Mechanisms, Fasteners and Snap-Fits
Joints for prototypes, screws and inserts in plastic, snap-fit strain, press fits and clearances.
Joints for prototypes
| Joint | How | Notes |
|---|---|---|
| Screws into plastic | Self-tapping screw into a pilot hole | Fine for a few assemblies; the thread wears out |
| Heat-set inserts | Melt a brass insert into a printed hole with a soldering iron | Strong, reusable threads; follow the insert maker’s hole size |
| Captive nut | A hex pocket traps a nut | Strong; no tapping; design the pocket into the part |
| Press fit | Slightly oversize part forced into a hole | Needs good tolerances (see the 3D Printing Tolerancing Guide) |
| Adhesives | Cyanoacrylate (superglue), epoxy, solvent weld for acrylic, hot glue | Match the glue to the plastic; epoxy for structural joints; ventilate with solvents |
| Snap-fits | A flexible hook or beam that clicks into place | No fasteners; one-piece design (see below) |
| Living hinge | A thin section that flexes repeatedly | Works in polypropylene; printed ones last a limited number of cycles |
| Tab and slot | Flat parts interlock | Common in laser-cut designs; allow for the kerf |
| Rivets, pins, zip ties | Mechanical fasteners for quick fixing | Fast for prototypes; not for repeated assembly |
Screws, inserts and nuts in plastic
Plastic threads are weak and wear out with repeated assembly. For a prototype that will be opened often, use a metal thread: a heat-set insert or a captive nut. For a one-off assembly, a self-tapping screw into a pilot hole is enough.
Worked example: an M3 screw into a printed boss
- The pilot hole is about 0.8 × the screw diameter: 0.82 × 3 = 2.5 mm (print it 0.1 mm smaller and ream, or drill). Engage at least 4.5 mm of thread (1.5 × diameter; 6 mm here is better).
- Tightening to 0.25 N·m gives a preload of F = T ÷ (K d) = 0.25 ÷ (0.2 × 0.003) = 417 N (K ≈ 0.2).
- The thread strips in shear: with a pitch diameter of 2.7 mm, 6 mm engaged and a plastic shear strength of 20 MPa (typical), and half of the circumference effective, F = 20 × π × 2.7 × 6 × 0.5 = 509 N. The margin over the preload is only 1.2: a screw tightened by feel will strip the thread. Use an insert or stop at a light snug.
- More about bolts and preload is in Bolted and Welded Joints.
| Thread | Insert OD (typical) | Printed hole diameter | Insert length | Boss outside diameter (min) |
|---|---|---|---|---|
| M2 | 3.2 mm | 2.8 to 3.0 mm | 4 mm | 5.6 mm |
| M2.5 | 3.6 mm | 3.2 to 3.4 mm | 4 mm | 6.0 mm |
| M3 | 4.6 mm | 4.2 to 4.4 mm | 5 mm | 7.0 mm |
| M4 | 5.6 mm | 5.2 to 5.4 mm | 6 mm | 8.0 mm |
| M5 | 6.4 mm | 6.0 to 6.2 mm | 7 mm | 8.8 mm |
Follow the insert maker’s hole size: values vary by brand and by material. Push the insert in slowly with a soldering iron at 200 to 250 °C for PLA, until it is flush, and let it cool.
Worked example: a captive nut pocket
- An M3 nut measures 5.5 mm across the flats and 2.4 mm thick. Allow 0.25 mm clearance each side: pocket across flats = 5.5 + 2 × 0.25 = 6.0 mm, i.e. a hexagon of 6.93 mm across the corners (the nut itself is 6.35 mm). Depth 2.7 mm. Slide the nut in from the side or pause the print to drop it in.
Press fits
A press fit holds by friction: the shaft is slightly larger than the hole, so the hole stretches and presses on the shaft. The interference should be small and the material strong enough.
Worked example: a printed press fit
- An 8 mm pin is pressed into a hole of the same PLA, with a diametral interference of 0.10 mm. For a solid pin in a large hub of the same material the contact pressure is p = E δ ÷ (2 d) = 3 500 × 0.10 ÷ (2 × 8) = 21.9 MPa. The hoop stress at the bore equals p: well below the 40 MPa strength.
- Holding force F = μ p π d L = 0.3 × 21.9 × π × 8 × 10 = 1 649 N for 10 mm of engagement. Plastics creep and relax: allow for half of that after some weeks (825 N).
- But printed holes are rarely accurate to 0.05 mm: print test coupons with holes in 0.05 mm steps to find the interference that works on your printer (see Strength, Accuracy and Fits).
