Chips, Annoying Pixies (Part 1)

Chip is the material that is stripped from the workpiece by the cutting tool during machining. Chips not only take away a large amount of cutting heat generated during processing, but also directly affect the processing process. Interested readers can extend to read the previous article of this official account "The Effect of Residual Chips in the Workpiece and Its Elimination and Discharge Method".

The author has encountered a very difficult chip breaking problem in his recent work. While consulting the data to find countermeasures, he also organizes what he sees and gains into an article to share with you.

 This article is the first part of the topic of chipping. It will introduce the formation and classification of chips conceptually. The next part will introduce some chip breaking methods and the author's practical experience.

1. The formation mechanism of chips

At the beginning, let’s talk about the chip formation mechanism and the concept of three cutting zones.

The formation process of chips is a plastic deformation process mainly caused by slip, which is produced by the cutting layer after being squeezed by the rake face of the tool. This has experienced three deformation zones:

The first deformation zone

When the rake face of the tool is pushed to the cutting layer, a stress field is generated in the cutting layer, and the closer it is to the cutting edge, the greater the stress. In the stress field, each point where the shear stress τ reaches the material yield strength τs can be found, and by connecting these points, the curve OA can be obtained. Since τ=τs here, the material to be cut starts to shear slip at the OA line, and the OA line is called the initial slip line.

Due to the strengthening phenomenon during plastic deformation, the stress τ must be continuously increased to continue the slip. Due to the extrusion of the blade, the shear stress it receives is constantly increasing, so the slip will continue to occur, and the slip will continue to increase.

When the flow direction of the material to be cut is parallel to the rake face, it will no longer slip, so OM is called the final slip line. The angle between the shear line in the shear zone and the free surface is 45°, and the width is generally 0.02 to 0.2 mm. Therefore, it can be regarded as a clipping plane, so it is called a clipping plane.

In summary, the material of the cutting layer becomes chips after passing through a shear deformation zone from OA to OM. This shear zone is commonly referred to as the I deformation zone. The size of the cutting deformation is mainly measured by the first deformation zone.

Deformation zone II

When the metal of the cutting layer turns into chips and flows out along the rake face after shear slippage, it is squeezed by the rake face to produce severe friction, which further deforms the chips, which forms the second deformation zone. The phenomena such as built-up edge and tool wear mainly depend on the deformation of the second deformation zone.

The third deformation zone

During the cutting process, the machined surface of the workpiece will be greatly deformed due to the extrusion and friction of the blunt part of the cutting edge and the flank, which is the third deformation zone. Due to the strong deformation, a work hardening layer will be formed on the surface of the workpiece, resulting in surface residual stress, and even accompanied by microscopic cracks, which will seriously affect the surface processing quality of the workpiece and the performance of the workpiece.

【Application Interpretation】

The above is the process of chip formation. Although the description of the three deformation zones is very abstract, it does condense the core of the metal cutting process:

Deformation zone I

It is the cutting deformation process, which corresponds to the "tool tip" in the geometric features of the tool. The sharper the tool tip, the smaller the cutting deformation. However, there is a dialectical relationship between the sharpness of the tool tip and the tool life. Although the sharp tool tip can reduce the amount of deformation, reduce the cutting force and reduce the cutting heat, it will also reduce the strength of the tool, which makes the tool easy to break. Due to the reduction of the wedge angle of the tool, the ability of the tool to dissipate heat will be reduced, so that the cutting heat will be concentrated on the tip of the tool, and the thermal damage of the tool will be accelerated.

Deformation zone II

It is the last but also the craziest stage when the chip leaves the tool: friction with the rake face. Readers who have read the previous "Tool Wear Topic" will remember that the crater wear is the unique wear pattern formed by the friction between the long chip material and the rake face of the tool. The rake face of the tool and its wear and chip breaking characteristics are optimized iteratively around the chips in this deformation zone.

Deformation zone III

It embodies the process of ironing the workpiece by the tool and forming the final surface. Because it is directly related to the quality of the machined surface, the tool wear standard also selects the wear amount of the flank as an indicator (here, the rake face is distressed for ten seconds). Therefore, this deformation zone is where the flank of the tool feature and its wiper and sharpening quality come into play.

Tool nose, rake face and flank face, these three features constitute the main features of metal cutting tools, and the three deformation zones have also become the basis of metal cutting principles. Readers who are interested in machining technology are requested to read further.

2.The classification of chips

After introducing the three microscopic deformation zones, let's introduce the macroscopic cutting classification.

a Ribbon chips

The appearance is fluffy and the roughness of the workpiece is low. Generally, when machining plastic metals, if the cutting thickness is small, the cutting speed is high, and the rake angle of the tool is large.

b Squeeze the chips

The appearance is serrated and the workpiece roughness is high. Generally, it occurs when the cutting speed is low, the rake angle of the tool is small, the cutting thickness is large, and the medium hardness plastic metal is processed.

c Unit chip

The workpiece surface quality is worse. When the shear stress in the extruded chip exceeds the strength limit of the metal, the crack occurs when the thickness of the chip is penetrated. 

d Break up chips

Generally, when processing metals with poor plasticity and low tensile strength, the cutting layer material often produces brittle cracks without plastic deformation. At this time, the cutting force fluctuates greatly and is concentrated on the cutting edge, which is easy to damage the tool. At the same time, the surface roughness of the workpiece is also high. Therefore, the generation of chipping chips should be avoided during production. The method is to reduce the cutting thickness and make the cutting Needle-shaped or flake-shaped, and at the same time appropriately increase the cutting speed to enhance the plasticity of the workpiece material.

【Application Interpretation】

Readers with organic machining experience have an intuitive understanding of chip morphology. Excluding the perspective of safety and environmental protection, everyone does not want to encounter ribbon chips and chipping chips. Generally speaking: we hope to break chips, but do not break them too much.

Banded chips usually wrap around the tool or remain in the workpiece (on), affecting subsequent automatic machining and measurement.

Chips are broken, although there is no worry about chip breaking, but there are concerns about impact. As mentioned in the previous topic on tool failure modes, the failure modes of tools include wear and damage. Generally speaking, they are end of life and accidents. Especially in occasions with large cutting impact such as intermittent machining, the cutting edge of the tool is subject to irregular damage. Shock, breakage can occur at any stage of tool life.

However, sometimes the damage and failure are dressed in the coat of wear: the impacted edge may not directly cause macro chipping, but in the form of micro chipping, resulting in a decrease in the sharpness of the cutting edge, and an increase in cutting force and cutting temperature, which makes microchipping. The wear rate of the chipped edge increases. If the micro chipping of the tool edge cannot be found in the early stage of the tool failure - this is really difficult, because there is no abnormality in the processing at this time - the tool will eventually show the appearance of wear failure .

For such problems, we need to use the means of big data analysis. Interested readers can refer to the previous article "Tool Mathematical Model Topic: Mining the Information Behind Tool Change Data", with the help of "tool change life distribution map", we can Discover discrete, irregular tool change life distributions due to impact damage.

This article preliminarily introduces the process and classification of chip formation, and interprets it based on the readers' own experience. In the next article, the author will share the found chip breaking methods, so stay tuned!