High-precision hardened tooth surface gear and processing method thereof

Abstract

The application discloses a high-precision hard tooth surface gear machining method, comprising the following steps: adopting a disc cutter to pre-compensate and mill the tooth blank which is forged; the profile of the disc cutter is obtained by the development method and the forming method; after the milling, the gear is carburized and quenched, stress relieved and formed and ground; after the formed and ground, the gear is subjected to surface finishing machining. The application can reduce the tooth part allowance difference of the gear after heat treatment, and the gear prepared by the application has the tooth surface hardness difference of less than or equal to 1HRC, the hardening layer depth difference of less than or equal to 0.2mm and good part fatigue strength.

Description

Technical Field

[0001] This invention relates to a high-precision hardened gear and its machining method, belonging to the field of gear machining technology. Background Technology

[0002] The conventional manufacturing process for hardened gear teeth is: gear blank machining → carburizing and quenching → finishing of non-tooth parts → gear grinding. In existing technologies, after carburizing and quenching, the gear teeth are not machined except for grinding; only the non-tooth parts are compensated for and removed. However, when the gear module is small, the impact on the fatigue resistance of the gear teeth is minimal. But as the gear module and shape increase, and the heat treatment parameters improve, the difference in allowance after heat treatment will increase accordingly. This increased difference in allowance will affect the final gear quality. For example, it will cause uneven hardness and carburized layer depth; it increases machining risk and the risk of cracking; and it will also significantly impact grinding efficiency and the final tooth surface quality, thereby reducing the gear's fatigue strength and mass production capacity. Summary of the Invention

[0003] This invention provides a method for machining high-precision hardened gear teeth, which can reduce the difference in allowance of gear teeth after heat treatment.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is: a processing method for high-precision hardened gears, comprising the following steps: pre-compensation milling of forged gear blanks using a disc milling cutter; the forming profile of the disc milling cutter is obtained by generating and forming methods; after milling, the gear is carburized, quenched, and stress-relieved, and then formed and ground; after forming and grinding, the gear is surface-finished.

[0005] Furthermore, the forming profile of the disc milling cutter is obtained through generating and forming methods, specifically including the following steps: obtaining the pre-machining undercutting amount Q of the disc milling cutter and the initial transition arc radius R of the disc milling cutter according to the generating and forming methods; obtaining the process deformation amount S according to the finite element simulation of heat treatment deformation; and obtaining the adjusted transition arc radius R' of the disc milling cutter by substituting the process deformation amount S into the pre-machining undercutting amount Q.

[0006] Furthermore, the step of obtaining the pre-machining depth Q of the disc milling cutter and the initial transition radius R of the disc milling cutter according to the generating method and the forming method specifically includes the following steps: within the effective involute development area, superimpose the milling allowance q on the finished involute tooth profile as the post-milled involute tooth profile; draw the finished involute starting circle according to the effective involute development, wherein the finished involute starting circle intersects the finished involute tooth profile at points B and B'; and draw the straight line segment K parallel to the y-axis respectively. And a straight line segment K', which passes through point B and intersects the milled involute tooth profile at point A, and a straight line segment K' passes through point B' and intersects the milled involute tooth profile at point A'; draw an arc tangent to the straight line segment K, the straight line segment K', and the tooth root circle, which is the top edge transition arc of the disc milling cutter, and the radius of the arc is the initial transition arc radius R of the disc milling cutter; the distance between the straight line parallel to the straight line segment K passing through point A and the straight line segment K is the pre-machining undercut Q of the disc milling cutter.

[0007] Furthermore, after pre-compensation milling, the actual deformation and precision of the heat-treated parts are judged by detecting the tooth accuracy, scanning the tooth profile, and the uniformity of the tooth allowance and gear accuracy to determine whether they meet the process requirements.

[0008] Furthermore, gears that meet the process requirements are subjected to both ordinary shot peening and enhanced shot peening at the tooth root.

