PRODUCT DESIGN

Grinding Process

The tool in the grinding process is the grinding wheel, which is generally composed of two materials: grits and bonding agent. The hard abrasive grits (small hard particles) are affixed to rotating hub or axis in the

general configuration of a wheel to erode material away from a workpiece as the wheel spins. Grinding wheels are formed into shape by casting and curing a slurry of bonding material with the grits. The cured wheel forms a three-dimensional matrix of grits held in place by the bonding agent. This yields a complex operation by nature, as the uneven and truly random distribution of grit surfaces of the wheel and the contact they make with the ground part are difficult at best to systematically model.

History and Perspective

Abrasive material removal (grinding) is one of the oldest machining technologies employed today, and has been utilized by people in the manufacturing of parts since the Stone Age (Malkin, 1989). A simplified grinding process can be thought of as milling using a “cutter” with a large number of teeth of irregular

shape, size, and spacing (Fig. 3.2.). Each grit can be seen as a cutting tooth with varying orientation and

sharpness. These grits are suspended in a bonding agent that holds the three-dimensional matrix of grits

together in a form. The grinding process can vary for many reasons including: wheel sharpness, wheel microstructure, workpiece material variation, loading of workpiece material on the wheel, and other phenomena that contribute to the changing nature of the grinding process. (Loading is the phenomena of the ground material becoming attached to or embedded onto the surface of the grinding wheel. This effect begins with the material filling in the voids around the grits, and if permitted to continue, the material can eventually cover the grits. This occurs more commonly with softer materials.)

The variable nature of the grinding process has been linked to the physical descriptions of the actions between the grits with the workpiece. Grinding is a complex operation that can be seen as three separate and concurrent process actions: (1) cutting, (2) plowing, and (3) rubbing (Hahn and Lindsay, 1971; Samuels, 1978; Salmon, 1992). The cutting action produces tangential forces that are related to material specific energy in the generation of chips. Although a small part of this energy is transferred to the chip as kinetic energy, the majority of the material specific energy is dispersed in several other ways including: friction, heat conduction, radiation, surface formation, residual stresses, etc. The second grinding process action is plowing. In plowing material is plastically deformed around the sides of dull grits, leading to ruts and uneven surfaces. No material is removed with plowing. Rubbing occurs as material loads onto the wheel or if the grits are exceptionally dull (causing attritious flat wear). Rubbing is a high friction condition with essentially no material removal and is the most detrimental of the three grinding actions.

The generated frictional heat (that can yield “burning”) from rubbing is dissipated into both the ground

part and the wheel. If coolant is not used, thermal damage may occur. Damage created from excess forces

or from plowing ruts can be hidden by the migration of material during rubbing, only to result in premature failure of the part in use. In general, all three grinding process actions occur simultaneously in varying distributions depending on the grinding wheel, material, and operating conditions. As stated previously grinding is stochastic by its nature, making detailed process analysis, modeling, and control difficult. This is also brought out by the variety and complexity of grinding models presented later.

COMPUTER-AIDED DESIGN,

ENGINEERING, AND MANUFACTURING

Systems Techniques And Applications

VOLUME

V I

Editor

CORNELIUS LEONDES

Boca Raton London New York Washington, D.C.

CRC Press

MANUFACTURING

SYSTEMS PROCESSE

LATHE

INTRODUCTION

The lathe is father of all machine tools; in early days it was equipped with a fixed tool rest and was used for woodworking. In operation, the lathe holds the job between two rigid supports called centres or by some chuck or face plate screwed to the nose or and of the spindle.

Function of lathe

The main function of lathe is to remove metal from a piece of work to give it the required shape and size. This is accomplished by holding the work securely and rigidly on the machine and then turning it against cutting tool, which will remove, metal from the work in the form of chips.

Types of lathes

Lathes of various designs and constructions have been developed to suit the various conditions of metal machining. But all of them employ the same fundamental principle of operation and perform the same function. The lathes are classified as follow.

