Engineering and fastener terminology

This section contains a useful list of terms and definitions.

A

AMS Aerospace Material Specifications: Material and process specifications for aircraft components conforming to recognised engineering and metallurgical practices in the aircraft industries. They are developed by the SAE aeronautics committee. All specifications are prefixed by "AMS". AN Dimensional standards for aircraft fasteners developed by the Aeronautical Standards Group. All drawings are prefixed by "AN" ASTM Standards developed by the American Society for Testing and Materials.
Acorn die: A form of threading die for use in screw machines. The cutting portion resembles an acorn.
Acorn nut: A blind tapped hex nut with an acorn shaped top. Provides sealing for projecting threaded parts.
Age hardening: A process of aging that increases hardness and strength, and ordinarily decreases ductility. Age hardening usually follows rapid cooling or cold working.
Air Force-Navy aeronautical standard drawings: Dimensional standards for aircraft fasteners developed by the Aeronautical Standards Group. All drawings are prefixed by "AN".
American standards: Dimensional standards for fasteners, etc., developed by the American Society of Mechanical Engineers (ASME).
Angle of head: In countersunk heads, the included angles of the conical under portion or bearing surface, usually 82 or 100 degrees.
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B

Balls: Used in "quick release pins" where permanent assemble and disassemble action takes place. Provides a reliable holding fastener.
Barrel nut: An internally threaded screw having a slotted head.
Bent bolt: A cylindrical rod having a thread at one end and the other end bent to some desired conformation; also, a bent cylindrical rod having threads at both ends.
Bevel: A surface not at right angles to the rest of the piece.
Binding head: Rounded top surface and slightly tapered sides. The bearing surface is flat with annular undercut optional.
Blind Rivet: A rivet designed for use where only one side of the work is accessible. View our full range of Blind Rivets here.
Blunt start: The removal of the partial thread at the entering end of thread.
Body of bolt or screw: Also designated grip. The blank portion of the bolt or screw which is between the thread and the head.
Bolt blank: A headed rod or bar intended for a subsequent threading operation.
Bolt end: A headless rod threaded at one end and assembled with a square nut, designed to be welded or otherwise fastened to a part or structure.
Brinell hardness test: A method of determining the hardness of metallic materials by applying a known load to the surface of the material through a hardened steel ball of known diameter. The diameter (or depth) of the resulting impression in the metal is measured. The Brinell hardness number is taken as a quotient of the applied load divided by the area of the surface impression which is assumed to be spherical.
Broaching: A generating process whereby metal is removed with a multiple-point tool, usually a bar, with tooth height increasing from the starting end. When the broach is pulled or pushed through or over the work, each tooth removes a clip of uniform thickness, in contrast to a milling cutting tooth which removes a wedge-shaped chip.
Button die: Small adjustable dies for screw machines and similar work, supported in a ring holder.
Burnish: To smooth or polish by a rolling or sliding tool under pressure.
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C

