Fig. 983.
It follows then that a tool whose sides are at a given angle will cut a different angle of thread for every variation in the amount of clearance. But whatever the amount of clearance may be, the tool will produce correct results providing that the gauge to which the tool is ground is held level, as in [Fig. 983] at a, and not at an angle as at b.
The tool, however, must be set at the correct height with relation to the work, and its top surface must point to the work axis to produce correct results.
Fig. 984.
Suppose, for example, that in [Fig. 984] a is a piece of work, its horizontal centre being represented by the dotted line c, and its centre of revolution being at c. Now suppose d is a screw-cutting tool cutting a depth of thread denoted by e. g is another lathe tool having teeth of the same form and angle as d, but lifted above the horizontal centre of the work. The depth of thread cut by g is denoted by f, which is shallower, though it will be seen that the point of g has entered the work to the same depth or distance (of the tool point) as d has. It is obvious, however, that for any fixed height, a tool suitable to cut any required depth or angle can be made, but it would be difficult to gauge when the tool stood at its proper height.
Fig. 985.