Shaping & Forming Metals
Categories of Shaping Processes
Metal shaping processes are grouped by whether they remove material, deform it without removing any, or reform it from a liquid or granular state. Understanding these categories helps you select the most efficient process for a given product and scale of production. The three main categories are wastage (material is removed), deforming/reforming (material is reshaped without significant removal) and fabricating (joining, covered in Lesson 4).
Marking Out
Before any cutting or shaping can begin, the metal must be marked out accurately. Marking out transfers measurements and cutting lines onto the metal surface. Key tools include:
- Engineer’s blue (layout fluid) — a dye applied to the metal surface so that scribed lines are clearly visible against the dark background.
- Scriber — a hardened steel tool used to scratch a fine line into the metal surface, following a rule or template.
- Centre punch — used with a hammer to mark the exact centre of a hole before drilling; the indent prevents the drill bit from wandering at the start of the cut.
- Engineer’s square — ensures lines are scribed at exactly 90° to a datum edge, critical for accurate right-angle cuts.
- Vernier calliper — a precision measuring instrument capable of measuring internal and external dimensions to 0.02 mm; essential for close-tolerance engineering work.
Wastage Processes
Wastage processes remove material to achieve the desired shape. They are also called subtractive processes. The material removed (swarf, off-cuts) is waste that must be managed and, ideally, recycled. Common wastage processes for metals include:
- Sawing — hand hacksaw or powered bandsaw used to cut metal to length or cut out profiles. A hacksaw blade should have at least three teeth in contact with the material at all times, or the teeth can snag and break.
- Filing — removes small amounts of material to achieve a smooth surface or a precise fit. Files are classified by their cut (single-cut, double-cut, rasp) and their coarseness (rough, bastard, second-cut, smooth). Always use a file handle to prevent injury from the tang.
- Drilling — a rotating drill bit removes material to form a cylindrical hole. The centre punch indent ensures accurate positioning. A pillar drill (bench drill) provides more accurate and safer drilling than a hand-held drill, with depth stops to control hole depth.
- Milling — a rotating multi-tooth cutter moves across the workpiece surface to remove material and create flat faces, slots or profiles. A vertical milling machine allows precise three-axis movement (X, Y, Z). CNC milling machines are programmable and used for high-volume, complex components.
- Turning (lathe work) — the workpiece rotates against a stationary cutting tool. A centre lathe can produce cylindrical shapes (shafts, pins), taper sections and threaded forms. Operations on a lathe include rough turning, facing (flattening the end), parting off (cutting to length) and thread cutting.
- Laser cutting — a high-powered laser beam melts and vaporises metal along a precise path controlled by a computer. Produces extremely accurate cuts with minimal kerf width and little heat-affected zone in thin sheet metal. Suitable for intricate profiles that would be impossible or very slow with mechanical cutting.
- Plasma cutting — a jet of ionised gas (plasma) at temperatures exceeding 20,000 °C melts through thicker plate steel. Faster and cheaper than laser cutting for thick material, but with a wider kerf and rougher edge finish. Widely used in shipbuilding and structural steelwork.
On a lathe, the workpiece rotates and the cutting tool is stationary. On a milling machine, the cutter rotates and the workpiece is moved. Confusing these two is a common error in GCSE answers describing machining operations.
Casting
Casting processes involve pouring or injecting molten metal into a mould where it solidifies into the desired shape. They can produce complex three-dimensional forms that would be very difficult to achieve by machining. The two main casting methods at GCSE are:
- Sand casting — a pattern (often made from wood or metal) is pressed into compacted sand to form a cavity. The pattern is removed and molten metal is poured in through a runner. After cooling, the sand mould is broken apart and the casting removed. Sand casting is relatively low-cost but produces a rough surface finish that usually requires machining. It is suitable for one-off and low-volume production and can produce very large castings (e.g. engine blocks, manhole covers).
- Die casting — molten metal is forced under high pressure into a reusable steel die. The die produces castings with a smooth surface finish and high dimensional accuracy. Die casting is suited to high-volume production of small-to-medium components in low-melting-point non-ferrous metals (zinc, aluminium, lead alloys). Examples include carburettor bodies, toy car shells and electrical connector housings.
Deforming and Reforming
Deforming processes reshape metal without removing material, using compressive or tensile forces. They are efficient because almost no material is wasted:
- Forging — a heated billet of metal is shaped by compressive forces, applied by hammering (drop forging) or squeezing (press forging) between shaped dies. Forging aligns the grain structure of the metal along the profile of the component, producing excellent strength and fatigue resistance. Car crankshafts, spanners and connecting rods are forged because the aligned grain makes them stronger than equivalent castings.
- Press forming (pressing) — sheet metal is placed between a punch (male die) and a die (female die); the punch descends under hydraulic pressure and forces the sheet into the die cavity. Used for very high-volume production of identical parts: car body panels, sinks and baking tins are all press-formed. The process is extremely fast once tooling is set up.
- Bending — metal sheet or bar is bent over a former or in a folder (pan brake) to produce angled or curved forms. A sheet metal folder uses a clamping beam and a folding beam to produce clean, accurate bends without thinning the material excessively. Simple, low-cost process suitable for one-off and batch work.
Sand casting uses a disposable mould broken after each casting; tooling costs are low and very large or complex components can be produced, but surface finish is rough and dimensional accuracy is limited. Die casting uses an expensive reusable steel die; tooling costs are very high (£10,000–£100,000+), but once the die is made, parts are produced rapidly with excellent surface finish and close tolerances. Die casting is therefore only economical for high-volume production (tens of thousands of parts) where the tooling cost is spread over many units. For a one-off or prototype, sand casting (or CNC machining from solid) is far more practical.
An important limitation of die casting is that the steel die cannot withstand the very high temperatures of ferrous metals such as steel; die casting is therefore restricted to lower-melting-point non-ferrous alloys. Sand casting can be used with virtually any metal, including cast iron and steel.
Forging is preferred over casting whenever a component must withstand repeated impact, bending or fatigue loading during its service life. Forging aligns the metal’s grain structure along the profile of the part, so the strongest direction of the grain runs through the sections that must resist the greatest forces — this is why car crankshafts and connecting rods are always forged rather than cast. Casting is better suited to components with complex internal cavities or very large cross-sections where forging dies would be impractical to make or too expensive for the production volume.
Forging produces a stronger component than casting for the same shape because the forging process aligns the metal’s grain structure along the profile of the component. A forged spanner is stronger than a cast one because its grain follows the shape of the jaws, resisting the bending forces applied in use.