Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. Countless everyday objects are constructed of the material. Thicknesses can vary significantly, although extremely thin thicknesses are considered foil or leaf, and pieces thicker than 6 mm (0.25 in) are considered plate.
Sheet metal is available as flat pieces or as a coiled strip. The coils are formed by running a continuous sheet of metal through a roll slitter.
The thickness of the sheet metal is called its gauge. The gauge of sheet metal ranges from 30 gauge to about 8 gauge. The higher the gauge, the thinner the metal is.
There are many different metals that can be made into sheet metal, such as: Aluminum, brass, copper, steel, tin, nickel and titanium. Sheet metal has applications in car bodies, airplane wings, medical tables, roofs for building and many other things.
Gauge
The sheet metal gauge (sometimes spelled "gage") indicates the standard thickness of sheet metal for a specific material. As the gauge number increases, the material thickness decreases.
Sheet metal thickness gauges for steel are based on a weight of 41.82 pounds per square foot per inch of thickness. This is known as the Manufacturers' Standard Gage for Sheet Steel.[1] For other materials, such as aluminum and brass, the thicknesses will be different. Thus, a 10 gauge steel sheet which has a thickness of 0.1345 inches will weigh 41.82*0.1345 = 5.625 pounds per square foot.
Examples: 16 ga CRS is 2.5 pounds per square foot. For 18 ga CRS the weight is 2.0 pounds per square foot and for 20 ga CRS the weight is 1.5 pounds per square foot.
Forming processes
Bending sheet metal with rollers
Deep drawing
Main article: Deep drawing
Deep drawing is a type of drawing process where the depth of the part is more than half its diameter. Deep drawing is used for making automotive fuel tanks, kitchen sinks, 2 piece aluminum cans, etc. Deep drawing is generally done in multiple steps called draw reductions. The greater the depth, the increased reductions required. Deep drawing may also be accomplished with fewer reductions by heating the workpiece, used in sink manufacture for example.
In many cases, special material that has been rolled at the steel mill in both directions can aid in the deep drawing process. Material that has been rolled in both directions has a more uniform grain structure and is referred to as "draw quality" material. Draw quality material will often improve deep drawing (limiting tearing).
Cutting
Cutting sheet metal can be done in various ways from hand tools called tin snips up to very large powered shears. With the advances in technology, sheet metal cutting has turned to computers for precise cutting.
Most modern sheet metal cutting operations are now based either on CNC (Computer numerical control) Lasers cutting or multi-tool CNC punch press.
CNC laser involves moving a lens assembly carrying a beam of laser light over the surface of the metal. Oxygen or nitrogen or air is fed through the same nozzle from which the laser beam exits. The metal is heated and then burnt by the laser beam, cutting the metal sheet. The quality of the edge can be mirror smooth, and a precision of around 0.1mm can be obtained. Cutting speeds on thin (1.2mm) sheet can be as high as 25m a minute. Most of the laser cutting systems use a CO2 based laser source with a wavelength of around 10um; some more recent systems use a YAG based laser with a wavelength of around 1um.
Punching is performed by moving the sheet of metal between the top and bottom tools of a punch. The top tool (punch) mates with the bottom tool (die), cutting a simple shape (e.g. a square, circle, or hexagon) from the sheet. An area can be cut out by making several hundred small square cuts around the perimeter. A punch is less flexible than a laser for cutting compound shapes, but faster for repetitive shapes (for example, the grille of an air-conditioning unit). A typical CNC punch has a choice of up to 60 tools in a "turret" that can be rotated to bring any tool to the active punching position. A modern CNC punch can take 600 blows per minute.
A typical component (such as the side of a computer case) can be cut to high precision from a blank sheet in under 15 seconds by either a punch or a laser CNC machine.
Spinning
Spinning is used to make axis-symmetric parts by applying a work piece to a rotating mandrel with the help of rollers or rigid tools. Spinning is used to make rocket motor casings, missile nose cones, and satellite dishes, for example.
Press brake forming
Forming metal on a pressbrake
This is a form of bending, used for long and thin sheet metal parts. The machine that bends the metal is called a press brake. The lower part of the press contains a V shaped groove. This is called the die. The upper part of the press contains a punch that will press the sheet metal down into the v shaped die, causing it to bend. There are several techniques used here, but the most common modern method is "air bending". Here, the die has a sharper angle than the required bend (typically 85 degrees for a 90 degree bend) and the upper tool is precisely controlled in its stroke to push the metal down the required amount to bend it through 90 degrees. Typically, a general purpose machine has a bending force available of around 25 tonnes per metre of length. The opening width of the lower die is typically 8 to 10 times the thickness of the metal to be bent (for example, 5mm material could be bent in a 40mm die) the inner radius of the bend formed in the metal is determined not by the radius of the upper tool, but by the lower die width. Typically, the inner radius is equal to 1/6th of the V width used in the forming process.
The press usually has some sort of back gauge to position depth of the bend along the workpiece. The backgauge can be computer controlled to allow the operator to make a series of bends in a component to a high degree of accuracy. Simple machines control only the backstop, more advanced machines control the position and angle of the stop, its height and the position of the two reference pegs used to locate the material. The machine can also record the exact position and pressure required for each bending operation to allow the operator to achieve a perfect 90 degree bend across a variety of operations on the part.
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