Screws come in a variety of
shapes and sizes for different purposes.
U.S. Quarter coin (diameter 24 mm) shown
for scale.A screw is a shaft with a helical
groove or thread formed on its surface.
Its main uses are as a threaded fastener
used to hold objects together, and as a
simple machine used to translate torque
into linear force. It can also be defined
as an inclined plane wrapped around a shaft.
Contents
1 Threaded fastener
2 Differentiation between bolt and screw
2.1 Other fastening methods
3 Materials and strength
4 Mechanical analysis
5 Tensile strength
6 Types of screws and bolts
7 Shapes of screw head
8 Types of screw drive
8.1 Combination drives
9 Tamper-resistant screws
10 Tools used
11 Mechanics of use
12 Thread standards
12.1 ISO metric screw thread
12.2 Whitworth
12.3 British Association screw threads (BA)
12.4 Unified Thread Standard
12.5 Others
13 History
14 Legal issues
15 See also
16 References
17 External links
Threaded
fastener
A screw used as a threaded fastener consists
of a shaft, which may be cylindrical or
conical, and a head. The shaft has a helical
ridge or thread formed on it. The thread
is essentially an inclined plane wrapped
around a shaft. The thread mates with a
complementary helix in the material. The
material may be manufactured with the mating
helix (taps and dies), or the screw may
create it when first driven in (a self-tapping
screw). The head is specially shaped to
allow a screwdriver or wrench (British English:
spanner) to grip the screw when driving
it in. It also stops the screw from passing
right through the material being fastened
and provides compression.
Screws can normally be removed
and reinserted without reducing their effectiveness.
They have greater holding power than nails
and permit disassembly and reuse.
A screw that may be tightened
by turning it clockwise is said to have
a right-hand thread. Screws with left-hand
threads are used in exceptional cases, when
the screw is subject to anticlockwise forces
that might undo a right-hand thread. Examples
include rotating items such as the left
hand grinding wheel on a bench grinder or
the left hand pedal on a bicycle (both looking
towards the equipment).
Threaded fasteners are traditionally
made by a cutting action such as taps and
dies provide, however recent advances in
tooling allows them to be made by rolling
the blank (a section of rod) between two
specially machined dies. The thread form
and shape of the fastener are squeezed onto
the blank. This method work hardens the
threads and saves material. A rolled thread
is obvious after manufacture because the
outside diameter of the thread is greater
than the diameter of the blank material.
Bicycle spokes, which are just very long
thin bolts, always use rolled threads for
strength.
Differentiation between bolt and screw
A capscrew with a nut, washer and locknut.A
screw, by definition, is not a bolt. A bolt
is designed such that a nut (or other turning
device) is required for operation. A bolt
is not designed to be turned. What most
people refer to as a bolt is in fact a 'cap
screw', which is designed to be turned (or
screwed). Cap screws may, or may not be
used with nuts. The distinction is subtle,
but significant in the design of the fastener.
See also the article on the
bolt manufacturing process.
Other
fastening methods
When screws and bolts cannot be used, nailing,
riveting, roll pins, pinned shafts, welding,
soldering, brazing, gluing, and duct tape
(taping) are some alternatives.
Another alternate is the threaded
insert. Examples include HeliCoil [1] and
Keensert [2].
Materials
and strength
Screws and bolts are made in a wide range
of materials, with steel being perhaps the
most common, in many varieties. Where great
resistance to weather or corrosion is required,
stainless steel, titanium, brass or bronze
may be used, or a coating such as brass,
zinc or chromium applied. Electrolytic action
from dissimilar metals can be prevented
with aluminium screws for double-glazing
tracks, for example. Some types of plastic,
such as nylon or Teflon, can be threaded
and used for fastening requiring moderate
strength and great resistance to corrosion
or for the purpose of electrical insulation.
Even porcelain and glass can have molded
screw threads that are used successfully
in applications such as electrical line
insulators and canning jars.
The same type of screw or
bolt can be made in many different grades
of material. For critical high-tensile-strength
applications, low-grade bolts may fail,
resulting in damage or injury. On SAE-standard
bolts, a distinctive pattern of marking
is impressed on the heads to allow inspection
and validation of the strength of the bolt.
However, low-cost counterfeit fasteners
may be found with actual strength far less
than indicated by the markings. Such inferior
fasteners are a danger to life and property
when used in aircraft, automobiles, heavy
trucks, and similar critical applications.