Cap nut: A blind tapped hex nut with an acorn-shaped top. Provides sealing for projecting thread parts.
Cap screws: Term used to describe hexagon head, slotted head, square head, and socket head cap screws.
Captive screws: Where the shank above the threaded portion is undercut lower than the threaded portion.
Carbide: A compound of carbon with one or more metallic elements.
Carbide tools: Tools with cutting points of tungsten, tantalum, or other alloys of sintered carbides.
Carbon steel: Any steel made by combining carbon with iron.
Carriage Bolts: A round or flat head bolt with a square neck to prevent rotation. Used in wood assemblies.
Case harden: Hardening a ferrous alloy so that the outer portion or case is made substantially harder than the inner portion or core. Typical processes used for case hardening are carburizing, cyaniding, carbonitriding, nitriding, induction hardening, and flame hardening.
Casting: 1. A term applied to the act of pouring molten metal into a mould. 2. The metal object produced by such pouring.
Castle Nut: A hexagon nut having a cylindrical portion at the end opposite to the bearing face, with opposed slots in this portion which are parallel to the axis, designed for insertion of a cotter to secure the nut in place when used with a drilled fastener.
Centreless grinding: Grinding the outside or inside diameter of a round piece not mounted on centres.
Chamfer: (v) To bevel a sharp external edge. (n) A bevelled edge.
Chasing threads: Cutting screw threads by moving a tool along the axis of the work to be threaded.
Chatter: Rough or unsatisfactory surfaces on work. It is usually caused by a slight jumping of the tool away from the work or of the work away from the tool.
Check Nut: A double chamfered hexagon machine screw nut Class 3B fit. (AN 316).
Chip: The metal removed by a tool.
Chuck: Device for holding work in machine tools.
Class of thread: Class of threads are distinguished from each other by the amount of tolerance or tolerance and allowance specified. Classes 1A, 2A, and 3A apply to external threads, and Classes 1B, 2B and 3B apply to internal threads.
Clevis: U-shaped shackle for connecting a rod to a pin.
Clevis Pins: A headed dowel pin with a drilled shank that accepts a cotter pin. Used as a fastener where more than temporary holding actions is required. (ASA B 5.20-1958).
Clinch Nut: A nut having a pilot which, after insertion in a hole, is clinched or staked in place to prevent rotation. View our range of Clinch Nuts here.
Cold heading: Forcing metal to flow cold into dies to form thicker sections and more or less intricate shapes. The operation is performed in specialized machines where the metal, in the form of a wire or bar stock, may be upset or headed in certain sections to a larger size and, if desired, may be extruded in other sections to a smaller diameter than the stock wire.
Cold swaging process: A method of reducing or forming steel or other material while cold, by drawing to a point or reducing the diameter, as may be required.
Cold work: To deform metal stock by hammering, forming, drawing, etc., while the metal is at ordinary room temperature.
Columbium: A metal which may be added to chrome-nickel stainless steel to improve its welding and general heat-resistant qualities, by preventing carbide precipitation.
Companion flanges: Shaft attached collars of 18-8 stainless steel into which a threaded piece may be joined. Has standard I.P.S. threads.
Comparator: A device for inspecting screw threads and outlines by comparing them with a greatly enlarged standard chart.
Cone point: A point in the form of a cone, commonly having an included angle of 90 degrees or 118 degrees when applied to set screws.
Copper steel: When any minimum copper content is specified, the steel is classed as copper steel. The copper is added to enhance erosion resistance of the steel.
Corrosion: Gradual chemical or electrochemical attack on a metal by atmosphere, moisture, or other agents.
Cotter Pin: Used as a locking device in clevis pins, castellated nuts, and slotted nuts with drilled bolts. Ends available in various types such as "extended mitre end", etc.
Counterbore: (v) To enlarge a hole to a given depth. (n) 1. The cylindrical enlargement of the end of a drilled or bored hole. 2. A cutting tool for counterboring, having a piloted end of the size of the drilled hole.
Countersunk head: A head, the underside of which is bevelled to fit a flaring hole. The bearing surface of other types of heads is generally perpendicular to the body axis.
Crest: That surface of the thread which joins the flanks of the thread and is farthest from the cylinder or cone from which the thread projects.
Crest clearance: As in a thread assembly, the distance, measured perpendicular to the axis, between the crest of a thread and the root of its mating thread.
Crest truncation of thread: The distance, measured perpendicular to the axis, between the sharp root and the cylinder or cone which bounds the root.
Cup point: A point in the form of a cone, commonly having an included angle of 90 degrees, with a conical depression in the end commonly having an included angle of 118 degrees. The contact area is a circular ridge which has considerable holding power with slight penetration, applied to set screws generally.
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D

