Engineering Glossary

An extensive A-Z reference of terms used across mechanical engineering, manufacturing, materials, fasteners, limits and fits, and engineering drawings. Where a term has its own dedicated page on this site, it's linked directly.

A • B • C • D • E • F • G • H • I • J • K • L • M • N • O • P • Q • R • S • T • U • V • W • X • Y • Z

A

Abrasive

A hard material (such as aluminium oxide or silicon carbide) used to grind, cut, or polish a softer material.

Additive Manufacturing

Building a part layer by layer from a digital model, e.g. 3D printing, as opposed to removing material (subtractive manufacturing) or forming it (casting, forging).

Allowance

The intentional difference in size between mating parts, chosen to give a specific type of fit. See Limits, Fits and Tolerances.

Alloy

A metal made by combining two or more elements, at least one of which is a metal, to improve properties such as strength or corrosion resistance.

Anchor Bolt

A bolt cast or fixed into concrete or masonry to anchor a structure or piece of equipment to it.

Anneal

A heat treatment process that softens a metal, relieves internal stress, and improves ductility by heating and then slowly cooling it.

Anodic Index

A ranking of metals by their electrochemical potential, used to predict galvanic corrosion risk when dissimilar metals are in contact. See Anodic Index.

Anodising

An electrochemical process that thickens the natural oxide layer on a metal, usually aluminium, to improve corrosion resistance and allow dyeing. See Anodising.

Annulus

A flat ring shape, the area between two concentric circles.

Arbor

A shaft or spindle used to hold and rotate a cutting tool or workpiece, commonly on a milling machine.

Austenitic Stainless Steel

A family of stainless steels (e.g. 304, 316) with a face-centred cubic structure, giving good corrosion resistance and formability; not hardenable by heat treatment.

B

Backlash

The small amount of clearance between mating gear teeth or a screw and nut, allowing them to move without binding.

Barth Key

A parallel key with a keyway cut so the key bears on its sides rather than its top and bottom, similar in principle to a Pratt & Whitney key.

Basic Size

The theoretical exact size from which allowances and tolerances are applied to define the limits of size for a feature. See Limits, Fits and Tolerances.

Bearing

A component that allows constrained relative motion between two parts while reducing friction, typically supporting a rotating shaft. See Bearing Dimensions.

Belleville Washer

A conical, disc-shaped spring washer used to apply a controlled, constant preload or to take up axial clearance.

Bending Moment

The internal moment that causes a beam or structural member to bend, equal to the sum of forces multiplied by their distance from a given point.

Bevel Gear

A gear shaped like a cone, used to transmit motion between two shafts whose axes intersect, typically at 90°.

Bill of Materials (BOM)

A structured list of all the parts, components and quantities required to build an assembly.

Blanking

A press operation that cuts a flat shape out of sheet metal in a single stroke; the cut-out piece is the finished part.

Black Oxide

A conversion coating, usually on steel, that gives a black finish with mild corrosion resistance and reduced light reflection. See Black-Oxidising.

Bolt Circle (PCD)

The diameter of the circle on which a set of bolt holes are evenly positioned around a centre point.

Bolt Grade

A marking system (e.g. 8.8, 10.9, Grade 5) that identifies a fastener's mechanical strength properties. See Bolt Grades and Markings.

Brazing

A joining process that melts a filler metal (above 450°C) into the gap between two closely fitted parts, without melting the parts themselves.

Brinell Hardness

A hardness scale based on pressing a hardened ball into a material's surface and measuring the indentation. See Hardness Comparison.

Broaching

A machining process that uses a toothed tool, pulled or pushed through a workpiece, to cut an internal or external profile such as a keyway or spline.

Burr

A rough, raised edge or fragment of material left on a workpiece after cutting, drilling, or machining.

Button Head Screw

A cap screw with a low-profile, rounded head, usually driven with a hex or Torx socket.

C

CAD

Computer-Aided Design; software used to create precise 2D drawings or 3D models of parts and assemblies.

Callout

A note or symbol on a drawing that specifies a dimension, tolerance, finish, or other requirement for a feature.

Cap Screw

A generic term for a bolt designed to be tightened directly into a threaded hole rather than used with a nut, such as a socket head cap screw.

