Aluminum Case Manufacturing Tolerance and Dimensional Stability

A medical device startup in Boston was shipping 50 portable diagnostic analyzers to hospitals across the US. Each analyzer was packed in a custom aluminum case with CNC-machined EVA foam. The cases were built to a drawing that specified internal dimensions of 420mm × 310mm × 180mm. When the first shipment arrived at a hospital in Chicago, the biomedical engineer opened the case and found the analyzer couldn’t fit. The case interior measured 428mm × 316mm × 184mm — 8mm over in length, 6mm over in width, 4mm over in height. Not much, except the foam cavity was machined for the analyzer at 420mm × 310mm × 180mm. The analyzer was 422mm × 312mm × 181mm. The case was too big for the foam, and the foam was too small for the analyzer. All 50 cases had to be re-machined. Cost: $4,200 in foam rework, plus 3 weeks of delay, plus a penalty clause for late hospital delivery that cost $8,000. Total: $12,200. The root cause? The case manufacturer’s tolerance on internal dimensions was ±5mm, and the foam shop machined to nominal dimensions without accounting for case wall variation.

Dimensional tolerance is the most underdiscussed specification in custom case procurement. Buyers focus on material, foam type, and hardware — and they should. But if the case interior doesn’t match the equipment and the foam cavity, nothing else matters. This guide covers how manufacturing tolerances affect custom aluminum cases, what tolerance levels to expect from different production methods, and how to specify tolerances so your equipment actually fits.

Why Tolerance Matters in a Custom Case

An off-the-shelf case doesn’t have a tolerance problem — it’s a fixed size and you buy equipment to fit it, or you buy foam that’s slightly oversized and cut to fit. But a custom case is built to your drawing. If the case comes out 5mm wider than the drawing, that’s not a “close enough” situation. It’s a problem because:

• The foam cavity is machined to the drawing, not to the actual case. If the case interior is 5mm over, the foam cavity (machined to nominal) won’t reach the case walls. The equipment shifts around in the cavity and the case is no longer protecting anything
• Stacking and racking dimensions matter. If you have a rack system or a flight case stack, 2mm over on every case means the whole stack doesn’t fit the rack. A single case is flexible. A system of cases is not
• Sealing depends on flatness. If the case lid panel warps by more than 1.5mm across a 600mm span, the gasket doesn’t seal consistently. IP rating drops from IP65 to IP54 or worse
• Equipment fit is binary. Your instrument either fits or it doesn’t. There’s no “close enough” when the device slides into a CNC-machined cavity

Where Tolerance Goes Wrong in Case Manufacturing

1. Panel Shearing and Cutting

Aluminum panels are sheared to size on a guillotine. Typical shearing tolerance is ±0.5mm to ±1mm per cut. This is tight and usually not the problem. But if the panel is oversized (e.g., 5052-H32 in 5mm thickness), shear tolerances can be ±1.5mm on longer pieces.

2. Folding and Bending

Panels are folded on a press brake to form the case walls. Bending introduces springback — the material doesn’t stay exactly where you put it. Typical bending tolerance on a 5052 panel is ±1mm to ±2mm across a 400mm bend. This is where most dimensional variation comes from.

3. Assembly and Riveting

Panels are riveted or screwed together at the corners. Each rivet pulls the panels together slightly differently. Assembly tolerance is typically ±1mm to ±2mm on the final external dimensions. Internal dimensions depend on whether the panels are inset or overlap.

4. Welding (if welded construction)

Welded aluminum cases have weld shrinkage. The weld pool cools and contracts, pulling the panels together. Welded cases can have ±2mm to ±4mm dimensional variation. This is why riveted cases are more dimensionally consistent than welded ones for precision applications.

5. CNC Foam Machining

Foam is CNC-machined on a router. Typical CNC tolerance on EVA foam is ±0.3mm to ±0.5mm. This is tight — much tighter than the case itself. The problem isn’t the foam machining. It’s that the foam is machined to the drawing, not to the actual case.

Typical Tolerance Levels by Construction Method

How to Specify Tolerances on Your Case Drawing

If you’re sending a drawing to a case manufacturer, here’s what to specify:

1. Internal Dimensions with Tolerance

Don’t just write “420 × 310 × 180.” Write “420 ± 1.5 × 310 ± 1.5 × 180 ± 1.0.” This tells the shop what tolerance they need to hold. If you don’t specify, they’ll hold shop-standard (usually ±3mm) and you’ll get what you get.

2. Flatness Requirement

For cases that need a gasket seal, specify lid flatness: “Lid flatness ≤ 1.0mm across 500mm.” This tells the shop they need to use a rigid lid panel (4-5mm) and proper reinforcement, not a 3mm panel that flexes.

3. Corner Squareness

For rack cases or stackable cases, specify corner squareness: “90° ± 0.5° at all corners.” This ensures the case fits a rack system or stacks straight.

4. Foam Machining Process

Tell the foam shop: “Machine foam after case assembly, using the actual case interior as the reference.” This adds a step (measure each case, set up the CNC) but eliminates the fit problem. If you’re buying case and foam from the same supplier, this is standard. If you’re buying them separately, you need to specify it.

Temperature and Humidity Effects on Dimensional Stability

Aluminum expands and contracts with temperature. The coefficient of thermal expansion for 5052 aluminum is approximately 23.6 × 10⁻⁶ /°C. For a 500mm panel, that’s 0.012mm per °C. Over a 40°C temperature range (from a warehouse in winter to a truck in summer), the panel grows by about 0.5mm. This is small — but if your foam cavity is a tight fit, it matters.

EVA foam has a much higher coefficient of thermal expansion — roughly 70-90 × 10⁻⁶ /°C. Over the same 40°C range, a 420mm foam cavity changes by about 1.4mm. In practice, this means:

• In cold weather, the foam cavity shrinks slightly and the equipment fits tighter
• In hot weather, the foam cavity expands and the equipment may have more play
• This is usually not a problem for equipment that fits with 1-2mm clearance. It is a problem if the equipment is a press-fit in the foam

Humidity affects wood-core flight cases (plywood / aluminum laminate) more than all-aluminum cases. Wood swells by 2-4% with a 10% change in humidity. If you’re using a wood-core case in a humid environment, specify the interior dimensions at the expected humidity level, not at dry-room conditions.

Quality Control: What to Ask Your Manufacturer

When you’re evaluating a case manufacturer for a custom run, ask these questions:

• “What’s your standard tolerance on internal dimensions?” — If they say “within 5mm” or “pretty close,” walk away. A professional shop should quote ±2mm as standard and ±1mm as a premium option
• “Do you machine foam after case assembly, or to the drawing?” — Machining after assembly is the right answer. Machining to the drawing is the source of the fit problem
• “What’s your flatness specification on the lid?” — If they don’t know, they haven’t thought about gasket sealing. The right answer is “≤1mm across 500mm for IP-rated cases”
• “Do you do first-article inspection?” — For a run of 20+ cases, they should build one case, measure it, send you photos and dimensions, and get your approval before building the rest. This catches tolerance problems at case #1, not case #50
• “What’s the corner squareness tolerance?” — For rack-mount cases, ±0.5° is the minimum. If they can’t hold that, your rack case won’t slide into the rack

We build custom cases for test & measurement instruments, medical devices, surveying equipment, and portable electronics. Every case goes through first-article inspection: we build one, measure all critical dimensions, send the customer a dimension report, and get sign-off before production. It adds 2 days to the lead time but eliminates the 3-week delay that comes from re-machining 50 sets of foam. Tolerance isn’t a specification to save on — it’s the foundation that makes everything else fit.