Trigger: a lever with return

You squeeze a trigger and expect it to come back on its own. That expectation — so automatic you never think about it — hides two mechanisms in a single part: a lever that pivots to multiply your force or your travel, and an elastic element that returns it to rest when you let go. In FDM the trigger is also a textbook case of everything that goes wrong with parts that move: a pivot that seizes, a root that splits along its layers, an integrated spring that fatigues. It packs those three problems into a few centimeters and runs them through a load cycle hundreds of times over. Designing it well means deciding, before you draw anything, where you want the play to live, which way the layers point, and what the elasticity has to do.

The lever: what you multiply, and what it costs

A trigger is a lever turning on a pivot, and at equilibrium it obeys a single sum: the input moment equals the output moment. The finger pushes at some distance from the pivot — the input arm. The output (the rod that pushes the glue stick, the plunger you actuate, the catch you release) acts at another distance, the output arm. Since the moment is force times arm and the two moments are equal, the output delivers more or less force than the input according to the ratio of the arms: that is the mechanical advantage.

What you cannot have at the same time is gain in force and gain in travel, and the reason is not the equality of moments but the conservation of work. The tip of the long arm travels a longer path than the tip of the short one through the same rotation, and since work — force times displacement — is conserved from one side to the other, what you multiply in force you pay for in travel, and vice versa. If the finger arm is longer than the output arm, you multiply force: the finger travels far and pushes little, and the output moves little but pushes hard. That is what you want in a glue gun, where you have to overcome the resistance of the molten filament. Invert it so the output arm is the long one and you multiply travel: a short tap of the finger turns into a wide displacement of the output, at the cost of demanding more force from the finger. Those two arms are the first thing you size, before any printing detail, because together they fix both the stroke and the effort.

That said, the class of lever is not free: it constrains which trade-off you can pick. A first-class lever puts the pivot between the finger and the output, like a seesaw, and depending on where you place it, it multiplies force or travel as you like. A third-class lever applies the finger between the pivot and the output, so the input arm is always the shorter one: it multiplies travel and reduces force, no exception — its gain is always less than one. If what you need is push, a third-class lever won't give it to you no matter how you tune it: you have to go to a first-class lever with the finger on the long arm. The arithmetic of the arm ratio doesn't change; what changes is which ratios each arrangement lets you reach.

The return: integrated flexure or elastic band

For it to come back on its own, you have to store energy when you squeeze and release it when you let go. You have two families, and the choice decides how long the trigger lasts.

The integrated flexure is a cantilever beam or a serpentine spring printed into the same part, bent by the trigger's motion and recovering its shape when you release. Its virtue is assembly: it all comes out of a single print, with nothing to put together. The price is fatigue. An FDM plastic bent over and over at the same spot accumulates damage in the outer fiber of the bend. Since that fiber usually coincides with a weld between layers, the flexure ends up cracking there long before the rest of the part shows the slightest symptom. It holds up well if you size it with margin — long arm, working deflection well below the material's limit — and poorly if you shrink it to fit.

The elastic band housed in a channel flips the balance: it adds a part to assemble and a housing to design, but the element that fatigues is no longer your printed plastic but a rubber meant to stretch. It takes many more cycles than an FDM flexure and, if it ever gives out, you replace it without reprinting anything. The rule of thumb is simple: for a toy or an occasional actuation, the integrated flexure keeps things simple and is enough; for a tool that will be squeezed daily for years, the band more than justifies the extra assembly.

The pivot: a turning fit, not a machining fit

The trigger's rotation rests on a pivot, and in FDM you have two ways to solve it: a pin printed in place (print-in-place), which comes off the bed already articulated, or a metal axle inserted into a hole with turning clearance. The printed pin saves assembly but inherits all the roughness and imprecision of the process on the contact surface; the inserted axle gives a cleaner bearing and, if you add a bushing too, a much more durable rotation.

Whichever you choose, the gap between axle and hole is a turning fit per side, not the zero clearance a machining shop would call good. And here the margin is narrow in both directions. With too little play the pivot seizes: remember that the process fattens the axle and narrows the hole, so a gap that looks roomy on screen can come out as interference in the part. With too much play the trigger develops slop — it rocks on the axle instead of turning cleanly — and loses its feel. A reasonable starting value is around 0.2 mm per side; reserve 0.3 mm for relatively large or metal axles, and drop toward 0.15 mm on a small-diameter print-in-place pin, where that same gap matters proportionally more and worsens the slop. The exact figure comes from your printer, once you've measured it, not from a table; Tolerances for moving parts works it out in detail, including the crucial difference between reasoning per side and reasoning per diameter.

