SHEET METAL SCREWS- Sheet metal screws
with (a) sharp-pointed ends; and (b) fewer threads per
inch than type AB screws; and (c) deeper threads with
better gripping power than type AB. The Industrial Fasteners
Institute incorrectly labels type A an “obsolete” thread
though it is universally preferred in 18-8 stainless over
type AB, especially by the marine industry.
“AB” SHEET METAL SCREWS - Sheet metal screws with pointed
ends similar to type-A screws and thread dimensions similar
to type-B. Type AB screws are seldom used in stainless.
AN - Stands for Air Force - Navy.
ANSI - Stands for American National Standards
ASME - Stands for American Society of Mechanical
NOMENCLATURE (B1.1 AND ALL)
- The various “B” numbers are standards set by ASME regarding
various aspects of fasteners. The most common number,
B1.1, deals with dimensions and thread tolerances; B1.2
with gages and their use; B1.3 with various systems for
gaging threads; B1.7 with definitions of terms.
ASTM - Stands for American Society for
Testing and Materials.
193-194 - ASTM 193 are chemical and physical
specifications for hex head cap screws, studs, and bolts
made of steel and stainless steel. ASTM 194 refers to
nuts. The commonly used stainless is called grade 8, referring
to 304 material to certain specifications, and grade 8M
referring to 316 material. The major differences between
ASTM and commercial stainless fasteners are: (a) 304 material
must be used for manufacturing grade 8, not simply 18-8;
(b) ASTM generally refers to heavy hex heads and heavy
nuts, though semi-finished hex heads and finished nuts
may be supplied with the permission of the buyer; (c)
cold formed material will require carbide solution treatment
or annealing to reduce hardness to meet ASTM requirements.
HARDEN - To use modified heat treatments
at various temperatures over a period of time to harden
and strengthen a fastener.
QUALITY - Fasteners made with a particularly
high level of attention in manufacture and inspection.
STEEL - A mixture (or alloy) of ordinary
steel added to other metals besides carbon with the specific
purpose of attaining certain characteristics such as higher
strength. A few exceptions to this definition exist, however,
so that a chromium content above 4% is not considered
alloy steel and above 12% is considered stainless steel.
ALUMINUM - The most abundant metal in the earth,
aluminum is blueish and silvery-white, very light, malleable,
and ductile with high heat and electrical conductivity.
It is non-magnetic and one-third the weight of steel with
good corrosion resistance against certain chemicals and
acids but weak resistance against other elements such
as sea water.
ANNEAL - To heat metal in order to lower
its hardness. The term anneal refers to the heat treatment
given all 300 series stainless and most 400 series stainless
by the steel mill after the raw material has been completed
but before fasteners are manufactured. Anneal also refers
to the heat treatment given 400 series stainless fasteners
after their manufacture (also called hardening and tempering)
to lower hardness and increase toughness. For example,
fasteners of 410 stainless may score over 200,000 psi
after manufacture and be too brittle. By annealing at
1000 degrees F., tensile strength would reduce to 125,000-150,000
psi, while annealing the same material to 500 degrees
F. would bring tensile to 160,000-190,000 psi.
AUSTENITIC - Refers to 300 series stainless,
the most popular of the stainless alloys accounting for
85% - 90% of stainless fasteners sold. Named for Sir Robert
Williams Austen, an English metallurgist, austenitic stainless
is a crystal structure formed by heating steel, chromium,
and nickel to a high temperature where it forms the characteristics
of 300 series stainless steel. An “AUSTENITE” is a molecular
structure where 8 atoms of iron surround one atom of carbon,
thus limiting the corrosive effects of the carbon. Austenitic
fasteners have the highest level of corrosion resistance
in the stainless family, cannot be hardened by heat treatment,
and are non-magnetic for practical purposes.
most popular of austenitic grades is known generically
as “18-8 stainless” and includes grades 302, 302HQ, 303,
304, 305, and XM-7. Typical industries using 18-8 fasteners
include: food, dairy, wine chemical, pulp and paper, pharmaceutical,
boating, swimming pool, pollution control, electronic,
medical and hospital equipment, computer, textile.
316 stainless has added nickel and especially molybdenum.
The molybdenum (called moly) sharply increases corrosion
resistance to chlorides and sulfates, including various
sulfurous acids in the pulp industry. It has superior
tensile strength at high temperatures compared to 18-8.
