CNC machining battery tray: how the part is made and where it fails

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  What a battery tray actually has to do
• 2
  Wall thickness, ribs and pocket depth that survive machining
• 3
  How 5-axis machining handles tray features
• 4
  Tolerances that matter and the ones that do not
• 5
  Coolant channels, fins and thermal paths
• 6
  Material choice and what each one costs you
• 7
  When to machine a battery tray and when not to

A battery tray is rarely just a box. It carries the mass of the cells, holds them at a fixed pitch, keeps coolant away from electronics and often acts as part of the chassis load path. On a 60 kWh pack the cells alone can weigh over 400 kg, so the tray sees static load, road vibration and crash acceleration. That mix is why a CNC machining battery tray is usually specified as a structural part with sealing and thermal duties layered on top.

Three features drive most of the cost. The first is the sealing face, a continuous groove or land that must stay flat across the whole perimeter. The second is the cell locating pattern, where hole pitch and depth control how the modules sit. The third is the coolant or air path, which needs wall thickness held tight so the plate does not flex under pressure.

Material choice follows from those duties. Aluminium 6061-T6 and 6082 give a good stiffness-to-weight ratio and machine cleanly. 5052 and 5083 are easier to form if the tray is later welded or bent. Stainless 304 and 316L appear in marine and chemical environments. Titanium TC4 (Ti-6Al-4V) is used when weight matters more than cost, for example in aerospace and high-end motorsport packs.

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  Structural dutyCarries cell mass and crash load; needs stiff ribs, not just thin walls.
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  Sealing dutyFlat perimeter face plus a defined groove for the gasket or sealant bead.
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  Thermal dutyCoolant channels or fins with controlled wall thickness.

Wall thickness is the first number to settle. For aluminium trays we normally hold 1.5–3 mm on non-structural walls and 3–6 mm on load-bearing side walls. Below 1.5 mm the part starts to chatter during roughing, and a thin floor will bow when the clamps release. If your design calls for a 1 mm floor over a 300 mm span, expect to add ribs or accept a stress-relief step.

Ribs should be as deep as the tray allows and spaced roughly 4–6 times their thickness apart. A 4 mm rib on a 25 mm pitch is a workable ratio for a 600 mm long aluminium tray. Corners inside pockets need a radius at least one third of the cutter diameter, otherwise the tool has to stop and the corner is either left as a sharp witness or cut with a smaller, slower tool.

Pocket depth drives both cycle time and distortion. Shallow pockets up to 20 mm deep cut in one or two passes. Deep pockets over 60 mm need stepped roughing and more coolant, and they release more internal stress from the plate. On large trays, a roughing pass followed by a stress-relief pause and a finishing pass is the usual way to keep flatness after the part leaves the machine.

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  Aluminium walls1.5–3 mm non-structural, 3–6 mm structural.
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  Rib pitchAbout 4–6 times rib thickness.
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  Corner radiusAt least one third of the cutter diameter.

A battery tray is mostly a 3-axis part with a few angled or curved features. The sealing face, cell pockets and mounting holes are all reachable from the top. Where 5-axis earns its place is on sloped end faces, curved side walls and angled coolant ports, which can be cut in one setup instead of three. That removes stack-up error between setups, which is usually the largest single contributor to a leaking seal.

Setup count is the quiet cost driver. A tray cut in three setups needs three datum references and three chances for a chip to sit under the part. On our 16 simultaneous 5-axis centers we aim to finish a tray in two setups: one for the main cavity and perimeter, one for the underside ribs or mounting pads. For parts up to 750 × 1,150 × 550 mm we can often do the whole job in a single tombstone setup.

Roughing removes 60–80% of the material in most trays, so toolpath strategy matters more than the finishing pass. Adaptive clearing with a constant chip load keeps heat out of the cutter and reduces the chance of work-hardening 304 stainless. For long trays we use the 4,000 × 400 × 150 mm travel machine, which lets the cutter stay in one continuous pass along the length instead of repositioning mid-face.

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  Typical setupsTwo for most trays; one when the part fits a tombstone.
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  Roughing share60–80% of material removal time.
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  Large travelUp to 4,000 × 400 × 150 mm in one setup.

Not every dimension on a tray needs the same tolerance. The sealing face flatness and the cell locating holes carry the function. Bolt holes are usually clearance holes and can sit at ±0.2 mm without any effect. Tightening a non-critical dimension just adds inspection time and cost with no gain in pack performance.

For critical features we hold ±0.005 mm where the design needs it, and Ra 0.8–1.6 μm on sealing surfaces. A gasket face that is too rough will not seal even if it is perfectly flat, because the surface peaks prevent the elastomer from wetting the full face. A face that is too smooth can also cause issues with some sealants, which need a light tooth to grip. Ra 0.8–1.6 μm is the usual working band.

Flatness across a full tray is harder than a small diameter tolerance. Over a 1,000 mm length, thermal drift during machining can move the part by more than the tolerance you are trying to hold. That is why we measure after the part has cooled, and why a stress-relief step is often cheaper than trying to hold flatness in one continuous cut.

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  CriticalSealing face flatness, cell hole pitch and depth, datum pads.
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  Non-criticalClearance holes, cable routing slots, cover mounting.
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  Sealing finishRa 0.8–1.6 μm on gasket and sealant faces.

Thermal management is where a tray stops being a bracket. Cold-plate style trays use channels cut into the base, closed by a bonded or welded plate. Machined channels give a defined cross-section and a smooth wall, which keeps pressure drop predictable. A cast channel can vary in section along its length, and that variation shows up as uneven cell temperature.

Channel geometry is a trade-off. Wider, shallower channels give lower pressure drop and more contact area. Narrower, deeper channels fit more path length into the same footprint but raise pumping losses. A common starting point for a 400 mm wide aluminium cold plate is a 6–10 mm wide channel at 4–8 mm depth with a 8–12 mm rib between passes.

Wall thickness under the channel is the number to watch. If the floor between the coolant and the cell is too thin, the plate deflects under pressure and the thermal interface material loses contact. We usually keep at least 2 mm of aluminium under a channel running at 1–2 bar. Above that, we check the deflection with the customer rather than guess.

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  Channel start point6–10 mm wide, 4–8 mm deep, 8–12 mm rib.
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  Floor thicknessAt least 2 mm under a 1–2 bar channel.
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  Surface finishSmooth machined walls keep pressure drop predictable.

Aluminium is the default for EV and energy storage trays. 6061-T6 machines well, welds acceptably and takes anodizing. 6082 is similar with slightly better corrosion behaviour in some environments. 5052 and 5083 are used when the tray will be formed or welded after machining, because they crack less in the heat-affected zone. 7075 gives the highest strength but is harder to weld and costs more per kilogram.

Stainless 304 and 316L come in for marine, food and chemical exposure. They machine at roughly half the feed rate of aluminium, so cycle time and tool wear go up. 17-4PH (SUS630) is used when you need corrosion resistance plus high strength, for example in a bolted structural joint. Titanium TC4 sits at the top of the cost range and is reserved for weight-critical aerospace and motorsport packs.

Plastics and composites appear on covers rather than structural trays. PEEK and carbon fibre hold stiffness at high temperature but do not replace an aluminium base plate. On trays that see fire exposure, the material spec is set by the safety standard, not by the machining shop, and we work to the drawing.

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  Default6061-T6 or 6082 for machined structural trays.
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  Weldable5052, 5083 when the tray is formed or welded after cutting.
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  Corrosive304, 316L, 17-4PH for marine and chemical duty.