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CNC cutting

2025-07-09

Cutting refers to the use of cutting tools (including tools, grinding tools and abrasives) to cut off the excess material layer on the blank or workpiece into chips, so that the workpiece obtains the specified geometric shape, size and surface quality.

Introduction

Any cutting process must have three basic conditions: cutting tools, workpieces and cutting motion, which are called the three elements of cutting. The cutting tool should have a cutting edge, and its material must be harder than the workpiece. Different tool structures and cutting motion forms constitute different cutting methods. The cutting methods with tools with fixed blade shape and number of blades include turning, drilling, boring, milling, planing, broaching and sawing; the cutting methods with grinding tools or abrasives with unfixed blade shape and number of blades include grinding, grinding, honing and polishing.

Cutting is a processing method that uses the relative movement of the cutting tool and the workpiece to remove the excess metal layer from the blank (castings, forgings, profiles, etc.) to obtain machine parts with dimensional accuracy, shape and position accuracy, and surface quality that fully meet the requirements of the drawing. Most of the parts processed by casting, forging and welding are blanks of parts, which are rarely used directly on the machine. Further cutting is required to meet the technical requirements of the parts. Cutting is divided into two parts: mechanical processing (referred to as machining) and benchwork. Mechanical processing refers to the cutting of parts by workers operating machine tools.

Cutting is the most important processing method in mechanical manufacturing. Although the manufacturing precision of blanks is constantly improving, and the processing technologies such as precision casting, precision forging, extrusion, and powder metallurgy are increasingly widely used, cutting still occupies an important position in mechanical manufacturing due to its wide range of adaptability and ability to achieve high precision and low surface roughness.

Classification of cutting

There are many classification methods for cutting of metal materials. The following three are common.

1. Distinguish by process characteristics

The process characteristics of cutting are determined by the structure of the cutting tool and the relative motion form between the cutting tool and the workpiece. According to the process characteristics, cutting processing can generally be divided into: turning, milling, drilling, boring, reaming, planing, slotting, broaching, sawing, grinding, honing, super finishing, polishing, gear processing, worm gear processing, thread processing, ultra-precision processing, benchwork and scraping, etc.

2. According to the removal rate and accuracy

It can be divided into: ① Roughing: using a large cutting depth, most or all of the machining allowance is cut off from the workpiece through one or a few passes, such as rough turning, rough planing, rough milling, drilling and sawing, etc. Roughing has high processing efficiency and low processing accuracy. It is generally used as pre-processing and sometimes as final processing.

② Semi-finishing: Generally used as an intermediate process between roughing and finishing, but it can also be used as final processing for parts on the workpiece that do not require high accuracy and surface roughness. ③ Finishing: Use fine cutting to make the machined surface achieve higher accuracy and surface quality, such as fine turning, fine planing, fine reaming, fine grinding, etc. Finishing is generally the final processing.

④ Finishing: It is carried out after finishing, and its purpose is to obtain a smaller surface roughness and slightly improve the accuracy. The machining allowance of finishing is small, such as honing, grinding, superfinishing and superfinishing. ⑤ Modification: The purpose is to reduce the surface roughness to improve the corrosion resistance, dustproof performance and appearance, but it does not require improved accuracy, such as polishing, sanding, etc.

⑥ Ultra-precision machining: In cutting-edge technology fields such as aerospace, laser, electronics, and nuclear energy, some particularly precise parts are required, with an accuracy of IT4 or above and a surface roughness of no more than Ra 0.01 microns. This requires special measures for ultra-precision machining, such as mirror turning, mirror grinding, and mechanical chemical polishing with soft abrasive particles.

3. Distinguish by surface formation method

During cutting, the machined surface of the workpiece is obtained by the relative movement of the cutting tool and the workpiece. According to the surface formation method, cutting can be divided into 3 categories. ① Tool tip trajectory method: The surface geometry required by the workpiece is obtained by relying on the trajectory of the tool tip relative to the workpiece surface, such as turning the outer circle, planing the plane, grinding the outer circle, and turning the forming surface with a template. The trajectory of the tool tip depends on the relative movement of the cutting tool provided by the machine tool and the workpiece.

