Metal Cutting

# Drill Tool Geometry 371

Drill Tool Geometry provides an overview of each tool angle for a drill, including point angle and helix angle, and details the impact that each angle has on a cutting operation. Changing the size of each cutting angle offers a tradeoff between cutting edge strength and cutting forces. Cutting tool angles must be optimized to each unique combination of workpiece material, tool material, and part feature.

Proper drill geometry can prolong tool life, optimize finished part quality, and greatly improve productivity. After taking this class, users will be able to identify and implement proper tool geometry for dill cutting processes. Improper drill tool geometry leads to premature tool wear and failure, poor surface finish, and slower speed and feed rates. Poor drill geometry can also cause deflection, which creates holes at incorrect locations and with poor tolerance. These issues increase manufacturing costs, create waste and scrapped parts, and slow production rates.

• Difficulty Advanced

• Format Online

• Number of Lessons 24

• Language English

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Course Outline
• Introduction to Drill Geometry
• Twist Drills
• Drill Parts
• Helix Angle
• The Drill Point
• Point Angle
• Web Thickness
• Drill Basics Review
• Workpiece Material and Machinability
• Drill Operation and Selection
• Selecting a Drill
• Drill Speed
• Drill Feed
• Drill Speed and Feed in Action
• Speed and Feed Review
• Deflection
• Rigidity
• Length-to-Diameter Ratio
• Regrinding
• Drill Tool Geometry Review
• Spade Drills
• Indexable Insert Drills
• Modular Drills
• Final Review
Objectives
• Describe drill geometry.
• Describe twist drills.
• Identify important components of a drill.
• Describe helix angles and how they affect a drilling operation.
• Identify important components of the drill point.
• Describe the point angle and how it affects a drilling operation.
• Describe how web size affects a drilling operation.
• Describe machinability.
• Describe a typical drilling operation and some of the considerations involved in selecting a drill.
• Describe drill speed and how it affects a drilling operation.
• Describe drill feed and how it affects a drilling operation.
• Define deflection.
• Describe rigidity.
• Describe length-to-diameter ratio and how it affects rigidity and deflection.
• Describe drill regrinding.
• Describe spade drills.
• Describe indexable insert drills.
• Describe modular drills.
Glossary
Vocabulary Term
Definition

alloys

A uniform mixture of two or more materials. Alloys must have a metallic component as one of the materials in their composition.

aluminum

A silver-white metal that is ductile, light, and thermally conductive. Aluminum is a soft metal that can tear when drilled, which reduces hole accuracy.

axial forces

A force created parallel to the drill axis. Axial forces decrease as a point angle decreases.

axis

An imaginary straight line passing through the center of an object. The axis is the line around which the drill rotates as it turns.

bearing surfaces

The portion of the drill that makes contact with the sides of a hole. The bearing surface is along the margins on a twist drill.

body

The area of the drill that extends from the toolholder. The body is the component of the drill that enters the workpiece, and it is therefore subject to tool wear.

body diameter clearance

The space between the margin and the rest of the land. The body diameter clearance provides space for cutting fluid to reach the workpiece and for chip removal.

boring

The process of enlarging an existing hole with a single-point tool. Boring is most often performed on a lathe.

brass

An alloy of copper and zinc. Brass has a tendency to grab a drill during drilling.

carbide

A common cutting tool material used to make both indexable inserts and solid cutting tools. Carbide tools are very hard and wear resistant.

cast irons

An alloy of iron, carbon, and silicon that contains at least 2.0% carbon. Cast iron is a hard, brittle material that creates short, discontinuous chips.

castings

A metal part that is formed by pouring molten metal into a mold. Castings can be hard and brittle, which means they should be drilled with heavy-duty drills.

center drilling

A drilling process in which a preliminary hole is made in a workpiece in order to guide the drill used to finish the hole. Center drilling can help prevent a drill from walking.

chatter

Vibrations of the cutting tool that cause surface imperfections on the workpiece. Chatter reduces the quality of surface finish on a part and can be decreased by using more rigid tools and setups.

chipbreakers

A feature or device designed to prevent chips from forming into long pieces. Chipbreakers can either be indentations on the surface of the drilling insert or a wafer clamped above the insert in the toolholder.

chips

An unwanted piece of metal that is removed from a workpiece. Chips form when a tool cuts or grinds metal.

chisel edge

The edge at the end of the web that connects the cutting lips. The chisel edge makes the initial penetration into the workpiece.

