Designing enclosures from flat panels

A rectangular enclosure has six faces but rarely six different parts: base and lid are usually identical, so are the two sides, and only front and back differ through their cut-outs. Six faces thus become three or four drawings — that is the first step, and it saves more time than any optimisation afterwards.

All parts lie flat on the same sheet. What stands upright in the finished enclosure is one surface beside another when you draw it. This translation between the three-dimensional object and the flat drawing is the actual design step.

Internal or external dimension — decide that first

Before you enter any dimension: is the internal or the external size fixed? You cannot have both. If a circuit board, a display or a power supply has to fit inside, the internal dimension is given and the external one follows. If the enclosure has to fit into a rack or onto an existing footprint, it is the other way round.

The most common design error is switching from one way of thinking to the other halfway through the drawing. Then exactly one material thickness is missing at the end — with 3 mm acrylic the board jams, with 5 mm it no longer fits at all.

Finger joints: the joint that aligns itself

The most widespread corner joint for panel enclosures is the box joint, usually called finger joints: alternating tabs and recesses along the edge that interlock. It has three advantages — it aligns the parts by itself during assembly, it considerably increases the glued area, and it holds together even without adhesive.

Two to three times the material thickness has proven a good finger width: with 3 mm material, roughly 6 to 9 mm. Narrower fingers look more delicate but break out more easily, especially in brittle acrylic. It is also important that each edge begins and ends with a tab — otherwise a thin tongue of material stands at the corner and snaps off the first time you assemble it.

The finger depth always corresponds exactly to the material thickness of the opposing wall. If it is smaller, the tab does not sit flush; if it is larger, it protrudes and has to be reworked.

Material thickness is not as exact as it sounds

Here lurks the error that defeats the cleanest dimension chain: “3 mm acrylic” does not mean the sheet is 3.00 mm thick. Cast acrylic is manufactured with a considerable thickness tolerance — ± (0.4 + 0.1 × thickness) mm is permissible. At 3 mm that is up to ±0.7 mm; the sheet may therefore lie anywhere between 2.3 and 3.7 mm.

Anyone drawing a groove or a finger recess at exactly 3.0 mm will therefore get, depending on the batch, a joint that jams or one that rattles — from an identical file. That is not a manufacturing fault but the material standard.

Three ways to deal with it: measure the actual thickness of your sheet and draw accordingly. Or design joints that tolerate variation — screwed rather than slotted, or with an adhesive gap. Or use precision cast acrylic, manufactured to ±0.1 mm; for fits that genuinely have to be right, that is the most direct route.

The tolerance chain

Every slotted joint brings play with it. With tab and slot it is twice the kerf — around 0.7 mm with 3 mm acrylic. Across four corners that adds up to an enclosure that visibly sits askew, even though every individual part is correct.

So either you compensate for the kerf in the drawing, or you plan for adhesive that fills the play. Both together is the most robust. How the compensation works arithmetically is covered in the article on kerf.

How does the lid come off again

An enclosure finger-jointed and glued all round is stable but final. For anything containing electronics you need access. Three approaches have proven themselves: the lid is screwed down, in which case you need threaded inserts or glued-on strips with holes — acrylic itself does not hold a cut thread. Or the lid is inset and rests on strips running round the inside. Or the enclosure is held together by threaded rods passing through all the panels; you often see this construction in amplifiers and power supplies.

In every case: fixing points belong in the drawing before the fingers are distributed. Retrofitting a hole rarely puts it where material is still left.

Cut-outs for connectors

Sockets, switches and displays almost always sit in the front or back panel. Take the dimensions from the component data sheet, not from a caliper on a sample — and remember that a cut-out comes out larger than drawn because of the kerf. With round sockets that is welcome clearance; on a display window it is visibly annoying.

Ventilation slots are cheaper than any other form of cooling, but they cost cutting length and therefore money. A few longer slots are cheaper than many short ones for the same open area.

How to proceed

1. Fix the internal dimension — what has to go inside, with how much clearance.
2. Decide which walls sit outside and which fit between them.
3. Work through the dimension chain once completely and note the external sizes.
4. Distribute the fingers, with a tab at both ends of every edge.
5. Enter cut-outs, holes and fixing points.
6. Compensate for the kerf and order a single pair of corners as a test cut before the whole enclosure runs.

Generators as a starting point

For standard shapes you do not have to draw anything by hand. Free generators such as boxes.py produce finger-jointed boxes including cut-outs as a ready vector file, with an adjustable kerf setting. We have been pointing customers to them for years when they ask for a starting point.

Two things nevertheless remain your job: the generator’s kerf value has to match our material — there is a dedicated test-cut generator for that. And you enter the cut-outs for your specific components yourself afterwards.