Carburizing

A case hardening process that introduces carbon into the surface of a low-carbon steel part, then heat treats it to harden just the outer case.

Carriage Bolt

A bolt with a smooth, domed head and a square section under the head that bites into the mating material, preventing the bolt from spinning as the nut is tightened. Also called a coach bolt.

Case Hardening

A heat treatment that hardens only the outer surface (case) of a part, leaving a tougher, more ductile core.

Casting

A manufacturing process where molten material is poured into a mould and allowed to solidify into the desired shape.

Centrifugal Casting

A casting process where molten metal is poured into a rotating mould, using centrifugal force to press the metal outward and form a hollow, cylindrical part such as a pipe.

Centroid

The geometric centre of a shape's area, the point at which the shape would balance if it were a flat plate of uniform thickness.

Chamfer

A flat, angled surface machined onto an edge or corner, usually at 45°, to remove a sharp edge or aid assembly.

Charpy Test

An impact test that measures the energy absorbed by a notched sample when struck by a swinging pendulum, used to assess toughness.

Circlip

A retaining ring that fits into a groove on a shaft or bore to hold components in place axially. See Circlips.

Clearance Fit

A fit where the hole is always larger than the shaft, guaranteeing a gap between them so the parts can move or be assembled freely. See Limits, Fits and Tolerances.

CNC

Computer Numerical Control; the automated control of machine tools via a computer following pre-programmed instructions.

Coach Bolt

See Carriage Bolt.

Coefficient of Thermal Expansion

A measure of how much a material's dimensions change per degree of temperature change.

Concentricity

A geometric tolerance requiring the median points of a feature to share a common axis with a datum feature. See Geometric Tolerances.

Corrosion Resistance

A material's ability to withstand chemical or electrochemical attack, such as rusting, over time.

Countersink

A conical recess machined around a hole so that a countersunk screw or rivet head sits flush with the surface.

Creep

The slow, gradual deformation of a material under sustained load, usually significant only at high temperatures or over long periods.

Critical Speed

The rotational speed at which a shaft's natural frequency is excited, causing resonance and potentially damaging vibration.

Cup Point Set Screw

A set screw with a small concave recess at its tip, giving a positive bite into the mating shaft. See Set Screws.

D

Datum

A theoretically exact point, axis, or plane used as a reference for dimensioning and tolerancing other features. See Geometric Tolerances.

Deburring

The process of removing burrs from a machined or cut part.

Deep Drawing

A sheet metal forming process that pulls a flat blank into a die cavity to form a hollow shape, such as a can or cup, deeper than its diameter.

Deflection

The displacement of a structural element, such as a beam, under an applied load.

Density

A material's mass per unit volume, typically expressed in kg/m³ or g/cm³.

Die Casting

A casting process where molten metal is forced under high pressure into a reusable steel mould (die).

Dimensional Tolerance

The permissible amount a dimension may vary from its specified value while still being acceptable.

Dog Point Set Screw

A set screw with a small cylindrical projection at its tip, designed to locate into a matching hole or slot in the shaft. See Set Screws.

Dowel Pin

A precisely sized cylindrical pin, pressed into reamed holes in two parts, used to locate them accurately relative to each other. See Dowel Pin Dimensions.

Drive Fit

A light interference fit that allows two parts to be assembled by hammering or pressing them together, relying on friction to hold them in place.

Ductility

A material's ability to deform plastically, typically stretching, before it fractures.

Duplex Stainless Steel

A stainless steel with a mixed austenitic-ferritic structure, giving higher strength and better stress-corrosion resistance than standard austenitic grades.

Durometer

An instrument (or the scale it produces, e.g. Shore A) used to measure the hardness of rubber, plastic, and other elastomeric materials.

Dynamic Load

A load that changes in magnitude, direction, or point of application over time, as opposed to a static load.

E

Eccentricity

The offset distance between two features, such as a shaft and a bore, that are intended to share the same axis.

EDM (Electrical Discharge Machining)

A machining process that removes material using rapid electrical sparks between an electrode and the workpiece, able to cut very hard materials and intricate shapes.

Elastic Limit

The maximum stress a material can withstand and still return fully to its original shape once the load is removed.

Elastomer

A polymer with elastic, rubber-like properties, able to stretch significantly and return to its original shape.

