Milling shank ball
Milling shank balls involves CNC machining one end of a bar into a spherical shape, while retaining the shank at the other. This process is widely used in the manufacture of parts such as valve balls, universal joint balls, and decorative balls. The structural characteristic of a shank ball is that the sphere and shank are coaxial. The diameter accuracy, roundness, and surface quality of the sphere directly impact the part’s assembly performance and functionality. For example, the roundness error of a valve ball must be controlled within 0.01mm to ensure sealing performance. Milling is the primary CNC machining method for shank balls. Compared to turning, milling can achieve high-speed cutting using multi-edged tools, resulting in higher efficiency and being particularly suitable for medium-volume production. However, due to the curved surface of the sphere, the milling process must ensure that the tool path matches the spherical surface, placing high demands on machine tool accuracy and programming capabilities.
The milling process for shank-mounted balls requires staged precision control. A typical process involves: bar stock cutting → rough turning of the shank and ball blank (allowing a 1-2mm allowance) → heat treatment (e.g., tempering) → rough milling of the sphere (allowing a 0.3-0.5mm allowance) → semi-finish milling of the sphere (allowing a 0.1-0.2mm allowance) → finish milling of the sphere → inspection. During the rough milling stage, a high-speed steel end mill or carbide end mill is used to remove the majority of the excess material through layered cutting. This ensures the basic shape of the sphere, with dimensional tolerances within ±0.2mm. During the semi-finish and finish milling stages, a ball-end milling cutter with a ball nose radius equal to 1/2-2/3 of the sphere’s radius is used to ensure smooth transitions between curved surfaces. For spheres with high precision requirements (roundness ≤ 0.005mm), grinding or polishing is performed after finish milling to further reduce the surface roughness to below Ra0.025μm. The shank processing must be completed before the sphere processing. The shank is used as the positioning reference. The coaxiality error of the shank should be ≤0.02mm, otherwise it will cause the sphere to be eccentric.
The clamping method for milling shank balls must ensure positioning accuracy and CNC machining stability. Small-diameter shank balls (diameter <50mm) can be clamped with a three-jaw self-centering chuck, which is tightened with a tailstock center to provide rigid support. Large-diameter shank balls (diameter >100mm) require a specialized fixture, such as a V-block with a pressure plate, positioned using the shank’s outer diameter and end face. The positioning surfaces must be finely machined to a surface roughness of Ra ≤ 1.6μm. During clamping, the sphere’s center of rotation must be aligned with the machine tool’s spindle axis. This can be done using a dial indicator. Radial runout should be controlled within 0.01mm to avoid eccentricity and out-of-roundness. For thin-walled shank balls (wall thickness <5mm), an elastic clamp or rubber pads should be used in the clamping area to reduce deformation caused by clamping force. The clamping force should be sufficient to prevent the workpiece from loosening, and can be adjusted through trial cutting.
When milling shank balls, tool selection and cutting parameters must be tailored to the material and CNC machining stage. Tool material selection depends on the workpiece material: When CNC machining steel, use carbide (such as YT15) for rough milling, and coated carbide (such as TiAlN-coated) for fine milling. When CNC machining cast iron, use YG8 for rough milling and YG6X for fine milling. When CNC machining non-ferrous metals (such as copper and aluminum), high-speed steel or diamond tools can be used. The diameter of the ball end mill is determined by the diameter of the ball, generally 1/3-1/2 the diameter of the ball. For example, for a 50mm diameter ball, a 16-20mm diameter ball end mill should be used. The tool’s rake angle should be 5°-10°, and the clearance angle should be 8°-12°. The cutting edge should be sharp and free of chipping. Cutting parameters: Rough milling: cutting speed 80-150 m/min, feed 0.1-0.2 mm/r, depth of cut 0.5-1 mm; finish milling: cutting speed 150-250 m/min, feed 0.05-0.1 mm/r, depth of cut 0.1-0.3 mm. High-pressure cooling with a flow rate of 20 L/min or higher is required for the cutting fluid. Extreme-pressure emulsion is used for steel CNC machining, while kerosene or specialized aluminum alloy cutting fluid is used for aluminum alloy CNC machining to reduce friction and cool the tool.
The CNC machining method and precision control for milling shank spheres are critical to ensuring quality. When CNC machining on a conventional milling machine, the workpiece angle must be adjusted using a manual oscillator or indexing head, allowing the tool to cut different parts of the sphere at each cut, gradually forming the spherical surface. This method is suitable for single-piece, small-batch production, offering lower efficiency but lower equipment costs. When CNC machining on a CNC milling machine, the tool is programmed to move along the longitude and latitude of the sphere, using circular interpolation to achieve continuous cutting of the curved surface. This efficiency is 5-10 times higher than that of a conventional milling machine, and the roundness error can be controlled to within 0.005mm. Precision control focuses on the sphere’s roundness, diameter accuracy, and coaxiality with the shank. Roundness can be ensured by adjusting the tool path’s step size (generally ≤0.1mm); a smaller step size results in a smoother surface. Diameter accuracy is calibrated using trial cuts, measuring the diameter after each cut until the required diameter is achieved. Coaxiality can be achieved with a single clamping operation using the shank as the reference, avoiding errors from multiple clampings. For batch production, a dedicated spherical milling machine can be used, equipped with automatic feeding and detection devices to achieve fully automatic processing of shank balls, with dimensional consistency up to ±0.01mm, meeting the use requirements of high-precision parts.
