Twist Drills
Session 8
Twist Drills
• End-cutting tools
• Used to produce holes in most types
of materials
• Two helical grooves, or flutes, are cut
lengthwise around body of drill
• Provide cutting edges and space for
cuttings to escape during drilling process
2
Twist Drill Parts
• Most made of high-speed steel
• Replaced carbon-steel drills for two
reasons
• Can be operated at double the cutting speed
• Cutting edge lasts longer
• Stamped with letters H.S or H.S.S.
• Carbide-tipped drills
• Speeds for production have increased up
to 300% over high-speed drills
3
Three Main
Parts
of a Drill
Point
Body
Shank
4
Drill Shank
• Straight-shank drills
• Held in drill chuck
• Up to ½ in.
in diameter
• Tapered-shank drills
• Fit into internal taper of drill press spindle
• Tang provided on end to prevent drill from
slipping
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Drill Body
• Portion of drill between shank and point
• Consists of number of parts for cutting
• Flutes
• Two or more helical grooves cut around
body of drill
• Form cutting edges, admit cutting fluid,
allow chips to escape hole
• Body Clearance
• Undercut portion of body between margin
and flutes
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Drill Body
• Margin
• Narrow, raised section on body of drill
• Extends entire length of flutes
• Provides full size to drill body and edges
• Web
• Thin partition in center of drill,
extends full length of flutes
• Forms chisel edge at cutting
end of drill
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Drill Point
• Consists of chisel edge, lips, lip clearance,
and heel
• Chisel edge
• Chisel-shaped portion of drill point
• Lips
• Cutting edges formed by intersection of flutes
• Must be of equal length and have same angle
• So drill run true and not cut hole larger than drill
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Drill Point
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Lip Clearance
• The relief ground on point of drill extending
from cutting lips back to the heel
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Drill Point Characteristics
The use of various point angles and lip
clearances, in conjunction with thinning
of the drill web, will allow:
1. Control size, quality and straightness of
drilled hole
2. Control size, shape and formation of chip
3. Control chip flow up flutes
4. Increase strength of drill's cutting edges
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Drill Point Characteristics
5. Reduce rate of wear at cutting edges
6. Reduce amount of drilling pressure
required
7. Control amount of burr produced
8. Reduce amount of heat generated
9. Permit use of various speeds and feeds for
more efficient drilling
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Conventional Point (118º)
• Most commonly used drill point
• Gives satisfactory results for most
general-purpose drilling
• Lip clearance of 8º to 12º for best
results
• Too much weakens cutting
edge and causes drill to chip
• Too little results in use of
heavy drilling pressure
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Long Angle Point 60º - 90º
• Used on low helix drills for drilling of
nonferrous metals, soft cast irons,
plastics, fibers, and wood
• Lip clearance generally from 12º to 15º
• Flat may be ground on
face of lips to prevent
drill from drawing itself
into the soft material
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Flat Angle Point 135º -150º
• Used to drill hard and tough materials
• Lip clearance on flat angle point drills
only 6º to 8º to provide as much
support as possible for cutting edges
• Shorter cutting edge
tends to reduce friction
and heat during drilling
15
Four Systems of Drill Sizes
•
•
•
•
Fractional
Number
Letter
Millimeter (Metric)
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Four Systems of Drill Sizes
• Fractional
• Range from 1/64 to 4 in. (steps of 1/64th )
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Four Systems of Drill Sizes
• Number
• Range from #1 (.228 in.) to #97 (.0059 in.)
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Four Systems of Drill Sizes
• Letter
• Range from A to Z (A = .234 in., Z = .413
in.)
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Four Systems of Drill Sizes
• Millimeter (Metric)
• Straight-shank standard (0.5 to 20 mm)
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Types of Drills
• Wide variety manufactured to suit
specific drilling operations and materials
• Design of drills vary
•
•
•
•
•
Number and width of flutes
Amount of helix or rake angle of flutes
Shape of land or margin
Shape of flute: straight or helical
Whether helix is right-hand or left-hand
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Twist Drills
• Manufactured from four main materials
•
•
•
•
•
Carbon-steel drills
High-speed steel drills – Uncoated
High-speed steel drills – Coated
High Cobalt Content
Cemented-carbide drills
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Twist Drills
• Carbon-steel drills
• Used in hobby shops not for machine shop
work
• Lowest cost of all drills
• Cutting edges wear down quickly
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Twist Drills
• High-speed steel drills - Uncoated
•
•
•
•
Used extensively in machine shop work
Replaced High Carbon Steel Drills
Cutting edges withstand more heat and wear
Most commonly used
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Twist Drills
• High-speed steel drills - Coated
• Used extensively in machine shop work
• Supplements uncoated High Speed Steel
Drills
• Cutting edges withstand much more heat
and wear
Black Oxide
Titanium Nitride
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Twist Drills
• High Cobalt High-Speed Steel drills
• Used in machine shop work
• 5% Cobalt allows cutting edges to withstand
more heat and wear
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Twist Drills
• Cemented-carbide drills
• Operated at high speeds, withstand higher
heat, and can drill hard materials
• Carbide Tipped
• Solid Carbide
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General-Purpose Drill
• Has two Helical flutes
• Designed to perform well on wide variety of
materials, equipment and job conditions
• Can be made to suit different conditions and
materials by varying point angle, speeds and
feeds
• Straight-shank drills called general-purpose
jobbers length drills
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Low-Helix Drill
• Developed primarily to drill brass and
thin materials
• Used to drill shallow holes in some
aluminum and magnesium alloys
• Can remove large volume of chips
formed by high rates of penetration
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High-Helix Drills
• Designed for drilling deep holes in
aluminum, copper, die-cast material,
and other metals
• Material where chips have tendency to jam
• High helix angle (35º to 45º)
• Wider flutes assist in clearing chips from
hole
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Core Drill
• Three or four flutes
• Used to enlarge cored, drilled, or
punched holes
• Produced in sizes from ¼ to 3 in.
