IKEA Skådis: the pegboard and its hooks

You have a Skådis panel on the wall and you want to print your own accessories: a tray for the screwdriver, a spool holder, a bin for your fasteners. You model a hook to the size you read off the hole (5 mm), print it, and it won't go in. You file it, still won't go, file more, and suddenly it slides in, but it wobbles and drops off at the first knock. The problem isn't your design: you handed a nominal dimension to a machine that doesn't print nominal dimensions, and you conflated the geometry that gets the hook in with the geometry that holds it. Those are two different things, and the Skådis keeps them apart on purpose.

This article is about that separation: what a Skådis panel actually measures, how its two-stage hook works, and how much clearance you need to give in FDM so that an accessory goes in, stays put, and doesn't tip under load.

The Skådis panel and its dimensions

The Skådis is IKEA's pegboard: a painted fibreboard panel, roughly 5 mm thick, with a regular pattern of round holes and, between them, oblong slots. It isn't a classic pegboard of identical holes: here two kinds of hole sit side by side with different jobs, and that mix is what gives the system its characteristic hook.

The round holes are the main anchor point. They measure around 5 mm in diameter and follow a square grid with a 40 mm pitch, both horizontally and vertically. Between the round holes sit oblong slots, meant for the two-pronged hooks that insert and rotate. The accessory goes in straight through the holes and then drops or turns so that it catches behind the panel.

The two-stage hook

The key to the Skådis – and to almost any accessory that works well – is that insertion and retention are two separate movements, and therefore two separate geometries. The hook goes in straight through the hole, with the arm perpendicular to the panel, and, once past the thickness of the board, drops or turns so that a chin catches behind it. At rest, the accessory hangs from that chin, not from friction in the hole.

Design them separately. The insertion geometry is the post or the tip that passes through the 5 mm hole: it has to pass through with clearance, without being forced. The retention geometry is the shoulder – the chin – that, once the hook has dropped, rests against the back face of the panel and takes the weight. If you try to make a single shape do both jobs, you end up with a hook that either won't go in or won't hold.

To keep it from tipping, the second stage matters as much as the first. A single-pronged hook rotates about its own axis: load the tray on one side and the whole thing swings and unhooks. That's why two-pronged accessories hook into two holes at once, or combine a round hole at the top with a bearing point lower down, resting against the panel. Two contact points spaced apart turn a pivot into a fixed joint: the moment that would try to rotate it splits into two reactions and the accessory stays put.

In FDM the post prints oversized: give it clearance

This is where the naive approach fails. A 5 mm hole won't take a 5 mm post printed in FDM, and not by a small margin. As Holes, pivots and first-layer squish explains, the process has a constant, signed bias: holes come out small and bosses come out big. The panel's hole is painted fibre, factory-moulded, so that side you can't change (it measures 5 mm) – but your printed post grows relative to the dimension you drew: first-layer squish, the bead width biting into the curve, and ooze all swell it. That excess concentrates in the first few layers, next to the bed; along the shaft the post comes out close to nominal. Model 5 mm and the base will come out 5.2 or 5.3 mm, and it won't go in.

Work in per-side terms, as in Real printed clearances. You want the post to go in loose (a running fit, with play, not a press fit), and it's the chin that holds, not the hole. So give it a generous per-side clearance and leave it room for the bias. For PLA with a 0.4 mm nozzle, aim for a modelled post of about 4.4–4.6 mm in diameter, which leaves 0.2–0.3 mm of gap per side on paper; the process will eat part of it and the part will be left with just enough play to go in by hand and drop without snagging.

Making it carry the load: orientation, walls and fillet

These accessories carry weight, and FDM has a well-known Achilles heel: adhesion between layers is weaker than the bond within a layer, as Layer adhesion and anisotropy explains. A printed part breaks sooner at the joint between layers than through solid material.

In a cantilevered hook, the critical section is the root, where the arm meets the body, and there the arm is loaded in bending: the load doesn't pull along the arm, it bends it, and that bending stretches the top face of the root. If that stretched face coincides with a joint between layers, the crack starts right there.

The rule is to print the hook lying down, with the arm horizontal on the bed, so that the layers run along the arm. That way the tension from bending stays within the layer, following the beads, and doesn't cross the joints. If you print the hook standing up, with the arm pointing upwards, every layer is a joint transverse to that tension: the weight works in exactly the mode that peels the layers apart, and the hook tears off at the root without warning.

Laying the hook down has a price you should plan for. The post, which was a cylinder, now prints resting on its side: it comes out slightly oval – sagging at the bottom, the vertical diameter a touch smaller than the horizontal – instead of round. For a round hole that matters. Size the clearance against the major axis of that ellipse, or give it an extra couple of tenths of a millimetre of gap, so the post goes in despite the ovalisation.

The chin also pays a price for lying flat: printed flat, that shoulder jutting out at the back becomes an overhang or bridge and usually calls for support. Design it with a self-supporting chamfer (under about 45° from vertical) so it comes out clean without support, or accept the support and clean it off afterwards.

At the root, on top of that, put a fillet. The sharp corner where the arm meets the body is a stress concentrator: the force piles up on that edge and the crack starts there. A generous fillet spreads that stress over a curve instead of a point and greatly raises the load the hook takes before failing. It's free in the model and it makes the difference between a hook that flexes elastically and one that snaps. Ribs, gussets and fillets covers this.

Orientation and fillet count for nothing on a hollow hook. On a small part hanging weight, the number of perimeters and the infill govern strength more than anything else: a well-oriented hook with two perimeters and 15% infill breaks all the same. Go up to four or five perimeters and leave the arm and the root practically solid; that's where the load-bearing section is. Walls, perimeters and infill spells it out.

Trays and boxes: hook at two points

With the decisions settled – insertion clearance, retention at two points, orientation and fillet, walls – you can now design real accessories. Anything that carries appreciable load – a tray, an organiser box, a jar holder – must hook at two or more spaced points, never at just one. Two hooks at the top and a lip bearing lower down against the panel form a triangle of reactions that doesn't tip: the weight of the contents creates a moment, and that moment needs two points to balance. With a single hook, the tray tilts outwards and dumps its contents.

Here the sensitivity to pitch returns. Two rigid hooks 40 mm apart demand that the panel have exactly 40 mm of pitch; with the tight clearance of a hole, half a millimetre of error is already enough to put the second point out of position. The solution is the one the panel itself offers: make the second hook elongated or slot-shaped, or seat it in one of the Skådis oblong slots rather than a round hole. That slot gives tolerance in the direction of the pitch – the hook finds room even if the real pitch isn't spot on – without releasing the part. Fix the first point and let the second float along its slot.

A common case is the Skådis-to-another-system adapter: a part with Skådis hooks at the back and, at the front, the interface of another wall standard – a grid, a rail, another pegboard. Here two hole patterns live in a single part, and each face has its own dimensions: the back, the 40 mm pitch and 5 mm holes of the Skådis; the front, whatever the other system measures. Measure the two separately and don't assume they share a grid, because they almost never do.

Once you have the hook geometry settled, the exact clearance value for your printer comes from a test coupon, not from this table. Print two or three posts with diameters stepped 0.1 mm apart, try them in a real hole in the panel, and keep the one that goes in loose and drops without seizing. That's your value, and you reuse it across all your accessories until you change material or nozzle. The full method is in Real printed clearances.