Caution
The two previous drilling techniques should
not be attempted until considerable experience in
lathe operation has been acquired.
Accuracy requires a centered starting point
for the drill. A starting point made with a center
drill is adequate for most drilling jobs. Holes over
1/2″ (12.5 mm) in diameter should not be drilled
until a pilot hole has been made. The drill
employed for this purpose should have a diam-
eter slightly larger than the dead center of the
larger drill’s tip. Ample clearance must be
provided in the back of the work to permit the
drill to break through without striking the chuck,
Figure 28-4.
28.1.2 Reaming on a Lathe
Reaming is an operation used to make a hole
accurate in diameter and finish, Figure 28-5. The
hole is first drilled slightly undersized to allow
450 Modern Metalworking
Work
Compound
Tailstock
Chuck
Taper shank of drill
Figure 28-2.
Drills larger than 1/2″ (12.5 mm) in diameter are
usually fitted with a self-holding taper that fits into
the tailstock spindle opening.
Taper shank
drill
Drill
holder
Tailstock
center
Tool holder
Compound rest
Taper shank
drill
Tailstock
center
Lathe
dog
Compound
rest
Tool holder
Figure 28-3.
A—When a drill shank is too large to be fitted into
the tailstock, a lathe dog can be used to keep it from
revolving. The tail of the dog is supported by the
compound rest. This type of drilling requires care to
prevent the drill from slipping off the tailstock center
when the full diameter breaks through the work.
B—A commercial drill holder can also be used when
using large taper shank drills.
Clearance
Figure 28-4.
There must be enough clearance between the back of
the work and the chuck face to permit large diameter
drills to break through the work without damaging
the chuck.
Chucking reamer
Reamer at 1/2 to 2/3
drilling speed
Figure 28-5.
A chucking reamer can be mounted in a Jacobs chuck
to finish a hole.