English
English
Views: 0 Author: Site Editor Publish Time: 2025-06-03 Origin: Site
An ideal machining program should not only ensure the production of qualified parts that meet the drawing requirements but also fully utilize the functional advantages of CNC machine tool. As highly efficient automated equipment, CNC machines typically achieve 2 to 3 times the efficiency of conventional machines. To fully leverage this advantage, a comprehensive process analysis of the workpiece must be conducted before programming, selecting an economical and reasonable process plan based on specific machining conditions. Improper design of CNC machining processes is a significant factor affecting machining quality, production efficiency, and cost control. This article, based on production practice, explores common process issues in CNC turning and their corresponding solutions.
When machining parts on CNC machines, operations tend to be concentrated, and as much as possible, all operations should be completed in a single setup. The division of operations generally follows two principles:
· Accuracy Assurance Principle
Due to the advantage of operation concentration in CNC machining, rough and finish machining are usually completed within one setup to ensure overall part accuracy. However, when thermal deformation or turning force deformation significantly affects accuracy, rough and finish machining should be separated to avoid cumulative errors.
· Production Efficiency Improvement Principle
To reduce tool changes and save tool change time, machining areas requiring the same tool should be completed before switching to another tool. Additionally, idle tool movements should be minimized by planning tool paths to achieve the shortest route when machining multiple areas with the same tool.
In actual production, CNC machining operations are often divided based on tool type or machining surfaces to balance accuracy and efficiency.
CNC programs describe the tool’s motion relative to the workpiece. In turning, the workpiece surface shape is determined by the envelope of the turning edge, but programming usually only describes the trajectory of a selected representative point on the tool, known as the tool reference point. Theoretically, any point on the tool can be chosen, but for programming convenience and accuracy assurance, the selection follows certain rules:
· For end mills, the reference point is the intersection of the tool axis and the tool bottom surface;
· For ball nose mills, it is the center of the ball;
· For drills, it is the drill tip;
· For turning tools, it is the hypothetical tool tip or the center of the tool tip arc.
When selecting the tool reference point, choose a point that is easy to measure and consistent with the tool length preset point; it should relate to dimensions requiring high accuracy or difficult to measure; the chosen point should directly reflect the tool’s limit position in the program’s motion commands. Programmers should maintain consistent selection habits and avoid frequent changes.
When machining an external cylindrical surface with a large allowance requiring multiple layered turns, the end position of each turn should be reasonably arranged to avoid sudden overload on the tool at the endpoint. Typically, the endpoint of each turn is progressively retracted by a small distance to prevent instantaneous impact on the main turning edge, thereby extending the life of roughing tools. If all turning layers end at the same axial position, the precision CNC lathe tool is prone to accelerated wear or damage due to load spikes.
Thin-walled workpieces, especially those made of difficult-to-machine materials, often exhibit “spring pass” during machining, causing dimensional changes such as increased outer diameter or decreased inner diameter. This phenomenon mainly results from elastic deformation of the workpiece and is closely related to the depth of turn during machining. Using the “equal depth of turn” method, tool compensation values can be adjusted through trial turns to offset dimensional errors caused by elastic deformation. The method involves performing a trial turn at half the intended depth of turn, measuring the dimension deviation, adjusting the tool compensation accordingly, and then proceeding with finish machining to ensure dimensional accuracy.
CNC turning is automated; poor chip breaking performance severely hinders stable machining. Priority should be given to improving the chip-breaking ability of the tool itself and selecting reasonable turning parameters to avoid continuous long ribbon chips. Ideal chips are moderate in length, spiral or segmented, facilitating chip evacuation and collection. If chip breaking is inadequate, program pauses can be arranged to force chip breaking, or chip breakers can be used to enhance chip control. When using clamped indexable inserts, the chip breaker and insert can be pressed simultaneously with a clamp plate to improve chip breaking. For internal turning, turning with the tool’s front face downward can improve chip evacuation.
CNC machining places higher demands on tools, requiring good rigidity, accuracy, dimensional stability, durability, excellent chip breaking and evacuation performance, and easy installation and adjustment. CNC turning machine common tool materials include high-speed steel and ultrafine-grain carbide, with widespread use of clamped indexable inserts.
Among indexable turning inserts, diamond-shaped inserts are most widely used, with common angles of 80°, 55°, and 35°.
· The 80° diamond insert offers a good balance of strength, heat dissipation, and durability, suitable for facing, external turning, internal holes, and step machining, with high positioning accuracy ideal for CNC turning.
· The 35° diamond insert has a small tip angle, reducing interference, and is suitable for complex profiles and groove machining.
For machining deep grooves, grooving tools are commonly used. If the tool width equals the groove width, a single pass can complete the turn; if the tool is narrower, multiple passes are required. The reasonable turning sequence is to turn the center first, then the left and right sides. This avoids difficulties caused by mismatched radii between tool corners and groove bottom corners and balances tool load to extend tool life. Clamped grooving tools should use straight turning paths and avoid lateral movements.
CNC machining programs are instruction files that control the entire automated machining process. The program includes not only the part’s process steps but also turning parameters, tool paths, tool dimensions, and machine movements. The quality of the process plan directly affects machine efficiency and part quality. Therefore, special attention should be paid to process design in production practice to ensure efficient and high-quality CNC machining.
