In metal cutting, some chips are rolled into a spiral shape and break on their own when they reach a certain length; some chips are broken into C-shaped or 6-shaped shapes; some are rolled up like clockwork; and some are broken into needles or small pieces. , splashing everywhere, affecting safety; some strip-shaped chips are wrapped around the tool and workpiece, which can easily cause accidents. Poor chip removal status will affect the normal progress of production.
Real masters can judge the processing status based on the shape of chips. Today we will share some theoretical information.
What chips can tell you:
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Factors affecting chips
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1. Workpiece material
The alloy elements, hardness, and heat treatment status of the workpiece material affect chip thickness and chip curl. Mild steel forms larger chips than hard steel; hard steel is less likely to curl than mild steel; the thickness of chips that are not easily curled is thin; but when the thickness of mild steel chips is too large, it is not easy to curl. At the same time, the shape of the workpiece is also an important influencing factor.
2. Geometric parameters of tool cutting area
Reasonable geometric parameters of the 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 an optimal value for different materials being processed; the main declination angle directly affects the chip thickness and width, and a large main declination angle is easy to break the chip; the tool tip arc radius is related to the chip thickness and width as well as chip flow. Direction, a small arc radius is suitable for finishing, and a large radius is suitable for roughing.
The width of the chip breaker should be selected in proportion to the feed amount. If the feed amount is small, choose a narrow one. If the feed amount is large, choose a wide one. The depth of the chip breaker should be inversely proportional to the feed amount. If the feed amount is small, choose a deep one. Choose a shallow feed.
3. Cutting amount
The three factors of cutting amount will limit the chip breaking range. The factors that have a greater impact on chip breaking are the feed rate and the amount of back cutting, while the cutting speed has the least impact on chip breaking within the conventional cutting speed. The feed amount is proportional to the chip thickness; the amount of back cutting is proportional to the chip width; the chip speed is inversely proportional to the chip thickness. As the cutting speed increases, the effective chip breaking range becomes narrower.
4. Machine tools
Modern CNC machine tools use the NC editing function to periodically change the feed amount to achieve the purpose of forced chip breaking, which is often called "programmed chip breaking". This method has high chip breaking reliability, but low cutting economy. It is often used in processes where chips are difficult to break by other methods, such as deep grooves on the end face of turning machines.
5. Cooling and lubrication status
With the addition of cutting fluid, the effective chip breaking range becomes wider, especially the chip breaking at small feeds is prone to curling. Using the high pressure of cutting fluid to break and remove chips is an effective method in some processing methods. For example, in deep hole machining, high-pressure cutting fluid can discharge chips from the cutting area.
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The formation process of chip shape
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The formation process of ribbon chips can be divided into three stages:
1. Basic deformation stage: the deformation of the chips occurs when the cutting layer metal and the cutting edge of the tool begin to contact and turn into chips and separate from the workpiece material;
2. Curl deformation stage: upward curl, lateral curl, tapered curl in both A and B directions;
3. Additional deformation and fracture stages.
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Classification of chips
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Due to different workpiece materials, cutting conditions vary. The shapes of chips generated during the cutting process are diverse. The shapes of chips are mainly divided into four types: ribbon, nodule, granular and crushed, as shown in the figure.
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1. Strip chips
This is the most common type of chip. Its inner surface is smooth and its outer surface is hairy. When processing plastic metals, such chips are often formed under working conditions of small cutting thickness, high cutting speed, and large tool rake angle. Its cutting process is balanced, the cutting force fluctuates less, and the machined surface roughness is smaller.
2. Nodular chips
Also called squeeze chips. Its outer surface is jagged and its inner surface is sometimes cracked. Such chips often occur when the cutting speed is low, the cutting thickness is large, and the tool rake angle is small.
3. Granular chips
Also called unit chips. During the chip formation process, if the shear stress on the shear surface exceeds the fracture strength of the material, the cracks will expand to the entire surface, and the chip units will fall off from the material being cut, forming granular chips. As shown in Figure c.
The above three types of chips are only possible when processing plastic materials. Among them, the cutting process of band-shaped chips is the smoothest, and the cutting force of unit chips fluctuates the most. The most common ones in production are strip chips, sometimes cracked chips are obtained, and unit chips are rare. If the conditions for chip extrusion are changed, such as further reducing the tool rake angle, reducing the cutting speed, or increasing the cutting thickness, unit chips can be obtained. On the contrary, strip-shaped chips can be obtained. This shows that the shape of the chips can be transformed according to the cutting conditions. By mastering its changing rules, you can control the deformation, shape and size of chips to achieve the purpose of chip curling and chip breaking.
4. Crushing chips
This is chips from brittle materials. The shape of this chip is irregular and the machined surface is uneven. From the perspective of the cutting process, the chip deforms very little before breaking, and the chip formation mechanism is different from that of plastic materials. Its brittle fracture is mainly due to the stress on the material exceeding its tensile limit. Such chips are often obtained when processing brittle and hard materials, such as high-silicon cast iron, white iron, etc., especially when the cutting thickness is large. Because its cutting process is very unstable, it is easy to damage the tool and damage the machine tool, and the machined surface is rough, so it should be avoided in production. The method is to reduce the cutting thickness so that the chips become needle-shaped or flake-shaped; at the same time, increase the cutting speed appropriately to increase the plasticity of the workpiece material.
The above are four typical types of chips, but the shapes of chips obtained at the processing site are diverse. In modern cutting processing, the cutting speed and metal removal rate have reached very high levels, and the cutting conditions are very harsh, often producing a large amount of "unacceptable" chips.
Appropriate measures should be taken during cutting to control the curl, flow and breakage of chips to form an "acceptable" good chip shape. The most widely used chip control method in actual machining is to grind a chip breaker on the rake face or use a pressure block chip breaker.
We often encounter some chip breaking situations during processing. Chip entanglement not only affects processing, but sometimes requires special shutdown for processing, resulting in low efficiency. Let’s learn about its chip breaking skills.
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Chip handling principles and methods
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1. After increasing the feed, the chips become thicker, which is beneficial to chip breaking.
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2. The radius of the tool tip fillet becomes smaller and the chip thickness increases, which is beneficial to chip breaking.
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3. Reduce rake angle
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Chip compression ratio = hc /h. The larger the compression ratio, the easier it is to break chips, but at the same time, the cutting resistance also increases. The compression ratio is related to the linear speed Vc. When Vc decreases, the compression ratio increases, so reducing the linear speed is also beneficial to chip breaking. The rake angle is reduced, the chip deformation is large, and the compression ratio is increased, which is beneficial to chip breaking.
4. Adopt sharp edge treatment form
As can be seen from the figure below, under the same feeding conditions, the blade edge is blunt and sharp, which is beneficial to chip breaking.
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Increasing the main deflection angle will make the chips thicker and facilitate chip breaking:
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5. Protruding chip breaker
Promote chip breaking: Rubbing against the protrusions of the chip breaker creates dents on the surface of the chip, and the chip thickness increases significantly → it promotes chip breaking and is highly damaging.
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Strong damage resistance: the contact area with the chips becomes smaller, and the chips are smoothly discharged due to smooth contact with the protrusions → less damage to the tool.
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6. The curl radius of chips becomes smaller
