Buyer guide

Efficiency With CNC Aluminium Casting Machining Service

Casting and machining are usually quoted as two separate jobs. When one supplier runs both, the part gets cheaper, straighter and faster to finish. This guide shows engineers and buyers how to judge a CNC aluminium casting machining service before sending an RFQ.

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Near-net-shape blanks±0.005 mmNo MOQISO 9001 / IATF 16949

efficiency with cnc aluminium casting machining service

On this page

1
What matters most
2
Machining from solid vs. casting plus machining
3
Why the split decides your unit cost
4
Where the tight numbers belong
5
A rough casting is not a machined blank
6
Choosing the alloy and the casting route
7
What a supplier needs to quote accurately
8
Five checks before you award the job

Key takeaways

What matters most

Cast first, machine secondA near-net blank leaves 0.5–2 mm of stock. Machining from solid plate turns 50–80% of the material into chips.

Split tolerances by featureCast faces can sit at ±0.5 mm. Only sealing faces, bores and bearing seats need the tight numbers.

Ask where the datum comes fromA casting with no machined datum pad forces the machinist to probe a rough skin. That adds setup time.

Check the quote covers both stepsTooling, heat treatment, machining and inspection should appear as separate lines. One blended price hides risk.

Selection table

Machining from solid vs. casting plus machining

Pick per feature, not per part.

Factor	Machined from solid plate	Casting then CNC	Best fit
Material removed	50–80% becomes chips	10–30% becomes chips	Casting for >2 kg parts
Wall thickness	Any, down to 1 mm	3–5 mm typical minimum	Solid for thin ribs
Internal passages	Difficult, deep drilling only	Cast-in as cores	Casting for manifolds
Tolerance on bores	±0.005 mm reachable	±0.005 mm after machining	Both, after finishing
Setup count	1–2 per face	3–5, datum is rough	Solid for one-offs
Tooling cost	None	Pattern or die up front	Casting above 500 pcs
Lead time, first part	3–5 days	Tooling plus 3–5 days	Solid for urgent prototypes

The verdict

If most of your part is as-cast and only a few faces carry tolerance, casting plus machining wins. If the part is small, thin-walled or needed this week, machine it from solid and skip the tooling.

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How the two steps divide work

Why the split decides your unit cost

A CNC aluminium casting machining service does one thing well: it puts metal only where the part needs it. The foundry forms the bulk geometry, then the machining center cuts the surfaces that carry a tolerance. That division of labor is the whole cost story.

Take a 1.8 kg aluminium housing. Machined from 6061 plate, the blank starts near 6 kg. Most of that 4 kg leaves as chips, and the cutter spends minutes in air before it ever touches the part. Cast as a near-net blank, the same housing arrives at 2.1 kg with 0.8 mm of stock on the machined faces. Cycle time drops, and so does the material invoice.

The saving is not automatic. If the casting arrives with 4 mm of stock, or with a warp that only shows up after heat treatment, the machinist removes the same volume a plate job would. The blank has to be designed for the machining step, not just for the mold.

1
Stock allowance0.5–2 mm per machined face is the working range. Below 0.5 mm, casting variation breaks through.
2
Draft angle1–3° on as-cast walls. Steeper draft costs nothing and releases cleaner.
3
Machining allowance padsAdd 3–5 mm pads where a face will be milled flat. They give the cutter a clean entry.

Tolerance strategy

Where the tight numbers belong

The most common RFQ mistake is a drawing where every dimension carries the same tolerance block. That forces the shop to machine every cast surface, which removes the cost advantage you came for.

Split the drawing. As-cast surfaces can live at ±0.5 mm and Ra 3.2–6.3 μm. Bores, sealing faces, bearing seats and dowel holes get the tight callouts. On our 5-axis centers we hold ±0.005 mm (±0.0002 in) on those features, with Ra 0.8–1.6 μm as a standard machined finish and Ra 0.2–0.8 μm when a seal or a bearing needs it.

Mark which faces stay as-cast. If the drawing does not say, the shop will machine them to be safe. You pay for that, and the part gains nothing.

1
Datum padsMachine two or three pads in the first operation. Every later setup locates from them.
2
Geometric calloutsFlatness and position tolerances drive setup count more than size tolerances do.
3
Porosity limitsState whether a sealedcavity is pressure-tight. It changes the casting process, not the machining.

Fixturing

A rough casting is not a machined blank

Fixturing is where quotes diverge most. A casting has no flat face, no square corner and no reliable hole. The first operation has to create those references before anything else can be cut.

Shops that run castings every week keep soft jaws and dedicated fixtures on the shelf. Shops that mostly machine plate will probe the rough skin, dial it in by hand, and lose an hour per setup. That hour lands in your price and in your lead time.

Ask how the first operation is held. If the answer is a vise and a dial indicator, expect variation across the run. If the answer is a cast-specific fixture with three-point location, the run will repeat.

For large parts, our floor handles up to 4,000 mm in one setup on the long-travel machines, and Ø400 mm rotary tables cover round features on 4-axis work. That matters when a housing has bores on four sides.

1
First opFace and bore the datum pads. Everything downstream inherits their accuracy.
2
Second opLocate on the machined pads, not on the casting skin.
3
Spring passesCast aluminium moves after the first cut. A light finishing pass holds the bore.