Electroless Plating

A plating process that deposits a metal coating, commonly nickel, using a chemical reaction rather than an electric current, giving a very even coating thickness. See Electroless Plating.

Electropolishing

An electrochemical process that removes a thin layer of material from a metal surface, leaving it smoother, cleaner, and more corrosion resistant. See Electropolishing.

Electroplating

Depositing a thin layer of metal onto a part's surface using an electric current, to improve appearance, corrosion resistance, or wear resistance. See Electroplating.

Endurance Limit

The maximum stress level below which a material can theoretically withstand an infinite number of load cycles without fatigue failure.

Engineering Drawing

A formal, standardised technical drawing that fully defines a part's shape, size, tolerances, and finish requirements.

Expansion Anchor

A fastener that grips the inside of a pre-drilled hole in masonry or concrete by expanding as it's tightened.

Extrusion

A process where material is pushed through a shaped die to produce a long part with a constant cross-section.

Eye Bolt

A bolt with a looped head, used as a fixed lifting or anchoring point for a rope, chain, or shackle.

F

Fastener Abbreviation

Standardised shorthand (e.g. CSK, SHCS, HDG) used to describe fastener types and finishes on drawings and in parts lists. See Fastener Abbreviations.

Fastener Torque

The recommended tightening torque for a given fastener size and grade, chosen to achieve the correct clamping force without overloading it. See Fastener Torque Information.

Fatigue

The progressive weakening and eventual cracking of a material caused by repeated cyclic loading, often at stresses well below its ultimate strength.

Feather Key

A parallel key fixed to a shaft that allows a mating component to slide axially along it while still transmitting torque.

Feature Control Frame

The rectangular box on a drawing containing a GD&T symbol, tolerance value, and datum references for a geometric tolerance. See Geometric Tolerances.

Ferrous Metal

A metal or alloy that contains iron as its main constituent, such as steel or cast iron.

Fillet

A rounded, concave corner where two surfaces meet, used to reduce stress concentration or ease manufacturing.

First Angle Projection

An orthographic projection convention, common in Europe and the UK, where each view is placed on the opposite side of the front view to the direction it was viewed from. See First Angle and Third Angle Projection.

Fit

The relationship between the sizes of two mating parts (clearance, transition, or interference), which determines how they assemble and behave. See Limits, Fits and Tolerances.

Flame Hardening

A localised surface hardening process that heats a part with a direct flame before quenching, hardening only the treated area.

Flange

A projecting rim or ring, often bolted, used to connect pipes, shafts, or structural sections together.

Flash

Excess material that escapes from a mould or die at the parting line during casting, moulding, or forging, usually removed afterwards.

Flat Head Screw

A screw with a flat, countersunk head that sits flush with the surrounding surface once fitted. See Screw Head Types.

Flatness

A geometric tolerance controlling how much a surface may deviate from a perfect plane. See Geometric Tolerances.

Force Fit

A heavy interference fit requiring considerable force, a press, or thermal expansion of one part to assemble.

Forging

Shaping metal using localised compressive force, typically by hammering or pressing, often while the metal is hot.

Free Fit

A generous clearance fit used where accuracy isn't essential, such as on unsupported, easily removed parts running at low speed.

Fretting

Wear damage that occurs at the contact area between two surfaces under load, caused by small, repeated relative motion.

Fundamental Deviation

In the ISO limits and fits system, the deviation closest to the basic size, used to locate a tolerance zone relative to the zero line.

G

Galling

A severe form of adhesive wear where material transfers between two sliding surfaces, common with similar or soft metals under high contact pressure.

Galvanising

Coating steel with a layer of zinc, usually by hot-dip immersion, to protect it from corrosion. See Galvanising and Zinc Plating.

Gasket

A compressible material placed between two mating surfaces to create a seal and prevent leakage.

Gauge (Sheet Metal)

A numerical designation of sheet metal thickness; a lower gauge number means a thicker sheet. See Sheet Metal Gauge Chart.

Gear Ratio

The ratio of the number of teeth (or rotational speeds) between two meshing gears.

Geometric Dimensioning and Tolerancing (GD&T)

A symbolic language used on engineering drawings to precisely define the allowable variation in a part's form, orientation, location, and runout. See Geometric Tolerances.

