Process Selection

Casting vs Forging vs CNC Machining: How to Choose for Your Part

Casting, forging, and CNC machining — three fundamentally different ways to make a metal part. This guide compares them on cost, strength, tolerances, lead time, and helps you decide which process is right for your application.

GRDH Precision · October 2026 · 12 min read

Manufacturing process selection — casting, forging, and CNC machining comparison

TL;DR — Key Takeaways

Casting: best for complex shapes, internal cavities, thin walls; widest material range; lowest per-piece cost at volume.
Forging: best for strength-critical parts; superior toughness and fatigue resistance; limited to simpler shapes.
CNC Machining: best for tight tolerances, low volumes, simple shapes; highest precision, highest material waste.
Most industrial parts use a combination: cast/forge the blank, CNC finish critical surfaces.
Decision driver: complexity → casting, strength → forging, precision/volume → CNC.

1. The Three Processes at a Glance

Before diving into comparison, let's define each process in one sentence:

Casting

Molten metal is poured into a mold cavity and allowed to solidify. The part takes the shape of the cavity. Includes sand casting, investment casting, die casting, etc.

Forging

Solid metal (usually heated) is shaped by compressive force using dies or tools. The metal is deformed, not melted. Includes open die, closed die, and ring rolling.

CNC Machining

Material is removed from a solid block (billet, bar, plate) using rotating or cutting tools controlled by computer. Includes milling, turning, drilling, grinding.

2. Side-by-Side Comparison

Factor	Casting	Forging	CNC Machining
How Shape Is Created	Molten metal fills mold cavity	Solid metal deformed by force	Material removed from block
Geometry Complexity	Very high (internal cavities, thin walls)	Low-Medium (no undercuts)	Medium (5-axis allows complexity)
Strength/Toughness	Lowest (porosity, dendritic grain)	Highest (grain flow aligned)	Medium (base material properties)
Tolerance	±0.05–0.25 mm/25mm	±0.25–1.5 mm	±0.005–0.05 mm
Surface Finish	Ra 1.6–12.5 μm	Ra 3.2–12.5 μm	Ra 0.4–3.2 μm
Tooling Cost	Low (sand) to High (investment)	High (closed die)	None
Lead Time (first part)	3–8 weeks	2–8 weeks	1–3 weeks
Minimum Volume	1 (sand) / 100 (investment)	1 (open die) / 200 (closed die)	1
Material Utilization	60–90%	70–95%	30–60% (high waste)
Max Part Size	Unlimited (sand)	~500 kg (closed die)	Limited by machine

3. Casting: When Complexity Wins

Casting is the only process that can create internal cavities, complex external geometry, and thin walls in a single operation. If your part has features that cannot be machined (internal passages, undercuts, honeycomb structures), casting is the only option.

Casting Sub-Processes

Process	Best For	Tolerance	Min Volume
Sand Casting	Large, simple parts	±0.25 mm/25mm	1 pc
Investment Casting	Complex, precise parts	±0.05 mm/25mm	~100 pcs
Die Casting	High-volume aluminum/zinc	±0.05 mm/25mm	~10,000 pcs
Lost Foam	Complex internal geometry	±0.2 mm/25mm	~500 pcs

Advantages of Casting

Only process for complex internal geometry
Widest material range (any castable metal)
Lowest per-piece cost at volume
Can produce very large parts (sand casting)

Disadvantages of Casting

Lower strength due to porosity and grain structure
Looser tolerances than CNC or forging
Higher defect risk (porosity, shrinkage, inclusions)
Tooling lead time for investment/die casting

4. Forging: When Strength Matters

Forging produces the strongest metal parts. The compressive force aligns the grain flow with the part's geometry, creating a continuous grain structure that follows the part's contours. This results in superior tensile strength, toughness, impact resistance, and fatigue life compared to cast or machined parts.

