Advantages of CNC Aluminium Casting Machining Service
A cast aluminium part rarely comes off the die at its final tolerance. This page explains what CNC machining adds after casting, which tolerances and finishes are realistic, and when a cast-then-machine route beats machining from solid bar. Written for design engineers and sourcing teams comparing processes.
Cast first, machine second
Casting gives the near-net shape. CNC machining gives the dimensions the drawing actually calls for.
What the machining step actually fixes
A die-cast or sand-cast aluminium part starts with as-cast surfaces and as-cast tolerances. Those tolerances are loose, and they move with die wear, cooling rate and gate placement. Machining brings the part back to nominal where it matters, and it does so on surfaces the casting process cannot control. The casting supplies the bulk shape; the cutter supplies the fit.
Three things change once the part goes on a machine. Datum faces become flat and repeatable, so downstream fixtures and assemblies line up. Bores and bores-to-face relationships hold position instead of drifting. Sealing and mating surfaces reach a finish that gaskets and O-rings can actually seat against.
The advantages of CNC aluminium casting show up most clearly on parts with a mix of free-form outer geometry and tight internal features. The casting handles the outer curve, the cutter handles the bore, the thread and the face. Neither process alone gets there cleanly.
- 1Datum controlMachined faces give the next operation something true to hold.
- 2Feature positionHoles, slots and bosses hold location relative to each other.
- 3Sealing surfacesGasket and O-ring seats reach a finish the casting cannot produce.
Tolerances you can hold on cast aluminium
As-cast tolerances on aluminium die casting typically sit around ±0.1 mm on small features and loosen as the part grows. That is fine for a housing outline. It is not fine for a bearing bore or a pilot diameter. After machining, GreatLight holds ±0.005 mm (±0.0002 in) on critical features, provided the casting leaves enough stock and the setup is stable.
Stock allowance is the number to agree on early. Too little stock and the cutter skims a cold shut or porosity that was hiding under the skin. Too much and cycle time climbs for no reason. A common allowance on machined faces runs 0.3–0.8 mm on small parts and up to 1.5 mm on larger castings. The right figure depends on draft angle, parting line position and how much distortion the part sees during cooling.
Porosity is the other variable. A cast skin can look solid and still hide gas pockets just below the surface. When a bore breaks into porosity, the part fails pressure or leak testing no matter how good the dimensions are. That is why we check raw material and monitor in process rather than trusting the last gate only.
As-cast versus machined: what changes
Typical values for aluminium castings that receive a CNC finishing operation.
| Property | As-cast | After CNC machining |
|---|---|---|
| Dimensional tolerance | Around ±0.1 mm, looser on large parts | ±0.005 mm on critical features |
| Surface finish | Ra 3.2–12.5 μm | Ra 0.8–1.6 μm standard; Ra 0.2–0.8 μm fine |
| Datum faces | Draft and parting line present | Flat, repeatable, ready for fixturing |
| Hole and bore position | Drifts with die wear | Held to drawing across the run |
| Threads | Usually cast or formed only | Cut or rolled to class |
| Sealing surfaces | Rough, gasket may not seat | Machined flat, gasket seats reliably |
| Wall thickness | Set by die design | Unchanged; only skin removed |
Complex shapes the casting cannot finish
Casting is good at sweeping curves, ribs and varying wall sections. It struggles with sharp internal corners, deep cross-holes and any feature that needs a true radius or a controlled angle. Those are exactly the features engineers keep adding to real parts.
A 5-axis machining center reaches faces a 3-axis setup cannot, and it does so without re-fixturing the part four times. GreatLight runs 16 simultaneous 5-axis centers alongside 12 four-axis mills and 27 three-axis machines. The choice is not about prestige; it is about how many setups the part needs and how much positional error each setup adds.
Undercuts, compound angles and ports that meet at odd angles are common reasons a cast part ends up on a 5-axis machine. If the feature can be reached from one direction, a 3-axis cut is faster and cheaper. Save the 5-axis time for the parts that genuinely need it.
- 1Compound anglesPorts and bosses that meet at non-orthogonal angles.
- 2Deep cross-holesReached from one setup instead of three.
