After the work has been properly chucked, rough turn the end and the
largest diameter to within ¹⁄₃₂ of an inch of the finished size.
All cast iron has a hard surface, or scale, from ¹⁄₆₄ to ¹⁄₃₂ of an
inch deep so that it is necessary to run the lathe slower for the first
cut than for those made after the scale has been removed. In taking
this first cut the tool should be set deep enough to permit the point
to cut under the scale.
=Speed of Lathe.=—The speed of the lathe in taking the roughing cut on
work of this size should be about right if the belt is on the smallest
step of the cone and the back gears are used. After the scale is
removed, the lathe may be run faster.
A beginner will require experience before being able to determine the
proper speeds and feeds for the different kinds of lathe work.
=Advantage of Roughing Inside.=—As the inside of piece A must fit the
outside of the piece B, the 1⅛ inch hole, the threads, and the taper
must be machine true with each other, or else A will not fit into B
properly. Now if the taper should be finished and the work moved in the
chuck before the threads and the 1⅛ inch hole are finished, they would
not be true with each other. For this reason it would be well to rough
bore the inside to within ¹⁄₃₂ of an inch of the finished size before
any of these three parts are finished.
=Roughing Inside.=—To rough bore the taper use a regular turning tool.
Set the compound rest to the correct angle and feed the tool in at that
angle.
If the lathe is not provided with a compound rest, the taper may be
rough bored by turning both feeds by hand and following the cored
surface as closely as possible.
The cored hole in the rough casting, Fig. 19, is ¹⁵⁄₁₆ of an inch in
diameter which allows ³⁄₁₆ of an inch for finishing the 1⅛ inch hole
and ⁵⁄₁₆ for the portion where the threads are to be cut.
=Use of Flat Drill.=—To rough bore the hole a 1¹⁄₁₆ flat, or lathe,
drill is used as shown in Fig. 21. The holder A is clamped in the tool
post so that the slot in it will hold the drill at the center of the
lathe. If the drill is held above or below the center, the hole will be
drilled larger than the drill. To prove that the slot in the holder is
at the center, move it close to the tail-stock center. After the holder
is properly set, move it as close to the work as possible and feed the
drill into the exercise by turning the hand-crank on the tail-stock.
This drill removes the hard surface, or scale, and also trues up, or
centers, the hole to within ¹⁄₆₄, or ¹⁄₃₂ of an inch. Now enlarge the
portion of the hole where the threads are to be cut with a 1³⁄₁₆ drill.
To determine when this drill has been fed in far enough, mark on the
drill with a piece of chalk the distance from the end of the work to
the point where the recess is to be cut. By sighting across the end of
the work the operator can then see when the drill has been fed in the
proper distance.
=Use of Boring Bar.=—To cut the square shoulder where the threads begin
and the recess where they end, use a tool and boring bar, as shown in
Fig. 22, held in the tool post. The width of this tool is ⁵⁄₃₂ of an
inch so that it will be necessary to take two cuts to make the recess
wide enough. Such a narrow tool is used because it is less liable to
chatter.
[Illustration: Fig. 22]
This tool is ground with clearance at the sides as well as at the front
and it should also be noticed that it is wider at the cutting edge than
back close to the boring bar. This is done so that when the tool is fed
into the work there will be little, or no chance of its binding on the
side.
To obtain the correct setting for the tool, move the boring bar into
the hole and bring it up close to one side. The tool should then be
adjusted until its cutting edge is parallel to the elements of this
surface.
The work is now all roughed out so that it makes very little difference
which of the three fitting parts is finished first.