Precautions when milling cylindrical gears
Milling cylindrical gears is a key method in gear processing, suitable for single-piece, small-batch production or repair. The quality of this process directly impacts the gear’s transmission accuracy, smoothness, and service life. Due to the complex tooth profile and high precision requirements of gears, the milling process requires careful attention to tool selection, clamping and positioning, indexing accuracy, cutting parameters, and quality inspection. Oversight in any of these areas can result in profile errors, uneven pitch, or excessive surface quality. Understanding the essentials of cylindrical gear milling can effectively improve processing efficiency and quality, reduce scrap rates, and meet the requirements of gears of varying precision grades.
When milling cylindrical gears, the tool selection must strictly match the gear parameters to ensure tooth profile accuracy. The gear milling cutter should be selected based on the gear’s module (m) and tooth profile angle (α). Standard gear milling cutters have a module range of 0.5-20mm and a tooth profile angle of 20° (standard). The cutter size is selected based on the number of teeth (1-8). The smaller the number of teeth, the larger the size. For example, a size 1 cutter is used for a 12-13 tooth gear, while a size 8 cutter is used for a 135 tooth rack. This is because a cutter with the same module can only accurately machine gears within a certain tooth range. Exceeding this range will result in tooth profile errors. For example, using a size 8 cutter to machine a 100-tooth gear can result in tooth profile errors exceeding 0.05mm. For non-standard gears (such as gears with displacement), a custom-made profile cutter is required, and its tooth profile is determined based on the displacement coefficient. The selection of tool materials should be based on the workpiece material and processing requirements. When processing steel parts, use high-speed steel milling cutters (such as W18Cr4V) or carbide milling cutters (such as YT15). When processing cast iron, use YG8 milling cutters. The tool edge must be sharp and free of chipping. The front angle should be 5°-10° and the back angle should be 8°-12° to ensure smooth cutting.
The clamping and positioning accuracy of a gear blank directly impacts the gear’s geometric and positional tolerances, emphasizing the selection of datums and the clamping method. The inner hole and end face should be the preferred positioning datums for the gear blank. The tolerance between the inner hole and the spindle is H7/g6, ensuring coaxiality ≤ 0.01mm. The end face must be precision-machined to a flatness ≤ 0.01mm and a perpendicularity ≤ 0.01mm/m relative to the inner hole to ensure that the gear meets radial and end runout requirements. During clamping, use a three-jaw chuck or center to clamp the spindle. For larger diameter gear blanks, a pressure plate should be used to hold the end face in place to prevent vibration and displacement during CNC machining. The clamping force should be uniform and moderate. Excessive force can easily cause deformation of the gear blank, while insufficient force can result in looseness. Trial cutting can be used to adjust the clamping force to ensure no noticeable movement during CNC machining. For thin-walled gear blanks, a specialized fixture or copper padding in the clamping area should be used to increase the contact area and minimize deformation. Deformation should be kept within 0.01mm.
Indexing accuracy is key to ensuring uniform tooth pitch. Gear milling requires precise indexing using an indexing head. After each tooth is machined, the workpiece is rotated 360°/z (z = the number of teeth). Indexing errors must be kept within ±5°, otherwise cumulative pitch errors will occur. When using a universal indexing head, the number of indexing handle revolutions must be calculated. For example, for an 18-tooth gear, the number of indexing handle revolutions n = 40/z = 40/18, which is ≈ 2.222 revolutions, or 2 revolutions plus 7.111 hole pitches (assuming a 54-hole indexing plate). Before indexing, check the indexing head’s positioning accuracy. Radial runout during idle operation should be ≤ 0.01mm, axial play ≤ 0.005mm, and the indexing fork’s positioning error should be ≤ 0.01mm. During indexing, rotate the handle slowly to avoid impact. Once the desired position is reached, gently reverse the handle to eliminate backlash before tightening to ensure accurate positioning. For batch production, an automatic indexing device can be used with an indexing accuracy of ±1′, greatly improving indexing efficiency and consistency.
The reasonable selection of cutting parameters has a significant impact on the surface quality and tool life of the gear, and needs to be determined according to the gear module, material and precision requirements. The cutting speed is selected according to the tool material. When processing steel parts with a high-speed steel milling cutter, the cutting speed is 15-30m/min, and when processing cast iron, the cutting speed is 20-40m/min; when processing steel parts with a carbide milling cutter , the cutting speed is 50-100m/min, and when processing cast iron, the cutting speed is 60-120m/min. The feed rate is calculated as the feed per tooth, and is 0.1-0.2mm/z for rough milling and 0.05-0.1mm/z for fine milling. Gears with larger modules use smaller values to avoid deformation of the tooth shape caused by excessive cutting forces. The cutting depth is divided into rough milling and fine milling. During rough milling, most of the allowance is removed at one time, leaving a 0.1-0.3mm allowance for fine milling; during fine milling, the cutting depth is equal to the allowance to ensure the quality of the tooth surface. The cutting fluid needs to be fully sprayed into the cutting area. Extreme pressure emulsion is used for processing steel parts. Cast iron can be cut dry or cooled with kerosene to reduce friction and lower the cutting temperature, avoiding burns or sticking on the tooth surface.
Quality inspection and error correction of milled cylindrical gears must be carried out throughout the entire CNC machining process to ensure that the gears meet the design requirements. During CNC machining, the tooth thickness must be checked at any time and measured at the pitch circle with a tooth thickness vernier caliper. The tooth thickness deviation should be controlled within the design range (e.g., h11). After rough milling, the tooth pitch should be checked by measuring the distance between two adjacent teeth with a vernier caliper. The deviation should be ≤0.05mm. After fine milling, the tooth profile should be checked by comparing it with a tooth profile template. The error should be ≤0.01mm, and the surface roughness should be Ra≤3.2μm. Common quality problems and solutions: If the tooth profile error is too large, it may be due to improper milling cutter number selection or tool wear. The appropriate milling cutter needs to be replaced and resharpened. If the tooth pitch is uneven, the indexing head accuracy needs to be checked and calibrated to eliminate the indexing gap. If chatter marks appear on the tooth surface, the cutting speed needs to be reduced, the tool rigidity needs to be increased, or the clamping force needs to be adjusted. If the gear blank is deformed, aging treatment needs to be used to eliminate internal stress before re-CNC machining. For gears with higher precision requirements (grade 7 and above), shaving or honing is required after milling to further improve the tooth profile accuracy and surface quality to meet the requirements of high-precision transmission.
