What is the essence of metal cutting process?

Metal cutting theory is a theory about the change law of basic physical phenomena in metal cutting process, which is summarized from production practice and cutting experiments. These basic physical phenomena include: cutting deformation, cutting force, cutting temperature and tool wear. Learning and mastering these laws can improve the productivity and quality of cutting and reduce the production cost.

First, cutting deformation

Metal cutting process is essentially a process of producing chips and forming machined surfaces. Chip generation and surface formation are two closely related aspects in metal cutting. Cutting deformation is to discuss the cutting process from these two aspects. Therefore, learning cutting deformation is the basis of learning other physical phenomena.

First, the cutting method

When cutting, when the workpiece material is fixed, the shape of the generated chips and the characteristics of forming the machined surface are largely determined by the cutting method. Cutting methods are determined by the movement between the cutting edge of the tool and the workpiece, and are divided into three ways: right-angle cutting, oblique cutting and ordinary cutting.

Second, cut the deformed outline

(a) the basic form of the chip:

In metal cutting, due to the different workpiece materials, tool geometry and cutting parameters, all kinds of chips will appear. But from the point of view of deformation, it can be summarized into four basic forms (as shown in Figure 3-2).

1. The band chip is continuous, the bottom layer contacting the rake face is smooth, and the back is thatched.

2. The back of the extruded potato chips is serrated, and sometimes there are cracks on the inner surface.

3. When the unit chip cuts materials with high plasticity, such as lead, annealed aluminum and pure copper, the chip is easy to form adhesion on the rake face, and it is not easy to flow out, resulting in great deformation, which makes the material reach the fracture limit and form a large deformation unit, thus becoming this kind of chip.

4. When chips cut brittle materials such as cast iron and brass, flaky or granular chips will be formed.

When cutting, in the process of producing strip chips, the cutting force changes little, the cutting process is stable and the machined surface quality is good. However, the chips become long, which affects the normal work of the machine tool and the safety of workers, so the chip breaking measures should be taken. In the process of producing chips and unit chips, the cutting force fluctuates greatly, especially unit chips, which may produce vibration during its formation and affect the machining quality; When cutting cast iron, the roughness value of the machined surface becomes larger because the broken chips crack at the graphite boundary.

(B) debris accumulation tumor

In a certain cutting speed range, when cutting steel, 4 ~ 6 brass, aluminum alloy and other materials, a kind of deposit will appear on the rake face near the cutting edge to replace the cutting edge, which is called chip accumulation tumor. Because the chip tumor will change the actual rake angle when the tool works, when it changes and falls off, it will also affect the surface roughness and tool wear. Therefore, chip accretion is often studied as an important physical phenomenon in cutting. According to the experiment, the chip accumulation tumor has the following characteristics: the chemical properties are the same as the workpiece material; The hardness increase is about 2 ~ 4 times that of the base material; It is firmly combined with the rake face; There is a certain critical cutting temperature for disappearance or shedding; Recurrent instability, growth and shedding.

1. Occurrence and growth of exfoliated tumor The occurrence and growth model of exfoliated tumor was obtained by electronic scanning. When the chips flow out through the rake face, part of the metal in the stagnation layer of the second deformation zone is separated from the matrix under appropriate temperature and pressure conditions, and firmly adheres to the rake face, becoming the core of chip accumulation tumor. Bonding is a state in which metal atoms attract each other and combine within the range of their forces. The conditions are as follows: solubility of two metals; Bonding is metal bonding, necessary temperature and sufficient contact time. Temperature plays a decisive role in bonding. Due to this structure, due to work hardening, its hardness is much higher than that of chips. It can be seen that bonding and work hardening are necessary conditions for the growth of debris accumulation tumors.

2. The desquamation and disappearance of the chip-forming tumor When the cutting temperature rises to a certain critical value, the chip-forming tumor disappears, which is about .500 ~ 600℃, which is equivalent to the recrystallization temperature of metal materials of 560℃. At this time, due to high temperature, the ductility of metal increases, work hardening disappears, deposits soften and are taken away by chips, and chips fall off or disappear. Therefore, the main reason of chip falling off and disappearing is also the cutting temperature.

In the actual cutting process, due to the influence of various factors, the cutting temperature will change constantly, so even under the same cutting conditions, the chip accumulation tumor is in an unstable state, sometimes growing and sometimes falling off.

3. Measures to control debris accumulation tumor. Chip accumulation tumor replaces cutting edge, which can protect cutting edge and increase the actual rake angle of tool, which is its advantage; However, the irregular and unstable shape of the chip-accumulating tumor directly affects the roughness of the machined surface, and when cutting with cemented carbide tools, the chip-accumulating tumor may peel off cemented carbide particles and aggravate tool wear, which is its shortcoming. It can be said that chip accumulation does more harm than good to cutting. Therefore, when finishing, it is necessary to avoid the generation of chip accumulation tumors; Even in rough machining, such as using cemented carbide tools, it is not desirable to produce debris accumulation tumors. But if we can master its laws, control and use them, we can also turn disadvantages into advantages, such as silver-white chip turning tools. Usually, the following measures are mainly adopted for control:

1) reduces the ductility of the material, improves the hardness and reduces the formation of stagnation layer;

2) control the cutting speed u to control the cutting temperature:

At low speed (below 10m/ rain), due to the low temperature (below 300℃), it will not cause adhesion and debris accumulation tumor, so it will not form debris accumulation tumor. Usually, small surface roughness can be obtained by fine turning threads with high-speed steel cutter or reaming holes with reamer at low speed.

At high speed (above 100 m/min), due to the high temperature (above 500 ~ 600℃), the work hardening of the debris accumulation tumor disappears, and the debris accumulation tumor disappears. Small surface roughness can also be obtained by high-speed cutting.

At a medium speed (cutting medium carbon steel at a speed of 20 ~ 30m/min), the chip accumulation is the largest and the surface roughness is the largest at a suitable temperature (about 300 ~ 400℃).

Or by using cutting fluid and increasing the rake angle (γ). & gt35 degrees), reducing the cutting thickness and other methods to reduce or even eliminate chip accumulation tumor.

5. Welding components should try to choose profiles.

In a welded structure, welding elements are usually welded together. If the profile can be selected reasonably, the welding process can be simplified and the deformation of the weldment can be effectively prevented. The weldment shown in Figure 3-24a is made of three steel plates with four welds. Figure 3-24b shows that the same weldment is assembled by two channel steels, and the welding process can be simplified and welding deformation can be reduced only by welding in sections at the joint. If the appropriate I-beam can be selected, the welding process can be completely omitted.