Holemaking Overview 271

abrasive

Consisting of hard particles used to wear down, rub away, or machine material. Abrasive tools and materials are often used in finishing operations.

adjustable boring head

A boring tool that is capable of being adjusted to different hole diameters. Adjustable boring heads are used for boring operations on a mill and may be either single-point or multi-point tools.

aluminum

A lightweight metal that is silvery white in color. Aluminum tends to produce long, stringy continuous chips.

bearings

A friction-reducing device that allows one moving part to glide past another moving part. Bearings are often cylindrical in shape.

blind hole

A hole that begins on one side of a workpiece and ends within the workpiece. Blind holes only have one opening.

boring

A cutting operation in which an existing hole is enlarged. Boring can be done on either a lathe or a mill.

boring bar

A long bar used to position a single-point cutting tool for boring operations. Boring bars cut the inside surface of a hole to enlarge it.

broach

A multi-point cutting tool that creates specific shapes in the interior of an existing hole. Broaches used in linear broaching have a series of cutting teeth, while broaches used in rotary broaching are shaped like the form they create.

broaching

A cutting operation that uses a multi-point cutting tool to change the shape or dimensions of a hole. Broaching can also be used to remove material from a workpiece surface.

broaching machine

A machine that uses a multi-point cutting tool to shape and finish either the interior of a hole or the surface of a workpiece. Broaching machines may remove large amounts of material from a workpiece.

BTA drills

A drill with multiple cutting edges and a hollow body. BTA drills, also called single tube systems, evacuate both chips and cutting fluid through the hollow drill body.

burrs

A rough, sharp edge remaining on a part after machining. Burrs pose an injury risk and interfere with the fitting of parts.

bushing

A hardened steel tube used to guide drills, reamers, and other cutting tools. Bushings are used with BTA drills to seal the entrance of a hole.

cast iron

A metal consisting of iron, over 2.11% carbon, and 1 to 3% silicon. Cast iron offers heat resistance and compressive strength.

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, also called spot drilling, can help prevent a drill from walking.

center drills

A type of drill with both a pilot drill and a countersinking drill. Center drills are used to start a hole to ensure that the hole is machined in the right location.

chamfer

To eliminate a sharp 90° corner on a workpiece by cutting angles into the edge. Chamfering removes burrs and facilitates assembly.

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.

chip

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

clearance

An intentional amount of space left between two components. Clearance is required to prevent tool damage in some holemaking operations.

concentric

Sharing a common center with another object or feature. The cutting tool should be concentric with the existing hole in both countersinking and counterboring.

continuous chips

A chip that does not break apart and instead continues to fold in on itself. Ductile metals tend to create continuous chips.

copper

A highly conductive, reddish-brown metal. Copper is a soft, ductile metal that tends to produce long, stringy continuous chips.

counterbore

A flat-bottomed recess added to the opening of a hole. Counterbores provide a space for the head of a flat-faced fastener to rest.

counterboring

A holemaking operation that enlarges one end of a previously drilled hole to a certain depth. Counterboring allows room for the head of a fastener such as a nut or bolt.

countersink

An enlarged, angled surface added to a hole opening. A countersink differs from a counterbore because it leaves a tapered opening.

countersinking

An operation that cuts a conical surface into a hole. Countersinking allows a fastener head with a matching conical angle to rest flush with the workpiece surface.

crests

The highest point of a thread. The crests of the threads on a tap determine its major diameter.

cross-hatched

Marked by very fine intersecting lines. A cross-hatched surface finish helps to lubricate a part more evenly.

cutting edges

The part of the cutting tool that engages the workpiece and removes material in the form of chips. A tool may have a single cutting edge or multiple cutting edges.

cutting fluid

Any fluid used to cool, lubricate, or clear chips during metal cutting. Cutting fluid lengthens tool service life and improves surface finish quality.

cutting force

A force generated by the motion of the cutting tool and the resistance of a workpiece. The cutting force of a drill bit is affected by its point and helix angles.

