A heat treatment step (S20) for heat treating a cold-forged shaft portion (3);
A first machining step (S30) of cutting the head part (2) by CNC machining;
A second processing step (S40) of cutting a worm (4) on a shaft part (3) by CNC machining;
A worm grinding step (S50) for grinding the gear tooth profile of the worm (4) that was cut by the second processing step (S40);
A joint grinding step (S60) for grinding bearing joints (2a) (3a) on both sides of the base material;
A warm brush processing step (S70) for brushing off burrs or foreign substances remaining on the surface after processing;
A method for manufacturing a worm shaft for MDPS, characterized by sequentially performing a worm burnishing step (S80) in which a worm, cleaned by brushing in a worm brush processing step (S70), is processed on the surface of a worm using a rolling tool to finally process the worm to a desired surface roughness.
In Article 1;
The second machining step (S40) of machining a worm (4) on the shaft portion (3) is,
A method for manufacturing a worm shaft for MDPS, characterized by fixing a cutting bit to a tool holder and enabling the tool holder to move left and right and back and forth through CNC precise numerical control.