Besides pulp and paper, typical industries using 316 are:
photographic and other chemicals, ink, textile, bleach,
metals in the 300 series include 309, 310, 317, 321, and
347. With superior corrosion resistance at elevated temperatures,
these metals are used for furnace parts, high temperature
containers and processing equipment, aircraft parts such
as collector rings, exhaust systems, and equipment for
very corrosive compounds of sulfuric, nitric, citric,
and lactic acids.
“B” - Referring to sheet metal screws,
type-B indicates a blunt point with more threads per inch
and smaller thread depth than type-A screws.
SURFACE - The part of a fastener such as the
washer face of a nut or under the head of a machine screw
that actually comes in contact with the part it fastens.
BEVEL - A small slant, usually describing a flat washer which is square
and thicker on one side than the other
HEAD - Old term for pan head, “binder”
has now come to mean “binding” head screws rather than
BLANK - A fastener where one or two stages
of manufacturing have been performed, but the fastener
has not been finished.
BODY – The unthreaded portion of the shank.
BOLT – An externally threaded fastener which requires a nut to secure fastened
BRASS - The most common alloy of copper,
brass is basically two-thirds copper, one-third zinc.
It is non-magnetic with good strength and toughness, high
electrical conductivity, and an attractive lustrous finish.
It has good corrosion resistance but not in salt water.
Brass is commonly used by the electrical and communications
industries, builders hardware, and some marine applications.
BROACH – Using sharp edges to cut material
and push it away, broach usually refers to the socket
drive on socket screws.
Where the shoulder of a screw is perceptibly smaller in
diameter than the threaded portion (technically the minor
diameter or less).
PRECIPITATION - Carbon that breaks loose from its
bond within the stainless solution when material is heated
between 800 - 1400 degrees F. Under severe corrosive conditions,
it can result in extra oxidation and surface corrosion.
See SOLUTION ANNEALED.
CARBON - Adds strength to stainless steel,
but also lowers corrosion resistance. The more carbon
there is, the more chromium must be added, because carbon
offsets 17 times its own weight in chromium to form carbides,
thus reducing the chromium available for resisting corrosion.
STEEL - Ordinary steel with no significant
additions besides carbon.
CASE DEPTH – That area of a fastener, measured from the surface inward,
which has a different hardness requirement that its core.
CASE HARDENED – Heat treated fastener in which the surface is harder than
OF COMPLIANCE – A certification that a fastener meets the description or standard to which
it was sold.
CHAMFER - A slight rounding on the end of
a fastener or the edges of a hex nut for ease of assembly
or smoother appearance.
HEAD - Old term for fillister head.
CHROMIUM - A blue-white metal, chromium is
the most important element providing corrosion resistance
in stainless steel. By adding 12% chromium to ordinary
steel, stainless steel is formed. Chromium offsets the
corrosive effects of carbon found in steel and is the
primary factor in the ability of stainless to form a passive
film on its surface providing corrosion resistance.
CLAMP LOAD – The total load across the joint interface in service. This
may vary during service life.
1A – Solution annealed in finished condition
to meet ASTM A103 specification.
FORMING or COLD HEADING or COLD WORKING - When fasteners are produced without heating or small heat below the
recrystallization temperature (so the raw material bond
of stainless remains unchanged) by pressing metal wire
against various dies at high speed to form a fastener’s
head or basic shape. Cold working causes an increase in
tensile strength and hardness (known as work hardening)
and a decrease in ductility.
A - Means that fasteners should be solution
B - Means strain hardened to meet certain
minimum tensile requirements.
COPPER - A reddish metal that is an excellent
conductor of heat and electricity. It is malleable, ductile,
and non-magnetic with low to average strength and good
corrosion resistance. Brass and silicon bronze, composed
mainly of copper, gain their strength from the addition
of other metals.
HARDNESS – The resistance a fastener material
has to being permanently deformed, measured at a spot
deeper than the case depth.
CRACKS – A clean, crystalline fracture which
passes through or across the grain boundaries without
inclusion of foreign elements.
STRENGTH - A measure of the resistance of fasteners
to stress under elevated temperatures. At higher temperatures,
a fastener can change in dimension under the same load,
and that is called creep. Creep can cause the loosening
of fasteners as temperature increases.
CORROSION - Refers to joints and crevices in
a fastener assembly where lack of oxygen caused by limited
space or by surface grease prevents the passive film on
stainless from forming.
THREADING - Forming threads on a fastener by
cutting away and actually removing the unneeded metal.
DEBURR- To remove chips,
burrs, or other imperfections through a secondary operation
such as grinding.