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.

discontinuous chips

A chip that easily fractures from the workpiece into small, separate pieces. Brittle materials tend to create discontinuous chips.

double tube systems

DTS. A drill with multiple cutting edges and an inner and outer tube within its body. Double tube systems, also called ejector drills, introduce cutting fluid through the outer tube and evacuate chips and cutting fluid through the inner tube.

drill bit

A multi-point cutting tool used to machine round holes in workpieces. Drill bits come in many shapes, the most common being the twist drill bit.

drill press

A machine tool that uses a drill to penetrate the surface of a workpiece and make a round hole. Drill presses can be used for a variety of holemaking operations.

drilling

A holemaking process in which a rotating cutter makes a round hole in the workpiece to a certain depth. Drilling is often the first step in a series of holemaking operations.

ductile

Able to be stretched, drawn, or formed without fracturing. Ductile metals tend to be soft and produce long, stringy chips.

ejector drills

A drill with multiple cutting edges and an inner and outer tube within its body. Ejector drills, also called double tube systems, introduce cutting fluid through the outer tube and evacuate chips and cutting fluid through the inner tube.

expansion reamers

A type of reamer that expands slightly as an adjustment screw is turned. Expansion reamers can be expanded and re-sharpened after the cutting edges have worn down.

fastener

A device that holds objects together or locates them in relation to one another. Common fasteners include screws, bolts, and rivets.

features

A defining characteristic. Features include shapes, lines, threads, and other elements machined into a workpiece.

flutes

A straight or helical recessed feature on the periphery of a cutter or drill. Flutes allow chips to flow away from the cut.

gun drills

A long drill with V-shaped flutes used for deep hole drilling. Gun drills have a hollow tube to deliver cutting fluid through the drill body.

helix angle

The angle formed by the slope of the edge of a flute and a line parallel to the drill centerline. The helix angle affects how a drill evacuates chips, with larger angles better suited for softer materials and smaller angles better suited for harder materials.

high-carbon steel

A metal alloy made up of iron and more than 0.5% carbon. High-carbon steel has high strength and hardness.

holemaking

A machining operation that creates, shapes, or finishes a hole. Holemaking is one of the most common machining operations.

honing

A precision abrasion process in which a relatively small amount of material is removed from a surface by means of an abrasive. Honing is used to obtain a desired finish or produce extremely accurate features.

honing stones

A bonded abrasive tool that may be hand-held or mounted onto a tool. Honing stones mounted on multi-stroke honing tools are used to finish the inside of cylindrical workpieces.

indexable

Having multiple cutting 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.

inserts

A replaceable cutting bit that has multiple cutting edges. Inserts can be rotated when one edge is excessively worn.

keyways

A rectangular slot or groove that is machined down the length of a hole. Keyways are sometimes created by internal broaches.

lathe

A machine tool that holds and supports a cylindrical workpiece at one or both ends and rotates it while a cutting tool removes material. Lathes are often used to perform holemaking operations such as drilling and boring.

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.

linear broaching

A broaching operation in which a non-rotating broach with a series of progressively larger cutting teeth moves through a workpiece in a single pass. Linear broaching is performed with a specialized broaching machine.

locate

To establish a designated location. Using a spot drill to locate the next drilling operation helps to prevent deflection and walking.

machine tools

A power-driven machine that uses a cutting tool to create chips and remove metal from a workpiece. Machine tools include lathes, mills, and drill presses.

major diameter

The diameter from crest to crest of an external thread or the diameter from root to root of an internal thread. The major diameter of the external threads on a tap will create a matching major diameter on the internal threads within a hole.

mandrel

The rotating shaft on which honing stones or other abrasives are mounted. Mandrels are the core of both single-stroke and multi-stroke honing tools.

mating part

The object that a workpiece has been manufactured to fit. Common mating parts for workpiece holes include screws and bolts.

mill

A machine tool that uses a rotating cutting tool to remove material from a workpiece. Mills, or milling machines, are often used to perform holemaking operations such as drilling and tapping.

minor diameter

The diameter from root to root of an external thread or the diameter from crest to crest of an internal thread. The minor diameter of the external threads on a tap will create a matching minor diameter on the internal threads within a hole.

modular drills

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.

multi-point cutting tool

A cutting tool that has two or more cutting edges. Multi-point cutting tools used in holemaking include drills, reamers, and taps.

multi-stroke honing tool

A honing tool that oscillates in and out of the workpiece while rotating. Multi-stroke honing tools have adjustable diameters and create a cross-hatched surface finish.

oscillates

Moves back and forth repeatedly. Multi-stroke honing tools oscillate and rotate to create a cross-hatched surface finish.

overhang

The distance a shank extends from a secured toolholder base. The overhang of a boring bar determines how deeply it can cut into a workpiece.

peck drilling

A drilling operation that periodically retracts the tool to clear chips or flood the hole with coolant. Peck drilling is often used for holes that are three or four times deeper than the drill diameter.

