Casting and CNC: How to Choose the Right Process

Casting pours or injects molten metal into a mold cavity. The part comes out close to its final shape, so almost no material is cut away. Die casting pushes aluminum or zinc alloy into a hardened steel die under pressure. Sand casting and investment casting handle steel, stainless, and higher-melting alloys at lower production speed. The mold is the product definition. Change the part and you cut a new one.

CNC machining starts from a solid billet, bar, or plate and removes material with rotating cutters and single-point tools. The program defines the geometry, so a design change is an edit and a re-post, not a new tool. This is why machining dominates prototypes and low-volume work: there is no tooling to amortize.

In practice the two are rarely competitors. Roughly nine out of ten castings that end up in a machine frame get at least one machining operation, because the features that need to seal, spin, or bolt down are not held by the mold alone.

The choice question is therefore not casting or CNC. It is how much of the shape should come from the mold and how much from the cutter.

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  Casting sets the shapeMold or die defines the near-net geometry; cost sits mostly in tooling.
• 2
  CNC sets the accuracyCutter path defines size, flatness, and surface finish; cost sits in cycle time.
• 3
  Most parts use bothCast the bulk, machine the fits, bores, and sealing faces.

Casting handles internal cavities, cooling fins, and organic ribs that a cutter cannot reach. A single aluminum housing with internal oil passages and a 2.5 mm wall is a straightforward die casting. Cut the same housing from 6061 plate and you either split it into parts or spend hours on a long-reach tool that still cannot form the internal channel.

Thin walls are casting's home ground, but only within limits. Die cast aluminum holds 1.5–2.5 mm walls in small parts and 3–4 mm in large ones. Below that the metal freezes before it fills the cavity. Casting also leaves draft on every vertical face so the part can be ejected, and draft changes the geometry you designed.

Machining wins on tight features and on anything that must be flat. A 0.05 mm flatness callout on a gearbox face, a bore held to ±0.005 mm, or a threaded port that must not leak belong on a machine. Cast surfaces also carry porosity, and porosity does not seal.

One more trade-off: casting needs volume to pay for the die. A single-cavity aluminum die typically justifies itself somewhere in the low thousands of parts, depending on part size. Under that, machining a billet is usually cheaper.

Sharp internal corners are another machining problem. A cutter leaves a corner radius equal to its own radius, so a square internal pocket needs either a smaller tool, a deeper reach, or a casting to get there in the first place.

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  Choose casting forThin walls, internal channels, ribs, large housings, high annual volume.
• 2
  Choose machining forPrototypes, tight tolerances, flat sealing faces, low volume, no tooling budget.
• 3
  Avoid casting whenAnnual volume is under a few thousand pieces or the design is still changing.

A die casting as pulled from the die holds roughly ±0.05 mm on small dimensions and loosens as the part grows. Machining holds ±0.005 mm (±0.0002 in) on features we control. That is a tenfold gap, and it is the single number that decides most process calls in a design review.

Surface finish follows the same pattern. Die cast surfaces sit around Ra 1.6–3.2 μm as cast. Machined surfaces land at Ra 0.8–1.6 μm, and fine finishing reaches Ra 0.2–0.8 μm on a turned or milled face. If a bearing seat or an O-ring groove has a finish callout, it gets machined.

Porosity is the hidden variable. Gas entrapment during injection leaves voids that may not show until the part is machined open or pressure-tested. Vacuum-assisted die casting reduces gas porosity, but it does not remove it. For pressure-tight parts, plan a machining allowance and a leak test.

Shrinkage is the other one. Aluminum shrinks about 0.6 percent as it solidifies, and the die is cut oversized to compensate. That compensation is uniform, so thick and thin sections in the same part cool at different rates and pull different amounts. Machining after casting fixes the result, which is why a machining allowance of 0.3–0.8 mm is standard on cast faces that need to be true.

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  As-cast toleranceRoughly ±0.05 mm on small features, looser on long dimensions.
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  Machined tolerance±0.005 mm on controlled features, verified by 100% inspection.
• 3
  Machining allowance0.3–0.8 mm typical on cast faces that must be cut true.

The classic production route is cast the blank, machine the critical features. A cast aluminum housing arrives at the machine with 0.5 mm of stock on the faces that matter and no stock on the faces that do not. The first operation establishes a datum, the following operations hold position from that datum, and the part leaves with a machined bore, a flat gasket face, and tapped holes.

This route is usually the cheapest way to make a mid-volume part with tight features. The mold pays for the overall shape; the machine pays only for the small fraction of the surface that needs accuracy. A part with 90 percent organic geometry and ten critical features is a good fit.

The route also fixes casting defects at the same time. Machining removes the skin where most porosity concentrates, and it opens the part so a leak test can confirm the result. Castings that are fully machined on sealing faces pass pressure tests far more often than as-cast parts.

Where the combination does not pay: low volume, a design still in flux, and parts with no critical features at all. If nothing on the drawing needs better than ±0.1 mm, machine the whole thing from billet or leave it as cast. Adding setup and fixturing to a part that does not need it only adds cost.

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  Stock allowance0.3–0.8 mm on machined faces, more on faces that must clean up fully.
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  Datum strategyMachine one face first, then locate every later operation from it.
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  Skip the mix whenNo feature needs better than ±0.1 mm or the design is still changing.

Die casting is limited by melting point and by how the alloy flows. Aluminum alloys such as ADC12 and A380 dominate, with zinc alloys used for small, thin, dimensionally tight parts. Magnesium AZ91D is castable but needs care because of oxidation. Steel and stainless are not die cast at production scale; they go through investment or sand casting instead.

Machining has no such limit. We cut aluminum 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C, and 17-4PH; steels including 1018, 1045, 4130, 4140, 4340, A36, and tool steel; copper and brass grades such as C101, C110, C36000, and beryllium copper; titanium TA1, TA2, and TC4; Inconel and magnesium; plus engineering plastics like POM, PEEK, PC, and PA.

That range matters when a part has to survive heat or load. A 17-4PH bracket or a Ti-6Al-4V mount cannot be die cast at all in practical terms, so the design goes straight to machining. Conversely, a thin-walled zinc housing at 50,000 pieces a year is not a sensible machining job.

If the alloy is not listed for casting but the geometry needs a mold anyway, the usual answer is investment casting of the intended alloy, followed by CNC finishing on the critical faces. That gives up some of die casting's cycle speed and keeps the material the design calls for.

• 1
  Castable at volumeADC12, A380, zinc alloys, magnesium AZ91D.
• 2
  Machined from solidStainless, tool steel, titanium, Inconel, copper, plastics.
• 3
  Neither clean fitInvestment cast the alloy, then CNC the sealing and bearing faces.