Threads and Snap Fits That Print

A printed thread works when three numbers are right: the pitch is coarse enough for the nozzle, the nut carries a calibrated diametral clearance, and the two helices are clocked so their flanks do not collide. A printed snap fit works when the hook's undercut is smaller than the deflection the arm can survive. Rig Cad has a Thread node for the first and an expression for the second; the demo proves both with an interference check and a readout.

Thread lives under Solids. It generates a helical thread form along local Z, growing from the base unless centerZ is on, and it has two personalities selected by type: external produces a threaded shaft, and internal_cut produces the cutter shape you subtract from a body to make a nut or a tapped hole.

Table: Thread node options.

The node also reports computed values: minor diameter, thread depth, lead angle and threads per length. Those appear in Object Properties and can be read by other expressions.

The demo bolt is a hexagonal polygon emitted as a solid with extrudeHeight, unioned with an external Thread positioned at headHeight. The nut is the same polygon minus an internal_cut Thread whose length is nutHeight + 2 so it clears both faces, the through-cutter rule from Oversize the Cutter applied to a helix.

tolerance is diametral. Put it on the cutter, bound to a calibrated variable, and leave the bolt nominal; the demo's threadClearance defaults to 0.3 mm, which is a common starting point for a 0.4 mm nozzle in PLA, and the Fits, Clearances, and Tolerances article explains why that number belongs to a printer profile rather than to the model.

A correct clearance is not proof that the pair assembles. Two helices only mesh when they are in phase: the nut's thread start must sit a whole number of turns from the bolt's, or the flanks land on each other. The demo derives the clocking from the geometry and applies it only when assembled is on:

nutPhase = mod((threadLength - nutHeight - 1) / threadPitch, 1) * 360
Nut.transform.rotation.z = assembled ? nutPhase : 0

The numerator is the axial distance between the two thread starts; its fractional part in pitches, times 360, is the angle the nut must turn.

Table: Interference check between Bolt_Thread and Nut on the demo, M8 by 1.25, clearance 0.3.

The Interference check computes the exact Boolean intersection of two solids, which is the only honest way to read a helix; one cross-section slice is phase-dependent and can show a clean gap on a pair that jams a quarter turn later.

Table: Thread rules of thumb for FDM. Calibrate on your own machine.

A test coupon of three or four nuts at different threadClearance values, printed in the production orientation, is a ten-minute print and settles the number for that material. Record it as the profile value, as Versioned Production recommends.

A cantilever snap fit is an arm that bends past a hook and springs back. The arm survives if the strain at its root stays below the material's allowable value, and for a straight arm of constant thickness the deflection that reaches that strain is^[1]

$$y_{max} = \frac{\varepsilon\, L^{2}}{1.5\, t}$$

where $L$ is the free length, $t$ the root thickness and $\varepsilon$ the allowable strain. The hook's undercut must be less than $y_{max}$. The demo exposes exactly that:

maxDeflection = allowableStrain * armLength^2 / (1.5 * armThickness)   (readout)
hookMargin    = maxDeflection - hookHeight                              (readout)

allowableStrain is a select whose values are guide numbers: 2 percent for PLA, 3.5 for PETG, 4 for ABS and ASA, 6 for nylon. They are conservative single-flex values; a clip that must cycle hundreds of times wants roughly half. With the defaults, an 18 mm PLA arm 1.6 mm thick may deflect 2.7 mm, so a 1.2 mm hook leaves 1.5 mm of margin. Switch to PLA at 30 mm and 1 mm and the same hook fails; the readout says so before the print does.

Three more rules do most of the remaining work.^[2] Taper the arm to half its root thickness at the tip and it can deflect about 1.67 times further at the same strain. Put a fillet of at least half the thickness at the root, using the blend techniques, because the sharp inside corner is where clips crack. Print the arm flat so the bend happens along the layers, not across them; a clip printed standing up delaminates at the root.

The insertion face of the hook wants a 25 to 35 degree ramp and the retention face 45 degrees for a releasable clip or 90 for a permanent one. The demo's hook is a plain box so the numbers stay legible; angle its faces with a path profile from Sketches to Solids when you build a real one.

The hinge in Fits, Clearances, and Tolerances is the same calibrate-then-record discipline applied to a pin and bore. The parameter-space rules that keep a customer from ordering an arm that cannot flex are in Beyond Sliders. The clip's arm thickness is a wall, and Shell, Hollowing, and Wall Thickness has the extrusion-width rule it must respect.