1. Speed lathe

 Wood working

 Centering

 Polishing

 Spinning

2.Engin lathe

 Belt drive

 Individual motor drive

 Gear head lathe

3.Bench lathe

4.Tool room lathe

5.Capstan and turret lathe

6.Special purpose

Wheel lathe

 Gap bed lathe

 T-lathe

 Duplicating lathe

7.Automatic lathe

The speed lathe

 The speed lathe, in construction and operation, is the simplest of all types of lathes. It consists of a bed, a headstock, and a tailstock and tool post mounted on an adjustable slide. There is no feed box, lead screw or conventional type carriage. The tool is mounted on the adjustable slide and is fed into work purely by hand control. This characteristic of the lathe enables the designer to give high spindle speeds, which is usually, range from 1200 to 3600 r.p.m. As the tool is controlled by hand, the depth of cut and thickness of chip is very small.

 Light cut and high speed necessitate the use of this type of machine where cutting force is minimum such as in wood working, spinning, centering, polishing, etc.

The engine lathe or center lathe

This lathe is most important member of lathe family and is most widely used. Similar to the speed lathe, the engine lathe has got all the basic parts, e.g. bed, headstock, and tailstock. But the headstock of an engine

lathe is much more robust in construction

and it contains additional mechanism of driving the lathe spindle at multiple speeds.

The engine lathe that can feed the cutting tool both in cross and longitudinal direction with reference to the lathe axis with help of a carriage feed rod and lead screw.

The bench lathe

This is a small lathe usually mounted on bench. It has practically all the parts of an engine lathe or speed lathe and it performs almost all the operations, its only difference being in size this is used for small and precision work.

The tool room lathe

A tool room lathe having features similar to an engine lathe is much more accurately built and has a wider range of spindle speeds ranging from a very low to a quite high speed up to 2500 r.p.m.This lathe is mainly used for precision work on tools, dies, and gauges and in machining work where accuracy is needed

Numerically Controlled Lathes

In these lathes the path of the tools to produce the desired shape, along with other auxiliary functions like speed/feed changes, turret indexing, tail stock positioning, coolant supply, etc., are controlled by pre-programmed numerical data input in the form of punched tape to an electronic control system. One of the latest developments is the computer control of NC machines which is popularly called the computer numerical control (CNC). With the concept of numerical control, the configuration of lathes has undergone radical changes with features like slant bed design infinitely variable seed headstock, antifriction lead screws, auto-tool changing, etc. Numerically controlled lathes are available in various versions. The control system employed on NC lathes enable straight cut and continuous path control.

The cutting tools on a CN C turning center are usually mounted in a device called a turret. The turret is then mounted on a heavy cross slide and carriage similar to that of a conventional lathe. The turret can be indexed very rapidly to automatically change to the next tool.

A number of smaller CNC lathes have also come to market in recent years to compete with the turret lathes and automatic screw machines.

fr:NETTUR TECHNICAL TRAINING FOUNDATION

Machine tool

Machine tool is a machine which is moved by the manpower (Electrical, mechanical, hydraulic and pneumatic) are used to create a shape, size, and precision or accuracy (according to the design) with remove metal from a coupon or a workpiece or specimen (Workspieces) in the form of anger. Machine tools are factory equipment

to produce the machines, instruments, tools and tooling for the entire needs. It could be argued that the machine tool is the mother of all machine.

Most machine tools perform four functions, namely:

a. Keeping a job

b. Maintain cutting tools

c. Moving one or more of the rotational movement or motion back and forth

d. Provides a feed movement to rotational movement or back and forth

3.2 Classification

Machine tools can be classified in several ways.

A. Based on the scope of application, machine tools can be classified into:

a. General use machine tools

Machine tools for general use or a universal machine tools widely used to make various specimens with broad coverage includes only the pieces can make it, small lot production, and for repairs. Machine tools used for a particular scope of work is widely known with the name of multi-purpose machine tools (multipurpose). That

included into the machine tools for general use is plain turning lathes, turret lathes, milling machines, drilling machines, grinding machine and so on.

b. Single-use machine tools

This type of machine tools used to create a certain type machining operations, eg, broaching, thread cutting, gear shaping and hobbing machine, machine for machining pistons, crankshaft, for turning the camshaft and cam Contours on camshafts and so on.

c. Machine tools for limited use

Machine tools capable of this type for an operation on a narrow various kinds of workpieces, for example, automatic cutting off machines.

d. Machine tool production

Machine tools of this type are widely used manufacturing lots production, mass production, high-production features and stiffness. That included in this type of machine is a multi-tool lathes, single and multi-spindle automatic, semi-automatic lathe, plunge-cut cylindrical grinder, centreless, planer-type milling machine, thread rolling machine for tap production, numerically controlled machine tool, and so on.

