Turning Machining: Principles, Equipment and Techniques

1. Selection of Turning Equipment and Cutting Tools

1.1 Turning Principles and Lathes

Turning is a machining process where a cutting tool, typically a non-rotary tool bit, describes a helix toolpath by moving linearly while the workpiece rotates. This process is primarily used to produce cylindrical parts, although many variations exist for complex shapes. In turning operations, the workpiece rotates as the primary motion, while the cutting tool performs feed motions that can be linear or curved. Different feed methods in turning produce different workpiece surfaces, and in principle, the surfaces formed by turning are always coaxial with the axis of rotation of the workpiece, which is crucial for parts of injection molds that require precise cylindrical features.

During turning, as the cutting tool continuously engages with the material, the metal originally on the initial sliding surface OA不断向刀具靠近, as shown in Figure 3-3. When the sliding process reaches the final sliding surface OE position, the stress and strain reach their maximum values. When the shear stress exceeds the ultimate strength of the material, the material is fractured. Beyond the OE surface, the cutting layer separates from the workpiece and flows out along the rake face. This deformation process is critical to understand for optimizing tool life and surface finish, especially in precision components like parts of injection molds that require tight tolerances.

The typical machining operations that can be completed by turning are shown in Figure 3-4. The workpiece profiles that can be formed include inner and outer cylindrical surfaces, end faces, conical surfaces, spherical surfaces, elliptical cylindrical surfaces, grooves, helical surfaces, and other special profiles. Each of these operations requires specific tooling and parameter settings to achieve the desired results, whether for general machining or precision parts of injection components.

The selection of appropriate turning equipment depends on several factors including the size and material of the workpiece, required precision, production volume, and specific features to be machined. For small to medium-sized parts, bench lathes or engine lathes are commonly used, while large workpieces may require heavy-duty lathes or vertical turning centers. CNC lathes offer superior precision and repeatability, making them ideal for complex parts and production runs, including precision parts of injection molds that demand consistent quality across multiple components.

Cutting tool selection is equally critical in turning operations. The tool material must be chosen based on the workpiece material, with considerations for hardness, toughness, and heat resistance. Common tool materials include high-speed steel (HSS), carbide, ceramic, and cubic boron nitride (CBN). Each material has its advantages: HSS tools are more韧性 and less expensive, while carbide tools offer higher cutting speeds and longer life, making them suitable for high-volume production of parts of injection components.

Tool geometry also plays a vital role in turning performance. The rake angle, relief angle, and nose radius all affect cutting forces, surface finish, and tool life. Positive rake angles reduce cutting forces but may compromise tool strength, while negative rake angles provide greater strength for heavy cuts. The nose radius influences surface finish—the larger the radius, the better the finish, which is particularly important for parts of injection molds where surface quality directly affects product release and final part appearance.

2. Workpiece Clamping and Positioning

2.1 Concept of Lathe Fixtures

Lathe fixtures are devices used on lathes to hold workpieces. Their function is to locate the workpiece so that it achieves the correct position relative to the lathe and cutting tool, and to clamp the workpiece securely. Proper fixturing is essential for ensuring dimensional accuracy, repeatability, and safety during machining operations, especially when producing critical components like parts of injection molds that require precise alignment.

Lathe fixtures can be divided into two main categories: general-purpose fixtures and special-purpose fixtures. General-purpose fixtures are those that can hold two or more types of workpieces. Examples of general-purpose fixtures on lathes include self-centering chucks (three-jaw chucks), independent chucks (four-jaw chucks), spring collets, and universal mandrels. These versatile fixtures are cost-effective for small to medium production runs and are frequently used for machining various parts of injection molds and components.

Special-purpose fixtures are specifically designed for machining a particular workpiece in a specific operation. During the turning process, fixtures are used to hold the workpiece being machined, so they must ensure the positioning accuracy of the workpiece and be as convenient and quick to load and unload as possible. General-purpose fixtures should be prioritized when selecting fixtures. Special-purpose fixtures should only be used when general-purpose fixtures cannot be used or cannot guarantee the positioning accuracy required for the machining process of the workpiece. Special-purpose fixtures offer higher positioning accuracy but also come with higher costs, making them justifiable only for high-volume production or extremely precise components like critical parts of injection molds.

The key principles of workpiece positioning in turning include:

• Locating the workpiece securely to prevent movement during cutting
• Ensuring proper alignment with the machine's axis of rotation
• Minimizing workpiece deformation under clamping forces
• Providing sufficient access for the cutting tool to all machined surfaces
• Allowing for quick and repeatable loading/unloading

For parts of injection molds and similar precision components, additional considerations include maintaining strict concentricity between features, preventing marring or damage to finished surfaces, and ensuring consistent positioning for multi-operation machining. Fixture design must account for these factors while balancing clamping force and workpiece integrity.

Mandrels are another important category of lathe fixtures, used primarily for machining the external surfaces of workpieces that have already been bored or drilled. They provide support from the inside of the workpiece, ensuring concentricity between the internal and external surfaces. This is particularly valuable for parts of injection molds where precise wall thickness and concentricity are critical to proper function.