Alloy and process

Choosing the alloy and the casting route

Alloy choice follows the process, not the other way round. Gravity and low-pressure casting suit 6061, 6063 and 6082 where you need weldability and good machined finish. High-pressure die casting runs ADC12 and similar grades, which fill thin walls fast but trap more gas.

Trapped gas shows up later as porosity at a machined surface. If your part has a sealing face, say so early. The foundry may switch to vacuum-assisted casting or move the gate so gas sits away from the cut.

For prototypes, a cast urethane or vacuum-cast part can stand in for the aluminium unit during fit checks. It will not carry load and it will not hold ±0.005 mm, but it answers the assembly question before tooling is cut.

1
6061 / 6082General machined parts. Good corrosion resistance, easy to anodize.
2
ADC12Thin-wall die castings at volume. Expect some porosity in thick sections.
3
7075High strength, but rarely cast. Use it when the part is machined from solid.

What to send

What a supplier needs to quote accurately

A quote built on a STEP file alone will be wrong. The shop cannot see which faces are critical, so it assumes all of them are.

Send the 3D model, a 2D drawing with the tolerance split marked, the annual volume and the expected ramp, and the alloy or its service conditions. Note any post-processing: anodizing, electroless nickel, powder coat, laser marking. Anodizing adds 5–15 μm per surface and will shift a press-fit bore if the shop does not plan for it.

Volume changes the answer more than any other input. One prototype and 10,000 parts a year are different processes and different suppliers. Our floor runs both: no minimum order quantity, from a single prototype up to 10,000+ part runs, with quotation and a free DFM analysis back within 12 hours and production able to start within 24 hours.

If the design is still open, ask for the DFM review before the price. Moving a rib or adding a datum pad costs minutes at that stage and real money after the mold exists.

1
Model plus drawingSTEP for geometry, PDF or DXF for tolerances and notes.
2
Volume profilePrototype, pilot and annual volumes separately.
3
Finish specName the finish and the surfaces it covers. Masking decisions follow.

Buyer checklist

Five checks before you award the job

Run these in order. Each one can send the RFQ back for revision.

1
1. Mark the as-cast facesGo through the drawing and label every surface that can stay rough. Target ±0.5 mm and Ra 3.2–6.3 μm on those. If more than 60% of the surface area still carries a tight tolerance, the casting route is not paying off.
2
2. Add datum pads to the modelPlace two or three pads, 3–5 mm thick, where the first operation can face them flat. Put them on a surface the mold can form without a side action. This single change removes probing time from every setup.
3
3. Set stock allowance per faceSpecify 0.5–2 mm on machined faces and 3–5 mm on pads. Anything under 0.5 mm risks a casting variation breaking through the skin and scrapping the part.
4
4. Ask for the setup planRequest the number of operations, the fixture type per operation, and the machine class. Three setups on a 4-axis mill is a different price from one setup on a 5-axis center. Compare like with like.
5
5. Confirm inspection and documentationAsk what is measured, on which features, and what report comes with the shipment. First article, in-process checks and final inspection should be named. We inspect 100% before shipment and supply reports on request.

FAQs

Questions buyers ask before an award

At what volume does casting plus machining beat machining from solid?

Below roughly 500 parts, tooling usually dominates and solid machining wins on total cost. The crossover depends on part size, how much material the casting saves, and how many faces need machining.

For a 1–3 kg housing with a simple mold, the casting route typically pays back between 500 and 1,000 parts. Above that, the per-part material and cycle-time saving compounds.

Can one supplier handle both the casting and the machining?

Yes, and that is usually the better setup. One supplier owns the stock allowance, the datum pads and the tolerance split, so nobody blames the other side when a bore comes in out of position.

If you split the work, you must specify the blank drawing yourself and inspect the incoming castings. That is real work and it needs to be in the schedule.

What tolerance can be held on a machined casting?

On machined features, ±0.005 mm (±0.0002 in) is achievable, with Ra 0.8–1.6 μm as a standard finish and Ra 0.2–0.8 μm where a seal or bearing requires it.

As-cast surfaces are a different class. Plan on ±0.5 mm and Ra 3.2–6.3 μm, and do not put a sealing face on an as-cast surface.

Does anodizing change the fit?

It does. Anodizing builds 5–15 μm per surface depending on the type. A bore that is 20 μm oversize after coating will not hold a press-fit pin.

Tell the shop at quote stage which surfaces are coated and which are masked. It is a one-line note that saves a rework cycle.

How do you handle porosity at a machined face?

State the pressure-tightness or cosmetic requirement on the drawing. The foundry then adjusts gate position, uses vacuum assistance, or changes the process.

If porosity appears anyway, a machined pocket can sometimes be moved a few millimeters to a sounder section. That is a design decision, so it needs the engineer's sign-off.

What certifications should a casting and machining supplier hold?

For general industrial work, ISO 9001:2015 covers the quality system. Automotive programs usually require IATF 16949:2016, and medical work calls for ISO 13485:2016.

If you send drawings and models, add ISO 27001:2022 to the list. It covers how the supplier protects your data. We hold all four, and we sign an NDA on request.