Mechanical analysis
Rotating screw and fixed troughA screw is
a specialized application of the wedge or
inclined plane. It contains a wedge, wound
around a cylinder or shaft, that either
fits into a corresponding inclined plane
in a nut, or forms a corresponding inclined
plane in the wood or metal as it is inserted.
The technical analysis (see also statics,
dynamics) to determine the pitch, thread
shape or cross section, coefficient of friction
(static and dynamic), and holding power
of the screw is very similar to that performed
to predict wedge behavior. Wedges are discussed
in the article on simple machines.
Critical applications of screws
and bolts will specify a torque that must
be applied when tightening. The main concept
is to stretch the bolt, and compress the
parts being held together, creating a spring-like
assembly. The stretch introduced to the
bolt is called a preload. When external
forces try to separate the parts, the bolt
sees no strain unless the preload force
is exceeded.
As long as the preload is
never exceeded, the bolt or nut will never
come loose (assuming the full strength of
the bolt is used). If the full strength
of the bolt is not used (e.g., a steel bolt
threaded into aluminum threads), then a
thread-locking adhesive may be used.
If the preload is exceeded
during normal use, the joint will eventually
fail. The preload is calculated as a percentage
of the bolt's yield tensile strength, or
the strength of the threads it goes into,
or the compressive strength of the clamped
layers (plates, washers, gaskets), whichever
is least.
Tensile
strength
Rusty hexagonal bolt headsScrews and bolts
are usually in tension when properly fitted.
In most applications they are not designed
to bear large shear forces. For example,
when two overlapping metal bars joined by
a bolt are likely to be pulled apart longitudinally,
the bolt must be tight enough so that the
friction between the two bars can overcome
the longitudinal force. If the bars slip,
then the bolt may be sheared in half, or
friction between the bolt and slipping bars
may erode and weaken the bolt (called fretting).
For this type of application, high-strength
steel bolts are used and these should be
tightened with a torque wrench.
High-strength bolts usually
have a hexagonal head with an ISO strength
rating (called property class) stamped on
the head. The property classes most often
used are 8.8 and 10.9. The number before
the point is the tensile ultimate strength
in MPa divided by 100. The number after
the point is 10 times the ratio of tensile
yield strength to tensile ultimate strength.
For example, a property class 5.8 bolt has
a nominal (minimum) tensile ultimate strength
of 500 MPa, and a tensile yield strength
of 0.8 times tensile ultimate strength or
0.8(500) = 400 MPa.
Tensile yield strength is
M10, property class 8.8 bolt can very safely
hold a static tensile load of about 15 kN.
Types
of screws and bolts
Combination flanged-hex/Phillips-head screw
used in computersA hex cap screw has a protruding
hexagonal head, designed to be driven by
a spanner or wrench.
A socket cap screw has a hexagonal recessed
drive, usually with a cylindrical head,
but can also be found with a rounded button
head or a countersunk flat head. Socket
cap screws can be torqued more tightly than
other drives without stripping, and they
are usually made from a high strength steel
alloy.
A Wood screw has a tapered shaft, allowing
it to penetrate undrilled wood, or a wall
plug in brickwork etc.
Lag screw, lag bolt or coach screw refers
to a large wood screw with a hexagonal head,
driven by a wrench rather than a screwdriver.
A Machine screw has a cylindrical shaft,
threaded its entire length, and fits into
a nut or a tapped hole.
Self-tapping screws or thread cutting screws
have sharp threads that cut into a material
such as sheet metal or plastic. They are
sometimes notched at the tip to aid in chip
removal during thread cutting.
A Self-drilling screw is similar to a self-tapping
screw, but has a drill-shaped point to cut
through the material without prior drilling.
Thread rolling screws have a lobed (usually
triangular) cross section. They form threads
by pushing outward during installation.
They may have tapping threads or machine
threads.
A Drywall screw is a specialized self-tapping
screw designed to bind drywall to wood or
metal studs, but it has proven to be a versatile
construction fastener.
A Set screw, used to prevent loosening due
to vibration, is available with thumb screw,
square head, Hex head (inset socket) and,
most commonly, headless (a grub screw in
UK parlance, designed to be inserted flush
with or below the surface of the work piece).
Alternatively defined to be a screw whose
thread reaches the head (if any), as opposed
to a bolt.
Dowel screw is a wood screw with two pointed
ends and no head, used for making hidden
joints between two pieces of wood.
A stud is similar to a bolt but without
the head. Studs are threaded on both ends.
In some cases the entire length of the stud
is threaded, while in other cases there
will be an unthreaded section in the middle.
