Fabrication, Fitting, Marking, Shearing, Bending and Pipe/Tube Work
What this domain tests
Marking and measuring technique, blank-length calculations for bends, shearing clearance, and pipe layout including offsets and mitres.
Core rules
- Marking and checking. Steel rule, scriber, dividers and trammel, surface plate, V-block, centre punch, vernier caliper and micrometer. Datum and centre lines must be transferred before cutting, not after.
- Bend allowance. BA = (π/180) × bend angle × (R + K·T), where K is the neutral-axis factor: about 0.33 for R/T ≤ 2, 0.4 for 2 < R/T ≤ 4, 0.5 for R/T > 4. For a 90° bend, the flat blank length = outside leg A + outside leg B − 2(R + T) + BA.
- Shearing. Blade clearance about 5–8% of thickness for mild steel; too little clearance gives secondary shear and a ragged edge, too much gives roll-over and burr. Shear force F = L × t × τ, with τ roughly 0.8 of tensile strength.
- Pipe work. Circumference = πD for wrap-around templates. For an offset, travel = offset / sin θ; at 45° that is offset × 1.414. Mitre cuts use the parallel-line development method; branch intersections use the same method with the pipe divided into equal parts.
Worked example
Find the flat blank for a 90° bend, outside legs 50 mm each, inside radius 3 mm, thickness 3 mm, R/T = 1 so K = 0.33. BA = (π/2) × (3 + 0.33×3) = 1.5708 × 3.99 = 6.27 mm. Setback per side = R + T = 6 mm, so total deduction = 12 mm. Blank length = 50 + 50 − 12 + 6.27 = 94.27 mm. Setting out 100 mm would over-run the bend lines by about 5.7 mm; the neutral axis sits inside the mid-thickness, which is why the blank is shorter than the outside dimensions less only the setbacks.
Traps
Using K = 0.5 regardless of R/T; measuring legs to the outside and forgetting the setback; taking pipe offset as offset × cos θ; setting shear clearance as a fixed millimetre value.