From the Loftsmith Workshop

Snap-fit test print: 7 keys, 0.2 to 1.4 mm interference

Find out how tight your printer lets a snap-fit be before you commit to a real part. Seven little keys press into a bar, each one a touch snugger than the last, so you can feel where it clicks and where it fights back. Change the starting gap and the step between keys.

8 parts · 21 g · 148 × 16 × 33 mm

by Loftsmith Workshop

Dark grey bar holding a row of seven orange numbered keys, from 0.2 to 1.4
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No adjustable sizes

Ask for the sizes you want to tweak, e.g. “make the width adjustable”.

Change it in the Studio

Parts

  • frame · PLA (or the filament you are testing) · Upside down, top face on the bed (printRotate 180, 0, 0), so the catch face under the bar prints as a clean top surface and the windows are vertical

    147.5 x 15.7 x 19.4 mm, about 13 g and 23 min. Raised tag on the front records filament, beam length (L), beam thickness (T) and catch angle (A).

  • key-0.2 · PLA (or the filament you are testing) · Flat, label up

    0.2 mm interference, 0.33% strain, about 1 g and 6 min

  • key-0.4 · PLA (or the filament you are testing) · Flat, label up

    0.4 mm interference, 0.67% strain

  • key-0.6 · PLA (or the filament you are testing) · Flat, label up

    0.6 mm interference, 1.00% strain

  • key-0.8 · PLA (or the filament you are testing) · Flat, label up

    0.8 mm interference, 1.33% strain

  • key-1.0 · PLA (or the filament you are testing) · Flat, label up

    1.0 mm interference, 1.67% strain

  • key-1.2 · PLA (or the filament you are testing) · Flat, label up

    1.2 mm interference, 2.00% strain (at the PLA limit)

  • key-1.4 · PLA (or the filament you are testing) · Flat, label up

    1.4 mm interference, 2.33% strain, past the PLA limit so it carries a notch in the head

Print settings

Layer height
0.2 mm
Walls
2
Infill
15%
Supports
None
Notes
At these settings the whole set is about 22 g and 1 h 8 min (frame about 13 g and 23 min, each key about 1 g and 6 min). Print every part in the filament you want to test and set the Filament slider to match. 3 walls is about 29 g and 1 h 30 min if you prefer, but the prongs print as solid perimeter

Assembly

  1. Print the frame upside down and the keys flat with their labels up, all in the filament you want to test.
  2. Stand the frame on its feet. Push each key straight down into a window until its shoulder sits on the bar and the barbs click under it.
  3. Compare the keys: which go in by hand, which click and hold firmly, which feel loose, and which whiten or crack. A notch in a key's head means its estimated strain is past half the filament's elongation at break.
  4. To take a key out, lift the frame, pinch the two prong tips together under the bar and pull the key up. Or pull it straight out to feel the release force of the catch angle.
  5. Use the interference of the key you liked, together with the beam length (L), thickness (T) and catch angle (A) on the frame's tag, in your own lid, clip or battery door.

Good to know

  • Strain and push or pull forces are textbook cantilever estimates from typical printed properties (PLA: E 3000 MPa, 4% elongation), not measurements. Your printer's tolerance, line width and elephant's foot change the real interference, which is exactly what the print shows.
  • Fatigue is not tested: a key that survives a few clicks may still crack after hundreds of cycles. For latches you open often, choose a key well inside the limit.
  • The notch marks keys past half the elongation at break, a one-time snap limit. It is not a guarantee that unnotched keys survive or that notched keys break.
  • The Filament slider changes the strain limit, labels and force estimates, not the key shape. Print in the filament you selected.
  • With several stiff settings combined (2.4 mm beams, 10 mm beam length and PETG), the hardest key needs over 100 N to push in. Change one stiff setting at a time.
  • The engine checks rigid geometry and the straight insertion path only. The flex of the prongs as they pass the catch is not simulated.
I printed this Remix

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