Gib Head Key

A parallel or taper key with a protruding head, allowing it to be driven out easily once fitted. See Gib Head Keys.

Grain Structure

The arrangement and size of individual crystals within a metal, which strongly influences its mechanical properties.

Grinding

A machining process that uses a rotating abrasive wheel to remove material and achieve a fine surface finish or tight dimensional tolerance.

Grub Screw

A headless screw, usually with a hex socket drive, used to lock a component such as a collar or pulley onto a shaft. See Set Screws.

H

Half-Moon Key

See Woodruff Key.

Hardness

A material's resistance to localised plastic deformation, such as indentation or scratching. See Hardness Comparison.

Heat Treatment

Controlled heating and cooling of a metal to alter its mechanical properties, such as hardness, strength, or ductility.

Helix Angle

The angle between a helical feature, such as a thread or helical gear tooth, and a line parallel to the axis.

Hex Bolt

A bolt with a six-sided head, designed to be tightened with a spanner or socket, typically used with a nut.

Hex Cap Screw

A hex-headed bolt manufactured to tighter tolerances than a standard hex bolt, often used to thread directly into a tapped hole.

Hole Basis System

A limits and fits system where the hole's tolerance is held constant and the shaft size is varied to achieve the required fit. See Hole Basis Limits and Fits.

Honing

A finishing process using bonded abrasive stones to improve the geometric accuracy and surface finish of a bore, commonly used on cylinder bores.

HSS (High Speed Steel)

A tool steel alloy that retains its hardness at high temperatures, commonly used for drill bits and cutting tools.

Hydraulic Press

A machine that uses a hydraulic cylinder to generate a large compressive force from a much smaller input force.

Hygroscopic

Describes a material that readily absorbs moisture from the surrounding air.

I

Impact Strength

A material's ability to absorb energy and resist fracture under a sudden, high-rate load.

Inclusion

A small particle of foreign material, such as an oxide or slag, trapped within a metal during solidification.

Induction Hardening

A surface hardening process that heats a part rapidly using an induced electric current before quenching, hardening only the outer layer.

Injection Moulding

A manufacturing process where molten plastic is injected under pressure into a mould cavity and allowed to cool and solidify.

Interference Fit

A fit where the shaft is always larger than the bore, requiring force or thermal expansion to assemble, and relying on friction to hold the parts together.

Investment Casting

A precision casting process, also called lost-wax casting, where a wax pattern is coated in ceramic, melted out, and the resulting cavity filled with metal.

IP Rating

A two-digit code defining a product's degree of protection against solid objects and liquid ingress. See IP Ratings.

ISO

The International Organization for Standardization, which publishes many of the standards referenced throughout this site.

ISO Tolerance Band

A standardised tolerance zone, defined by a letter and number (e.g. H7, g6), that fixes the size limits for a hole or shaft. See ISO Tolerance Bands.

IT Grade (Tolerance Grade)

A number (e.g. IT6, IT7) in the ISO system that defines the width of a tolerance zone; a lower grade number means a tighter tolerance.

Izod Test

An impact test similar to the Charpy test, but with the notched sample held as a cantilever rather than supported at both ends.

J

J-Bolt

A bolt bent into a J shape, commonly cast into concrete to provide a fixing point, or used as a hook fastener.

Jig

A work-holding device that both positions a workpiece and guides the cutting tool during machining.

Journal

The section of a rotating shaft that is supported by a bearing.

Just-in-Time Manufacturing

A production strategy where materials and components arrive only as they're needed, minimising inventory.

K

Keyway

A slot machined into a shaft and matching bore to accept a key, preventing relative rotation between them. See Parallel Keys.

Kinematic Viscosity

A fluid's resistance to flow under gravity, equal to its dynamic viscosity divided by its density.

Knurling

A machining process that produces a patterned, textured surface, often to improve grip on a knob or handle.

L

Lag Bolt (Lag Screw)

A heavy-duty, coarse-threaded fastener with a hex or square head, driven directly into wood or a pre-drilled hole without a nut.

Lapping

A fine finishing process using an abrasive slurry to achieve a very smooth, flat, or precisely fitted surface.