- A variety of small or large disfigurations
in a fastener such as pits (slight depressions on the
surface), tool marks, voids (small cracks), laps, folds
and seams (slightly bunched or folded material at the
corners of a fastener), and inclusions (a slight non-metallic
impurity in the metal). Minor discontinuities are permissible
in both commercial fasteners and those made to various
MS and other specs.
DRAWING - Where raw material shaped like wire
is pulled through a die to reduce its diameter to that
needed for the particular fastener being manufactured.
TORQUE – A screw shall form a mating internal
thread in a test plate, without damaging its own thread
with the application of a rotational force not in excess
of the drive torque.
DUCTILITY - The ability of a fastener to deform
before breaking (for example, and elastic would be more
ductile than a diamond). Ductility is a measurement similar
“18.8” - 300 series stainless steel having
approximately (not exactly) 18% chromium and 8% nickel.
The term “18-8” is used interchangeably to characterize
fasteners made of 302, 302HQ, 303, 304, 305, 384, XM7,
and other variables of these grades with close chemical
compositions. There is little overall difference in corrosion
resistance among the 18-8 types, but slight differences
in chemical composition do make certain grades more resistant
than others against particular chemicals or atmospheres.
“18-8” has superior corrosion resistance to 400 series
stainless, is generally non-magnetic, and is hardenable
only by cold working.
ELONGATION - Stretching a fastener
to the point that it breaks. The percent of elongation
at rupture (same as measure of ductility) is determined
by dividing the total length after stretching to the original
length. Elongation decreases as strength and hardness
CONDUCTIVITY - Metals carry electric currents with
varying capacities. As a relative guide to the conductivity
of different metals, with electrolytic copper rated at
101 under the International Annealed Copper Standard at
68 degrees F., 18-8 stainless rates is rated at 5; silicon
bronze 651 at 12; and brass at 27.
MINIMUM – The minimum depth an anchor must
be installed to meet the minimum pull-out values. It is
the distance measured from the concrete surface to the
bottom of the anchor.
ETCH – A chemical process that cleans
and brightens aluminum after heat treatment.
EXTRUDING - When cold forming produces a fastener
before threading with two different diameters. The portion
with the larger diameter is the shoulder; the smaller
portion will be roll threaded. In the extruding process,
a manufacturer starts with raw material equal to the shoulder
diameter and pushes part of it through a die, reducing
the diameter of the portion which will later be roll threaded.
F594 - F593 is a specification
for stainless hex head cap screws; F594 is for stainless
nuts. Compared to regular stainless fasteners, F593 and
F594 call for: (a) tensile requirements about 20% higher
than that of commercial 18-8 or stainless hex caps and
nuts to MS specifications (MS35307-8, MS34649-50); (b)
both a minimum and a maximum tensile and hardness requirements
while commercial and MS fasteners do not have a maximum;
(c) chemical requirements that are somewhat bizarre, eliminating
many commonly used mixtures of 300 or 18-8 stainless while
FATIGUE - Metal failure due to stresses that
push first in one direction and then another. FATIGUE
CORROSION is caused by repeated stress in a corrosive
atmosphere and is generally not associated with stainless.
STRENGTH - Measures the endurance of a fastener
by showing the load it can accept without breaking under
repeated load cycles.
FERRITIC - Comprising less than 5% of stainless
fasteners, mainly type 430, it is magnetic and not hardenable
by heat treatment. Though containing no nickel, ferritic
stainless has a high chromium content providing greater
corrosion resistance than martensitic stainless but much
less then austenitic. It is mainly used by the automotive
and building industries for decorative trim, architectural
hardware, handrails, moldings on various products.
FERROUS – That which contains iron; usually
refers to fasteners containing more iron than any other
FILLET – Concave junction at two intersecting
surfaces of a fastener.
FIT - Normally referring to threads, fit
is a measure for the tightness of mating parts.
CORROSION - Occurs when vibration causes a stainless
fastener to continually rub against another surface, resulting
in the passive oxide film on stainless rubbing off. Fretting
corrosion might occur in high tensile fasteners such as
BODY DIAMETER - When the shoulder of a fastener
equals the outside or major diameter of the threaded portion.
(also called SEIZING) - When two metals or fasteners stick together and cannot be easily loosened.
In tightening fasteners, for example, pressure builds
on threads as metals rub against each other, and the passive
film preventing corrosion on stainless may not form due
to lack of oxygen.
HOT-DIP GALVANIZING – The process of coating iron or steel with zinc by
means of hot dipping.
