Turning & Related Operations

Lecture-01Conventional Machining

NIKHIL R. DHAR, Ph. D.DEPARTMENT OF INDUSTRIAL & PRODUCTION ENGINEERINGBUET BANGLADESH

Conventional Machining

Turning & Related Operations

Drilling & Related Operations

Milling & Related Operations

Shaping & Related Operations

Grinding & Related Operations

Department of Industrial & Production Engineering 30/3

Introduction

Production or manufacturing of any object is a value addition process by which raw material of low utility and value due to its irregular size, shape and finish is converted into a high utility and valued product with definite size, shape and finish imparting some desired function ability. Machining is an essential process of semi-finishing and often finishing by which jobs of desired shape and dimensions are produced by removing extra material from the preformed blanks in the form of chips with the help of cutting tools moved past the work surfaces in machine tools.Machined parts can be classified as rotational or non-rotational. A rotational workpart has a cylindrical or disk-like shape. The characteristics operation that produces this geometry is one in which a cutting tool removes material from rotating workpart. Examples include turning and boring. Drilling is closely related except that an internal cylindrical shape is created and the tool rotates in most drilling operations.A non-rotational (also called prismatic) workpart is block-like or plate like. This geometry is achieved by linear motions of the workpart, combined with either rotating or linear tool motion. Operations in this category include milling, shaping, planning and sawing

Rotational Non-rotational


Introduction

Each machining operation produces a characteristics geometry due to two factors

the relative motions between the tool and the workpart and the shape of the cutting tool. These operations may be classified as generating

and forming.

In generating, the geometry of the workpart is determined by the feed trajectory of the cutting tool. Examples of generating the work shape in machining include straight turning, taper turning, contour turning, peripheral milling and profile milling.In forming, the shape of the part is created by the geometry of the cutting tool. In effect, the cutting edge of the tool has the reverse of the shape to be performed on the part surface. Form turning, drilling, and broaching are examples of the case. Forming and generating are sometimes combined in one operation, such as in thread cutting on a lathe and slot milling.


(a) Straight turning (b) Taper turning (c) Contour turning

(d) Plain milling (e) Profile turning

Generating Shape in Machining


(a) (c)(b)

Forming to create shape in machining (a) form turning (b) broaching and (b) drilling

Forming Shape in Machining


(a) (b)

Combination of forming and generating to create shape (a) slot milling and (b) thread cutting

Forming & Generating Shape

NIKHIL R. DHAR, Ph. D.DEPARTMENT OF INDUSTRIAL & PRODUCTION

ENGINEERINGBUET BANGLADESH

LECTURE-02TURNING & RELATED OPERATIONS


Introduction

Turning is the process whereby a centre lathe is used to produce "solids of revolution". It can be done manually, in a traditional form of lathe, which frequently requires continuous supervision by the operator, or by using a computer controlled and automated lathe which does not. This type of machine tool is referred to as having computer numerical control, better known as CNC and is commonly used with many other types of machine tool besides the lathe.

When turning, a piece of material (wood, metal, plastic even stone) is rotated and a cutting tool is traversed along 2 axes of motion to produce precise diameters and depths. Turning can be either on the outside of the cylinder or on the inside (also known as boring) to produce tubular components to various geometries. Although now quite rare, early lathes could even be used to produce complex geometric figures, even the platonic solids; although until the advent of CNC it had become unusual to use one for this purpose for the last three quarters of the twentieth century. It is said that the lathe is the only machine tool that can reproduce itself.


Basic Turning Operations


Facing

Contour turning

Form turning


Facing: The tool is fed radially into the rotating work on one end to create a flat surface on the end.

Contour turning: Instead of feeding the tool along a straight line parallel to the axis of rotation as in turning, the tool follows a contour that is other than straight, thus creating a contoured form in the turned part.

Form turning: In this operation, sometimes called forming, the tool has a shape that is imparted to the work by plunging the tool radially into the work.