chisel edge angle

The angle between the chisel edge and a cutting lip, as viewed from the end of the drill. The chisel edge angle influences feed rates and hole tolerance.

chuck

A clamping workholding device that grips the shank of a mounted drill. Chucks commonly have three or four jaws that adjust to the drill diameter.

clearance angle

The slope of the side of a spade drill. The clearance angle provides space for the chips to exit the workpiece.

collet

A slotted workholding device that grips the shank of a mounted drill. A collet is designed to hold a tool with specific dimensions.

concentric

Two circular or cylindrical objects that share a common center or axis. Two objects that are concentric are aligned and on-center with one another.

conical

Having a shape like a cone. The point of a typical twist drill is conical.

copper

A reddish metal that is very ductile, thermally conductive, and corrosion resistant. Copper is a soft metal that creates long, stringy chips and should be drilled with drills that have high helix angles.

countersinking

The cutting of a beveled edge at the end of the hole so that the head of a fastener can rest flush with the workpiece surface. Countersinking requires specially designed drill bits or points.

cutting edges

The portion of the tool that performs the actual metal removal during a cutting operation. The cutting edges of a drill are located on the drill lips.

cutting fluid

Any fluid used to cool or lubricate a metal cutting process such as drilling. A cutting fluid can be an oil- or water-based liquid, gas, or paste.

cutting forces

The various stresses involved in a machining process. Cutting forces are determined by a combination of speed and feed rate, drill geometry, workpiece material, and other factors.

cycle times

The time it takes to make a part or complete one step in the process of making a part. Drilling cycle times can be reduced by using modular drills, which have easily replaceable drill points.

deflection

The unintended movement or deviation of a drill due to the application of mechanical force. Deflection of a drill can cause inaccurate hole location and dimensions.

dial indicator

A device that measures the angles at the drill point to assure the drill point is symmetrical and on-center. A dial indicator is essential for ensuring that a drill has been properly reground.

diameter

The distance from one edge of a circle to the opposite edge. The diameter of a drill helps determine the proper speed and feed at which operators should use the drill.

drill

A tool used to penetrate the surface of a workpiece and make a round hole. A drill is a multi-point cutting tool.

drill geometry

The angles and shapes formed by a drilling tool that indicate the jobs for which it is best suited. Drill geometry describes the drill's physical dimensions and attributes and cutting capabilities.

drill point

The tip of a drill. The drill point contains the cutting edges, called the lips, of the drill.

drilling

The process of using a multi-point tool to produce a hole in a workpiece. Drilling is often the first in a series of holemaking operations.

ductile

Able to be stretched, drawn, or formed without fracturing. Ductile metals are easier to cut but are prone to tearing when drilled.

extended length

The length that a drill extends from the toolholder. Extended length greatly influences the likelihood of drill deflection.

extrudes

To shape metal through the use of force. The chisel edge of the drill extrudes the workpiece to begin shaping the hole.

fasteners

A device that holds two or more objects together. Common fasteners include screws, bolts, and rivets.

fast-spiral drills

A drill with a helix angle between 35 and 40 degrees. Fast-spiral drills, also called high-helix-angle drills, have excellent chip evacuation for drilling deep holes.

feed

The rate at which the drill moves into the workpiece. Drilling feed is measured in either inches per minute (ipm), or millimeters per minute (mm/min), or inch per revolution (ipr), or millimeters per revolution (mm/rev).

feed

The rate at which the drill moves into the workpiece. Drilling feed is measured in either inches per minute (ipm), or millimeters per minute (mm/min), or inches per revolution (ipr), or millimeters per revolution (mm/rev).

feed pressure

The force applied to the drill to enable it to penetrate the workpiece. Feed pressure generally increases as the hardness of the workpiece increases.

fiber-reinforced plastics

A composite material made of polymers strengthened through the addition of a web-like matrix of fibers. Fiber-reinforced plastics should be drilled with low-helix-angle drills because they have a tendency to grab the drill.

finishing

A final metal cutting pass that emphasizes tight tolerances and smooth surface finish. Though finishing operations can be performed with a drill, usually they are done through procedures such as boring or reaming.

flex

To bend due to the application of mechanical force. When drills flex, it can lead to issues, such as deflection, which lowers hole quality and reduces tool life.

flutes

A helical recess that winds up the length of a drill body. Flutes enable the evacuation of chips from the cutting area during drilling.

forgings

A workpiece formed by compressing metal between two dies to achieve a specific shape. Forgings are often hard and must be drilled with heavy-duty drills.