Laser Cutting

A thermal cutting process that uses a focused, high-powered laser beam to melt or vaporise material along a precise path.

Lead

The axial distance a screw thread advances in one complete turn.

Least Material Condition (LMC)

The condition where a feature contains the least amount of material permitted by its tolerance, e.g. the largest hole or smallest shaft.

Limits and Fits

A standardised system defining the permissible size limits of mating parts to achieve a desired type of fit. See Limits, Fits and Tolerances.

Locational Fit

A fit intended primarily to locate mating parts accurately, which may be clearance, transition, or interference depending on the accuracy required.

Locknut

A nut designed to resist loosening under vibration or dynamic loading, through friction, deformation, or a secondary locking feature.

Lower Deviation

In the ISO limits and fits system, the algebraic difference between the minimum permissible size of a feature and its basic size.

Lubrication

The introduction of a substance, usually oil or grease, between moving surfaces to reduce friction and wear.

M

Machinability

How easily a material can be cut or shaped by machining, considering tool wear, surface finish, and cutting speed.

Machine Screw

A small, uniformly threaded screw designed to mate with a tapped hole or nut, rather than to cut its own thread.

Malleability

A material's ability to deform under compressive force, such as rolling or hammering, without fracturing.

Master Gauge

A precisely made reference standard used to calibrate or check other measuring instruments or gauges.

Material Strength

A general term for the various measures (tensile, yield, shear, etc.) describing how much load a material can withstand. See Material Strength.

Materials Cross Reference

A table matching equivalent material grades between different national and international standards. See Materials Cross Reference.

Maximum Material Condition (MMC)

The condition where a feature contains the maximum amount of material permitted by its tolerance, e.g. the smallest hole or largest shaft.

Metal Properties

The physical and mechanical characteristics, such as density, strength, and thermal conductivity, that define a metal's behaviour. See Metal Properties.

Metallurgy

The science of the physical and chemical behaviour of metals and their alloys.

Micrometer

A precision hand tool used to measure small distances, typically to within a few microns.

Milling

A machining process that uses a rotating multi-point cutter to remove material from a workpiece, typically held on a moving table.

Modulus of Elasticity

Also called Young's Modulus; a measure of a material's stiffness, defined as the ratio of stress to strain within its elastic range.

Modulus of Rigidity

Also called the shear modulus; a measure of a material's stiffness in shear.

Moment of Inertia

A geometric property of a cross-section describing its resistance to bending, used in beam deflection and stress calculations.

MTBF (Mean Time Between Failures)

A reliability metric giving the average operating time between failures of a repairable system.

N

NDT (Non-Destructive Testing)

Inspection methods, such as ultrasonic or dye penetrant testing, that evaluate a material or part without damaging it.

Neutral Axis

The line within a beam's cross-section, under bending, along which the material experiences neither tension nor compression.

Nitriding

A case hardening process that diffuses nitrogen into a steel's surface at relatively low temperature, producing a hard case with minimal distortion.

Nominal Size

The size used to identify a feature or part for general reference, which may differ slightly from its exact basic size.

Non-Ferrous Metal

A metal or alloy that does not contain iron as a main constituent, such as aluminium, copper, or brass.

Normalising

A heat treatment where steel is heated above its critical temperature and then air-cooled, refining its grain structure.

O

O-Ring

A circular, elastomeric ring used as a seal, fitted into a groove and compressed between two mating parts. See O-Rings.

O-Ring Groove

A machined channel sized to house an O-ring and control how much it's compressed when the assembly is fitted. See O-Ring Grooves.

Ovality

The deviation of a round feature, such as a tube or bore, from a perfect circle, usually expressed as the difference between its maximum and minimum diameter.

Oxidation

A chemical reaction between a material and oxygen, often resulting in corrosion such as rust on steel.

P

Passivation

A chemical treatment, commonly used on stainless steel, that removes free iron from the surface to improve corrosion resistance. See Passivation.

Piercing

A press operation that punches a hole through sheet metal; unlike blanking, the punched-out slug is scrap and the surrounding sheet is the finished part.

Pitch (Thread)

The distance between corresponding points on adjacent thread crests, measured parallel to the thread axis.

Pitch Circle Diameter (PCD)

For gears, the diameter of the theoretical circle at which two gears effectively mesh; also used to describe a bolt circle.

