Milling helical cylindrical gears
Milling helical gears uses a milling machine to create gear teeth with a helix angle. Helical gears are widely used in transmission systems for automobiles, machine tools, and construction machinery due to their smooth transmission, strong load-bearing capacity, and low noise. Machining is more challenging than for spur gears, requiring precise control of parameters such as the helix angle, lead, and tooth profile accuracy. High-quality gear machining is achieved through the proper selection of milling cutters, adjustment of machine tool transmission relationships, and optimization of cutting parameters.
The helix angle characteristics of helical cylindrical gears determine the selection and installation of milling cutters, which is the prerequisite for ensuring tooth profile accuracy. The tooth direction of helical teeth is a helix, and its normal tooth profile differs from the end face tooth profile. Therefore, a milling cutter that matches the gear’s normal module and pressure angle must be selected. The module of the milling cutter should be equal to the gear’s normal module, and the pressure angle should be equal to the gear’s normal pressure angle (standard value 20°). The milling cutter number is selected based on the number of teeth on the gear, similar to spur gears. That is, an involute gear milling cutter of the corresponding number is selected based on the tooth number range to ensure that the milling cutter tooth profile matches the theoretical tooth profile of the gear. The milling cutter must be installed so that its axis is aligned with the helix direction of the gear. For horizontal milling machines, the angle must be adjusted by rotating the worktable or milling head. The angle value is equal to the helix angle of the gear. Use a sine rule or angle ruler to measure during adjustment, and the error should not exceed ±5′. For vertical milling machines, this can be achieved by tilting the tool bar. The tool bar tilt angle is also equal to the helix angle. After installation, the radial runout of the milling cutter needs to be checked with a dial indicator to ensure that it does not exceed 0.01mm.
The helical feed motion of the worktable is crucial for milling helical gears. Adjusting the helical gear to match the lead and helix angle requires adjustment via the gear pulley. The relationship between the lead Pz and the helix angle β of a helical cylindrical gear is Pz = πd/tanβ (d is the pitch circle diameter). During milling, the milling machine’s feed system achieves helical feed of the worktable, meaning that the worktable moves one lead axially for each workpiece rotation. For standard milling machines, the gear pulley ratio is calculated to adjust the transmission relationship: i = 40Pss / Pz (Psss is the lead screw pitch). Gear pulley selection must ensure a transmission ratio error of no more than 0.01%. The gear rotation direction must also be considered to ensure the helix direction matches the drawing requirements (left-hand or right-hand). After adjusting the gear pulley, a test run should be conducted to measure the actual worktable travel distance. After 10 workpiece rotations, the deviation between the worktable travel distance and the theoretical value (10 × Pz) should be within 0.05 mm. Otherwise, the gear pulley needs to be readjusted. For CNC milling machines, the helix parameters can be set directly through programming without the need for pulley adjustment, but program simulation and trial cutting are required to ensure the accuracy of the helix angle.
The milling process for helical cylindrical gears is carried out in stages, with tooth profile accuracy gradually improved by controlling cutting parameters and feed patterns. Rough milling primarily removes the bulk of the stock, using lower cutting speeds (15-25 m/min for high-speed steel milling cutters, 40-60 m/min for carbide milling cutters), larger feeds (0.1-0.2 mm/r), and a depth of cut (1-2 mm). A layered milling method is employed, with a radial feed of 0.5-1 mm after each layer. Simultaneously, the table moves in a spiral feed motion to ensure even cutting on both sides of the tooth slot. Semi-finishing milling requires adjustment of cutting parameters, increasing the cutting speed by 10%-20%, reducing the feed to 0.05-0.1 mm/r, and controlling the depth of cut to 0.3-0.5 mm. At this point, the tooth thickness is measured through trial cutting, and the radial feed is adjusted based on the measurement results to ensure a tooth thickness deviation within ±0.015 mm . During fine milling, a high-speed and low-feed method is used, with a cutting speed ( 25-35m/min for high-speed steel milling cutters and 60-80m/min for carbide milling cutters), a feed rate of 0.03-0.05mm/r, and a cutting depth of 0.1-0.2mm. After fine milling, the tooth surface roughness must reach Ra3.2μm or above, and the tooth profile error must not exceed 0.01mm.
The control of the helix angle and tooth direction accuracy is a difficult point in the milling of helical gears, which requires precise machine tool adjustment and measurement. The helix angle can be measured with a universal gear measuring instrument or a gear helix angle measuring instrument. During measurement, 3-5 points are taken at different circumferential positions of the gear and the average value is calculated. The helix angle error should be controlled within ±10′. If the helix angle is found to be out of tolerance during measurement, the inclination angle of the workbench or milling head needs to be readjusted. For every 1′ angle adjustment, the helix angle changes by approximately 0.017mm/m. The control of tooth direction accuracy requires ensuring the parallelism of the tooth surface and the gear axis. It can be checked by measuring the tooth direction error. The tooth direction error does not exceed 0.015mm/100mm. For high-precision gears, an involute tester is required for tooth direction measurement. In addition, the cumulative pitch error of the gear also needs to be controlled. The cumulative pitch error refers to the maximum difference between the actual arc length and the theoretical arc length between any two tooth surfaces on the same side of the pitch circle. For grade 8 precision gears, the cumulative pitch error should not exceed 0.03mm. It can be measured by a pitch meter. When measuring, 12 points are evenly selected on the circumference of the gear to calculate the error value.
