We utilize several tools for Metal Fabrication, here
are some backgrounds and descriptions on a few of the most common types:
The English Wheel
"The English Wheel is a manually operated
metalworking apparatus that allows a craftsman to form smooth, compound curves
from flat sheets of metal, such as aluminum or mild steel.
The machine is
shaped like a large, closed letter "C". At the ends of the C, there are two
wheels. The wheel on the top is called the rolling wheel, while the wheel
on the bottom is called the anvil wheel. (Some references refer to the
wheels by their position: upper wheel and lower wheel.) The anvil
wheel usually has a smaller radius than the rolling wheel. Although larger
machines exist, the rolling wheel is usually 8 cm (3 inches) wide or less, and
usually 25 cm (9 inches) in diameter, or less.
The rolling (top) wheel is flat in cross section, while
the anvil (bottom) wheel is domed.
The depth of the C-shaped frame is called the throat.
The largest machines have throat sizes of 120 cm (48 inches), while smaller
machines have throat sizes of about 60 cm (24 inches). The C stands vertically
and is supported by a frame. The throat size usually determines the largest size
of metal sheet that the operator can place in the machine and work easily. On
some machines, the operator can turn the top wheel and anvil 90 degrees to the
frame to increase the maximum size of the work piece. Because the machine works
by an amount of pressure between the wheels through the material, and because
that pressure changes as the material becomes thinner, the lower jaw and cradle
of the frame that holds the anvil roller is adjustable. It may move with a
hydraulic jack on machines designed for steel plate, or a jackscrew on machines
designed for sheet metals. As the material thins, the operator must adjust the
pressure to compensate.
A properly equipped machine has an assortment of anvil
wheels. Anvil wheels, like dollies used with hammers in panel beating (which are
also known as anvils) should be used to match the desired crown or curvature of
the work piece.
The operator of the machine passes the sheet metal
between the anvil wheel and the rolling wheel. This process stretches the
material and causes it to become thinner. As the material stretches, it forms a
convex surface over the anvil wheel. This surface is known as crown. A high
crown surface is very curved, a low crown surface is slightly curved. The
rigidity and strength in the surface of a workpiece is provided by the high
crown areas. The radius of the surface, after working, depends on the degree
that the metal in the middle of the work piece stretches relative to the edge of
the piece. If the middle stretches too much, the operator can recover the shape
by wheeling the edge of the piece. Wheeling the edge has the same effect in
correcting mis-shape due to overstretching in the middle, as shrinking directly
on the overstretched area by the use of heat shrinking or eckold type shrinking.
This is because the edge holds the shape in place. Strength and rigidity is also
provided by the edge treatment such as flanging or wiring, after the fabrication
of the correct surface contour has been achieved. The flange is so important to
the shape of the finished surface, that it is possible to fabricate some panels
by shrinking and stretching of the flange alone, without the use of surface
stretching or shrinking at all.
The pressure of the contact area, which varies with the
radius of the dome on the anvil wheel and the pressure of the adjusting screw,
and the number of wheeling passes determines the degree to which the material
stretches. Some operators prefer a foot adjuster in order to be able to maintain
a constant pressure over the varying sheet metal thickness for smoothing, while
using both hands to manipulate the work piece. This style of adjuster is also
helpful for blending the edge of high crown areas that are thinner, with low
crown areas that are relatively unstretched. A drawback of the foot adjuster, is
that it can foul very longitudinally curved panels, such as cycle type mudguards
(wings/fenders), as used on pre-WW2 sports cars and on the Lotus / Caterham 7.
To address this problem, there are wheeling machines that have a hand adjuster
close beneath the anvil yoke in order to allow such panels to curve underneath
unobstructed. This type of machine typically has a diagonal lower 'C' shaped
frame, that curves lower to the floor, instead of the horizontal and long
vertical adjuster shown in the picture. A third type of adjuster moves the top
wheel up and down with the bottom anvil wheel left static.
The operator makes several passes over an area on the
sheet in order to form the area correctly. He may make additional passes with
different wheels and in different directions, (at 90 degrees for a simple double
curvature shape, for example), in order to achieve the desired shape. Using the
correct pressure and appropriate anvil wheel shape and pattern of accurate,
close to overlapping wheeling passes, makes the use of the machine something of
an art in order to produce a piece of steel, aluminium or other sheet metal with
a particular physical shape. Too much pressure results in a finished product
that is undulating, marred and stressed, while too little pressure causes work
to progress very slowly.
