A variety of drills are manufactured to suit different operations, workpiece
materials, hole dimensions and special applications, such as surgery. The design of
drills may vary in the number and width of the flutes, the amount of helix or rake
angle of the flutes, or the shape of the land and margins. In addition, the flutes may
be straight or helical, and the helix may be a right-hand or left-hand. Within the
general classification of drills flat drills, twist drills and number of the special drills
are discussed below.
A:) Twist drill
The most commonly used drill is the general-purpose twist drill, which has two
helical flutes. This drill is designed to perform well on a wide variety of materials, equipment, and job conditions. The general-purpose drill can be made to suit different conditions and materials by varying the point angle.
B:) Low-helix twist drill
The low-helix twist drill was developed primarily to drill brass and thin materials.
This modification of a twist drill is used to make shallow holes in some aluminum
and magnesium alloys. Because of its design, the low helix drill can remove the
large volume of chips formed by high rates of penetration when it is used on
machines such as lathes.
C:) High-helix twist drills
High-helix twist drills are designed for drilling deep holes in aluminum, copper,
die-cast material, and other metals where the chips have a tendency to jam in a
hole. The high helix angle (35 to 45°) and the wider flutes of these drills assist in
clearing chips from the hole, but weaken the drill body. It is used for cutting softer
metals and other low strength materials.
D:) Core drill
A coredrill, designed with three or four flutes, is used primarily to enlarge cored,
drilled, or punched holes. Due to greater number of teeth the core drill has
advantages over the twist drills in productivity and finish. A core drill is named because its first use was in drilling out the hole left by a casting core, a cylinder placed in a mould for a casting that leaves an irregular hole in the product.
E:) Spade drills
Spade drills or flat drills are similar to gun drills in that the cutting end is a flat
blade with two cutting lips. Spade drills are usually clamped in a holder and are
easily replaced or resharpened. They are available in a wide range of sizes from
very small microdrills to drills up to 300mm in diameter. Some of the smaller
spade drills have replaceable carbide inserts.
F:) Gun drill
Gun drills belong to the pressurized coolant family of hole making tools. They are
outstanding for fast, precision machining regardless of hole depth. As a rule, a gun
drill can hold hole straightness within 0.8mm per 1m of penetration, even when the tool is reasonably dull. For most jobs a gun drill can cut from 12 to 25 meters in
alloy steel before re-sharpening is necessary.
G:) Center drill
Center drills are used in metalworking to provide a starting hole for a larger-sized
drill or to make a conical indentation in the end of a workpiece in which to mount a
lathe center. In either use, the name seems apt, as the drill is either establishing the
center of a hole or making a conical hole for a lathe center.
H:) Trepanning drill
A trepaning drill is a hollow drill that cuts a ring and leaves a center core.
Trepanning drills usually have multiple carbide inserts and rely on cutting fluid to
cool the cutting tips and to flush chips out of the hole. Because trepanning drills
don’t remove all the metal from a hole less horsepower is needed to make the hole.
I:) Ejector drill
An ejector drill is used for deep hole drilling of medium to large diameter holes
(about 20mm up to about 100mm diameter). As a rule, the drill head is screwed to
the body by means of a four-start square thread. The drill body is essentially a tube
within a tube with cutting fluid traveling down between the tubes. Chip removal is
back through the center of the drill.
materials, hole dimensions and special applications, such as surgery. The design of
drills may vary in the number and width of the flutes, the amount of helix or rake
angle of the flutes, or the shape of the land and margins. In addition, the flutes may
be straight or helical, and the helix may be a right-hand or left-hand. Within the
general classification of drills flat drills, twist drills and number of the special drills
are discussed below.
A:) Twist drill
The most commonly used drill is the general-purpose twist drill, which has two
helical flutes. This drill is designed to perform well on a wide variety of materials, equipment, and job conditions. The general-purpose drill can be made to suit different conditions and materials by varying the point angle.
B:) Low-helix twist drill
The low-helix twist drill was developed primarily to drill brass and thin materials.
This modification of a twist drill is used to make shallow holes in some aluminum
and magnesium alloys. Because of its design, the low helix drill can remove the
large volume of chips formed by high rates of penetration when it is used on
machines such as lathes.
C:) High-helix twist drills
High-helix twist drills are designed for drilling deep holes in aluminum, copper,
die-cast material, and other metals where the chips have a tendency to jam in a
hole. The high helix angle (35 to 45°) and the wider flutes of these drills assist in
clearing chips from the hole, but weaken the drill body. It is used for cutting softer
metals and other low strength materials.
D:) Core drill
A coredrill, designed with three or four flutes, is used primarily to enlarge cored,
drilled, or punched holes. Due to greater number of teeth the core drill has
advantages over the twist drills in productivity and finish. A core drill is named because its first use was in drilling out the hole left by a casting core, a cylinder placed in a mould for a casting that leaves an irregular hole in the product.
E:) Spade drills
Spade drills or flat drills are similar to gun drills in that the cutting end is a flat
blade with two cutting lips. Spade drills are usually clamped in a holder and are
easily replaced or resharpened. They are available in a wide range of sizes from
very small microdrills to drills up to 300mm in diameter. Some of the smaller
spade drills have replaceable carbide inserts.
F:) Gun drill
Gun drills belong to the pressurized coolant family of hole making tools. They are
outstanding for fast, precision machining regardless of hole depth. As a rule, a gun
drill can hold hole straightness within 0.8mm per 1m of penetration, even when the tool is reasonably dull. For most jobs a gun drill can cut from 12 to 25 meters in
alloy steel before re-sharpening is necessary.
G:) Center drill
Center drills are used in metalworking to provide a starting hole for a larger-sized
drill or to make a conical indentation in the end of a workpiece in which to mount a
lathe center. In either use, the name seems apt, as the drill is either establishing the
center of a hole or making a conical hole for a lathe center.
H:) Trepanning drill
A trepaning drill is a hollow drill that cuts a ring and leaves a center core.
Trepanning drills usually have multiple carbide inserts and rely on cutting fluid to
cool the cutting tips and to flush chips out of the hole. Because trepanning drills
don’t remove all the metal from a hole less horsepower is needed to make the hole.
I:) Ejector drill
An ejector drill is used for deep hole drilling of medium to large diameter holes
(about 20mm up to about 100mm diameter). As a rule, the drill head is screwed to
the body by means of a four-start square thread. The drill body is essentially a tube
within a tube with cutting fluid traveling down between the tubes. Chip removal is
back through the center of the drill.
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