Why a PCB fits in CAD but not in the enclosure.

Start with the actual parts

Record the exact board revision, outline, thickness and mounting-hole positions. Measure the tallest parts on both sides, including solder joints and fitted headers. A generic 3D model can omit a connector or show a different component variant.

Make a simple envelope for every item that occupies space: the board, display, sensor, switch, cable plug and fasteners. Mark the surfaces that must line up. For example, a display opening should align with the visible display area, which may be smaller than its glass or circuit board.

Check access, not just empty space

An opening large enough for a USB socket may still block the cable plug. Model the plug body and allow the approach needed to insert and remove it. Check the real cable you intend to use and allow room for its bend without loading the connector.

Describe the assembly sequence: install the board, connect the cable, position the display, then close the lid. At each step, check whether fingers, a screwdriver or a connector still have access. A part that fits in its final position may have no practical way to get there.

Separate clearance from tolerance

Clearance is the space intentionally left between parts. Tolerance describes the permitted variation in a dimension. If two nominal dimensions are identical, manufacturing variation can turn a neat CAD fit into interference.

Use a clearly labeled calculation before choosing a gap. In a hypothetical one-dimensional example, a cavity is 50.0 ± 0.3 mm wide and a board is 49.0 ± 0.2 mm wide. The smallest total free width is 49.7 − 49.2 = 0.5 mm. That does not guarantee 0.25 mm on each side: mounting position also matters. These are teaching values, not recommended production tolerances.

Agree tolerances with the fabricator for the selected material and process. Printed, machined and molded parts behave differently. Check hole position, part distortion and mounting variation as well as nominal size. The linked manufacturing reference explains why printing accuracy varies by technology.

Test the risky interface first

Before printing a full box, make a small coupon containing the connector opening or mounting detail you are unsure about. Test it with the actual part and keep the result with the design revision. A quick fit sample can expose an assumption before a larger build.

For the assembled prototype, record whether the lid closes without force, the board is securely supported, connectors remain accessible and wires are not trapped. Do not use a tighter screw to compensate for a misaligned mount; review the geometry and fastening method.