CNC Machining of Die-Cast Parts: Finishing Gigacast EV Body Castings

A large integrated die casting is not a finished body part when it leaves the mold. The casting captures the overall shape, but the mating faces, locating holes, and functional holes that decide whether the rest of the body bolts up correctly still depend on CNC machining of die-cast parts to reach assembly-grade tolerance. As gigacasting consolidates many stamped panels into single aluminum structures, this machining step quietly becomes the gatekeeper of final precision. This guide focuses on that step: where integrated die casting is applied on an EV body, what the castings are made of, and why CNC machining — not the casting alone — sets the tolerances that matter for assembly.

For a foundational explainer of the process itself, see our companion article, What Is Gigacasting? How Integrated Die Casting Changes EV Body Manufacturing. Here we go a layer deeper into the machining and tolerance side.

Table of Contents

• Integrated die casting in brief
• Where die-cast body parts need machining
• Material and surface requirements before machining
• Dimensional tolerance and CNC machining of die-cast parts
• The outlook for machining large castings
• Frequently asked questions
• Conclusion

Integrated die casting in brief

Integrated die casting — popularly called gigacasting — is a high-pressure die casting (HPDC) process that forms a large, single-piece structural aluminum part in one shot. Molten aluminum is injected through one or more gates into a shot sleeve, forced under high pressure to fill the mold cavity, then held under pressure while it cools. One casting replaces an assembly of many stamped panels. Two enablers made it practical at body scale: large-volume lightweight aluminum from electrification, and immersion-free (heat-treat-free) alloys that let complex castings reach the needed mechanical properties without full-heat-treatment distortion. Industry sources describe HPDC machines large enough to cast structural parts of 100 kg or more.

That single-piece outcome is exactly what creates the machining requirement. Consolidating parts replaces an assembly tolerance stack-up with a single-part tolerance — but the part only reaches that accuracy after its critical features are machined. The rest of this article concentrates on that finishing work; the process fundamentals and the full advantage-versus-trade-off comparison are covered in the companion explainer linked above.

flowchart LR
    A[Molten aluminum] --> B[Inject into shot sleeve]
    B --> C[High-pressure mold fill]
    C --> D[Hold pressure and cool]
    D --> E[Demold casting]
    E --> F[CNC machine faces and holes]
    F --> G[Assembly-ready body part]

Where die-cast body parts need machining

Integrated die casting earns its advantage in body zones that are structurally complex, carry high loads, and share well across platforms — and those same zones concentrate the machined interfaces. The upper body is generally not a fit: lower load requirements and styling-driven variation keep commonality low. The lower body is the candidate, split into the front structure (engine-bay) zone, the rocker zone, and the rear structure zone. Rocker sections have simple cross-sections and must adapt to different wheelbases, so roll-forming usually suits them better. The front and rear zones are structurally complex, demand high mechanical performance, and suit platform sharing, so they are the natural home for integrated die casting on the body-in-white — and the parts that carry the most machined locating and mounting features.

Compared with the traditional stamping-and-welding body-in-white — hundreds of stamped panels held together by hundreds of fasteners — these castings collapse the part count dramatically (one reported program saw roughly 30% fewer lower-body parts and around 10% lower mass). What matters for machining is that each large casting then arrives at the line needing precise machined faces and holes before it can be assembled.

flowchart LR
    A[Vehicle body] --> B[Upper body]
    A --> C[Lower body]
    B --> B1[Not applied: low load, low commonality]
    C --> C1[Front structure zone: integrated die casting]
    C --> C2[Rocker zone: roll-forming]
    C --> C3[Rear floor zone: integrated die casting]

Front structure (engine-bay) zone

The first plan for the engine-bay zone integrated the rails, wheel housings, and front bulkhead into a single casting. Detailed analysis changed that. Powertrain variety — combustion, full-electric, and hybrid — means the bay’s interior space is likely to vary, and styling changes also affect the bulkhead. To keep the design flexible, the program split the zone into separately developed left and right sides, settling on three castings per side. Each casting brings its own set of machined mating faces and holes, joined to the structure with self-piercing rivets (SPR), flow drill screws (FDS), bolting, and structural adhesive.

Rear floor zone

The rear floor zone consolidates the rails, wheel housings, and the central connecting panel into one casting. The mounting points for the rear structure are all designed onto this single casting, which improves assembly precision and supports platform sharing — and concentrates the machined locating and mounting features onto one part. It uses the same joining mix: SPR, FDS, bolting, and structural adhesive.

Joining methods drive machining accuracy

Because integrated castings still connect to surrounding sheet metal and sub-structures, multi-material joining is essential. The four methods used across both zones — self-piercing rivets, flow drill screws, bolting, and structural adhesive — combine mechanical fastening with bonded joints, well suited to mixed aluminum-and-steel assemblies where welding alone is not ideal. The precision of the machined holes and faces directly determines whether these joints land where the design intends, which is why the machining tolerances in the next sections are not cosmetic — they govern assembly.