Deburring: The removal of burrs by a secondary operation such as grinding.
Deep drawing: Forming shaped articles or shells by forcing sheet metal into a die.
Die: 1. One of a pair of hardened metal blocks for forming, impressing, or cutting out a desired shape. 2. (thread). A tool for cutting external threads. Opposite of tap.
Die casting: A very accurate and smooth casting made by pouring a molten alloy usually under pressure into a metal mould or die.
Die chaser: The separate cutting tools used in die heads, which cut the screw threads.
Dog point: A cylindrical extension, or pilot, of diameter smaller than the minor diameter of the thread, commonly equal to about D/2 in length, with a conical section between it and the thread; usually used as a pilot in assembling or as the end of a set screw projecting into a fairly deep hole or slot.
Double end stud: Threaded at both ends with standard Class 2A threads to take nut assembly.
Dowel Pins: Used as a holding pin with chamfered on one end. Drilled headed dowel pin (clevis pin) used - frequently removed fastener.
Dowel Screw: Both ends have Gimlet points, threaded similar to a Lag Bolt. The centre section has a plain unthreaded shoulder. Used in joining two wooden members such as a leg to a tabletop.
Drill: (v) To sink a hole with a drill, usually with a twist drill. (n) A pointed cutting tool rotated under pressure.
Drilled head or shank: Used in AN Bolts and Machine Screws, etc.
Drive Screw: A piloted, multiple threaded screw with a large helix angle, used for permanent applications. It forms a mating thread as it is hammered or pressed into a prepared hole.
Driver head: A head, on a bolt or screw, designed for driving the fastener by means of a tool other than a wrench, such as a screwdriver.
Drop forging: Forming metal, usually under impact, by compression within dies designed to produce the required shape.
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E

Electroplating: Structural fasteners are normally electroplated to BS1706: 1990 and a thickness of Fe/Zn 8 (previously Zn 3). This coating is sufficient to ensure protection from corrosion during on-site storage, before assembly. Higher grade fasteners must always be hydrogen de-embrittled.
Elevator Bolt: A flat, plain, circular countersunk head bolt with a square neck to prevent rotation.
Endurance limit: The maximum stress that a metal will withstand without failure during a specified large number of cycles of stress.
Escutcheon Pin: A piece generally used to join a plate or shield, to cover or protect softer or more delicate materials or actions.
Expansion Bolt: A combination of a "lag bolt" and an internally threaded split sleeve, designed for fastening to stone or concrete by inserting a sleeve into a hole in the concrete and expanding to a tight fit in the hole by turning the lag both with a wrench.
Eyebolt: A bolt having a head in the form of an open or closed anchor ring, or of a flattened and pierced section, with or without a collar or shoulder under the head.
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F

Face: To machine a flat surface perpendicular to the axis at rotation on a lathe.
Fatigue strength: The stress to which a metal can be subjected for a specified number of cyclic changes of stress.
Feather Key: A parallel key fastened in either the shaft or in the hub of a member sliding on it. When fastened in the shaft, it must be long enough to hold or drive the sliding member in any of its positions on the shaft. When in the sliding member the key need only be as long as the hub.
Fillet: A rounded filling of the internal angle between two surfaces.
Fillister head: Rounded top surface, cylindrical sides, and a flat bearing surface.
Fin: A thin projecting rib.
Finished Hexagon Bolts: A washer faced or chamfered bearing surface with a close body tolerance.
Fin Neck Carriage Bolt: A plain, circular, oval head bolt with two oppositely located fins to prevent rotation.
Fit: The general term used to signify the range of tightness which may result from the application of a specific combination of allowances and tolerances in the design of mating parts. See "Class of Thread".
Flat head: Flat top surface and a conical bearing surface.
Following flank: The flank of a thread opposite to the leading flank.
Forge: To shape metal while holt and plastic by a hammering or forcing process. Dies used in process.
Form of thread: The profile of a thread in an axial plane for a length of one pitch.
Free machining: The property that makes machining easy because of the forming of small chips, a characteristic imparted to steel by sulphur, etc.
Full annealing: Annealing a ferrous alloy by austenitizing and then cooling slowly through the transformation range.
Full size body: The body of a bolt or screw which has a diameter between the minimum and maximum limits of the major diameter of the thread.
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G

