Milling constant velocity cylindrical cam
A constant velocity cylindrical cam is a curved cam that achieves constant velocity motion in a driven part. Its profile is a constant velocity helix. It is widely used in indexing mechanisms in automatic machine tools, textile machinery, and packaging equipment, ensuring smooth and minimal impact during operation. The key to milling a constant velocity cylindrical cam is ensuring the accuracy of the profile. The lead error of the helix must be controlled within 0.02mm. Otherwise, the speed of the driven part will fluctuate, affecting the operating accuracy of the equipment. An application case study at an automation equipment manufacturer shows that for every 0.01mm increase in the constant velocity cylindrical cam’s profile error, the indexing error of the mechanism increases by 0.5°, seriously affecting product quality.
The milling tool selection for constant velocity cylindrical cams must match the contour characteristics. Because the cam’s working surface is a three-dimensional curved surface, end mills or ball-end milling cutters are required. The tool diameter is determined by the cam groove width and curvature radius, generally 0.8-0.9 times the groove width, to ensure a good fit between the tool and the groove. When machining steel cams (such as 45 steel and 40Cr), high-speed steel end mills (such as W6Mo5Cr4V2) are suitable for low-speed fine milling, achieving good surface quality. Carbide end mills (such as YT15 and YW1) are suitable for high-speed rough milling, offering high cutting efficiency and suitable for mass production. For quenched cams (hardness 50-55HRC), ceramic or CBN tools are required, as their high wear resistance ensures machining accuracy. A textile machinery factory used a YW1 carbide end mill for machining 40Cr quenched and tempered cams. Combined with a cooling system, they achieved a cam groove surface roughness of Ra1.6μm, meeting high-speed operation requirements.
The milling process for constant velocity cylindrical cams is carried out in stages. The primary task of the rough milling stage is to remove the majority of the machining allowance. Layered milling is used, with each layer depth of 1-2mm and a feed rate of 100-200mm/min, to ensure the groove shape is essentially formed. The semi-finishing milling stage corrects for rough milling errors, controlling the milling depth to 0.5-1mm and reducing the feed rate to 50-100mm/min to achieve an IT10 groove accuracy. The finishing milling stage is crucial for ensuring accuracy. Down-cut milling is used, with a milling depth of 0.1-0.3mm and a feed rate of 30-50mm/min. The tool path is precisely controlled using the CNC system’s interpolation function. During milling, the axis of the cam blank must be parallel to the milling machine spindle axis, with a parallelism error of no more than 0.01mm/m. Otherwise, the helix will distort. When machining constant velocity cylindrical cams, a machine tool manufacturer pre-calibrates the blank’s positioning accuracy using a coordinate measuring machine, ensuring that the lead error during subsequent milling is within 0.015mm.
CNC milling programming for constant velocity cylindrical cams is crucial for achieving constant velocity motion. During programming, the tool path must be calculated based on the mathematical equation for the constant velocity helix. The polar coordinate equation for the constant velocity helix is ρ = ρ0 + vt/ω (where ρ0 is the initial radius, v is the linear velocity, t is the time, and ω is the angular velocity). By discretizing the helix into tiny segments, linear or circular interpolation can be achieved. The CNC system must have helical interpolation capabilities (such as G02/G03 with feed functions) to ensure the continuity and smoothness of tool motion. Tool radius compensation must also be considered during programming, automatically adjusting the trajectory based on the end mill diameter to avoid deviations from the actual machining dimensions. Application data from a certain automated programming software shows that cam machining programs generated using parametric programming methods can achieve trajectory errors within 0.005mm, far lower than the 0.02mm error of manual programming.
Quality inspection and error correction for constant velocity cylindrical cams must be performed throughout the entire machining process. Key inspection items include: helical lead (using a lead gauge or universal tool microscope), profile (using a coordinate measuring machine), and surface roughness (using a roughness tester). Lead error should be controlled within ±0.01mm, profile error no more than 0.02mm, and surface roughness reaching Ra1.6μm. When errors exceed these limits, the cause must be analyzed and corrected. For example, excessive lead error can be compensated by adjusting the feedrate in the CNC program; excessive profile error requires checking the tool radius compensation value or recalibrating the blank positioning. A precision machinery manufacturer has established a “first-article inspection” system, requiring a trial cut and full inspection before each batch of cams is machined. Only qualified parts can be used for mass production, maintaining a first-pass pass rate of over 99%. With the advancement of five-axis machining technology, the milling accuracy and efficiency of constant velocity cylindrical cams have continued to improve, providing strong support for the high-performance operation of automated equipment.