e. Specialized machine tools

This type of machine tools used to create a form similar but different sizes. This type of machine tools also perform the process machining multiple surfaces on different areas. The advantage of these machine tools are able to do change from one job to another job. This can be done because install the head where there is an additional angle can be changed in the horizontal plane or vertical plane or vice versa. This machine widely used to produce large lot.

f. Machine specific tooling

This type of machine tools in design and manufactured individually with the intent to establish something on the machining operation the particular and the particular workpieces as well. Machine type These include machines for sharpening round whorls dies, mengerinda The slanted edges around the whorls dies, marking around the whorls type stalk dies and tooling, for whorls by die tap, for grinding flute on tap and reamer, tap chamfer, flutes on a twist drill, and so on. This type of machine is widely used in the production-lot production

large as well as mass.(Nafsan Upara)

PLASTICITY - STRESS–STRAIN RELATIONSHIP - ELASTIC LIMIT

PLASTICITY is that state of matter where permanent deformations or strains may occur without fracture. A material is plastic if the smallest load increment produces a permanent

deformation. A perfectly plastic material is nonelastic and has no ultimate strength in the ordinary meaning of that term. Lead is a plastic material. A prism tested in compression will deform permanently under a small load and will continue to deform as the load is increased, until it flattens to a thin sheet. Wrought iron and steel are plastic when stressed beyond the elastic limit in compression. When stressed beyond the elastic limit in tension,

they are partly elastic and partly plastic, the degree of plasticity increasing as the ultimate strength is approached.

STRESS–STRAIN RELATIONSHIP gives the relation between unit stress and unit strain when plotted on a stress–strain diagram in which the ordinate represents unit stress and the abscissa represents unit strain. Figure 5 shows a typical tension stress–strain curve for medium steel. The form of the curve obtained will vary according to the material, and the curve for compression will be different from the one for tension. For some materials like

cast iron, concrete, and timber, no part of the curve is a straight line.

PROPORTIONAL LIMIT is that unit stress at which unit strain begins to increase at a faster rate than unit stress. It can also be thought of as the greatest stress that a material can stand without deviating from Hooke’s law. It is determined by noting on a stress–strain diagram the unit stress at which the curve departs from a straight line.

ELASTIC LIMIT is the least stress that will cause permanent strain, that is, the maximum unit stress to which a material may be subjected and still be able to return to its original form upon removal of the stress.

JOHNSON’S APPARENT ELASTIC LIMIT. In view of the difficulty of determining precisely for some materials the proportional limit, J. B. Johnson proposed as the ‘‘apparent elastic limit’’ the point on the stress–strain diagram at which the rate of strain is 50% greater than at the origin. It is determined by drawing OA (Fig. 5) with a slope with respect to the vertical axis 50% greater than the straight-line part of the curve; the unit stress at which the line O A which is parallel to OA is tangent to the curve (point B, Fig. 5) is

the apparent elastic limit.

YIELD POINT is the lowest stress at which strain increases without increase in stress. Only a few materials exhibit a true yield point. For other materials the term is sometimes used as synonymous with yield strength.

YIELD STRENGTH is the unit stress at which a material exhibits a specified permanent deformation

or state. It is a measure of the useful limit of materials, particularly of those whose stress–strain curve in the region of yield is smooth and gradually curved.

ULTIMATE STRENGTH is the highest unit stress a material can sustain in tension, compression, or shear before rupturing.

RUPTURE STRENGTH, OR BREAKING STRENGTH, is the unit stress at which a material breaks

or ruptures. It is observed in tests on steel to be slightly less than the ultimate strength because of a large reduction in area before rupture.

MODULUS OF ELASTICITY (Young’s modulus) in tension and compression is the rate of change of unit stress with respect to unit strain for the condition of uniaxial stress within the proportional limit. For most materials the modulus of elasticity is the same for tension and compression.

MODULUS OF RIGIDITY (modulus of elasticity in shear) is the rate of change of unit shear stress with respect to unit shear strain for the condition of pure shear within the proportional limit. For metals it is equal to approximately 0.4 of the modulus of elasticity.

MECHANICAL DESIGN

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz