Knowledge contained in machining chips

In metal cutting, some chips are rolled into spiral shape and break automatically when they reach a certain length; Some chips are broken into C-shape and 6-shape; Some of them are in the shape of strips; Some are broken into needles or small pieces, splashing around, affecting safety; Some banded chips are wrapped around the tool and workpiece, which is easy to cause accidents. Poor chip removal will affect the normal production.

Factors affecting chip

1. Workpiece materials
The alloy elements, hardness and heat treatment state of workpiece materials affect the chip thickness and chip curl. The thickness of chips formed by soft steel is larger than that of hard steel; Hard steel is less prone to curl than mild steel; The thickness of chips that are not easy to curl is thin; But when the chip thickness of mild steel is too large, it is not easy to curl. At the same time, the shape of the workpiece is also an important factor.
2. Geometric parameters of tool cutting area
Reasonable geometric parameters of tool cutting area are the most commonly used methods to improve the controllability of chip formation and the reliability of chip breaking.
The rake angle is inversely proportional to the chip thickness and has the best value for different processed materials; The main deflection angle directly affects the chip thickness and width, and the chip is easy to break when the main deflection angle is large; The arc radius of the tool tip is related to the chip thickness and width and the direction of chip flow. Small arc radius is suitable for finishing and large radius is suitable for rough machining.
The width of the chip breaker is selected in proportion to the feed rate, with narrow feed rate and wide feed rate; The selection of chip breaker depth is inversely proportional to the feed rate, with small feed rate selecting deep and large feed rate selecting shallow.
3. Cutting parameters
The three factors of cutting parameters will limit the chip breaking range. The feed rate and back-cutting rate have great influence on chip breaking, while the cutting speed has the least influence on chip breaking within the conventional cutting speed. The feed rate is proportional to the chip thickness; The amount of back-eating knife is proportional to the chip width; The chip speed is inversely proportional to the chip thickness, which increases the cutting speed and narrows the effective chip breaking range.
4. Machine tools
Modern CNC machine tools use NC editing function to change the feed periodically to achieve the purpose of forced chip breaking, which is usually called “program-controlled chip breaking”. This method has high chip breaking reliability, but low cutting economy. It is often used in processes that are difficult to break chips by other methods, such as circular deep grooves on the end face of the car.
5. Cooling and lubrication state
With the addition of cutting fluid, the effective chip breaking range is widened, especially in the case of small feed chip breaking, which is easy to curl. It is an effective method in some machining methods to use the high pressure of cutting fluid to break and discharge chips. For example, in deep hole machining, high pressure cutting fluid can discharge chips from the cutting area.

Formation process of chip shape

The formation process of banded chips can be divided into three stages:
1. Basic deformation stage: the deformation caused by the chip when the cutting layer metal and the cutting edge of the tool start to contact and become chips and leave the workpiece material;
2. Curling deformation stage: upward curling, lateral curling, and conical curling in both direction A and direction B;
3. Additional deformation and fracture stage.

Classification of chips

Due to different workpiece materials and different cutting conditions, the chip shapes generated in the cutting process are various. Chip shapes are mainly divided into four types: banded, nodular, granular and crumbling.

1. Strip chip
This is the most common kind of chip. Its inner surface is smooth and its outer surface is fluffy. When processing plastic metals, such chips are often formed under the working conditions of small cutting thickness, high cutting speed and large rake angle of tools. Its cutting process is balanced, the fluctuation of cutting force is small, and the machined surface roughness is small.
2. Nodular chip
Also known as crush chip. Its outer surface is serrated and its inner surface is sometimes cracked. This kind of chip often occurs when the cutting speed is low, the cutting thickness is large and the rake angle of the tool is small.
3. Granular chips
Also known as unit chip. In the process of chip formation, if the shear stress on the shear surface exceeds the fracture strength of the material, the crack spreads to the whole surface, and the chip unit falls off from the cut material to form granular chips. As shown in figure C.
The above three kinds of chips can only be obtained when processing plastic materials. Among them, the cutting process of strip chip is the most stable, and the cutting force fluctuation of unit chip is the largest. The band chip is the most common chip in production, and sometimes the chip is crushed, but the unit chip is rare. If the conditions of chip extrusion are changed, such as further reducing the rake angle of the tool, reducing the cutting speed or increasing the cutting thickness, the unit chip can be obtained. On the contrary, banded chips can be obtained. This shows that the shape of chips can be transformed with cutting conditions. By mastering its changing law, the deformation, shape and size of chips can be controlled to achieve the purpose of chip rolling and chip breaking.
4. Break chips
This is the chip of brittle material. The shape of this chip is irregular, and the machined surface is scraggy. From the cutting process, the chip deformation before fracture is very small, which is different from the chip formation mechanism of plastic materials. Its brittle fracture is mainly due to the stress of the material exceeding its tensile limit. Processing brittle and hard materials, such as high-silicon cast iron, white iron, etc., especially when the cutting thickness is large, such chips are often obtained. Because its cutting process is very unstable, it is easy to destroy the tool and the machine tool, and the machined surface is rough, so it should be avoided in production. The method is to reduce the cutting thickness and make the chips needle-like or flaky; At the same time, the cutting speed is appropriately increased to increase the plasticity of the workpiece material.
The above are four kinds of typical chips, but the shapes of chips obtained at the processing site are various. In modern cutting, the cutting speed and metal removal rate have reached a high level, and the cutting conditions are very bad, which often produces a lot of “unacceptable” chips.
Appropriate measures are taken in cutting to control the curling, outflow and breaking of chips, so as to form an “acceptable” good chip shape. The most widely used chip control method in practical machining is to grind a chip breaker on the rake face or use a briquette chip breaker.

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