Sheet metal: Wikis

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Sheets of stainless steel cover the Chrysler Building

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. For decorative uses, important sheet metals include silver, gold, and platinum (platinum sheet metal is also utilized as a catalyst.)

Sheet metal has applications in car bodies, airplane wings, medical tables, roofs for building and many other things. Sheet metal of iron and other materiales with high magnetic permeability, also known as laminated steel cores, has applications in transformers and electric machines. Historically, an important use of sheet metal was in plate armor worn by cavalry, and sheet metal continues to have many decorative uses, including in horse tack.

Contents

Materials

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Stainless steel

The three most common stainless steel grades available in sheet metal are 304, 316, and 410.[1]

Grade 304 is the most common of the three grades. It offers good corrosion resistance while maintaining formability and weldability. Available finishes are #2B, #3, and #4. Note that grade 303 is not available in sheet form.[1]

Grade 316 offers more corrosion resistance and strength at elevated temperatures than 304. It is commonly used for pumps, valves, chemical equipment, and marine applications. Available finishes are #2B, #3, and #4.[1]

Grade 410 is a heat treatable stainless steel, but does not offer as good corrosion resistance. It is commonly used in cutlery. The only available finish is dull.[1]

Aluminium

The four most common aluminium grades available as sheet metal are 1100-H14, 3003-H14, 5052-H32, and 6061-T6.[1]

Grade 1100-H14 is commercially pure aluminium, so it is highly chemical and weather resistant. It is ductile enough for deep drawing and weldable, but low strength. It is commonly used in chemical processing equipment, light reflectors, and jewelry.[1]

Grade 3003-H14 is stronger than 1100, while maintaining the same formability and low cost. It is corrosion resistant and weldable. It is often used in stampings, spun and drawn parts, mail boxes, cabinets, tanks, and fan blades.[1]

Grade 5052-H32 is much stronger than 3003 while still maintaining good formability. It maintains high corrosion resistance and weldability. Common applications include electronic chassis, tanks, and pressure vessels.[1]

Grade 6061-T6 is a common heat treatable structural aluminium alloy. It is weldable, corrosion resistant, and stronger than 5052, but not as formable. Note that it loses some of its strength when welded.[1]

Gauge

The sheet metal gauge (sometimes spelled "gage") indicates the standard thickness of sheet metal for a specific material. For most materials, as the gauge number increases, the material thickness decreases.

Sheet metal thickness gauges for steel are based on the weight of steel, allowing more efficient calculation of the cost of material used. The weight of steel is 41.82 pounds per square foot per inch of thickness; this is known as the Manufacturers' Standard Gage for Sheet Steel.[2] For other materials, such as aluminium and brass, the thicknesses will be different.

Rate of change in thickness vs gauge number
Standard sheet metal gauges
Gauge Steel[3] Galvanized steel Stainless steel Aluminium Zinc[3]
3 0.2391 (6.0731) - - - 0.006
4 0.2242 (5.6947) - - - 0.008
5 0.2092 (5.3137) - - - 0.010
6 0.1943 (4.9352) - - - 0.012
7 0.1793 (4.5542) - 0.1875 0.1443 0.014
8 0.1644 (4.1758) 0.1681 (4.2697) 0.1719 0.1285 0.016
9 0.1495 (3.7973) 0.1532 (3.8913) 0.1563 0.1144 0.018
10 0.1345 (3.4163) 0.1382 (3.5103) 0.1406 0.1019 0.020
11 0.1196 (3.0378) 0.1233 (3.1318) 0.1250 0.0907 0.024
12 0.1046 (2.6568) 0.1084 (2.7534) 0.1094 0.0808 0.028
13 0.0897 (2.2784) 0.0934 (2.3724) 0.094 0.072 0.032
14 0.0747 (1.8974) 0.0785 (1.9939) 0.0781 0.0641 0.036
15 0.0673 (1.7094) 0.0710 (1.8034) 0.07 0.057 0.040
16 0.0598 (1.5189) 0.0635 (1.6129) 0.0625 0.0508 0.045
17 0.0538 (1.3665) 0.0575 (1.4605) 0.056 0.045 0.050
18 0.0478 (1.2141) 0.0516 (1.3106) 0.0500 0.0403 0.055
19 0.0418 (1.0617) 0.0456 (1.1582) 0.044 0.036 0.060
20 0.0359 (0.9119) 0.0396 (1.0058) 0.0375 0.0320 0.070
21 0.0329 (0.8357) 0.0366 (0.9296) 0.034 0.028 0.080
22 0.0299 (0.7595) 0.0336 (0.8534) 0.031 0.025 0.090
23 0.0269 (0.6833) 0.0306 (0.7772) 0.028 0.023 0.100
24 0.0239 (0.6071) 0.0276 (0.7010) 0.025 0.02 0.125
25 0.0209 (0.5309) 0.0247 (0.6274) 0.022 0.018 -
26 0.0179 (0.4547) 0.0217 (0.5512) 0.019 0.017 -
27 0.0164 (0.4166) 0.0202 (0.5131) 0.017 0.014 -
28 0.0149 (0.3785) 0.0187 (0.4750) 0.016 0.0126 -
29 0.0135 (0.3429) 0.0172 (0.4369) 0.014 0.0113 -
30 0.0120 (0.3048) 0.0157 (0.3988) 0.013 0.0100 -
31 0.0105 (0.2667) 0.0142 (0.3607) 0.011 0.0089 -
32 0.0097 (0.2464) - - - -
33 0.0090 (0.2286) - - - -
34 0.0082 (0.2083) - - - -
35 0.0075 (0.1905) - - - -
36 0.0067 (0.1702) - - - -
37 0.0064 (0.1626) - - - -
38 0.0060 (0.1524) - - - -
Thickness is in inches except for values in parentheses which are in millimeters

