Die Casting Machining – CNC Finish for Cast Parts
Secondary CNC machining of aluminum and zinc die cast parts: drilled, tapped, bored and milled to ±0.02mm exactly where as-cast tolerances fall short. Casting and machining in one supply chain means datum choices, machining allowances and fixture design are decided together, not discovered late.
Free Consultation±0.02mm
Machined Tolerance
Cast + Machine
Process Chain
0.5-1.5mm
Machining Allowance
5-10 Days
Lead Time
Materials
| Grade | Properties | Typical Applications |
|---|---|---|
| ADC12 / A383 (Aluminum Casting Alloy) | The standard aluminum die casting alloy (Al-Si-Cu): excellent fluidity, good machinability, tensile 250-320 MPa. Castings machine cleanly with carbide tooling; silicon content abrasive but manageable with correct insert grades. | Housings, valve bodies, pump bodies, gearbox cases, power tool bodies, connector bodies |
| ALDC1 / A356 (High-Integrity Aluminum) | Lower iron and higher purity casting alloy for better mechanical properties and polishability; heat-treatable (T6). Preferred where castings carry structural load or must be pressure-tight. | Structural castings, leak-critical valve bodies, automotive chassis components |
| Zamak 3 / Zamak 5 (Zinc Alloy) | Excellent castability with very fine detail; tensile 250-280 MPa. Machines exceptionally well – the least abrasive casting material we process, with superb surface finish after cutting. | Precision small parts, connector bodies, decorative hardware, lock components, gear housings |
| ZA-8 / ZA-12 / ZA-27 (Zinc-Aluminum) | Higher-strength zinc alloys reaching 300-420 MPa; ZA-27 approaches bearing-bronze properties. Machinability remains excellent, with attention to tool geometry for higher aluminum content. | Load-bearing bushings, structural small parts, wear components, gear blanks |
| Magnesium AZ91D (Case by Case) | Lightest structural casting alloy (1.81 g/cm³). Machines easily but requires fire-safety protocols and chip handling discipline; quoted case by case. | Housings, structural brackets and aerospace-grade lightweight castings |
| Cast Iron & Brass Castings | We also machine sand- and investment-cast steel and brass components: abrasive but standard work with correct tooling, typically interrupted cuts requiring rigid setups. | Valve bodies, pump impellers, plumbing components, machinery castings |
Process Parameters
| Parameter | Standard Range |
|---|---|
| As-Cast Dimensional Tolerance | IT11-IT13 typical (CT5-CT7 precision grade achievable) |
| Machined Dimensional Tolerance | ±0.02mm standard; ±0.01mm on critical features |
| Machining Allowance (Per Face) | 0.5-1.5mm depending on casting size and flatness |
| Positional Accuracy (Machined Holes) | ±0.05mm with fixtures; ±0.02mm with CNC positioning |
| Surface Roughness (Machined) | Ra 1.6μm – 3.2μm; Ra ≤0.8μm with finishing pass |
| Flatness (Machined Sealing Faces) | ≤0.05mm over 100mm |
| Thread Capability (Post-Cast) | M2 – M30 tapped or thread-milled |
| Maximum Part Envelope | 500mm x 400mm x 300mm depending on casting |
| Typical Machining Content | Sealing faces, bolt patterns, bearing bores, ports, mounting faces |
| Production MOQ | 500 pcs cast + machined; machining-only from 50 pcs |
Surface Finishes
As-Cast (Shot Blasted)
Standard shot blasting removes casting skin and gives a uniform grey matte surface. The baseline finish for internal castings and any surface that will be further machined or coated.
Machined Faces (As-Cut)
Surfaces finished by CNC cutting at Ra 1.6-3.2μm – the functional finish for sealing faces, mounting interfaces and bearing bores. Tool marks are directional and acceptable unless cosmetic.
Vibration / Barrel Finishing
Mass finishing of small castings to deburr, round edges and produce a consistent satin appearance – the standard route for high-quantity small parts before plating.
Powder Coating & E-Coat
Polyester powder or electrophoretic coating for corrosion protection and color; e-coat reaches internal surfaces better and is common for automotive castings. Applied after machining with masked critical faces.
Plating (Zinc / Nickel / Chrome)
Electroplating of zinc castings for decorative and corrosion protection – chrome plating for wear and appearance on hardware and lock components. Requires excellent surface preparation.
Chromating & Passivation
Chemical conversion coatings for aluminum and zinc castings: chromate for corrosion resistance and paint adhesion base, passivation for stainless inserts and machined stainless features.
