The 1⅛ inch hole has been drilled with a ¹¹⁄₁₆ inch lathe drill, but
as such a tool cannot be relied upon to drill true to center, or size,
it is necessary to turn it out with a boring tool. With this tool
the hole can be bored true to center and within .01 of an inch of the
finished size.
The boring bar used in this case is the same as shown in Fig. 22, but
the cutter has a rounded point and is similar to the tool used for
outside turning except that it is ground with less clearance.
To insure accuracy and conserve time, the hole is then finished with a
shell reamer held in the lathe as shown in Fig. 23.
[Illustration: Fig. 23]
=Reaming the Hole.=—Before starting the reamer, the hole should be
bored at the end, for a distance of about ⅛ of an inch, to the size
which will just permit the reamer to enter. This diameter must be
calipered very carefully and should be tested with the reamer itself.
The rest of the hole is then bored about .01 of an inch smaller in
diameter to allow enough material for finishing with the reamer. Since
the reamer used in this case cuts on the sides as well as on the end,
the hole must be bored true to center in order to be reamed true.
If the reamer has a tapered shank, it is held in the lathe by a square
shank socket and wrench, as shown in Fig. 23, and is fed into the work
by turning the hand-crank on the tail-stock.
[Illustration: Fig. 24]
In case the reamer has a straight shank, it is held as shown in Fig.
24. Here a dog is fastened to the end of the reamer and prevented from
turning by a tool clamped at an angle in the tool post. The end of the
tool presses against the dog near the shank of the reamer so that as
the reamer is fed into the work the carriage of the lathe is forced
along with it. This causes the tool to hold the end of the reamer
against the center of the tail-stock.
When reaming work in a lathe, if the tail-stock is off center the hole
will be reamed too large at the front end.
=Accurate Boring with Boring Bar.=—In turning out holes with a boring
bar, if all the cuts are started from one end, that end will be bored
larger than the other. In case the hole is to be reamed, the reamer
will correct this, but if the hole is to be finished with the boring
bar it will be necessary to bore the hole from both ends. This is done
by reversing the feed of the carriage.
=Speed of Lathe.=—The speed of the lathe for reaming should be slower
than when using the boring bar. If the belt is on the second smallest
step of the cone with the back gears in, the lathe should have about
the right speed for reaming. When using the boring bar, the belt should
be on the largest step of the cone without the back gear.