Trre OnIGINAL HONAE WOOOWORKING RruN ... - Wood Tools

abiliry to hold a cutting edge. That's
the first thing to remember about
drills - that the rotational speed
(rpm), feed rate, bit rype, hole diameter,
and workpiece properties must
be balanced to avoid heat build-up
or tool failure. For deep holes especially,
backing the bit out periodically
to clear chips will help dissipate
the heat before it's a problem.
Keep in mind also that the forces
involved change rapidly for larger
holes. It's the area of the circle, not
the diameter, that represents the
material you're removing. For
example, a 1"-dia. hole is four times
larger than a 1/2" hole, though its
diameter is only hvice as great. For
safery's sake, large-diameter bits have
to rotate at significantly slower
speeds than smaller drills do.
TWIST DRILLS: IllE BASIC M0DEL
Twist drills are easily the most common
rype around today, in part
because they are so versatile. Most
are made from high-speed steel,
which can withstand high temperatures
without "burning." They can
be used to drill almost any metal,
plastic. wood, or composite material.
but in larger sizes (over l/2") Because rwist drills are designed
for use with metal, they feature a
slighdy blunted nose that won't
deform under pressure against a
hard suface. The tradeoff for this
feature is a tendency to spin away
from the intended hole center, a
habit known as walking or skating.
Recently, manufacturers have
been grinding new tip profiles on
rwist drills to make them stay put
when starting a hole. Called "pilotpoints"
or "bullet-points," these do
improve a bit's precision, but their
complex geometry usually means
do-it-yourself sharpening isn't an
option.'Whatever their shortcomings,
standard rwist drills can at least
be sharpened easily on a bench
grinder or with an inexpensive drill
sharpening accessory.
Titanium-nitride (TiN) coatings
on some bits add hardness and a
self-lubricating properry but this
gold-colored microthin coating
improves metal-drilling only, with
little or no difference in wood.
W0OD-BORlltlG BfiS
'Wood
is softer and machines differently
than metal, and specialized bits
they have evolved for drilling it.
can get pretty expensive, especially For instance, drill bits designed
relative to other rypes ofbits.
for wood have a center point that
TWIST DRILT BIili
DRITL BIT ANATOMY
Chip
llfter
Auger blt
WOOD BORING BITS
liil-coated bit Standard bit Bullet-point bit Brad-poirt bit Spade bit
Self-feed screw polnt
(not for drlll press use)
Cutting
spur
Chlp
clearance
flute
Twlst drlll
contacts the workpiece immediately
to keep the bit placement precise.
Also, cutting spurs score the wood at
the perimeter of the hole so the
fibers at its edge are cut cleanly
rather than torn.
Forstner blt
Forstner bit Muftispur bit Ship auger