Design for Manufacturing (DFM)
- Specify machining allowance (0.5-1.5mm per machined face) in the casting drawing – without it the machining stock may cut into porosity or fail to clean up the cast surface
- Choose machining datums on the casting drawing: machined features are positioned from cast surfaces, and which surface is the datum decides whether your bolt pattern lines up with your assembly
- Design cast features so the machined interface stays away from porosity-prone zones: thick-to-thin transitions, last-fill corners and gate areas hide internal shrinkage
- Prefer flat, accessible machining zones: faces reachable from one or two tool directions machine faster and hold flatness better than multi-axis contoured interfaces
- Put critical dimensions on machined features and general dimensions on as-cast geometry – mixing the two wastes money, because casting tolerance is looser than machining tolerance
- Design bolt circles and port positions with fixture access in mind: clamps must hold the casting without covering the features being cut
- Add spot faces or bosses where threads enter cast walls: threads cut directly into thin or unsupported cast sections strip under assembly torque
- Avoid machining through cast skins into internal cavities unless sealing is intended – breakthrough points need defined geometry, not "wherever the tool exits"
- Consider assembly direction when marking machined holes: a hole accessible from the casting back side may be impossible to machine without a second fixture
- Send the casting drawing and the machining drawing (or one combined drawing) with critical features marked; our free DFM review covers allowance, datums, porosity risk and fixture strategy
Tooling & Capacity
| Type | Lead Time | Suitable For |
|---|---|---|
| Die Casting Tool (New or Existing) | 4-6 weeks new tool; machining-ready immediately with existing tool | Producing the casting with designed-in machining allowances and clean, accessible machining zones |
| Machining Fixtures | 1-2 weeks | Repeatable positioning of cast parts for drilling, tapping and milling; soft jaws and dedicated fixtures per part family |
| CNC Milling (3-axis / 4-axis) | 5-10 days per batch | Sealing faces, bolt patterns, ports, pockets, mounting faces and multi-face machining of cast housings |
| CNC Turning / Boring | 5-10 days per batch | Bearing bores, cylindrical ports, internal threads and concentric features on cast parts with rotational interfaces |
Quality Assurance
- Incoming casting inspection: dimensional spot-check, visual check for porosity, cold shuts, flash and surface defects before machining
- Machining allowance verification on first parts: confirms the casting cleans up without breakthrough or excessive stock removal
- First Article Inspection with CMM on machined features: position, diameter, flatness and thread verification with full report
- Flatness check on sealing faces with surface plate or CMM – the functional interface most often rejected in cast-plus-machined assemblies
- Thread inspection with go/no-go gauges on all post-cast tapped features
- Porosity surveillance: machined faces inspected for exposed porosity at sealing surfaces; leak-tested where pressure integrity is specified
- In-process dimensional checks on batch production, with process adjustments recorded
- Visual inspection of machined surfaces for chatter, dwell marks and burrs against the approved boundary sample
- Final dimensional report with measured values shipped with every batch; casting lot and machining records retained for traceability
Applications
- Automotive: transmission housings, valve covers, sensor bodies, brackets, wiper motor housings, charging components
- Power tools: gearbox housings, motor bodies, chuck bodies, handle frames
- Hydraulics and pneumatics: valve bodies, manifold blocks, pump housings, cylinder caps
- Electronics: connector bodies, heat sink frames, shielding housings, enclosure frames
- Appliance: pump bodies, motor end caps, gear cases, control housings
- Industrial equipment: bearing housings, gear cases, pulley bodies, pump components
- Hardware and lock components: precision small zinc castings with machined interfaces
- Lighting: lamp housings, heat sink bodies, reflector frames with machined mounting faces
Why Choose FHY Precision
Cast and machine in one chain: datums, allowances and fixture strategy are designed with the casting, not retrofitted after the first batch fails to clean up
DFM across both processes: we review casting and machining drawings together, so porosity-prone zones, gate locations and machining directions are compatible by design
Machining where tolerance actually matters: as-cast geometry stays loose and cheap, while sealing faces, bores and bolt patterns are machined to ±0.02mm
Fixture engineering in-house: dedicated fixtures and soft jaws designed per part family, so batch-to-batch positioning stays consistent at volume
Porosity-aware machining: machined faces are inspected for exposed porosity; pressure-critical parts are leak-tested before shipment
One quality system: casting inspection, machining FAI and final reports sit in the same traceability chain – one supplier to hold accountable
Machining-only service too: send us your castings from elsewhere and we will machine them with the same controls (from 50 pieces)
Honest feasibility: when a feature is better cast to near-net shape instead of machined – or vice versa – we say so before tooling, because it changes your cost more than any negotiation
FAQ
1. Why do die cast parts need machining at all?
Die casting holds general shape well but not fine tolerance: as-cast features typically run IT11-IT13, which is too loose for sealing faces, bearing bores, bolt patterns and threaded interfaces. Machining adds precision exactly where assembly demands it, while the casting provides the complex shape cheaply. The art is choosing which features to cast and which to machine – we design that split with you.
2. Can you machine castings we supply from another foundry?
Yes – machining-only service starts at 50 pieces. Send castings plus drawings; we inspect incoming condition (porosity, flash, dimensional spread) before machining and flag anything that will affect clean-up or yield. If your castings arrive out of allowance, we will tell you before cutting rather than after.
3. How much machining allowance should the casting have?
0.5-1.5mm per machined face is the practical range: enough to guarantee clean-up of the cast surface and any heat-treat distortion, not so much that cycle time and material waste climb. Large flat faces need more allowance; small bores need less. We specify allowance per face on the casting drawing during DFM.