Clearances for printed and cut parts
| Fit | FDM (per side, mm) | Resin / SLA (per side, mm) | Laser-cut (after kerf, mm) |
|---|---|---|---|
| Press fit (parts stay together) | −0.05 to −0.15 | 0 to −0.1 | 0 to −0.05 |
| Snug sliding fit | 0.10 to 0.20 | 0.05 to 0.10 | 0.05 |
| Free sliding / rotating | 0.25 to 0.40 | 0.15 to 0.25 | 0.10 |
| Print-in-place gap | 0.30 to 0.50 | 0.20 to 0.30 | n/a |
| Easy fit of a lid or box | 0.20 to 0.30 | 0.15 | 0.10 |
Worked example: a laser-cut tab and slot
- A tab of 10 mm is to fit a slot with a press fit. The laser removes a kerf of 0.2 mm, so a slot drawn at 10 mm is cut 10.2 mm wide and a tab drawn at 10 mm is cut 9.8 mm: a loose fit.
- For a snug fit with 0.05 mm of interference the cut slot must be 0.05 mm narrower than the cut tab (9.8 mm). The slot is cut 0.2 mm wider than drawn, so draw it at tab − 2 × kerf − interference = 10 − 0.4 − 0.05 = 9.55 mm. Cut a test strip of slots at 0.05 mm steps to find the value for your laser and material (see Kerf, Accuracy and Designing Parts).
Snap-fits
A cantilever snap-fit is a beam that bends as the parts are pushed together, and then springs back. Its bending strain at the root must stay below what the material can survive many times:
ε = 1.5 × h × δ ÷ L²
Worked example: a printed snap-fit
- First attempt: L = 15 mm, h = 2 mm, δ = 1.5 mm: ε = 1.5 × 2 × 1.5 ÷ 15² = 4.5 ÷ 225 = 2.00 %.
- Redesign: lengthen the beam to 20 mm, thin it to 1.5 mm, and cut the deflection to 1 mm: ε = 1.5 × 1.5 × 1 ÷ 20² = 2.25 ÷ 400 = 0.56 %, nearly four times lower.
- Guideline: brittle plastics such as PLA need about 1 % or less for repeated use (a conservative starting point, not a data-sheet limit); tougher ones such as nylon and PETG tolerate more. Test a few before committing.
Strain depends on L squared, so length is the most powerful change: doubling it cuts the strain by four. Taper the beam (thicker at the root), add a fillet at the root to avoid a stress concentrator, and print it so the layers run along the beam (see Designing for 3D Printing), never across it.
Worked example: how far can the hook flex, and how hard is the click?
- Maximum deflection for an allowable strain: δ = ε L² ÷ (1.5 h). For L = 20 mm, h = 1.5 mm and ε = 1 %: δ = 0.01 × 400 ÷ (1.5 × 1.5) = 1.78 mm.
- The force to deflect the beam by 1.0 mm: P = E b h³ δ ÷ (4 L³) = 2 100 × 6 × 3.38 × 1.0 ÷ (4 × 8 000) = 1.33 N (PETG, E = 2 100 MPa, width 6 mm).
- With a 30° lead-in ramp on the hook and a friction coefficient of 0.3, the insertion force is P (μ + tan α) ÷ (1 − μ tan α) = 1.41 N: a light click. A steeper ramp or a stiffer beam makes the click harder.
| Material | Allowable strain | Maximum deflection (L = 20, h = 1.5 mm) | Notes |
|---|---|---|---|
| PLA | 1 % (typical, repeated use) | 1.8 mm | brittle: avoid repeated snapping |
| PETG | 3 % (typical, repeated use) | 5.3 mm | tougher; common choice |
| ABS / ASA | 3 % (typical, repeated use) | 4.4 mm | good; warps |
| Nylon (PA12) | 5 % (typical, repeated use) | 8.9 mm | excellent fatigue; absorbs moisture |
| Polypropylene (molded) | 5 % (typical, repeated use) | 8.9 mm | living hinges |
| Acetal (POM) | 4 % (typical, repeated use) | 7.1 mm | springy, low friction |
Living hinges
A living hinge is a thin section of the part that flexes repeatedly. In polypropylene a 0.3 to 0.5 mm hinge lasts for hundreds of thousands of bends; in printed PLA it cracks within a few cycles.
Worked example: strain in a hinge
- A hinge 0.5 mm thick and 2.0 mm long (in the direction of bending) bent through 90° (1.57 rad) has a strain of about ε = t θ ÷ (2 L) = 0.5 × 1.57 ÷ (2 × 2.0) = 20 %.
- That is fine for polypropylene (it can stretch much more) but far above the 1 to 3 % limit of PLA and PETG: a printed hinge needs a long, thin section (increase L to 10 mm and the strain falls to 3.9 %), a tough material such as nylon, or a separate pin hinge.