Plasma Cutting

A thermal cutting process that uses a jet of ionised gas (plasma) to melt and blow away material from an electrically conductive workpiece.

Plastics Properties

The physical and mechanical characteristics that define how a plastic material behaves under load, heat, or chemical exposure. See Plastics Properties.

Plasticity

A material's ability to undergo permanent deformation without fracturing.

Poisson's Ratio

The ratio of transverse strain to axial strain when a material is stretched or compressed elastically.

Porosity

The presence of small voids or pores within a material, often a defect in castings or welds.

Powder Coating

Applying a dry powder finish electrostatically and then curing it under heat to form a hard, protective coating.

Powder Metallurgy

A manufacturing process that compacts metal powder into a shape and then sinters it, forming a solid part without fully melting the metal.

Pratt & Whitney Key

A parallel key with fully rounded ends, fitted into a keyway cut with a matching rounded-end profile.

Precipitation Hardening

Also called age hardening; a heat treatment that strengthens certain alloys by forming fine particles within the metal's structure over time.

Precision

The repeatability of a measurement or process; how closely repeated results agree with each other, as distinct from accuracy.

Press Fit

See Interference Fit.

Proof Load

The maximum load a fastener can withstand without any permanent deformation.

Push Fit

A close clearance fit that allows two parts to be assembled by hand, with little to no perceptible play.

PVD / CVD (Physical / Chemical Vapor Deposition)

Coating processes that deposit a thin, hard film onto a surface from a vapour phase, either physically (PVD) or via a chemical reaction (CVD).

Pyrometer

An instrument used to measure high temperatures, often without direct contact with the material.

Q

Quality Assurance (QA)

The systematic activities used to ensure a product or process meets specified quality requirements.

Quenching

Rapidly cooling a heated metal, typically in water, oil, or air, to achieve a desired hardness or microstructure.

R

Rack and Pinion

A gear system that converts rotational motion (pinion) into linear motion (rack), or vice versa.

RAL Colour

A European colour matching standard widely used for paints, powder coatings and plastics. See RAL Colours.

Reaming

A finishing operation that enlarges and smooths an existing hole to a precise diameter.

Residual Stress

Internal stress that remains in a part after the original cause, such as machining or heat treatment, has been removed.

Rivet

A permanent mechanical fastener consisting of a shaft and head, inserted through aligned holes and then deformed on the opposite side to clamp the parts together. See Round Head Rivets.

Rockwell Hardness

A hardness scale based on the depth of penetration of an indenter under a set load. See Hardness Comparison.

Roll Forming

A continuous process that bends long strips of sheet metal into a desired cross-section by passing them through a series of shaped rollers.

Root Cause Analysis

A structured method for identifying the underlying cause of a problem or failure, rather than just its symptoms.

Round Key

A cylindrical key, usually fitted into a drilled, part-round hole straddling the joint between shaft and hub.

Running Fit

A clearance fit generous enough to allow free rotation or sliding between mating parts, typically with a film of lubricant.

Runout

A geometric tolerance controlling how much a surface may deviate as a part is rotated about a datum axis. See Geometric Tolerances.

S

Sacrificial Anode

A more reactive metal deliberately connected to a structure so it corrodes preferentially, protecting the main structure.

Saddle Key

A key that bears only on the hub's keyway, with a curved underside sitting against a plain (un-keyed) shaft, relying on friction rather than a positive fit.

Sand Casting

A casting process where molten metal is poured into a mould made from compacted sand, typically formed around a pattern.

Sandblasting

Propelling abrasive particles at a surface under pressure to clean it or prepare it for a coating.

Screw Driving Types

The various drive recesses (slotted, Phillips, Pozidriv, Torx, hex socket, etc.) used to turn a screw. See Screw Driving Types.

Screw Head Types

The various head shapes (countersunk, pan, round, button, socket cap, etc.) used on screws for different applications. See Screw Head Types.

Selective Assembly

A manufacturing approach where parts are measured and sorted into size groups, then matched together to achieve a tighter effective fit than either part's tolerance alone would allow.

Self-Drilling Screw

A screw with a drill-point tip that drills its own pilot hole and forms a thread in one operation, without a separate drilling step.

