How CNC Machining Compares to 3D Printing

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  Key takeaways
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  Subtractive and additive, side by side
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  Material range and what it does to the part
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  Tolerance, surface finish and inspection
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  Lead time, cost curve and volume
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  Design freedom versus design discipline
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  How to run the selection in seven steps
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  CNC machining compares to 3D printing by criterion

Any decision about how CNC machining compares to 3D printing starts with one fact: machining removes material from solid stock, printing adds it layer by layer. That single difference drives nearly every downstream trade-off, from the grain structure inside the part to whether a tool can physically reach a feature.

In machining, a cutter follows a toolpath and leaves a surface defined by the tool nose radius and the feed rate. The part you hold is cut from a billet that was rolled, forged or extruded, so its mechanical properties match the mill certificate. Nothing about the process changes the metal. You are subtracting from a known material.

In printing, the part grows from a powder bed or a resin vat. Each layer bonds to the one below it, and that bond line is the weak plane. A printed bracket can be strong in-plane and noticeably weaker across the layer stack. Orientation on the build plate becomes a design variable, not an afterthought.

The practical consequence: when a load path must be isotropic and the drawing carries a real tolerance, machining is usually the safer call. When the geometry is organic, internal or otherwise unreachable by a cutter, printing is the only call.

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  Machining keeps the parent materialWrought aluminium, stainless and titanium hold their certified properties after cutting.
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  Printing builds anisotropy inLayer direction and build orientation change strength, so test coupons must match orientation.
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  Tool reach sets the limitDeep pockets, undercuts and internal channels often cannot be cut at all.

CNC covers the full metal catalogue. We machine 6061-T6, 7075, 2024 and 6082 aluminium, 303, 304, 316L and 17-4PH stainless, 1018, 1045, 4140 and 4340 steel, plus C36000 brass, beryllium copper, Ti-6Al-4V, Inconel and magnesium AZ31B. Plastics run from POM and PEEK to PC, PMMA and carbon fibre filled grades.

3D printing splits into two families. Resin processes such as SLA and DLP handle photopolymers well and give fine surface detail, but the cured resin is not a structural engineering plastic. Powder processes such as SLS and SLM handle nylon, TPU, and a narrower set of metals including stainless, aluminium and titanium alloys.

The gap shows up in heat and load. A machined 17-4PH valve body can be heat treated to a specified condition and inspected to that condition. A printed metal part can also be heat treated, but the starting density, residual porosity and layer bonding must be controlled first, and the qualification work is heavier.

For wear surfaces, threads under load and press fits, machined metal is the default. For lightweight ducting, jigs, covers and low-load housings, printed nylon is often enough and saves weeks.

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  Metals: machining covers more gradesTool steel, Inconel, beryllium copper and 4340 are routine on a mill, rare on a printer.
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  Resin is not a substitute for PEEKSLA parts creep under sustained load and lose stiffness as temperature rises.
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  Printed metal needs orientation controlSupport removal and build angle affect fatigue life on load-bearing parts.

This is where the two processes separate most clearly. Our machining tolerance is ±0.005 mm (±0.0002 in) on critical features, with a finish range from Ra 0.2–0.8 μm on fine surfaces through Ra 0.8–1.6 μm on standard high-quality faces to Ra 1.6–3.2 μm as-machined.

A well-run SLS or SLA printer holds roughly ±0.1 mm on small features, and that number degrades with part size and with resin shrinkage. Printed metal from SLM can reach tighter values on some features, but you pay for it in post-machining, stress relief and inspection time.

Surface finish follows the same pattern. Printed parts carry layer stair-stepping unless they are sanded, tumbled or coated. Machined faces come off the tool with a measurable Ra, and that measurement can be recorded on a report.

For a bearing bore, a seal groove, a spigot fit or anything that appears on a first-article inspection report, plan on machining. For a shroud or a bracket that only needs to be the right shape, printed tolerance is usually fine.

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  Bearing seats and seal groovesMachine them. Printed surfaces are too rough and too variable to hold a press fit.
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  Cosmetic coversPrinting plus bead blasting is often enough, and much faster to iterate.

For one prototype, printing is often faster to first part because there is no fixture and no toolpath to verify. The trade-off is that printing time scales with part volume: a big printed housing can occupy a machine for a full day.

Machining front-loads effort into programming and setup, then runs fast. Once the first article is signed off, per-part cycle time is stable and predictable. That is why the cost curve for machining falls quickly with quantity, while printing stays close to flat.

In our shop, quotation and a free DFM analysis come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. There is no minimum order quantity, so a single prototype and a 10,000-part run both go through the same quoting route.

The honest rule: below roughly 10 to 20 units of a simple plastic part, printing usually wins on cost. Above that, or as soon as tolerance tightens, machining takes over. For metal parts the crossover often comes earlier, because printed metal carries its own post-processing cost.

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  Setup dominates small quantitiesOne-off machined parts cost what they cost because of programming and fixturing.
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  Print time scales with volumeA tall build can run overnight; nesting many parts improves the economics.
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  Post-processing is a real line itemSupport removal and finishing on printed metal can rival the build cost.

Printing removes the cutter from the equation. Internal lattices, conformal cooling channels, hollow sections and blended organic ribs all become possible, and weight drops without losing shape. That is genuine design freedom.

Machining imposes rules that experienced designers learn to work with. Avoid deep pockets narrower than four times the tool diameter. Keep floor radii matched to a standard cutter. Give tapped holes enough wall around them. Design for the axis count you are buying.

With 16 simultaneous 5-axis centers and a Ø400 mm rotary table, we cut angled faces, port geometry and compound curves in one setup, which removes the stacked-tolerance error of multiple fixtures. Maximum processing size is 4,000 mm, with travels from 500 × 310 × 200 mm up to 4,000 × 400 × 150 mm.

The best results often come from mixing. Print a near-net blank with the internal channels already formed, then machine the mounting faces, bores and threads. You get the internal geometry printing does well and the fits machining does well.

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  Print internal, machine externalConformal channels inside, bearing bores and sealing faces outside.
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  Keep radii standardMatching floor radii to an available cutter removes a whole EDM or small-tool operation.