Allowance |
An intentional difference between the maximum
material limits of mating parts. It is also the minimum clearance
(positive allowance) or maximum interference (negative allowance)
between such parts. |
Annealed |
When a fastener has been heated and cooled to make it free
of hardness caused by working or pervious heat treatment. |
Blank |
A fastener in some intermediate stage of manufacturing,
yet not completed. |
Burr |
A small undesirable amount of material extending out from
the edge of a hole, slot shoulder or end of a screw resulting
from a machining operation. |
Case Hardened |
A fastener of ferrous material having a surface which has
been hardened more than the core. |
Class of Thread |
An alphanumerical designation recognized by the fastener
industry to indicate a standard grade of allowance and/or
tolerance for a thread. Also known as "the fit"
of the thread. |
Countersink |
A bevel on a flat surface or a flare at the end of a hole;
usually measured in degrees such as 82° for commercial
applications and 100° for aerospace. |
Decarburized |
A fastener that has a lower carbon content on the surface
than at its core. |
Dimension |
Numerical values expressed in units of measure that are
indicated on a drawing along with lines, symbols, and notes
to define the characteristics of a fastener. |
Ductility |
The ability of a fastener to deform before it fractures.
The most meaningful test for ductility is passing the wedge
test in conjunction with a Rockwell hardness reading within
the specification. The lower the ratio between yield and tensile,
the more ductile the fastener. |
External Threads |
Threads that are formed on the outside of a fastener such
as a screw. Designated as "2A" or "3A"
fit. |
Fastener |
A mechanical device used to hold tow or more objects in
a definite position with respect to each other until intentionally
loosened. |
Fatigue |
The maximum stress a fastener can withstand for a specified
number of repeated applications or cycles of load until the
extreme is reached. |
Finish |
Commonly used to describe the condition of the surface of
a fastener as a result of chemical or organic treatment following
manufacturing. |
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. |
Gage |
A device used to check whether a thread is within the maximum
or minimum tolerance limits |
Grip |
The unthreaded portion of the fastener. It can also be measured
as the thickness of the material which the fastener was designed
to secure. |
Hardening |
A method of heat treating metals by heating to a temperature
within or above the critical range, then holding at that temperature
for a given length of time - cooling rapidly, usually by quenching
in oil or water. |
Heat |
Treat An operation involving heating and cooling of a metal
fastener to obtain desirable conditions or properties. |
High Strength Fastener |
A fastener having high tensile and shear strengths attained
through combinations of materials, work-hardening and heat
treatment. |
Inclusions |
Particles of non-metallic impurities contained in materials. |
Internal Threads |
Threads formed on the inside of a fastener such as the threads
of a nut. Designated as "2B" or "3B" fit. |
Knurling |
A process that produces an intentional roughened surface
of hash marks or diamond patterns by means of a forming tool
called a knurl. |
Laps |
Surface defects caused by folding over fins or sharp corners
into the surface of the material during manufacturing. |
Lead |
The distance a screw will advance in one 360? rotation of
the threads. |
Left-Hand Threads |
Threads that wind in a counter-clockwise direction. Designated
by the size call-out followed by "LH". |
Mechanical Properties |
Those properties which involve a relationship between strain
and stress. |
Non-Ferrous Material |
Any metal that does not contain an appreciable amount of
iron such as copper, brass and aluminum. |
Non-magnetic |
Materials that have a maximum magnetic permeability of 2.0
(air=1.0) for a field strength of H=200 oersteds using a magnetic
indicator per MIL-I-17214. Generally associated with the 18-8
stainless steel series. |
Passivation |
A process of removing particles and surface impurities from
stainless steel by chemicals (usually a nitric acid dip).
Recommended process to improve corrosion resistance. |
Physical Properties |
The properties that define the characteristics of the material
or the fastener itself. |
Plain |
Refers to the finish of a fastener. "Plain" indicates
no supplementary surface treatments have been done to the
fastener. Non-corrosion resistant parts may be oiled and still
be referred to as plain. |
Plating |
The application of a metallic deposit or corrosion inhabiting
coating on the surface of the fastener by electrolysis, impact
or other suitable means. |
Proof Load |
The tension applied load that a fastener can withstand without
deformation. It is expressed as an absolute value instead
of a minimum/maximum value. To pass as acceptable, the after
load length must be the same as the original length within
a small tolerance. |
Punching |
The process of trimming or removing material with dies in
a press. |
|
Quality |
` Denotes the suitability f a fastener for the purpose for
which it is intended. It should not be confused with precision
or workmanship as it is possible that precision parts of good
workmanship and finish can be poor quality if they fail to
perform as they were intended to. Also good quality parts
do not always require precision or fine finish to serve as
intended. Optimally a quality fastener should possess all
of the beneficial characteristics above. |
Right Hand Threads |
Threads that wind in a clockwise direction. All threaded
fasteners have right-hand threads unless noted otherwise. |
Rockwell Hardness |
A test measure using either a steel ball or a diamond shero-conical
penetrator. The depth of the indentation is measured and a
hardness number relating tot eh depth is assigned. The higher
the number, the harder the material. |
SAE Specifications |
Standards developed by the Society of Automotive Engineers
Incorporated from which many screw thread standards conforming
to Unified and American Standards are derived. |
Shear Strength |
The maximum transverse load a fastener can withstand just
before it fractures. There is a single shear test concerned
with one transverse plane and a double shear which pertains
to two planes. In fasteners it may be assumed to be approximately
60% of specified minimum tensile strengths. |
Torsional Strength |
A load measured in terms of torque requiring a fastener
to be twisted off of it axis until it snaps apart. At the
point the measurement is taken in torque pounds. This practice
is generally reserved for tapping screws and some stainless
steel metric screws. |
Toughness |
Also called impact strength, it is the ability of fastener
to accept punishment by absorbing energy in the form of impact
or shock loading. This test is usually reserved for low temperature
applications and aerospace fasteners. |
Undercut |
Short length flat and oval head screws may have 30% of the
head height reduced by shaving the countersink, thereby increasing
thread length. Binder head machine screws may be undercut
on special request, to allow wire to pass beneath the head. |
Yield Strength |
The tension applied load at which a fastener has been stressed
beyond its elastic limit and becomes deformed. This test has
been somewhat replaced by the Proof Load test. Yield strength
is expressed in Ksi (thousand pounds per square inch), PSI
(pounds per square inch) or Mpa (Mega Pascal) in metric.
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