Fitting, Assembly, Alignment and Scraping of Machine Components
What this domain tests
Bench skills – filing, sawing, drilling and tapping, reaming, dowelling – plus the alignment and hand-scraping techniques used when rebuilding machine tools.
Core rules
- File grades from coarsest: rough, bastard, second cut, smooth. Single-cut files for finishing and sharpening, double-cut for fast metal removal. Pin files for slots and keys.
- Hacksaw blade pitch: coarse (14–18 TPI) for thick sections and soft metals, fine (24–32 TPI) for thin tube and sheet so teeth do not strip.
- Tap drill size ≈ major diameter − pitch. Reaming allowance is typically 0.1–0.2 mm undersize before reaming.
- Tapping: use the correct tap wrench, keep the tap square, back off a quarter turn every half turn to break the chip, and use tapping compound.
- Scraping: apply marking compound (Prussian blue) to the reference surface, rub the part, and scrape only the blue high spots. Typical acceptance for machine ways is 20–25 contact spots per 25 mm square, with 70–80 % bearing area.
- Alignment: correct angular error first, then parallel offset. Use dial indicators on a test mandrel and check run-out at 90° intervals.
Worked example
To tap an M12 × 1.75 thread in mild steel, tap drill = 12 − 1.75 = 10.25 mm, so use a 10.25 mm or 10.2 mm drill. After tapping, check with the GO thread plug gauge, which must screw in freely by hand.
Traps that cost marks
- Using the outside diameter as the tap drill size, which leaves a shallow, weak thread.
- Scraping low spots instead of blue high spots, which makes the surface worse.
- Correcting parallel offset before angular error.
- Forcing a tap without backing off, causing it to snap in the hole.