Joining and Assembly
Permanent and Temporary Joining
Engineering assemblies require parts to be joined. The choice of method depends on whether the joint needs to be dismantled later, the materials being joined, the load the joint must carry and the cost of the process. Joining methods divide into two families: permanent (riveting, soldering, brazing, welding — joint cannot be undone without damaging the parts) and temporary (threaded fastenings such as bolts and screws — joint can be taken apart and reassembled). Understanding this distinction, and the temperature and filler requirements of each thermal process, is the key to answering exam questions correctly.
Riveting
Rivets are used to permanently join two or more sheets of metal. Pop (blind) rivets are used where only one side of the joint is accessible — common in aircraft skins and bodywork. Solid rivets require access to both sides: a bucking bar supports the tail while a rivet gun deforms it into a second head.
The correct sequence for riveting is one of the most-tested topics in the examination. Many students omit the marking-out stage and go straight to drilling, which means they lose marks. The full sequence is:
- Mark out — use a scriber and rule to mark the rivet positions on the top sheet.
- Centre-punch each marked position to prevent the drill wandering.
- Clamp the sheets together in the correct position.
- Drill the rivet holes through both sheets using a twist drill of the correct diameter.
- De-burr the holes to remove sharp edges.
- Insert the rivet through the aligned holes.
- Form the tail — use a rivet snap (or for pop rivets, the rivet gun) to peen (deform) the tail into the second head, clamping the sheets together.
The riveting sequence always starts with marking out. Skipping this stage — going directly from “select rivet” to “drill” — is the single most common error on joining questions and costs marks every year.
Threaded Fastenings (Temporary Joining)
Bolts, machine screws and set screws are threaded fastenings. A bolt passes through clearance holes in both parts and is secured by a nut on the reverse; a machine screw threads into a tapped hole in one part. Threaded fastenings are temporary: they can be removed with a spanner or screwdriver, which makes them essential for assemblies that need maintenance access — such as engine covers, gearbox housings and electrical panels. Locking devices (spring washers, nyloc nuts, thread-locking adhesive) prevent vibration-induced loosening.
Soft Soldering
Soft soldering uses a filler metal with a melting point below 450 °C — traditionally a tin-lead alloy, now typically lead-free tin-copper or tin-silver for health reasons. The soldering iron heats the joint; flux is applied to remove oxide films and allow the solder to flow; the filler melts and fills the gap by capillary action. The parent metal does not melt. Soft solder is used for electronic circuit-board connections and for joining copper water pipes (plumbing). The joint is weaker than the parent metal and not suitable for structural applications.
Hard soldering (also called silver soldering) uses a filler alloy containing silver, with a melting point between 450 °C and 900 °C. The process is otherwise similar to soft soldering — the parent metal is not melted — but the joint is significantly stronger and more heat-resistant. Hard soldering is used to join copper and brass components in plumbing and jewellery.
Brazing uses a brass filler rod (typically 60% copper, 40% zinc) with a melting point around 870 °C to join steel or copper parts. A blowtorch or oxy-acetylene torch heats the joint area to the correct temperature (cherry-red for steel). Flux is applied to prevent oxidation. The filler rod is touched to the hot joint and melts into the gap by capillary action — the engineer must NOT melt the filler directly in the flame. The parent metal is not melted. This is the critical distinction: brazing uses a filler rod at a temperature below the parent metal’s melting point. Welding (below) melts the parent metal itself.
A common exam error is confusing soft soldering and brazing. The temperature is the key: soft solder < 450 °C; brazing > 450 °C (typically 870 °C for brass filler). Another error is confusing brazing with welding: in brazing, the parent metal remains solid; in welding, the parent metal melts.
Welding
Welding is the only joining process listed here in which the parent metal melts. An electric arc or oxy-acetylene flame generates temperatures above the melting point of the steel (above 1,500 °C), creating a molten pool at the joint. A filler rod (or the wire electrode in MIG welding) adds material to the pool, which solidifies into a continuous fusion bond on cooling.
Common welding processes:
- MIG (Metal Inert Gas) welding — a continuously fed wire electrode melts into the joint, shielded by an inert gas such as argon to exclude oxygen. MIG is fast, highly controllable and widely used in vehicle body repair and fabrication workshops. BAE Systems uses MIG welding for structural steel fabrication on ships and aircraft ground equipment.
- Arc (manual metal arc) welding — a coated electrode is clamped into a holder; the arc between electrode and workpiece melts both the electrode coating (which provides a flux) and the parent metal. Slower than MIG but requires no gas supply, making it portable for site work.
Soft soldering < 450 °C, parent metal not melted. Brazing > 450 °C (brass filler rod), parent metal not melted. Welding: parent metal melts — this is what distinguishes it from all other thermal joining methods.
Soft soldering: temp < 450 °C | filler: tin alloy | parent metal: does NOT melt | materials: copper, steel, PCB | use: electronics, plumbing
Hard soldering / silver soldering: temp 450–900 °C | filler: silver alloy | parent metal: does NOT melt | materials: copper, brass | use: plumbing, jewellery
Brazing: temp ~870 °C | filler: brass rod | parent metal: does NOT melt | materials: steel, copper | use: pipework, bicycle frames, small engineering assemblies
Welding (MIG/arc): temp > 1500 °C | filler: electrode wire | parent metal: MELTS | materials: steel (MIG/arc), aluminium (MIG) | use: structures, vehicles, fabrication