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ME Question of the day - Date 27 august 2020


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When the friction between tool and chip is high while machining ductile materials, some particles of chip adhere to the tool rake face near the tool tip. When such sizeable material piles up on the rake face, it acts as a cutting edge in place of the actual cutting edge. This is termed as built up edge (BUE). By virtue of work hardening, BUE is harder than the parent work material. As the size of BUE grows larger, it becomes unstable and parts of it get removed while cutting. The removed portions of BUE partly adhere to the chip underside and partly to the machined surface. This causes finished surface to be rough. However, since the cutting is being carried by the BUE and not the actual tool tip, the life of the cutting tool increases while cutting with BUE. That way BUE is not harmful while rough machining.
The conditions that normally induce the formation of BUE are low cutting speed, high feed and low rake angle. One of the prerequisites for the formation of BUE is the work hardenability of the workpiece material. Higher the work hardenability, rougher is the machined surface produced.
Chip if adheres to the cutting tool, because of the differential thermal properties can fracture the tool bit at the surface where it is contacting.
It increases cutting forces because the rake angle is masked by the BUE.
The broken segments of BUE adheres to workpiece and the tool thereby surface finish is affected.
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Regardless of tool being used in machining the chip forming process occurs by a mechanism known as plastic deformation. The deformation can be visualized as shearing. As the cutting tool engages with the work piece the material directly ahead of the tool is sheared and deformed under tremendous pressure. The deformed material then seeks to relieve its stress by fracturing and flowing into the space above the tool in the form of chip. As  a result of high temperature, pressure and frictional resistance formed against the flow of chip along the chip-tool interface , small particle tend to adhere at the edge of cutting tool while chip shears away, resulting to the formation of built of edges or BUE. As the cutting process continues more particles try to adhere to the cutting tool and larger BUE results. The BUE increase in size and become more unstable. Eventually a point is reached where fragments are torn off. Portions of these fragment break off and stick to both the chip and work piece.The build-up and breakdown of the built-up edge occur rapidly during a cutting action and cover the machined surface with a multitude of built-up fragments. These fragments to and score the machined surface, resulting in poor surface finish. We can sum up to following points to avoid BUE while machining:

i) BUE is more likely to be formed when the cutting speed is low. this is because at high cutting speed the metal moving away from the work piece becomes hot enough to recover before seizing on to the tool preventing the formation of BUE.

ii) BUE is also formed if the metal being cut is one that work-hardens and is reluctant to recover. A BUE will not form with pure metals since they do not work-harden much. Conversely, alloys, such as steel, do work-harden and recover less so they are prone to forming a BUE.

iii) Destiny Tool, through a combination of rake face geometry, carbide substrate and concentric tolerance is able to enable the chip to more readily "separate from itself" which not only improves MRR, but also reduced heat into the end mill and thereby extends tool life as the feed rate increases.

 

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On 8/27/2020 at 9:52 AM, admin said:

Why a built up edge on a tool is undesirable ?

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Beacause built up edge makes a extra cavity on tool which distruct the workpiece as well as unusual flaws comes on surface of workpiece.

It also degrade the accuracy of finishing of the product

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On 8/27/2020 at 9:52 AM, admin said:

Why a built up edge on a tool is undesirable ?

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The material in a built up edge is work-hardened and abrasive. When pieces break away from the cutting tool they stick to the workpiece, causing damage to the surface. The larger the BUE, the rougher the resulting surface.This leads to premature tool wear.

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On 8/27/2020 at 9:52 AM, admin said:

Why a built up edge on a tool is undesirable ?

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As it degrades machinability, surface finish, precision & accuracy and tool life. Also producing vibrations and marks on the work surface. It may also lead to tool failure.

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