pilot

The end of the cutting tool that helps guide the cutter straight into a hole. Pilots are smaller than the rest of the tool and concentric with the drilled hole.

pitch

The distance between a point on an individual thread and the corresponding point on the next thread. The pitch of a tap should be the same as the pitch of the intended mating fastener.

pitch diameter

The diameter of an imaginary cylinder that intersects a threaded object at the point where the width of the threads and the width of the spaces between the threads are the same. The pitch diameter of a tap is smaller than the major diameter but larger than the minor diameter.

point angle

The angle formed by the cutting edges of a 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.

print

A document that contains all the instructions necessary to manufacture a part. Prints include a drawing, intended dimensions, and notes about the part.

reamers

A multi-point cutting tool that smooths or enlarges a previously drilled hole. Reamers may have straight or helical flutes.

reaming

The process of enlarging or smoothing an existing hole with a multi-point tool. Reaming is a more precise operation than boring and creates a smoother surface finish.

roots

The lowest point of a thread. The roots of the threads on a tap determine its minor diameter.

rotary broaching

A broaching operation in which a broach moves through a workpiece while rotating with a slight wobble. Rotary broaching is performed on common machine tools such as lathes, mills, or drill presses.

roundness

The degree of circularity in a cylindrical hole. The roundness of a hole can be improved by operations such as boring, reaming, and honing.

self-reversing tapping attachment

A device used in tapping applications that reverses the direction of rotation of a tap without the operator having to stop and reverse the spindle. Self-reversing tapping attachments allow machinists to retract taps without slowing down production.

shank

A cylindrical part of a tool or device opposite the cutting edges. Shanks that are shorter and thicker provide more rigid support for cutting tools.

shoulder

A flat step or plane in between two surfaces. Shoulders within holes provide space for nuts and screws to sit below the top surface of a workpiece.

single tube systems

STS. A drill with multiple cutting edges and a hollow body. Single tube systems, also called BTA drills, evacuate both chips and cutting fluid through the hollow drill body.

single-point cutting tool

A cutting tool that has a single cutting edge. Single-point cutting tools are primarily used for operations on a lathe.

single-stroke honing tool

A honing tool that makes a single pass through a workpiece while rotating. Single-stroke honing tools create a very smooth surface finish.

sleeve

A tubular part designed to fit over another part. The sleeve on a single-stroke honing tool is coated with an abrasive material.

slug

The cylindrical core of material left behind by a trepanning operation. Slugs may be saved and re-used in other machining operations.

spade drills

A drill with a wide blade at the tip capable of drilling very large holes. The width of a spade drill's blade is often larger than the drill body.

spindle

The part of a machine tool that rotates. On a lathe, the spindle holds the workpiece, and on a mill, the spindle holds the cutting tool.

splines

Teeth added to shafts and interconnecting components that ensure they move together. Splines fit loosely and provide flexible movement for many devices.

spot 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. Spot drilling, also called center drilling, can help prevent a drill from walking.

spot drills

A short, sturdy drill used to start a hole and accurately locate it. Most spot drills have a 90° tip.

spotface

A shallow flat-bottomed recess added to the opening of a hole. Spotfaces allow the head of a flat-faced fastener to sit flush with a workpiece surface.

tapered

Gradually decreasing in size from one end of an object to the other. The tapered threads on a tap perform the main cutting operation.

tapping

The process of cutting internal threads in a workpiece with a multi-point tool. Tapping is required for holes intended to fit a threaded fastener or mating part.

taps

A multi-point cutting tool that machines internal threads in a workpiece. Taps have helical external threads that taper at the end of the tool.

through hole

A hole that begins on one side of a workpiece, extends through it, and exits on another side. Through holes have openings on both sides of a part.

tolerances

The range of acceptable deviations from the measurements specified in a print. Tolerances allow a machined part to differ slightly from its intended dimensions.

tool wear

The erosion of tool material as a result of friction. Tool wear degrades the accuracy and precision of a tool.

trepanning

A holemaking operation that uses a hollow rotating tool to cut all the way through a workpiece, leaving behind a slug. Trepanning can only be used to create through holes but uses less power than conventional drilling.

twist drill

A drill characterized by helical flutes along its length and two cutting edges at the drill point. Twist drills are the most common type of drilling tool.

walking

The tendency of a drill to move off-center from the intended hole at the beginning of a drilling operation. Walking is prevented by spot drilling prior to the main drilling operation.