Taper turning

Chamfering

Cutoff

Taper turning: Instead of feed the tool parallel to the axis of rotation of the work, the tool is fed at an angle, thus creating a taper cylinder or conical shape.

Chamfering: The cutting edge of the tool is used to cut an angle on the corner of the cylinder, forming what is called a “chamfer”.

Cutoff: The tool is fed radially into the rotating work at some location along its length to cut off the end of the part. This operation is sometimes referred to as parting.



Boring

Drilling

Threading

Knurling

Boring: A single point is fed linearly. parallel to the axis of rotation, on the inside diameter of an existing hole in the part.

Threading: A pointed tool is fed linearly across the outside surface of the rotating workpart in a direction parallel to the axis of rotation at a large effective feed rate, thus creating threads in the cylinder.

Drilling: Drilling can be performed on a lathe by feeding the drill into the rotating work along its axis. Reaming can be performed in a similar way.

Knurling: This is not a machining operation because it does not involve cutting of material. Instead, it is a metal forming operation used to produce a regular cross hatched pattern in the work surface.





Types of LatheSpeed Lathes

Limited to headstock, tailstock, and simple tool post. Limited to 3-4 speeds High spindle speeds, For light work such a wood turning, metal polishing, or metal spinning

Engine Lathes Most common type Variable in design from low to high power designs Broad range of lengths up to 60ft long

Tool room Lathes Specialized Engine lathe with greater accuracy. Broader range of speeds and feeds Greater versatility for tool and die manufacturing

Turret Lathes Turret on tool post rotates to position a variety of tools Capstan wheel used to pull to away from work piece to position next tool A number of tools set up on machine, each brought up in quick succession to complete

the part in a single setup


Lathe ComponentsBed: supports all major componentsCarriage: slides along the ways and consists of the cross-slide, tool post, apronHeadstock – Holds the jaws for the work piece, supplies power to the jaws and has various drive speedsTailstock – supports the other end of the workpieceFeed Rod and Lead Screw-Feed rod is powered by a set of gears from the headstock


Lathe Design and Terminology

Bed Gray cast for vibration dampeningHeadstock assembly Spindle Transmission Drive motorTailstock assembly Longitudinal way clamp Transverse way clamp Quill for cutting tools, live centers,

or dead centersQuick-change gearbox Powers Carriage Assembly

movement with lead screw

Carriage Assembly Fixed to cross slide Holds tool post at variable orientations Provides longitudinal and transverse movement of toolingWays Provides precise guidance to carriage assembly and tailstock


Computer Numerical Controls (CNC) LatheIn the most advanced lathes, movement and control of the machine and its components are actuated by computer numerical controls (CNC); the features of such a lathe are shown in the following Figure.


Vice: This holds the material to be cut or shaped. Material must be held securely otherwise it may 'fly' out of the vice when the CNC begins to machine. Normally the vice will be like a clamp that holds the material in the correct position. Guard: The guard protects the person using the CNC. When the CNC is machining the material small pieces can be 'shoot' off the material at high speed. This could be dangerous if a piece hit the person operating the machine. The guard completely encloses the dangerous areas of the CNC. Chuck: This holds the material that is to be shaped. The material must be placed in it very carefully so that when the CNC is working the material is not thrown out at high speed. Motor: The motor is enclosed inside the machine. This is the part that rotates the chuck at high speed. Lathe Bed: The base of the machine. Usually a CNC is bolted down so that it cannot move through the vibration of the machine when it is working. Cutting Tool: This is usually made from high quality steel and it is the part that actually cuts the material to be shaped.