friction

A force that resists the movement of two objects sliding against each other. Friction causes heat to form in the areas where the objects make contact.

general-purpose drill

A drill with a standard web size, typically between 15 and 20% of the drill diameter. A general-purpose drill is often used on high-production drilling of cast iron, steel, and nonferrous metals.

grabbing

The tendency of a metal to hold onto a drill during drilling. Grabbing can lead to poor surface finish and tolerance and can be reduced by using drills with low helix angles.

grinding

The use of an abrasive to wear material away to achieve highly accurate measurements. Grinding can achieve tight tolerances and finishes.

grinding

The use of an abrasive to wear material away to achieve highly accurate measurements. Grinding can achieve tight tolerances and high-quality finishes.

hard

Resists penetration, indentation, and scratching. Hard metals generate greater cutting forces when drilled.

hardness

A material's ability to resist indentation or scratching. Increasing hardness generally lowers the machinability of a metal.

heat

The accumulation of an elevated temperature. Heat is often the result of friction and can cause tool degradation.

heavy-duty drill

A drill with a larger web, typically between 20 and 40% of the drill diameter. Heavy-duty drills are used for drilling steel forgings, hard castings, and high-hardness ferrous alloys.

helical

Having a spiral shape. Most drills have helical flutes.

helix angle

The angle formed by the slope of the edge of a flute and a line parallel to the drill axis. Helix angles greatly affect the jobs for which a drill should be used.

high-carbon steel

A plain carbon steel that contains more than 0.45% carbon. High-carbon steels are extremely strong and hard, making them difficult to drill.

high-helix-angle drills

A drill with a helix angle between 35 and 40 degrees. High-helix-angle drills, also called fast-spiral drills, have excellent chip evacuation for drilling deep holes.

high-production

A machining operation that involves creating a large number of parts quickly. High-production drilling is often performed on steel, cast iron, and other common metals.

horsepower

A unit of power used to describe machine strength. One horsepower equals 33,000 foot-pounds (ft-lbs) of work per minute or 746 watts.

inches per minute

ipm. An English unit of measurement for feed that indicates how many linear inches a drill travels in one minute. Inches per minute corresponds to the metric measurement millimeters per minute (mm/min).

inches per revolution

ipr. An English unit of measurement for feed that indicates how many linear inches a drill travels into the workpiece in one revolution. Inches per revolution corresponds to the metric measurement millimeters per revolution (mm/rev).

indexable

A cutting tool with multiple edges that can be rotated into place. For indexable inserts, when one cutting edge wears out, an operator can turn the insert to expose a new cutting edge.

indexable insert drill

A drill with cutting inserts clamped to a steel body. Indexable insert drills are among the most cost-effective drills because of their high metal removal rate.

inserts

A removable, geometric cutting bit that has multiple cutting edges. Inserts are used with indexable insert drills.

ipm

Inches per minute. An English unit of measurement for feed that indicates how many linear inches a drill travels in one minute. Drill feed can be measured in ipm, which corresponds to the metric measurement millimeters per minute (mm/min).

ipr

Inches per revolution. An English unit of measurement for feed that indicates how many linear inches a drill travels into the workpiece in one revolution. Drill feed can be measured in ipr, which corresponds to the metric measurement millimeters per revolution (mm/rev).

L/D ratios

Length-to-diameter ratio. A numerical value comparing the length of a cylindrical tool or workpiece with its diameter. Higher L/D ratios offer less rigidity.

land

The area of a drill between flutes. The land is cut back slightly on both sides to leave room for chips to exit the cutting area.

left-hand twist drills

A drill that rotates counterclockwise. A left-hand twist drill is mostly used to remove broken right-hand threaded bolts.

length-to-diameter ratio

L/D ratio. A numerical value comparing the length of a cylindrical tool or workpiece with its diameter. Higher length-to-diameter ratios offer less rigidity.

lip face

The flat surface on the side of the drill lip that is fed into the workpiece radially. The position of the lip face helps determine if a drill is right-handed or left-handed.

lip relief angle

The measurement between a line tangent to the outer edge of the lip and a line perpendicular to the axis. The lip relief angle measures the clearance behind the cutting lip.

lips

The cutting edges of a drill that extend from the chisel edge to the periphery. The lips perform the actual metal removal during drilling.

low-carbon steel

A plain carbon steel that contains less than 0.30% carbon. Low-carbon steels are generally tough, ductile, and fairly easy to drill.

low-helix-angle drills

A drill with a helix angle between 15 and 20 degrees. Low-helix-angle drills, also called slow-spiral drills, have high rigidity and can withstand greater cutting forces.

