Milling of conical teeth
Conical teeth are gear teeth whose tooth profiles follow a conical surface. They are commonly found in components such as bevel gears and bevel worms. Their complex tooth profiles require high machining precision, and milling is one of the primary methods for machining conical teeth. Milling conical teeth requires a dedicated milling machine or a modified general-purpose milling machine, combined with appropriate tools and fixtures to achieve complex relative motion between the tool and the workpiece to produce the desired tooth profile. Understanding the milling principles, tool selection, and process parameters for conical teeth is crucial to ensuring machining quality.
The principle of bevel gear milling is based on either generating or forming methods. Generating utilizes the principle of gear meshing, simulating the meshing motion of a pair of bevel gears between the tool and the workpiece. The tool’s tooth profile gradually creates a conjugate tooth shape on the workpiece. This method offers high machining accuracy and is suitable for bevel gears requiring high precision, such as straight and spiral bevel gears. In generating milling, the tool is typically a disc or finger-shaped milling cutter, with the tool axis at a specific angle to the workpiece axis (equal to the angle between the two axes). The tooth profile is generated by rotating the worktable and feeding the tool. Forming uses a forming milling cutter that directly mills the tooth profile, precisely matching the tooth groove shape of the bevel gear. This method is simple to operate and highly efficient, but its machining accuracy is significantly affected by tool accuracy. It is therefore suitable for bevel gears with lower precision requirements or for single-piece, small-batch production. The tooth profile of the forming milling cutter must be designed and manufactured based on the tooth profile of the large end of the bevel gear.
Tool selection for bevel gear milling depends on tooth profile parameters, material, and machining accuracy. For rough milling of straight bevel gears, a high-speed steel disc bevel gear milling cutter can be used. These cutters are categorized by module series, with each cutter suitable for a specific range of gear teeth. The cutter’s tooth profile angle aligns with the gear’s pressure angle (typically 20°). High-speed steel cutters offer excellent sharpness and are suitable for machining materials such as mild steel and cast iron. For fine milling of bevel gears or machining high-strength materials (such as alloy structural steel), carbide cutters such as tungsten-cobalt carbides (YG8, YG6) or tungsten-titanium-cobalt carbides (YT15, YT14) should be selected. Carbide cutters offer excellent wear resistance and can withstand high cutting speeds, improving machining efficiency and surface quality. For curved bevel gears, a dedicated curved bevel gear milling cutter is required. The cutter’s tooth profile is arc-shaped and conjugate to the workpiece’s tooth profile. The cutter’s diameter and angle must be precisely designed based on the gear parameters to ensure meshing accuracy.
Clamping and adjustment during conical gear milling are critical to ensuring machining accuracy. The workpiece must be clamped to ensure that the apex of the cone’s indexing cone coincides with the center of rotation of the milling machine’s indexing head. For cone gear parts with shafts, the shaft neck can be clamped between the indexing head’s chuck and the tailstock center. The tailstock height can be adjusted to align the workpiece axis with the indexing head axis. For parts with holes, a spindle is used for positioning. The clearance between the spindle and the hole should be less than 0.01 mm to ensure positioning accuracy. After clamping, the workpiece’s radial runout and axial play should be checked with a dial indicator. Runout should be controlled within 0.02 mm. The milling machine table angle should be adjusted so that the workpiece’s indexing cone generatrix is parallel to the milling machine table. For straight conical gears, the table should be rotated an angle equal to the workpiece’s indexing cone angle. For curved conical gears, the angle between the tool axis and the workpiece axis should also be adjusted to meet the gear’s helix angle requirements. In addition, the height of the tool needs to be adjusted so that the cutting edge of the tool passes through the indexing cone surface of the workpiece to ensure that the tooth depth meets the requirements.
When setting cutting parameters for tapered gear milling, the workpiece material, tool material, and machining stage must be considered. The cutting speed is selected based on the tool and workpiece materials. When milling steel with high-speed steel tools, the cutting speed should be 15-30 m/min; when milling cast iron, the speed should be 20-40 m/min. When milling steel with carbide tools, the cutting speed can be increased to 50-100 m/min; when milling cast iron, the speed should be 60-120 m/min. The feed rate is determined based on the tooth surface roughness requirements. For rough milling, the feed rate should be 0.1-0.3 mm/tooth to quickly remove machining stock; for finish milling, the feed rate should be 0.05-0.1 mm/tooth to ensure surface quality (generally requiring Ra 3.2-6.3 μm). The back-cut should be applied in multiple passes based on the machining stock. Each back-cut should be 1-2 mm for rough milling and 0.2-0.5 mm for finish milling. During the milling process, sufficient cutting fluid must be used. For steel parts, emulsion can be used, and for cast iron, kerosene can be used to reduce cutting temperature, reduce tool wear, and improve tooth surface quality.
Common quality issues and solutions during the milling of conical gears are as follows: Excessive tooth profile error is usually caused by improper tool angle adjustment, tool wear, or workpiece clamping offset. This can be resolved by readjusting the tool angle, replacing the tool, or re-clamping the workpiece. Excessive pitch error is often caused by insufficient indexing head accuracy or incorrect indexing operation. The indexing head’s transmission accuracy needs to be checked and calibrated regularly, and operation must strictly follow the indexing requirements. Unsatisfactory tooth surface roughness may be caused by a blunt tool edge, excessive feed rate, or insufficient cooling . The tool should be reground or replaced, the feed rate reduced, and cooling and lubrication increased. After milling, the conical gears need to be inspected. A tooth profile template should be used to check whether the tooth profile meets the requirements. A pitch gauge should be used to measure the pitch error. A dial indicator should be used to check the tooth direction error. For conical gears with higher precision requirements, a gear measuring instrument should also be used for comprehensive inspection. Through reasonable tool selection, precise clamping adjustment, and optimized cutting parameters, the milling quality of conical gears can be guaranteed to meet the accuracy and performance requirements of gear transmission.