[0009] Furthermore, after carburizing and quenching the gear, hardened tooth surface milling and semi-finish milling are performed to relieve stress; during hardened tooth surface milling, the residual oxide scale state after milling is checked and adjusted online.

[0010] Furthermore, abrasive flow or rolling finishing techniques are used to perform surface finishing on the gears.

[0011] A high-precision hardened gear is prepared using the above-mentioned machining method.

[0012] This invention forms the pre-machining undercut Q of the gear teeth by superimposing the process deformation amount S, such as heat treatment, on the top edge transition arc of the milling cutter. By analyzing the deformation law of heat treatment, the top edge transition arc is adjusted to reduce the difference in the allowance of the gear teeth after heat treatment.

[0013] The gears obtained by the method of this invention have a tooth surface hardness difference of ≤1HRC and a hardened layer depth difference of ≤0.2mm. By optimizing the cutting tools, the tooth root fatigue strength can be increased by about 5% compared with the theoretical calculation. By finishing, the compressive stress on the tooth surface can be increased by 35%-45%, which can further improve the fatigue strength of the parts. Attached Figure Description

[0014] Figure 1 This is a flowchart illustrating a high-precision hardened gear machining method provided in an embodiment of the present invention.

[0015] Figure 2 This is a schematic diagram of the disc milling cutter in this invention;

[0016] Figure 3 This is a diagram illustrating the design method of the disc milling cutter in this invention. Detailed Implementation

[0017] To better understand the essence of the present invention, the present invention will be further described below in conjunction with specific embodiments and accompanying drawings.

[0018] This invention is particularly applicable to high-precision gear parts with large modules, significant heat treatment deformation, high tooth surface hardness (≥60HRC), and precision machining using gear grinding. Specifically, it relates to a machining method for high-precision hardened tooth surface gears, such as... Figure 1 As shown, it includes the following steps:

[0019] Step one involves pre-compensating milling the forged gear blank. Unlike existing technologies that use hobbing or a combination of milling and hobbing to process gears, this invention employs a disc milling cutter with a corresponding hob-shaped profile designed for direct gear milling.

[0020] The design of the disc milling cutter specifically includes the following steps:

[0021] S1.1. Initial tool design, such as Figure 2 and Figure 3 As shown:

[0022] a. Within the effective involute development area, the finished involute tooth profile is superimposed with the milling allowance q to form the milled involute tooth profile;

[0023] b. Based on the effective involute development length L of the finished product, calculate the starting circle diameter d of the finished involute according to formula (1). nf Draw the starting circle of the involute in this way, where d b The base circle diameter of the part is given. The starting circle of the finished involute intersects the tooth profile of the finished involute at points B and B'.

[0024] (1)

[0025] c. Draw line segments K and K' parallel to the y-axis respectively. Line segment K passes through point B and intersects the milled involute tooth profile at point A. Line segment K' passes through point B' and intersects the milled involute tooth profile at point A'.

[0026] d. Draw an arc that is tangent to the three phases of line segment K, line segment K', and tooth root circle. This arc is the top edge transition arc. The point of tangency between the top edge transition arc and line segment K is point C, and the point of tangency between the top edge transition arc and line segment K' is point C'.

[0027] e. The distance between the line passing through point A and parallel to line segment K and line segment K is the pre-machining depth Q of the milling cutter of the present invention, Q=q+S, where S is the process deformation amount.

[0028] This invention superimposes a process deformation amount S, such as heat treatment, onto the transition arc of the milling cutter's top edge to form the pre-machining undercut amount Q. When the pre-machining undercut amount Q is known, for different process deformation amounts S, the Q value can be satisfied by adjusting the size of the transition arc radius R. Therefore, it effectively avoids the disadvantage of directly using a hob design, where the difference in allowance after heat treatment will increase accordingly.

[0029] S1.2. Analyze the deformation law of heat treatment to obtain the process deformation amount S.

