Battery Housing Machining: Gasket Grooves, Flatness and Leak Test Prep
A battery pack enclosure is a pressure vessel that is not allowed to be one. It has to keep water at IP67 out for the life of the vehicle, vent a cell thermal event without becoming a projectile, carry 400 to 700 kg of modules over 200,000 km of vibration, and still come apart for service. The machined features that make that possible are almost all in one place: the gasket groove and the face it sits in.
If you are sourcing battery housing machining, most of the cost is in the weldment and the coating. Most of the risk is in a groove that is 0.1 mm too shallow, a sealing face that is flat to 0.3 mm over a meter, and a vent port drilled after the final flatness check. Those three things decide whether the pack passes its leak test on the first attempt.
What actually seals a pack
The seal is a compressed elastomer, usually silicone or EPDM cord in a machined groove, sometimes a molded profile gasket. What holds the seal is not the lid bolt torque. It is the groove volume and the squeeze percentage, which together determine whether the elastomer fills the groove and pushes against both faces.
Two numbers control it. Squeeze, the percentage of cord diameter compressed by the lid, should land between 20 and 30 percent for a static face seal. Fill, the ratio of gasket cross-section area to groove cross-section area, should land between 80 and 90 percent. Below 20 percent squeeze, a cold EPDM gasket will not follow a 0.2 mm face wave. Above 35 percent, the elastomer takes a compression set and will not reseal after the lid comes off for service.
That is why groove depth is a +0.05/−0.00 mm feature and groove width is usually ±0.1 mm. A depth tolerance of ±0.15 mm looks reasonable on a drawing and will put half your packs outside the squeeze window.
| Cord diameter | Groove width | Groove depth | Squeeze | Fill ratio |
|---|---|---|---|---|
| 3.0 mm | 3.8 mm +0.1 | 2.15 mm ±0.05 | 28% | 87% |
| 5.0 mm | 6.3 mm +0.1 | 3.55 mm ±0.05 | 29% | 88% |
| 6.0 mm | 7.5 mm +0.1 | 4.30 mm ±0.05 | 28% | 88% |
| 8.0 mm | 10.0 mm +0.1 | 5.75 mm ±0.05 | 28% | 87% |
Note that these are cord seals in a rectangular groove. A molded profile gasket has its own groove geometry and usually needs 25 to 30 percent compression on the sealing bead rather than on the whole section.
Flatness over a meter is harder than a tight groove
A 1.2 m long pack lid machined from 6061 extrusion will move 0.3 to 0.5 mm after machining if the extrusion was not stress-relieved. The groove depth can be perfect and the face still leak, because the cord cannot compress uniformly across a wave of that amplitude.
Specify flatness on the sealing face, not parallelism to the opposite side. For an IP67 pack with a 6 mm cord, 0.15 mm over the full perimeter is achievable on a large gantry mill with a single fixturing. For IP68 or for a pack that will be submerged, hold 0.10 mm and check it with a laser or a surface plate and feeler gauges after the part is unclamped.
Machine the sealing face and groove in one setup, in that order, so depth references the actual face. Do not deburr the groove by hand afterward: a 0.2 mm rolled lip holds the gasket off the face and creates a leak path no detector will explain.
Surface finish on the sealing face should be Ra 1.6 to 3.2 µm. Smoother is not better here: a mirror finish on aluminum holds water between the gasket and the face, and the resulting crevice corrosion shows up two winters later.
See also: Sensor Housing Machining: Wall Thickness, Sealing Faces and Thread Coaxiality and Harmonic Drive Housing Machining: Keeping Three Bores on One Axis.
Ports, vents and threads
Every pack has at least a pressure-relief vent, a service port, and often a coolant inlet. These are small features with outsized consequences. A vent port that is drilled after the sealing face is machined can raise a burr on the inner face that traps a module; a service port thread that is cut with a worn tap will leak at 30 kPa even with a sealing washer.
Cut threads with a thread mill, not a tap, on any port in a housing wall thinner than 8 mm. A formed or cut tap displaces material and can bulge the wall, which shows as a high spot on the sealing face 20 mm away. Thread mill the M12 × 1.5 vent boss and the M10 × 1.5 test port, and keep 12 mm of full thread engagement.
The boss face where the vent seals needs its own flatness callout, typically 0.05 mm, and a spotface that is perpendicular to the thread axis within 0.05 mm. A vent that sits 0.2 mm off perpendicular will seal once and weep after the first thermal cycle.
Machining after welding
Most aluminum pack housings are friction stir welded or MIG welded from extrusions and a floor plate. FSW moves a 1.2 m housing 1 to 3 mm at the ends, and MIG moves it more. Any sealing face or bearing feature finished before welding has to be recut afterward.
The practical sequence is: weld, stress-relieve at 180 °C for 4 hours, machine the mounting datums, then machine the sealing face, groove, and all port bosses in one setup. Leave 2 mm on the sealing face and 1.5 mm in the groove before welding. If the housing is large enough that no single machine covers it, use a gantry mill or split the operations at a feature that has no sealing function.
Wall thickness at the flange matters more than most drawings suggest. A 4 mm flange pulls 0.5 mm out of flat from one weld pass; 6 mm moves half that, and the extra material costs less than the straightening you avoid.
Coating, masking and the final check
Powder coating over a sealing face is a leak. So is e-coat, and so is any primer that bridges the groove. Mask the groove, the sealing face, and every threaded port before coating, and plug the ports rather than relying on a screw threaded in place.
Leak test preparation belongs in the machining plan, not at the end of the line. Specify a 20 to 30 kPa pressure decay test with a stabilized fill, or a helium test at 1 × 10⁻⁵ mbar·L/s if the customer wants a tighter number. Both need clean, dry, burr-free internal surfaces; a chip sitting in a groove will show as a leak and cost a day of rework.
For programs moving from prototype to series, it helps to machine the first articles with the production fixture and the production sequence, even at 20 pieces. A shop that does electric vehicle enclosure work will usually quote the groove and face as a single CNC milling operation for that reason.
Frequently asked questions
How tight should the gasket groove depth be?
±0.05 mm on a 6 mm cord groove, with the depth referenced to the sealing face that was machined in the same setup. Anything looser pushes the squeeze outside the 20 to 30 percent window.
Can a 1.2 m housing be machined flat without a gantry mill?
Yes, with a large bed mill and a fixture that supports the part at the points it will see in the vehicle. Clamping a flexible housing on a flat table and machining it flat produces a part that springs 0.3 mm when released.
What leak test should be specified?
A 20 to 30 kPa pressure decay test for IP67 production, or helium at 1 × 10⁻⁵ mbar·L/s for tighter requirements. Specify the test pressure, stabilization time, and the maximum decay rate, not just the IP rating.
Does powder coating need to be removed from the groove?
Yes. Mask the groove and the sealing face, and plug every threaded port. Coating thickness of 60 to 80 µm in a groove reduces the volume available for the gasket and changes the squeeze by more than the groove tolerance.
Related reading
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