– The process of coating iron or steel with zinc through
an electric current. This results in a somewhat smoother,
shinier finish than hot-dipping.
– The process of coating iron or steel with zinc at room
temperature where the zinc powder becomes cold welded
to the metal parts. It results in a more uniform finish
than hot-dipping and greatly reduces the chance of hydrogen
embrittlement which can occur in electro-galvanizing.
CORROSION - An accelerated degree of corrosion
occurring when two different metals are in contact with
moisture, particularly sea water. All metals have what
is termed a specific electric potential, so that low level
electric current flows from one metal to another. A metal
with a higher position in the galvanic series (see below)
will corrode sacrificially rather than one with a lower
position, meaning stainless, for example, will corrode
before gold. The further apart the metals on the chart,
the more electric current will flow and the more corrosion
will occur. No serious galvanic action will occur by combining
the same metals, only dissimilar ones. To prevent galvanic
corrosion, use insulation, paint or coatings when separating
dissimilar metals; or put the metal to be protected next
to a metal which is not important in the assembly, so
it can corrode sacrificially.
listed first will corrode due to galvanic reaction before
those at end of paragraph: magnesium, zinc, aluminum 1100,
cadmium, aluminum 2024, steel and iron, lead, tin brass,
copper, bronze, monel, 304 and 316 stainless (passive),
silver, titanium, graphite gold.
POINT - A threaded cone point usually having
a point angle of 45-50 degrees.
GRIP - The unthreaded part of a fastener.
GRIP RANGE – The minimum and maximum thicknesses of materials a rivet
can join together.
HARDNESS - Normally stated
in terms of Rockwell or Brinell scale of measurement,
hardness shows resistance of a fastener to rough marks
and abrasions, can indicate yield strength and brittleness,
and has a direct relationship to tensile strength in alloy
steel fasteners. However, for stainless, brass, and silicon
bronze, the correlation between hardness and tensile or
yield is tenuous with no definite relationship.
uses surface heat treatment on ferrous material to cause
a harder outside surface than the center. Through-hardening
hardens the entire fastener. Bright hardening calls for
heat treatment without oxygen, so no oxides are formed
on the material surface.
POINT - A chamfer at the end of a fastener
formed at the time of heading but before threading.
TREATMENT - Heating often combined with cooling
at controlled temperatures in order to strengthen and
harden a fastener.
FORGING - Heating metal to red-hot temperatures
or temperatures above the recrystallization point to soften
it before shaping a fastener. Hot forging is primarily
used when the diameter of the metal is to large for cold
forming or the quantity required is to small to economically
set up a cold-forming machine.
EMBRITTLEMENT – The condition of a fastener which
has had hydrogen introduced into its steel, causing it
to be substantially less ductile and prone to sudden and
IFI - Stands for Industrial Fasteners
INDUCTION HARDENED – A heat-treated fastener that has undergone a selective hardening
process, using induction coils, to further strengthen
a part of the fastener.
SAMPLING – Random samples of fasteners taken
at different process points in the manufacture for testing
ISO - Stands for International Organization
CORROSION - A technical term describing corrosion
at grain boundaries (various outside portions) of a fastener.
It can occur when fasteners are heated above 800 degrees
during use, such as welding, which changes the chromium-carbon
bond in stainless, thus allowing increased oxidation and
corrosion. To prevent intergranular corrosion, low carbon
stainless should be used, or material should be annealed
and quenched after exposure to elevated temperatures,
so the carbon is put back into a austenitic stainless
NUT - A thinner nut that
is “jammed” against another nut to prevent loosening.
ENGAGEMENT – The distance from the head surface
of a socket to that depth to which the hex wrench will
KNURL - A rough or decorative
surface on part of a fastener.
LEAD - A heavy malleable
ductile metal that increases machineability.
LEAD THREAD – The thread length from where it starts to where it becomes
full-sized. This distance is usually one-half the fastener
HAND THREAD - Opposite of commonly used fasteners.
With left hand thread, a nut would be tightened on a bolt
by turning it counter-clockwise.
OF ENGAGEMENT – The length of full-sized fastener
threads that engage in the nut material. The length of
the lead thread is not counted in the length of engagement,
since its reduced size minimizes any performance benefits.
The length of engagement is usually expressed in relationship
to the nominal diameter of the screw (e.g. 2 to 2-1/2
diameters of engagement).
PENETRANT TEST – Dipping fasteners into a dye and
then viewing under ultraviolet light to look for cracks.
