Reasons for producing scrap when turning special-shaped surfaces
Surface turning involves turning parts with irregular curved surfaces, such as handles, spheres, and ellipses. These parts have complex shapes and require high machining precision, making them prone to scrap. Analyzing the causes of scrap in surface turning is crucial for improving machining quality and reducing production costs. Common causes of scrap include improper machining methods, inappropriate tool selection, insufficient operator skills, measurement errors, and insufficient equipment accuracy.
Improper machining methods are one of the main causes of scrap when turning contoured surfaces. There are many methods for turning contoured surfaces, including two-handed control, template-based machining, and template-based machining. Different methods are suitable for different part shapes and precision requirements. The two-handed control method involves the operator simultaneously operating the lathe’s longitudinal and transverse feed handles, moving the tool along the required trajectory to produce the contoured surface. This method is suitable for parts with simple shapes and low precision requirements, but it requires high operator skill. Inexperienced operators can easily produce defects such as unevenness and dimensional inconsistencies in the contoured surface. The template-based machining method uses a template device to control the tool’s trajectory, offering high machining accuracy and making it suitable for mass production. However, the manufacturing accuracy of the template directly affects machining quality. If the template’s shape is insufficient or improperly installed, the resulting contoured surface will not meet the drawing requirements. The template-based machining method uses a template tool that matches the contoured surface shape. It is suitable for machining regular contoured surfaces such as arcs. However, if the template tool’s shape is insufficient or the angle of installation is incorrect, the contoured surface will exhibit shape errors. Therefore, choosing the appropriate processing method according to the shape and precision requirements of the parts is the prerequisite for avoiding waste.
Improper tool selection can also lead to scrap when turning contoured surfaces. Contoured surface machining places special demands on tool geometry and material properties. Improper tool selection can compromise machining quality and efficiency. For example, when machining contoured surfaces in plastic materials, a tool with a larger rake angle should be selected to reduce cutting forces, reduce chip deformation, and prevent tool sticking. When machining brittle materials, a tool with a smaller rake angle should be selected to increase tool strength and prevent edge chipping. Tool grinding quality is also crucial. A blunt cutting edge and high surface roughness can cause defects such as scratches and tears on the contoured surface, impacting surface quality. Furthermore, insufficient tool rigidity can cause vibration during machining, resulting in ripples on the contoured surface. This impact is particularly pronounced when machining slender, thin-walled contoured parts. Therefore, the tool material and geometry should be appropriately selected based on the workpiece material, contoured surface shape, and machining accuracy requirements, and tool grinding quality should be ensured.
The operator’s skill level and operational standards directly impact the machining quality of feature-shaped surfaces, and insufficient operator skills are a common cause of scrap. CNC Machining feature-shaped surfaces using the two-handed control method requires the operator to skillfully coordinate longitudinal and transverse feeds and accurately control the tool’s trajectory. Improper control of feed rate and cutting speed can result in dimensional and shape accuracy deviations from the feature-shaped surface. For example, when machining a sphere, if the longitudinal feed is too fast and the transverse feed is too slow, the sphere will appear flat; if the transverse feed is too fast and the longitudinal feed is too slow, the sphere will appear pointed. Furthermore, a lack of operator responsibility, such as failure to measure workpiece dimensions promptly during machining or incorrect measurement methods, can also lead to scrap. For example, if the shape of a feature-shaped surface is not checked using a standard template during machining, and the shape does not meet requirements until after machining is complete, scrap will result. Therefore, strengthening operator skills training and improving operational standards and accountability are important measures to reduce scrap on feature-shaped surfaces.
Measurement errors are also one of the reasons for scrapping special-shaped surfaces. Special-shaped surfaces have complex shapes and are difficult to measure. If the measuring tool is not properly selected or the measurement method is incorrect, the measurement results will be inaccurate, and the machined parts will not meet the drawing requirements. For example, when measuring the diameter of a sphere, if an ordinary caliper is used, it is difficult to accurately measure its maximum diameter, which is prone to measurement errors. When measuring an elliptical surface, if the appropriate measurement reference is not selected, the major and minor axis dimensions of the ellipse will be inaccurate. In addition, the measurement environment can also affect measurement accuracy. For example, temperature changes can cause thermal deformation of the workpiece and measuring tools, resulting in increased measurement errors. Therefore, appropriate measuring tools, such as templates and special measuring tools, should be selected according to the shape characteristics of the special-shaped surface, and the correct measurement method should be adopted. At the same time, factors such as the temperature and humidity of the measurement environment should be controlled to reduce measurement errors.
Inadequate equipment accuracy is one of the root causes of scrap when turning special-shaped surfaces. The lathe’s spindle rotation accuracy, guideway straightness, and feed system transmission accuracy all affect the machining accuracy of special-shaped surfaces. Inadequate spindle rotation accuracy can cause radial runout during workpiece rotation, resulting in roundness errors in the machined special-shaped surface. Large guideway straightness errors can cause tool offset during feed, affecting the shape accuracy of the special-shaped surface. Play or uneven transmission in the feed system can lead to uneven tool feed, causing waviness or dimensional errors on the special-shaped surface. For example, when machining special-shaped surfaces using the profiling method, if the lathe’s cross-feed guideway has straightness errors, the profiling accuracy will decrease, resulting in distorted shape of the machined special-shaped surface. Therefore, regular lathe maintenance and servicing, along with timely adjustment and repair of equipment accuracy, should be performed to ensure that all lathe precision indicators meet machining requirements and provide reliable equipment support for special-shaped surface machining.
