Machinery Repairman

Figure 6-45.—Boring center hole.
If you must center a piece very accurately, bore
the tapered center hole after you center drill to correct
any run-out of the drill. You can do this by grinding a
tool bit to fit a center gauge at a 60° angle. Then, with
the toolholder held in the toolpost, set the compound
rest at 30° with the line of center as shown in figure
6-45. Set the tool exactly on the center for height and
adjust the tool to the proper angle with the center
gauge as shown at A. Feed the tool as shown at B to
correct any runout of the center. The tool bit should
be relieved under the cutting edge as shown at C to
prevent the tool from dragging or rubbing in the hole.
For center drilling a workpiece, the combined
drill and countersink is the most practical tool.
Combined drills and countersinks vary in size and the
drill points also vary. Sometimes a drill point on one
end will be 1/8 inch in diameter and the drill point on
the opposite end will be 3/16 inch in diameter. The
angle of the center drill is always 60° so that the
countersunk hole will fit the angle of the lathe center
point.
In center drilling, use a drop or two of oil on the
drill. Feed the drill slowly and carefully to prevent
breaking the tip. Use extreme care when the work is
heavy, because it is then more difficult to “feel” the
proper feed of the work on the center drill.
If the center drill breaks in countersinking and
part of the broken drill remains in the work, you must
remove the broken part. Sometimes you can jar it
loose, or you may have to drive it out by using a
chisel. But it may stick so hard that you cannot easily
remove it. If so, anneal the broken part of the drill
and drill it out.
The importance of having proper center holes in
the work and a correct angle on the point of the lathe
centers cannot be overemphasized. To do an accurate
job between centers on the lathe, you must
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Figure 6-46.—Examples of center holes.
countersink holes of the proper size and depth and be
sure the points of the lathe centers are true and
accurate.
Figure 6-46 shows correct and incorrect
countersinking for work to be machined on centers.
In example A, the correctly countersunk hole is deep
enough so that the point of the lathe centers does not
come in contact with the bottom of the hole.
In example B of figure 6-46, the countersunk hole
is too deep, causing only the outer edge of the work to
rest on the lathe center. Work cannot be machined on
centers countersunk in this manner.
Example C shows a piece of work that has been
countersunk with a tool having too large an angle.
This work rests on the point of the lathe center only.
It is evident that this work will soon destroy the end of
the lathe center, thus making it impossible to do an
accurate job.
MOUNTING THE WORK.—Figure 6-47
shows correct and incorrect methods of mounting
work between centers. In the correct example, the
driving dog is attached to the work and rigidly held by
the setscrew. The tail of the dog rests in the slot of the
drive plate and extends beyond the base of the slot so
that the work rests firmly on both the headstock center
and tailstock center.
In the incorrect example, the tail of the dog rests
on the bottom of the slot on the faceplate at A, thereby
pulling the work away from the center points, as
shown at B and C, causing the work to revolve
eccentrically.