m/min

Meters per minute. A metric measurement of speed that accounts for the number of linear meters that a point on the edge of a drill travels in one minute. Drill speed is measured in m/min, which corresponds to the English measurement surface feet per minute (sfm).

machinability

The ability of a metal to be cut and shaped by machining processes such as drilling, milling, or turning. Machinability describes the ease or difficulty inherent in cutting a specific material.

margin

A portion of the land that is not cut away. Margins guide the drill into the hole and maintain the drill diameter.

metal removal rate

MRR. The volume of metal removed in a given amount of time. Metal removal rate is measured in cubic inches per minute or cubic centimeters per minute.

meters per minute

m/min. A metric measurement of speed that accounts for the number of linear meters that a point on the edge of a drill travels in one minute. Meters per minute corresponds to the English measurement surface feet per minute (sfm).

millimeters per minute

mm/min. A metric unit of measurement for feed that indicates how many linear millimeters a drill travels in one minute. Millimeters per minute corresponds to the English measurement inches per minute (ipm).

millimeters per revolution

mm/rev. A metric unit of measurement for feed that indicates how many linear millimeters a drill travels into the workpiece in one revolution. Millimeters per revolution corresponds to the English measurement inches per revolution (ipr).

mm/min

Millimeters per minute. A metric unit of measurement for feed that indicates how many linear millimeters a drill travels in one minute. Drill feed can be measured in mm/min, which corresponds to the English measurement inches per minute (ipm).

mm/rev

Millimeters per revolution. A metric unit of measurement for feed that indicates how many linear millimeters a drill travels into the workpiece in one revolution. Drill feed can be measured in mm/rev, which corresponds to the English measurement inches per revolution (ipr).

modular drill

A drill consisting of an interchangeable and disposable point mechanically attached to a drill body. Modular drills can greatly increase production efficiency but tend to be less rigid than solid tools.

nickel alloys

A metal containing a significant percentage of nickel. Nickel alloys are hard and should be drilled with low-helix-angle drills.

nonferrous

A material that does not contain a significant amount of iron. Common nonferrous metals include aluminum and copper.

off-center

When the components of a tool or part are improperly aligned with that tool or part's axis. When a drill flexes, it is forced off-center and will not create a hole in the desired location or of the desired dimensions.

on-center

When the components of a tool or part are properly aligned with that tool or part's axis. A reground drill must be on-center or it will not perform correctly.

operators

A person trained to run a specific machine. Operators are responsible for helping ensure that a machining process runs properly, efficiently, and safely.

perpendicular

Two lines or objects that meet at a right (90°) angle. The lip relief angle is determined by measuring the angle between a line tangent to the outer edge of the lip and a line perpendicular to the axis of the drill.

plastics

A lightweight, corrosion resistant material composed of larger polymer molecules. Plastics create long, stringy chips and should be drilled with drills that have high helix angles.

point angle

The angle formed by the cutting edges, or lips, of the drill. The point angle greatly affects how a drill cuts, with larger angles better suited for harder materials and smaller angles better suited for softer materials.

proportional

A constant ratio or relationship between two or more values. The likelihood of drill deflection is proportional to extended length relative to its diameter.

radial forces

A force created perpendicular to the drill axis. Radial forces increase as the point angle decreases.

reaming

The use of a multi-point cutting tool to smooth or enlarge a previously drilled hole. Reaming is often a finishing process.

regrinding

The process by which a tool is resharpened with an abrasive after extensive use. Regrinding, also known as sharpening, can help a tool perform optimally again, but it also reduces tool life.

removable drill tip

The interchangeable portion of the drill that performs the actual cutting. The removable drill tip determines the drill geometry.

revolutions per minute

rpm. A unit of measurement that indicates the number of revolutions a drill makes in the spindle in one minute. Revolutions per minute is an important factor in determining drill speed.

right-hand twist drills

A drill that rotates clockwise. Right-hand twist drills are by far the most common drills.

rigid

Stiff and inflexible. Rigid drills are often required when drilling harder materials.

rigidity

The quality of a workpiece, drill, machine, or machine setup characterized by being stiff and inflexible. A drill with good rigidity is often required for drilling harder materials.

rigidity

The quality of a workpiece, machine, or machine setup characterized by being stiff and inflexible. A drill with good rigidity is often required for drilling harder materials.

rpm

Revolutions per minute. A unit of measurement that indicates the number of revolutions a drill makes in the spindle in one minute. Drill speed is calculated in part using rpm.

