Manufacturing Processes: Casting, Forging, Welding and Sheet Metal
What the exam tests
Process selection, pattern and die allowances, simple welding and sheet-metal calculations, and recognition of defects.
Core rules
- Sand casting sequence: pattern → mould with sand and binder → core → gating and riser → pour → shakeout → fettling. Pattern allowances are shrinkage, draft, machining and distortion.
- Shrinkage allowance is added to the pattern, not the drawing. Steel shrinks about 20 mm/m, so a 500 mm steel casting needs a pattern near 510 mm; grey cast iron shrinks about 10 mm/m.
- The riser feeds the casting during solidification and must freeze last; chills and padding control directional solidification.
- Forging: open-die (upsetting, drawing down) and closed-die with flash and trimming. Hot working refines grain and needs less force; cold working gives better finish and strength through work hardening.
- Arc welding needs the molten pool shielded from air. Electrode E 6013 means 60 ksi tensile strength, all-position, rutile coating. TIG uses a non-consumable tungsten electrode with separate filler; MIG uses a continuous wire.
- Weld defects: porosity, slag inclusion, lack of fusion, undercut, spatter, distortion.
- Sheet metal: blanking cuts the piece out, piercing cuts a hole in it. Bend allowance must be added to the flat length.
Worked example
A 90° bend in 2 mm mild steel with inside radius 2 mm and bend allowance 3.5 mm, legs 40 and 30 mm: flat length = 40 + 30 − 2 × (2 + 2) + 3.5 = 65.5 mm. The subtraction removes the corner material already counted in both legs.
Traps
- Adding shrinkage to the finished drawing size instead of the pattern.
- Confusing blanking with piercing.
- Ignoring bend allowance, so the flat blank comes out oversize.