Galvanise: To treat with a bath of lead and zinc to prevent rusting.
Galvanising: Long term protection for structural fasteners is obtained by galvanizing to BS729 with a specified minimum thickness range of 0.4mm for the coating. Normal coating thicknesses range from 70 to 100mm; again, the nut thread is over tapped by 0.4mm, to accommodate the coating. High temperature galvanizing, developed in the 1970's, results in a high-quality surface finish with few of the imperfections of the traditional galvanizing process. Acid free cleaning processes make hydrogen de-embrittlement for fasteners unnecessary.
Gimlet point: A taper-threaded point; applied to wood screws, Type A tapping screws, lag bolts, etc.
Grind: To finish or polish a surface by means of an abrasive.
Grip: The unthreaded portion of a bolt or screw.
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H

Half dog point: The same as a dog point but half as long; used on short screws for the same purposes as the dog point, but in a shallower hole or slot.
Hanger Bolt: One end is gimlet pointed and has a wood screw thread. The other end consists of a coarse machine screw thread. The centre section is unthreaded.
Hardenability: In a ferrous alloy, the property that determines the depth and distribution of hardness induced by quenching.
Hardening: A method of heat-treating metals by heating to a temperature within, or above, the critical range, holding at that temperature for a given time, and then cooling rapidly, usually by quenching in oil or water.
Hardness: Resistance to plastic deformation by indentation, penetration, scratching or bending.
Head: The preformed, enlarged end of a bolt, screw, pin, or rivet, provided with a bearing surface which is usually either flat or conical.
Header point: A chamfer point, usually of 45 degrees included angle, forming by a die at the time of heading and prior to threading.
Head marking: Used to identify the material used in a fastener such as a bolt or cap screw. The marking is either raised or indented to specifications.
Heat treating: An operation involving the heating and cooling of a metal to obtain certain desirable conditions or properties.
Height of thread: The distance, measured perpendicular to the axis, between the major and minor cylinders or cones, respectively.
Heli-Coil: Coil of wire used as an insert to accept a screw or bolt and adding holding power by forcing itself between the fastener and the walls of the recess when the fastener is driven in.
Hexagonal: A recessed hexagon socket in the head of a cap or set screw to add greater tightening and loosening power. Used with a Hex key wrench.
Hexagon head: Flat top surface with hexagonal sides and with a flat bearing surface (six sides).
Hook Bolt: A "bent bolt" having the unthreaded end bent to form a hook, such as a round bend, square bend, right-angle bend, or acute-angle bend hook bolt.
Hot forming: Working operation such as bending and drawing sheet and plate, forging, pressing, and heading, performed on metal heated to temperatures above room temperature.
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I

Immunise: To remove small particles of iron or grit from the surface of stainless steel by pickling in an acid solution.
Impact test: A test to determine the energy absorbed in fracturing a test bar at high velocity. The test may be in tension or in bending, or it may properly be a notch test if a notch is present, creating multiaxial stresses.
Included angle of thread: The angle between the flanks or the thread measured in an axial plane.
Incomplete thread: On straight threads, that portion at the end having roots not fully formed by the lead or chamfer on threading tools.
Ingot: Steel formerly in a molten state, transferred to an ingot mould to solidify.
Interference fit: A thread fit having limits of size so prescribed that an interference always results when mating parts are assembled.
Internal thread: A thread on the internal surface of a hollow cylinder or cone.
International metric thread system: A thread form like the American standard, excepting the depth which is greater. There is a clearance between the root and mating crest fixed at a maximum of 1/16 the height of the fundamental triangle or 0.054 x pitch. A rounder root profile is recommended. The angle in the plane of the axis is 60 degrees and the crest has a flat like the American standard equal to 0.125 pitch.
Iron: Primarily the name of a metallic element. In the steel industry, iron is the name of the product of a blast furnace containing 92 to 94 percent iron. Other names for blast furnace are pig iron and hot metal.
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J