screws

A threaded device used for fastening parts or transferring motion. Broken screws can be removed using a left-handed twist drill.

service life

The length of time a drill is expected to be operational before it must be replaced. Service life can be extended through selection of proper drill tool geometry.

setup

The arrangement of tooling and fixturing on a manufacturing machine. A drilling setup with low rigidity may require a more rigid tool.

sfm

Surface feet per minute. An English measurement of speed that accounts for the number of linear feet that a point on the edge of a drill travels in one minute. Drill speed is measured in sfm, which corresponds to the metric measurement meters per minute (m/min).

shank

The portion of a drill that allows the drill to be held and driven. The shank is on the opposite end of the drill from the point.

sharpening

The process by which a tool is resharpened with an abrasive after extensive use. Sharpening, also known as regrinding, can bring a tool back to optimal performance, though tool life will be reduced.

slow-spiral drills

A drill with a helix angle between 15 and 20 degrees. Slow-spiral drills, also called low-helix-angle drills, have high rigidity and can withstand greater cutting forces.

soft

Deforming easily when subjected to stress. Soft metals include aluminum and copper.

spade drill

A drill with a wide blade at the tip capable of drilling very large holes. A spade drill's blade width often exceeds the diameter of the drill body.

speed

The rate at which the drill rotates in relation to the workpiece. Drilling speed is usually expressed in surface feet per minute (sfm) or meters per minute (m/min).

spindle

The part of the machine tool that rotates. In drilling applications, the spindle holds the drill.

stability

The ability to remain firmly in position. Drilling machines with good stability create more accurate holes.

steels

An alloy of iron and carbon containing less than 2.0% carbon. Steels often contain other elements to enhance various aspects of the metal.

strength

A metal's ability to resist outside forces attempting to break or deform the metal. Increasing strength makes a metal more difficult to drill.

surface feet per minute

sfm. An English measurement of speed that accounts for the number of linear feet that a point on the edge of a drill travels in one minute. Surface feet per minute corresponds to the metric measurement meters per minute (m/min).

tang

The optional flattened end on some drill shanks that locks into the machine workhead and allows the drill to be rotated and driven securely. A tang allows a drill to be mounted on a lathe.

tangent

A line, line segment, or ray that touches a circle or object at exactly one point. The lip relief angle is determined by measuring the angle between a line tangent to the outer edge of the lip and a line perpendicular to the axis of the drill.

taper shank drills

A drill with a tang or tapered shank. Taper shank drills are used when the drill must be mounted on a lathe.

thrust

The amount of force or drive exerted on a drill while it turns. Thrust occurs when the drill is accelerated in a linear direction.

titanium

A silver-gray, strong, lightweight metal known for its corrosion resistance and strength-to-weight ratio. Titanium tends to work harden and should be drilled using drills with high helix angles.

tolerance

An unwanted but acceptable deviation from a given dimension defined by a blueprint. Tolerance is improved through the correct selection of a chisel edge angle when drilling.

toolholder

A device used to hold a cutting tool in place during machining. A toolholder can be a range of devices, including a milling cutter and a spindle.

torque

The amount of force exerted to rotate a drill and cut a hole in a workpiece. More rigid drills can be subjected to greater levels of torque.

torsion strength

The ability of a rotating drill to withstand the forces of workpiece resistance. Torsion strength is particularly important when drilling hard metals such as high-carbon steel.

twist drills

A drill characterized by helical flutes along its length and a cutting point at the tip. Twist drills are the most commonly used type of drill.

walk

To deviate from an intended path. The tendency of a drill to walk can be eliminated by center drilling prior to the drilling operation.

wear

The erosion of material as a result of friction. Wear is common in tools that operate at excessively high speeds.

wear lands

A worn portion of the drill near the cutting edges. Wear lands appear gradually due to abrasion and other cutting forces.

web

The central portion of the drill body that joins the lands. The web forms the chisel edge at the cutting point on a drill.

web thinning

The grinding of the web and chisel edge to shrink them and return a drill to its original specifications. Web thinning greatly improves the performance of a reground drill.

work harden

To increase in hardness due to plastic deformation during a cold working or machining process. Work hardening materials should be drilled using a bit with high helix angles.

workhead

The component of a drilling machine that houses the spindle. The workhead holds and rotates the drill.

workpiece

A part that is subjected to one or more manufacturing procedures, such as machining, welding, or casting, Workpiece material is a key consideration when determining proper drill geometry.