CHAPTER 1

CHAPTER 4MACHINING PROCESSES4.1Basic principles

The process of metal removal goes back many years. It is a process in which a wedge-shaped tool engages a work piece to remove a layer of material in the form of a chip . Even with all of the sophisticated equipment and techniques used in todays modern industry, the basic mechanics of forming a chip remain the same. As the cutting tool engages the work piece, the material 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. Machining is the broad term which describes the removal of material from a work piece.4.1.1 Advantages of machining More dimensional accuracy may be required than the accuracy provided by casting, shaping, or in some forming process alone.Parts may have external and internal profiles that cannot be produced by forming or shaping processes.Heat treated parts may undergo distortion and require additional finishing operations such as grinding.Special surfaces or textures may be needed.4.1.2 Disadvantages of Machining Removal processes waste material. Require more energy and labor than forming and shaping operations. Removing a volume of material is time consuming. Improper techniques can affect the surface quality or properties of the product.4.1.3 Variables in the Cutting Process

The quality resulted from any machining operation depends on: Tool material and its condition. Tool shape, surface finish, and sharpness. Work piece material, condition, and temperature. Cutting conditions (such as speed, feed and depth of cut). Type of the cutting fluid.

Changing any of the above variables will affect:

Type of chip produced. Force and energy dissipated in the cutting process. Temperature rise in the work piece. Wear and failure of the tool. Surface finish of the produced part.

4.1.4 Cutting Conditions

The main Cutting Motions for Metal Cutting Processes are:Main cutting motion (R) This is represented in either Linear cutting speed in m/min~ or in rotational cutting speed in rpm Feed motion (S) This is represented in either mm/min or in mm/rev.Depth of Cut (a) This is represented in mm.

Fig. 4.1: Main cutting motions in several cutting processes

4.1.5 Cutting Tool Materials

Cutting tools must have the following characteristics in order to produce the required quality and economical parts:Hardness (particularly at elevated temperatures).Toughness, so that impact forces will not chip the tool.Wear resistance for a long tool life.Chemical stability and inertness.

Some types of the Tool Materials Carbon Steels High Speed Steel (HSS) Tungsten Carbide Ceramics Diamonds Cemented Oxides

4.2 Turning Operations

Drilling

DrillingOne of the most important and essential tools in any metalworking shop is the drilling machine or drill pressit is often used for reaming, boring, tapping, counter boring, countersinking, and spot facing. DrillingAll drilling machines operate on the same basic principle. The spindle turns the cutting tool, which is advanced either by hand or automatically into a work piece that is mounted on the table or held in a drill press vise.

Types of drill presses

1- General types of drill presses

Types of drill presses

2- Vertical or Pillar 3- Radial Arm

Types of drill presses

4- Gang Type

Types of drill presses

5- Multi Spindle 6- Numerical Control Type

Drill NomenclatureThe most important type of drill is the twist drill. The important nomenclature listed below

twist drill parts.Typical drill press operations

4.4 Milling

MillingMilling is the process of machining flat, curved, or irregular surfaces by feeding the work piece against a rotating cutter containing a number of cutting edges.The milling machine consists basically of a motor driven spindle, which mounts and revolves the milling cutter, and a reciprocating adjustable worktable, which mounts and feeds the work piece.Milling machinesvertical or horizontal.

Types of milling machinesKnee and ColumVertical2.HorizontalHorizontal with vertical head attachmenUniversal (table rotates in plan view) and is used for helical millingRam & Turret - Light weight machine tool with slotter on one end of turret. No power feeds.Special Purpose - For production usage. Usually more rigid construction Milling CuttersMilling cutters are usually made of high-speed steel and are available in a great variety of shapes and sizes for various purposes.The basic types includeEnd Mills - The face and sides at the bottom end of this tool are used for plunge cutting (two flutes) and side and end cuts (four flute).Plain - These mills are used to cut with the sides only. They are generally mounted on an arbor. Side or Side & Face

Milling CuttersFace - This cutter is held on a spindle nose.

Up-Cut Milling & Down - Cut Milling

Up-Cut Milling Down - Cut Milling

Down - Cut Milling This method has advantages,The cutter forces are directed into the table, which reduces fixture forces, and allows thinner work piecesThere is less radial pressure on the arborBetter surface finishes obtained because there is no dig-in

Milling Operations

4.5 Shaping & planningBoth the shaper and the planer are single point tools that cut only in straight lines. They both make the same types of cuts. The shaper handles relatively small work. The planer handles work weighing up to several tons. The cutting stroke of the shaper is made by moving the tool bit attached to the ram. The cutting stroke of the planer is achieved by moving the work past a stationary tool bit.4.5.1 The Shaper

4.6 GRINDING Grinding has two main uses,As a surface finishing process for flat or cylindrical features that have already been cut.To cut complex surfaces with high tolerances or hard materials.The basic process involves an abrasive wheel spinning at high speed that is brought into contact with the work surface, thus giving a smooth finish.

4.6.1 Machine TypesTypical categories of grinders include, SurfaceCylindricalInternal Center Center less

Surface

Cylindrical

Centerless

Interna Grinding

4.6.2Grinding wheels

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