Logical Lock Box — 4-Digit Programmable Combination Box

A fully 3D-printed, programmable 4-digit combination lock built almost entirely from compliant mechanisms — no keys, no batteries, no bought hardware. This isn't just another storage box; it's a functional mechanical puzzle and a fun engineering demonstration that lets you build and program your own working combination lock.

Unlike many printable locks that rely on metal springs or store-bought hardware, the Logical Lock Box uses carefully designed compliant spring elements integrated directly into the printed parts. The result is a satisfying mechanical lock that's fun to print, enjoyable to assemble, and genuinely fascinating to watch in action through its transparent lid.

Each of the 4 Dials is programmed during assembly to set your own unique combination. Once built, just scramble the dials to lock the box, and dial in your combination to open it back up.

Key Features

• 100% 3D printed — no screws, magnets, or metal springs required
• Snap-together assembly
• Print-in-place compliant mechanisms (sprung tumblers, sprung detents)
• Fully programmable 4-digit combination — set during assembly, reprogram anytime by disassembling
• Thousands of possible combinations
• Partially clear lid (print with AMS/multicolor) so you can watch the lock mechanism work
• Compact footprint: 6.93" x 3.21" x 1.74" (176 x 81.6 x 44.2mm)
• A practical storage box, a conversation piece, and a nice demonstration of compliant mechanical design

How It Works Each of the 4 Gears has a missing tooth. How you seat that Gear relative to its Dial during assembly is what sets your combination. Spin the dials to any other number and the Gear notches misalign with the spring-loaded Tumblers, locking the Latch Bar in place. Dial in the correct code and the notches line up, the Tumblers drop free, and the Latch Bar slides open.

Using It

• Set all 4 Dials to your combination
• Slide the Latch Bar from locked to unlocked
• Lift the lid
• To lock back up: close the lid, slide the Latch Bar to locked, then scramble the dials

Assembly & Programming A fully illustrated assembly and programming guide is included in the files, walking you through every step — from printing through programming your custom combination. The short version: build the frame and snap the two-piece Latch Bar together, seat the Dials, set your combination by how you seat each Gear onto its Dial (tilt onto the post, press down), then press all 4 Gears home without rotating the Dials. Flip the lid over to confirm your combo is showing correctly before you close it up, snap in the Inside Cover, then join the Lid and Box — the hinge notches only mate one way, so it can't go together wrong.

Note: there are a few small print-stabilization bridges holding the compliant tumblers in place for printing — just flex those free before you start assembly.

A small dab of PTFE or silicone lubricant on the Dial/Gear posts and the Latch Bar contact surfaces keeps everything moving smoothly.

Print Settings

• Material: PLA, or any material
• Nozzle: 0.4mm
• Layer Height: 0.2mm
• Line Width: 0.4mm
• Walls: 4 (1.6mm)
• Top/Bottom: 5 (1.0mm)
• Detect Thin Walls: On
• Wall Generator: Arachne
• Infill: 40% Cubic
• Brim/Raft: No
• Support: None required — break-away supports are built into the model
• Print Orientation: As per included STL/3MF files

Functional designs like this one need more strength than typical slicer defaults provide, so give your settings a look before you print.

Dimensions

• Length: 6.93" (176mm)
• Width: 3.21" (81.6mm)
• Height: 1.74" (44.2mm)

Whether you're after a fun weekend build, an educational look at compliant mechanisms, or you just appreciate clever mechanical design, I hope you enjoy building the Logical Lock Box as much as I enjoyed designing it. Happy printing!

Related Links

Join the LogicalPlanet Discord https://linktr.ee/logicalplanet

Visit my blog for more details and other designs https://www.logicalplanet.com/

Revisions and Updates I occasionally update and revise my designs. When I do, I'll document the changes here. Check back periodically for the latest revision notes or if you're experiencing any issues with a model. Please email me at logicalplanet3d@gmail.com if you would like me to make a revision or have comments about this design.