Workshop Practice, Fitting, Bench Work and Assembly
What the exam tests
Sequence of bench operations, correct tool selection, and the arithmetic behind drilling, tapping and fitting a job to size.
Core rules
- Typical sequence: study the drawing → mark out with surface plate, scriber and divider → punch → saw → file from rough to smooth → drill → tap or ream → deburr → check.
- Hacksaw blade selection: 14 TPI for soft thick sections, 32 TPI for thin tube and sheet. At least three teeth must touch the work at all times.
- Files are graded by cut - rough (bastard), second cut, smooth - and must never be used without a handle; clean them with a file card.
- Tap drill size = major diameter − pitch. M10 × 1.5 takes an 8.5 mm drill, leaving roughly 75 % thread.
- Taps come as taper, second and bottoming; back the tap half a turn for every full turn and use tapping compound.
- Reaming removes 0.1–0.3 mm and controls size and finish only; the drilled location must already be correct.
- Scraping and lapping are for fit and finish of mating surfaces, not for removing heavy stock.
- Assembly follows the drawing's fits: a clearance fit assembles by hand, an interference fit needs pressing, heating or cooling.
Worked example
A bracket needs an M12 × 1.75 tapped hole in a 12 mm plate. Tap drill = 12 − 1.75 = 10.25 mm, so the standard 10.2 mm drill is used. Full thread depth is about 1.5 × diameter = 18 mm, longer than the plate, so a bottoming tap is needed and the drawing's minimum full-thread depth must be checked before drilling.
Traps
- Quoting the tapping size as the finished hole size on the drawing.
- Sawing without a starting notch, which snaps blades.
- Filing across the marked line instead of up to it.
- Assuming reaming can move a hole to position.