- 3Thin ribsMachined without the chatter a long 3-axis tool would cause.
Which aluminium grades suit which route
Casting alloys and wrought alloys behave differently under the cutter. ADC12 is the workhorse die-casting grade: it flows well, fills thin walls and machines cleanly, but it is not heat-treatable to high strength. A380 and A360 sit in the same family. They are the right pick for housings, brackets and covers where stiffness and cost matter more than peak strength.
When the part needs strength, the route changes. A 6061-T6 or 7075 billet gives higher mechanical properties than any common casting alloy, and it machines to a better finish. GreatLight stocks 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12. The trade-off is real: billet costs more per part at volume, and it wastes material on parts with a lot of internal void.
A useful rule: if the part is mostly solid and needs high strength, machine from billet. If the part is a shell with ribs and a few precision features, cast it and machine the features. Hybrid thinking beats picking one process for the whole plant.
Where the cost actually goes
Casting spreads its cost across the die. A die is expensive once, then cheap per part. Machining is the opposite: cheap to set up, and the cost scales with cycle time on every unit. That is why the cast-then-machine route wins at volume and loses at one-off.
Volume is not the only lever. Wall thickness, draft angle and how many faces need machining all move the number. A part with three machined faces is a different job from one with twelve. When we quote, the DFM analysis points at which features are driving cycle time and whether a small design change removes a setup.
GreatLight runs no minimum order quantity, from a single prototype to 10,000+ part runs. For prototypes, machining from billet is often faster than waiting on a die. For production, the casting pays for itself and the machining step holds the tolerances. The crossover point depends on part size, alloy and how many features need to be true.
Inspection and traceability on cast parts
A cast part can pass a dimensional check and still fail in service because of internal porosity. Dimensional inspection alone is not enough. GreatLight checks raw material on arrival, monitors during machining and runs a final inspection before shipment. Reports are available on request.
GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The first two cover general and automotive quality systems, the third covers medical device work, and the fourth covers information security for customer files. Uploads are secure and confidential, and an NDA is available on request.
GreatLight operates three wholly-owned plants across 7,600 m² with 150 technicians, based in Dongguan with a Singapore factory. The qualification rate runs at 99.99%, and historical late-delivery probability sits below 2%. Those numbers matter when a cast part feeds a line that cannot wait.
Common questions
How much stock should I leave on a cast surface for machining?
On small parts, 0.3–0.8 mm is a workable starting point. Larger castings often need up to 1.5 mm because distortion grows with size.
The exact figure depends on draft angle, parting line position and cooling behavior. We confirm it during DFM analysis before cutting.
Can you machine a casting that a different supplier produced?
Yes, if the casting meets the agreed stock allowance and the material is sound. We inspect incoming parts and flag porosity or cold shuts before machining.
Send the drawing and we will tell you which faces need stock and which are already close enough.
What surface finish can I expect on machined cast aluminium?
Ra 0.8–1.6 μm is standard for machined faces, and Ra 0.2–0.8 μm is available where a sealing or bearing surface needs it.
As-machined at Ra 1.6–3.2 μm is fine for non-critical faces and costs less cycle time.
Does the casting alloy affect how well it machines?
It does. ADC12 and A380 machine cleanly and hold a good finish. High-silicon grades wear tools faster and can leave a duller surface.
If the part needs both high strength and a fine finish, 6061-T6 or 7075 from billet is usually the better call.
When should I skip casting and machine from solid?
Low volume, high strength, or a part with very few internal voids all point to billet. Prototypes almost always fall here.
Once volume climbs and the part is a shell with ribs, casting plus a machining pass usually wins on cost.
How do you handle porosity that shows up during machining?
We monitor in process and pull parts that break into gas pockets on critical features. A leak or pressure test catches what a caliper cannot.
If porosity is consistent, the fix is usually on the casting side: gate placement, cooling or a change in wall section.
Send the casting drawing and we will review the machining stock
Quotation and free DFM analysis within 12 hours. No minimum order quantity, from one prototype to 10,000+ part runs.
12-hour quote100% inspectionNDA on request±0.005 mm
