One-Off Casting Versus Machining from Solid: Which Is Right for Your Project?
When you need a single metal part, should you cast it or machine it from billet? Here's how to make the right call for your application.
The Decision Every Engineer Faces
You have a part to make. Just one. The CAD model is ready, the material spec is defined, and the timeline is tight. Two paths forward: cast it or machine it from solid stock.
Both approaches produce functional metal parts. Both can achieve tight tolerances. Both work for one-off production. But the economics, lead times, and material outcomes differ significantly—and choosing wrong costs you time and money.
This guide breaks down when casting wins, when machining wins, and how to evaluate the decision for your specific project.
Understanding the Fundamentals
Machining from Solid
Machining starts with a block of material (billet, bar stock, or plate) and removes material until the final geometry emerges. The process is subtractive—you're paying for material that becomes chips.
Strengths:
No tooling required
Familiar process for most machine shops
Excellent for prismatic geometries
Tight tolerances achievable in a single setup
Limitations:
Material waste increases with complexity
Internal features require multiple setups or EDM
Some geometries are impossible to machine
Per-part cost scales with machining time
One-Off Casting
Casting creates near-net-shape parts by pouring molten metal into a mold. Modern technologies like 3D sand printing and robocasting eliminate traditional tooling economics for single-piece production.
Strengths:
Material efficiency—only pay for what's in the part
Complex internal passages possible
Cast material properties differ from wrought
Economics favor complex geometries
Limitations:
Requires pattern or mold development
Some secondary machining typically needed
Minimum wall thickness constraints
Requires foundry expertise
When Machining Wins
Simple, Prismatic Parts
A rectangular bracket with a few drilled holes machines faster than it casts. When geometry consists of flat surfaces, simple pockets, and standard features, CNC machining offers the most direct path to a finished part.
Rule of thumb: If the part fits in a vise and machines complete in under two hours, machining likely wins.
Material Already in Stock
When you have suitable billet on the shelf and a CNC available, machining eliminates lead time entirely. For emergency repairs or same-day needs, this availability advantage often overrides cost considerations.
Extremely Tight Tolerances Throughout
Castings typically hold ±0.030" to ±0.060" in the as-cast condition. Critical dimensions require secondary machining. If your part has tight tolerances on every surface, you're machining it anyway—sometimes starting from billet makes more sense than casting and machining.
Wrought Material Properties Required
Some applications specifically require wrought material properties. Rolled or forged stock has directional grain structure that differs from cast microstructure. If your specification calls for wrought material, casting isn't an option.
When Casting Wins
Complex Geometries with High Material Removal
Consider a housing with internal passages, ribs, and variable wall sections. Machining from solid might require removing 80% of the starting material. You're paying for:
The material you remove (often the majority)
The time to remove it
Tool wear from extended cutting
Multiple setups for internal features
Casting produces near-net-shape parts. You pay for the material in the final part plus a machining envelope for critical surfaces.
Example: A pump housing with internal flow channels. Machining from solid: 4,500inmaterial,18hoursofmachinetime.Castingwithmachinedinterfaces:1,800 total including pattern development.
Internal Passages and Undercuts
Some geometries simply cannot be machined from solid. Internal cooling channels, undercut features, and enclosed cavities require casting or multi-piece fabrication.
3D sand printing creates these features directly in the mold. No cores to assemble, no parting line constraints, no geometry limitations imposed by traditional pattern-making.
Cast Material Properties Needed
Certain applications benefit from cast microstructure:
Gray iron provides vibration damping for machine bases
Cast stainless steels offer corrosion resistance configurations unavailable in wrought form
Aluminum castings achieve specific strength-to-weight ratios
If your material specification calls for a cast alloy (ASTM A48, A536, A216, A351, etc.), machining from solid isn't an alternative—it's a different material.
Large Parts Where Material Cost Dominates
As parts grow larger, material cost increasingly dominates the total. A 200-pound casting starts with roughly 220 pounds of metal. The same part machined from solid might require a 600-pound billet.
At current steel prices, that difference alone can exceed the cost of pattern development and casting.
The Economics Breakdown
Cost Structure: Machining
FactorImpactRaw materialBillet cost × buy-to-fly ratioMachine timeHourly rate × cycle timeToolingCutting tools, fixturesSetupProgramming, first-article
Scaling behavior: Costs are relatively linear. The 10th part costs about the same as the 1st.