FLANK – The top portion of each thread
which applies the preload or load pressure to the mating
LOCKNUT – A nut constructed to resist loosening
when subjected to vibration or axial load. A prevailing
torque type locknut achieves its locking action without
being against another nut or a bearing surface, but by
a controlled distortion in its threads or by means of
another locking element (i.e. a nylon ring) built into
the nut. A free-spinning locknut achieves its locking
action when tightened against another surface.
LOT – A particular size of fastener processed
from the same raw material heat and same production process.
SAMPLING – Random samples taken from the same
lot of fasteners for quality inspection. Maximum lot size
for inspection purposes should not exceed 250,000 pieces;
thus, lots over 250,000 would require additional samples.
MS - Stands for Military Standards. The
overriding characteristic of MS fasteners compared to
commercial products is the extensive inspection and lot
traceability for MS, guaranteeing the chemical, physical
and dimensional qualities. While commercial fasteners
may look similar and happen to pass many tests given MS
products, the commercial fasteners lack the pedigree of
guaranteed quality for chemical, physical and dimensional
aspects that users who order MS fasteners rely on.
MACHINEABILITY - Same as free machining. Refers to
the malleable characteristics of metal when cutting or
forming on screw machines.
MAGNETISM - As related to stainless fasteners,
300 series stainless is non-magnetic in its raw material
condition. Cold working can sometimes induce traces of
magnetism in 300 series, depending on the severity of
cold working and chemical composition of the stainless.
A rise in magnetism is related to an increase in tensile
strength and work hardening caused by the heat and friction
of cold forming and does not reduce corrosion resistance
or cause any molecular change in austentic raw material.
A higher portion of nickel can increase stability in stainless,
thus decreasing work hardening and any possibilities of
magnetism. Brass and silicon bronze are non-magnetic.
PERMEABILITY test simply determines the level
DIAMETER - Largest or outside diameter of the
BREAK LOAD – The axially applied load required
to break the mandrel while a rivet is being set.
MANGANESE - A non-magnetic metal which improves
strength and hardness.
MARTENSITIC - Comprising approximately 5% of stainless
fasteners, martensitic refers mainly to stainless types
410, 416, and 420. Named for Robert Martens, a German
metallurgist, martensitic grades have a high carbon content
which reduces corrosion resistance, but allows a sharp
increase in tensile strength after heat treatment.
Because of its high tensile strength, martensitic
stainless is used for highly stressed parts such as control
rod mechanisms, valves, shafts, pump parts under high
stress. Martensitic stainless is magnetic, contains no
nickel, loses toughness in very cold temperatures, and
may have tendency to become brittle. Its corrosion resistance
is not as good as austentic or ferritic stainless, so
martensitic fasteners are used in mild atmospheres.
FROM BAR (also called MACHINING) - Made
on a screw machine or lathe by cutting material away from
the original piece of metal. It is used for manufacturing
very large diameters which cannot be cold formed and for
small quantities where it would not be economical to set
up cold forming equipment. However, machining can interrupt
the grain of metal causing a lessening in tensile and
DIAMETER - The inside or smallest diameter
of the screw threads.
– The amount of rotational force a fastener must endure
before failure occurs. This presumes the screw is driven
into a proper size hole.
MOLYBDENUM – Nicknames moly, molybdenum is a
metal added to 316 stainless steel, sharply increasing
its corrosion resistance to chlorides and sulfates, especially
various sulfurous acids in the pulp industry. Molybdenum
helps reduce hardness and increase tensile strength at
higher temperatures. Molybdenum is also added to Marutex®
self-drilling screws made of 410 stainless to significantly
increase corrosion resistance.
MONEL - Invented by the International Nickel
Co. and composed basically of two-thirds nickel, one-third
copper, monel has good strength, excellent corrosion resistance
against salt water and in high temperatures, and is very
MUNTZ – A form of brass with about 60%
BRONZE(also called NAVAL BRASS) - Basic brass with a small addition of tin for added corrosion resistance
against salt water.
NAS - Stands for National Aerospace Standards.
NICKEL - A metal added to 300 series stainless
to provide corrosion resistance, increased strength in
both high and low temperatures, and increased toughness
in low temperatures. Nickel lowers the effects of work
hardening, thus reducing traces of magnetism caused by
cold forming and making material flow more freely in manufacturing.
DIAMETER – The major diameter of a screw,
or, in tri-round fasteners, the “c” dimension.