It may be anchored in concrete, for example,
with only the threads on one end exposed.
(See also: screw anchor, wedge anchor.)
A carriage bolt or coach bolt has a domed
or countersunk head, and the shaft is topped
by a short square section under the head.
The rib neck carriage bolt has several longitudinal
ribs instead of the square section, to grip
into a metal part being fixed.
A stove bolt is similar to a carriage bolt,
but usually used in metal. It requires a
square hole in the metal being bolted to
prevent the bolt from turning.
The Superbolt is a form of very large fastener
where tension in the bolt is developed by
a special nut containing individual jackscrews.
This is applied in large structural joints
where hand tools or portable tools are insufficient
to develop the required tension of a simple
bolt and nut.
Acme screw form has threads that are stronger
and broader than standard V-profile threads,
making them much better for load carrying,
linear actuating (also see ball screws),
and quick threading. It is often used for
vise screws.
A Shoulder Screw is a screw used for revolute
joints in mechanisms and linkages. The main
shank of the screw is smooth and works as
a suitable bearing surface. At the end of
the cylindrical bearing surface is a reduced
diameter thread.
Shapes
of screw head
(a) pan, (b) button, (c) round,
(d) truss, (e) flat, (f) oval
pan head: a low disc with chamfered outer
edge.
button or dome head: cylindrical with a
rounded top.
round head: dome-shaped, commonly used for
machine screws.
truss head: lower-profile dome designed
to prevent tampering.
flat head or countersunk: conical, with
flat outer face and tapering inner face
allowing it to sink into the material.
oval or raised head: countersunk with a
rounded top.
bugle head: similar to countersunk, but
there is a smooth progression from the shaft
to the angle of the head, similar to the
bell of a bugle.
cheese head: disc with cylindrical outer
edge, height approximately half the head
diameter.
fillister head: cylindrical, but with a
slightly convex top surface.
socket head: cylindrical, relatively high,
with different types of sockets (hex, square,
torx, etc.).
mirror screw head: countersunk head with
a tapped hole to receive a separate screw-in
chrome-plated cover, used for attaching
mirrors.
headless (set or grub screw): has either
a socket or slot in one end for driving.
Some varieties of screw are manufactured
with a break-away head, which snaps off
when adequate torque is applied. This prevents
tampering and disassembly and also provides
an easily-inspectable joint to guarantee
proper assembly.
Types
of screw drive
Slotted
Phillips
Pozidriv
Torx
Hex
Robertson
Tri-Wing
Torq-Set
Spanner Head
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Modern screws employ a wide variety of drive
designs, each requiring a different kind
of tool to drive in or extract them. The
most common screw drives are the slotted
and Phillips; hex, Robertson, and TORX are
also common in some applications. Some types
of drive are intended for automatic assembly
in mass-production of such items as automobiles.
More exotic screw drive types may be used
in situations where tampering is undesirable,
such as in electronic appliances that should
not be serviced by the home repair person.
Slot head has a single slot,
and is driven by a flat-bladed screwdriver.
The slotted screw is common in woodworking
applications, but is not often seen in applications
where a power driver would be used, due
to the tendency of a power driver to slip
out of the head and potentially damage the
surrounding material.
Phillips
vs. Frearson
BNAE driver bitCross-head, cross-point,
or cruciform has a "+"-shaped
slot and is driven by a cross-head screwdriver,
designed originally for use with mechanical
screwing machines. There are five types:
The Phillips screw drive has slightly rounded
corners in the tool recess, and was designed
so the driver will slip out, or cam out,
under high torque to prevent over-tightening.
The Phillips Screw Company was founded in
Oregon in 1933 by Henry F. Phillips, who
bought the design from J. P. Thompson. Phillips
was unable to manufacture the design, so
he passed the patent to the American Screw
Company, who was the first to manufacture
it.
A Reed & Prince or Frearson screw drive
is similar to a Phillips but has a more
pointed 75° V shape. Its advantage over
the Phillips drive is one driver or bit
fits all screw sizes. It is found mainly
in marine hardware and requires a special
screw driver or bit to work properly. The
tool recess is a perfect cross, unlike the
Phillips head, which is designed to cam
out. It was developed by an English inventor
named Frearson in the 19th century and produced
from the late 1930s to the mid-1970s by
the former Reed & Prince Manufacturing
Company of Worcester, Massachusetts a company
which traces its origins to Kingston, Massachusetts
in 1882, and was liquidated in 1990 with
the sale of company assets.
