The CNC vs casting question is mostly about volume and geometry. Casting pours molten metal into a mold to make a near net shape, and it requires tooling. CNC machining removes material from wrought bar, plate or forgings and needs no hard tooling. For low quantities, machining almost always wins. For higher volumes of complex shapes, casting plus finish machining often gives the lower total cost. This guide explains where the crossover sits and how to plan for it.
CNC vs Casting: Comparison Table
Figures are typical for a complex housing-type part with heavy stock removal. Cost is a relative per-part index with tooling amortized over the quantity, where one machined part at quantity 1 equals 100.
| Factor | CNC machining from wrought stock | Investment casting plus finish machining | Die casting plus finish machining |
|---|---|---|---|
| Relative cost, qty 1 | 100 | 1,500 to 2,500 | 6,000 to 10,000 |
| Relative cost, qty 10 | 55 | 180 to 280 | 600 to 1,000 |
| Relative cost, qty 100 | 35 | 40 to 60 | 70 to 110 |
| Relative cost, qty 1,000 | 28 | 18 to 26 | 10 to 18 |
| Tooling | None beyond fixtures | Wax pattern tooling, moderate | Hardened steel die, high |
| Typical tolerance | ±0.001 to ±0.005 in | About ±0.005 in per inch as cast | ±0.002 to ±0.005 in as cast on small features |
| Surface finish | Ra 32 to 125 µin | Ra 125 to 250 µin as cast | Ra 32 to 125 µin as cast |
| Materials | Nearly any wrought alloy or plastic | Steels, stainless, nickel alloys, bronze, aluminum | Aluminum, zinc, magnesium |
| Lead time | Days to weeks | 6 to 12 weeks for tooling and first articles | 10 to 20 weeks for tooling |
| Strength | Full wrought properties, no porosity | Good, possible porosity and shrink | Moderate, porosity common |
| Typical use case | Prototypes, low and mid volume, critical parts | Complex shapes, mid volume, harder alloys | High volume aluminum and zinc housings |
When CNC Machining Wins
- Prototypes and low volumes. Without tooling, machined parts are ready in days and changes cost only a program edit.
- Critical, pressure-containing or fatigue-loaded parts. Wrought material has no porosity, so leak paths and hidden voids are not a concern.
- Designs still changing. A casting pattern locks the geometry. Machining keeps options open until the design settles.
When Casting Wins
- Higher volumes of complex shapes. When a machined part would remove 70 to 90 percent of the starting block, a near net casting saves material and cycle time.
- Internal passages and organic shapes. Cores create internal cavities that would need several machining setups or could not be machined at all.
- Stable, mature designs. Once the design is frozen, tooling cost spreads over years of production.
Common Mistakes
- Moving to casting before the design is stable, then paying for tooling changes.
- Leaving too little machining stock on cast surfaces. Typical allowance is 0.060 to 0.125 in on sand castings and 0.020 to 0.060 in on investment castings.
- Not defining casting datums that the machine shop can locate from repeatably. Poor datum strategy causes wall thickness problems after machining.
- Skipping porosity requirements on sealing surfaces. Porosity exposed by machining can cause leaks unless impregnation or radiography is specified.
Finish Machining Castings
Most castings still need machining. Bores, bearing fits, threaded holes, sealing faces and mounting surfaces are cut after casting to reach drawing tolerances. Horizontal machining centers with pallet changers are well suited to this work because a fixtured casting can reach several faces in one setup while the next pallet is loaded. The first article is where casting datums, machined datums and wall thickness are confirmed, usually with a CMM report.
How This Applies at Wexmar
Wexmar does not pour castings. We machine parts from wrought bar and plate, and we finish machine customer-supplied castings from your foundry. Our six horizontal machining centers with pallet changers handle multi-face casting work, and our lathes handle round castings and turned features. First article inspection in AS9102 format and CMM reports are available. See CNC milling, inspection and quality, and the guide on CNC vs injection molding for the plastic equivalent of this decision.
Getting a Quote
Whether you need machined parts from solid or finish machining on castings, send the drawing and casting model if you have one. You can upload a drawing for an instant estimate, reviewed by a human estimator, or call (315) 400-2654.