The final process in the fabrication of a panel, after
the correct surface contour has been achieved, is provided by the edge treatment
such as flanging or wiring. There will be too much or too little metal in the
flange, this will pull the panel out of shape after the flange has been turned,
so it needs to be stretched or shrunk in order to pull the surface shape back to
the correct contour.
Working with an English wheel is easier for many applications than manually
hammering the steel, and is usually more appropriate for smooth curves than
using an pneumatic hammer, it may used for
planishing to a smooth
final finish after these processes."
The Planishing Hammer (Stake)
Planishing (from the Latin planus,
"flat") is a metalworking technique used to smooth sheet metal.
After a piece of metal has been roughly formed by
techniques such as sinking or raising, the surface will have irregular
indentations and bumps. To remove these imperfections, the piece is hammered
between a flat or slightly curved
hammer and a special
forming object known as a planishing stake. Using repeated,
relatively soft blows, the piece is smoothed toward the curvature of the stake.
Since planishing hammers are generally in contact with
the outside surface of the piece, they have rounded edges and are kept polished
to avoid marring the work.
A 1/2" drive pistol-grip air impact wrench
An impact wrench (also known as an air wrench,
air gun, or just gun in some contexts, as well as rattle gun
in some countries) is a socket wrench power tool designed to deliver high torque
output with minimal exertion by the user, by storing energy in a rotating mass,
then delivering it suddenly to the output shaft.
Compressed air is the most common power source,
although electric or hydraulic power is also used, with cordless electric
devices becoming increasingly popular in recent times.
Impact wrenches are widely used in many industries,
such as automotive repair, heavy equipment maintenance, product assembly (often
called "pulse tools" and designed for precise torque output), major construction
projects, and any other instance where a high torque output is needed.
Impact wrenches are available in every standard socket
wrench drive size, from small 1/4" drive tools for small assembly and
disassembly, up to 3.5" and larger square drives for major construction.
Impact wrenches are one of the most commonly used air tools, and are found in
virtually every mechanic's shop.
In operation, a rotating mass (the hammer) is
accelerated by the motor, storing energy, then suddenly connected to the output
shaft (the anvil), creating a high-torque impact. The hammer mechanism is
designed such that after delivering the impact, the hammer is again allowed to
spin freely, and does not stay locked. With this design, the only reaction force
applied to the body of the tool is the motor accelerating the hammer, and thus
the operator feels very little torque, even though a very high peak torque is
delivered to the socket. This is similar to a conventional hammer, where the
user applies a small, constant force to swing the hammer, which generates a very
large impulse when the hammer strikes an object. Energy is stored over time,
allowing a very strong, but short output impulse to be generated from a
relatively weak, but constant input force. The hammer design requires a certain
minimum torque before the hammer is allowed to spin separately from the anvil,
causing the tool to stop hammering and instead smoothly drive the fastener if
only low torque is needed, rapidly installing/removing the fastener.
Compressed air is the most common power source for
impact wrenches, providing a low-cost design with the best power-to-weight
ratio. A simple vane motor is almost always used, usually with four to
seven vanes, and various lubrication systems, the most common of which uses
oiled air, while others may include special oil passages routed to the parts
that need it and a separate, sealed oil system for the hammer assembly. Most
impact wrenches drive the hammer directly from the motor, giving it fast action
when the fastener requires only low torque. Other designs use a gear reduction
system before the hammer mechanism, most often a single-stage planetary gearset
usually with a heavier hammer, delivering a more constant speed and higher
"spin" torque. Electric impact wrenches are available, either mains powered, or
for automotive use, 12-volt or 24-volt DC-powered. Recently, cordless electric
impact wrenches have become common, although their power outputs are still
significantly lower than corded electric or air-powered equivalents. Some
industrial tools are hydraulically powered, using high-speed hydraulic motors,
and are used in some heavy equipment repair shops, large construction sites, and
other areas where a suitable hydraulic supply is available.
Sizes and styles
A variety of impact wrenches, in all common sizes
from 1/4" to 1", of different styles, including inline, butterfly, and
Impact wrenches are available in all sizes and in
several styles, depending on the application. 1/4" drive wrenches are commonly
available in both inline (the user holds the tool like a screwdriver, with the
output on the end) and pistol grip (the user holds a handle which is at right
angles to the output) forms, and less commonly in an angle drive, which is
similar to an inline tool but with a set of bevel gears to rotate the output 90
degrees. 3/8" impacts are most commonly available in pistol grip form and a
special inline form known as a "butterfly" wrench, which has a large, flat
throttle paddle on the side of the tool which may be tilted to one side or the
other to control the direction of rotation, rather than using a separate
reversing control, and shaped to allow access into tight areas. Regular inline
and angle 3/8" drive impact wrenches are uncommon, but available. 1/2" drive
units are virtually only available in pistol grip form, with any inline type
being virtually impossible to obtain, due to the increased torque transmitted
back to the user and the greater weight of the tool requiring the larger handle.
