What Is CNC Aluminium Casting Machining?

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  Key takeaways
• 2
  How CNC aluminium casting machining works
• 3
  What the process can and cannot hold
• 4
  Alloy choice and what it does to the cut
• 5
  Design rules that keep cast parts machinable
• 6
  Fixtures, clamping, and inspection
• 7
  Cast blank versus solid billet: which route fits

CNC aluminium casting machining is a two-stage route. First, molten aluminium is poured into a mold to create a near-net shape. Then that blank goes onto a CNC mill or lathe, where cutters remove 0.3 to 3 mm of stock to reach the drawing. The casting supplies the bulk geometry. The CNC work supplies the accuracy.

The mold itself comes in two main forms. Die casting pushes metal into a hardened steel die under pressure, which suits ADC12 and A380 and gives cycle times measured in seconds. Sand or gravity casting pours into a bonded sand mold or a permanent steel mold, cooling slower and leaving a coarser skin. Which route you pick is decided by quantity and wall thickness, not by preference.

After the pour, the part is trimmed, shot-blasted, and sometimes heat-treated. T6 treatment on a cast alloy raises hardness but also moves the part. A cast-then-aged blank can shift 0.2 to 0.5 mm across a 300 mm span. That shift is why finish machining happens after heat treatment, never before.

Only then does the CNC stage start. Roughing clears the skin at 2 to 4 mm depth of cut. Semi-finishing brings the part to 0.2 to 0.5 mm of nominal. Finishing takes the last 0.1 to 0.3 mm and sets the surface. If the drawing calls for Ra 0.8–1.6 μm on a sealing face, that face gets its own finishing pass with a fresh insert.

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  Near-net shapeCasting leaves 0.5–3 mm of stock on machined faces.
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  Stress reliefHeat treatment goes before the finishing cut, not after.
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  Skin removalThe hard as-cast skin must be cut, not rubbed.
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  Single setup where possibleFewer re-clamps means fewer datum errors.

Tolerances depend on how many setups the part needs. A face machined in one setup on a 3-axis machine can hold ±0.005 mm with a stable fixture. The same feature split across two setups drifts to ±0.025 mm or worse, because the second setup inherits the first setup's error. If your drawing stacks tight tolerances across two opposite faces, expect to pay for a 5-axis setup or a custom fixture.

Surface finish follows the same logic. A cast skin sits around Ra 6.3–12.5 μm and looks mottled. A machined face at Ra 1.6–3.2 μm is the normal as-machined result. To reach Ra 0.8–1.6 μm you slow the feed and use a wiper insert. Below Ra 0.2–0.8 μm the part usually needs lapping or polishing after machining, not a slower cut.

Porosity is the limit that no machining strategy fixes. Die-cast ADC12 traps gas, and those voids sit anywhere from the skin down to the core. You may cut 0.5 mm and see nothing, then cut another 0.5 mm on the same face and open a hole 2 mm across. Sealing surfaces, pressure-tight cavities, and thin optical walls are the features that fail first.

Wall thickness matters too. Die casting holds 1.5 to 2 mm walls reliably, and 1 mm only with good flow design. Thin cast walls flex when the vise closes and chatter when the cutter passes. A 1.2 mm rib on a 200 mm part can push a 0.5 mm flatness call out of reach, even though the same rib machined from billet would hold it.

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  One setup, one datumTight tolerance calls belong on the same face group.
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  Cast skinRa 6.3–12.5 μm and non-uniform; always machine it off.
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  PorosityCannot be predicted from the outside of the part.
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  Thin wallsBelow 1.5 mm, clamping and cutting loads dominate.

Cast alloys and wrought alloys behave differently at the spindle. ADC12 and A380 are die-casting alloys: high silicon, good flow, low ductility. They machine fast but chip abrasively, and the silicon particles dull edges quicker than 6061 does. Expect shorter insert life and slightly worse surface finish on the same parameters.

Wrought grades like 6061-T6, 6082, and 7075 are not cast, but they appear in the same shops and often in the same assembly. A common mistake is to specify a cast alloy for a bracket that will be machined from billet anyway. If the part has no draft angles, no ribs deeper than 3× the wall, and no volume above a few hundred pieces, billet is the cheaper answer.

Magnesium AZ31B and AZ91D sit in a separate category. They cast and machine well and weigh about a third less than aluminium, but chips burn. That means dedicated tooling, chip management, and a shop that already runs magnesium. Not every supplier will quote it.

Heat treatment changes the cut too. A T6 cast blank is 20 to 40% harder than the as-cast state, so roughing feed drops and tool wear climbs. Plan the process window around the treated condition, since that is the state the finishing cut sees.

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  ADC12 / A380Die-cast grades; abrasive chips, shorter tool life.
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  6061-T6 / 6082Wrought grades for billet parts; clean chips, good finish.
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  7075Higher strength, more residual stress after heavy cuts.
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  AZ31B / AZ91DMagnesium; light but needs fire-safe chip handling.

Add machining stock on every face that carries a tolerance. Die casting holds roughly ±0.1 mm on a 100 mm dimension, which is nowhere near a ±0.005 mm call. A 0.8 to 1.5 mm allowance on machined faces gives the cutter something to remove and hides casting variation at the same time.

Avoid machining a tight feature into a cast wall that sits next to a thick boss. The mass difference drives uneven cooling and the wall pulls. If a boss must sit close to a thin wall, add a fillet and a gradual transition so heat leaves the section evenly.

Put datum targets on the casting itself. Three small pads, machined first and used for every later setup, beat clamping on a raw as-cast surface every time. Once the pads are cut, every subsequent operation references the same geometry.

Keep deep pockets and long bores out of the cast design unless they are cored. A 40 mm deep pocket cut into a cast pocket means a long reach tool and chatter. Core the pocket in the mold to within 1 mm of final size and let the CNC take the last pass.

Call out which faces are cosmetic. Casting leaves flow lines and ejector pin marks. If a face is visible on the finished product, the drawing should say so, so the shop can plan a finishing pass or a bead blast.

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  Stock allowance0.8–1.5 mm on machined faces.
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  Datum padsMachine three pads first; reference them everywhere.
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  Cored pocketsCore to 1 mm of final size, then finish cut.
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  Cosmetic facesMark them on the drawing, not in an email.

A cast blank is not a machined blank. The as-cast surface is uneven, so a vise closes on high points and the part rocks. Soft jaws machined to the actual casting profile, or a dedicated fixture with three-point support, remove that problem. We cut soft jaws from the first-off casting of each lot rather than from the CAD model.

Clamping force is the next trap. Cast aluminium yields at lower stress than wrought 6061. Over-tightening a thin section bows it, the cutter removes material from a bowed part, and the part springs back after unclamping. Light clamping with support underneath beats heavy clamping on a bridge.

Inspection should follow the same plan as the machining. First-article inspection checks the datum pads and the critical faces. In-process checks catch tool wear on long runs. Final inspection covers the drawing before the part leaves the shop. Reports are available on request.

For pressure-tight parts, add a leak test after machining. Porosity that survived the finishing cut shows up here, and it is better to find it at the bench than at the customer's assembly line.

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  Soft jaws from real castingsCut them from a first-off part, not the model.
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  Light clamp, full supportCast sections bow under heavy vise pressure.
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  First articleCheck datums before running the batch.
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  Leak testFor any part that must hold pressure.