We all enjoy the crisp flavor filled chips, be it of potato or banana. Did you know there are chips in machining process as well?
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Chips are a byproduct of any machining or cutting process.
In the process of machining when the tool is performing a cutting operation, e.g. turning, then some material from the workpiece is gradually removed and the desired profile is created. The material which is getting removed is nothing but the machining chip formed.
Chips are formed from tearing and shearing of the workpiece material.
During tearing, compression of the workpiece material adjacent to the cutting tool face occurs, creating a crack that runs alongside the cutting tool and towards the workpiece body. Through this process, the chip gets highly deformed whereas the workpiece material remains under-formed. While the cutting is happening intermittently, the workpiece material does not move over the tool face.
In case of chip formation via shearing, the metal at the tip of the cutting tool is under severe stress as the tool is moved ahead. This causes internal shearing action in the workpiece material. The shearing happens in such a way that the cutting edge yields and flows plastically in the form of chip.
When the compression limit of the metal under the tool edge has been exceeded, there is first a compression of the metal under the tool edge, followed by a separation of that same metal. In chip formation by sheaaar, there is general movement of the chip over tool face.
Types of chips formed
As the name suggests, the chips are in the form of either a long string or bend into a roll like a spring without any fracture. A continuous chip is generally produced when cutting of ductile materials like aluminum and low carbon steel. The chip at time can get so long that comes in contact with the workpiece after spiraling back. This could affect the finishing of the surface hence a chip breaker is used in such cases. High back rake angle favors the continuous chip by providing resistance to chip flow.
In the process of machining, the deformed material rapture ahead of the tool when compressive force reaches the fracture limit of the material. The discontinuous chips are the fragments of these plastically deformed workpieces. These can be loosely connected or entirely separated from the workpiece. The material having low ductility such as brass has this type of plastic chip formation.
The majority of the heat generated due to friction between the workpiece and the tool is carried out by the chip. This improves the life of the tool. For brittle material, discontinuous chip gives a fair surface finish, lower power consumption and increase tool life. But in ductile materials, it causes a poor surface finish and excessive tool wear. Discontinuous chips are convenient to handle and dispose off.
Built Up Edge
The built up edge (BUE) occurs when an accumulation of material is present against the rake face and seizes to the tool tip, thus separating it from the chip.
At the initial contact of the tool with the surface, the shear force is strongest. This causes the first layer of the workpiece to seize on it work-hardens more than the remaining metal. Due to this work hardening, the first layer becomes stronger than the adjacent metal moving away from the workpiece. As a result, the hardened layer gets welded to the tip of the tool. And with the repetition of the process, a built up edge is formed on the tool.
Is Built Up Edge good?
The built up edge gets welded on the tip of the tool which effectively changes the tool geometry and rake steepness. The area of contact between the chip and the cutting tool is decreased. This leads to (1) reduction in the power demand of the cutting operation and (2) increase in tool life as the cutting is done by the built up edge instead of the tool itself.
But this is not all, Built up edges have negative impact on the quality of the workpiece.
A work piece undergoing excessive work hardening.
Bad surface finish, since pieces of the BUE eventually break off and stick to the workpiece. These broken pieces can become problematic because, due to the work hardening, they become extremely hard and therefore abrasive.
A reduction in the dimensional control of the process, due to the dynamically changing geometry of the cutting tool.