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Drilling machines. 5. Grinding machines.
Boring machines.
Machining of large-volume production parts is best accomplished by screw machines. These machines can do turning, threading, facing, boring and many other operations. Machining can produce symmetrical shapes with smooth surfaces and dimensional accuracies not generally attainable by most fabrication methods.
Screw-machined parts are made from bar stock or tubing fed intermittently and automatically through rapidly rotating hollow spindles. The cutting tools are held on turrets and tool slides convenient to the cutting locations. Operations are controlled by cams or linkages that position the work, feed the tools, hold them in position for the proper time, and then retract the tools. Finished pieces are automatically separated from the raw stock and dropped into a container.
Bushings, bearings, nuts, bolts, studs, shafts and many other simple and complex shapes are among the thousands of products produced on screw machines. Screw machining is also used to finish shapes produced by other forming and shaping processes.
Most materials and their alloys can be machined — some with ease, others with difficulty. Machinability involves three factors: 1. Ease of chip removal. 2. Ease of obtaining a good surface finish. 3. Ease of obtaining good tool life.
Exercise 5.5. Complete the table:
Название станка
Операция
Exercise 5.6Complete the sentences choosing the suitable part from the second column. Translate them.
1. There are...
2. They are...
3. These machine-tools can perform...
4. Finished parts possess...
5. A lot of simple and complex shapes...
6. Screw-machining is also used...
7. Most engineering materials can be machined...
a) symmetrical shapes, high dimensional accuracies and smooth surfaces.
b) for finishing operations.
c) five general categories of machine-tools.
d) can be produced on screw machines.
e) turning, milling, grinding, boring, etc. operations.
f) by machine-tools.
g) lathes, drilling, boring, milling and grinding machines.
Exercise 5.7Answer the questions to the text "Metal Cutting".
1. When did the study of metal cutting start? 2. What is the purpose of metal cutting? 3. What machines are called "machine-tools"? 4. List the general categories of machine-tools. 5. What is the function of the spindle? 6. Where are cutting tools held? 7. By what means are cutting operations controlled? 8. List products produced on screw machines. 9. What are the general advantages of machining over other fabrication methods?
TEXT E. FACTORS AFFECTING MACHINABILITY
Machinability is generally assumed to be a function of tool edge life. The main factors which influence the behaviour, and thus the life of the edge of a cutting tool, are:
— the mechanical characteristics of the material being machined, such as its strength, hardness and metallurgical structure;
— the state of the casting, involving the skin finish, critical dimensions, machining allowances, slag inclusions, the presence of scabs, rust, dirt, etc.;
— the nature of the machining techniques being used;
— the characteristics of the machine-tool being used, such as machine efficiency, available power, and the rigidity of the setup.
Other factors aside, it is primarily the structure of the metal which determines its resistance to the cutting action of the tool, i. e. the potential rate of metal removal, and the resulting abrasion on the tool, i. e. the life of the cutting edge.
Structure, strength and machinability are interrelated to some extent — in general, increased strength implies reduced machinability. This basic relationship must be understood, otherwise difficulties may be experienced in the machine shop if the designer has specified a material with a higher strength than is necessary. Nevertheless, care should be taken in rating machinability on the basis of strength. For example, nodular irons are normally considerably stronger than flake-graphite types, but are likely to be easier to machine. It is therefore recommended that structure, rather than strength, be adopted as the basis for machining practice.
Hardness provides a more reliable guide to machinability than does strength, for hardness depends mainly on the matrix structure of the casting. Again, however, the relation is of a general nature only, for it is possible to have a metal which exhibits a low hardness value, but which has a very abrasive action on the cutting tool. For example, the presence of hard phosphide particles embedded in a soft, ferritic matrix reduces tool life considerably.
Exercise 5.8 Answer the questions :
1. What are the main factors influencing the tool edge life? 2. Does the structure of the material influence machinability? In what way? 3. What does increased strength result in? 4. Why is hardness more reliable in determining machinability of a material than strength?
UNIT 6
MACHINE-TOOLS
Text A: MACHINE-TOOL
Text B: LATHE
Дата: 2019-02-25, просмотров: 487.