Factors Governing the General Shape of a Turning Tool and the Contour of its Cutting Edge:
The most important points in any turning tool are:
(a) The cutting edge of the tool must have a certain shape or contour. Some turning tools have straight rounded corners as at (i) (ii) and (iii) of Fig. 12.100, whereas others have curved edges as at (iv) and (v) of Fig. 12.100.
(b) The end of the tool, or the surface below the cutting edge must have a certain inclination to provide clearance or relief.
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(c) The face of the tool or the chip-bearing surface must be given certain amount of inclination on rake.
Why is chip thickness affected by tool shape, irrespective of feed? (Refer Fig. 12.100)
The chip thickness for a given amount of feed and the efficiency of the tool may be affected considerably by the contour of the cutting edge. In the case of tool (i), the thickness of the chip is equal to the feeding movement per revolution.
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With tool (ii), the chip thickness t is somewhat less than the feeding movement due to the inclination of the cutting edge. With tool (iii), the chip thickness is further reduced for a given feeding movement because the cutting edge is more nearly parallel to the axis of the work than in the case of tool (ii). If the side edge angle S is 45°, the chip-thickness t will be seven-tenths of the chip thickness from a tool like (i) which has no side edge angle.
If this angle is further increased to 60°, the chip thickness will be only 1/2 that resulting from the use of tool (i). Theoretically tool (iii) is capable of higher cutting speeds than tool (i) because for a given feeding movement per work revolution the chip is thinner; or, if the cutting speed is the same in each case, tool (iii) can be used longer than the tool (ii) without sharpening, especially in turning tough steel.
In actual practice, however, tool (ii) or some intermediate form may be preferable to shape (iii). This is particularly true when turning parts that are not sufficiently rigid to withstand the lateral thrust resulting from the cutting with a tool having large side edge angle.
The thrust from tool (i) is largely endwise, whereas tool (iii) exerts considerable lateral thrust which tends to bend the work, consequently, chattering may result, and this is not only destructive to edge of the tool but may necessitate a reduction of cutting speed. A tool of general shape (ii), or one having a side edge angle varying from 10 or 15° to 25 or 30°, is extensively used, because it gives some reduction in chip thickness without introducing excessive lateral thrust against the work.
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Cutting tools frequently used have curved edges as shown at (iv) and (v). Shape (iv) is similar in action to (ii) whereas shape (v) is more like tool (iii). Experiments have shown that a cutting edge of large radius like (v) is capable of higher speeds than a tool (iv). This is also because tool (v), for a given feed, removes a thinner chip than tool (iv), the principal being the same as that referred to in connection with the straight edge tools.
Tool (vi) having a flat or straight cutting edge along the end or nose is used for finishing cuts wherever a rough turned surface can be finished by using coarse feed. The feed per revolution should be somewhat less than the width of the tool. However, parts should be sufficiently rigid to prevent the tool from gouging in.