Fig. 949.
In [Fig. 949], for example, let e be the washer or ring under the tool, and f therefore the fulcrum from which the tool will bend. Let the horizontal dotted line a represent the centre of the work, and it is plain that to whatever amount the tool may spring under the pressure of the cut, its motion from this spring will be in the direction of the dotted arc h, causing the tool to dip deeper into the work in proportion as the tool point is set above the work centre line a. Now the amount of tool spring will even under the most rigid conditions vary in a heavy cut with every variation in the depth of cut or in the hardness of the metal. Furthermore, as the cutting edge of the tool becomes dulled from use, its spring will increase, because the pressure required to force it to its cut becomes greater, and as a result when the conditions are such that a perceptible amount of tool spring or deflection occurs, the work will not be turned cylindrically true. Obviously the work under these conditions will be most true when the tool point is set level with the line a, passing through the work axis.
There are two advantages, however, in setting the tool above the work centre: first it severs the metal easier; and second, it enables the employment of more bottom rake without increasing the bottom clearance.
Fig. 950.