NON-FERROUS - Metals without iron. Brass and silicon
bronze are non-ferrous; stainless is often characterized
as non-ferrous, but it is not non-ferrous.
NYLON - Light and low in strength compared
to metal fasteners, nylon in non-magnetic, good for insulation,
and corrosion resistant against many chemicals.
HEAD - Old term for truss
PASSIVATING - A very confusing
term, since the common usage has taken on a different
meaning than the technical definition. In past experience,
users (including engineers) of commercial fasteners seldom
mean the old technical terminology. Technically, passivating
is not cleaning but is a process of dipping fasteners
into a nitric acid solution to rapidly form a chromium
oxide on the surface of the material, creating a passive
film that protects stainless from further oxidation (see
PASSIVE FILM). The purpose of passivating is to remove
both grease left from manufacturing and traces of steel
particles which may have rubbed off manufacturing tools
onto the fastener. In common commercial parlance (meaning
non-military and aerospace), passivating means cleaning
to users, and the terms “passivating” and “cleaning” are
used interchangeably. A wide range of cleaning methods
using different mixtures containing nitric, phosphoric
and other acids or simply exposing cleaned stainless fasteners
to air for a period of time will result in a “passivated”
condition. For fasteners that have been properly cleaned,
it is impossible to determine the method of cleaning or
passivation that was used.
FILM - The major characteristic of stainless
is its ability to form a thin layer of protection, called
a “passive film”, on its outside surface. This film results
from a continual process of low-level oxidation, so oxygen
from the atmosphere is needed for the passive film to
exist. Once formed, it prevents further oxidation or corrosion
from occurring. Even if chipped or scratched, a new passive
film on stainless will form.
GAUGE DEPTH – A range of measurements which determine
the acceptability of a recess in the head of a screw.
It is measured from a plane where the edge of the recess
wings meet the top of the head’s surface, downward into
PHOSPHORUS - A non-metallic substance that lowers
the rate of oxidation, thereby helping resist corrosion.
PICKLING - Removing surface impurities by using
HOLE SIZE – An opening of sufficient size for
a specific fastener to be properly installed.
POINT - Similar to a “B” point, a pilot
point is a small (perhaps 1/8” - 1/4”) unthreaded blunt
portion at the end of a sheet metal or drive screw.
PITCH - The distance between two adjacent
threads measured at the outside diameter of the threads.
CYLINDER – A volume parallel to the fastener
axis whose diameter is equidistant between the major and
DIAMETER - Approximately in-between the major
and minor diameters.
CORROSION - Pitting indicates deep corrosion
in localized spots on a fastener. Dirt or grease on certain
portions of a fastener may block oxygen from that surface,
thus impeding the passive film which protects stainless
PLATING – The application of a metallic deposit
on the surface of a fastener for protective and/or decorative
purposes (see plating and coatings chart link).
TAPER LENGTH – The length of the pointed portion
of the fastener measured parallel to the axis, from the
end of the point to the first full form thread.
HARDENED STAINLESS STEEL
- Type 630 stainless, little used, expensive
and not sold as commercial products, it combines corrosion
resistance of 300 series stainless with high tensile
of 400 series.
PRELOAD – The initial load put on a fastener
once assembly is complete.
LOAD - A test load that a fastener must
undergo without showing significant deformation. It is
usually 90% of yield strength.
QUENCH - To cool suddenly
and rapidly after heating.
STRESS – Forces that propagate from the
fastener towards the outside diameter of the joint.
DEPTH – The distance measured, axially,
from the plane where the edge of the recess wings meet
the top of the head’s surface to the bottom of the recess.
OF AREA - A measurement like
elongation which is related to the tensile strength of
a fastener. While elongation measures the length of a
fastener stretched to its breaking point compared to its
original length, reduction of area measures the diameter
of a fastener just before breaking compared to its original
HARDNESS TEST – Test designed to measure the hardness
of the fastener, based on an alphabetical-numerical scale.
The higher the number, the harder the fastner.
tests are utilized to test for decarburization and carburization
and to determine the amount of resistance to permanent
deformation during the testing procedure. They also assure
that heat treating was performed to specification.
standards require socket screw products meet specific
Rockwell Hardness Standards. Socket screws are typically
in the “C” scale, which is the hardest Rockwell designation,
but the scale designation is dependent on the size of
the socket screw.
THREADING - Forming threads on a fastener by
pushing or rolling dies against the fastener without any
removal of metal. Roll threading, as opposed to cut threading,
hardens the material making the threads stronger.
