Tc 9-524 fundamentals of machine tools

BORING WITH THE LATHE
Boring is the enlarging and truing of a hole by removing
material from internal surfaces with a single-point cutter bit.
On the lathe, boring is accomplished in either of these two
methods:
Mounting the holder and boring tool bar with cutter bit on
the tool post and revolving the workpiece.
Mounting the workpiece in a fixed position to the carriage
and revolving the boring tool bar and cutter bit in a chuck
attached to the headstock spindle. (This is a special
process and not used in most machine shops).
Mounting Workpiece for Boring
The workpiece may be supported in a chuck or fastened to a
faceplate for boring operations depending upon of the material
to be machined. When boring is to be performed on the ends
of long stock, the workpiece is mounted in a chuck and a
steady rest is used to support the right end near the cutter bit.
Some boring operations require the use of special chuck-
mounted mandrels to hold workplaces that cannot be
successfully mounted otherwise.
Purpose for Boring
Boring is necessary in many cases to produce accurate holes.
Drilled holes are seldom straight due to imperfections in the
material which cause drills to move out of alignment.
Therefore, where accuracy is important, drilled holes are
usually made undersize and then bored or reamed to the
proper dimensions. Boring is also useful in truing large holes
in flat material. In this case, the hole is cut undersize using a
bandsaw or trepanning tool and is trued to proper dimensions
by boring.
Boring Cutter Bit Setup
The cutter bit used for boring is similar to that used for
external turning on the lathe. The bit is usually held in a soft
or semisoft bar called a boring tool bar. The boring tool bar
(Figure 7-11) is supported by a cutting tool holder which fits
into the lathe tool post.
Boring tool bars are supplied in several types and sizes for
holding different cutter bits. The bit is supported in the boring
tool bar at a 90°, 30°, or 45° angle, depending upon the nature
of the workpiece being bored. Most general boring is
accomplished with a 90° cutter bit. The bit is mounted at a 30°
or 45° angle to the axis of the boring tool bar when it is
necessary to cut up to the bottom of a hole or finish the side of
an internal shoulder. It is desirable that the boring tool bar be
as large as possible without interfering with the walls of the
hole. The cutter bit should not extend far beyond the boring
tool bar and the bit securely in the bar, yet not have the shank-
end protrude far from the bar.
The cutter bits used for boring are shaped like left-hand
turning and facing cutter bits. Greater attention must be given
to the end clearance angle and the back rake angle because of
the curvature of the hole (Figure 7-97).