In order to remove more weight of metal from a piece of work in a given time consistent with a desired degree of accuracy and finish on a heavy and rigid lathe, it is essential that the lathe tools be made from the most suitable material and shaped to the best advantage.
Of course, the factors like speed, feed, depth of cut, work support etc. do contribute towards the output and general efficiency; but good result in turning can be obtained only when the tools are ground or lapped to correct cutting and clearing angles, and are securely and correctly held in the tool holder. In a cutting tool, the cutting edge may be formed on the end of a solid or one-piece tool, or the tool may consist of a holder with an inserted bit or cutter.
A common type of front tool used on lathe for turning operations is shown in Fig. 12.73 and the various tool angles and clearances are shown in it. It may be noted that there is no standard shape or generally accepted tool form for turning cylindrical surfaces. As different notations are followed by different countries for cutting tools, generally a lot of confusion arises. However, here first we shall be explaining the British System and then the system followed in U.S.A.
Cutting Tool Shapes:
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For best cutting action, the tool bit must have a keen, properly shaped cutting edge. The shape of tool is decided by the metal to be machined and the type of cut, i.e., roughing or finishing. Usually the cutter bits are ground to cut in one direction only.
Fig. 12.69 shows the action of left-hand and right-hand roughing cutting tool. It should be remembered that left hand cutting tool cuts efficiently when it travels from left to right, and right hand cutting tool when it travels from right to left.
Referring to shape of tool in Fig. 12.69, it will be seen that cutting edge is straight and is provided with a small rounded nose in order to permit deep cuts at heavy feeds. The slight size relief angle is made to provide ample support to the cutting edges.
Fig. 12.70 shows the shape and action of finishing cutting tools. It will be noted that the nose of finishing tool is more rounded and honed with a fine oil stone after grinding to produce a smooth finish. For best results a light cut with a fine feed is used.
Fig. 12.71 shows the shape and action of facing tool. It will be seen that the facing tool is ground to prevent interference with the tail-stock centre. The tool point is set at a slight angle to the work face with the point leading slightly.
Fig. 12.72 shows the shape and cutting action of round nose tool. It is basically designed for lighter turning and is ground flat on the face without back or side rake to permit cutting in either direction.