Material and surface requirements before machining

Alloy and mechanical properties

In this program, the casting body material was a C611-type alloy. The mechanical requirements were a yield strength of at least 120 MPa, tensile strength above 250 MPa, and elongation of at least 10%, with a specified face hardness of 70 HRB. For the threaded inserts on the castings, the pushout strength reached 30–80 kN or more, depending on thread size and the specific product requirement. These figures reflect one vehicle program’s specifications rather than universal industry values, but they show the property window an immersion-free casting alloy must hit to serve as a structural body part.

Surface treatment and defect inspection

Compared with conventional stamped sheet, a casting leaves the mold with a layer of release agent on its surface. Bonding faces need laser cleaning, and the release agent has to be chosen so the paint shop’s cleaning process can remove it well enough to meet electrocoat (e-coat) quality. Surface quality is controlled against an inspection standard. The program’s appearance criteria are summarized below:

Dimensional tolerance and the role of CNC machining

Integrating many parts into one casting has a direct quality benefit: the old assembly stack-up tolerance becomes a single-part tolerance. Instead of accumulating error across many joints, the part carries its own dimensional accuracy. But casting alone does not reach the tightest tolerances. The program applied CNC machining to upgrade the tolerance grade on selected high-precision dimensions, and the contrast between machined and as-cast features is significant:

The pattern is clear. A machined face holds profile to 0.6 mm versus 1.6 mm as-cast, and a machined hole holds diameter to a 0.1 mm band versus 0.5 mm as-cast, with positional accuracy improving from 1.0 mm to 0.6 mm. For locating holes and critical mounting interfaces, this difference decides whether downstream assembly fits correctly. This is the heart of CNC machining of die-cast parts: large gigacast structures need rigid, high-accuracy machining of mating faces, locating holes, and functional holes to convert a sound casting into an assembly-ready body part. Only the features that interface with other components are machined to the tight grades; non-critical surfaces stay as-cast. UBright builds CNC machines and turnkey lines aimed at exactly this class of large-aluminum-part machining.

The outlook for machining large castings

Integrated die casting is spreading: major automakers have rolled out their own lines, and concepts for casting an entire lower body, or even a full body-in-white frame, are reportedly being studied by some carmakers and equipment builders. Every step in that direction enlarges the parts that must be machined and tightens the interfaces that hold a vehicle together. As castings grow, the machining demand grows with them — bigger work envelopes, more locating and mounting features per part, and less tolerance for fixture or spindle error. The casting press gets the headlines, but the CNC machining of die-cast parts is what keeps these large structures buildable.

Frequently asked questions

Why do gigacast parts still need CNC machining?

A casting captures the overall shape but cannot hold the tightest tolerances as-cast. CNC machining tightens the critical faces and holes: in one reported program a machined face held profile to 0.6 mm versus 1.6 mm as-cast, and machined holes held a 0.1 mm diameter band versus 0.5 mm, with position improving from 1.0 mm to 0.6 mm. Locating holes and mounting interfaces depend on this step for correct assembly.

Which features on a die-cast body part get machined?

The features that interface with other parts: locating holes that set the part’s datum, machined mounting and functional holes, and machined faces where panels or sub-assemblies mate. Non-mating faces and non-critical holes are often left as-cast because their looser tolerances are acceptable, which keeps machining time focused on the features that govern fit.

What does CNC machining of die-cast parts require from the machine?

Large castings demand a big, rigid work envelope, stable fixturing of a thin-walled aluminum part, and accuracy that comfortably beats the target tolerances (for example, holding hole position to 0.6 mm and a 0.1 mm diameter band). Thermal stability and repeatable datums matter because the whole point of the casting is to carry single-part accuracy into assembly.

What are the main downsides of integrated die casting?

The two main trade-offs are a high technology barrier — advanced presses and alloys are costly to adopt — and difficult repair, since damage to a large integrated casting can require replacing the whole part instead of a small panel. The full pros-and-cons comparison is in the companion explainer linked above.

Conclusion

Integrated die casting reshapes how aluminum car bodies are built, but the casting only becomes an assembly-ready structural part after CNC machining brings its critical faces and holes into tolerance. The machined locating holes, mounting holes, and mating faces — not the as-cast surfaces — are what let one large casting carry single-part accuracy into the body line. If you are planning CNC machining of die-cast parts or other large, high-precision aluminum components, UBright can help with drawings review, machining requirements, equipment selection, and turnkey line planning — reach out to discuss your application.

References

• Giga Press — Wikipedia — Supports the description of high-pressure die casting of large single-piece structural castings and the mold/cooling cycle.
• How megacasting could revolutionize car manufacturing — Hydro — Supports HPDC scale-up to mega/gigacastings and structural part weights up to 100 kg or more.