J-Bolt: A "bent bolt" having the unthreaded end reverse bent to approximately a semicircle.
Jackson head: A machine screw with a small oval head (trimmed oval head).
Jam Nut: 1. A second nut forced or jammed against the main nut to prevent loosening. 2. A thin nut.
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K

Key: A small block or wedge inserted between shaft and hub to prevent circumferential movement.
Key slot: The recess cut for Woodruff keys produced by sinking a milling cutter of the right diameter and width into a shaft.
Keyway: A groove or slot cut to fit a key. A key fits into a key seat and slides into a keyway.
Knurl: A roughened surface produced by contact with a wheel which forces metal above the surface while making indentations below the surface.
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L

Lag Bolt: A fastener having a square head, with a gimlet or cone point, and a thin, sharp, coarse-pitch thread, designed for insertion in wood or other resilient materials and producing its own mating thread.
Lead error: A variation in the distance between the threads of a screw.
Left-hand thread: A thread is a left-hand thread, if, when viewed axially, it winds in a counter clockwise and receding direction. All left-hand threads are designated LH.
Length of fastener: The length of a flat bearing surface fastener is the distance, in a line parallel to the axis, from the bearing surface to the extreme point. The length of a countersunk head fastener is the distance, in a line parallel to the axis, from the largest diameter of the bearing surface to the extreme point.
Lentil Head Screw: Obsolete term for oval head screw.
Liquid honing: A metal finishing process by forcing a stream of liquid, in which a very fine abrasive is suspended, against the surface to be finished.
Lock Nut: A nut which, in addition to serving the purposes of an ordinary nut, has a special means for gripping a threaded member so that a relative rotation between the nut and the threaded companion member is prevented in use.
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M

MS specifications: Military Standard
Machineability: Relative ease or difficulty in forming.
Magnaflux: A magnetic method of determining surface and subsurface defects in metals.
Major diameter: On a straight thread, the diameter of the coaxial cylinder which would pass through the crests of an external thread or the roots of an internal thread.
Master gage: A thread-plug gage which represents the physical dimensions of the nominal or basic size of the part. It clearly establishes the minimum size of the threaded hole and the maximum size of the screw at the point at which interference between mating parts begin.
Maximum material limit: The maximum limit of size of an external dimension or the minimum limit of size of an internal dimension.
Mechanical properties: Those properties of a material that reveal the elastic and inelastic reaction when force is applied, or that involve the relationship between stress and strain; for example, the modulus of elasticity, tensile strength, and fatigue limit.
Micro-inch: One millionth of an inch. Used in measuring imperfections of surface finishes.
Milling process: A machining process whereby a surface is generated with a rotating toothed cutter. Each tooth takes an individual chip.
Minor diameter: On a straight thread, the minor diameter is the diameter of the coaxial cylinder which would pass through the roots of an external thread.
Monel: A corrosion resisting metal consisting essentially of two-thirds nickel and one-third copper.
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N