Tolerances

During the rolling process the rollers bow slightly, which results in the sheets being thinner on the edges.[1]

Steel sheet metal tolerances[1]
Gauge Nominal [in] Max [in] Min [in]
10 0.1345 0.1405 0.1285
11 0.1196 0.1256 0.1136
12 0.1046 0.1106 0.0986
14 0.0747 0.0797 0.0697
16 0.0598 0.0648 0.0548
18 0.0478 0.0518 0.0438
20 0.0359 0.0389 0.0329
22 0.0299 0.0329 0.0269
24 0.0239 0.0269 0.0209
26 0.0179 0.0199 0.0159
28 0.0149 0.0169 0.0129
Aluminium sheet metal tolerances[1]
Thickness [in] Sheet width
36 in [in] 48 in [in]
0.018–0.028 0.002 0.0025
0.029–0.036 0.002 0.0025
0.037–0.045 0.0025 0.003
0.046–0.068 0.003 0.004
0.069–0.076 0.003 0.004
0.077–0.096 0.0035 0.004
0.097–0.108 0.004 0.005
0.109–0.125 0.0045 0.005
0.126–0.140 0.0045 0.005
0.141–0.172 0.006 0.008
0.173–0.203 0.007 0.010
0.204–0.249 0.009 0.011
Stainless steel sheet metal tolerances[1]
Thickness [in] Sheet width
36 in [in] 48 in [in]
0.017–0.030 0.0015 0.002
0.031–0.041 0.002 0.003
0.042–0.059 0.003 0.004
0.060–0.073 0.003 0.0045
0.074–0.084 0.004 0.0055
0.085–0.099 0.004 0.006
0.100–0.115 0.005 0.007
0.116–0.131 0.005 0.0075
0.132–0.146 0.006 0.009
0.147–0.187 0.007 0.0105

Forming processes

Bending sheet metal with rollers

Deep drawing

Deep drawing is a type of drawing process where the depth of the part being made 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 10 um; some more recent systems use a YAG based laser with a wavelength of around 1 um.

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 press or a laser CNC machine.

Perforating

Perforating is a cutting process that punches multiple small holes close together in a flat workpiece. Perforated sheet metal is used to make a wide variety of surface cutting tools, such as the surform.

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.

Roll forming

A continuous bending operation for producing open profiles or welded tubes with long lengths or in large quantities.

Rolling

Stamping

Includes a variety of operations, such as punching, blanking, embossing, bending, flanging, and coining; simple or complex shapes formed at high production rates; tooling and equipment costs can be high, but labor costs are low.

Alternatively, the related techniques repoussé and chasing have low tooling and equipment costs, but high labor costs.

Bending

The equation for maximum bending force is,

F_{max} = k \frac{(UTS)Lt^{2}}{W},

where k is a factor taking into account several parameters including friction, and L and t are Length and thickness of sheet metal respectively. The variable W is opening width of a V-die or Wiping die.

Ironing

Wheeling

Incremental sheet forming

Decambering

Fasteners

Fasteners that are commonly used on sheet metal include:

See also

References

Notes

  1. ^ a b c d e f g h i j k l m Sheet metal material, http://www.precisionsheetmetal.com/home/materials.htm, retrieved 2009-07-26 .
  2. ^ Oberg, p. 2522.
  3. ^ a b Oberg, p. 2502.

Bibliography

External links


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