inding, the important point is the clearance
line (B). As shown in this figure, it has an angle of 10 degrees, so
that in placing the tool in the holder it is obvious it cannot be placed
very high above the center, particularly when used on small work. The
top rake is ground at an angle of 60 degrees from the vertical. The arc
of the curved end depends on the kind of lathe and the size of the work.

The tool (Fig. 25), with a straight cutting edge, is the proper one to
rough off cast iron. Note that the top rake (C) is 70 degrees, and the
clearance 15 degrees.

THE CUTTING ANGLE.--Wrought iron, or mild steel, will form a ribbon when
the tool wedges its way into the material. Cast iron, on the other hand,
owing to its brittleness, will break off into small particles, hence the
wedge surface can be put at a more obtuse angle to the work.

In grinding side-cutters the clearance should be at a less angle than 10
degrees, rather than more, and the top rake should also be less;
otherwise the tendency will be to draw the tool into the work and swing
the tool post around.

DRILLS.--Holders for grinding twist drills are now furnished at very low
prices, and instructions are usually sent with the machines, but a few
words may not be amiss for the benefit of those who have not the means
to purchase such a machine.

Hand grinding is a difficult thing, for the reason that through
carelessness, or inability, both sides of the drill are not ground at
the same angle and pitch. As a result the cutting edge of one side will
do more work than the other. If the heel angles differ, one side will
draw into the work, and the other resist.

[Illustration: _Fig. 26. End view._ _Fig. 27. Side view._]

WRONG GRINDING.--When such is the case the hole becomes untrue. The
sides of the bit will grind into the walls, or the bit will have a
tendency to run to one side, and particularly if boring through metal
which is uneven in its texture or grain.

Figs. 26 and 27 show end and side views of a bit properly ground. If a
bit has been broken off, first grind it off square at the end, and then
grind down the angles, so that A is about 15 degrees, and be sure that
the heel has sufficient clearance--that is, ground down deeper than the
cutting point.

CHISELS.--A machine shop should always have a plentiful supply of cold
chisels, and a particular kind for each work, to be used for that
purpose only. This may seem trivial to the boy, but it is real