Common fixtures for gear hobbing and gear installation
Gear hobbing is one of the most commonly used methods in gear processing. The gear tooth profile is produced through the generating motion of the hob and the gear blank. It has the advantages of high efficiency, high precision, and a wide range of applications. It is widely used in the processing of spur and helical cylindrical gears. The quality of gear hobbing is closely related to the accuracy of the fixture and the correct installation of the gear. The proper selection of gear hobbing fixtures and the correct installation of the gear can effectively ensure the gear pitch accuracy, tooth direction accuracy, and radial runout of the gear ring. Common fixtures for gear hobbing include spindle fixtures, chuck fixtures, and center fixtures. Different types of fixtures are suitable for gears with different structures and precision requirements. Gear installation must follow the principles of accurate positioning, reliable clamping, and minimal deformation.
Spindle fixtures are one of the most widely used fixtures in gear hobbing. They are suitable for machining gears with holes, and are particularly well-suited for high-volume production and applications requiring high precision. Spindle fixtures primarily consist of a spindle, nut, and washer. The spindle and the inner bore of the gear blank utilize a transition fit or a small interference fit. The nut and washer axially clamp the gear blank to ensure coaxiality between the spindle and the gear blank. The spindle’s positioning reference surface should be precision-machined, with an outer diameter typically requiring an h6-h7 accuracy and a surface roughness of Ra ≤ 0.8μm. The center holes at both ends serve as auxiliary references to ensure the spindle’s rotational accuracy. Depending on the shape and size of the gear’s inner bore, spindles can be classified as cylindrical, tapered, or splined. Cylindrical mandrels are suitable for gears with cylindrical bores, offering high positioning accuracy and easy installation. Tapered mandrels are suitable for gears with less precise bores or tapered bores. The self-locking action of the tapered surface allows for positioning and clamping, resulting in less accurate centering but more reliable clamping. Splined mandrels are suitable for gears with splined bores, transmitting greater torque and ensuring circumferential positioning accuracy. Mandrel-type fixtures achieve positioning accuracy of 0.01-0.03mm and are suitable for machining precision gears of grades 6-8.
Chuck fixtures are suitable for machining gears without holes, large gears, or gears with low internal bore precision. They primarily include three-jaw self-centering chucks, four-jaw single-action chucks, and specialized chucks. Three-jaw self-centering chucks center and clamp the outer diameter of a gear blank through the synchronized movement of its three jaws. They are easy to operate and offer a centering accuracy of 0.05-0.1mm, making them suitable for machining small and medium-sized gears with low precision requirements. Four-jaw single-action chucks feature individually adjustable jaws, capable of gripping irregularly shaped gear blanks. Through alignment, they achieve high positioning accuracy (0.01-0.03mm). They are suitable for single-piece, small-batch production, and machining large gears, but they are complex to operate and require long setup times. Specialized chucks are designed based on the gear’s structural characteristics. For example, flanged gears can be positioned using specialized flange-type chucks, which use the flange surface and outer diameter to ensure axial and radial accuracy, making them suitable for mass production and high-precision gear machining. The clamping force of chuck-type fixtures is relatively large, so it is necessary to control the clamping force to avoid deformation of the gear blank. Especially for thin-walled gears, soft jaws should be used or the clamping area should be increased to evenly distribute the clamping force.
Center-type fixtures are suitable for machining long-shaft gears or gears requiring positioning at both ends. They primarily consist of a front center, a rear center, a dial, and a heart-shaped collet. The front center is fixed to the spindle, while the rear center is mounted on the tailstock. The gear blank is positioned using center holes at both ends and rotated by the dial and heart-shaped collet. Center-type fixtures offer high positioning accuracy, ensuring minimal radial and face runout (typically ≤0.02mm). They are suitable for machining gears with precision grades 7 and above, and are particularly well-suited for gear blanks with larger aspect ratios. Centers are categorized as dead centers and live centers. Dead centers offer superior rigidity and high positioning accuracy, but generate significant frictional heat and are therefore suitable for low-speed machining. Live centers are supported by bearings, resulting in low friction and suitable for high-speed machining, but with slightly lower positioning accuracy. When using a center-type fixture, the center holes at both ends of the gear blank must be precisely machined, typically using Type A or Type B center holes with a surface roughness Ra ≤ 0.8μm and a center hole angular error ≤ 30°, to ensure positioning accuracy. For gears without center holes at either end, the center holes can be machined first and then removed after machining is complete.
A combination fixture is a fixture composed of a series of standardized fixture components. It is suitable for multi-variety, small-batch production and new product trial production. It has the advantages of high flexibility, strong versatility and reusability. The combination fixtures commonly used in gear hobbing include base plates, positioning parts, clamping parts, guide parts, etc., which can be quickly assembled into the required fixture according to the structure and size of the gear. The positioning accuracy of the combination fixture can reach 0.02-0.05mm, which can meet the processing requirements of 6-9 grade precision gears. When using a combination fixture, it is necessary to select appropriate positioning elements according to the positioning reference of the gear. For example, when positioning with the end face and the outer circle, a combination of locating pins and V-blocks can be selected; when positioning with the hole and the end face, a combination of mandrels and locating sleeves can be selected. The assembly of the combination fixture should ensure that the connection between the components is firm and reliable, and the parallelism error between the positioning surface and the work table is ≤0.01mm/m to avoid affecting the processing accuracy of the gear. Combination fixture