Advantages of Forging

Highest strength and fatigue resistance
Improved impact toughness
No internal porosity (unlike casting)
Better material utilization than CNC
High repeatability for production runs

Disadvantages of Forging

Cannot produce complex internal geometry
High tooling cost for closed die forging
Limited part size for closed die (~200 kg)
Requires secondary machining for precision features

Open Die vs Closed Die Forging." data-zh="关于开模锻与闭模锻的详细对比，请参阅我们的文章：开模锻 vs 闭模锻。">For a detailed comparison of open die vs closed die forging, see our article: Open Die vs Closed Die Forging.

5. CNC Machining: When Precision Is Critical

CNC machining is the most precise manufacturing process. It removes material from a solid block using rotating cutting tools, following a computer-generated toolpath. It can achieve tolerances down to ±0.005 mm and surface finishes down to Ra 0.1 μm with grinding.

CNC Machining Processes

Process	Best For	Tolerance
3-Axis Milling	Prisms, pockets, flat features	±0.01 mm
4-Axis Milling	Cylindrical features, curved surfaces	±0.01 mm
5-Axis Milling	Complex 3D surfaces, impellers	±0.02 mm
CNC Turning	Round parts, shafts, bushings	±0.005 mm
EDM (Electrical Discharge)	Hard materials, intricate shapes	±0.002 mm

Advantages of CNC Machining

Tightest tolerances (±0.005–0.05 mm)
Best surface finish (Ra 0.1–3.2 μm)
No tooling cost — ideal for prototypes and low volume
Fastest turnaround for first parts

Disadvantages of CNC Machining

High material waste (30–70% becomes chips)
Highest per-piece cost at volume
Cannot create internal undercuts without 5-axis
Limited by tool access (deep pockets, small holes)

6. Decision Framework: How to Choose

Use the following questions to guide your process selection:

Question 1: What Is Your Part's Geometry?

Complex internal cavities, thin walls, undercuts → Casting
Simple prismatic or cylindrical shape → CNC Machining
Moderate complexity, no internal features → Forging or Casting

Question 2: What Are Your Strength Requirements?

High fatigue, impact, or load-bearing requirement → Forging
Moderate strength with corrosion resistance → Casting (stainless steel)
Standard structural requirements → Any (choose by cost)

Question 3: What Tolerances Do You Need?

Tighter than ±0.05 mm → CNC Machining (or cast/forge + CNC finish)
±0.05–0.25 mm → Investment Casting or Forging
Looser than ±0.25 mm → Sand Casting or Open Die Forging

Question 4: What Is Your Annual Volume?

1–50 pieces → CNC Machining (no tooling)
50–500 pieces → Sand Casting or Open Die Forging + CNC
500+ pieces → Investment Casting or Closed Die Forging

The Hybrid Approach: Cast/Forge + CNC

For most industrial parts, the optimal solution is a combination: use casting or forging to create a near-net-shape blank, then CNC machine the critical surfaces to tight tolerances. This gives you the cost advantage of casting/forging with the precision of CNC. The machining allowance is typically 0.5–3 mm, far less than machining from solid bar.

7. Real-World Examples

Part Type	Chosen Process	Why
Stainless steel valve body	Investment Casting + CNC	Complex internal flow passages, pressure-tight requirement
Steel connecting rod	Closed Die Forging + CNC	High fatigue strength, moderate geometry
Aluminum gear housing	Die Casting + CNC	High volume, complex shape, light weight
Large bearing block	Sand Casting + CNC	Large size, simple geometry, low volume
Precision shaft	CNC Turning	Simple shape, tight tolerance, low volume
Wind turbine hub	Sand Casting	Very large, complex, moderate volume

8. Cost Comparison Example

Consider a steel bracket, 3 kg, annual volume 1,000 pieces, requiring ±0.05 mm tolerances on mounting surfaces:

Process	Tooling	Unit Cost	CNC Finish	Total for 1,000 pcs
CNC from bar	$0	$45	Included	$45,000
Sand Cast + CNC	$1,500	$12	$15	$28,500
Investment Cast + CNC	$8,000	$10	$8	$26,000
Closed Die Forge + CNC	$6,000	$11	$10	$27,000

In this example, investment casting + CNC is the most cost-effective at 1,000 pieces. At 100 pieces, CNC from bar would be cheapest ($4,500 vs. ~$10,000+ for cast/forge options due to tooling).