Two mechanical stops complete the pivot, and they are not optional. A start-of-stroke stop defines the rest position and gives the flexure its starting shape; an end-of-stroke stop arrests the travel before the lever demands more deflection from the flexure than it can take at once. It's worth not confusing the flexure's two failure limits: fatigue is damage that accumulates cycle by cycle and is controlled by sizing — long arm, low working deflection — not with a stop; a one-off over-extension is the tearing of a single hard squeeze, and that is what the stop does protect you from. Without that end stop, the user who squeezes all the way drives the spring past its limit in one motion and breaks it: the stop turns a predictable abuse into a dead blow against solid plastic.

Orientation: keep the finger's load from splitting the layers

Layered printing has a strong direction — along the beads — and a weak one — between one layer and the next, where only the weld of one to the next holds. In a trigger, that fact decides two orientations at once.

The first is the pivot pin. Orient it parallel to the layers, so the axle runs along the beads over its whole length. If you print it upright, perpendicular to the bed, its cylindrical surface is formed by the stack of layers and rides on the inter-layer welds, exactly where it wears and chips first; laid down, the turning surface is continuous along the bead and lasts much longer.

There's an interaction worth keeping in mind on print-in-place pivots: the orientation you pick for strength affects the real turning clearance. In a pin that comes off already articulated, the axle and the hole are printed together, and the slicer fixes the gap one way in the XY plane — parallel to the layers — and differently in Z — between layers, where the layer staircase and elephant's foot get in the way. Laying the pin down to gain strength puts the turning interface into mixed XY/Z planes, and the effective clearance is no longer the same in every direction. That's no reason to print it upright — wear rules — but it is a reason to measure your articulated pivot's clearance in the final orientation, not on a vertical test coupon that doesn't reflect it.

The second orientation, and the most critical, is that of the arm root. The finger's force bends the lever and loads its junction with the trigger body. If you print the part so that bending pulls two layers apart, the root works in tension between layers — the weak plane — and delaminates, unzipping after a handful of squeezes. Orient the part so the arm bends in the plane of the layers, following the beads, and the same load that used to delaminate the part now bears on sound material. It's the same logic that governs any articulated part and that Layer orientation for motion covers in detail: orientation isn't a finish, it's what decides which way the part's weakness points.

Three failure modes and how to design them out

A trigger fails in three places, and naming them is the first step to designing them out.

The first is fracture of the lever root. Where the finger arm meets the pivot hub there's an abrupt change of section, and a sharp corner there acts as a stress concentrator: the load you thought was spread out spikes locally and opens a crack on the first hard push, even if the arm was oversized everywhere else. The defense is a generous fillet at that junction, as large as the geometry allows; on the order of a third of the arm's thickness already cuts down the bulk of the concentration, and going beyond that is safe: an oversized radius does no harm, it only eats into usable arm length. Without that radius you've drawn, without meaning to, the line along which it's going to break.

The second is fatigue of the return flexure, already covered: the integrated spring that cracks along the outer fiber after many cycles. You fight it with a long, lightly loaded flexure, with a radius at its own root, and by orienting it — like the arm — to bend in the plane of the layers and not between them. If the calculation asks you to stress it too much, that's the signal to move to the elastic band.

The third is wear of the printed pivot. A plastic axle turning on a plastic hole hundreds of times files down the contact surface, the gap grows, and the trigger that started out precise ends up sloppy. The fix isn't geometry but hardware: a metal axle and, better still, an embedded bushing or sleeve that gives the rotation a surface that doesn't wear. How to house that bushing without cracking the hole or leaving it loose is covered by Embedded hardware: magnets, bearings, and inserts.

A trigger is, ultimately, a pivot and a spring sharing one body, and both are designed around the same figure: the real clearance of your printer. Measure it once before you tune the turning fit, set your pivot gap with it, and reuse it across all your mechanisms; Tolerances for moving parts walks you from what the joint must do to the clearance you set.