Three Ways to Make a Metal Part
Every metal part is made by one of three fundamental approaches: casting (pour liquid metal into a shape), machining (cut solid metal into a shape), or fabrication (bend and join metal pieces into a shape). Choosing the right one depends on quantity, geometry, material, and cost.
Casting
Melt metal, pour into a mold, let it solidify. The mold defines the shape.
When to Cast
- Complex internal geometry — passages, cavities, undercuts that can’t be machined
- Large parts — engine blocks, pump housings, machine bases
- High volume — die casting cranks out thousands of identical parts per day
- Near-net shape — minimal machining needed after casting
Casting Methods
| Method | Quantity | Accuracy | Cost per Part | Tooling Cost |
|---|---|---|---|---|
| Sand Casting | 1–1,000 | ±0.030″ | $$ | $ |
| Investment (Lost Wax) | 100–10,000 | ±0.005″ | $$ | $$ |
| Die Casting | 10,000+ | ±0.002″ | $ | $$$$ |
| Permanent Mold | 500–50,000 | ±0.015″ | $ | $$$ |
Limitations
- Porosity (trapped gas) — can be a problem for pressure-containing parts
- Tooling lead time (4–12 weeks for molds/dies)
- Material properties generally lower than wrought (forged/rolled) equivalents
Machining
Start with a solid block (or near-net forging/casting) and cut away everything that isn’t the part.
When to Machine
- Tight tolerances — ±0.001″ or better
- Low-to-medium quantity — 1 to 5,000 pieces
- High-strength materials — wrought bar stock is stronger than castings
- Fast turnaround — no tooling required, program and cut
- Prototyping — one-off parts in production material
Limitations
- Material waste (buying a 5 lb block to make a 1 lb part)
- Cost per part doesn’t scale well — Part #10,000 costs almost as much as Part #1
- Some geometries are impossible (fully enclosed cavities, sharp internal corners)
Fabrication
Cut, bend, and weld sheet, plate, tube, and structural shapes into a finished assembly.
When to Fabricate
- Large structures — frames, enclosures, tanks, trailers
- Sheet metal parts — brackets, housings, panels, chassis
- Mixed materials / shapes — tube welded to plate welded to angle
- When weight matters — hollow structures are lighter than solid machined blocks
Limitations
- Weld distortion — heat from welding warps parts
- Lower precision than machining (±0.030″ typical for welded assemblies)
- Cosmetic limitations — welds need grinding/finishing
Decision Matrix
| Factor | Casting | Machining | Fabrication |
|---|---|---|---|
| Quantity: 1–10 | Sand only | ✓ Best | ✓ Best |
| Quantity: 100–1,000 | ✓ Good | ✓ Good | ✓ Good |
| Quantity: 10,000+ | ✓ Best | Expensive | Expensive |
| Tolerance: ±0.001″ | Needs machining | ✓ Best | Not achievable |
| Internal cavities | ✓ Best | Limited | Limited |
| Large structures | Expensive | Not practical | ✓ Best |
| Lead time | Weeks (tooling) | Days | Days–weeks |
Hybrid Approaches
Often the best solution combines methods:
- Cast + machine: Cast the rough shape, machine critical surfaces. Common for housings and manifolds.
- Fabricate + machine: Weld the structure, then machine mounting surfaces and bores. Common for machine bases and fixtures.
- 3D print + machine: Print the complex shape in metal, machine the critical features. Emerging approach for aerospace.
Not sure which approach? Describe your part — we’ll recommend the most cost-effective manufacturing method.