NAS drawings and specifications: Dimensional and material standards for aircraft fasteners developed by the National Aerospace Standards Committee. All drawings and specifications are prefixed by "NAS".
Nail point: A sharp pyramidal point of approximately 30 degrees or 45 degrees included angle.
Naval brass: A corrosion resistant metal containing 60 per cent copper, 39 1/4 per cent zinc and 3/4 per cent tin.
Neck: A portion reduced in diameter between the ends of a shaft.
Nickel: A silver-white metal usually used as an alloy in steel, bronze, brass and cast iron. It tends to increase corrosion resistance.
Nipple: A short piece of threaded pipe used to connect two fittings.
Nitriding: A surface hardening process used on ferrous metals by heating the metal in contact with ammonia gas or other nitrogenous material.
Non-Ferrous metal: Metals or alloys without an appreciable amount of iron. Examples are aluminium, brass, copper, etc.
Non-magnetic: Steels with enough manganese or nickel to render the steel non-magnetic. 18-8 (300 series chrome-nickel steel) is non-magnetic when annealed. Type 316 is non-magnetic in all conditions.
Normalise: To remove internal stresses by heating a metal piece to its critical temperature and allowing it to cool very slowly.
Nut: A metal block (solid nut) or sleeve having an internal thread made to assemble with the external thread on a bolt, screw, or other threaded part. It may be a fastening means, an adjusting means, a means for transmitting motion, or a means for transmitting power with large mechanical advantage and non-reversible motion.
Nylon fasteners: Fasteners made of a material that has a low dielectric constant and relatively high tensile strength, enabling it to resist high voltage at commercial frequencies. It can also operate at continuous temperatures as high as 250 degrees F. Any temperatures above have an effect to oxidize material. Electric and electronic equipment manufacturers are finding many corrosion-resistant applicable for this type of fastener.
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O

One-Way Head Screw: A round head screw which is slotted but has side clearances at diagonally opposite sides of the slot so that the screw can be driven only in the direction of assembly, designed for preventing tampering.
Open Washer: A washer with one side open to be removed or put under a nut without removing the nut.
Oval Binding Head Screw: Obsolete term for a truss head screw.
Oval head: Rounded top surface and a conical bearing surface with an included angle usually of 82 degrees.
Oval point: A rounded end which is used, particularly for set and adjusting screws, to apply pressure without cutting action.
Oven Head Screw: Obsolete term for truss head screw.
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P

Pan Head: Flat top surface rounded into cylindrical sides, and a flat bearing surface. The recessed pan head has a rounded top surface blending into cylindrical sides and a flat bearing surface.
Passivate: A process of surface treatment for neutralizing stainless steels. An oxidizing solution, such as nitric acid is applied to the surface. This strengthens the normal protective film which helps in resisting corrosion. It also removes any foreign substance which might cause local corrosion.
Pattern Nuts: Special nuts usually furnished in plain or chamfered face unless otherwise specified, and threads are unified Coarse or unified Fine, Class 2B (also small and extra small).
Peening: The stretching of metal by hammering or rolling the surface.
Phillips recessed head: A type of screw drive having a cross recess with a wide centre opening, steep walls, and a blunt conical bottom. Permits easy and rapid entrance of the 4-winged driver.
Physical properties: Those properties familiarly discussed in physics, including those described under mechanical properties; for example, density, electrical conductivity, co-efficient of thermal expansion.
Pinch point: A sharp conical point, usually of 45 degrees included angle, formed by a pinching operation.
Pipe Caps: A cap that threads onto a pipe (like a nut) to seal one end.
Pipe fittings: The term applying to all forms of connecting parts which join pieces of pipe together.
Pipe Plugs: A short piece of threaded pipe, Slotted, Square Head or Socket, used to close one end of a fitting.
Pipe screw thread: American Standard pipe threads are tapered 1 inch in 16, or 3/4 inch per foot. They are 60-degree threads, of National form with flat or rounded top and bottom.
Pitch: The distance, measured parallel to its axis, between corresponding points on adjacent thread forms in the same axial plane and on the same side of the axis.
Pitch diameter: On a straight thread, the diameter of the coaxial cylinder, the surface of which would pass through the thread profiles at such points as to make the width of the groove equal to one-half of the basic pitch. On a perfect thread this occurs at the point where the widths of the thread and groove are equal.
Pitch line: A generator of the cylinder or cone of a screw thread specified in the definition of pitch diameter.
Plain sheared point: The end of a fastener cut approximately flat and square to the axis, without chamfer.
Planer Head Bolt: A bolt having a large low square head, designed for insertion in T-slots of planer, shaper, or milling machine tables.
Plas-Tech^®: This is a registered trade name owned by TR Fastenings. Plas-Tech^® is assigned to our proprietary fasteners for plastics.
Plate: The electro-mechanical coating of a metal piece with a very different metal.
Polish: To make smooth or lustrous by friction with a very fine abrasive.
Polymate^®: Is a registered trade name owned by TR Fastenings. The Polymate^® product is 10.9 grade twin start threadforming screw for plastics with a 40-degree inclusive thread angle. The nominal metric thread diameter options are 2.2 through 6.0.
Precipitation hardening: Hardening caused by the precipitation of a constituent from a supersaturated solid solution.
Precision Machine Screws: Slotted machine screws, milled from bar, cut thread, and are machined finished Class 3A fits.
Process annealing: Heating a ferrous alloy to a temperature close to, but below, the lower limit of the transformation range and then cooling, to soften the alloy for further cold working.
Punch: (v) To perforate by pressing a non-rotating tool through the work.
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Q

