CNC Machining vs. Traditional Machining: Key Differences

Comparison of CNC and Traditional Machine Tools

Compared with traditional machine tools, CNC machine tools differ significantly not only in overall performance but also in structural and functional complexity.

Key Structural Features of CNC Machine Tools

Enclosed Machining Space and Main Drive System

CNC machine tools can use higher cutting speeds, feed rates, and coolant-lubricant pressures. For machining safety and environmental protection, they are usually equipped with an enclosed machining space. Chips produced during machining remain inside this enclosure and are then conveyed outside by the machine-mounted chip conveyor. The coolant-lubricant used in machining is collected and treated inside the machine before being recirculated.

From an ergonomic perspective, a large machine guard door with a wide opening lets the operator enter the work area easily and observe the machining space clearly. CNC machining commonly requires a constant cutting speed. These speed changes are generally achieved with continuously variable-speed motors. In modern machine tools, such motors are integrated directly into the drive spindle. On CNC milling machines, this configuration maintains excellent drive characteristics even when the overall speed range is divided into several gear ranges.

Servo Drives, Linear Motion, and Multi-Axis Expansion

To machine complex workpiece geometries, each CNC axis is normally driven by a separate digital servo motor. As a result, linear drives are increasingly used. They provide high acceleration, short response delays, and wear-free operation, and can position each axis accurately over long travel distances.

Adding more controllable axes further expands the range of workpiece shapes that can be machined. On a CNC milling machine, for example, a two-axis NC table and a work spindle can provide precisely defined rotary motion as a feed or transverse-feed motion. Programmable tailstocks, driven tool turrets, and main and subspindles also extend the ability of CNC lathes to perform complete machining.

Importance of Ball Screws and Thermal-Stability Design

To transmit the computational accuracy of the control system and the positioning accuracy of servo motors precisely to the motion between the workpiece and cutting tool, machine tools commonly use ball screws as transmission elements. Ball screws operate with virtually no backlash and can continuously transmit very slow motion without stick-slip. Their associated guideways combine the advantages of sliding and rolling friction.

CNC machine tools can generally achieve a very high material-removal rate per unit time, Q. Their structural components therefore experience high mechanical and thermal loads. Modern machine-tool builders address this through specialized structural designs and new materials. In milling-machine design, particular attention is paid to reducing the mass of moving components. To remove chips, coolant-lubricant, and the resulting heat more effectively, CNC lathes generally use inclined bed guideways. The accessibility and motion of the machine table are also important features of the CNC universal milling-machine structure.

Tool Management and Inspection Systems

Tool changing on CNC machine tools can be automated in most cases. To ensure that every required tool can be called during program execution, lathes are equipped with at least one tool turret, while milling machines use a tool magazine and automatic tool changer. Tool turrets and tool magazines on newer machines are commonly servo-driven and equipped with inspection systems, giving the machine higher positioning accuracy and faster tool-change speeds.

Expanded Capability Through Automation Interfaces

Modern CNC machine tools are commonly equipped with standardized interfaces for connecting peripheral automation equipment. Adding workpiece pallet changers, workpiece storage units, and workpiece handling systems can further increase the level of automation in cutting operations.

Each controlled axis of a CNC machine tool is equipped with an electronic measuring system. By using the results of travel and positioning movements, the control system can obtain the real-time position of the machine slide, rotary table, or spindle. In recent years, conventional machine-tool spindles have also increasingly been equipped with such measuring systems. This enables functions such as linear control and substantially improves ease of operation. In addition, technologies developed specifically for CNC machine tools, including continuously variable-speed spindles, servo motors, and dedicated guideway and transmission units, are gradually being applied to conventional machines. Traditional machine tools such as lathes and milling machines are now most commonly found in the vocational-training departments of industrial enterprises.