Trade Theory
Trade Theory tests the principles behind your trade: why a tool cuts efficiently, how a material responds to heat or load, and how a process is controlled to meet a specification. Questions often target tool geometry, cutting parameters, fits and tolerances, heat treatment, and inspection.
Core rules
- Cutting speed is the surface speed of the workpiece or cutter, expressed in m/min. Feed is the relative movement per revolution (turning) or per tooth (milling). Depth of cut is the radial or axial engagement. These are independent settings.
- Tool geometry: rake angle controls cutting force and chip flow; clearance angle prevents rubbing; positive rake suits ductile work, negative rake suits interrupted cuts and carbide.
- Fits: clearance, transition, and interference. Hole-basis H7/g6 is a clearance fit; H7/p6 is an interference fit.
- Heat treatment: annealing softens and relieves stress; normalizing refines grain; hardening forms martensite; tempering restores toughness.
Worked example
A 50 mm diameter mild steel bar is turned with an HSS tool at a recommended cutting speed of 30 m/min. Find the spindle speed. N = 1000V/(πD) = 1000 × 30/(π × 50) = 30000/157.08 ≈ 191 rpm. On a step-speed lathe with 180 and 220 rpm, select 180 rpm because the surface speed then stays below the recommended value. The reasoning: cutting speed is a surface property, so a larger diameter needs a lower spindle speed.
Traps
Using radius instead of diameter in N = 1000V/(πD); mixing feed per revolution with feed per minute; assuming a deeper cut raises cutting speed; omitting the 1000 conversion from m to mm; confusing rake with clearance.