9. Sourcing from China: Process Selection Support

When sourcing from China, choosing the right process is even more important because it determines which type of supplier you need. A sand casting foundry cannot do investment casting; a forging shop cannot do CNC machining at scale. Matching the process to the right supplier type avoids wasted time and poor quotes.

How GRDH Precision Helps

We review your drawing and recommend the optimal manufacturing process based on geometry, material, tolerance, volume, and cost. We then identify and audit qualified Chinese suppliers for that specific process, compare quotes on an apples-to-apples basis, and manage production and quality control through delivery.

How to Source Investment Castings From China
• How to Source Forged Parts From China
• How to Source CNC Machined Parts From China" data-zh="工艺特定的采购指南，请参阅：
• 如何从中国采购熔模铸件
• 如何从中国采购锻件
• 如何从中国采购 CNC 加工件">For process-specific sourcing guides, see:
• How to Source Investment Castings From China
• How to Source Forged Parts From China
• How to Source CNC Machined Parts From China

10. Conclusion

There is no single "best" process — the right choice depends on your part's geometry, strength requirements, tolerance, and volume. Casting wins on complexity and material range, forging wins on strength, and CNC machining wins on precision. For most industrial parts, the optimal solution is a hybrid: cast or forge the near-net-shape blank, then CNC machine critical surfaces.

Not Sure Which Process Fits Your Part?

Send us your drawing and we will recommend the right process — casting, forging, or CNC — from qualified Chinese suppliers.

Start a Project →

Frequently Asked Questions

What is the difference between casting, forging, and CNC machining?

Casting pours molten metal into a mold cavity. Forging deforms solid metal under compressive force (usually heated) to shape it. CNC machining removes material from a solid block using cutting tools. Casting is best for complex shapes, forging for strength-critical parts, and CNC for tight tolerances and low volumes.

Which process is strongest: casting, forging, or machining?

Forging produces the strongest parts because the compressive force aligns the grain flow with the part geometry, improving tensile strength, toughness, and fatigue resistance. Machined parts retain the base material's properties but have no grain flow optimization. Castings are generally the weakest of the three due to porosity and dendritic grain structure, though heat treatment can improve properties.

Which process is cheapest?

It depends on volume and complexity. For simple shapes and low volumes (1–50 pcs), CNC machining from bar stock is usually cheapest (no tooling). For complex shapes and medium-high volumes (200+), casting or forging is cheaper despite tooling cost because per-piece cost is much lower. For large simple parts, sand casting is cheapest.

Which process has the best tolerances?

CNC machining achieves the tightest tolerances (±0.005–0.05 mm), followed by investment casting (±0.05 mm/25 mm), closed die forging (±0.25–0.5 mm), and sand casting (±0.25 mm/25 mm). For the tightest tolerances, CNC machining is required — but cast or forged blanks can be used as the starting point to reduce material and machining cost.

How do I decide which process to use for my part?

Use this decision framework: (1) If your part has complex internal geometry or thin walls → casting. (2) If your part needs high strength, fatigue resistance, or impact toughness → forging. (3) If your part needs very tight tolerances or is a simple shape in low volume → CNC machining. (4) For parts needing both complex shape and high strength, consider casting + CNC machining or forging + CNC machining.

Can a single part use multiple processes?

Yes — most industrial metal parts use a combination. A common approach is casting or forging to create a near-net-shape blank, then CNC machining to finish critical surfaces to tight tolerances. This combines the cost advantage of casting/forging with the precision of CNC machining.