Secondary Operations
Post-processing: deburring, blasting, coating, plating, heat treat and marking
Secondary operations convert a raw machined/printed part into a finished deliverable: deburring and edge-breaking, surface prep (blasting), protective or cosmetic coatings (anodize, powder, paint, plating), heat treatment, and identification (laser marking, silkscreen). They add cost per part but are what make parts functional, durable and traceable — specify them explicitly on the drawing.
1. Mechanical finishing
Deburring (vibratory, tumbling, manual) removes edge burrs from machining and stamping — free-ish on CNC, always required for safety and coating adhesion. Bead/sand blasting textures and homogenizes surfaces (Ra 0.8–1.6 µm typical) and hides tool marks; polishing takes surfaces to Ra 0.2–0.4 µm for optics and seals.
Shot peening compresses the surface for fatigue life — specified on springs, axles and aerospace brackets. Grinding and lapping correct geometry and achieve the tightest tolerances (≤ ±0.01 mm).
2. Coatings and plating
Aluminum: anodize Type II (5–25 µm, decorative, dyed), Type III hard coat (25–75 µm, wear); chromate conversion under paint. Steel: zinc plating (5–12 µm), nickel (hardness/anti-corrosion), black oxide (cosmetic), phosphate+oil. Stainless: passivation (citric/nitric) to restore oxide and remove embedded iron.
Polymers: powder coat (25–100 µm) and paint for appearance/UV; dyeing for printed nylon; clear coat for acrylic. Electroless nickel covers complex geometry uniformly; electroplating needs line-of-sight. Salt-spray ratings (ASTM B117 / ISO 9227 hours) define corrosion targets.
3. Heat treatment
Steels: quench-and-temper (4140/4340) to target hardness; case hardening (carburizing, nitriding) for wear surfaces; annealing before machining for softness. Aluminum: T6 aging for strength; stress-relief before final machining for stability. Titanium: annealing and stress-relief.
Specify hardness range (e.g. 28–32 HRC) rather than vague “hardened”, and note any distortion allowance — heat treatment moves thin features.
4. Marking and documentation
Laser marking (engraved, 2D DataMatrix, serial numbers) is permanent and preferred for traceability; silkscreen and pad printing for labels; embossing for plastics in mold. FAI and material certificates (EN 10204 3.1) ship with orders that require them.
Each secondary operation is a cost line on the quote; combining operations (anodize + laser mark) in one vendor pass saves money. Mark the drawing with every operation, its spec (e.g. MIL-SPEC anodize class, salt-spray hours) and the cosmetic face.
Quick reference
| Operation | Typical spec | Effect |
|---|---|---|
| Deburr / vibratory | All edges | Safety, coating adhesion |
| Bead blast | Ra 0.8–1.6 µm | Matte, uniform surface |
| Anodize Type II | 5–25 µm, dyed | Corrosion + color |
| Hard anodize III | 25–75 µm | Wear, insulation |
| Zinc plate | 5–12 µm + clear/chromate | Steel corrosion |
| Passivation | Citric / nitric | Stainless oxide restore |
| Q&T 4140 | 28–32 HRC | Strength |
| Laser mark | DataMatrix | Traceability |
Sources & references
- ASTM B117 / ISO 9227 — Salt spray
- EN 10204 — Material certificates
- MIL-A-8625 — Anodize classes
- Xometry Learning Center — Secondary Operations (public engineering reference)