Meccano and Erector: the metal grid of holes

Before there were tubes that snapped together, construction toys bolted. Meccano was born in 1901 as perforated steel strips and plates joined with a screw and nut, and in the United States the same principle sold for decades under the name Erector. The cleverness of the system isn't in any single part but in the grid: every hole falls on a regular lattice, so any strip can bridge any two points on another. If you want to print a part that joins an inherited box of Meccano — a bespoke plate, a bracket you can't buy — your only real job is to reproduce that grid precisely enough for a screw from the system to pass through your holes and find its nut on the other side. And that's exactly where FDM lays its usual trap: the hole you draw isn't the hole you get.

The system: perforated steel that bolts together

Meccano and Erector are metal construction systems. The parts are strips, plates, angles and brackets of sheet steel, all with through-holes, and the joint is always the same: a screw crosses the holes of two parts and a nut closes it from behind. There's no friction fit between parts as there is with the tubes or studs of other systems; here the friction comes from the screw, and what you have to nail down is the position of the holes, not their grip.

That difference changes what you're chasing when you print. In a snap-fit system, a tenth of a millimetre either way on a peg decides whether it grips or wobbles. Here the screw is metal and doesn't change; what decides whether your part assembles is that its holes land exactly where the system expects them and that they're wide enough for the screw to enter without forcing. Grid accuracy and clearance for the screw: those are the two parameters, and they're independent.

The grid in inches: pitch and diameter

Meccano is an imperial system, designed in inches long before anyone thought in millimetres. The grid pitch — the centre-to-centre distance between adjacent holes — is half an inch, that is, 12.7 mm. That number governs the whole system: a "five-hole" strip measures four pitches between its first and last hole, the holes in plates fall on a square lattice of 12.7 mm, and two parts only mate if their grids share that same pitch. If your printed plate places the holes at 12.5 mm, the error compounds: at the first hole you won't notice it, but by the fifth you're a whole millimetre out of step and the screw no longer enters.

The through-hole diameter is around 4.3 mm (roughly 0.170 in), sized to let the system screw pass freely. Classic Meccano isn't metric: its threaded hardware is a 5/32 in Whitworth thread, 32 threads per inch, specific to the system. The screw has a shank of the order of 4.0 mm (5/32 in ≈ 3.97 mm), so the hole is always slightly larger than the shank — it's a clearance hole, not a thread — and that clearance is what lets the nut clamp, and what lets two parts be angled against each other before you fasten them.

Why the nominal hole comes out tight

Here's where the FDM bias comes in, the same one that governs any printed fit and is covered in full in Real printed clearances: holes come out small. The nozzle lays down a bead 0.42–0.48 mm wide, the squash of the first layers closes the mouth of the hole, and cooling contraction shrinks it; on a small circle like this a smaller effect adds to them: the facets of the contour (the circle is approximated as a polygon) cut just inside the true chord. All of these effects pull the same way — they narrow the hole — and the first two dominate. A hole drawn at 4.3 mm, the system nominal, doesn't come out at 4.3 mm: it comes out at 4.1 or 4.15, and the 4 mm screw already scrapes as it passes.

You do the sums per side. The screw is the fixed part; you only control the hole. For the screw to pass with the play of a comfortable clearance hole, you need about 0.2 mm of gap per side, that is, a hole of roughly 4.4 mm diameter in the model. In well-calibrated PLA, that gives a clearance hole which turns smoothly and lets you set the part's angle before you tighten the nut. If you want it tighter, 4.3 mm; below 4.2 mm you risk the bias eating the gap so the screw won't enter. Remember that the safe side to fail on is the loose one: a hole 0.1 mm wider than it should be only leaves a little play that the nut closes anyway; a hole too narrow forces you to reprint the whole plate. And if you force it, you don't delaminate — you split the perimeters of the hole from the inside, because in a flat plate the wedge of the screw pushes in the plane of the layers, not between them.

Orientation: plate flat, holes vertical

How you rest the part on the bed decides whether your holes come out round and whether your strip holds. Both push towards the same orientation, and happily it's the natural one.

Print the plate flat on the bed, with its broad face against it. That leaves the holes vertical, their axis perpendicular to the bed, printed layer by layer as a stack of circles. A vertical hole comes out cylindrical and round; the bias narrows it a little, but that loss is predictable and you compensate for it by opening the diameter. A lying hole — with its axis parallel to the bed — is another story: its upper half prints as an overhang, sags without support and comes out oval, with the top collapsed. That distortion isn't fixed by opening the dimension, because it isn't a uniform shrinkage but a sag, and a screw that needs a round hole won't forgive an oval. With the plate flat you don't even have to think about it: all the holes come out vertical in one go.

Strength: a printed strip is not a steel strip

Let's be plain: a strip printed in PLA is far less stiff than the steel one it replaces. Meccano sheet is under a millimetre thick and still withstands bending and tension because steel is some 50 to 60 times stiffer than PLA (200 GPa against about 3–3.5 GPa). If you copy the metal thickness in plastic, you get a strip that bends by hand and, worse still, breaks along a layer line the moment you load it the wrong way.

FDM is anisotropic: the part is strong along the layers, in the plane of deposition, and weak between layers, where the bond is only the thermal adhesion of one bead to the one below. A strip printed flat has its layers stacked vertically, so a tension along the strip — the normal use, pulling on one end — runs along the layers and holds well. What it won't forgive is the bending that tries to peel the layers apart. Orient the part so the main loads run in the plane of the layers, and beef it up where the metal was thin: a useful printed strip wants 3 or 4 mm of thickness, not the scant millimetre of steel, and a large plate appreciates a rib or a raised edge that stiffens it without turning it into a brick. You're swapping a stiff, thin material for a flexible, thick one; the design has to reflect that.

And if the part is going to live under sustained load, count on the creep of PLA: under constant stress it flows cold, yields little by little over time, and a bolted joint that carries weight all day ends up relaxing. For a structural support working continuously, a PETG or an ABS will give you longer-lasting stiffness and loosen far less at the joint.

What to print in plastic and what to leave in steel

If you're going to bolt plastic to a box of Meccano, don't compete with steel at what steel does well. A long straight strip, under bending, will always come out better in metal. What FDM gives you for free is geometry that sheet metal won't give you easily: plates with irregular shapes cut to the measure of your build, brackets with angles that aren't 90°, mounts that wrap a motor or a sensor and carry the grid of holes exactly where you need it, ready to bolt to the frame. This is where thick printed plastic wins, because such a part in metal would demand cutting, bending and drilling, whereas you lift it off the bed with the holes already in place.

The rule is simple: reproduce the 12.7 mm pitch with positional accuracy — compensating for contraction on a long plate — give the hole about 4.4 mm so the screw passes freely, print the part flat with the holes vertical, and thicken the material until its stiffness makes up for what it lacks against steel. When you get into the hardware — which screw to use, when to fit an insert instead of a through-nut — the detail is in Metric fasteners: M3/M4/M5 patterns, bosses and inserts.