A basic knowledge of how sheet metal is worked will enable you to carry out a large range of repairs and projects. There are five main processes involved in forming sheet metal into shapes: bending, shearing and stretching can usually be accomplished without elaborate equipment, while deep drawing and spinning in most cases will require machinery.
Types of sheet metal
Mild and galvanized steel, aluminium, duralumin, zinc, brass, copper and phosphor bronze are usually available in a thickness range of 4mm down to 0.3mm (0.157-0.012in) — or from No 8 to No 30 gauge. The most common sheet size is 1200mm x 600mm (4 x 2ft). Brass and copper can be obtained in perforated sheet, the standard thickness being 0.55mm (0.021in/No 24 gauge or SWG); the perforations range from 0.8mm up to 6.35nun (0.031 to 0.250in).
Most jobs will require sheet metal no thicker than 1.6mm (0.063in). Sheet up to this size is easily worked cold, except possibly duralumin which may fracture on tight bends. This and mild steel may have to be worked at red heat to obtain very sharp bends. Non-ferrous metals must be bent in stages and annealed in between; they can be used where a decorative finish is required or where corrosion problems arise. Most steel sheet is protected in the same way, the most common forms used being:
A thin film of tin protects steel sheet from rusting; the thin steel sheet is ‘hot-dipped’ into a bath of molten tin. When working with tinplate, you must cover scratches or joints by ‘tinning’ with solder to restore the protection. Tinplate is sold in a number of sizes; the most common is 700 x 500min (28 x 20in). Sheet thickness is indicated by ‘strength marks’ such as IC, IXX, IXXX ; for general work IXX is suitable, this being 0.46mm (0.018in) thick. Terneplate This is similar to tinplate but cheaper because the coating is tin alloyed with lead. Terneplate has a dull surface and is unsuitable for use in situations where it might come into contact with food, because of its lead content.
The most suitable sheet metal for outdoor use, such as for water storage tanks or garden equipment, it is easily identifiable by its mottled coating produced by hot-dipping sheet steel into zinc.
Sheet iron This is not in fact iron, but sheet steel and comes in thicknesses up to 5mm (0.2in). It is supplied ‘black’ or `cold-rolled’; the latter is finished with a smooth, bright surface.
Marking out sheet metal
Most marking out on sheet metal can be done with a pencil; you can use a scriber, but if bend lines are scribed the metal is likely to crack when bent. You will also need a rule, a good metal straight-edge and an engineer’s try square. For circles you will need a pair of compasses or dividers — and to set out irregular curves a draughtsman’s Ilexicurve’. For very complicated shapes, it is best to draw a paper pattern full size and stick this to the metal before cutting; use an adhesive which enables the paper pattern to be easily removed.
Annealing sheet metal
Sheet metal which has been bent and restraightened, crushed, distorted or fractured, may work-harden and become difficult to handle. Some types can be annealed (softened) by heat treatment to restore them to their original workable state. This process is difficult if the sheets are large; but moderately sized sheets can be heated with a blowtorch as long as you apply an even heat to the sheet and do not overheat. Copper, brass and steel should be heated until they become a dull red colour; brass and steel should be allowed to cool slowly while copper can be quenched in hot water. Aluminium should be heated carefully because it has a low melting point. Coat it with a thin layer of soap and heat gently and evenly; when the soap turns a dark brown colour, you should remove the heat source and allow to cool slowly. Immerse zinc in boiling water for a few minutes and rework while it is still hot. Duralumin must be heated carefully until it is dark red and quenched in water; the temperature is critical so try to experiment with scrap pieces first. Tinplate, terneplate and galvanized steel cannot be heat-treated because their protective coatings may be damaged by high temperatures.
Tools for cutting sheet metal
Cutting tools for sheet metal are used in the same way as a pair of pliers. The length of handle determines the leverage, so it is advisable to buy the largest you can afford to obtain the best possible leverage. Never let the tips of the tool pass each other during the cut since this will cause the metal to distort and kink. Whichever tool you buy, remember it is not a general-purpose cutting tool — for example it will become damaged if used to cut wire.
Universal tinsnips This type is satisfactory if you intend to buy only one cutting tool. These will cut straight lines and curves in sheet metal. Monodex/Goscut snips These are patented tools for cutting straight lines or curves without distorting the surrounding metal.
These will also cut outside curves. Curved snips These cut inside curves.
Long cuts in sheet metal can be made with a fine-tooth hacksaw blade or with a piercing saw; clamp the metal inside a batten along the line of the cut to prevent vibration. After cutting, use a dreadnought file to clean up the edge. Sawing metal has been covered earlier in the Course.
Tools for bending sheet metal
Most of the tools you will need should already be in your tool kit; these include pliers for cutting wire and soldering equipment of the kind described earlier in the Course.
This can be easily made or you can improvize. Make formers from blocks of scrap hardwood.
