0 Introduction
An ideal machining program should not only ensure the production of qualified workpieces that meet the design specifications but also fully utilize and enhance the functions of CNC machine tools. CNC machines are highly efficient automated equipment, boasting efficiency levels 2-3 times higher than conventional machines. To fully leverage this advantage, it is essential to perform a thorough process analysis on the workpiece before programming and choose the most economical and rational process plan based on specific conditions. Inadequate consideration of CNC machining processes can significantly impact machining quality, production efficiency, and processing costs. This article aims to explore and summarize some process issues in CNC turning based on practical production experience.
1 Division of CNC Machining Processes
When machining parts on CNC machines, processes are relatively concentrated. Ideally, all operations should be completed in a single clamping. Common principles for dividing processes include:
Ensuring Accuracy
CNC machining allows for process concentration, enabling rough and finish machining to be completed in one clamping to ensure part accuracy. However, if thermal deformation and cutting force deformation significantly affect machining accuracy, rough and finish machining should be performed separately.
Improving Production Efficiency
To reduce the number of tool changes and save tool change time in CNC machining, all areas requiring the same tool should be completed before switching to another tool. Additionally, empty travel should be minimized, and the tool should follow the shortest path to reach various machining areas.
In practical production, CNC machining processes are often divided according to tools or machining surfaces.
2 Selection of Tool Position Points for Turning Tools
In CNC machining, the CNC program should describe the tool's movement trajectory relative to the workpiece. In CNC turning, the formation of the workpiece surface depends on the position and shape of the moving cutting edge envelope. However, only the trajectory of a selected point on the tool system needs to be described in the program. This point is known as the tool position point, which represents the tool's location. The machining trajectory described by the program is the movement path of this point.
Theoretically, any point on the tool can be chosen as the tool position point in CNC turning. However, to facilitate programming and ensure machining accuracy, the selection of the tool position point follows certain rules and techniques. Generally, the following rules are observed:
- Choose a point on the tool that can be directly measured, ensuring consistency with the point measured during tool length presetting.
- If possible, the tool position point should directly relate to dimensions with high accuracy requirements or those difficult to measure.
- The chosen tool position point should allow for the tool's extreme position to be directly reflected in the program's movement commands.
- Programmers should adopt a habitual tool position point selection method, avoiding frequent changes.
- The selected tool position point should be graphically marked in the tool adjustment diagram.
3 Termination Position of the Tool in Layered Cutting
When the machining allowance of an external cylindrical surface is substantial, multiple passes of layered cutting are required. From the second pass onward, it is crucial to prevent a sudden increase in the depth of cut at the endpoint. As shown in Figure 2, for tools with a 90° principal angle, a reasonable arrangement is to slightly advance the endpoint of each pass by a small distance \( e \) (e = 0.05). If \( e = 0 \), and each pass terminates at the same axial position, the tool's main cutting edge may experience an instant heavy load impact. Arranging the endpoints of layered cuts in a staggered manner helps prolong the life of roughing tools.
4 Determining Tool Compensation Values During "Letting the Tool"
For thin-walled workpieces, especially those made of difficult-to-cut materials, the "letting the tool" phenomenon is severe, leading to dimensional changes in the workpiece, typically resulting in larger outer diameters and smaller inner diameters. This is mainly caused by the elastic deformation of the workpiece during machining. The degree of "letting the tool" is closely related to the depth of cut. By using the "constant depth of cut method" and making small adjustments with tool compensation values, the impact of "letting the tool" on machining accuracy can be minimized.
5 Chip Breaking During Turning
In CNC turning, if the chip-breaking performance of the tool is poor, it will severely hinder normal machining operations. To address this issue, it is essential to enhance the tool's chip-breaking performance and reasonably select the tool's cutting parameters to avoid producing long, continuous chips that obstruct machining. Ideal chips in CNC turning are spiral or conical chips with a length of 50-150 mm and a small diameter, which can be easily discharged and collected. If chip breaking is not ideal, the program can include pauses for forced chip breaking, or use chip breakers to enhance chip breaking effectiveness.
6 Selection of Insert Shapes for Indexable Tools
Compared with conventional machining methods, CNC machining imposes higher requirements on tools, needing good rigidity, high accuracy, dimensional stability, durability, and excellent chip-breaking and chip-removal performance. Additionally, tools must be easy to install and adjust to meet the high-efficiency demands of CNC machines. Tools used in CNC machines often utilize materials suitable for high-speed cutting, such as high-speed steel and ultra-fine grain carbide, and use indexable inserts.
7 Tool Path for Grooving
When machining deeper grooves on CNC lathes, grooving tools are commonly used. If the tool width matches the groove width, the grooving tool makes a single cut. For wider grooves, multiple passes are required. The optimal cutting path is to first cut the middle and then the sides, as shown in Figure 1. This ensures balanced loading on the cutting edges and even tool wear.
8 Conclusion
CNC machining programs are directive documents for CNC machines, dictating the entire machining process, including the technological process, cutting parameters, tool paths, tool dimensions, and machine movements. The detailed process planning directly impacts machine efficiency and part quality, warranting significant attention in practical production.