Quench hardening: Hardening a ferrous alloy by austenitizing and then cooling rapidly enough so that some or all the austenite transforms to martensite.
Quenching: Rapid cooling. When applicable, the following more specific terms should be used: direct quenching, fog quenching, hot quenching, interrupted quenching, selective quenching, spray quenching, and time quenching.
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R

Ream: To finish a drilled or punched hole very accurately with a rotating fluted tool of the required diameter.
Reamer: Tool used for enlarging holes previously formed by drilling or boring.
Recessed head: A screw head, having specially formed indentations or recesses centred on the head, into which a suitably formed driver fit.
Relief: The amount one plane surface of a piece is set below or above another plane, usually for clearance or for economy in machining.
Ribbed Neck Carriage Bolt: A plain, circular, oval head bolt with a serrated neck to prevent rotation.
Right-hand thread: A thread is a right-hand thread if, when viewed axially, it winds in a clockwise and receding direction.
Rivet: A headed metal fastener of some malleable material used to join parts, as metal plates, of structures and machines by inserting the shank through a hole in each piece and forming a head on the headless end.
Rivet Bolt: A bolt designed as a substitute for rivets, having a button head, a ribbed shank, a UNC thread, or a special thread such as a Dardalet thread or Lok-Thread (TM).
Rockwell Hardness Test: A measure of hardness by determining the depth of penetration of a penetrator into the specimen under certain fixed conditions of test. The penetrator may be either a steel ball or a diamond sphero- conical penetrator. The hardness number is related to the depth of indentation and the higher the number the harder the material.
Rod Ends: The ends of connecting rods which fit over crank pins or cross head pins.
Roll threading (or thread rolling): Applying a thread to a bolt or screw by rolling the piece between two grooved die plates, one of which is in motion, or between rotating grooved circular rolls.
Rolled point: The point frequently produced by the cupping of the last 1 to 1 1/2 threads by the thread rolling pressure. This type of point is not produced intentionally but is considered as an alternative form of the plain sheared point.
Root: That surface of the thread which joins the flanks of adjacent thread forms and is identical with or immediately adjacent to the cylinder or cone from which the thread projects.
Round head: Semi-elliptical top surface and flat bearing surface.
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S

SAE specifications: Standards developed by the Society of Automotive Engineers, Inc.
SAE standard screw threads: The SAE Screw Thread Standard, as revised in 1954, conforms with the Unified and American Standard (ASA B1.1-1949).
Secondary operations: Operations which include punching, shaving, trimming, drilling, sawing, milling, pointing, tapping, and polishing, etc., are generally called secondary.
Self-Locking: Where binding action is self-induced on the body of the bolt, screw, or nut to keep it rigid and prevent movement and vibration.
Self-Tapping Screw: Screws that can tap its own hole when driven into a material such as wood or sheet materials.
Semi-Finished Hexagon Bolts: Bolts with standard sizes ranging from 1/4" through 4". Class 2A tolerance. The bearing surface is washer faced.
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T

