Metal Fabrication


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."

Re-Printed from



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.

Re-Printed from



Impact Hammer

A 1/2" drive pistol-grip air impact wrench

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.

Power source

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 pistol grip.

A variety of impact wrenches, in all common sizes from 1/4" to 1", of different styles, including inline, butterfly, and pistol grip.

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 drive

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.


Hammer mechanisms

The pin clutch mechanism from a 3/4" drive impact wrench.

The pin clutch mechanism from a 3/4" drive impact wrench.

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.
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 accessories

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 bruised knuckles.

Re-Printed from


Welding  <Click the Link!