o be bored. Ordinarily, one or two roughing
cuts and one finishing cut would be sufficient, unless the rough bore
were considerably below the finish diameter. As previously explained,
the speed and feed must be governed by the kind of material being bored
and the diameter of the cut. The power and rigidity of the boring
machine and the quality of the steel used for making the cutters also
affect the cutting speed and feed. As the finishing cut is very light, a
tool having a flat cutting edge set parallel to the bar is ordinarily
used when boring cast iron. The coarse feed enables the cut to be taken
in a comparatively short time and the broad-nosed tool gives a smooth
finish if properly ground.

[Illustration: Fig. 9. Cylinder mounted on Horizontal Machine for
Boring]

The coarse finishing feed is not always practicable, especially if the
boring machine is in poor condition, owing to the chattering of the
tool, which results in a rough surface. The last or finishing cut should
invariably be a continuous one, for if the machine is stopped before the
cut is completed, there will be a ridge in the bore at the point where
the tool temporarily left off cutting. This ridge is caused by the
cooling and resulting contraction and shortening of the tool during the
time that it is stationary. For this reason independent drives are
desirable for boring machines.

Facing arms are attached to the bar on either side of the cylinder for
facing the flanges after the boring operation. The turning tool of a
facing arm is fastened to a slide which is fed outward a short distance
each revolution, by a star-wheel that is caused to turn as it strikes
against a stationary pin. By facing the flanges in this way, they are
finished square with the bore.

When setting a cylinder which is to be bored it should, when the design
will permit, be set true by the outside of the flange, or what is even
better, by the outside of the cylinder itself, rather than by the rough
bore, in order that the walls of the finished cylinder will have a
uniform thickness. The position of very large cylinders, while they are
being bored, is an important consideration. Such cylinders should be
bored in the position which they will subsequently occupy when
assembled. For example, the cylinder for a large horizontal engine
should be bored while in a horizontal position, as the bore is liable to
spring to a slight oval shape when the cylinder is placed horizontal
after