Cost Structure: Casting
FactorImpactPattern/moldOne-time development costMetalCasting weight + risers/gatesProcessingMolding, melting, finishingMachiningSecondary operations on critical surfaces
Scaling behavior: First-part cost includes pattern amortization. Subsequent parts drop significantly.
The Crossover Point
For simple parts, machining wins at low quantities. For complex parts, casting often wins even at quantity one—especially with patternless technologies like 3D sand printing and robocasting that eliminate traditional tooling costs.
Traditional rule of thumb: Casting becomes economical above 5-10 pieces.
Modern reality with 3D sand printing: Casting can win at quantity one for complex geometries.
Decision Framework
Answer these questions to guide your choice:
1. What's the buy-to-fly ratio?
Calculate: (Starting billet weight) ÷ (Finished part weight)
Ratio < 2:1 → Machining likely wins
Ratio 2:1 to 5:1 → Evaluate both options
Ratio > 5:1 → Casting likely wins
2. Are there internal features?
No internal features → Machining viable
Simple internal features → Either approach
Complex internal passages → Casting required
3. What material properties matter?
Wrought required → Machining only
Cast alloy specified → Casting only
Either acceptable → Evaluate economics
4. What's the timeline?
Same-day emergency → Machine from stock
1-2 weeks → Either approach possible
Standard lead time → Evaluate economics
5. Will you need more?
Truly one-time → Weight economics heavily
Possible repeat orders → Casting creates reusable tooling
Future production → Casting validates design for scale
The One Off Castings Advantage
At One Off Castings, we've eliminated the traditional barriers that pushed one-off projects toward machining:
No pattern cost barrier: 3D sand printing creates molds directly from CAD without patterns. Complex geometries don't require complex tooling investments.
Rapid turnaround: Robocasting systems produce precision molds without weeks of pattern development. We match or beat typical machining lead times.
Engineering support: Our simulation tools (MAGMA, NovaCast) optimize designs before metal is poured. You get first-article success, not trial-and-error iteration.
Secondary machining included: We machine critical surfaces in-house. You receive finished parts, not rough castings requiring additional vendors.
Material breadth: Gray iron, ductile iron, carbon steel, stainless steel, aluminum—we pour the alloys your application requires.
Making the Call
The casting-versus-machining decision isn't about preference—it's about matching the manufacturing method to your specific part geometry, material requirements, and project economics.
For simple parts with low material removal, machining remains the efficient choice. For complex geometries, internal features, or large parts where material cost dominates, modern casting technologies deliver better economics even at quantity one.
Not sure which approach fits your project? Send us your CAD file and we'll provide a casting quote alongside your machining estimates. The comparison often surprises engineers who haven't evaluated casting for one-off work.
One Off Castings specializes in short-run metal casting and rapid prototyping from our facility in Jonesboro, Arkansas. We help engineers evaluate casting versus machining decisions with honest assessments of when each approach makes sense.
Frequently Asked Questions
What is the buy-to-fly ratio and why does it matter?
The buy-to-fly ratio is starting material weight divided by finished part weight. A ratio of 5:1 means 80% of material becomes waste chips. Higher ratios favor casting economics since you only pay for metal in the final part, not material removed during machining.
When should I machine a part instead of casting it?
Machine from solid when parts are simple prismatic shapes, require wrought material properties, need same-day turnaround from stock material, or have less than 2:1 buy-to-fly ratio. Simple parts with minimal material removal machine faster than they cast.
When is casting better than machining for a single part?
Casting wins for complex geometries with internal passages, parts requiring high material removal (buy-to-fly ratio above 3:1), large parts where material cost dominates, and applications requiring specific cast alloy properties like gray iron's vibration damping.
Can castings achieve the same tolerances as machined parts?
As-cast tolerances typically range ±0.030" to ±0.060". Tighter tolerances require secondary machining on critical surfaces. Combined casting with CNC finishing achieves ±0.001" on specified features while keeping overall costs lower than machining from solid.
Why can't some geometries be machined from solid?
Internal cooling channels, enclosed cavities, and undercut features cannot be accessed by cutting tools. These geometries require casting where cores create internal features, or multi-piece fabrication and welding. 3D sand printing handles complex internal passages in single molds.
How do material properties differ between cast and machined parts?
Machined parts retain wrought properties from rolling or forging with directional grain structure. Cast parts have equiaxed grain structure with properties specific to the casting alloy. Some applications specifically require cast properties—gray iron for damping, cast stainless for corrosion resistance configurations unavailable in wrought form.