[0030] The deformation amount S during heat treatment is obtained from finite element simulation of heat treatment deformation.

[0031] The effects of the process deformation S include: expansion and contraction affecting the pressure angle; expansion, contraction, and part twisting affecting the direction and size of the gear helix angle; the bulging or concave shape of the teeth affecting the type and size of gear tooth profile modification; and expansion, contraction, and saddle-shaped deformation affecting the tooth root cutting structure and transition arc parameters, etc.

[0032] S1.3. Based on the deformation pattern obtained from step S1.2, optimize the parameters of the disc milling cutter initially designed in step S1.1, and adjust the corresponding machining process parameters according to the workpiece machining standards during gear milling.

[0033] By incorporating the deformation patterns and extent of heat treatment, pre-compensation is performed to address the corresponding deformation effects. Since heat treatment increases the helix angle θ (in degrees), the tooth profile increases ε (in mm), and the tooth thickness increases λ (in mm), for external teeth, the cutting tool needs to increase the undercut (Q) and decrease the involute starting circle diameter. Furthermore, during tooth milling, the corresponding machining parameters are adjusted according to the workpiece machining standards.

[0034] Finite element analysis was used to compare the normal tooth root strength of gears machined by hobbing and disc milling, and the new gear parameters and corresponding disc milling profiles for milling before heat treatment were optimized.

[0035] After pre-compensation milling using the tool designed in this invention, the actual tooth root structure will be further optimized based on theoretical calculations, and the actual strength will be better than the theoretical calculations, thereby improving the bending fatigue strength of the tooth root. Furthermore, after heat treatment, the uniformity of the tooth allowance can also be effectively improved, which will correspondingly improve the uniformity of tooth surface hardness and hardened layer depth after grinding, thereby improving the contact fatigue strength of the tooth surface.

[0036] S1.4. Based on the actual deformation and accuracy of the parts after heat treatment, verify the applicability of the tool design parameters and the optimization of the machining process parameters by checking whether the tooth accuracy and the scanned tooth profile, the uniformity of the tooth allowance and the gear accuracy are consistent with the process design.

[0037] Step two: After milling, carburizing and quenching are performed, followed by ordinary shot peening and root strengthening shot peening to increase the compressive stress in the root portion, thereby improving the bending fatigue strength of the root.

[0038] Step 3: After finishing the non-tooth parts, use a disc milling cutter with no top edge, side edge with pressure plate and flat mounting structure to mill the hard tooth surface. During milling, the residual oxide scale after milling is checked and adjusted online.

[0039] S4. Stress relief after semi-finish milling, followed by shaping and grinding.

[0040] Because of the semi-finish milling of the hardened tooth surface, the grinding allowance and its differences are greatly reduced, which can significantly reduce the preset safety grinding amount and shorten the grinding time. Furthermore, since the subsequent finishing process improves the surface roughness, grinding wheels with relatively small grit size and larger porosity and structure can be selected to improve the heat exchange efficiency of the grinding process, which can effectively increase the grinding speed and reduce the risk of grinding burn.

[0041] The gear component is made of 18CrNiMo7-6 material, with a module of 20mm, number of teeth of 40mm, and tooth width of 400mm. Its heat treatment is carburizing and quenching, with a surface hardness ≥60HRC. During grinding, a grinding wheel with an abrasive grit size of 60, large porosity, and a microstructure density ≥10 can be used. Due to the reduction in grinding allowance and the improvement in allowance uniformity, the grinding depth in each grinding stage can be appropriately reduced to 0.008-0.015mm, while the stroke speed can be increased to over 5000mm / min. Appropriately increasing the grinding fluid flow rate and pressure can effectively improve grinding efficiency and reduce the risk of burns.

[0042] S5. Use abrasive flow or rolling finishing technology to improve the surface roughness of gear machining, improve surface stress distribution, and improve contact fatigue strength.