Self-Tapping Screw

A screw that cuts or forms its own matching thread as it's driven into a pre-drilled, untapped hole.

Set Screw

A headless (or shouldered) screw used to lock a component in place, commonly against a shaft, without a separate nut. See Set Screws.

Shaft Basis System

A limits and fits system where the shaft's tolerance is held constant and the hole size is varied to achieve the required fit. See Shaft Basis Limits and Fits.

Shear Strength

The maximum stress a material can withstand before failing under a sliding, or shearing, force.

Sheet Metal Screw

A self-tapping screw with a sharp point and coarse thread, designed to form its own thread in thin sheet material.

Shim

A thin piece of material used to fill a gap, adjust alignment, or set clearance between two parts.

Shot Peening

Bombarding a metal surface with small round media to induce beneficial compressive residual stress, improving fatigue resistance.

Shoulder Bolt

A bolt with a precisely ground, unthreaded shoulder section, used as a precision pivot or locating spigot rather than purely as a clamping fastener.

Shrink Fit

An interference fit assembled by heating the outer part (or cooling the inner part) so it temporarily expands or contracts enough to fit, then grips tightly once the temperature equalises.

Sintering

Compacting and heating a powdered material, below its melting point, so the particles bond into a solid mass.

Sliding Fit

A close clearance fit that allows two parts to slide relative to each other with minimal play, without needing to rotate freely.

Slenderness Ratio

The ratio of a column's effective length to its radius of gyration, used to assess its tendency to buckle.

Socket Head Cap Screw

A cylindrical-headed cap screw driven with a hex key, commonly used where a compact, high-strength fastener is needed.

Spanner Jaw Size

The across-flats dimension of a bolt head or nut, used to select the correct size spanner or wrench. See Spanner Jaw Size Comparison.

Spline

A series of ridges or teeth on a shaft that mesh with matching grooves in a bore, transmitting torque while sometimes allowing axial movement. See Splines.

Stainless Steel Surface Finish

A standardised grade (e.g. 2B, No. 4, mirror) describing the texture and reflectivity of a stainless steel sheet's surface. See Stainless Steel Surface Finishes.

Stamping

A general term for press operations, such as blanking, piercing, and forming, that shape sheet metal using a die.

Static Load

A load applied gradually and then held constant, without significant change in magnitude or direction over time.

Straightness

A geometric tolerance controlling how much a line or axis may deviate from being perfectly straight. See Geometric Tolerances.

Stress Relieving

A heat treatment that reduces residual stress in a part, without significantly changing its hardness or microstructure.

Stud Bolt

A fastener threaded on both ends (or its full length), used with a nut on each end or threaded into a tapped hole at one end and a nut on the other.

Surface Roughness

A measure of the fine, closely spaced irregularities on a surface, commonly quantified as Ra. See Surface Texture.

Swaging

A forming process that shapes a workpiece by hammering or pressing it radially, often used to reduce a diameter or form a fitting onto a cable or tube.

T

Tangent Key

A pair of keys fitted at 120° to each other in tapered keyways, used on some large shafts to transmit torque in both rotational directions.

Taper Key

A key with a slight taper along its length, driven into a matching tapered keyway to lock a hub axially as well as rotationally. See Taper Keys.

Tapping

Cutting an internal thread into a hole using a tap. See Tap Size Chart.

Tempering

A heat treatment carried out after hardening that reduces brittleness by reheating the part to a lower temperature, trading some hardness for toughness.

Tensile Strength

The maximum stress a material can withstand while being stretched before it fails.

Thermal Conductivity

A measure of how readily a material conducts heat.

Thread Designation

The standardised code (e.g. M10x1.5, 3/8-16 UNC) used to fully specify a screw thread's size, pitch, and standard. See Thread Designations.

Thread Equivalent

A cross-reference identifying which thread sizes across different standards are directly interchangeable or comparable. See Thread Equivalents.

Third Angle Projection

An orthographic projection convention, common in the US, where each view is placed on the same side of the front view as the direction it was viewed from. See First Angle and Third Angle Projection.

Through Bolt

A bolt that passes completely through an assembly and is secured with a nut on the far side, rather than threading into either part.

Thumb Screw

A screw with a knurled or winged head designed to be tightened by hand, without tools.

