Sheet metal is everywhere. It's inexpensive, durable, and lightweight, easy to work with and can be folded, bent, curved, cut, shaped, punched, deformed, or otherwise machined into an enormous variety of shapes. It's used to make computer cases, ovens, washers and dryers, electrical enclosures, flashing and coping cap for roofing, HVAC ductwork, food and beverage cans, and hundreds of other things. It comes in a variety of alloys to give it various properties such as strength, resistance to corrosion, and aesthetic appeal. Sheet metal is stronger and less brittle than plastic, longer-lasting and easier to shape than wood, and much more convenient in applications where plate steel would be overkill.
Sheet metal occupies a rather nebulous region between foil and plate steel. Sheet metal is usually referred to by gauge, foil by mils (thousandths of an inch) and plate steel by fractions of an inch. In general, anything thinner than 3/16" (188 mils or 4.8mm) and thicker than 30 gauge (16 mils or 0.4mm) can be considered sheet metal, although it really depends on the application and the individual rather than any kind of standard.
Gauge is a semi-arbitrary measure for the thickness of sheet metal. Rather than a reference to the actual thickness of the sheet, it refers to the weight of the sheet. 20 gauge mild steel sheet metal is actually slightly thinner than 20 gauge stainless steel sheet metal, and 20 gauge sheet aluminum is thinner still. There is no equation for converting gauge to sheet metal thickness, but, as with wire gauge, the material becomes thinner as the gauge number increases. A gauge, calipers, or micrometer can be used to judge the gauge of sheet metal, but experienced craftsmen can differentiate them at a glance. Of course, these are US standards. Metric standards are a lot easier to use, simply calling everything by its thickness in millimeters.
Although the term sheet metal usually refers to galvanized mild steel, it can also be made of stainless steel, aluminum, anodized aluminum, brass, copper, and many other materials. Each has various strengths and weaknesses. Brass and copper are often used for decorative purposes. Anodized aluminum is very lightweight and resistant to corrosion. Stainless steel is extremely durable, resistant to abrasion, and unharmed by many chemicals, but is expensive and difficult to work with. Mild steel is a good, reliable general-purpose material, but since mild steel is unattractive and prone to rust in many applications, it is usually painted or galvanized.
Sheet metal is made in steel mills, which bring either recycled steel or raw metal ores in one side and slide sheet metal out the other. In between the material unloading and product shipping doors the material goes through a number of processes. First, it is heated in huge vats until molten, either with a blast furnace or an electric arc furnace. Because molten steel is extremely heavy, material doesn't have to be very buoyant to float to the top. Rust, slag, and other impurities float to the top where they can be disposed of. Once molten, other materials such as titanium, nickel, or carbon are added to the mix to create various alloys. Next, a plug is removed from the bottom of the molten steel vat, which pours into a caster that cools and shapes the liquid metal into a six inch thick slab. The slab is then passed between two huge, powerful press rollers which squeeze the steel into progressively thinner and thinner sheets with each back and forth pass.
In a hot rolling process, the rolling takes place immediately after the slab comes out of the caster and the steel is kept at a high temperature, making it easier to press into thinner sheets. Cold rolling means that it is pressed after it has cooled down, which makes a stronger steel by introducing work hardening (high strains produced by pressing the cold steel) into the process. After cold rolling, the steel can be annealed (re-heated and then cooled) which makes it less brittle and improves ductility. Cold rolling also improves the accuracy of the finished thickness, since hot rolled steel shrinks as it cools, due to thermal expansion. When the steel reaches its desired thickness, it is trimmed to length and transferred to a cooling bed, where it is considered a finished product once cooled to safe temperatures. If the sheet is very long, it will be coiled up into a roll before shipping.
A workroom for cutting and shaping sheet metal is called a tin shop. Although actual tin is rarely used for anything these days the techniques and tools for working sheet metal are almost identical to those once used to work tin, so the name has stuck. Tin shops use a variety of tools to make things such as metal boxes, ductwork, and pipes out of various thicknesses of sheet metal. Thicker sheet metal is heavier and harder to work with but stronger. Heavy gloves must be used for working with sheet metal because the thin edges are very sharp, and machining the pieces creates burrs and metal splinters which can puncture flesh very easily. Depending on the tool being used, other personal protective equipment might include goggles, earplugs, steel-toed boots, or even full face shields.
Sheet Metal Tools:
- Abrasive Water Jet: A high-pressure jet of water with abrasive material mixed in, used to cut sheet metal to a finished edge with no additional filing or tooling necessary.
- Brake: A machine used to fold or crease sheet metal to a specified angle. This can also be done by hand by nailing blocks of wood in place and using a rubber or wooden mallet to work the sheet into shape.
- Drill: A rotary tool with replaceable bits of different diameters used to cut holes in sheet metal and other materials. Large holes can be drilled with a bit called a hole saw, which is basically a round saw blade used like a drill bit to cut a circle out of the work surface. Only use bits designed to drill metal when working with sheet metal, they are different from plastic, wood, and masonry bits.
- File: Used to finish edges, remove burrs, and round off sharp corners that could otherwise be dangerous.
- Nibbler: A tool used to cut sheet metal by taking very small "bites" out of it at a time. Used to cut shapes out of sheet metal that would be difficult with tin snips.
- Plasma Torch: A very hot cutting tool that slices the sheet metal by melting it. A plasma table is a computer-controlled plasma torch that can be used for very intricate work or large numbers of sheets that need to be cut the same way.
- Press: A usually hydraulic machine that can either stamp shapes out of sheet metal, or deform it over a mold.
- Punch: A stamp-like tool used to punch holes into sheet metal. They come in various shapes and sizes. Large diameter punches are hollow and cut a circle out of the sheet metal.
- Shear: A long blade used to cut sheet metal by cutting down across its entire length.
- Tin Snips: Very large, strong scissors used to cut sheet metal by hand. These also come in electric and pneumatic models to make cutting easier.
- Saw: Any of several kinds of serrated blades used to cut sheet metal; examples include the hacksaw, table saw, circular saw, and Sawzall. Only use blades designed to cut metal when working with sheet metal, they are different from plastic, wood, and masonry blades.
Sheet Metal Fasteners:
- Bolt: Usually only used on very thick sheet metal, bolts can either be used with a nut on the other side or screwed in to a threaded hole in the sheet metal. Thinner sheet metal cannot be threaded and usually requires a washer to spread out the holding surface area.
- Machine Screw: A small bolt used for thinner sheet metal.
- Rivet: A small, non-removable fastener that mushrooms out on both sides of the sheet metal to hold it.
- Sheet Metal Screw: A short, thick screw with coarse threading and a large head that sits flat against the sheet metal. The coarse threading bites the sheet metal securely and the large head offers a wide surface area to hold it.
- Weld: Melts the sheet metal and a welding material together in order to create a strong, uniform joint once cooled. Spot welders are often used for sheet metal.