Milling straight rack
A spur rack is a rack with a tooth line parallel to the axis. It can be considered the expanded form of a spur gear with infinite diameter. It works with spur gears to convert linear motion into rotational motion. It features a simple structure, high transmission efficiency, and ease of manufacturing, making it widely used in machine tools, lifting equipment, conveying machinery, and other fields. Spur racks have an involute tooth profile, with the tooth direction aligned along the length of the rack. The key to milling is ensuring pitch accuracy, tooth profile accuracy, and tooth direction parallelism. Commonly used equipment is a horizontal milling machine or a rack milling machine, coupled with an indexing device for continuous indexing. While the machining process is simpler than that of a helical rack, strict control of process parameters is still required.
The basic parameters of a spur rack include module m, pressure angle α, pitch p, addendum height ha, root height hf, and total tooth depth h. These parameters determine the rack’s dimensions and meshing performance. The pressure angle of a standard spur rack is 20°, and the module ranges from 0.1mm to 50mm, with a common module of 1mm-10mm. Module m is the fundamental parameter that determines the tooth profile: pitch p = πm, addendum height ha = m, root height hf = 1.25m, and total tooth depth h = ha + hf = 2.25m. For example, a spur rack with a module m = 3mm has a pitch p = 9.425mm, addendum height 3mm, root height 3.75mm, total tooth depth 6.75mm, and tooth thickness s = p/2 ≈ 4.712mm. These parameters must be determined through calculation before machining and serve as the basis for tool selection and machine tool adjustment.
The tool selection for milling spur racks must match the module and pressure angle. Commonly used tools are disc gear milling cutters or finger milling cutters. Disc gear milling cutters are suitable for spur racks with a module ≤10mm. They are supplied in sets of 8 or 15, depending on the module. Each cutter corresponds to a specific range of teeth. For example, a No. 2 cutter is suitable for gears or racks with 12-13 teeth (a rack can be considered to have an infinite number of teeth, corresponding to a No. 1 or No. 2 cutter). Finger milling cutters are suitable for large spur racks with a module >10mm. They can mill the full tooth height in a single pass, resulting in higher efficiency. The tool material selection depends on the workpiece material. When machining steel, high-speed steel milling cutters (such as W18Cr4V) or carbide milling cutters (such as YT15) are used. When machining cast iron, YG-class carbide milling cutters are used. The tooth profile accuracy of the milling cutter must meet the requirements. For fine milling, use a Class A cutter to ensure a tooth profile error of ≤0.01mm.
The clamping and positioning of spur racks must ensure that the tooth direction is parallel to the worktable feed direction. Flat-nose pliers or pressure plates are commonly used for clamping. The bottom surface of the workpiece must be planed or ground to a flatness of ≤0.02mm/m to ensure smooth clamping. During clamping, use a dial indicator to calibrate the side of the workpiece so that it is parallel to the longitudinal feed direction of the worktable, with a parallelism error of ≤0.01mm/100mm to avoid tooth deviation. For long spur racks (length > 1000mm), segmented clamping is required. After clamping, each segment must be aligned with the previous segment, with a docking error of ≤0.03mm to ensure rack continuity. The positioning reference is usually a side face and end face of the workpiece. The end face must be perpendicular to the side face, with a perpendicularity error of ≤0.01mm/100mm to ensure uniform tooth pitch distribution.
The milling process parameters for spur racks are divided into roughing and finishing stages. Roughing aims to remove the majority of the stock. The cutting speed is 20-30 m/min (for high-speed steel cutters) or 50-80 m/min (for carbide cutters), with a feed rate of 0.15-0.25 mm/r/min. The depth of cut is two-thirds of the full tooth height, leaving a 0.5-1 mm allowance for finishing. A higher feed rate can be used during roughing to improve efficiency, but excessive cutting forces that could cause workpiece deformation must be avoided. Finishing aims to ensure tooth profile accuracy and surface quality. The cutting speed is increased to 30-40 m/min (for high-speed steel cutters), the feed rate is reduced to 0.05-0.1 mm/r/min, and the depth of cut is the remaining allowance. Climb milling is used to minimize tooth flank scratches. Cutting fluid is required during milling, and when machining steel, an extreme pressure emulsion (8%-10%) should be used to cool and lubricate the tool and workpiece to prevent built-up edge. After each tooth is milled, the workbench is fed longitudinally by a distance of a tooth pitch. The feed accuracy must be controlled within ±0.01mm to ensure uniform tooth pitch.
Quality inspections must cover pitch, tooth profile, tooth direction, and surface quality. Pitch is measured with a pitch gauge or vernier caliper, with adjacent pitch errors ≤ 0.02mm and cumulative pitch errors ≤ 0.05mm/1000mm. Tooth profile is checked with a tooth profile template or involute tester, with profile errors ≤ 0.015mm. Tooth direction is measured on the tooth surface with a dial indicator, with parallelism error between the tooth direction and the rack length ≤ 0.01mm/100mm. Surface roughness must be Ra ≤ 3.2μm (rough milling) or Ra ≤ 1.6μm (finishing milling). For critical spur racks, tooth thickness deviation is also tested, with an upper deviation of + 0.02mm and a lower deviation of – 0.02mm to ensure proper meshing with the spur gear. Through strict process control and comprehensive quality inspections, the transmission accuracy and reliability of spur racks are guaranteed to meet the operational requirements of mechanical equipment.
