What Is a Sketch
The mechanic must not only know how to read the language of the industrial world but must also know how to write it. He must be able to give his own ideas and describe an object to someone else. It is often necessary to make just a part for a machine. However, there is no time to make a drawing of it with instruments, no time either for a draughtsman to make a drawing. Sometimes only one piece of a machine part is to be made and, therefore, it would be inconvenient to have the draughtsman make a mechanical drawing.
Sketches are important because they show ideas and dimensions of a machine part. The cleverest idea, however, may result in a great mistake if the sketch is poorly done. We must learn to make clean sketches to be easily read by others as well as by ourselves.
Machine Elements
A machine consists of a number of machine elements.
The most useful machine element in power transmission  is the gear. The gear is virtually a w
heel with a number of teeth. Gears must work in pairs. A pair of gears with different number of teeth will increase or decrease the speed of rotation
Pulleys and belts are very common in power transmission. They can also change speeds on machine tools. Gears or pulleys rotate on shafts. The shaft must rest in bearings. Sometimes there may be couplings between shafts. We may use clutches to start or stop shaft motion.
Some machine elements may be very large. Others may be very small. However, each of them plays a part in the construction of a machine.
Cutting Tools
The metal cutting tool separates chips from the workpiece in order to cut the part to the desired shape and size. There is a great variety of metal cutting tools. Each of them is designed to perform a particular job or a group of metal cutting operations in an efficient manner. For example, a twist drill is designed to drill a hole which has a particular size, while a turning tool may be used to turn a variety of cylindrical shapes.
There are three basic types of metal cutting tools: single-point tools, multiple-point tools, and abrasives. A single-point metal cutting tool has a single cutting edge and is used for turning, boring, shaping, and planning. The most common machine tool that employs the single-point cutting tool is the engine lathe. Multiple-point tools have two or more cutting edges such as drills, reamers, and milling cutters. The cutting edge is that part of the tool where cutting is actually done. Grinding wheels are an example of abrasive cutting tools. Each grinding wheel has a lot of abrasive grains which act as very small cutting tools.
Limits and Tolerances
Machine parts are manufactured so they are interchangeable. In other words, each part of a machine or mechanism is made to a certain size and shape so it will fit into any other machine or mechanism of the same type. To make the part interchangeable, each individual part must be made to a size that will fit the mating part in the correct way. It is not only impossible, but also impractical to make many parts to an exact size. This is because machines are not perfect, and the tools become worn. A slight variation from the exact size
pulleysis always allowed. The amount of this variation depends on the kind of part being manufactured. For example, a part might be made 6 inches long with a variation allowed of 0.003 ( three thousandths ) inches above and below this size. Therefore the part could be 5.997 to 6.003 inches and still be the correct size. These are known as the limits. The difference between upper and lower limits is called the tolerance.
When the tolerance is in both directions from the basic size, it is known as a bilateral tolerance ( plus and minus ). Sometimes the limit is allowed in only one direction. This is known as unilateral tolerance. For example, a hole may have to be 2 inches minus 0.000 inches, or plus 0.005 inches. This means that the hole must not be smaller than 2 inches, but can be as large as 2.005 inches.
Surface Finish
Within recent years a great deal of attention has been given to the invesigation of the surface texture produced by various machining and finish operations, and, although we have not yet reached the stage when surface finish requirements can be specified and pro
duced just as it is possible for a given dimension, information is being built up which may in the end bring this about.
If a machine surface is reproduced to a large enough magnification it will be found to have a wevy profile, the character and shape of the waves varying for different machining methods. Equipment is now available which will produce an enlarged contour of a surface by traversing a needle-like probe over it, the instrument at the same time plotting on a chart a greatly magnified reproduction of the up-and-down movement of the probe with its longitudinal traverse.

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