Metal cutting

METAL CUTTING

Machining Processes: Machining Processes are performed in Metal Cutting Machines.

Machine Tools: It is a well-known fact that machine shop forms not only an important, but an indispensable part of a modern workshop. One should have a thorough knowledge of the different Metal Cutting Machines, also known as Machine Tools, their types, principal parts and their functions, and the various operations that can be performed, etc.

A Machine Tool is defined as a power-driven machine which is capable of cutting and removing metal in the form of chips to produce a desired shape.

Chips: It should be understood that every metal working machine cannot be called a machine tool for the reason that every machine tools is expected to remove the metal in the form of chips (Refer Figure). In fact, it would be difficult to name a product which does not depend in some way upon the machine tools.

Mother of All Machines: Machine tools are known as the Mother of All Machines. In a developing country like India where short and long term programmes have been undertaken in building up heavy industry, machine tools play a key role in bringing about self-generating economy in the country. Men in industry today, from company president to apprentice need a good speaking acquaintance with machine tools. It should, therefore, receive the highest priority in the field of mechanical engineering.

Classification of Machine Tools

Machine tools are classified according to the operations, i.e., type of the surface generated as follows:

1. Machine Tools for Cylindrical Work: In this case, the workpiece is rotated about an axis and tool is traversed in a definite path relative to the axis, or alternatively, the work may be at rest and the tool is rotated as well as traversed.

Various machine tools used for this type of work are Lathe, Turret and Capstan Lathes, Boring Machine and Cylindrical Grinder.

2. Machine Tools for Flat Surface Work: For generating a flat surface, the workpiece is moved past the cutting tool in a straight path and the tool is traversed in perpendicular direction, or alternatively, the tool is moved in straight path and work is traversed in perpendicular direction. The machine tools doing this type of operation are Shaping, Slotting, Planing and Broaching.

The flat surface could also be produced by having rotary motion of multi-cutting edge tool and translatory motion of job. The examples of machine tools employing such motions are Milling and Surface Grinding Machines.

 

1. LATHE

Lathe is the Mother of all Machine Tools. It is a basic machine tool, probably the most important one of all. Lathe was actually the first machine tool, the history of which dates back to 18th century. Later developments led to a number of amendments, as years passed, and the result is what we see today. Further, lathe formed the basis of production of all other machine tools which are the results of later developments.

 

a.  Working Principle

The main function of a lathe is to remove metal in the form of chips from a workpiece to give it the required shape and size. This is accomplished by holding the work securely and rigidly on the lathe and then rotating it against a cutting tool. The lathe is mainly used to machine cylindrical shapes.

Cutting Tool: Generally, Single Point Tool is used as the cutting tool. The tool material should be harder and stronger than the work piece material.

Lathe Operations: The main significance of a lathe lies in the fact that, besides its basic operations of Turning, Facing, Thread Cutting, etc., it can also perform the operations of other machine tools like Drilling, Boring, Milling, Gear Cutting, etc. Hence, it is called a Versatile Machine Tool.

 

b.  Main Parts of Center Lathe

Fig. 21 shows the principal parts of a Center Lathe. The parts are:

1. Bed

2. Headstock

3. Tailstock

4. Carriage: (a) Saddle, (b) Cross-slide, (c) Compound Rést (d) Tool Post (e) Apron

5. Feed Mechanism

6. Thread Cutting Mechanism

1. Bed: It is the base of the lathe. The headstock is mounted on the left end, the carriage in the middle and the tailstock at the right end of the bed. The bed has flat or inverted 'V' guideways. The carriage and the tailstock move along the guideways over the bed.

2. Headstock: Headstock carries a hollow spindle. A live center can be fitted into the hollow spindle. The live center rotates with the work piece and hence called Live Center. The spindle nose is threaded. Chucks or face plates can be attached to the spindle nose. The headstock has a gear box (speed changer). Power is transmitted from the headstock to the different parts of the lathe.

3. Tailstock: It is mounted on the bed at the right end. It is used for supporting the right end of the workpiece by means of a dead center. The dead center does not revolve with the work piece and hence called Dead Center. However, it can be moved axially by means of a handwheel.

Tailstock can be moved along the bed for supporting different lengths of workpieces and clamped at any position. The Tailstock is also used for holding drill and reamer for drilling and reaming operations.