ROOT – The base of the V thread. This
is the weakest point on a fastener because it has the
smallest cross sectional area.
DIAMETER - Refers to the minor diameter on
screws or the major diameter on nuts.
THREADS – The thread section that is between
the last scratch of thread and the fillet or body.
SAE- Stands for Society of Automotive
SCALE - A discoloring or oxidation on the
surface of hot forged fasteners.
SCREW – An externally threaded fastener
which does not require a nut to secure the fastened joint.
MACHINE - Cutting and removing material in
order to form a fastener.
SEAMS – A narrow, non-crystalline discontinuity,
which is usually inherent in the raw material. Seams are
usually straight or smooth-curved line discontinuities
running parallel to the product axis.
OPERATIONS - Less important than the major steps
of heading or cold forming fasteners, secondary operations
include grinding, polishing, drilling.
FINISHED HEX CAPS – The normally sold variety of stainless
hex head cap screws, semi finished have the same dimensions
as a finished fastener but with generally greater tolerances.
SHANK – That portion of a headed fastener
that lies between the head and the point.
SHEAR – Force that tends to divide an object
along a plane parallel to the opposing stresses.
STRENGTH - Measured by the push or pull against
the side of a fastener until the fastener breaks (for
example, moving an object continually against the side
of a screw that is protruding from a wall). As a rule
of thumb, shear strength is two-thirds of tensile strength.
DOUBLE SHEAR STRENGTH is applying a load against
the fastener in two places causing the fastener to break
into three pieces.
SILICON - A non-metallic substance that adds
strength and toughness to copper to help form a bronze
BRONZE - An alloy made of 95% - 98% copper
plus a small amount of silicon added for strength. Small
amounts of manganese and aluminum may also be added for
strength, and lead may be added for machineability. Silicon
bronze is non-magnetic with a high degree of thermal conductivity
and high corrosion resistance against sea water, gases,
and sewage. It is often used by the utilities industry
for pole line hardware and switchgear equipment, mine
sweeping, sewage disposal equipment, food machinery, marine
applications, plumbing and liquid handling. Surprisingly,
silicon bronze is only a low to moderate conductor of
electricity, though it is a better conductor than stainless.
DEPTH – Distance measured parallel to fastener
axis from the top of the head to the extreme end of the
ANNEALED - (same as CARBIDE SOLUTION ANNEALED)
- A process of heating and removing carbide precipitants
(carbon that has broken loose from its stainless steel
solution) by heating a finished fastener to over 1,850
degrees F. and cooling it quickly, usually in water, so
carbon content goes back into the stainless solution.
STEEL - With the addition of 12% chromium
to iron, stainless steel is formed. The chromium
protects the iron against most corrosion or red-colored
rust; thus the term “stainless” steel. The ability
of stainless to form a thin layer of protection on its
outside surface, called a “passive film”, is its most
important characteristic in preventing corrosion (see
The overriding purpose of stainless
steel is to provide corrosion resistance against: (a)
atmospheric conditions such as carbon dioxide, moisture,
electrical fields, sulfur, salt, and chloride compounds;
(b) natural and artificially produced chemical, (c) extremes
of weather where cold temperatures cause brittleness and
hot temperatures reduce strength and increase corrosion.
For more information, see AUSTENITIC, MARTENSITIC,
FERRITIC, and PRECIPITATION HARDENING.
STAMPING - Punching out parts with dies, usually
referring to flat washers.
HEAD - Old term for truss head.
HARDENED - To increase hardness and strength
by (a) cold working of raw material by a steel mill or
(b) cold forming by a fastener manufacturer can sharply
increase tensile strength and hardness, so that ordinary
material from a steel mill may often be used. However,
fasteners that are milled from bar will decrease in strength
and hardness, so that raw material would need to be strain
hardened by a steel mill before milling fasteners.
CORROSION - Occurs when corrosion causes a highly
stressed part (one that is pushed to its maximum tensile
strength) to crack. Except for heat treated 400
series stainless, stress corrosion does not normally apply
to austenitic stainless, brass, or bronze, since these
metals are relatively ductile and not normally used
for high tensile operations.
STUD – A headless fastener with threads
at both ends of the shank.
SULFUR - A non-metallic substance found in
large quantities by itself or combined with other elements
to form sulfates and sulfides. It improves machineability,
and helps cool material and prevent galling, but its presence
lowers the corrosion resistance of stainless.