Tolerances and stack-up
Every dimension varies. When several parts are stacked, the variations add. Worst case adds all the tolerances directly (guaranteed, but pessimistic); root-sum-square (RSS) adds them in quadrature (statistically realistic for many parts, but not a guarantee). See Tolerance Stack-up Analysis for the full method.
Worst case = Σ ti, RSS = √(Σ ti²)
Worked example: four parts in a row
Four parts are stacked, each with a tolerance of ±0.2 mm. Worst case: 4 × 0.2 = ±0.8 mm. RSS: √(4 × 0.2²) = ±0.4 mm.
If the assembly will not work with ±0.8 mm but will with ±0.4 mm, specify tighter tolerances on the critical parts, add an adjustment (a shim, a slot, a spring) or measure and sort the parts, rather than hoping (a Monte Carlo check is on CAD, Simulation and Digital Prototypes).
- Design for the process: know what your process can hold (see Rapid Fabrication: Choosing a Method).
- Add adjustment, not precision, for fits that matter: slotted holes, shims, and screws against a stop cost far less than tight tolerances.
- Allow for movement of the material: wood swells and shrinks with humidity, plastics expand with temperature (aluminum 23 µm/m·K, PLA about 70 µm/m·K: over 100 mm and 30 K a PLA part grows 0.2 mm).
- Print or cut test coupons (a row of holes in 0.1 mm steps) on your own machine to find real clearances.
Mechanisms: linkages, gears and guides
- Linkages can be tested in card and pins first (see Low-Fidelity Prototyping: Mock-ups and Models and the Grashof check in CAD, Simulation and Digital Prototypes).
- Gears: printed involute gears of module 1 to 2 run well at light loads; check the tooth bending stress (see Gears and Power Transmission).
- Bearings and guides: a printed hole on a steel pin is fine for a prototype; use real bearings or bushings when friction or play matters.
- Springs: printed springs creep; use a real spring or a flexure designed to the strain limit above.
- Fasteners and shafts: use standard rods, screws and bearings, and design the printed parts around them.
Troubleshooting
| Problem | Likely cause | Try |
|---|---|---|
| Parts do not fit together | Tolerances stack up, process error, wrong clearance | Print or cut test coupons; add adjustment; revisit the stack-up |
| Snap-fit breaks on first use | Strain too high, layers across the beam, brittle plastic | Lengthen or thin the beam, reduce deflection, orient layers along the beam, try PETG or nylon |
| Screw strips the plastic thread | Too much torque, shallow engagement | Use a heat-set insert; deeper boss; lower torque |
| Press fit too loose or splits the part | Interference off; hole not round | Calibrate with test coupons; ream the hole; add a chamfer |
| Hinge cracks | Strain too high for the material | Longer, thinner hinge or a tougher material; a pin hinge |
| Mechanism binds | Clearances too small; misaligned axes | Increase the clearance; check the alignment; add a bushing |
Related
- Prototyping Techniques: the overview of this guide.
- Rapid Fabrication: Choosing a Method: what each process can hold.
- Designing for 3D Printing.
- Fits and Tolerances.
FAQ
How do I put a threaded hole in a 3D print?
Use a heat-set brass insert (the best), a captive nut pocket, or a self-tapping screw for a one-off. Print the hole to the size recommended for the insert or screw.
How much clearance for a sliding fit in a printed part?
About 0.25 to 0.4 mm per side on FDM, 0.15 to 0.25 mm on resin: print a test coupon on your own printer.
How do I design a snap-fit that does not break?
Keep the bending strain below the material’s limit: lengthen and thin the beam, reduce the deflection, orient the layers along the beam and use a tough material such as PETG or nylon.
Can I make a living hinge by 3D printing?
Only with care: a long, thin section in nylon or PETG lasts a modest number of cycles. Polypropylene molded hinges last far longer.
Worst case or RSS for a tolerance stack?
Worst case when every assembly must work; RSS or Monte Carlo when a small fraction of failures is acceptable and the errors are independent.
References
- Bayer Material Science / Covestro, Snap-Fit Joints for Plastics: A Design Guide; Bonenberger, P. R., The First Snap-Fit Handbook, Hanser.
- Shigley, J. E. and Mischke, C. R., Mechanical Engineering Design, McGraw-Hill (press fits, bolts); Rao, R. K., heat-set insert manufacturers’ data (hole sizes).
- Machinery’s Handbook, Industrial Press (fits, threads).
Last reviewed: October 2026. Allowable strains, clearances and strengths are typical: test on your own printer, material and settings.