If you use an ordinary hammer to bend metal, it will leave dents and marks. A tins-man’s mallet is made from boxwood or hide and it has a slightly domed striking surface.
This hammer has an angled striking edge used for tucking in the edges of seams. Creasing hammer This can be used for forming grooves in metal; it has a rounded striking edge. Seaming and grooving tools Both can be easily made from mild steel. A seaming tool is used to close down joints, or squash them flat; a grooving tool is used to close up wired edges.
A small, shaped anvil used to form special curves and shapes, such as a cone, in metal. A set of stakes will prove expensive, but it is often possible to improvize. The tapered portion of a car axle shaft, for example, can be held in a vice and used to form small rings and curves. A half-moon stake is used to form an edge on a circular base.
This stake, used for making sharp folds, is one you will be able to make yourself. Use a 100-300mm (4-12in) long piece of hardwood, mitre one edge and fix a handle to the narrower face.
Bending sheet metal
Most work with light gauge materials can be done mainly with your hands; the mallet need only be used for the final stages of bending. Very ductile metals which can be bent upon themselves without cracking are said to have a ‘zero bend radius’.
In general the radius of a bend should be no less than three to five times the thickness of the metal being bent. When metal is bent, the surfaces become distorted because the metal is compressed on the inside of the bend and stretched on the outside — the metal at the bend will therefore be both thinner and weaker. The stronger the metal is, the greater the bend radius required.
When marking out you will have to make allowance for the length of metal taken up by the bend. If you know the bend radius required, the bend length is found by multiplying the bend radius by the angle of bend in degrees and dividing by 60 (or more accurately 57.3). This formula is satisfactory for thin sheets of metal, say up to 2mm (0.080in); for thicker sheets, calculate the radius from the middle of the sheet thickness.
When metal is bent, it will tend to spring back once the bending force has been removed. The amount of spring-back will depend upon the thickness of the metal and the bend radius. You can only determine spring-back effects by trial and error; it may be necessary to bend on a smaller radius than originally planned to overcome the effects of spring-back.
To bend metal properly, it is important to support the metal on both sides as near as possible to the line of the bend. If you have no bending bars, two hardwood battens held in place by G-clamps will provide the support. Bend the metal gradually along the bending line (with your hands on light materials and with a mallet on heavier metals). If you attempt to bend the metal from one end only, it will buckle and fold.
These can be made from hardwood formers. When using soft metals the former can be made exactly to the required shape; with harder metals, make the former slightly undersize to allow for spring-back. Small work can be formed with a mallet, while larger work will require stretch-forming methods with a hydraulic jack providing the stretching force.
The edges of sheet metalwork must be formed for safety and strength; make sure you allow enough metal for this when marking out. The simplest edge is formed by bending the metal to a right-angle in a bending bar and closing it down with a hatchet stake; tuck the edge in with a paning hammer or flatten it down with a mallet. If you do use a mallet, work carefully to ensure you do not produce unsightly folding in the metal.
This technique will give a better finish to an edge, but it is more difficult to accomplish. First calculate the allowance needed; this is twice the diameter of the wire you intend to use plus four times the thickness of the metal sheet. Use a bending bar with a radius on the bend side of the metal to ensure you make a curve rather than a sharp bend. Alternatively file a piece of scrap metal to the radius of the wire and fold the metal over this. Insert the wire into the bend and close the fold with a mallet, then a paning hammer.
One problem encountered when wiring is that rectangular objects cannot be wired unless the box shape has already been formed; this means you will have to find a way of supporting the box shape while you wire its edges. There is no universal solution to this problem. When making cylindrical objects, wire the edges when the metal is flat and then bend it to its cylindrical shape. Always use wire which matches the material of the sheet metal otherwise the work may corrode, especially if used as a container for liquids.
A number of different joints are used in sheet metalwork. Whichever you choose, it is essential to mark out accurately. If the allowance you leave for joints differs on different parts, the work will have a poor appearance. Some joints are quite strong without soldering; but if the work is to be liquidproof, all joints will have to be soldered or filled with epoxy resin. A joggled lap joint gives a smooth finish to one side of the work but depends on solder for its strength. Folded and grooved seams give strong joints and folded seams can be used for circular work. A box seam is strong and it will give good external appearance to rectangular work.
Repairing damaged sheet
If a sheet becomes distorted through cutting or because it has been subjected to localized heating. You can repair the damage by hammering around the area in a circle. Move gradually towards the centre of the damage in decreasing circles and reduce the number and intensity of the hammer blows as the circle gets smallpr. Hammering will always harden metal, so it will have to be annealed after the damage is repaired.
This kind of repair work requires a planishing hammer; this has one round and one square face – both are convex. A 1 kg (35oz) hammer is suitable and you may find it useful for other applications – to repair dents in car bodywork, for example.