Toggle Bolt

A fastener with a spring-loaded, folding wing nut that opens up behind a hollow wall or panel, spreading the load once the bolt is tightened.

Tolerance Stack-Up

The cumulative effect of individual part tolerances on the overall fit or function of an assembly.

Torque

A rotational, or twisting, force, equal to the applied force multiplied by the distance from the axis of rotation.

Torsion

The twisting of an object, such as a shaft, due to an applied torque.

Total Indicator Reading (TIR)

The full difference between the maximum and minimum readings recorded by a dial indicator as a part is rotated, often used to assess runout.

Transition Fit

A fit where the tolerance zones of the hole and shaft overlap, so depending on the actual sizes produced, the assembly may end up as either a small clearance or a small interference fit.

Trepanning

Cutting an annular groove to remove a solid core from a workpiece, rather than drilling it out completely.

True Position

A geometric tolerance defining the allowable variation in the location of a feature from its theoretically exact position. See Geometric Tolerances.

Turning

A machining process where a workpiece is rotated against a stationary cutting tool, typically on a lathe, to produce cylindrical features.

U

U-Bolt

A U-shaped bolt, threaded on both straight ends, used to clamp around a pipe, rod, or axle.

Ultimate Tensile Strength (UTS)

The maximum stress a material can withstand in tension before necking and eventual failure.

Ultrasonic Testing

A non-destructive testing method that uses high-frequency sound waves to detect internal flaws or measure thickness.

Undercut

A recessed feature machined into a part, often to allow clearance for an adjoining feature or to terminate a thread.

Unified Thread

The UNC/UNF imperial screw thread standard used mainly in North America. See Unified Threads.

Upper Deviation

In the ISO limits and fits system, the algebraic difference between the maximum permissible size of a feature and its basic size.

Upsetting

A forging process that increases a workpiece's diameter by compressing its length.

V

Vacuum Impregnation

A process that seals the microscopic pores of a casting or sintered part by drawing a sealant into them under vacuum, preventing leaks. See Vacuum Impregnation.

Vernier Caliper

A precision measuring instrument used to measure internal, external, and depth dimensions using a sliding vernier scale.

Vibration Damping

The dissipation of vibrational energy from a system, reducing the amplitude of oscillation over time.

Viscosity

A fluid's resistance to flow; higher viscosity fluids flow more slowly.

Void

A small gas pocket or cavity trapped within a casting or weld, considered a defect.

W

Washer

A thin, flat (or shaped) component placed under a bolt head or nut to spread load, prevent surface damage, or resist loosening. See Metric Washers.

Waterjet Cutting

A cutting process that uses a high-pressure jet of water, often mixed with an abrasive, to cut through material without heat-affecting it.

Wear Resistance

A material's ability to withstand surface damage from friction, abrasion, or repeated contact.

Weld Symbol

A standardised symbol used on drawings to specify the type, size, and location of a weld. See Welding Symbols.

Wire EDM

A variant of electrical discharge machining that uses a thin, continuously fed wire electrode to cut intricate profiles through a workpiece.

Wire Gauge

A numbered scale used to specify the diameter of wire; comparisons vary between standards such as AWG and SWG. See Wire Gauge Comparison.

Woodruff Key

A semicircular key that fits into a matching curved keyway, commonly used on tapered shafts. See Woodruff Keys.

Work Hardening

The strengthening of a metal caused by plastic deformation, such as bending or rolling, at room temperature.

Wringing Fit

An extremely close, zero-clearance fit where parts can only be assembled by "wringing" them together, such as gauge blocks.

Wrought

Describes a metal that has been shaped by mechanical working, such as rolling or forging, rather than casting.

X

X-Ray Inspection

A non-destructive testing method that uses X-rays to reveal internal defects, such as porosity or cracks, inside a casting or weld.

Y

Yield Strength

The stress at which a material begins to deform plastically and will no longer return to its original shape.

Young's Modulus

See Modulus of Elasticity.

Z

Zinc Plating

Applying a thin layer of zinc, usually electrolytically, to protect steel from corrosion. See Galvanising and Zinc Plating.

Zero Line

In a tolerance chart, the line representing the basic size, from which upper and lower deviations are measured.

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