27. Read the texts again and make notes under the following headings. Then use your notes to talk on the topics:

1. Metals: Properties. 2. Alloys: Properties. 3. Plastics and their Usage. 4. Use of Metals in Automobiles. 5. Different Processes Used with Metal.

UNIT VII. MECHANICAL ENGINEERING

B efore you start: Think it over. Agree – Disagree

“Restlessness and discontent are the first necessities of progress.”

Edison T.A.(1847 – 1931), US inventor

“If a thing is worth doing it is worth doing well.”

Anon.

Reading, Vocabulary & Creative Practice

1. Words to be remembered (Texts: a, b).

feeding shaving carve whittle precision tiny screw machine tool turning planing milling drilling power saw grinding metal working edge lathe tapered

plain screw thread bed headstock dead centre compound rest tempered steel tool-post cutting tool chuck turret bar resetting trace appliance reinforced cab rollover crawler

driveline running gear dump truck share ripper seeder potato digger disk harrow perambulator car spare slotting machine mortiser planer interurban freight traffic trailer coach caravan

2. Translate the sentences. Pay attention to the words in italics.

1. He carved the wood into the shape of a bird.

2. Scientific instruments have to be made with great precision.

3. These screws are very big, give me the tiny ones, please.

4. This machine tool is used for cutting and shaping only metals

5. This knife has a very sharp edge.

6. She had long tapered fingers.

7. Chuck is a small apparatus for holding a tool in a machine.

8. He traced the picture by drawing its lines on transparent paper placed on top of it.

9. Dishwashers and washing machines belong to household appliances.

10. Dump trucks are used for carrying and emptying heavy loads of soil.

11. We have a large market share but the competition is growing fast.

12. We need a spare part because this one is damaged.

13. The perambulator was of great help to her.

14. Steel is tempered by heating it and then putting it into cold water.

15. We went to Paris by coach.

16. Caravans contain cooking and sleeping equipment.

Part I

3. Read text a carefully. Pay attention to the words in bold type.

A. Machine Tools

A machine tool is a power-driven tool that changes the size, shape, or finish of metal objects. A true machine tool does at least four things: It holds a workpiece, holds a cutting tool, moves one or both of these objects, and provides a feeding movement for the tool or the workpiece.

The cutting tool makes a new part by shaving metal from an original piece. An average machining operation reduces the original weight by approximately 50 percent. A machine tool carves a piece of metal in much the same way that a pocket knife is used to whittle a toy sailboat from a piece of wood.

An outstanding feature of a modern machine tool is its great precision. It can cut metal to an accuracy of millionths of an inch. This figure is approximately equal to 1/300 of the thickness of a human hair. Such precision makes it possible to produce two parts or thousands of parts that are exactly alike.

There are more than 500 kinds of machine tools at work in industry today. They range from small bench devices to complicated machines that weigh hundreds of tons. Machine tools perform thousands of operations primarily on metal objects. They produce tiny screws and great turbine rotors with precision.

In principle machine tools are simple. They perform seven basic operations on metals. These operations are turning, planing, milling, drilling, power sawing, grinding, and metal working.

Turning Operations. The lathe is the machine tool used to perform turning operations. It produces a cutting action by rotating a workpiece against a fixed tool having a single edge. The line of cut forms a cylindrical surface. The principle of the lathe has been known and used for hundreds of years. It probably developed from an early combination of the potter's wheel and a knife. A lathe is the most versatile machine tool. It can machine a workpiece to a round, a concentric, an eccentric, or a tapered shape. It can cut plain or tapered screw threads inside or outside. With the proper attachments, a lathe can perform milling or grinding operations. It does all work with precision.

P rincipal Parts of the Lathe. The bed is the frame that supports the tool and the workpiece. The headstock supplies mechanical power. It contains a cone pulley that provides variable operating speeds. The tailstock controls the dead centre that supports the end of the workpiece. The carriage moves and controls the tool. A compound rest holds the tool post and the cutting tool. The cutting tool is made of tempered steel or of steel tipped with alloys such as the carbides of tungsten, boron, titanium, or tantalum.

In operation the workpiece may have a hole drilled in each end and be held rigidly between centres. The workpiece may also be held in chucks. In either case the workpiece rotates, and the tool is fed into it.

Types of Lathes. The engine lathe is the type most widely used. It has back gears to provide the low speed and high torque required to make heavy cuts on a large workpiece. An engine lathe is built with change gears and a lead screw for cutting threads. It also has powered feed in both longitudinal and cross directions.

A turret lathe is equipped with a toolholder called a turret. Cutting tools are mounted in bars that protrude from the turret. This makes it possible to use several different tools successively without resetting. The sequence of operations can be repeated as often as desired without resetting.

An automatic, or duplicating, lathe is used to reproduce, in a metal workpiece, the irregular shape of a model. A tracer point, remotely connected to the cutting tool, traces a pattern that follows the contours of the model. At the same time the cutting tool follows the same sequence of movements on the workpiece. The workpiece is thus cut to a duplicate shape.

Comprehension Check - I