4. What if machining exposes porosity in the casting?
Porosity in castings concentrates in last-fill corners, thick-to-thin transitions and gate areas. Our DFM review keeps machined sealing faces away from those zones where geometry allows. When porosity is exposed, the part is rejected – we do not fill and ship. For pressure-critical parts we leak-test, and we will show you where the risk zones are on the drawing before tooling.
5. What tolerance can you achieve on machined cast features?
±0.02mm standard on diameters, positions and lengths; ±0.01mm on critical features with CNC positioning. Flatness on sealing faces holds ≤0.05mm over 100mm. Positional accuracy on drilled holes is ±0.05mm with fixtures, ±0.02mm with full CNC positioning. As-cast surfaces stay at casting tolerance – we machine only what the drawing marks critical.
6. Do you cast and machine, or only one of the two?
Both, in one supply chain. We die cast aluminum (ADC12, ALDC, A356) and zinc (Zamak, ZA alloys) in-house, then machine the castings on our CNC equipment under the same quality system. We also machine castings supplied by customers. Whichever route you choose, one supplier owns the whole chain.
7. How do you choose which surfaces to machine?
The rule: surfaces that interface with other parts (sealing faces, bores, bolt patterns, mounting faces, threaded holes) get machined; surfaces that only exist for shape and volume stay as-cast. Every machined face costs money in allowance and cycle time, so the drawing should mark only functional interfaces. Our DFM review walks your drawing face by face and proposes the split.
8. What is the typical lead time for cast + machined parts?
With existing tooling: castings in 1-2 weeks, machining in 5-10 days per batch. New die casting tools: 4-6 weeks including trial casting, then machining. Heat-treated aluminum castings (T6) add about a week before machining. All dates confirmed in writing with the quotation.
9. What is the minimum order quantity?
Cast plus machined production typically starts at 500 pieces – die casting tooling and setup drive that floor. Machining-only work (your castings, our machines) starts at 50 pieces. Pilot programs sometimes run 100-200 pieces with existing tooling.
10. Can you leak-test machined castings?
Yes, where pressure integrity is specified – valve bodies, pump housings and manifold blocks are routinely leak-tested after machining. Test pressure and acceptance criteria come from your specification; we record results per piece or per sample lot as agreed, and any leaker is sectioned to confirm the cause before the batch continues.
11. Do zinc and aluminum castings machine differently?
Zinc castings machine more easily – less abrasive, finer surface finish, longer tool life. Aluminum castings contain silicon that abrades carbide, so insert grade and coating selection matter more; cutting parameters are adjusted to control built-up edge and surface tear. Both are standard work for us, with tooling libraries established per alloy.
12. Can you handle heat-treated castings?
Yes. Aluminum castings are commonly T6 heat-treated for strength, and we machine after treatment – which is why allowance must cover heat-treat distortion. Heat treatment adds about a week to the schedule. We verify post-treatment dimensions on first parts before running the batch.
13. How much machining allowance should a die casting have?
A practical guide: 0.3-0.5mm stock per side on critical sealing faces, 0.3-0.6mm on diameter for bored holes and bearing seats, 0.2-0.4mm on non-critical flats, 0.3-0.5mm on boss tops and pads. The honest answer is that there is no universal number – the right allowance depends on which faces get machined, how the part distorts after ejection, and the clamping strategy. Too much stock means long cycles, tool wear and heat; too little means the cutter hits as-cast porosity. We agree allowance per face at quotation, in writing.
14. How do you avoid exposing porosity when machining?
A die casting has a dense outer skin, typically about 0.5mm thick, that solidifies quickly against the die wall – the interior can carry gas porosity or shrinkage voids. Machining through that skin can expose pores that compromise sealing, finish and structure. Prevention is upstream: keep sealing and high-precision machined faces away from porosity-prone zones identified by fill and venting review, design gate and overflow placement with those faces in mind, and use mold flow analysis where the part is critical. A machining problem found after T1 is usually a casting design problem found too late.
15. What datum and fixturing strategy works for cast parts?
Start at the die. Datum surfaces are the reference the casting seats on in the fixture, and they must be agreed during tooling design – not after T1 samples reveal that the only flat surfaces are cosmetic. We use functional datums (mounting faces, gasket faces) as primary references and design the die to produce them flat enough and large enough for repeatable clamping. On the machining side: 3-2-1 location wherever possible, soft jaws or shaped nests that follow the casting form, clamp near supports, and rough-and-finish passes with clamp loads landing on solid material to control spring-back.
16. Which features are typically machined after casting?
Features whose dimensional or surface requirements exceed as-cast capability: bearing and dowel bores, sealing faces and O-ring grooves, threaded holes, mounting pads and cover faces with flatness or position requirements. Features that meet the drawing as-cast stay unmachined – machining everything wastes money. On your drawing, we ask you to clearly identify machined surfaces, their tolerances, roughness, minimum remaining wall thickness, cosmetic surfaces, permitted tool marks and deburring requirements, so the quotation covers the finished condition you actually need.