3. Turning Process Parameters

In turning operations, the rational determination of cutting parameters can ensure the quality of machined parts, extend the service life of machine tools and cutting tools, and maximize cutting efficiency. Increasing the feed rate and cutting speed of the machine tool can reduce the time required for cutting parts, but at the same time, the service life of the machine's cutting tools will be significantly shortened, and the surface quality of the machined parts will also decrease. The so-called "rational selection" refers to selecting cutting parameters that minimize machining time and thus obtain higher productivity while achieving the lowest machining cost, on the basis of making full use of existing conditions (including available machining equipment, processing range and power performance of processing equipment, wear resistance and hardness performance of existing tools, etc.) and meeting machining quality requirements. For machine tool cutting, the three elements of cutting conditions are closely related; changing any parameter will cause other related parameters to change. For example, increasing the cutting amount requires increasing the load on the cutting edge, which accelerates tool wear, which in turn increases processing costs and limits processing speed. Therefore, in practice, it is by no means as simple as using a value obtained from a calculation formula. Instead, it is necessary to comprehensively consider the calculated value and empirical value based on processing experience to reasonably select cutting parameters and obtain higher production efficiency and economic benefits with lower processing costs, especially for critical parts of injection molds where quality and efficiency must be balanced.

3.1 Spindle Speed

Determining a reasonable spindle speed is essential to achieve the appropriate cutting speed required for machining. Therefore, the spindle speed should be determined based on the cutting speed required for part processing and the diameter of the bar stock. From production practice, it can be found that except for thread processing, the spindle speed for machine turning only needs to consider the diameter of the part's processing area and be determined according to the cutting speed allowed by external conditions such as the processed part and tool material. This principle applies equally to general components and precision parts of injection molds, where maintaining optimal spindle speed is critical for surface finish and dimensional accuracy.

The formula for calculating spindle speed is:

Cutting speeds vary significantly depending on the workpiece material and tool material. For example, when machining carbon steel with carbide tools, typical cutting speeds range from 100-300 m/min, while for titanium alloys, speeds may be as low as 10-50 m/min. These parameters must be adjusted based on the specific application, whether machining structural components or precision parts of injection molds.

When machining parts of injection molds, which often use pre-hardened steels, spindle speeds are typically on the lower end of the range to ensure surface quality and dimensional stability. The higher hardness of these materials requires careful balance between cutting speed, feed rate, and depth of cut to prevent excessive tool wear and maintain precision.

3.2 Cutting Feed Rate

The distance that the tool moves in the feed direction per unit time is the feed rate, usually expressed in mm/min. Feed rate can also be expressed as feed per revolution (mm/rev), which is the distance the tool moves during one complete revolution of the workpiece. This parameter directly affects surface finish, chip formation, and cutting forces, making it crucial for both general machining and precision parts of injection components.

Generally, the principles for determining turning feed rates are as follows:

Feed rates typically range from 0.05 to 0.5 mm/rev for most turning operations, with smaller values used for finishing cuts and larger values for roughing. For parts of injection molds requiring excellent surface finish, finishing feeds may be as low as 0.02-0.1 mm/rev, depending on the tool nose radius and material.

It's important to note that feed rate has a significant impact on chip formation. Proper chip control is essential for safe operation, tool life, and surface quality. For automated production of parts of injection components, consistent chip formation is particularly important to prevent chip buildup and machine stoppages.

When determining feed rates for specific applications, including the machining of parts of injection molds, operators must consider not only the theoretical recommendations but also practical factors such as machine tool rigidity, workpiece fixturing stability, and power availability. A feed rate that is too high can cause excessive vibration, poor surface finish, and potential damage to the machine or tooling.

In modern CNC turning centers, feed rate optimization is often handled by computer algorithms that adjust parameters in real-time based on cutting conditions. This adaptive control technology helps maintain optimal cutting conditions even as tool wear occurs, ensuring consistent quality throughout production runs of parts of injection components and other critical parts.

For precision turning operations, especially those involving complex geometries in parts of injection molds, the feed rate may need to be reduced in specific areas to maintain accuracy. Internal corners, small radii, and thin-walled sections often require special parameter adjustments to prevent deflection and ensure dimensional integrity.

4. Advanced Turning Considerations

4.1 Cutting Fluids and Coolants

Proper application of cutting fluids is essential for optimizing turning operations. These fluids serve multiple purposes: cooling the cutting zone, lubricating the tool-workpiece interface, flushing away chips, and preventing corrosion. For machining parts of injection molds, which often use heat-resistant materials, effective cooling is critical to maintain dimensional stability and tool life.

There are several types of cutting fluids available, including emulsions, synthetic fluids, and cutting oils. The selection depends on the workpiece material, tool material, and cutting conditions. For example, synthetic fluids are often used for high-speed machining of aluminum parts of injection components, while cutting oils may be preferred for heavy-duty machining of alloy steels.

4.2 Surface Finish Requirements

Surface finish in turning operations is primarily determined by feed rate, tool nose radius, and cutting speed. A general rule is that finer surface finishes require lower feed rates and larger nose radii. For parts of injection molds, surface finish is particularly important as it directly affects the appearance and release properties of the molded parts. Critical surfaces may require additional finishing operations such as grinding or polishing after turning.

4.3 Automation in Turning Processes

Modern turning operations increasingly utilize automation to improve productivity, consistency, and safety. CNC lathes with bar feeders can run unattended for extended periods, producing high volumes of consistent parts. Robotic loading/unloading systems further enhance automation, making them ideal for mass production of parts of injection components. These systems can handle complex sequences of operations, ensuring each part meets exact specifications.

For manufacturers producing parts of injection molds and related components, automation offers significant advantages in terms of repeatability and precision. Complex geometries can be machined with consistent accuracy across multiple workpieces, ensuring proper fit and function in the final assembly.