Using the template to turn special-shaped surfaces
Using a template to turn a contoured surface is a method that utilizes a template to control the tool’s trajectory, enabling the turning of complex curved surfaces (such as spheres, ellipsoids, and parabolas). This method is suitable for the efficient CNC machining of contoured surfaces in mass production, offering significantly higher precision and efficiency than two-handed control methods. It is widely used in industries such as automotive, tractor, and instrumentation. The template transfers the contour of the template onto the workpiece through mechanical transmission, allowing the tool to follow the template’s curved trajectory, resulting in a contoured surface that matches the template’s shape. This method does not require advanced operator skills and offers stable and consistent CNC machining quality.
The template assembly consists of a template, rollers, brackets, slides, and tool connection mechanism. The precision of each component directly impacts the CNC machining quality of the feature surface. The template is the core component, and its working surface’s curvilinear shape perfectly aligns with the workpiece’s feature surface’s generatrix. High precision is required, with surface roughness Ra ≤ 0.8μm and straightness ≤ 0.01mm/m. Made of quenched and tempered 45 steel (220-250HBW) or cast iron (HT300), the working surface is ground. For templates requiring high precision, aging treatment is required to eliminate internal stresses. The rollers, which contact the template and transmit the template’s curvilinear motion to the slides, are typically 10-20mm in diameter and made of bearing steel (GCr15). After quenching, the rollers have a surface roughness Ra ≤ 0.1μm to reduce friction and wear with the template. The bracket secures the template and supports the slide. It must be rigid enough to prevent deformation during CNC machining. The clearance between the slide and bracket must be ≤0.01mm to ensure smooth, non-binding movement. The tool connection mechanism transmits the slide’s motion to the tool, aligning its trajectory with the template curve. The connection requires high rigidity and transmission accuracy, with a transmission error of ≤0.005mm.
Adjusting a profiled surface using a master plate is a complex but crucial process, directly determining CNC machining accuracy. First, secure the master plate to the lathe bed or slide box, ensuring the master plate’s reference surface is parallel to the lathe spindle axis, with a parallelism error of ≤0.01mm/m. This can be corrected using a dial indicator. Adjust the contact pressure between the roller and the master plate. Too little pressure will result in poor contact, while excessive pressure will increase wear on the master plate and roller. This pressure is typically adjusted using springs or counterweights, keeping it between 5-10N. Mount the tool on the tool holder, aligning the tool tip with the center of the roller, with a height difference of ≤0.02mm. Otherwise, form errors will occur. For example, when CNC machining a spherical surface, a 0.1mm height difference will result in a 0.1mm radius error. Adjust the relative position of the workpiece and the master plate so that the workpiece’s center of rotation coincides with the master plate’s center of symmetry. For asymmetrical profiled surfaces, alignment should be performed according to the markings to ensure positional accuracy after CNC machining. Try cutting a workpiece, measure its shape and size, and compare it with the template. If there is an error, adjust the template position or tool height until it meets the requirements.
The CNC machining method for contoured surfaces using a profiling system depends on the type of surface. Common CNC machining methods include external and internal contours, each requiring different key operations. When CNC machining external contours (such as handle spherical surfaces or cam profiles), the workpiece is clamped in a three- or four-jaw chuck, aligned, and tightened. The profiling system is fixed to the side of the machine bed, with rollers in contact with the profiling plate. The tool is fed longitudinally by the slide box while also being fed transversely by the profiling system to form the contoured surface. The longitudinal feed rate must be controlled during CNC machining (generally 20-50 mm/min). Excessive feed can cause tool vibration, affecting surface quality, while too slow a feed reduces efficiency. When CNC machining internal special-shaped surfaces (such as the inner spherical surface of a bearing or the inner conical surface of a valve), the workpiece is clamped on a faceplate or chuck, the profiling device is installed on the slide box, and the tool is inserted into the workpiece through the tool rod. The roller drives the tool to perform radial feed under the action of the profiling. At this time, attention should be paid to the rigidity of the tool rod. The tool rod diameter should be as large as possible, and the extended length should not exceed 5 times the diameter to avoid bending and deformation of the tool rod.
The cutting parameters and tool selection for contoured surfaces using a molded tool should be determined based on the workpiece material and the surface accuracy. Tool materials should be high-speed steel (such as W18Cr4V) or carbide (such as YT15). Carbide tools should be used for steel, with a cutting speed of 80-120 m/min ; YG8 tools should be used for cast iron, with a cutting speed of 60-100 m/min; and high-speed steel tools should be used for non-ferrous metals, with a cutting speed of 100-150 m/min. Feed rates should be 0.1-0.2 mm/r, reducing to 0.05-0.1 mm/r for finish turning to ensure surface quality. Tool geometry should be suitable for curved surface CNC machining, with a rake angle of 5°-10°, a relief angle of 6°-8°, and primary and secondary rake angles of 45°-60°. The tool tip radius should be determined based on the curvature radius of the contoured surface, generally 1/5-1/10 of the curvature radius, to prevent interference between the tool tip and the workpiece surface. The cutting fluid needs to be fully lubricated and cooled. Extreme pressure emulsion is used for processing steel parts, kerosene is used for processing cast iron, and diesel is used for processing non-ferrous metals to ensure smooth cutting and surface quality.
Quality inspection and maintenance of profiled surfaces machined using a template are crucial for ensuring long-term CNC machining accuracy. Machined profiles must be inspected using templates, calipers, dial indicators, and other tools. Form error must be ≤0.02mm, dimensional tolerances must be within ±0.05mm, and surface roughness must be Ra ≤1.6μm. For symmetrical profiles, such as spheres, the radius and roundness must be measured. Radius error must be ≤0.03mm, and roundness error must be ≤0.01mm. The template assembly requires regular maintenance. The template working surface must be kept clean from iron filings and oil contamination. Clean it weekly with kerosene and check flatness and straightness monthly. Rollers must be regularly greased and replaced when wear exceeds 0.05mm. The mating surfaces between the slide rod and bracket must be regularly lubricated to ensure smooth movement. Profiles that are not used for an extended period should be coated with anti-rust oil and properly stored to prevent deformation and rust. Through strict quality inspection and equipment maintenance, the method of profiling special-shaped surfaces can stably produce high-precision complex curved surfaces to meet the requirements of mass production.
