We have now
defined the form of the upper face of the tooth. How to delineate the U
arms will be taken up later on, as, in the present case, the necessary
lines would confuse our drawing.
We would here take the opportunity to say that there is a great latitude
taken by makers as regards the extent of angular impulse given to the
cylinder, or, as it is termed, the "actual lift." This latitude governs
to a great extent the angle _A b g_, which we gave as sixty-four degrees
in our drawing. It is well to understand that the use of sixty-four
degrees is based on no hard-and-fast rules, but varies back and forth,
according as a greater or lesser angle of impulse or lift is employed.
In practical workshop usage the impulse angle is probably more easily
estimated by the ratio between the diameter of the cylinder and the
measured (by lineal measure) height of the impulse plane. Or, to be more
explicit, we measure the radial extent from the center _A_ between the
arcs _a k_ on the line _A b_, and use this for comparison with the outer
diameter of the cylinder.
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