What "rated pull" actually means
The number on a magnet listing is usually its pull force measured perpendicular to a thick, flat, ground steel plate with zero air gap. Your project almost never looks like that. A 1 mm layer of printed plastic between magnet and steel, a thin sheet of painted steel instead of a thick plate, or a load that slides sideways instead of pulling straight off — each one cuts holding force, often dramatically.
Sideways loading matters most for everyday builds. A magnetic tool strip on a wall carries tools in shear, and resistance to sliding depends largely on friction between the surfaces. Manufacturers commonly suggest that shear holding is a fraction of rated pull — often quoted around a fifth to a quarter on bare steel. Rubber-coated pot magnets exist precisely to raise that friction.
Grades, heat and why your magnet got weaker
Neodymium-iron-boron magnets were announced in 1984, developed in parallel at General Motors and at Sumitomo Special Metals, where Masato Sagawa led the work. Their grade numbers — N35, N42, N52 — refer to maximum energy product in mega-gauss-oersteds. Higher grades are stronger for the same size, but they are not the only lever: a slightly larger N35 magnet can outperform a smaller N52.
Heat is the quiet failure. Standard N-grade magnets are typically rated to about 80 °C. Glue one into a part that sits near a heated print bed, a radiator or a soldering station and it can lose strength permanently. Suffixed grades (M, H, SH and beyond) tolerate higher temperatures. If a build lives near heat, check the datasheet, not the listing title.
Alignment: magnets as locating features
A pair of magnets with opposite poles facing does two jobs at once: it holds and it centres. Lids drop into place, modules click onto a baseplate and swap-out tool heads find their position without pins. For better repeatability, combine magnets with a kinematic arrangement — three balls in three V-grooves, an idea associated with James Clerk Maxwell — and let the magnet supply only the preload. The balls locate; the magnet holds.
Minimise the gap. Put magnets flush with the surface or behind the thinnest wall you can print reliably. A steel washer as a target is cheaper than a second magnet and avoids polarity mistakes.
Use paired magnets or magnet-and-cone pairs to self-centre parts. Alternating polarities in a pattern can make a joint that only closes one way round.
Release is easiest by peeling or sliding rather than pulling straight off. A lever lip or a rotating cam that increases the gap releases a strong magnet with fingertip force.
Halbach arrays: one strong side
Arrange magnets so their magnetisation rotates step by step along a row and the field strengthens on one side while largely cancelling on the other. John Mallinson described the one-sided effect in 1973; Klaus Halbach developed arrays for particle accelerators at Lawrence Berkeley Laboratory. For hobby builds, a small array behind a panel can hold firmly on the front while staying weak behind — handy near electronics. Assembly is the hard part: the magnets fight you, so use a printed jig and clamp each magnet as you go.
Plating, corrosion and safety
NdFeB corrodes readily, so most magnets carry a nickel-copper-nickel coating. Chip it with a press fit or a pair of pliers and rust can start, especially in kitchens and bathrooms. Glue magnets into slightly oversized pockets instead of forcing them.
Small strong magnets are a serious swallowing hazard: two magnets attracting across intestinal walls can cause severe injury. Keep loose magnets away from children and pets, fully enclose them in finished builds and keep strong magnets away from pacemakers and other implanted medical devices. Large magnets can pinch skin hard enough to injure.
Illustrative dimensions — not verified project measurements
Our cable-holder prototype used a 10 × 3 mm disc behind a 0.6 mm printed skin. That worked for a phone cable; it would not hold a laptop charger brick.
Failure modes to expect
Magnet pops out of its pocket
The attraction to its partner exceeds the glue or press-fit grip. Use a captured pocket — print a pause and drop the magnet in, then cover it — or a backing lip.
Wrong polarity glued in
Mark one face of every magnet with a pen before assembly, using a reference magnet. It costs ten seconds and saves a reprint.
Grip fades over months
Check for heat exposure, corrosion under chipped plating or dust building up in the gap. Clean surfaces first; replace if strength has truly dropped.
Parts slide even though they "hold"
You are loading in shear. Add friction (rubber, a textured surface) or a mechanical ledge that carries weight while the magnet only resists tipping.
"Let the geometry carry the load and the magnet supply the intent."Puff Tinker Puzzle bench notes