The tool selection and determination of cutting parameters in NC machining are performed through human-computer interaction, which distinguishes it from conventional machine tool machining. To carry out NC machining effectively, programmers must have a good understanding of the basic principles of tool selection and cutting parameters, take into account the unique characteristics of digital machining in their programming, and be able to choose the appropriate tools and parameters correctly.
CNC machining tools must be designed to work with the high speed, efficiency, and automation of CNC machine tools. They generally include standard tools, standard tool holders, and a limited number of specialized tool holders. The tool holder is attached to the tool and mounted on the machine tool’s spindle, which has led to increased standardization and modularization.
NC cutting tools can be classified in a variety of ways.
1. Structure of NC machine cutting tools
1. Integral type;
2. Inlay type, which adopts welding or machine clamp connection. Machine clamp type can be divided into non indexable and indexable;
3. Special types, such as compound cutting tools, damping cutting tools, etc.
2. Materials used for manufacturing tools
1. High speed steel cutting tools;
2. Cemented carbide cutting tools;
3. Diamond cutting tools;
4. Other material cutting tools, such as cubic boron nitride cutting tools, ceramic cutting tools, etc.
3. Classification of cutting process
1. Turning tools, including outer circle, inner hole, thread, cutting tool and so on;
2. Drilling tools, including drill bits, reamers, taps, etc;
3. Boring tools;
4. Milling tools, etc.
In order to meet the requirements of CNC machine tools, the number of adjustable and durable tools has reached 40% ~ 90% of the total number of CNC machine tools in recent years.
4. Characteristics of NC tools
1. Good rigidity (especially rough machining tools), high precision, low vibration resistance and thermal deformation;
2. Good interchangeability, convenient for quick tool change;
3. High service life, stable and reliable cutting performance;
4. The size of the tool is easy to adjust to reduce the adjustment time of tool change;
5. The cutting tool shall be able to reliably break chips or roll chips, so as to facilitate the removal of chips;
6. Serialization and standardization to facilitate programming and tool management.
5. Tool selection
The tool selection in NC programming is carried out through human-computer interaction. The cutter and shank must be selected based on the machine tool’s processing capability, the workpiece material’s performance, the processing procedure, cutting parameters, and other relevant factors.
The general principle for tool selection is to choose tools that are easy to install and adjust, have good rigidity, high durability, and precision. When meeting processing requirements, try to choose a shorter tool holder to enhance tool processing rigidity.
When selecting a tool, make sure that its size matches the surface size of the workpiece to be processed. In production, an end mill cutter is frequently used to process the peripheral contour of flat parts. For milling flat surfaces, a carbide blade milling cutter should be used. During high-speed milling, a convex cutter or groove cutter should be chosen. For rough surface machining or rough hole machining, a corn milling cutter with a cemented carbide blade can be used. Ball end milling cutters, ring milling cutters, conical milling cutters, and disc milling cutters are often used for machining some three-dimensional profiles and variable angle profiles.
When machining free-form surfaces (such as molds), the cutting speed of the ball end tool is zero at its end, so to maintain machining accuracy, the cutting row spacing is often set to a dense distance. As a result, ball end tools are often used for surface finishing. Flat end cutters are superior to ball end cutters in terms of surface machining quality and cutting efficiency. Therefore, whenever possible, flat end cutters should be preferred for both rough and finish machining of curved surfaces.
The durability and accuracy of cutting tools have a significant impact on their cost. It’s important to keep in mind that, although using high-quality cutting tools may increase the cost of cutting tools, it can significantly reduce the overall processing cost by improving processing quality and efficiency.
On a machining center, different cutting tools are mounted on the tool magazine, and tool selection and change are performed according to the program specifications at any time. To ensure quick and accurate installation of standard tools for drilling, boring, expanding, milling, and other processes, it’s necessary to use a standard tool holder.
Programmers should be familiar with the tool holder’s structural size, adjustment method, and adjustment range used on the machine tool, in order to determine the radial and axial dimensions of the tool during programming.
6. The principle of tool arrangement shall be followed
In the machining process of economical NC machine tools, manual grinding, measurement, and replacement of cutting tools often take up a lot of auxiliary time. Therefore, it’s important to arrange the cutting tool order efficiently. The following principles should be followed:
- Minimize the number of tools used.
- Once a tool is clamped, all machining steps it can perform should be completed.
- Tools for rough and finish machining should be used separately, even if they have the same size and specification.
- Milling should be performed before drilling.
- The surface should be finished first, followed by the two-dimensional contour.
- Whenever possible, the NC machine tool’s automatic tool change function should be used to improve production efficiency.
7. Principle of reasonable selection of cutting parameters
During rough machining, productivity is typically improved, but consideration should also be given to economy and processing cost. In semi-finishing and finishing, cutting efficiency, economy, and processing cost should be considered while maintaining processing quality. The specific values should be determined based on the machine tool manual, cutting parameter manual, and experience.
The following factors should be considered:
Cutting depth t: If the machine tool, workpiece, and tool rigidity allow, t is equal to the machining allowance, which improves productivity. A finishing allowance should be reserved to ensure machining accuracy and part surface roughness. NC machine tools may have a slightly lower finishing allowance than conventional machine tools.
Cutting width L: L is generally proportional to the tool diameter D and inversely proportional to the cutting depth. In the machining process of economical NC machine tools, L is generally in the range of L = (0.6 ~ 0.9) D.
Cutting speed v: Increasing v improves productivity, but it also affects tool durability. The choice of v mainly depends on tool durability, which decreases as v increases. The cutting speed also depends on the processing material. For example, when milling alloy 30CrNi2MoVA with an end milling cutter, v can be about 8 m/min, while milling aluminum alloy with the same end milling cutter, v can be over 200 m/min.
Spindle speed n (R/min): The spindle speed is generally selected based on the cutting speed v. The calculation formula is: v = πnd/1000. The NC machine tool control panel typically has a spindle speed adjustment (magnification) switch, which can adjust the spindle speed by an integral multiple during machining.
Feed speed vF: vF should be selected based on the machining accuracy and surface roughness requirements of the parts, as well as the cutting tool and workpiece materials. Increasing vF improves production efficiency. When the surface roughness requirement is low, vF can be larger. During processing, vF can also be adjusted manually through the adjustment switch on the machine tool control panel, but the maximum feed speed is limited by the equipment stiffness and feed system performance.