CNC processing that supports AI: hardware parts that hold their geometry

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  Key takeaways
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  Which machined parts actually sit inside AI hardware
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  Tolerances and finishes that AI parts really need
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  Matching the process to the geometry
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  Material choices for thermal and structural AI parts
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  Why thin fins and long plates move after machining
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  Which process fits which AI hardware part

AI hardware is not one product. It is a stack of racks, boards, optics and actuators, and only some of that stack is machined. The machined parts are the ones that manage heat, hold an optical axis, or carry a load. A liquid cold plate under a GPU module is a good example: two plates bonded or friction-stir welded, with internal channels that must not leak and a mating face that must sit flat against the die package.

Edge inference boxes follow a similar pattern at a smaller scale. A machined aluminium housing doubles as a heat sink, so the wall thickness and fin geometry are thermal decisions as much as mechanical ones. Camera modules and lidar units add a different requirement: the bore that locates a lens must stay concentric with the sensor plane, often to 0.01 mm across the assembly.

Robotics adds moving parts. Harmonic drive housings, joint brackets and end-effector plates carry repeated loads and need bores that stay round after anodizing. None of these parts are exotic on their own. What makes them demanding is that several requirements land on the same face at once.

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  Thermal partsCold plates, heat spreaders, finned housings, vapour chamber frames
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  Optical partsLens barrels, sensor mounts, camera brackets, lidar housings
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  Structural partsRack rails, chassis frames, robot joint housings, end-effector plates
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  Interconnect partsBackplane brackets, connector panels, cable clamp blocks

The headline number is ±0.005 mm, and we hold it on features that justify it: bearing bores, dowel holes, optical seats, spigot diameters. Putting that tolerance on every dimension does not improve the part. It raises cost and slows inspection without changing function. A cold plate does not care if its outer profile is ±0.1 mm, but it cares a great deal about flatness across the die contact area.

Surface finish follows the same logic. A sealing groove for an O-ring usually runs Ra 0.8–1.6 μm so the elastomer can seat. An optical bore may need Ra 0.2–0.8 μm to control stray light and adhesive bond line. A structural bracket can stay as machined at Ra 1.6–3.2 μm and nobody will notice.

Then there is the requirement that rarely appears on the drawing: stability over temperature. An aluminium cold plate and a steel bracket expand at different rates. If the design bolts them together at 20 °C and expects alignment at 70 °C, no machining tolerance will save it. That is a design conversation, and it is worth having before the first chip is cut.

Three-axis milling handles most flat plates, covers and brackets, and it is the cheapest way to get there. Once a part has features on four or five faces, or an angled port that must meet an internal channel, the setup count becomes the cost driver. Every extra setup adds a fixture, a re-datum and a chance for error to stack.

Five-axis machining removes that stack. We run 16 simultaneous five-axis centers with travels of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, plus a Ø400 mm rotary table for parts that need continuous rotation. A cold plate with ports on two sides and a channel pattern on the back is a one-setup job on a five-axis machine. The same part on a three-axis machine is four setups and a fixture design.

Turning and mill-turn centers cover the round parts: lens barrels, spigots, connector bodies, shaft adapters. We run 16 mill-turn centers so a part can be turned and then milled without losing concentricity. For long, thin profiles such as heat pipes and extrusion-based housings, the 4,000 × 400 × 150 mm travel machines take parts up to 4,000 mm maximum processing size.

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  Flat plates and coversThree-axis milling, 27 machines available
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  Multi-face housings and cold platesFive-axis in one setup
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  Round optical and connector partsMill-turn, concentricity held across operations
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  Long profilesUp to 4,000 mm on large-travel machines

Aluminium dominates for a simple reason: it is easy to machine and it conducts heat well. 6061-T6 is the default for cold plates, housings and brackets. 7075 gives more strength where a bracket is thin, though it machines differently and anodizes to a slightly different colour. 2024 is used occasionally where fatigue matters. Copper conducts better than any aluminium grade but weighs three times as much and is far harder to machine, so it appears mostly in small, high-flux spreaders.

Stainless shows up where corrosion or cleanliness matters. 304 and 316L are common for medical-adjacent fixtures and wet environments. 17-4PH gives high strength with reasonable machinability for robot joint parts. Titanium TC4 (Ti-6Al-4V) is chosen for weight-critical actuator components, at a cost in tool life and cycle time.

Two material notes matter for AI hardware. First, beryllium copper machines to fine detail and conducts well, but the dust needs control, so it belongs in a shop with the right extraction. Second, carbon fibre and PEEK appear in lightweight brackets and insulators. Both cut cleanly with the right tooling but behave nothing like metal when you clamp them.

A cold plate with 0.8 mm fins is a spring. Clamp it hard on a vise and the fins close up. Release the clamp and they spring back to a shape that is no longer parallel to the base. The cutting forces were never the problem. The clamping was.

We deal with this in three ways. Soft jaws machined to the part profile spread the load. Vacuum fixtures hold a plate flat without point pressure. For very thin sections, we leave a sacrificial web and remove it in a light finishing pass, which lets the material relax before the final dimensions are cut.

Long plates have a second issue: residual stress in the stock. A 6061 plate that has been rolled and not stress-relieved will bow after the first face is milled away. The fix is either stress-relieved stock or a rough, relax, finish sequence. Both cost cycle time. Both are cheaper than a scrapped batch.

Thermal growth during cutting matters too. Aluminium expands about 23 µm per metre per °C. A 500 mm part that warms by 5 °C during roughing has moved roughly 0.06 mm. On a ±0.005 mm feature, that is the whole budget. We keep coolant flow steady and take finish cuts after the part has stabilised.