CNC Machining Castings: How Datum and Stock Drive Accuracy

• 1
  Key takeaways
• 2
  What CNC Machining Castings Actually Means
• 3
  Why the As-Cast Surface Is a Poor Datum
• 4
  Stock Allowance: The Setup Budget You Spend Once
• 5
  Rough, Stress-Relieve, Finish: The Sequence That Holds Tolerance
• 6
  Where Casting Plus Machining Stops Making Sense
• 7
  Six Steps We Run on Casting Jobs
• 8
  Casting vs Machining from Solid: Choosing the Route

CNC machining castings is the practice of taking a cast or die-cast blank and cutting the functional geometry into it. The casting supplies the near-net shape: ribs, bosses, internal passages, an outer skin that already looks like the finished part. The machining center supplies what a mold cannot: flatness, bore roundness, thread pitch, hole position and surface finish.

The two processes fail in different ways. A mold fails by wearing, flashing or shifting a parting line. A cutter fails by deflecting, chipping or following a bad datum. When you combine them, the casting error becomes an input to the machining error. That is the whole engineering problem in one sentence.

Readers of this page are usually engineers who already have a casting drawing and now need to decide how much material to leave for machining, which faces to cut first, and whether the casting route is cheaper than cutting from solid. Every number below comes from work we do daily on aluminum, iron and steel cast blanks.

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  Cast blankNear-net shape, skin hardness varies, tolerances typically looser than ±0.5 mm.
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  Machined featuresBores, faces, threads, sealing grooves, dowel holes; held to ±0.005 mm on our machines.
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  Bridge between themDatum choice, stock allowance and clamping strategy.

A machined face is flat to a few microns. An as-cast face is not, even on a good sand casting. It carries draft angle, parting-line mismatch, sand inclusions near the skin and a chill zone that is harder than the metal below. If you clamp on that face and measure from it, you are measuring the mold, not the part.

For a first operation, we usually locate on three cast pads that the foundry holds to a tighter tolerance, or on a machined area if the customer supplies one. When nothing is reliable, we machine a temporary datum: skim a face, then re-clamp on the skimmed surface and cut everything else from it. That costs one extra setup and removes a whole class of scrap.

On die castings the same logic applies with different numbers. Die-cast skin is dense and dimensionally repeatable, but it contains porosity just under the surface. A face that looks ready to use may open into a gas pocket once you take 0.3 mm off it. We plan the first cut deep enough to clear that skin, typically 0.4–0.8 mm on aluminum die castings.

The practical test is simple. If a face is defined on the drawing as cast, treat it as a reference only after you have measured it on the actual blank. Never trust a cast face to set the position of a precision bore on the far side of the part.

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  Good datum candidatesCast locating pads, a previously machined face, a bored hole held to ±0.1 mm.
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  Bad datum candidatesParting line, ejector-pin area, gate remnant, any raw draft surface.
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  When in doubtSkim, re-clamp, then cut the tight features.

Stock allowance is the layer of metal you leave between the cast surface and the finished surface. It is a budget: too little and the cutter misses a low spot or hits hard skin at full depth; too much and you spend cycle time removing material that only exists to be thrown away. Cutting it away also releases residual stress, which can move the part after the last pass.

For sand and investment castings, 2–4 mm per face is a normal starting point on medium parts. Small investment castings of a few hundred grams can run at 1–1.5 mm. Die castings sit lower: 0.5–1.0 mm per machined face is common because the die itself is accurate and repeatable. These are starting figures, not rules. A long slender casting with a warped parting line needs more room on the warped face and less on the others.

The number that matters is not the allowance on the drawing. It is the allowance on the actual blank you clamp. If the foundry ships a batch with 1.8 mm where the drawing says 3 mm, a pre-planned finishing pass will not clean up. We measure incoming blanks on critical faces when the geometry is thin or the supplier is new.

One more cost hides here. Extra stock means extra passes, and extra passes mean more clamping cycles on a part that is already thin. On a 3 mm wall casting, adding 1 mm of stock on each side can double the number of times the part is loaded and unloaded. That is where distortion is born.

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  Sand / investment1.5–4 mm per face, depending on part size and foundry capability.
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  Die casting0.5–1.0 mm per machined face; clear the 0.4–0.8 mm skin first.
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  Thin wallsKeep allowance on the low side and support the wall during cutting.

A casting arrives with residual stress locked into it from cooling. Machining removes material from one side, unbalances that stress, and the part bends toward the cut. On a gray iron housing, a bore that measured round in the rough state can go oval by 0.03 mm after the finish pass. The fix is not a better cutter. It is a sequence.

We rough with 0.5–1.0 mm of finishing stock left on tight features, then let the part rest or run a stress-relief cycle, then finish. On aluminum and thin-wall parts the rest can be short. On iron and steel castings we plan it into the schedule. The order of operations matters as much as the cutting parameters.

Clamping pressure is the second half of the story. A casting that is 3 mm thick at the wall will deflect under a vise long before the cutter touches it. We use soft jaws machined to the part contour, low-pressure clamping and support under the wall. For thin housings, a fixture plate with dedicated pads often beats a vise on both accuracy and cycle time.

Thermal growth is the third factor and the easiest to ignore. A 300 mm aluminum casting grows about 0.07 mm over a 10 °C shop swing. If the shop warms up between the rough and finish operations, the dimensions move with it. We finish tight features in the same thermal window as the measurement that signs them off.

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  RoughLeave 0.5–1.0 mm on tight features; take the cast skin in one pass.
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  Rest or relieveLet the part settle, or run a controlled stress-relief cycle.
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  FinishCut tight features after clamping forces are stable and the shop is at temperature.

Castings win when the geometry has internal passages, deep ribs or a shape that would need many hours of milling from billet. They lose when the part is small, simple and needed in low volume. If a bracket can be cut from a 6061 plate in 40 minutes, a pattern and a foundry lead time will not pay back.

Porosity is the hard limit. A casting can pass a visual check and still have a gas pocket 2 mm under a sealing face. Once you machine into it, the part leaks or the thread pulls out. For pressure-tight or vacuum-tight features, we ask the foundry for a higher grade of casting or we specify a machined insert.

Tolerance stacking is the other boundary. The casting holds the shape; the machining holds the accuracy. If a drawing asks for ±0.05 mm between two features that both sit on cast surfaces, the casting has to be good enough to support that, or both features must be machined in the same setup. There is no third option.

Finally, consider the material. Aluminum castings machine fast and hold finish well. Gray iron is stable and dampens vibration, which is why machine tool bases are still cast. Ductile iron and cast steel can be machined, but hard skin and inclusions shorten tool life, so we plan inserts and speeds accordingly.

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  Choose castingComplex internal geometry, housings, bases, runs above roughly 50 parts.
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  Choose solid stockSimple shapes, prototypes, tight-tolerance parts in low volume.
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  Walk away from castingPressure-tight thin sections with no room for a machined insert.