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Oil Hole Drills
• Have one or two oil holes running from
shank to cutting point
• Compressed air, oil, or cutting fluid can be
forced through when deep holes being
drilled
• Cutting fluid cools drill's cutting edges
and flushes chips out of hole
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Straight-Fluted Drills
• Recommended for drilling operations on
soft materials such as brass, bronze,
copper and various types of plastic
• Straight flute prevents drill from drawing
itself into material while cutting
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Deep Hole (Gun) Drills
• Used for producing holes from
approximately 3/8 to 3 in. in diameter
and as deep as 20 feet
• Consists of round, tubular stem, on end
is fastened flat, two-fluted drilling insert
• Cutting fluid forced through center of
stem to flush chips from hole
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Spade Drills
• Cutting end is flat blade with two
cutting lips
• Easily replaced or sharpened
• Available in wide range of sizes
• Micro to 12 inch diameter
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Hard-Steel Drill
• Used for drilling hardened steel
• Made from heat-resistant alloy
• As brought into contact with workpiece,
fluted, triangular point softens metal by
friction and then removes softened metal
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Step Drills
• Used to drill and countersink or drill
and counterbore different sizes of
holes in one operation
• Usually has two or more diameters
• Each size or step separated by square
or angular shoulder
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Saw-Type Hole Cutter
• Cylindrical-diameter cutter
with twist drill in center to
provide guide for cutting
teeth on hole cutter
• Made in various diameters
• Used for drilling
holes in thin materials
• Little burr produced
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Procedure to Grind a Drill
1. Wear approved safety glasses
2. Check grinding wheel and dress it to
sharpen and/or straighten wheel face
3. Adjust grinder tool rest so it is within
.060 in. of wheel face
4. Examine drill point and margins for
wear
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5. Hold drill near point with one hand,
other hand hold shank of drill slightly
lower than point
6. Move drill so it is approximately 59º to
face of grinding wheel
7. Hold lip or cutting edge of drill parallel
to grinder toolrest
8. Bring lip of drill against grinding wheel
and slowly lower drill shank
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Drilling Facts and Problems
•
•
•
•
•
•
•
Excessive speed
Excessive clearance
Excessive feed
Insufficient clearance
Cutting lips with unequal angles
Cutting lips with unequal in length
Loading and galling
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Excessive speed will cause wear at outer corners
of drill. This permits fewer regrinds of drill due
to amount of stock to be removed in
reconditioning.
Discoloration is warning sign of excess speed.
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Excessive clearance results in lack of support
behind cutting edge with quick dulling and poor
tool life. Despite initial free cutting action.
Clearance angle behind cutting lip for general
purposes is 8º to 12º.
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Excessive feed sets up abnormal end thrust,
which causes breakdown of chisel point and
cutting lips. Failure induced by this cause will
be broken or split drill.
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Insufficient clearance causes the drill to rub
behind the cutting edge. It will make the drill
work hard, generate heat, and increase end
thrust. This results in poor holes and drill
breakage.
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The web is the tapered central portion of the
body that joins the lands.
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Cutting lips with unequal angles will cause one
cutting edge to work harder than the other. This
causes torsion strain, bellmouth holes,
rapid dulling, and poor tool life.
47
Cutting lips unequal in length cause
chisel point to be off center axis and will
drill holes oversize by approximately
twice the amount
of eccentricity.
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Loading and galling is
caused by poor chip
removal with insufficient
dissipation of heat so that
material anneals itself to
the cutting edge and flute.
This condition frequently
results from using wrong
drills for the job or
inadequate cutting fluid
application.
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Characteristics of a
Properly Ground Drill
•
•
•
•
Length of both cutting lips equal
Angle of both cutting lips be the same
Lips should be free from nicks or wear
No sign of wear on margin
Note: Resharpen drill if it does not meet
all of these requirements.
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When to Regrind
• Color and shape of chips change
• More drilling pressure required
• Chips turn blue because of excessive
heat
• Top of hole out of round
• Poor finish produced in hole
• Drill chatters when it contacts metal
• Drill squeals and may jam in hole
• Excessive burr left around drilled hole
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Factors Causing
Premature Dulling of Drill
• Drill speed may be too high for
hardness of material being cut
• Feed may be too heavy and overload
cutting lips
• Feed may be too light and cause lips to
scrape rather than cut
• May be hard spots or scale on work
surface
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More Factors
• Work or drill may not be supported
properly, resulting in springing and
chatter
• Drill point may be incorrect for material
being drilled
• Finish on lips may be poor
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More Factors
•
•
•
Remove drill from wheel without
moving position of body or hands,
rotate drill one-half turn, and grind the
other cutting edge
Check angle of drill point and length of
lips with drill point gage
Repeat operations 6-10 until cutting
edges are sharp and lands are free
from wear nicks
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