Tap: A tool to cut or roll female threads in a pre-drilled hole. The tap has a slightly bigger diameter than the pilot hole to allow for form the thread.
Tap Bolt: A fully threaded bolt.
Tap, Cutting: A tool to remove material from the wall of a pre-drilled hole to form a thread. Generally, the pre-drilled hole size determines the final minimum diameter of the female thread.
Tap, Rolling: A tool to create a thread by deforming the material into which a hole has been created. It is recognised that rolling threads is preferential to cutting in steel fasteners, because of its higher integrity.
Thread cutting: The manufacture of a thread on a blank or in a hole by cutting away material. The cutter is often designed as a mating part and equipped with flutes to allow removal of the cut material.
Thread forming: A manufacturing process that involves the creation of internal threading by deforming the inner wall of the material.
Thread Forming Screw: A screw which will form a thread in the mating material. The thread profile has often a slightly triangular shape to allow thread rolling.
Thread rolling: The manufacture of an external thread to a fastener by rolling blanks between grooved plates. The grooves in the plate will deform the surface of the blank and form the eventual thread geometry.
Truss head: A domed head with an extra wide profile and low height.
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U

U-Bolt: A bolt in the shape of a U with both ends threaded.
Unified Coarse Thread (UNC): Standard imperial thread pattern used in the USA.
Unified Fine Thread (UNF): Standard imperial thread pattern used in the USA for fine thread.
U-Nut: A metal clip formed in the shape of a U, made of spring steel sheet material. One of the flat sides has a threaded profile, whereas the opposite side is equipped with a hole to pass a male fastener through.
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V

Vibrational loosening (self-loosening): Under vibration, bolted joints make small movements that cause the joint’s two sections to move repeatedly, creating friction between the bolt and joint threads which hold the joint together.
Vickers hardness: Vickers hardness tests are a commonly recognised hardness testing method for fasteners. A pre-defined diamond shape is pressed into the material. The hardness of the material’s ability to resist deformation. Depending on the shape and hardness range to measure, various loads can be used. E.g. HV0.1 can be used for micro hardness, whereas HV5 is used to measure core hardness.
Von Mises equivalent stress: A key output of FEA (Finite Element Analysis) defining the "uniaxial tensile stress that would create the same distortion energy as is created by the actual combined applied stresses". This can be used to support to predict potential high-risk areas, prone to failure.
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W

Whitworth thread: Imperial thread pattern. Now obsolete but might still be found in vintage equipment. Most applications can be converted to UNC or UNF, alternatively metric thread.
Wing Nut: A nut equipped with protruding wings for manual assembly.
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X

X-ray fluorescence (XRF): Identifying and measuring material content of metallic structure.
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Y

Yield strength: This refers to an indication of maximum stress that can be developed in a material without causing plastic deformation. It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of the elastic limit.
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Z

Zinc flake coatings: Mainly used in the light and heavy vehicle industries, the coating consists of an organic bonding material with finely dispersed zinc. These coatings, much alike a paint, are applied by dip-spin or spraying application after which they are cured. Geomet^®, Magni 565, Delta Protekt^® are some of well-known brand names and supplied by TR on products.
Zinc plating electrolytic: Zinc electroplating is a common method of protecting fasteners from the effects of corrosion. The zinc is applied to the fasteners by submersing these in a fluid and to apply an electrical current between the fastener and a zinc electrode. These can be either cathodic or anodic depending if the bath is alkaline, acid or cyanide based. Zinc coatings are often followed by a conversion coating to passivate the surface and improve corrosion protection. A further protection can be achieved by a topcoat.
Zinc plating mechanical: A zinc layer is deposited onto fasteners by tumbling these in a barrel with zinc powder and media. The zinc will be ‘hammered’ onto the surface to create a protecting layer against corrosion. Zinc coatings are often followed by a conversion coating to passivate the surface and improve corrosion protection. A further protection can be achieved by a topcoat.
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