3/4" drive impact wrenches are again essentially only available in pistol grip
form. 1" drive tools are available in both pistol grip and "D handle" inline,
where the back of the tool has an enclosed handle for the user to hold. Both
forms often also incorporate a side handle, allowing both hands to hold the tool
at once. 1.25" and larger wrenches are usually available in "T handle" form,
with two large handles on either side of the tool body, allowing for maximum
torque to be applied to the user, and giving the best control of the tool. Very
large impact wrenches (up to several hundred thousand foot-pounds of torque)
usually incorporate eyelets in their design, allowing them to be suspended from
a crane, lift, or other device, since their weight is often more than a person
can move. A recent design combines an impact wrench and an air ratchet, often
called a "reactionless air ratchet" by the manufacturers, incorporating an
impact assembly before the ratchet assembly. Such a design allows very high
output torques with minimal effort on the operator, and prevents the common
injury of slamming one's knuckles into some part of the equipment (generally
always sharp, pointy, or hot) when the fastener tightens down and the torque
suddenly increases. Specialty designs are available for certain applications,
such as removing crankshaft
pullies without removing
the radiator in a vehicle.
Various methods are used to attach the socket or
accessory to the anvil, such as a spring-loaded pin that snaps into a matching
hole in the socket, preventing the socket being removed until an object is used
to depress the pin, a hog ring which holds the socket by friction or by snapping
into indents machined into the socket, and a through-hole, where a pin is
inserted completely through the socket and anvil, locking the socket on. Hog
rings are used on most smaller tools, with though-hole used only on larger
impact wrenches, typically 3/4" drive or greater. Pin retainers used to be more
common, but seem to be being replaced by hog rings on most tools, despite the
lack of a positive lock. 1/4" female hex drive is becoming increasingly popular
for small impact wrenches, especially cordless electric versions, allowing them
to fit standard screwdriver tips rather than sockets.
Many users chose to equip their air-powered impact
wrenches with a short length of air hose rather than attaching an air fitting
directly to the tool. Such a hose greatly aids in fitting the wrench into tight
areas, by not having the complete coupler assembly sticking out the back of the
tool, as well as making it easier for the user to position the tool. An
additional benefit is greatly reduced wear on the coupler, by isolating it from
the vibration of the tool. A short length of hose also prevents the air fitting
from being broken off in the base of the tool if the user loses their grip and
the tool is allowed to spin.
Effects of impact
As the output of an impact wrench, when hammering, is a
very short impact force, the actual effective torque is difficult to measure,
with several different ratings in use. As the tool delivers a fixed amount of
energy with each blow, rather than a fixed torque, the actual output torque
changes with the duration of the output pulse. If the output is springy or
capable of absorbing energy, the impulse will simply be absorbed, and virtually
no torque will ever be applied, and somewhat counter-intuitively, if the object
is very springy, the wrench may actually turn backwards as the energy is
delivered back to the anvil, while it is not connected to the hammer and able to
spin freely. A wrench that is capable of freeing a rusted nut on a very large
bolt may be incapable of turning a small screw mounted on a spring. "Maximum
torque" is the number most often given by manufacturers, which is the
instantaneous peak torque delivered if the anvil is locked into a perfectly
solid object. "Working torque" is a more realistic number for continually
driving a very stiff fastener. "Nut-busting torque" is often quoted, with the
usual definition being that the wrench can loosen a nut tightened with the
specified amount of torque in some specified time period. Accurately
controlling the output torque of an impact wrench is very difficult, and even an
experienced operator will have a hard time making sure a fastener is not
undertightened or overtightened using an impact wrench. Special socket
extensions are available, which take advantage of the inability of an impact
wrench to work against a spring, to precisely limit the output torque. Designed
with spring steel, they act as large torsion springs, flexing at their torque
rating, and preventing any further torque from being applied to the fastener.
Some impact wrenches designed for product assembly have a built-in torque
control system, such as a built-in torsion spring and a mechanism that shuts the
tool down when the given torque is exceeded. When very precise torque is
required, an impact wrench is only used to snug down the fastener, with a torque
wrench used for the final tightening. Due to the lack of standards when
measuring the maximum torque, some manufacturers are believed to inflate their
ratings, or to use measurements with little bearing on how the tool will perform
in actual use. Many air impact wrenches incorporate a flow regulator into their
design, either as a separate control or part of the reversing valve, allowing
torque to be roughly limited in one or both directions, while electric tools may
use a variable speed trigger for the same effect.