② Forming tool method: referred to as the forming method, the forming surface is machined with a forming tool or a forming grinding wheel that matches the final surface contour of the workpiece. At this time, part of the forming movement of the machine tool is replaced by the geometry of the blade, such as forming turning, forming milling and forming grinding. Since the manufacturing of forming tools is relatively difficult, the cutting force that the process system formed by the machine tool-fixture-workpiece-tool can withstand is limited, and the forming method is generally only used to process short forming surfaces.

③ Development method: also known as rolling cutting method, during processing, the cutting tool and the workpiece perform relative development movement, the instantaneous center line of the tool (or grinding wheel) and the workpiece rolls purely against each other, and a certain speed ratio relationship is maintained between the two. The obtained machined surface is the envelope surface of the blade in this movement. Gear hobbing, gear shaping, gear shaving, gear honing and gear grinding (excluding profile grinding) in gear processing are all processing by the development method.

Types of cutting processing

Increasing the cutting amount to increase the material removal rate is the basic way to improve the efficiency of cutting processing. Commonly used high-efficiency cutting processing methods include high-speed cutting, strong cutting, plasma arc heating cutting and vibration cutting.

1. High-speed cutting

Generally refers to the cutting processing that can be achieved by using carbide tools. Cutting with a grinding speed of more than 45 meters per second is called high-speed grinding. The use of high-speed cutting (or grinding) can not only improve efficiency, but also reduce surface roughness. The cutting speed of ordinary steel can reach 200 meters per minute with carbide tools; 500 meters per minute with ceramic tools; and 900 meters per minute with diamond tools for turning non-ferrous metals.

The speed of ultra-high-speed cutting tested in the laboratory can reach more than 4000 meters per minute. Since the 1960s, the grinding speed has gradually increased from about 30 m/s to 45, 60, 80 and even 100 m/s; the grinding speed in the laboratory has reached 200 m/s. High-speed cutting (or grinding) requires the machine tool to have a high speed, high rigidity, high power and a process system with good vibration resistance; the tool is required to have reasonable geometric parameters and convenient fastening methods, and a safe and reliable chip breaking method must also be considered.

2. Powerful cutting

Refers to cutting with large feed or large cutting depth, generally used for turning and grinding (see slow feed grinding). The main feature of powerful turning is that in addition to the main cutting edge, the turning tool also has a secondary cutting edge parallel to the machined surface of the workpiece, which participates in cutting at the same time, so the feed rate can be increased by several times or even dozens of times compared to general turning. On general machine tools, as long as the power is sufficient and the process system rigidity is good, powerful cutting can be implemented. Compared with high-speed cutting, the cutting temperature of powerful cutting is lower, the tool life is longer, and the cutting efficiency is higher; the disadvantage is that the machined surface is rougher. During strong cutting, the radial cutting force is very large, so it is not suitable for processing slender workpieces.

3. Plasma arc heating cutting

The cutting method that uses the high temperature of the plasma arc to instantly heat the local cutting area of the workpiece to 800-900℃ often uses ceramic tools and is suitable for processing large parts. When cutting, the heating intensity of the plasma arc should be adjusted according to the material, size, cutting speed, cutting depth and feed rate of the workpiece. After proper adjustment, the temperature of the processed surface of the workpiece can be kept below 150℃ without causing metallographic changes. This method is suitable for processing hardened workpieces and difficult-to-process metal materials. The material removal rate can be increased by 2-20 times, and the cost can be reduced by 30-85%.

4. Vibration cutting

Cutting with low-frequency or high-frequency vibration along the tool feed direction can improve cutting efficiency. Low-frequency vibration cutting has a good chip breaking effect, and the chip breaking device can be omitted, which increases the blade strength. The total power consumption during cutting is reduced by about 40% compared with ordinary cutting with a chip breaking device. High-frequency vibration cutting, also known as ultrasonic vibration cutting, helps to reduce the friction between the tool and the workpiece, lower the cutting temperature, and reduce the adhesive wear of the tool, thereby improving the cutting efficiency and the quality of the machined surface, and the tool life can be increased by about 40%.