Technical Knowledge: Welder, Machinist and Turner Trades
This section combines welding practice with machining on lathe, drilling and milling machines, plus the measurement and safety rules that apply to both.
Welding essentials
- Arc welding uses a consumable electrode; the flux coating shields the molten pool.
- Gas welding uses a fuel-gas flame, usually oxy-acetylene.
- TIG uses a non-consumable tungsten electrode; MIG uses a continuous wire electrode.
- Common defects: porosity, slag inclusion, incomplete penetration, undercut, spatter and cracks.
- A long arc causes spatter and undercut; a short arc risks sticking.
Machining essentials
- Turning reduces diameter; facing produces a flat end; taper turning and thread cutting need the compound slide or gear train.
- Cutting speed V = pi x D x N / 1000, with D in mm, N in rpm and V in m/min.
- Feed is in mm/rev; depth of cut is half the diameter reduction.
Worked example
Find the spindle speed to turn a 50 mm diameter bar at 30 m/min. N = 1000V/(pi x D) = 1000 x 30/(3.14 x 50) = 30000/157 = 191 rpm. The machine is set to the nearest lower available speed, about 190 rpm.
Traps
Use diameter in mm with the 1000 factor; mixing metres and millimetres is the commonest error. Depth of cut is not the full diameter reduction. In welding, do not blame current alone for porosity; damp electrodes and dirty surfaces also cause it.
Key points
- V = pi DN/1000 with D in mm gives speed in m/min.
- Depth of cut = (initial diameter - final diameter)/2.
- TIG uses tungsten; MIG uses a wire electrode.
- Long arc causes spatter and undercut.
- Porosity can come from damp electrodes or contaminated surfaces.