Manufacturing Processes (Casting, Welding, Forming)
Manufacturing Processes (Casting, Welding, Forming)
The exam tests the sequence of sand casting, pattern allowances, weld types and defects, and hot versus cold forming operations.
Casting
Mould parts: cope (top), drag (bottom), parting line, sprue (vertical pouring passage), runner, gate, riser (feeds shrinkage), core (forms internal cavities), vent. Green sand is silica + clay + moisture. Pattern allowances: shrinkage (added, e.g. 10 mm/m for cast iron), draft (taper for easy withdrawal), machining (extra metal), distortion, rapping. Defects: blowhole, shrinkage cavity, hot tear, misrun, cold shut, sand inclusion.
Welding
Arc and gas welding are fusion processes. Soldering (below 450 °C) and brazing (above 450 °C) do not melt the parent metal. TIG uses a non-consumable tungsten electrode plus filler; MIG uses a consumable wire with shielding gas; resistance spot welding uses pressure and current with no filler. Defects: slag inclusion, porosity, undercut, lack of penetration, spatter.
Forming
Hot working means recrystallisation, lower forces and oxide scale; cold working gives strain hardening, a better finish and closer tolerance. Rolling, forging, extrusion, wire drawing and press work. In blanking the punched-out piece is the product; in piercing the hole is the product.
Worked example
A cast-iron block must finish 200 mm long; shrinkage allowance is 10 mm/m. Pattern length = 200 + (200 × 10/1000) = 200 + 2 = 202 mm. The allowance is added to the pattern, never to the finished casting.
Traps
- Applying shrinkage allowance to the finished casting instead of the pattern.
- Reversing blanking and piercing.
- Calling brazing a fusion process.
- Subtracting draft allowance instead of adding it.