Machining and casting are not rivals but two methods that win in different quantity and geometry regions; most of the time they are used together. In our article on the factors that determine CNC part prices we explained how machining cost arises; this article sets that cost against the casting alternative.
Brief Definition of the Methods
| Method | Tooling cost | Economic quantity | Tolerance (typical) | Surface Ra |
|---|---|---|---|---|
| CNC from solid | 0 (excluding fixtures) | 1–5,000 | ±0.01–0.05 mm | 0.8–3.2 µm |
| Sand casting | €500–5,000 | 10–5,000 | ±0.5–1.5 mm (CT10–12) | 12.5–25 µm |
| Gravity die casting | €5,000–25,000 | 500–50,000 | ±0.3–0.8 mm (CT8–10) | 6.3–12.5 µm |
| High-pressure die casting (aluminium/zinc) | €15,000–80,000 | 5,000–1,000,000+ | ±0.1–0.3 mm (CT6–8) | 1.6–6.3 µm |
| Investment casting | €3,000–20,000 | 100–20,000 | ±0.1–0.4 mm (CT5–7) | 3.2–6.3 µm |
The tolerances in the table are typical values based on ISO 8062 casting tolerance grades; casting tolerances are 10–50 times those of CNC. This is why functional surfaces are almost always machined after casting.
Break-Even Calculation
A simple model: Casting total = tooling + quantity × (casting unit + finish-machining unit); CNC total = quantity × CNC unit. Example: aluminium housing, 180×120×60 mm, EN AW-6082 / EN AC-46000.
- CNC from solid: 2.2 kg block, 25 min machining → ~€38/piece (at 500 pieces).
- Gravity die casting: tooling €12,000, casting €6/piece, finish machining (faces + holes, 7 min) €8/piece → €14/piece + tooling share.
- Break-even: 12,000 / (38 − 14) = 500 pieces. Below it CNC, above it casting + machining is economical.
In high-pressure die casting, with tooling at €40,000, unit €2.50 + machining €3, break-even is ~1,200 pieces; but tool life and the risk of freezing the design must be taken into account. Figures change with geometry and material; the rule does not: tooling cost ÷ unit difference = break-even quantity.
Decision Criteria
1. Quantity and forecast reliability
If the annual quantity is not 2–3 times break-even and the forecast is uncertain, CNC is safer; tooling is made once and remade when the design changes. For start-ups and new products the common path is CNC for the first 1–2 years, then a switch to casting once volumes settle; our prototype manufacturing page describes how we support this phase.
2. Geometry
Internal cavities, complex external forms, thin walls and organic shapes point to casting; prismatic, rotational and flat-faced parts are economical in CNC. The chip ratio (if 70%+ of the material is removed) is a strong signal in favour of casting.
3. Mechanical properties
Bar and plate material produced by forging/rolling is more homogeneous and has higher strength than castings; for parts under fatigue load and for pressure-bearing thin walls, CNC from solid is safer. Castings carry a porosity risk; pressure testing or X-ray may be required.
4. Material
Aluminium, cast iron, cast steel, brass and bronze are common in casting; stainless is possible by investment casting. High-strength heat-treatable steels (42CrMo4 +QT), 7075 aluminium and titanium are in practice machined from solid.
5. Time
CNC first part in 1–3 weeks; casting tooling 6–12 weeks + samples + corrections. For urgent projects CNC has no rival.
6. Surface and appearance
High-pressure die casting gives a good appearance; sand castings are painted or machined. A CNC surface is directly suitable for anodising; details in our surface roughness article.
The Hybrid Route: Casting + CNC Finishing
The great majority of housing parts in series production are made this way: raw casting, with functional surfaces (bearing bores, sealing faces, threads, mounting faces) machined on CNC. When buying this combination from a contract supplier:
- Are the foundry and the machine shop the same or different? Responsibility should sit at a single point.
- The casting drawing (machining allowances, datums, casting tolerance) and the machining drawing should be separate; datum surfaces cannot be as-cast surfaces.
- Tool ownership, storage and maintenance should be written into the contract; the tooling belongs to the customer.
- Acceptance criteria for porosity and internal defects should be defined; a pore opened on a machined surface is a matter of dispute.
Turkey has a strong foundry industry (Konya, Ankara, Bursa, Izmir) and machine shops work with foundries routinely; the regions are listed in our industry article.
Quick Decision Table
| Situation | Recommendation |
|---|---|
| <300 pieces/year, any geometry | CNC |
| 300–3,000 pieces, housing with complex internal cavities | Sand/gravity die casting + CNC finishing |
| >5,000 pieces, aluminium/zinc, thin walls | High-pressure die casting + CNC finishing |
| Fatigue load, pressure, high strength | CNC from solid (or forging + CNC) |
| Design not yet frozen | CNC; casting decision later |
| Parts needed in under 6 weeks | CNC |
For the Designer: Designing for Both Routes
When developing a new product with the quantity still unknown, designing the part so it can be produced both by CNC and by casting saves a great deal later:
- Uniform wall thickness and smooth transitions: Reduces shrinkage cavities in casting, vibration in CNC.
- External surfaces that can take a draft angle (1–2°): Required for casting; creates no extra cost in CNC.
- Grouping functional surfaces on a few planes: Lowers both the number of CNC set-ups and post-casting machining time.
- Datum surfaces that can carry a machining allowance: 2–3 mm allowance on the casting drawing, finished dimension on the machining drawing.
- Standard internal corner radii (R2–R3): Machinable with a milling cutter, reduces stress concentration in casting.
- Leaving holes and threads to machining: Cast holes should be designed only as pilot holes; precision holes are machined in any case.
A housing designed by these rules can be produced by CNC at 200 pieces in the first year and by casting at 5,000 pieces in the third year, and the assembly does not change between the two versions. Asking the contract supplier for design feedback (DFM) at an early stage lets you plan this transition at the cheapest point; more suggestions in our DFM article.
Frequently Asked Questions
Is a cast part weaker than a CNC part?
Generally yes; a cast structure may have porosity and lower ductility. With the right alloy and process control it is sufficient for most applications, but for critically loaded parts solid material is preferred.
Does the supplier cover the tooling cost?
Tooling is normally paid by the customer and belongs to the customer; some suppliers spread the tooling into the unit price in return for a quantity guarantee.
Can I have the casting made and machined in Turkey?
Yes; the machine shop can manage the foundry and deliver finished parts from a single point.
Will there be problems if I prototype by CNC and produce the series by casting?
If the geometry was designed for casting (parting line, draft angles, uniform walls) the transition is smooth; designing by casting rules from the start is important.
Conclusion
Low quantity, tight tolerance, high strength and speed point to CNC; high quantity, complex internal geometry and low unit cost point to casting; in the real world most series parts are a combination of the two. Send your part drawing and annual quantity via our quote form; we present the CNC-from-solid price and, where relevant, the casting + finishing alternative side by side. You can also look at our CNC milling scope.
