Pegboard: the wall panel we all grew up with

You hang a shop-bought hook on the garage pegboard and it holds anything you throw at it. You print one yourself, same shape, and the first time you reach for a wrench the shank pops out of the hole or the arm snaps clean off with a sharp crack. The geometry was right; everything else was wrong — how much the shank grows as it prints, which way the layers run through the arm, whether or not there is a fillet at the root. The pegboard is the oldest and most widespread wall-organisation system there is, and precisely because it is old, nobody spells out its dimensions: they are taken for granted. Here they are, and with them the translation into FDM clearances that makes a printed hook — a peg — actually hold.

What it is and why it resists printing well

A pegboard is a board — hardboard (high-density fibreboard), plywood or metal — with a regular grid of through holes. Accessories carry a shank shaped like a hook or a U that slots into those holes and is held in place by the thickness of the board itself. It is a purely mechanical system: no screws, no rails, just a hole and a bent rod passing through it.

That same simplicity is what makes it so treacherous to reproduce in FDM. The original accessory is made of steel wire or moulded plastic, processes that pin the shank dimension down to hundredths of a millimetre. When you lay down beads of molten material, the shank does not come out at the dimension you drew: it comes out thicker. Take the nominal hole diameter, model a shank to match, and it will not go in; force it and it goes in as a press fit, at which point you can no longer adjust the height of the hook. The reason for that thickening is the usual one in FDM, and Real printed clearances sets it out in full: holes shrink and posts grow, so a nominal fit always comes out tight.

The two families: inches and millimetres

There is no such thing as the pegboard: there are two lineages, and you tell them apart by the grid pitch. The original is imperial, North American, with holes at exactly one inch (25.4 mm) between centres. Within that family two hole gauges coexist: the classic 1/4 inch (6.35 mm), meant for thick wire hooks, and a finer variant, 3/16 inch (4.76 mm), common on lightweight DIY boards. The metric lineage, more common in Europe, works to a 25 mm pitch with holes of 5 to 6 mm.

The difference between 25.4 and 25 mm looks trivial — four tenths — but it accumulates. Every pitch drags those four tenths along, so between the first hole and the sixth you are already two millimetres out: enough that a long, multi-prong accessory will not line up with all its holes at once. That is why a peg has to be printed for your board, not for "the pegboard".

The two-prong peg: why one is almost never enough

A hook that grips through a single hole has an elementary physics problem: hang weight on it and the load sits in front of the board, generating a moment that tends to tip the accessory. The shank acts as a pivot, the tip of the hook pushes up against the top edge of the hole, and the whole thing rotates until it falls off. That is why well-designed pegboard accessories go through two holes: a lower one that carries the load-bearing shank and an upper prong that bears against the board one pitch higher up and absorbs that overturning moment. Two contact points, one pitch apart vertically, turn an unstable pivot into a rigid anchor.

When you design the printed peg, that upper prong has to land exactly one pitch above the lower shank — 25.4 mm or 25 mm depending on your board — to enter both holes at once. With two adjacent prongs, the shank clearance leaves ample room to absorb the difference between families: it is when you chain several prongs together that the drift accumulates and it stops seating. Getting the family wrong hurts here more than anywhere else, because the whole accessory lives on this pitch.

Shank clearance: how much gap to leave per side

This is where the theory turns into millimetres. The printed shank grows relative to what you draw, and the board hole is fixed, so you have to shave material off the shank on purpose to leave a gap. Always reason per side — the gap on each flank of the shank, measured on the radius — and convert to diameter only at the very end, as Real printed clearances insists: a clearance of 0.3 mm per side means a shank 0.6 mm smaller in diameter than the hole.

For a pegboard you want the peg to go in and out by hand, but without rattling once fitted, so you are after a locating clearance with a touch of margin. In PLA, starting from 0.25–0.35 mm per side off the nominal hole diameter is a good first cut: for a 6.35 mm hole, a shank modelled at around 5.7–5.8 mm diameter. With PETG, which oozes more and comes out slightly thicker, add 0.05–0.10 mm per side and stay at the loose end of the range. Overshoot on the loose side and the peg rattles a little but still holds; come up short and it will not go in — and on an accessory that passes some depth through the board, a tight shank jams halfway.

Orientation: what decides whether the hook holds or snaps

This is the step that breaks the most pegs, and it has nothing to do with clearance. A hook is a cantilever arm, and hanging weight on it puts that arm in bending: its upper fibres stretch and its lower fibres compress. In an FDM part the strength depends heavily on the layer direction, because the bond between layers is the weak point — the material sticks to itself, but that weld holds far less than a continuous bead.

Print the peg upright, with the hook pointing up and the layers stacked perpendicular to the arm, and the bending tension falls straight across the interface between layers. The moment you hang something, that tension prises apart two layers at the root of the hook and the arm snaps cleanly along a layer line. It does not bend, it gives no warning: it breaks.

The fix is to lay the peg down on the bed so the hook arm runs along the layers, with the beads following the axis of the arm. Then the bending stress travels along continuous beads, without crossing the weld between them, and the arm holds several times longer. The difference between a well- and badly-oriented cantilever is roughly 2 to 4 times — not an order of magnitude, but the difference between holding and not. It is the same principle that governs any hook, tab or part that carries hanging load: orient the part so the bending tension runs along the beads and does not cross the layer interface.

And finish the root of the hook with a fillet. The inner corner where the arm meets the shank is where all the bending stress concentrates; a sharp edge there is the perfect place for a crack to start. A generous fillet — 2 to 4 mm radius — spreads that stress over more material and lifts the breaking load noticeably. In FDM the fillet has a bonus: it stops the corner layers forming an abrupt angle, where the sharp change of path weakens adhesion.

A loaded cantilever does not live on infill, it lives on walls. It does little good to lay the arm down and add a fillet if the slicer leaves you a hook with two perimeters and 15% infill: the crack finds the void straight away. Go up to four or more perimeters or, better on a part this small, print the arm and shank solid (100% infill). The extra material weighs next to nothing and multiplies the continuous section that resists bending.

With the pitch measured on your board, the shank tuned per side, and the hook laid down with its fillet and its perimeters, you have an accessory that holds like the hardware-store one instead of dropping the load on the floor. If your wall has slots rather than holes, the system and its dimensions change entirely: that is covered in Wall Control: the slotted steel panel.