Checking Fixtures for EV Battery Trays: Designing for a 1.6 m Part

A checking fixture for a bracket is a solved problem. A checking fixture for an EV battery tray is not, because the tray breaks three assumptions the discipline was built on: the part is bigger than the fixture designer's reach, it is thin and floppy relative to its footprint, and its most important characteristic is not a hole — it is a large sealing plane whose flatness decides whether the finished pack passes a leak test.

Trays and covers commonly exceed 1.6 m across. Published requirements on this class of part include module mounting surface flatness around 0.3 mm and bottom flatness and hole position in the 0.5 mm range — over an envelope where the part's own weight and thermal state are measurable error sources. That combination is what makes the fixture design interesting.

What actually changes at this size

Datum strategy: 3-2-1 still applies, but where you put the three matters more

The 3-2-1 principle does not stop being true at 1.6 m: a primary plane removes three degrees of freedom, a secondary line removes two, a tertiary point removes the last. What changes is consequence. On a small part, moving a primary pad 20 mm is a detail. On a tray, moving it 20 mm changes how the structure sags between supports, and therefore changes the flatness number you report.

The trap is over-constraining. Adding clamps until the tray reads flat feels like progress and is actually the fixture manufacturing a passing result. Extra pads should support the part in its free state, not push it into shape. If a clamp is closing a gap, the gap is data — and you have just deleted it.

Checking flatness and sealing surfaces on the floor

Flatness of the sealing and mating surfaces is what integrates the tray into the body and keeps the pack sealed. Full-field flatness is a scanner or CMM job. What the floor needs is a fast, repeatable check that catches drift between those measurements — which a fixture does well if the characteristic is broken into the right discrete checks.

• Indicator or probe points distributed across the sealing plane, positioned by the drawing's datum frame rather than evenly for neatness.
• Go / no-go pins for the module mounting and body-attach holes, which are position characteristics and suit attribute checks.
• Profile details on critical contour and flange regions where a scan is too slow for line rate.
• CMM-ready reference points so the same fixture can carry the tray to coordinate measurement without a second setup.
• Clear pass criteria at each point — a fixture that produces numbers nobody has a limit for produces arguments, not decisions.

Where the fixture ends and the CMM or scanner begins

Battery tray programmes almost always run all three, and the mistake is using them interchangeably. Scanning gives full-field truth on a surface but is slow and usually off the line. A CMM gives traceable point data and settles disputes. The checking fixture gives fast, repeatable in-process confirmation that nothing has drifted since the last one.

A workable division: scan the first article and prove the fixture correlates to it. Use the fixture in process, at rate. Escalate back to the CMM when the fixture says something changed. That way the expensive equipment answers questions instead of counting parts.

Construction notes specific to large trays

See gauge and CMM inspection in practice

Frequently asked questions