A JIS (Japanese Industrial Standard) head,
commonly found in Japanese equipment, looks
like a Phillips screw, but is designed not
to cam out and will, therefore, be damaged
by a Phillips screwdriver if it is too tight.
The standard number is JIS B 1012:1985
French Recess, also called BNAE NFL22-070
for Bureau de Normalisation de l'Aéronautique
et de l'Espace, a French standards organization.
Pozidriv is patented, similar to cross-head
but designed not to slip, or cam out. It
has four additional points of contact, and
does not have the rounded corners that the
Phillips screw drive has. Phillips screwdrivers
will usually work in Pozidriv screws, but
Pozidriv screwdrivers are likely to slip
or tear out the screw head when used in
Phillips screws. Pozidriv was jointly patented
by the Phillips Screw Company and American
Screw Company.
TORX is a star-shaped "hexalobular"
drive with six rounded points. It was designed
to permit increased torque transfer from
the driver to the bit compared to other
drive systems. TORX is very popular in the
automotive and electronics industries due
to resistance to cam out and extended bit
life, as well as reduced operator fatigue
by minimizing the need to bear down on the
drive tool to prevent cam out. TORX screws
were found in early Apple Macintosh computers,
to discourage home repairs. TORX PLUS is
an improved version of TORX which extends
tool life even further and permits greater
torque transfer compared to TORX. A tamper-resistant
TORX head has a small pin inside the recess.
The tamper-resistant TORX is also made in
a 5 lobed variant. These TORX configurations
are commonly used in correctional facilities,
public facilities and government schools.
TTAP is an improved "hexalobular"
drive for without wobbling and stable stick-fit.
TTAP is backward convertible with generic
hexalobular drive.
Hex socket screwsHexagonal (hex) socket
head has a hexagonal hole and is driven
by a Hex Wrench, sometimes called an Allen
key or Hex key, or by a power tool with
a hexagonal bit. Tamper-resistant versions
with a pin in the recess are available.
Hex sockets are increasingly used for modern
bicycle parts because hex wrenches are very
light and easily carried tools. They are
also frequently used for self-assembled
furniture (e.g. from Ikea).
Robertson head, invented in 1908 by P.L.
Robertson, has a square hole and is driven
by a special power-tool bit or screwdriver.
The screw is designed to maximize torque
transferred from the driver, and will not
slip, or cam out. It is possible to hold
a Robertson screw on a driver bit horizontally
or even pendant, due to a slight wedge fit.
Commonly found in Canada in carpentry and
woodworking applications and in Canadian-manufactured
electrical wiring items such as receptacles
and switch boxes.
Square-drive head is an American clone of
the Robertson that has a square hole without
taper. Due to the lack of taper, the hole
must be oversize relative to the screwdriver,
and is much more likely to strip than the
Robertson.
Tri-Wing head has a triangular slotted configuration.
They were used by Nintendo on several consoles
and accessories, including the Game Boy,
Wii, and Wii Remote, to discourage home
repair.
Torq-Set or offset cruciform is an uncommon
screw drive that may be confused with Phillips;
however, the four legs of the contact area
are offset in this drive type.
Spanner drive uses two round holes opposite
each other, and is designed to prevent tampering.
Commonly seen in elevators in the United
States.
Clutch Type A or standard clutch head resembles
a bow tie. These were common in GM automobiles
of the 1940s and '50s, particularly for
body panels.
Clutch Type G head resembles a butterfly.
This type of screw head is commonly used
in the manufacture of mobile homes and recreational
vehicles.
Combination
drives
Some screws have heads designed to accommodate
more than one kind of driver. The most common
of these is a combination of a slotted and
Phillips head. Because of its prevalence,
there are now drivers made specifically
for this kind of screw head. Other combinations
are a Phillips and Robertson, a Robertson
and a slotted, and a triple-drive screw
which can take a slotted, Phillips or a
Robertson. The Recex drive system claims
it offers the combined non-slip convenience
of a Robertson drive during production assembly
and Phillips for after market serviceability.
Combination head screws are becoming more
and more popular.
Tamper-resistant
screws
Tamper-resistant TORX driverMany screw drives,
including Phillips, TORX, and Hexagonal,
are also manufactured in tamper-resistant
form. These typically have a pin protruding
in the center of the screw head, necessitating
a special tool for extraction. However,
the bits for many tamper-resistant screw
heads are now readily available from hardware
stores, tool suppliers and through the Internet.
What is more, there are many commonly used
techniques to extract tamper resistant screws
without the correct driver — for example,
the use of an alternative driver than can
gain enough purchase to turn the screw,
modifying the head to accept an alternative
driver or forming ones own driver by melting
an object into the head to mould a driver.
