Chaper 2 metal Removal Methods.

 
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Metal Removal

Cutting-Tool Materials

Metal Removal Methods

Machinability of Metals

Single Point machining

Turning Tools and Operations

Turning Methods and Machines

Grooving and Threading

Shaping and Planing

Hole Making Processes

Drills and Drilling Operations

Drilling Methods and Machines

Boring Operations and Machines

Reaming and Tapping

Multi Point Machining

Milling Cutters and Operations

Milling Methods and Machines

Broaches and Broaching

Saws and Sawing

Finishing Processes

Grinding Wheels and Operations

Grinding Methods and Machines

Lapping and Honing

2.1 Introduction

Even with all of the sophisticated equipment and techniques used in today's modern industry, the basic mechanics of forming a chip remain the same. As the cutting tool engages the workpiece, the material directly ahead of the tool is sheared and deformed under tremendous pressure. The deformed material then seeks to relieve its stressed condition by fracturing and flowing into the space above the tool in the form of a chip.

2.2 Cutting Tool Forces

A general discussion of the forces acting in metalcutting is presented by using the example of a typical turning operation. When a solid bar is turned, there are three forces acting on the cutting tool:

Tangential Force: This acts in a direction tangential to the revolving workpiece and represents the resistance to the rotation of the workpiece. In a normal operation, tangential force is the highest of the three forces and accounts for about 98 percent of the total power required by the operation.

Longitudinal Force: Longitudinal force acts in the direction parallel to the axis of the work and represents the resistance to the longitudinal feed of the tool. Longitudinal force is usually about 50 percent as great as tangential force. Since feed velocity is usually very low in relation to the velocity of the rotating workpiece, longitudinal force accounts for only about 1 percent of total power required.

Radial Force: Radial force acts in a radial direction from the center line of the workpiece. The radial force is generally the smallest of the three, often about 50 percent as large as longitudinal force. Its effect on power requirements is very small because velocity in the radial direction is negligible.

2.3 Chip Formation and Tool Wear

Regardless of the tool being used or the metal being cut, the chip forming process occurs by a mechanism called plastic deformation. This deformation can be visualized as shearing. That is when a metal is subjected to a load exceeding its elastic limit. The crystals of the metal elongate through an action of slipping or shearing, which takes place within the crystals and between adjacent crystals.

Most practical cutting operations, such as turning and milling, involve two or more cutting edges inclined at various angles to the direction of the cut. However, the basic mechanism of cutting can be explained by analyzing cutting done with a single cutting edge.

Chip formation is simplest when a continuous chip is formed in orthogonal cutting. In oblique cutting, a single, straight cutting edge is inclined in the direction of tool travel. This inclination causes changes in the direction of chip flow up the face of the tool. When the cutting edge is inclined, the chip flows across the tool face with a sideways movement that produces a helical form of chip.

2.3.1 Chip Formation

Metalcutting chips have been classified into three basic types:

Discontinuous Chip - Type 1: Discontinuous or segmented chips are produced when brittle metal such as cast iron and hard bronze are cut or when some ductile metals are cut under poor cutting conditions. As the point of the cutting tool contacts the metal, some compression occurs, and the chip begins flowing along the chip-tool interface. As more stress is applied to brittle metal by the cutting action, the metal compresses until it reaches a point where rupture occurs and the chip separates from the unmachined portion. This cycle is repeated indefinitely during the cutting operation, with the rupture of each segment occurring on the shear angle or plane. Generally, as a result of these successive ruptures, a poor surface is produced on the workpiece.

Continuous Chip - Type 2: The Type 2 chip is a continuous ribbon produced when the flow of metal next to the tool face is not greatly restricted by a built-up edge or friction at the chip tool interface. The continuous ribbon chip is considered ideal for efficient cutting action because it results in better finishes. Unlike the Type 1 chip, fractures or ruptures do not occur here, because of the ductile nature of the metal.

Continuous Chip with a Built-up Edge (BUE) - Type 3: The metal ahead of the cutting tool is compressed and forms a chip which begins to flow along the chip-tool interface. As a result of the high temperature, the high pressure, and the high frictional resistance against the flow of the chip along the chip-tool interface, small particles of metal begin adhering to the edge of the cutting tool while the chip shears away. As the cutting process continues, more particles adhere to the cutting tool and a larger build-up results, which affects the cutting action. The built-up edge increases in size and becomes more unstable. Eventually a point is reached where fragments are torn off. Portions of these fragments which break off stick to both the chip and the workpiece. 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 adhere to and score the machined surface, resulting in a poor surface finish.

Shear Angle: Certain characteristics of continuous chips are determined by the shear angle. The shear angle is the plane where slip occurs to begin chip formation.

Regardless of the shear angle, the compressive deformation caused by the tool force against the chip will cause the chip to be thicker and shorter than the layer of workpiece material removed. The work or energy required to deform the material usually...

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