Thin walls, round bores, and the clamping trap
I have scrapped enough EV motor housings to know the print always looks simpler than the part actually is. You start with an aluminum casting (typically A356-T6 for sand cast or A380 for high-pressure die cast) or 6061 billet for prototypes, and the stator bore needs to be round within 0.02 mm, straight within 0.03 mm over 200+ mm of length, with a wall as thin as 3.5 mm around the water jacket. If you clamp the housing too hard on the OD, the bore measures perfectly round on the machine and springs to an ellipse the moment you release the chuck.
The fix is fixturing that supports the part radially rather than squeezing it axially. On 5-axis DMG MORI equipment we use expanding mandrels that locate on a semi-finished bore, or custom soft jaws bored to match the casting OD at each chucking position. The machining sequence matters: rough all features leaving 0.5-1 mm stock, wait for thermal stabilization (or do a low-temp stress relief), then finish machine in the least-clamped state possible.
- Stator bore: typically H7 fit, roundness 0.02 mm for EV traction motors
- Bearing bores: concentric to stator bore within 0.02 mm, P6 tolerance common
- Coolant jacket wall: 3.5-5 mm with +/-0.5 mm allowance for casting variation
- Surface finish: Ra 1.6 on bearing bores, Ra 3.2 on stator bore for heat-shrink fit
Coolant jacket sealing surfaces
Coolant jackets are sealed by a press-fit sleeve, a gasketed cover, or friction-stir welding. The sealing face is a large-diameter, thin-wall annular surface that is extremely sensitive to clamping distortion and thermal growth. Aluminum grows at roughly 0.023 mm per meter per degree C, so a 250 mm housing that warms up 5 degrees from cutting heat grows 0.029 mm in diameter — enough to push a sealing face out of flatness spec.
I always plan the jacket sealing face as the last or second-to-last operation, after all heavy material removal is done. The face gets a spring pass: a very light cut at reduced feed with a fresh insert, after the part has sat in the fixture for at least ten minutes to equalize temperature. For cast housings, a pressure test at 1.5x operating pressure (typically 4.5-7.5 bar for water-glycol) verifies freedom from porosity leaks.
Billet vs cast for prototypes
For prototype and low-volume runs under 50 units, machining a housing from solid 6061-T6 billet is faster and cheaper than waiting for casting tooling. You lose the as-cast spiral coolant geometry — billet housings use a two-piece bolted design or a sleeve-sealed channel — but you skip a 6-10 week casting lead time and can iterate on bore size, mount pattern, or sensor bosses in days.
| Housing type | Volume | Lead time | Cost per part | Notes |
|---|---|---|---|---|
| Billet 6061/7075 | 1-50 units | 5-10 days | High unit cost | Fast iteration, no tooling, sleeve-sealed jacket |
| Sand cast A356-T6 | 50-500 units | 3-6 weeks (with tooling) | Moderate | Good for prototyping production geometry |
| HPDC A380 | 500+ units | 8-14 weeks (production tooling) | Lowest unit cost | Requires steel die, best for volume production |
| Gravity permanent mold | 200-2000 units | 5-8 weeks | Moderate-low | Better properties than sand, less tooling cost than HPDC |
Other automotive precision parts
Motor housings get the most attention, but an EV powertrain has dozens of tight-tolerance machined parts. Shift forks in reduction gearboxes — typically 6061 with hardened wear pads — need the fork faces parallel within 0.03 mm and the pad thickness held to ensure proper fork-to-sleeve clearance. Inverter housings require flat mounting surfaces for IGBT modules at Ra 1.6 finish and precise bolt patterns to give even clamp load on thermal interface material. Rotor shafts, end caps, resolver mounts, and sensor brackets all land in the +/-0.01 to +/-0.05 mm zone.
Post-machining and thermal management
Aluminum castings have residual stresses, and billet has forging stresses. Finishing a bore right after roughing cuts into stressed material that moves within 48 hours. A semi-finish pass leaving 0.3-0.5 mm stock, a dwell period (or stress relief at 170 C for two hours), then the finish pass eliminates most of this distortion.
Surface treatments: chromate conversion for basic interior corrosion protection; Type II anodize for exterior corrosion; Type III hard anodize on wear surfaces like shift fork pads (anodize adds 0.008-0.02 mm per surface — machine features undersize). Electroless nickel plating is used on some steel bearing races and connector interfaces.
Production inspection
We run Zeiss CMM inspection on every first article, and production lots receive sampling plans with Cpk reporting on critical dimensions. A standard EV housing inspection covers stator bore roundness and cylindricity, bearing bore size and concentricity, mounting face flatness, bolt-hole position, and coolant jacket pressure test results. FAI reports are provided at no charge on production-intent parts, with typical prototype lead times of 5-7 working days.
Frequently Asked Questions
What aluminum alloy is used for EV motor housings?
Production housings use A356-T6 (sand/gravity cast) or A380 (high-pressure die cast). Prototype and low-volume billet housings are typically 6061-T6 for machinability and thermal conductivity; 7075-T6 for higher strength at the cost of harder machining.
How tight are stator bore tolerances?
Stator bores typically require 0.02 mm roundness with an H7 fit (about +0.03/-0 mm on a 200-250 mm bore) for heat-shrink or slip-fit stator assembly. Bearing bores are tighter, often P6 interference fit.
Can I prototype a motor housing without casting tooling?
Yes. A billet 6061-T6 housing with a two-piece bolted design or sleeve-sealed coolant channels is standard for prototypes. Lead time from model to first article is typically 5-7 working days.
How do you prevent thin-wall distortion?
Use radial support (expanding mandrels, bored soft jaws), rough and finish in separate setups with a stress-relief dwell between, take spring passes on critical surfaces after thermal stabilization, and avoid excessive boring bar overhang that causes chatter.
Do EV housings need pressure testing?
Yes. Coolant jackets are pressure tested at 1.5x operating pressure (typically 4.5-7.5 bar for water-glycol) to verify freedom from casting porosity and seal integrity.