Thus, these special screws offer only modest
security.
One-way slotted screwThe slotted screw drive
also comes in a tamper-resistant one-way
design with sloped edges; the screw can
be driven in, but the bit slips out in the
reverse direction.
There are specialty fastener
companies that make unusual, proprietary
head designs, featuring matching drivers
available only from them, and only supplied
to registered owners.
Tools
used
The hand tool used to drive in most screws
is called a screwdriver. A power tool that
does the same job is a power screwdriver;
power drills may also be used with screw-driving
attachments. Where the holding power of
the screwed joint is critical, torque-measuring
and torque-limiting screwdrivers are used
to ensure sufficient and not excessive force
is developed by the screw. The hand tool
for driving cap screws and other types is
called a spanner (UK usage) or wrench (US
usage).
The knock-off bolt is a high
security screw that is extremely difficult
to remove. It is comprised of a counter-sunk
flat head screw, with a thin shaft and hex
head protruding from the flat head. The
hex head is used to drive the bolt into
the countersunk hole, then the wrench or
hammer is used to knock the shaft and hex
head off of the flat head, leaving only
a smooth screw head exposed. Removal is
facilitated by drilling a small hole part
way into the outer part of the head and
using a punch and hammer at a sharp angle
in an anti-clockwise direction. This type
of screw is used primarily in prison door
locks.
Mechanics
of use
When driving in a screw, especially when
the screw has been removed and is being
placed again, the threads can become misaligned
and damage, or strip, the threading of the
hole. To avoid this, slight pressure is
applied and the screw is driven in reverse,
until the leading edges of the helices pass
each other, at which point a slight click
will be felt (and sometimes heard.) When
this happens, the screw will often assume
a more aligned position with respect to
the hole.
Immediately after the 'click',
the screw may be driven in without damage
to the threading. This technique is useful
for re-seating screws in wood and plastic,
and for assuring the proper fit when screwing
down plates and covers where alignment is
difficult.
Thread
standards
See also: Screw thread
There are many systems for specifying the
dimensions of screws, but in much of the
world the ISO metric screw thread preferred
series has displaced the many older systems.
Other relatively common systems include
the British Standard Whitworth, BA system
(British Association), and the SAE Unified
Thread Standard.
ISO metric screw thread
The basic principles of the ISO metric screw
thread are defined in international standard
ISO 68-1 and preferred combinations of diameter
and pitch are listed in ISO 261. The smaller
subset of diameter and pitch combinations
commonly used in screws, nuts and bolts
is given in ISO 262. The most commonly used
pitch value for each diameter is known as
the "coarse pitch". For some diameters,
one or two additional "fine pitch"
variants are also specified, for special
applications such as threads in thin-walled
pipes. ISO metric screw threads are designated
by the letter M followed by the major diameter
of the thread in millimeters, e.g. "M8".
If the thread does not use the normal "coarse
pitch" (e.g., 1.25 mm in the case of
M8), then the pitch in millimeters is also
appended with a multiplication sign, e.g.
"M8×1" if the screw thread
has an outer diameter of 8 mm and advances
by 1 mm per 360° rotation.
The nominal diameter of a
metric screw is the outer diameter of the
thread. The tapped hole (or nut) into which
the screw fits, has an internal diameter
which is the size of the screw minus the
pitch of the thread. Thus, an M6 screw,
which has a pitch of 1 mm, is made by threading
a 6 mm shaft, and the nut or threaded hole
is made by tapping threads in a 5 mm hole.
Metric hexagon bolts, screws
and nuts are specified, for example, in
British Standard BS 4190 (general purpose
screws) and BS 3692 (precision screws).
The following table lists the relationship
given in these standards between the thread
size and the maximal width across the hexagonal
flats (wrench size):
ISO metric thread M1.6 M2
M2.5 M3 M4 M5 M6 M8 M10 M12 M16 M20 M24
M30 M36 M42 M48 M56 M64
wrench size (mm) 3.2 4 5 5.5 7 8 10 13 17
19 24 30 36 46 55 65 75 85 95
In addition, the following
non-preferred intermediate sizes are specified:
ISO metric thread M14 M18
M22 M27 M33 M39 M45 M52 M60 M68
wrench size (mm) 22 27 32 41 50 60 70 80
90 100
Whitworth
The first person to create a standard (in
about 1841) was the English engineer Sir
Joseph Whitworth. Whitworth screw sizes
are still used, both for repairing old machinery
and where a coarser thread than the metric
fastener thread is required. Whitworth became
British Standard Whitworth, abbreviated
to BSW (BS 84:1956) and the British Standard
Fine (BSF) thread was introduced in 1908
because the Whitworth thread was a bit coarse
for some applications. The thread angle
was 55° and a depth and pitch of thread
that varied with the diameter of the thread
(i.e., the bigger the bolt, the coarser
the thread). The spanner size is determined
by the size of the bolt not distance between
the flats. The most common use of a Whitworth
pitch nowadays is the standard photographic
tripod thread, which for small cameras is
1/4" Whitworth (20 tpi) and for medium/large
format cameras is 3/8" Whitworth (16
tpi).
