CNC Machining vs Metal Casting vs Forging: Process Selection Guide with Cost Data (2026)
CNC Machining vs Metal Casting vs Forging: Complete Process Selection Guide (2026)
Golden rules of process selection: CNC machining for 1–1,000 pieces with tight tolerances. Metal casting for 100–100,000 pieces with complex internal geometries. Forging for 500–50,000 pieces requiring maximum strength and fatigue resistance. The crossover points depend on part size, material, and complexity — this guide gives you the data to choose correctly.
Process Comparison Matrix
| Factor | CNC Machining | Sand Casting | Die Casting | Investment Casting | Forging |
|---|---|---|---|---|---|
| Ideal volume | 1–1,000 | 10–10,000 | 1,000–100,000 | 100–10,000 | 500–50,000 |
| Tooling cost | $0 | $1,000–5,000 | $10,000–100,000 | $2,000–10,000 | $5,000–50,000 |
| Per-part cost (100 pcs) | $30–150 | $10–50 | $3–15 | $15–80 | $15–60 |
| Per-part cost (10,000 pcs) | $15–80 | $5–15 | $1–5 | $5–25 | $5–20 |
| Tolerance | ±0.01mm | ±0.5–2mm | ±0.1–0.3mm | ±0.1–0.3mm | ±0.3–1mm |
| Surface finish (Ra) | 0.8–3.2 μm | 6.3–25 μm | 1.6–6.3 μm | 3.2–6.3 μm | 3.2–12.5 μm |
| Material strength | Wrought (high) | Cast (lower) | Cast (lower) | Cast (lower) | Wrought (highest) |
Strength Comparison: Wrought vs Cast Properties
Why forged and CNC-machined parts are stronger than cast parts: wrought materials have a continuous grain structure that follows the part shape. Cast materials have a random, discontinuous grain structure with potential porosity. For the same alloy, a forged or machined-from-billet part is typically 15–30% stronger and has 3–5x better fatigue life than the cast equivalent.
| Al 6061 Property | Wrought (CNC/Forged) | Cast (Permanent Mold) | Difference |
|---|---|---|---|
| Tensile Strength | 310 MPa | 235 MPa | -24% |
| Yield Strength | 276 MPa | 185 MPa | -33% |
| Elongation | 12% | 6% | -50% |
| Fatigue Limit (10⁷ cycles) | 95 MPa | 55 MPa | -42% |
When to Choose Each Process
- CNC Machining: Low to medium volumes, tight tolerances, high strength required, design still evolving, wide material selection needed
- Die Casting: High volumes (>5,000), aluminum/zinc/magnesium, complex thin-walled shapes, consistent appearance, lower strength acceptable
- Investment Casting: Medium volumes, complex shapes with undercuts, stainless steel or high-temp alloys, near-net-shape before CNC finish machining
- Forging: High volumes, maximum strength and fatigue life, structural/safety parts, predictable grain flow direction needed
- Sand Casting: Low to medium volumes, large parts (>500mm), low tooling budget, rough surfaces acceptable, iron and steel parts
Hybrid Approach: Cast/Forge Near-Net-Shape + CNC Finish
The most common high-volume strategy: cast or forge the part to near-net-shape, then CNC machine only the critical surfaces (bearing bores, mounting faces, sealing surfaces). This combines the low per-part cost of casting/forging with the precision of CNC where it matters. A typical automotive steering knuckle: forged to shape (~$12), then CNC machined on 3–5 surfaces (~$8) = $20 total, vs $80+ for full CNC from billet.
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