many CAD/CAM software packages offer automatic programming capabilities. These software packages typically provide prompts regarding process planning within the programming interface, such as tool selection, machining path planning, and cutting parameters. Once the programmer sets the relevant parameters, the software can automatically generate NC programs and transmit them to CNC machines for processing.
Therefore, tool selection and determination of cutting parameters in CNC machining are completed in an interactive environment, which is in stark contrast to conventional machining. This also requires programmers to understand the basic principles of tool selection and cutting parameter determination. Programmers must consider the characteristics of CNC machining to correctly select tools and cutting parameters during programming.
### Common Types and Characteristics of Tools in CNC Machining
CNC machining tools must be suited to the high speed and high automation characteristics of CNC machines. Generally, they include universal tools, standard tool holders, and a small number of specialized tool holders. The tool holders connect the tools to the machine's spindle, and as such, have gradually become standardized and serialized. There are various ways to classify CNC tools:
- **By tool structure:**
1. Integral type
2. Inserted type, connected by welding or mechanical clamping (mechanical clamping can be non-indexable or indexable)
3. Special types, such as composite tools and damped tools
- **By material:**
1. High-speed steel tools
2. Carbide tools
3. Diamond tools
4. Tools made from other materials, such as cubic boron nitride or ceramics
- **By cutting process:**
1. Turning tools (external, internal, threading, grooving, etc.)
2. Drilling tools (drills, reamers, taps, etc.)
3. Boring tools
4. Milling tools
To meet CNC machine requirements for durability, stability, ease of adjustment, and interchangeability, indexable insert tools have been widely adopted in recent years. These tools now constitute 30%–40% of all CNC tools, accounting for 80%–90% of the total metal removal volume.
### Requirements and Characteristics of CNC Tools
Compared to tools used on conventional machines, CNC tools have specific requirements and characteristics:
1. Good rigidity (especially for roughing tools), high precision, resistance to vibration, and minimal thermal deformation
2. Good interchangeability to facilitate quick tool changes
3. High durability and reliable cutting performance
4. Adjustable dimensions to reduce tool change adjustment time
5. Reliable chip breaking or curling for efficient chip removal
6. Standardization and serialization to facilitate programming and tool management
### Tool Selection in CNC Machining
Tool selection in CNC programming is conducted interactively. The correct selection of tools and tool holders depends on the machine’s capabilities, the material of the workpiece,
the machining process, cutting parameters, and other relevant factors. The general principles for tool selection are ease of installation and adjustment, good rigidity, durability, and high precision. To enhance tool rigidity, shorter tool holders should be selected whenever possible, provided they meet the machining requirements.
When selecting tools, their size should be appropriate for the surface dimensions of the workpiece. For example, flat end mills are commonly used for machining the contours of flat parts; carbide insert milling cutters are preferred for milling flat surfaces; high-speed steel end mills are used for machining bosses and grooves; corn milling cutters with carbide inserts are suitable for machining rough surfaces or pre-machined holes; ball nose end mills, ring mills, taper mills, and disc mills are typically used for machining complex surfaces and varying angle contours.
In free-form surface (mold) machining, the cutting speed at the end of a ball-end tool is zero. Therefore, to ensure machining accuracy, the cutting path spacing is usually set closer at the top end, making ball-end tools suitable for finish machining of surfaces. Flat-end tools, however, are superior in surface machining quality and cutting efficiency. Hence, flat-end tools should be preferred for both roughing and finishing of surfaces, as long as overcutting can be avoided.
The durability and precision of tools are significantly related to their cost. Notably, while high-quality tools increase tool costs, they also significantly enhance machining quality and efficiency, thereby reducing overall machining costs.
In machining centers, various tools are mounted in the tool magazine and selected and changed as per the program's instructions. Therefore, standard tool holders must be used to ensure that standard tools for drilling, boring, reaming, and milling can be quickly and accurately mounted onto the machine spindle or tool magazine. Programmers need to understand the structural dimensions, adjustment methods, and adjustment ranges of the tool holders used on the machine to determine the radial and axial dimensions of the tools during programming.
Currently, China's machining centers use the TSG tool system, which includes straight shank (three specifications) and tapered shank (four specifications) holders, covering a total of 16 types of tool holders for different applications.
In the processing of economical CNC machines, where tool sharpening, measurement, and replacement are mostly done manually, the auxiliary time is relatively long. Therefore, the arrangement of tool sequences must be reasonable. The general principles to follow include:
1. Minimize the number of tools.
2. Each tool should complete all possible machining steps after being clamped once.
3. Separate rough and finish machining tools, even if they have the same specifications.
4. Mill first, then drill.
5. Perform contour finishing after 3D surface finishing.
6. Utilize the automatic tool change function of the CNC machine as much as possible to improve production efficiency.
### Determining Cutting Parameters in CNC Machining
The principles for selecting cutting parameters reasonably are: prioritizing productivity during rough machining while considering economy and machining costs; balancing machining quality, cutting efficiency, and cost during semi-finishing and finishing. Specific values should be determined based on the machine's manual, cutting parameter manuals, and experience. Several factors need to be considered:
1. **Cutting depth (t):** When the machine, workpiece, and tool rigidity permit, t should equal the machining allowance, which is an effective measure to improve productivity. To ensure part precision and surface roughness, a certain amount of allowance should be left for finishing. The finishing allowance for CNC machines can be slightly less than that for conventional machines.
2. **Cutting width (L):** Generally, L is proportional to the tool diameter (d) and inversely proportional to the cutting depth. For economical CNC machines, L typically ranges between 0.6d and 0.9d.
3. **Cutting speed (v):** Increasing v is another measure to improve productivity. However, v closely relates to tool durability; as v increases, tool durability decreases sharply. Therefore, v is primarily determined by tool durability. Cutting speed also varies significantly with the material being machined. For instance, when milling alloy steel 30CrNi2MoVA with an end mill, v can be about 8 m/min, whereas for milling aluminum alloy with the same tool, v can exceed 200 m/min.
4. **Spindle speed (n):** Spindle speed is usually determined by cutting speed (v) using the formula v = πnd/1000. CNC machines typically have a spindle speed override switch on the control panel to adjust spindle speed by multiples during machining.
5. **Feed rate (vF):** vF should be chosen based on the required machining precision and surface roughness, as well as the tool and workpiece materials. Increasing vF can also improve productivity. When low surface roughness is required, a higher vF can be selected. During machining, vF can be manually adjusted using the override switch on the machine control panel, but the maximum feed rate is limited by the machine's rigidity and feed system performance.
With the widespread application of CNC machines in production and the formation of quantitative production lines, CNC programming has become a key issue in CNC machining. During the preparation of CNC programs, tool selection and cutting parameter determination must be done interactively. Therefore, programmers must be familiar with tool selection methods and principles for determining cutting parameters to ensure part quality and machining efficiency, fully leverage the advantages of CNC machines, and improve the economic efficiency and production level of enterprises.
