Abstract
Gear hobbing plays a pivotal role in the production of external gears, combining the principles of kinematics with hob profile conditioning technology. Despite the initial configuration of the hob profile being stipulated in relevant standards, alterations to the profile are often necessary to meet the gear’s strength and contact ratio requirements. Gears processed with modified profile tools undergo changes in geometry and wear characteristics. Currently, these modifications rely heavily on operator experience, lacking systematic standards and references. This paper summarizes the impact of hob profiles on wear mechanisms and presents tests conducted with various hob profiles and gear geometries.
1. Introduction
Modern high-performance gearboxes typically require specially designed gear geometries to cater to specific engineering applications, primarily aiming to enhance tooth root strength and reduce meshing noise. Modifying the pressure angle or augmenting the gear profile can strengthen the teeth, while increasing the contact ratio between gears helps minimize meshing noise. Generally, increasing tooth height or gear helix angle achieves a higher contact ratio; altering the meshing angle or adjusting the gear profile geometry enhances tooth root strength; or increasing the contact area and meshing degree between gears reduces meshing noise.
The design of the hobbing process aligns with the gear’s geometric requirements for hard machining or direct application, optimizing manufacturing costs. Tailored gears necessitate customized standardized hob profiles, which undergo wear during use, inevitably affecting gear machining accuracy.
2. Process Simulation
Transmission devices simulate the influence of cutting loads on hob profile wear, demonstrating a correlation between wear phenomena and tool life. Gear processing simulation software provides digitalized cutting length, chip thickness, clearance, and angle information for each individual point, along with the position generated by the engaging cutting edge. Furthermore, the maximum relative tool tip chip removal volume describes the deformation load in the rake face tip area. The theory related to one-dimensional load parameters cannot fully explain the wear behavior of the tested gearboxes. A feature of the hobbing process is the deviation from chip thickness along the cutting edge, with individual slice thicknesses ranging from 5 to 300 mm. Increasing the size of the cutting elements in finite element modeling (FEM) simulations does not reduce computation time.
3. Test Results
3.1 Influence of Tip Radius on Wear
Derived from a standard reference tool profile with a tip, the tip radii are ρaP0 = 0.2mn, where mn is the tool normal modulus. The wear effects of different tip radii, ρaP0,1 = 0.2mn, ρaP0,2 = 0.3mn, and ρaP0,3 = 0.4mn, are analyzed.
In cutting tests, increasing the tip radius affects tool life. Based on tool life, the horizontal feed cutting speed also increases with the radius. Changing the tip radius from 0.2mn to 0.4mn increases the Vc value by 7% to 30% depending on the frame conditions. During meshing, tip wear is more concentrated in the small tip radius profile of the rake face tip area.
The impact of the tip radius on gear wear is related to chip deformation. During hobbing, chip material flows over the rake face from both the tip and side directions. When the tip radius is small, chip material flows between the two tooth faces, where it compresses and deforms. With a larger tip radius, the deformation on the rake face decreases, reducing the ratio of slice volume generated in this tip area, meaning tools with larger tip radii reduce overall deformation loads.
3.2 Influence of Hob Profile Angle on Wear
In rapid cutting simulations of gear hobbing, when the boundary conditions set profile angles at ρaP0,1 = 15° and ρaP0,2 = 20°, such changes in gear profiles lead to different wear effects. During the finite element simulation of the machining process, the temperature changes at the tool tip during chip removal at a certain cutting arc (viewed through the rake face direction). As the temperature increases, tools with larger profile angles experience faster critical crescent wear, resulting in a 5% decrease in cutting speed.
Generally, larger profile angles result in increased clearance angles on the gear flank. Simulation shows that the effective clearance angle increases from the initial tip radius to the maximum tip radius across the entire flank range. The simulated flank clearance angle increases from 2.25° to 3.0°. When the flank angle increases, a larger cutting relief angle reduces the friction load between the tool’s flank face and the workpiece material, slowing flank wear.
Apart from the pattern of clearance angle changes, increasing the profile angle reduces the cutting length, leading to increased tool life. It is estimated that increasing the profile angle from 15° to 25° corresponds to a 6% to 18% increase in the tool’s feed cutting speed.
3.3 Influence of Gear Helix Angle on Wear
The helix angle, as a determinant geometric dimension of the workpiece, significantly affects tool wear. The experimental subjects include a standard gear with a helix angle of 25.8° and a spur gear with a helix angle of 0°.
In these two cases, the spur gear results in reduced tool life, with crescent wear leading to wear on the flank width of the rear face. Crescent wear appears earlier in the front flank area of the gear teeth. Additionally, flank wear develops faster, with the first cut of the gear causing more wear.
Special attention should be given to extreme flank wear, primarily due to the reduction in cutting length (+32%) and chip thickness (+5%) caused by the helix angle. Following this, the slice volume also increases, and the total load on the tool tip area of the chip increases accordingly, reflected in the load differences on the rake face,
When cutting horizontally and across the spindle axis, the speed is set to a lower level (Dvc10 = 8% to 32%), resulting in stable cutting volumes for both gears and helical gears.
4. Conclusion
Altering the hob profile or the angle of the gear helix during hobbing changes the tool life. Modeling the different wear mechanisms and causes of wear during the gear machining process, through finite element simulations, allows for the assessment of hob profiles without the need for process simulations when machining new gears.
Based on the evaluated gear machining tool design standards, applying optimized hobbing tools and selecting corresponding cutting parameters can reduce machining costs.