Milling straight bevel gears
Milling spur bevel gears is a crucial process for achieving power transmission between intersecting shafts. Spur bevel gears feature a tooth profile that tapers from the larger end to the smaller end, with the tooth lines converging at the cone apex. These gears offer a constant transmission ratio and compact structure, making them widely used in automotive rear axles, machine tool transmissions, and construction machinery. Due to their complex tooth profile and high precision requirements (particularly tooth line accuracy and contact accuracy), milling presents challenges such as difficulty adjusting tool angles, controlling indexing accuracy, and measuring tooth thickness. This requires specialized milling equipment and processes to achieve high-quality CNC machining through precise control of tool position and feed trajectory.
The tooth profile parameters and CNC machining datums for spur bevel gears must be clearly defined before milling to ensure CNC machining accuracy. Key parameters include module ( m , based on the large end), number of teeth ( z ), pitch angle (δ), tooth profile angle (α = 20° ), tooth width ( b ), and taper pitch ( R ). The module determines the gear dimensions. For example, for a gear with module m = 5mm , the large end pitch diameter d = mz = 5z . The pitch angle is determined by the transmission ratio i : δ₁ = arctan (1/i) , where δ₂ = 90° – δ₁ (δ₁ and δ₂ are the pitch angles of the two meshing gears, respectively). The CNC machining benchmarks are typically the large end face and inner bore. The inner bore tolerance is IT7 , with a surface roughness of Ra ≤ 1.6μm , a flatness of the large end face ≤ 0.01mm , and a perpendicularity to the inner bore ≤ 0.01mm/m to ensure gear positioning accuracy. The blank is typically 45 steel or 20CrMnTi forgings. After rough CNC machining, a quenching and tempering treatment (220-280 HBW for 45 steel) is required to improve cutting performance and overall mechanical properties.
The tool selection for milling straight bevel gears must match the tooth profile parameters and the CNC machining stage. Commonly used tools include bevel gear milling cutters, angle milling cutters, and form milling cutters. Bevel gear milling cutters are categorized by module and number of teeth (sizes 1-8). Similar to cylindrical gear milling cutters, selection should be based on the large-end module and number of teeth. For example, for CNC machining straight bevel gears with m=3mm and z=20, a size 3 bevel gear milling cutter should be used. The tool’s tooth profile angle is 20°, the blade width is greater than the tooth width, and the tool tip radius is ≤0.3mm to ensure a smooth transition to the tooth root. For rough milling, a high-speed steel milling cutter (such as W18Cr4V) should be used. The cutting edge can be ground with chip grooves to improve chip evacuation. For finish milling, an ultrafine-grain high-speed steel or carbide milling cutter should be used. The cutting edge is finely ground to a surface roughness of Ra ≤0.025μm to ensure tooth surface quality. For batch production, a special forming milling cutter can be used. Its tooth shape is designed according to the large-end tooth shape, and the tooth shape processing can be completed in one feed. The efficiency is 3-5 times higher than that of ordinary milling cutters.
The main methods for milling spur bevel gears are forming and generating. The forming method is suitable for single-piece, small-batch production, while the generating method is suitable for medium-volume and larger production. The forming method uses a milling cutter that matches the gear tooth shape. The tooth is divided by an indexing head, and each tooth groove is milled out sequentially. During CNC machining, the worktable is rotated an angle (equal to the workpiece’s root taper angle) so that the cutter axis is tangent to the root taper surface. During rough milling, the feed is based on the tooth depth at the large end, leaving a 0.1-0.3mm allowance for finishing. The cutter is fed across the tooth width, moving from the large end to the small end, at a feed rate of 0.1-0.2mm/r. During finish milling, the cutter position is adjusted to ensure the tooth thickness meets the requirements, at a feed rate of 0.05-0.1mm/r. The generating method uses the principle of gear meshing to form the tooth profile through the generating motion of the milling cutter and the workpiece. It needs to be performed on a dedicated bevel gear milling machine. The milling cutter simulates the tooth profile of another gear, and the workpiece rotates with the cradle to achieve the generating motion. It can process high-precision tooth profiles (grade 7 and above) with a tooth surface roughness of Ra ≤ 1.6μm.
The clamping and adjustment of milling straight bevel gears must ensure that the apex of the indexing cone coincides with the center of the machine tool cradle. This is the key to ensuring tooth accuracy. When clamping, the workpiece is placed on the mandrel, one end of the mandrel is inserted into the spindle hole of the indexing head, and the other end is tightened with the tailstock top. By adjusting the position of the tailstock, the apex of the workpiece is aligned with the center of the cradle. Special alignment tools (such as the cone top aligner) can be used. The deviation is controlled within 0.01mm . When adjusting the worktable angle, the root cone angle of the workpiece must be consistent with the worktable rotation angle. The root cone angle δ բ =δ-θ (δ is the indexing cone angle, θ =Root angle). Angular error should be ≤ ± 10 °; otherwise, the tooth profile error will be excessive. For straight bevel gears with bosses, a special clamp is required to clamp the bosses tightly to avoid vibration during CNC machining. The clamping force should be uniform to prevent workpiece deformation.
Precision inspection and quality control for milling straight bevel gears must cover tooth profile, pitch, tooth thickness, and contact accuracy. For tooth profile inspection, a tooth profile template is used to compare the large and small ends, with an error of ≤0.01mm. For pitch inspection, a universal angle ruler is used to measure the angle between adjacent teeth, with a deviation of ≤±5′. For tooth thickness inspection, a tooth thickness vernier caliper is used to measure at the large end pitch circle, with the deviation controlled within the h11 range. Contact accuracy is tested using a color coating method. The gear is meshed with a standard gear and rotated to inspect the contact spot, which should be distributed in the middle of the tooth width, with an area ≥60% and a height ≥50%. Common quality issues and solutions: If the tooth guide error is too large, the worktable angle and cone top position need to be readjusted. If the tooth thickness is uneven, the indexing accuracy needs to be checked and corrected. If the contact spot is biased toward the large or small end, the tool position needs to be adjusted. If there is excessive contact at the large end, move the tool toward the small end, and vice versa. Through strict clamping adjustment and quality inspection, high-precision milling of straight bevel gears can be achieved to ensure their smooth operation in the transmission system.
