Another fault of the careless workman is, to press the bit against the stone too hard. This cuts the stone more than it wears off the tool, and it is entirely unnecessary. Furthermore, it heats up the tool, which should be avoided.
CALIPERS.--A true workman, who endeavors to turn out accurate work, and preserve his tools, will never test the work with his calipers while the piece is turning in the lathe. A revolving cast iron disk will cut ruby, the hardest substance next to the diamond, so it is not the hardness of the material which resists wear, but the conditions under which it is used.
CARE IN USE OF CALIPERS.--The calipers may be of the most hardened steel, and the work turned up of the softest brass, the latter, when revolving, will grind off the point of the tool, for the reason that the revolving piece constantly presents a new surface to the point of the calipers, and when tests are frequently made, it does not take long to change the caliper span so that it must be reset.
As stated elsewhere, the whole energy of the lathe is concentrated on the bit or cutting tool, hence, in order to get the most effective work out of it requires care; first, in grinding; and, second, in setting.
MACHINE BITS.--It does not always matter so much whether you use a square, pointed, or a round-nosed bit, provided it is properly ground and set in the tool holder. As a rule, the more brittle the metal the less the top rake or angle should be.
In the chapter relating to the grinding of tools, references were made as to the most serviceable bits for the various metals. We are concerned here with the setting or holding of these articles.
[Illustration: _Fig. 30. Plain Hook Tool_ _Fig. 31. Plain Straight Tool._]
The two illustrations here given show a pair of plain bits, in which Fig. 30 represents a hook-shaped formation, and Fig. 31 a straight grind, without any top rake. The hooked bit would do for aluminum, or steel, but for cast iron the form shown in Fig. 31 would be most serviceable.
Then the side bits, such as the round-nosed, Fig. 32 and the square end, Fig. 33, may be ground hooked, or with a top rake, or left flat.
The too common mistake is to grind the lower or clearance side at too great an angle. Fig. 34 shows the correct angle, and the dotted line A illustrates the common tendency to grind the clearance.