The pin clutch mechanism from a 3/4" drive impact
The hammer mechanism in an impact wrench needs to allow
the hammer to spin freely, impact the anvil, then release and spin freely again.
Many designs are used to accomplish this task, all with some drawbacks.
Depending on the design, the hammer may drive the anvil either once or twice per
revolution (where a revolution is the difference between the hammer and the
anvil), with some designs delivering faster, weaker blows twice per revolution,
or slower, more powerful ones only once per revolution.
A common hammer design has the hammer able to slide and
rotate on a shaft, with a spring holding it in the downwards position. Between
the hammer and the driving shaft is a steel ball on a ramp, such that if the
input shaft rotates ahead of the hammer with enough torque, the spring is
compressed and the hammer is slid backwards. On the bottom of the hammer, and
the top of the anvil, are dog teeth, designed for high impacts. When the tool is
used, the hammer rotates until its dog teeth contact the teeth on the anvil,
stopping the hammer from rotating. The input shaft continues to turn, causing
the ramp to lift the steel ball, lifting the hammer assembly until the dog teeth
no longer engage the anvil, and the hammer is free to spin again. The hammer
then springs forward to the bottom of the ball ramp, and is accelerated by the
input shaft, until the dog teeth contact the anvil again, delivering the impact.
The process then repeats, delivering blows every time the teeth meet, almost
always twice per revolution. If the output has little load on it, such as when
spinning a loose nut on a bolt, the torque will never be high enough to cause
the ball to compress the spring, and the input will smoothly drive the output.
This design has the advantage of small size and simplicity, but energy is wasted
moving the entire hammer back and forth, and delivering multiple blows per
revolution gives less time for the hammer to accelerate. This design if often
seen after a gear reduction, compensating for the lack of acceleration time by
delivering more torque at a lower speed.
A stop-motion animation of a pin clutch hammer.
Normally the hammer rotates while the anvil remains stationary attached to
the fastener, but rotating the anvil more clearly demonstrates the action
of the pins.
Another common design uses a hammer fixed directly onto
the input shaft, with a pair of pins acting as clutches. When the hammer rotates
past the anvil, a ball ramp pushes the pins outwards against a spring, extending
them to where they will hit the anvil and deliver the impact, then release and
spring back into the hammer, usually by having the balls "fall off" the other
side of the ramp at the instant the hammer hits. Since the ramp need only have
one peak around the shaft, and the engagement of the hammer with the anvil is
not based on a number of teeth between them, this design allows the hammer to
accelerate for a full revolution before contacting the anvil, giving it more
time to accelerate and delivering a stronger impact. The disadvantages are that
the sliding pins must handle very high impacts, and often cause the early
failure of tool.
Yet another design uses a rocking weight inside the
hammer, and a single, long protrusion on the side of the anvil's shaft. When the
hammer spins, the rocking weight first contacts the anvil on the opposite side
than used to drive the anvil, nudging the weight into position for the impact.
As the hammer spins further, the weight hits the side of the anvil, transferring
the hammer's and its own energy to the output, then rocks back to the other
side. This design also has the advantage of hammering only once per revolution,
as well as its simplicity, but has the disadvantage of making the tool vibrate
as the rocking weight acts as an eccentric, and can be less tolerant of running
the tool with low input power. To help combat the vibration and uneven drive,
sometimes two of these hammers are placed in line with each other, at 180 degree
offsets, both striking at the same time.
Many other designs are used, but all of them accomplish
the same goal of allowing the hammer to spin freely of the anvil, allowing it to
be accelerated and store energy, then delivering that energy suddenly to the
anvil, before allowing the process to repeat.
Sockets and extensions for impact wrenches are made of
very hard metal, as any spring effect will greatly reduce the torque available
at the fastener. Even so, the use of multiple extensions, universal joints, and
so forth will weaken the impacts, and the operator needs to minimize their use.
Using non-impact sockets or accessories with an impact wrench will often result
in bending, fracturing, or otherwise damaging the accessory, as most are not
capable of withstanding the sudden high torque of an impact tool, and can result
in stripping the head on the fastener. Safety glasses should always be worn when
working with impact tools, as the strong impacts will generate high-speed
shrapnel if a socket, accessory, or fastener fails, unlike the steady torque of
a hand ratchet where a broken accessory usually does nothing worse than cause
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