Milling constant velocity disc cam
Constant velocity disc cams are key components for achieving constant reciprocating motion in mechanical components. They are widely used in applications such as automatic machine tools, textile machinery, and printing equipment. Their profile curves must ensure uniform velocity (zero acceleration) of the follower during motion, avoiding shock and vibration. The key to milling a constant velocity disc cam lies in precisely machining a profile that conforms to the theoretical curve (typically an Archimedean spiral or a constant velocity spiral), ensuring a follower motion error of ≤0.02mm and a surface roughness Ra of ≤1.6μm. Compared to conventional cams, the calculation of the constant velocity disc cam’s profile curve is complex, requiring strict control of the relationship between the tool path and the cam angle during milling. Failure to do so can result in follower speed fluctuations (exceeding 5%), impacting equipment operational stability. For example, in an automatic feed mechanism, a constant velocity cam profile error of 0.1mm can lead to uneven feed speeds and workpiece positioning errors. Therefore, milling a constant velocity disc cam requires systematic planning, including curve calculation, fixture design, and milling technology.
v of the follower of the constant velocity cam is constant, and the relationship between its displacement s and the angle θ (radian) is s=kθ ( k is the proportional constant, k=v/ω , ω is the cam angular velocity), the corresponding profile curve is an Archimedean spiral, and the polar coordinate equation is ρ = ρ₀ + s = ρ₀ + kθ (ρ₀ is the base circle radius). For example, if a constant velocity cam is designed with a follower velocity v = 50 mm/s and a cam speed n = 60 r/min (ω = 2πrad/s ), k = v/ω = 50/(2π) ≈ 7.96 mm/rad . When the rotation angle θ = π ( 180° ), the displacement s = 7.96 × π ≈ 25 mm , and the profile radius ρ = ρ₀ + 25 mm. The curve calculation requires discretization by rotation angle increments (usually 1°-5°), resulting in a series of coordinate points (ρ, θ) that serve as a reference for the tool path during milling. The calculated error must be ≤ 0.005 mm. For computer-aided manufacturing (CAM), tool paths can be generated by directly inputting curve equations. Manual calculations require using trigonometric tables or a calculator to calculate each point accurately, ensuring accurate coordinates. An automation equipment manufacturer achieved precise calculations, limiting the theoretical error of cam profiles to within 0.01mm, laying the foundation for subsequent milling.
The clamping and positioning of constant-velocity disc cams require guaranteed machining accuracy. Cam blanks are typically made of 45 steel, cast iron, or aluminum alloy. During clamping, the cam’s inner bore and end face serve as reference points, using a “one-side, two-pin” positioning method. The clearance between the inner bore and the positioning mandrel is maintained at ≤0.01mm, and the end face is clamped with a pressure plate (clamping force 5-8kN) to prevent axial movement during milling. For cams without an inner bore, a process hole is first machined as a positioning reference. The coaxiality between the process hole and the cam center must be ≤0.02mm. The positioning mandrel must be ground to a radial runout of ≤0.005mm and a perpendicularity of ≤0.01mm/m relative to the milling machine table. To ensure synchronization between the cam angle and the tool feed, the mandrel must be connected to a milling machine indexing head with an indexing accuracy of ≤±1′. The cam angle is controlled by cranking a handle (each rotation corresponds to 9° of cam rotation, and precise indexing is achieved using an indexing plate). A textile machinery factory uses precision indexing head positioning to control the cam’s angular error within ±30″, meeting the requirement of constant velocity motion.
The milling cutter selection and geometric parameter design must be adapted to the cam material and profile characteristics. When milling steel cams, a carbide end mill (such as YT15) with a diameter of φ10-φ20mm and 4-6 teeth is used for rough milling.
