This piece is first placed in the chuck, as shown in Fig. 28, and the
end rough turned to see if it is a good casting. The hole is then
drilled with a ²³⁄₃₂ inch twist drill and reamed out to size with a ¾
inch rose reamer.
[Illustration: Fig. 28]
=Centering Twist Drill.=—This drill will not bore a hole in the center
unless the point is controlled in some way. To do this, a cutting-off
tool is clamped in the tool post with its point well above the center
of the lathe and is then moved close to the point of the drill. As the
drill starts to cut, it wabbles a little on account of the point being
off center. The cutting-off tool is then gradually brought against
the drill which is at the same time being slowly fed into the work by
turning the hand-crank on the tail-stock. It is necessary to have the
drill centered true before it begins to cut the full diameter.
The drill should be placed in the tail-stock so that the cutting edges
are vertical. If they are horizontal, it will be difficult to make the
drill center.
If the hole in this piece were larger, it would be cast with a core and
then machined in the same manner as the 1⅛ inch hole in piece A, but
since it is cast solid, the hole can be machined more advantageously by
using a twist drill and a rose reamer.
=Reaming.=—After the hole has been drilled with the ²³⁄₃₂ drill, bore
it out with a small boring tool for about ⅛ of an inch from the end to
the diameter that will just fit over the reamer and insure its starting
true. Ream the hole with the reamer held in the same manner as the
twist drill in Fig. 28.
=Speed of Lathe.=—The lathe should run slower for reaming than for
drilling. The speed will be about right for this size reamer if the
belt is on the largest step of the cone without the back gears being
used. The speed for the drill may be much faster. With a high-speed
steel drill, the belt can be run on the second smallest step of the
cone. If the drill is made of carbon steel, a slower speed should be
used.
=Advantage of Rose Reamer.=—In drilling long holes like this, the drill
is very apt to get off center a little as it is fed deeper into the
work, even though it may have been started dead true.
The reamer used in this case is called a rose reamer, or rose bit, and
cuts on the end only. For this reason, if the hole is approximately
true, say within ¹⁄₆₄ of an inch, it will ream the hole straight and
true to size if it is once started true.