The real question in a machining-versus-casting comparison is not "which process is better" but "which process is cheaper for this geometry at this quantity, after all the finishing steps are included." Machining starts with no tooling and a straightforward per-part cost; casting starts with a mold and a low per-part cost that only wins once the tooling amortizes. The crossover exists, but it moves with geometry, material state, and how much post-cast machining the design needs. This guide gives a decision tree for the geometry questions and a step-by-step break-even calculation you can run with real quotes.
The Decision Tree: Filter Geometry Before Comparing Price
Run the geometry through these questions before any cost comparison, because a casting that cannot hold the required features is not an option regardless of price.
| Decision node | Lean CNC machining | Lean metal casting |
|---|---|---|
| Quantity | Below a few hundred units typical | Thousands, unless tooling is cheap or parts are large |
| Part size | Any; small and medium are economical | Large sand castings can win at low volume; small die castings need multi-cavity tooling |
| Wall thickness | Machined from solid; thin walls are a machining cost, not a casting rule | Needs uniform walls and draft; thin isolated walls are hard to fill |
| Internal cavities | Only where a cutter can reach | Possible as-cast, including curved and blind cavities |
| Key tolerances | Machined directly | As-cast tolerances are looser; critical faces need machining |
| Material state | Wrought stock with documented properties | Cast microstructure; heat treatment and inspection may be required |
| Design stability | Changes are cheap | Freeze before tooling; changes cost mold work |
No single node decides the process. Quantity is the gate for casting, but geometry filters first: a part that needs a tight, deep bore will need machining either way, and a part with an internal cavity no cutter can reach will need casting or another near-net process. If both filters point the same way, the comparison becomes arithmetic.
The Break-Even Calculation, Step by Step
The break-even quantity is where the total delivered cost of the two routes crosses. Use these steps with real quotes, and include every cost that appears after the part leaves the machine or the foundry.
- Quote the fully machined route at your forecast quantities. Machining per-unit cost usually declines as quantity rises, so ask for the curve, not a single number.
- Quote the casting route as a system: mold cost, per-part cast price, and every post-cast operation: machining critical faces, heat treatment, inspection, and any scrap allowance.
- Total the casting route: mold cost plus (per-part delivered cost × quantity).
- Total the machining route: (machined per-unit cost × quantity).
- Find the crossover: the quantity where the two totals are equal. Below it, machining is cheaper; above it, casting pays back the mold.
An illustrative example shows the shape of the math. Suppose a fully machined housing costs $95 per unit in small quantities. The casting route needs an $18,000 mold, and each casting costs $8, with $35 of post-cast machining, $7 of heat treatment, and $5 of inspection, for $55 delivered per unit. The crossover solves 95Q = 18,000 + 55Q, or Q = 450. At 500 units the casting route saves about 500 × (95 − 55) − 18,000 = $2,000; at 1,000 units the saving is $22,000. The numbers are illustrative, and your crossover will be different, but the structure of the calculation is the same for every part.
| Quantity | Machined route, total | Casting route, total | Cheaper route |
|---|---|---|---|
| 100 | $9,500 | $23,500 | Machining |
| 300 | $28,500 | $34,500 | Machining |
| 450 | ~$42,750 | ~$42,750 | Crossover |
| 700 | ~$66,500 | ~$56,500 | Casting |
| 1,000 | ~$95,000 | ~$73,000 | Casting |
Two corrections make the calculation realistic. Machining quotes usually drop per unit with volume, which shifts the crossover higher; multi-cavity molds and casting scrap shift it lower. Re-run the math whenever quantity, material price, or machining content changes, because a decision made for 2,000 parts may not hold at 8,000.
Material State and Porosity Risk
Machined parts start from wrought stock with documented, consistent properties. Cast parts have a cast microstructure that can differ in strength, ductility, and porosity, and heat treatment is often needed to reach the required properties. The difference matters most for structural parts where the material state is part of the requirement.
Porosity is the casting risk that shows up late. Shrinkage and gas can create internal voids that appear only under machining or loading, so structural castings need a quality plan: test coupons, heat treatment records, and possibly non-destructive inspection. If the application needs certified mechanical properties, confirm how the casting will be qualified before the mold is committed, because adding qualification after production starts is expensive.
Design rules differ for the same reason. Castings allow internal cavities and complex walls that machining cannot reach, but they also need draft, fillets, and consistent wall thickness so the metal flows and cools evenly. A design that works as a machined part may need geometry changes to cast well, and the drawing should reflect the cast state before the mold is cut.
The Hybrid Route: Cast Near-Net, Machine Critical Faces
Most production metal parts are hybrids: the body is cast near-net, and the critical faces, bores, threads, and datums are machined. This combines the low material cost of casting with the accuracy of machining, and it is usually the answer when the decision tree points in both directions.
Design the hybrid from the start. The drawing must allocate machining stock on critical surfaces, define the datums that machining will use, and specify as-cast tolerances for the foundry. The handoff between foundry and machine shop is where problems appear: castings that vary in wall thickness or warp in heat treatment frustrate machining fixtures. A CNC machining partner that handles the post-cast work should agree on the datum scheme and the inspection plan before the mold is approved, so the machining stock lands where the fixture expects it.
When Machining Stays the Answer
Machining remains the production process, not just the prototype process, in several clear cases: quantities below the crossover, designs that will change during the program, parts that need certified wrought properties, and geometries where the casting would require so much post-machining that the mold never pays back. A part with a tight tolerance bore, a threaded boss, and a thin wall may be cheaper fully machined than cast-and-machined, because the machining content is nearly the same either way.
6CProto Engineering Team
Treat machining and casting as stages of one production plan, not rivals. Machine the prototype to validate the design, quote the casting route with every post-cast operation included, and freeze the design before the mold is cut. The crossover that matters is the total delivered cost at your real quantity, and the as-cast part is never the final part.
Decision Rule
Run the geometry through the decision tree first, then compute the break-even with complete quotes. If the design is stable, the quantity clears the crossover, and the casting quality after machining meets the requirements, casting wins; otherwise machining is the safer route. For most parts the production answer is both: cast the near-net body and machine the critical faces.
FAQs
At what quantity does casting become cheaper than CNC machining?
It depends on mold cost, part size, material, and post-cast machining content. Small die-cast parts can cross over below 1,000 units, while large sand castings can justify tooling at much lower volumes. Calculate the crossover with the formula mold cost divided by the per-unit saving; your geometry determines both numbers.
Can a casting meet the same tolerances as a machined part?
As-cast, no: cast tolerances are looser and depend on draft, cooling, and the material. That is why critical faces are machined after casting. The drawing should define which features are machined and which are as-cast, with the datums that connect them.
What does porosity do to a cast part, and how is it checked?
Porosity can create internal voids that weaken the part or appear during machining. Structural castings are checked with test coupons, heat treatment records, and often non-destructive inspection. Define the qualification evidence, such as coupon tests and the NDT scope, before the mold is approved if the application is load-bearing.
Why do many production parts use both casting and machining?
Because the economics split: casting produces the complex body at low per-part cost, and machining produces the tight bores, threads, and datums that castings cannot hold. The hybrid route allocates machining stock and a datum scheme on the drawing so the two processes fit together.
Sources
- 6CProto CNC Machining Services
- 6CProto Rapid Prototyping
- 6CProto Industrial Equipment Manufacturing
- ISO 2768-1:1989 – General tolerances for linear and angular dimensions
- ISO 9001:2015 – Quality management systems