TANTALUM – A gray ductile metal with a high
melting point and excellent corrosion resistance against
TAP - To put internal threads in a hole
or in a nut.
BOLT - Fully threaded bolt.
TEMPER - To heat material after hardening
to a temperature of perhaps 1000 degrees F. and allow
to cool naturally in order to soften material and make
it less brittle. Or to heat to a lower temperature
of possibly 500 degrees F. to relieve stress in metal
without affecting the hardness.
STRENGTH - A common measure to compare the
strength of a fastener. It is the load needed to
pull the fastener apart.
STRESS AREA – Selected area or areas used to
calculate the tensile strength of an externally threaded
fastener, so the fastener strength is consistent with
the material strength. IT corrects for the notch and helix
effect of the threads and is a function of the pitch and
THREADS - Class 1 threads are a loose tolerance.
Class 2 threads comprise 90% of stainless fasteners and
are normal commercial tolerance. Class 3 threads
have a stricter tolerance and tighter fit such as socket
cap and set screws. No definite relationship exists
between tensile strength and tightness or looseness of
fit. The symbol “A” added to threads, such as 2A,
means external threads (screws), and “B” means internal
With the exception of 10/32 diameter,
which is extremely popular, coarse thread comprises 90-95%
of hex head cap screws and hex nuts sold in 18-8 stainless,
and perhaps 98% of other stainless items including machine
screws and socket products. Coarse threads are deeper
than fine threads with fewer threads per inch, so coarse
threads may have greater protection against thread stripping,
better tap in brittle materials, and better fatigue resistance,
while fine threads may have better fit in thin-walled
materials, higher torque strength, and increased tightness
ENGAGEMENT – The amount of thread tooth that
is filled by the application material. This measurement
is usually expressed as a percentage and is used to determine
optimal hole size.
PROFILE – The angle between the flank of
the thread and a line perpendicular to the axis.
PITCH – The distance between corresponding
points on adjacent threads in the same plane parallel
to the part’s axis and on the same side of the axis.
HARDENED – Heat treated fastener with uniform
hardness from the surface to the core.
TORQUE – The amount of rotational force
which is approximately midway between a fastener’s “drive
torque” and “ultimate torque”.
TIN - A malleable and ductile metal which
increases strength, hardness and corrosion resistance
against salt water when added to brass alloys.
TITANIUM - A silvery gray metal with high corrosion
resistance against salt waters, chlorides, and many acids.
It is strong, though lightweight, and very expensive.
TOLERANCE – The difference between the lower
and upper limits between which a fastener’s size(s) must
or TORSION STRENGTH
- Torque is the force used in twisting, such as tightening
a fastener. Torsion strength is the amount of force
needed to twist a fastener apart. Both measures
consider the amount of pressure applied to the fastener
and the length of the wrench used in the application.
TORSION – Twisting force applied to fastener.
TOUGHNESS - A fastener’s capacity to accept
various impacts and shocks.
TUMBLING - To flip fasteners around like clothes
in a dryer in order to clean fasteners and increase the
shininess of stainless. Soap or a cleansing solution
are often added.
UNC – Unified National Coarse Threads
UNF – Unified National Fine Threads
UNR - Indicates “unified” screw threads
to “inch” dimensions used in the U.S. as distinguished
from metric dimensions.
UNJ - A type of threads originating around
the 1950’s with a more rounded fit in order to prevent
cracks, reduce loosening due to vibration and slightly
increase strength. Possessing a tighter fit, UNJ
thread depth is smaller that the usual UN standards with
the minor diameter of external threads on screws and internal
threads on nuts both increasing. UNJ is used in
critical applications by the aerospace and automotive
BODY OR REDUCED BODY DIAMETER
- Where the shoulder of a fastener equals the pitch diameter
or less, which means the shoulder is smaller than the
outside diameter of the threads. It would indicate
that a fastener was not extruded during its manufacture.
- A circular rim on the underside of the head of a bolt
or on one side of a nut with the purpose of providing
a flat bearing surface for the bolt or nut to sit on.
A smooth washer face takes away any burrs or imperfections
caused by the manufacturing process.
HARDENED - An increased level of hardness caused
by cold forming fasteners.
YIELD - The resistance to a load pulling
on the middle of a fastener until the fastener shows permanent
STRENGTH - The amount of pressure required
to cause permanent deformity. When a fastener is stretched,
yield strength is the point where the fastener will not
return to its original length following testing. It is
measured in terms of pounds per square inch (psi) or megapascals
(MPa). Yield strength is often determined by the offset