Metric fasteners: M3/M4/M5 patterns, bosses and inserts

Almost everything you screw into a printed part ends up being an M3, an M4 or an M5. A monitor's VESA mount, the four holes on a Raspberry Pi, a 2020 profile rail, a tripod thread: they all speak this language, and they all take for granted that you already know how to make the hole the screw passes through, the boss that receives it, or the pocket the nut drops into. The rest of this track tells you where the holes go for each standard; this article tells you how to make each one in FDM, and why the figure printed in the screw's catalogue isn't the one you draw.

Clearance holes: the standard sets the starting point, not the final dimension

A clearance hole lets the screw pass without threading into it: the shank crosses cleanly and the head clamps against the surface, while the thread bites on the other side — in a nut, an insert or a boss. The metric standard (ISO 273, medium fit) sets these diameters: 3.4 mm for M3, 4.5 mm for M4 and 5.5 mm for M5. They aren't the screws' nominal diameters: they're slightly larger, just enough for the shank not to rub.

The problem is the usual one in FDM: the hole comes out smaller than you draw it. The wall bead closes in on the diameter, the first-layer squish narrows the mouth, and shrinkage pulls the whole thing in. Every one of these effects pushes the same way — the physics is in Holes, pins and first-layer squish — so a 3.4 mm hole on screen can end up 3.2 mm at the mouth and leave you an M3 that goes in with a scrape instead of passing freely.

The nut thickness in the table follows DIN 934; for M3 and M4 it matches ISO 4032, but the ISO 4032 M5 nut is thicker — 4.7 mm instead of 4.0 — so measure the one you're actually going to use before you trust the figure. The across-flats sizes (5.5 / 7.0 / 8.0) are the same in both standards.

Take the clearance values as a starting point, not a final dimension. In practice, for a vertical through-hole in PLA, it pays to draw the hole 0.2–0.3 mm wider than the table's nominal — around 3.6 mm for M3 — and confirm it with a test coupon, exactly as you do with any loose fit in Real printed clearances. If the screw has to go in and out many times, or you want margin for paint, push it up a little more; err on the loose side, because a loose through-hole still clamps down and a tight one won't let the screw through at all.

Bosses and pilot holes: threading into the plastic itself

A boss is a solid cylinder that stands proud to receive a screw along its axis. When you want the screw to cut its own thread in the plastic — self-tapping, with no nut or insert — the pilot hole isn't a clearance hole: it's narrower than the nominal diameter, so the thread flank has material to bite into and displace.

The pilot has to be narrower than the shank, but not as narrow as the drill diameter used to cut a thread in metal. That metal tap-drill hole — 2.5 mm for M3, 3.3 mm for M4, 4.2 mm for M5 — is too aggressive in plastic: a thread-forming screw doesn't remove material, it displaces it, and against so tight a wall it builds up hoop stress that splits the boss. For a thread-forming screw in plastic the target is around 0.8 times the nominal diameter — roughly 2.4 mm for M3, 3.2 mm for M4 and 4.0 mm for M5 — as a starting figure. And remember that the pilot, like any hole, shrinks on printing: if you have to correct, correct upward, not down, because an undersized pilot is exactly what cracks the wall. The exact value depends on the type of screw (forming versus cutting), the material and the torque: print two or three pilots stepped by 0.1 mm and test which one grips without splitting.

Heat-set inserts: the thread that lasts

When you want a reliable metal thread — one that takes torque and repeated assemblies without wearing out — the answer is a heat-set insert: a threaded brass bush that you melt into a hole with the tip of a soldering iron. The brass heats up, softens the plastic of the wall, sinks in square, and on cooling ends up anchored by the material that has flowed around its knurling. It's by far the most robust way to thread a printed part.

Here the hole isn't sized for the screw but for the insert: a plain cylinder, slightly narrower than the outer diameter of the bush, so that when it melts there's enough plastic to flow and grip it — without the insert sinking in loose or squeezing plastic out. That diameter depends on the insert's series and manufacturer — one maker's M3 doesn't call for the same hole as another's M3 — so there's no single reliable value here: check the datasheet for your insert and dial it in with a test. The full procedure — hole geometry, entry chamfer, iron temperature and orientation — is in Designing for heat-set inserts.

Captive nuts: a hexagon that traps the nut

The iron-free alternative is the captive nut: a hexagonal pocket inside the part that holds a standard metric nut, so the screw enters from one side and the nut can't turn because the flats of the hexagon lock it. It's cheap, reversible and very solid, and you see it right across the maker ecosystem: profile joints, enclosures, mounts.

The pocket is drawn to the nut's across-flats — 5.5 mm for M3, 7.0 mm for M4, 8.0 mm for M5, from the table above — plus a clearance, because the printed hexagon, like every hole in FDM, comes out tight and with its corners rounded off by the bead radius. Give it around 0.2–0.4 mm over the across-flats so the nut goes in without hammering it but without rattling either. For depth, make the pocket a touch shallower than the nut's thickness — your nut's, which changes with the standard — so that on tightening, the screw pulls the nut against the bottom and seats it instead of leaving it floating.

Orientation: round holes versus load-bearing strength

Two things depend on how you set the part on the bed, and both matter more than any tenth of a millimetre of clearance.

The first is roundness. A hole printed upright — its axis perpendicular to the bed — comes out cylindrical, because each layer is a circle the nozzle traces in full. The same hole laid down prints as a run of unsupported bridges on the inside: the roof sags and the hole comes out oval, collapsed at the top. An M5 through-hole laid flat stops being a round 5.5 mm and becomes a deformed slot that the screw enters crooked. A small horizontal hole, up to 5 or 6 mm, holds up reasonably well without support; above that the collapse shows. When you can't put the axis vertical, don't print the hole as a bare circle: give it a teardrop profile or a diamond- or hexagon-topped roof, or at least a top chamfer, so the material has something to hold onto and comes out round without support. It's the standard trick for horizontal holes, and it works the same for through-holes, pilots and insert bores.

The second is inter-layer adhesion, and it only shows up when the joint takes load or torque. An FDM part is strong within the layer and weak between layers: the join between stacked beads is its natural fracture plane. A screw under tension pulls along its axis and presses under the head; a boss taking torque sees radial stress. A boss printed upright, with its layers stacked crosswise to the screw's tension, shears off along a layer line; laying it down would align the layers with that tension, but then the hole prints on its side and loses its roundness. That's the conflict: what saves strength ruins geometry, and vice versa.

The real way out isn't to choose between the two — it's an insert. Set in a vertical hole — round and accurate — it spreads the screw's load across a knurled metal wall that grips several layers at once, instead of concentrating it on a formed thread in a single layer line. That way you keep the vertical axis that gives you the round hole and, at the same time, resolve the inter-layer weakness. When an upright self-tapped boss wouldn't hold, the insert is what reconciles orientation and strength.

With these four patterns — through-hole, boss, insert and captive nut — mastered and calibrated on your machine, the rest of this track is geometry: where the VESA mount's holes fall, the pitch of the holes on a Raspberry Pi or an Arduino, the slot of a 2020 profile, the thread of a tripod. They all rest on this vocabulary. The next link, when the thread has to last, is Designing for heat-set inserts.