[Illustration: _Fig. 36. Correct Angle_ _Fig. 37. Wrong Angle_]
Fig. 39 is still worse, and is of no value for any purpose except in polishing brass, where it would be serviceable. It would make a sorry looking job with aluminum. Brass requires a tool with very little top rake, and the point should be set near the center line C.
LATHE SPEED.--It is often a question at what speeds to run the lathe for different work. If you know the speeds of your lathe at low and high gear, you must also consider the diameter of the work at the cutting point.
The rule is to have the bit cut from 15 to 20 feet per minute for wrought iron; from 11 to 18 feet for steel; from 25 to 50 for brass; and from 40 to 50 for aluminum.
[Illustration: _Fig. 38. Too Low_ _Fig. 39. Improper Set_]
As a result, therefore, if, at low speed, a piece 10 inches in diameter, runs at the proper speed to cut at that distance from the center, it is obvious that a piece 5 inches in diameter should ran twice as fast. This is a matter which time and practice will enable you to judge with a fair degree of accuracy.
Observe this as a maxim: "Slow speed, and quick feed."
BORING TOOLS ON LATHE.--The lathe is a most useful tool for boring purposes, better for some work than the drilling machine itself. The work which can be done better on a lathe than on a drilling machine, may be classified as follows:
1. When straight and true holes are required.
2. In long work, where the lathe is used to turn up the article, and where the drilling can be done at the same time.
3. Anything that can be chucked in a lathe.
4. Where the work is long and cannot be fixed in a drilling machine. The long bed of the lathe gives room for holding such work.
[Illustration: _Fig. 40. Internal Set_ _Fig. 41. Set for Brass_]
THE RAKE OF THE DRILL.--A boring tool requires some knowledge in setting. It should have a greater top rake than for the outside work, and the cutting edge should also be keener, as a rule.
[Illustration: _Fig. 42.--Surface Gage._]
In this class of work the material bored must be understood, as well as in doing outside work.
The hooked tool, Fig. 40, is shown to be considerably above the center line, and at that point it will do the most effective cutting on steel. If, on the other hand, brass is operated on there should be no top rake, as illustrated in Fig. 41, thus assuring a smooth job.