British Association screw threads (BA)
A later standard established in the United
Kingdom was the BA system, named after the
British Association for Advancement of Science.
Screws were described as "2BA",
"4BA" etc., the odd numbers being
rarely used. While not related to ISO metric
screws, the sizes were actually defined
in metric terms, a 0BA thread having a 1
mm pitch. These are still the most common
threads in some niche applications. Certain
types of fine machinery, such as moving-coil
meters, tend to have BA threads wherever
they are manufactured.
Unified
Thread Standard
The United States has its own system, usually
called the Unified Thread Standard, which
is also extensively used in Canada and in
most other countries around the world. At
least 85% of the world's fasteners are dimensioned
to Unified thread dimensions, and the biggest
selection of fastener sizes and materials
are found supplied in this standard (Source:
World Fastener Review, Industrial Press,
2006). A version of this standard, called
SAE for the Society of Automotive Engineers,
was used in the American automobile industry.
The SAE is still associated with inch-based
fasteners by the public, even though the
U.S. auto industry (and other heavy industries
relying on SAE) have gradually converted
to ISO preferred series fasteners for some
assemblies from the 1970s onward, because
global parts sourcing and product marketing
favor international standardization. However,
all automobiles sold in around the world
contain both metric (engine assemblies)
and Imperial fasteners (for example, lug
nuts, oxygen sensors, internal electrical
assemblies, body fasteners, lamps, steering,
brake and suspension parts).
Machine screws are described
as 0-80, 2-56, 3-48, 4-40, 5-40, 6-32, 8-32,
10-32, 10-24, etc. up to size 16. The first
number can be translated to a diameter,
the second is the number of threads per
inch. There is a coarse thread and a fine
thread for each size, the fine thread being
preferred in thin materials or when its
slightly greater strength is desired.
The numbering system follows
a roughly logarithmic series where an increase
in each screw number size approximately
doubles the tensile strength of the screw
and is given by Dia = (#N X .013")
+ .060" Using this formula a #5 screw
has a major diameter of .125" (1/8"),
a #10 screw has a diameter of .190"
(or 3/16" in practical terms), etc.
The formula applies for screw thread numbers
#0 and higher, but does NOT apply to smaller
Unified miniature screw thread series. Typically
screws smaller than size #0 are supplied
in the Unified Miniature Series. The formula
for number sizes smaller than size #0 is
given by Dia = .060" - (#zerosize X
.013). So a #00 screw is .047" dia,
#000 is .034" dia, etc.
The number series of machine
screws include odd numbers (7, 9, etc.)
and extended up to #16 or more. Standardization
efforts in the late 19th and the early part
of the 20th century reduced the range of
sizes considerably. Now, it is less common
to see machine screws larger than #14 or
odd number sizes other than #1, #3 and #5,
even though #14 and #16 screws are still
available and are common use, although not
as common as sizes #0 thru #12.
Sizes 1/4" diameter and
larger are designated as 1/4"-20, 1/4"-28,
etc. the first number giving the diameter
in inches and the second number being threads
per inch. Most thread sizes are available
in UNC or UC (Unified Coarse Thread, example
1/4"-20) or UNF (example 1/4"-28
UNF or UF).
Others
Other thread systems include Acme thread
form, BSP (British standard pipe thread
which exists in a taper and non taper variant;
used for other purposes as well) and BSC
(British Standard Cycle) a 26tpi thread
form, CEI (Cycle Engineers Institute, used
on bicycles in Britain and possibly elsewhere),
British Standard Brass a fixed pitch 26tpi
thread, NPT (National pipe thread) and NPTF
(pipe threads), and PG (German: "Panzer-Gewinde"),
used in thin plate metal, such as for switches
and nipples in electrical equipment housings.
Taken From Wikipedia, the
free encyclopedia