Advantages of CNC Machines

CNC machines can be used continuously 24 hours a day, 365 days a year and only need to be switched off for occasional maintenance.CNC machines are programmed with a design which can then be manufactured hundreds or even thousands of times. Each manufactured product will be exactly the same.Less skilled/trained people can operate CNCs unlike manual lathes / milling machines etc.. which need skilled engineers.CNC machines can be updated by improving the software used to drive the machinesTraining in the use of CNCs is available through the use of ‘virtual software’. This is software that allows the operator to practice using the CNC machine on the screen of a computer. The software is similar to a computer game.CNC machines can be programmed by advanced design software such as Pro/DESKTOP®, enabling the manufacture of products that cannot be made by manual machines, even those used by skilled designers / engineers.Modern design software allows the designer to simulate the manufacture of his/her idea. There is no need to make a prototype or a model. This saves time and money.One person can supervise many CNC machines as once they are programmed they can usually be left to work by themselves. Sometimes only the cutting tools need replacing occasionally. A skilled engineer can make the same component many times. However, if each component is carefully studied, each one will vary slightly. A CNC machine will manufacture each component as an exact match.


Disadvantages of CNC Machines

CNC machines are more expensive than manually operated machines, although costs are slowly coming down.The CNC machine operator only needs basic training and skills, enough to supervise several machines. In years gone by, engineers needed years of training to operate centre lathes, milling machines and other manually operated machines. This means many of the old skills are been lost.Less workers are required to operate CNC machines compared to manually operated machines. Investment in CNC machines can lead to unemployment.Many countries no longer teach students how to use manually operated lathes / milling machines etc... students no longer develop the detailed skills required by engineers of the past. These include mathematical and engineering skills.


Reaming

Reams remove small amounts of material to ensure exact hole size and improve hole surface finishReams are either hand operated or machined at slow speedReam types

Shell reams Expansion reams Adjustable reams Tapered reams


Ream Geometry

Figure: Standard nomenclature for hand and chucking reamers.


Boring

Boring consists of producing circular internal profiles in hollow workpieces or on a hole made by drilling or another process and is carried out using cutting tools that are similar to those used in turning. However, because the boring bar has to reach the full length of the bore, tool deflection and hence maintenance of dimensional accuracy can be a significant problem. A vertical boring machine is similar to a lathe is shown in the following Figure.

Figure: Schematic illustration of the components of a vertical boring mill


Cutting Tools for Lathes

Tools consists of cutting surface and support Cutting surfaces can be of same material as support or a separate insert Supports materials must be rigid and strong enough to prevent tool deflection during cutting Cutting materials are typically carbides, carbide coatings, ceramics, or high carbon steels Inserts are used to decrease cost in that the insert is disposed of, and the support reused.

Figure: Common types of forged tool holders: (a) right-hand turning, (b) facing, (c) grooving cutoff, (d) boring, (e) threading.


Quick Change Tool Holders

Tool changing can take over 50% of manual lathe operations Quick Change holders are used to reduce manual tool change time and increase production


Workholding Devices for Lathes

Work pieces can be held by various methods Work piece mounted between centers Work piece mounted within a single chuck Work piece mounted within a collet Work piece mounted on a faceplate

Lathe CentersA lathe center hold the end of the work piece, providing support to preventing the work piece from deflecting during machiningLather centers can be mounted in the spindle hole, or in the tailstock quillLathe centers fall into two categories

Dead Center: solid steel tip that work piece spins against Live Center: centers contact point is mounted on bearings and allowed to spin

with work piece


Lathe ChucksLathe Chucks are adjustable mechanical vises that hold the work piece and transfer rotation motion from the drive motor to the work pieceLathe Chucks come in two basic types

Three-jaw self-centering chucksUsed to center round or hexagonal stock

Four-jaw independent chucksEach jaw moves independently to accommodate various work piece shapes


Lathe ColletsCollets are used to hold round stock of standard sizesMost accurate holding method for round stock

Run out less than 0.0005 inch Stock should be no more than 0.002 inch larger or 0.005 smaller than the collet Typically used for drill-rod, cold-rolled, extruded, or previously machined stock

Several types of lathe collets


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