Investment casting and CNC machining are both capable of producing high-quality precision metal parts. Choosing between them — or knowing when to combine them — depends on your part geometry, volume, tolerance requirements, and cost targets. Here is a practical framework for making the decision.
Investment casting (lost-wax casting) — what it is
Investment casting builds a wax model of the part, coats it in ceramic, burns out the wax, and pours molten metal into the resulting shell. It can produce complex internal geometries, thin walls, and fine external detail that would be prohibitively expensive to machine.
Key characteristics: - Can produce complex 3D geometry in a single operation - Near-net-shape output means less machining allowance (lower material waste) - Typical tolerances: ±0.2–0.5mm without machining, ±0.05–0.15mm with post-cast machining - Surface finish (Ra): 1.6–6.3 µm as-cast, better with post-processing - Suitable materials: aluminium alloys, stainless steel, carbon steel, titanium alloys, brass
CNC machining — what it is
CNC machining removes material from a solid billet or pre-cast/forged blank to achieve the desired geometry. It offers precise dimensional control and is best suited to parts with geometric complexity achievable by cutting rather than moulding.
Key characteristics: - Tight dimensional tolerances: typically ±0.01–0.05mm achievable routinely - Excellent surface finish: Ra 0.4–1.6 µm on machined surfaces - Short setup time for prototypes and small batches - Higher material cost (waste in machining from billet)
The volume crossover
Investment casting requires tooling: a metal die to form the wax pattern, costing $3,000–$25,000 depending on complexity. This tooling cost must be amortised over production volume.
At low volumes (under 200–500 pieces), CNC machining typically wins on total cost because there is no tooling investment. As volume grows, casting's lower per-unit variable cost (especially for complex shapes with significant machining time) makes it progressively more attractive.
A rough crossover for safety hardware (carabiners, connectors): - Under 200 units/year: CNC from billet - 200–1,000 units/year: evaluate geometry and material — casting may win - Over 1,000 units/year: investment casting with CNC finishing is typically optimal for complex shapes
Tolerance and finish comparison
If your part requires tolerances below ±0.1mm on critical dimensions, casting alone will not achieve it. Most load-bearing safety hardware uses a combination: investment casting achieves the near-net shape, and CNC machining finishes the critical interfaces (gate pocket, nose, hinge bore).
This hybrid approach — cast the bulk of the material, machine the precision features — is how most production carabiners are made. The cast body achieves the complex curved geometry; the CNC step finishes the gate housing bore, the nose, and any threaded features.
Application guide for safety hardware
Carabiners and connectors: Investment casting body + CNC finishing of gate housing and nose geometry. Sometimes forged body for highest-strength applications.
Descenders and pulleys: Cast housing; CNC-finished axle bore and friction grooves.
Rigging plates and anchor hardware: Cast for complex geometry; CNC for bolt holes and critical load-bearing interfaces.
Tactical gear hardware (buckles, D-rings, adapters): Often CNC from billet for short runs; casting for volume orders over 500 pieces.
Making the decision
Send us a drawing. The fastest path to a process recommendation is a DFM review of your specific part geometry. We will tell you whether casting, machining, or a hybrid approach makes sense, at what volume the economics shift, and what tolerances each process can reliably achieve.