Processes: cutting, shaping, joining, finishing

Resistant Materials – Processes: Cutting, Shaping, Joining, Finishing

1. The Design Process (Common Content)

All IGCSE 0445 candidates must be able to apply the design cycle when developing a product that uses resistant‑materials processes.

2. Material Properties and Selection

Understanding how material properties affect the choice of process is essential for both the exam and real‑world design.

3. Measurement, Mark‑out & Tool Selection

• Primary measuring instruments: steel rule, tape measure, vernier calipers, micrometre, depth gauge.
• Mark‑out tools: centre punch, scribe, marking gauge, combination square, layout fluid.
• Key concepts:
  • Establish datum lines and reference points before any cutting or shaping.
  • Check tolerances against the specification (e.g., ±0.1 mm for a milled slot).
• Fastener identification: thread pitch (mm), diameter (M6, M8, etc.), grade (8.8, 10.9) – essential for mechanical joining.

4. Cutting Processes

Cutting creates the raw shape from a larger work‑piece. Both high‑tech and low‑tech methods are examined.

Example Calculation – Saw Blade Peripheral Speed

For a circular saw the peripheral speed (v) is:

$$v = \pi D N$$

where D = blade diameter (m) and N = revolutions per minute (rpm).

If a 300 mm (0.30 m) blade runs at 5 000 rpm:

$$v = \pi \times 0.30 \times 5\,000 \approx 4\,712\ \text{m min}^{-1}$$

5. Shaping Processes

Shaping refines geometry after cutting. The table includes both machine‑based and manual techniques.

Design Formula – Bending Stress

For a rectangular beam:

$$\sigma = \frac{M\,c}{I}$$

• M – Bending moment (N·mm)
• c – Distance from neutral axis to outer fibre (mm)
• I – Second moment of area (mm⁴) = \(\frac{b h^{3}}{12}\) for a rectangle (b = width, h = depth)

6. Joining Processes

Joining creates permanent or semi‑permanent bonds. The table includes both high‑tech and low‑tech methods required by the syllabus.

Strength Check – Fillet Weld

Design shear strength of a fillet weld:

$$F_{v}=0.6\,\sigma_{f}\,a\,l$$

• \(\sigma_{f}\) – Tensile strength of base metal (MPa)
• a – Weld throat size (mm)
• l – Weld length (mm)

7. Finishing Processes

Finishing improves appearance, protects against corrosion, and can modify surface hardness.

Coating Thickness Check – Example

For a zinc galvanised coating, the required minimum thickness is 85 µm. Using a micrometre gauge:

Measured thickness = 0.090 mm → Pass (≥ 0.085 mm).

8. Health & Safety (H&S)

• General PPE: safety glasses/goggles, hearing protection, gloves, steel‑toe boots, lab coat or aprons.
• Cutting tools: keep blades sharp, use blade guards, never force the material, ensure work‑piece is firmly clamped.
• Laser cutting: wear laser‑rated goggles, use interlocked enclosure, avoid reflective materials.
• Water‑jet: wear water‑resistant PPE, ensure drainage, never point jet at people.
• Plasma & welding: wear welding helmet, use appropriate shielding gas, ensure adequate ventilation for fumes, keep fire‑extinguishers nearby.
• Adhesives & solvents: work in a well‑ventilated area, avoid skin contact, store according to MSDS.
• Noise: limit exposure to < 85 dB; use ear defenders for prolonged use of power tools.
• Emergency procedures: know location of first‑aid kit, fire blanket, eye‑wash station, and emergency stop buttons.

9. Sustainability & Life‑Cycle Considerations

• Material selection: prefer recyclable metals (aluminium, steel) or biodegradable polymers where appropriate.
• Energy use: compare electricity consumption of laser cutting vs. manual sawing; favour low‑energy processes for large‑volume production.
• Waste minimisation: nest parts on a sheet to reduce off‑cut, recycle off‑cuts, use scrap for prototype testing.
• Finishes: choose water‑based paints or powder coating (lower VOCs) over solvent‑based paints.
• End‑of‑life: design for disassembly (e.g., use mechanical fasteners rather than permanent adhesives) to facilitate recycling.

10. Testing & Evaluation

• Dimensional inspection: use calipers, micrometres, height gauges; compare against drawing tolerances.
• Surface‑finish inspection: visual check, surface‑roughness tester (Ra values), coating thickness gauge.
• Strength testing: tensile test for welded joints, shear test for rivets, pull‑out test for screws/bolts.
• Functional testing: load‑bearing tests, movement checks for moving parts, water‑tightness for sealed enclosures.
• Evaluation report: record results, assess against specification, suggest improvements (e.g., change welding parameters, select a different finish).

11. CAD/CAM Integration

• CAD: 2‑D orthographic drawings, isometric views, exploded views; use layers to indicate cuts, bends, and finishes.
• CAM: generate CNC code for laser, plasma, water‑jet, or milling; set feed rate, spindle speed, and tool path based on material properties.
• Simulation: virtual stress analysis for joints, deformation analysis for bends, and material usage optimisation (nesting).
• Example: A CAD model of a metal enclosure exported to a CAM program to produce G‑code for a CNC laser cutter, with a 0.2 mm kerf compensation.

12. Consolidated Example – Design of a Portable Laptop Stand

1. Brief: Lightweight, adjustable height, able to support 15 kg, aesthetically modern, recyclable.
2. Material selection: Aluminium alloy (density 2700 kg m⁻³, good strength‑to‑weight, recyclable).
3. Measurement & Mark‑out: Use calipers to mark cut‑outs to ±0.1 mm; centre‑punch for drill points.
4. Cutting: CNC laser cutting of 2 mm aluminium sheets (precision edges, complex cut‑outs).
5. Shaping: Press‑brake bending to 90° for legs; hand‑filing of edge tolerances.
6. Joining: Mechanical fastening with M4 × 20 mm socket head cap screws (grade 8.8) and optional snap‑fit hinges for height adjustment.
7. Finishing: Deburr edges, then anodise to a 25 µm thick oxide layer (corrosion resistance, colour).
8. H&S: Laser safety goggles, gloves for handling sharp edges, ventilation during anodising.
9. Sustainability: Aluminium is 100 % recyclable; design allows disassembly for material recovery.
10. Testing: Load test to 20 kg, measure deflection (< 2 mm); check anodise thickness with micrometre.
11. Evaluation: Meets all criteria; recommend adding rubber pads to improve grip.