4. Carriage: It is supported on the lathe bed-ways. It can move in a direction parallel to the lathe axis. The carriage is used for giving various movements to the tool by hand or by power. It carries Saddle, Cross-slide, Compound Rest, Tool Post and Apron.

a) Saddle: It carries the Cross Slide, Compound Rest and Tool Post. It is a H-shaped casting fitted over the bed. It moves along the guideways.

b) Cross-slide: It carries the Compound Rest and Tool Post. It is mounted on the top of the saddle. It can be moved by hand or may be given power feed through apron mechanism.

c) Compound Rest: It is mounted on the Cross-slide. It carries a circular base called Swivel Plate which is graduated in degrees. It is used during taper turning to set the tool for angular cuts. The upper part known as Compound Slide can be moved by means of a hand wheel.

d) Tool Post: It is fitted over the compound rest. Tool is clamped in the tool post..

e) Apron: Lower part of the carriage is termed as the Apron. It is attached to the saddle and hangs in front of the bed. It contains Gears, Clutches and Levers for moving the carriage by a hand wheel or power feed.

5. Feed Mechanism

Feed: The movement of the tool relative to the work piece is termed as Feed. The lathe tool can be given three types of feed, namely, Longitudinal, Cross and Angular.

Longitudinal Feed: When the tool moves parallel to the axis of the lathe, the movement is called Longitudinal Feed. This is achieved by moving the carriage.

Cross Feed: When the tool moves perpendicular to the axis of the lathe, the movement is called Cross Feed. This is achieved by moving the cross slide.

Angular Feed: When the tool moves at an angle to the axis of the lathe, the movement is called Angular Feed. This is achieved by moving the compound slide, after swiveling it at an angle to the lathe axis.

Feed Rod: It is a long shaft, used to move the carriage or cross-slide for Turning, Facing, Boring and all other operations except Thread cutting. Power is transmitted from the lathe spindle to the apron gears through the feed rod via a large number of gears.

6. Thread Cutting Mechanism: The Lead Screw is a long threaded shaft used as a master screw. It is brought into operation only when threads have to be cut. In all other times the lead screw is disengaged from the gear box and remains stationary. The rotation of the lead screw is used to traverse the tool along the work to produce screw threads. The half-nut makes the carriage to engage or disengage the lead screw.

 

c. Specifications of a Lathe

The size of a lathe is specified by

i) Overall Length of the bed,

ii) Largest Diameter of the workpiece that can be rotated without touching the guide-ways of the bed,

iii) Height of Centers from the bed, and

iv) Maximum Distance between the Centers.

 

d. Operations Performed on Lathe

Operations performed on a lathe either by holding the workpiece between centers or by a chuck are: Turning, Facing, Knurling, Forming, Taper Turning and Thread Cutting.

Operations performed by holding the workpiece by chuck or face plate are: Grooving, Parting-Off, Drilling, Reaming, Boring, Counter-Boring and Internal Thread Cutting..

1. Turning: (Fig. 22)

The workpiece may be held in a chuck as shown or between the centers. The turning tool is fed parallel to the lathe spindle axis and cylindrical surface is produced. It reduces the diameter of workpiece.

For Rough Turning the rate of feed of tool is fast and the depth of cut is heavy. For this, rough turning tool is used and the depth of cut may be from 2 to 5 mm.

For Finish Turning feed and depth of cut will be small. For this, a finish turning tool is used and the depth of cut may be from 0.5 to 1 mm.

2. Facing: (Fig. 23)

Facing is the machining of the end face of workpiece to make it flat. The workpiece may be held in a chuck as shown or between centers. A facing tool is fed perpendicular to the axis of rotation of workpiece reducing its length.

Only the face of the workpiece is machined in this process and hence the name facing.

3. Drilling: (Fig. 24)

It is the operation of making a hole in the workpiece. For drilling, the workpiece is held in a chuck or face plate on one side, whereas the other side remains free.

Drill: The tool used for drilling is called the Drill. From the tail stock, the dead center is taken out and drill is inserted in it. When the job rotates, the drill bit is fed into the workpiece by turning the tail stock hand wheel.

Lathe Tools

Cutting tools of a lathe have only single cutting edge or point and hence called Single Point Tools. Most lathe tools are derived from the operation for which they are designed. For instance, turning tool is meant for turning, facing tool for facing, etc.

Tool Materials: Cutting tools used for lathe work are made of :

1. High-Speed Steel (H.S.S),

2. High Carbon Steel,

3. Cemented Carbide and

4. Diamond.

Note: In forging, there is a continuous grain flow. In machining, the grain flow is broken. In casting, there is no grain flow.