CHAPTER (1)

General IntroductionGeneral Introduction

Dr. Ahmed Abou El-Wafa

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1.1. Classification of Manufacturing Processes

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A machine tool (metal cutting machine) is a device in which energy is utilized in deformation of material.

The machine tools are generally used for two aims;1- to produce certain form2- to finish surfaces.

1.1.1. Objectives of machine tools

1- to hold the workpiece (w.p.)2- to hold the cutting tool3- to produce a relative movement necessary to generate the required surface.

1.1.2. Machine tools drive systems

1- Electrical 2- Hydraulic 3- Mechanical 4- Pneumatic

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1.1.3. Machine tools workpiece holding devices

(a) For rotating w.p.

1- Chuck2- Between centers3- Faceplate4- Collet5- Arbor 6- Mandrel7- Special attachments

(b) For reciprocating w.p.

1- Vise2- T-slot3- Magnetic chuck.

To perform the machining operation, two motions are required:

1- Cutting motion2- Feed motion

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1.1.4. Cutting and feed movements

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1.2. Materials in Manufacturing

1.2.1. Workpiece materials

1- Metals2- Ceramics3- Polymers4- Composites (non-homogeneous mixtures of the above three types).

FIGURE 1.2 W.P. material types

Ceramics

Metals Metal- polymer composites

Metal- ceramic composites

Ceramic-polymer composites

Polymers

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MetalsMetals: - ferrous {e. g. steel (0.02 - 2%C) and cast iron(2 - 4%C)}

- non-ferrous {e. g. Al, Cu, Mn, Ag, Ti, etc.}

CeramicsCeramics: is a compound containing metallic (or semi-metallic) and non-metallic elements (O2 , N and C) as an example Al2O3 , SiC, WC, TiC and TiN.

PolymersPolymers: usually consists of C plus one or more other elements, such as H2 , N, O2 and Cl. Three types of polymers are exist; thermoplastic: a plastic that repeatedly can be softened by heating

and hardened by cooling, e.g. nylon, thermosetting: a class of polymers that, when cured using heat,

chemical, or other means, changes into a substantially infusible and insoluble material, e.g. epoxy, and elastomers: exhibit significant elastic behavior, e.g. natural rubber.

CompositesComposites:consists of particles of one phase (e. g. fiber) mixed in a second phase, called the matrix (e. g. epoxy). A composite is a material consisting of (usually) two phases that are processed separately and then bonded together to achieve properties superior to those of its constituents.

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1.2.2. Cutting tool materials

A cutting tool should have the following properties:1- Hardness to resist abrasion2- Red (hot) hardness (the ability to retain this hardness at high temp.) 3- Toughness to withstand shocks4- Ability to provide a good surface finish to reduce chip/tool friction.

Types of tool materials:

- Plain high-carbon steel - Plain high-carbon steel Its use is limited to hand tools.

- High-speed steel (HSS)- High-speed steel (HSS)Tungsten (W) is the alloying element responsible for providing the red hardness. HSS is used in medium speed range. The commonest type is 18-4-2 HSS, being the percentages of W, Cr, and V, respectively, present in steel containing 0.7% C.

- Stellite- StelliteIt is a non-ferrous cast alloy of cobalt (50%), chromium (28%) and tungsten (20%) (+ ≈ 2% C), containing iron only as an impurity.

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1.3. Manufacturing processes

They can be divided into two basic types; 1- Processing operations, and 2- Assembly operations.

- Cemented carbides- Cemented carbidesWC or BC bonded together by another material (cobalt) and then placed in a mould and heated to a very high temperature in the presence of H2 since H2 prevents damage by atmospheric O2. It can be used as brazed or clamped tips mounted on a tough medium steel shank.

- Ceramic tools- Ceramic toolsThe most common ceramic cutting tool material is Al2O3. They are used as clamped throwaway-tips only since they cannot be brazed to tool steel shank. They lend themselves to negative rake cutting.

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1.4. Machinability

The machinability of a material is defined as follows: "The most machinable material is the one which permits the removal of material with a satisfactory finish at lowest cost".

1.4.1. Factors affecting the machinability of materials

1- W.p. material2- Tool material3- Size and shape of tool4- Type of machining operation5- Size, shape and velocity of cut6- Type and quality of the used machine tool7- Type of lubricant used during machining operation8- Coefficient of friction between chip and tool9- Shearing strength of w.p. material.

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Web site:

http://www.staff.zu.edu.eg/awafa/http://www.staff.zu.edu.eg/awafa/

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