[0043] Taking the machining of a gear with a module of 20mm, a helix angle of 7°, and a tooth surface hardness of 60HRC as an example, the milling allowance q is 0.4mm, and the pre-machining undercut Q is 0.8mm.

[0044] The influencing factors of the process deformation S are: an increase of 2′ in the helix angle, an increase of 0.3mm in the tooth profile bulge, an average expansion of 0.5mm in the part, and an increase of 0.2mm in the tooth thickness. Therefore, it is necessary to adjust the tool pre-cutting amount Q' to 0.7mm and the milling allowance q' to 0.3mm. Based on the adjusted pre-cutting amount Q' and milling allowance q', the size of the transition arc radius R is adjusted to determine the forming profile of the disc milling cutter. During machining, the helix angle needs to be adjusted to 6°58′, and the tooth profile bulge needs to be reduced by 0.2mm.

[0045] The gear obtained by the method of this invention has a tooth surface hardness difference of ≤1HRC and a hardened layer depth difference of ≤0.2mm. By optimizing the cutting tool, the tooth root fatigue strength can be increased by about 5% compared with the theoretical calculation. By finishing, the compressive stress on the tooth surface can be increased by 35%-45%, which can further improve the fatigue strength of the part.

[0046] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of the claims of the present invention pending approval.

Claims

1. A method for machining high-precision hardened gear teeth, characterized in that, Includes the following steps: A disc milling cutter is used to pre-compensate the gear blanks; the forming profile of the disc milling cutter is obtained by generating and forming methods. After milling, the gear is carburized, quenched, and stress-relieved before being shaped and ground. After the gears are shaped and ground, the surface of the gears is finished. The forming profile of the disc milling cutter is obtained through a generating method and a forming method, specifically including the following steps: The pre-machining depth Q of the milling teeth and the initial transition radius R of the milling cutter are obtained based on the generating method and the forming method. The process deformation S is obtained from the finite element simulation of heat treatment deformation. After incorporating the process deformation amount S into the pre-machining undercutting amount Q, the transition arc radius R' after the disc milling cutter is obtained; The steps for obtaining the pre-machining depth Q of the milling cutter teeth and the initial transition radius R of the milling cutter based on the generating method and the forming method specifically include the following steps: Within the effective involute development area, the finished involute tooth profile is superimposed with the milling allowance q to form the milled involute tooth profile. Draw the starting circle of the finished involute based on the effective involute development of the finished product. The starting circle of the finished involute intersects the tooth profile of the finished involute at points B and B'. Draw line segments K and K' parallel to the y-axis respectively. Line segment K passes through point B and intersects the milled involute tooth profile at point A. Line segment K' passes through point B' and intersects the milled involute tooth profile at point A'. Draw an arc that is tangent to the three phases of line segment K, line segment K', and tooth root circle. This arc is the top edge transition arc of the disc milling cutter, and the radius of this arc is the initial transition arc radius R of the disc milling cutter. The distance between the line passing through point A and parallel to line segment K and line segment K is the pre-machining depth Q of the milling cutter.

2. The processing method according to claim 1, characterized in that: After pre-compensation milling, the actual deformation and precision of the heat-treated parts are judged by detecting the tooth accuracy, scanning the tooth profile, and the uniformity of the tooth allowance and gear accuracy to determine whether they meet the process requirements.

3. The processing method according to claim 2, characterized in that: Gears that meet the process requirements are subjected to ordinary shot peening and enhanced shot peening at the tooth root.

4. The processing method according to claim 1, characterized in that: After carburizing and quenching the gear, hardened tooth surface milling and semi-finish milling are performed to relieve stress. During hardened tooth surface milling, the residual oxide scale after milling is checked and adjusted online.

5. The processing method according to claim 1, characterized in that: Abrasive flow or rolling finishing techniques are used to perform surface finishing on gears.

6. A high-precision hardened gear, characterized in that, It is prepared by the processing method described in any one of claims 1 to 5.

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