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GeoCore

GeoCore is a 3D-printable building system for kids that turns simple cardboard tubes into a hands-on geometry set. Print the colourful hubs, cut rods (or reuse cardboard cores), and snap them together to make cubes, domes, bridges, and more. Each connector is designed to teach real 3D shapes, such as triangles, pentagons, and polyhedra, whilst staying strong, reusable and easy to print. Perfect for classrooms, STEM clubs, or home projects, GeoCore helps kids explore design, structure, and creativity, one printed piece at a time.

Overview

I came across hundreds of cardboard cores, and rather than throwing them away, I designed several types of hubs to connect them together.

If you have cardboard cores with an inner diameter of 25 mm, all you need to do is 3D print the hubs, supplied in STL format.

You can also automatically generate custom-sized HUBS and RODS with a Python script, see here for detailed instructions.

With these printed hubs and a handful of cardboard cores, you can build a wide range of 3D structures, from simple cubes and domes to complex geodesic frames and lattices. Each hub type provides a different connection geometry.

Hubs

(Hubs arranged in order, top-left to bottom-right.)

1. `straight_2.stl` \u2014 Inline Coupler (top 1, pink)

Connects two cores end-to-end in a straight line.
Extend a rods\u2019s length to build long beams and spans.

2. `elbow_90_2.stl` \u2014 Right-Angle Elbow (top 2, yellow)

Joins two cores at a 90\u00b0 angle.
Useful for corners, mazes, or rectangular frames.

3. `corner_cube_3.stl` \u2014 3-Way Orthogonal Corner (top 3, green)

Connectors in the +X, +Y, and +Z directions.
Build cubes, boxes, and scaffolds.

4. `tetra_3.stl` \u2014 3-Way ~60\u00b0 Connector (top 4)

Three pegs meet at tetrahedral angles (\u224860\u00b0).
Create tetrahedra, triangular lattices, or trusses.

5. `octa_4.stl` \u2014 4-Way Orthogonal Pair Connector (mid 1, orange)

Four pegs arranged in perpendicular pairs.
Build octahedra, space frames, or octet-truss modules.

6. `icosa_5.stl` \u2014 5-Way Icosahedral Vertex (mid 2, teal)

Five pegs radiate evenly from the center at about 63.43\u00b0 between each.
Assemble domes, geodesic spheres, and high-strength frameworks.

7. `dodeca_3.stl` \u2014 3-Way (Dodeca) (mid 3, light purple)

Three pegs meet at 108\u00b0, the golden-ratio angle of pentagons.
Construct pentagons, decagons, and \u03c6-based geometries.

8. `cubic_6.stl` \u2014 6-Way Cartesian Connector (center, blue)

Six pegs aligned along \u00b1X, \u00b1Y, \u00b1Z axes.
Build voxel grids, towers, bridges, and frame structures.

9. `trigonal_planar_3.stl` \u2014 3-Way Planar (bottom 1, yellow-green)

Three pegs in a single plane, equally spaced at 120\u00b0.
Use for hexagonal or triangular tiling patterns and Y-junctions.

10. `hex_planar_6.stl` \u2014 6-Way Planar (bottom 2, blue)

Six coplanar pegs forming a hexagon (60\u00b0 spacing).
Use to create honeycomb sheets or dense triangular meshes.

11. `tetrahedral_4.stl` \u2014 4-Way Regular Tetrahedral (bottom 3, green)

Four pegs arranged at 109.47\u00b0, matching tetrahedral geometry.
Used for diamond-like lattices, rigid trusses, or alternating with octa_4 for octet structures.(Hubs arranged in order, top-left to bottom-right.)

Printed Hubs

I used SuperSlicer (supports needed, 0.3 draft mode) and a Prusa i3 MK3S+ with generic PLA.

Hubs with cardboard cores

Print rods if you need them

Or use cardboard cores

Rods

I started with a box of cardboard cores. They are 102 mm long, with an inner diameter of 25 mm and outer diameter of 30 mm.

Ideally, we need a few different lengths. Initially, I didn\u2019t plan to use multiple lengths, but they are essential to create all the different shapes.

\u221a2 for square diagonals: In a square with side length L, the diagonal measures L\u00b7\u221a2, so a \u221a2\u00b7L rod fits perfectly across a square\u2019s corner-to-corner span.

\u03c6 for pentagon diagonals: In a regular pentagon with side length L, the long diagonal is \u03c6\u00b7L (golden ratio), so a \u03c6\u00b7L rod matches that vertex-to-vertex distance.

2\u00d7 for larger models: A 2\u00b7L rod simply doubles the base length to scale up spans and frames while keeping proportions consistent.

So if the base size is 102 mm:

L = 102 mm
\u221a2 \u00b7 L \u2248 144 mm
\u03c6 \u00b7 L \u2248 165 mm
2L = 204 mm

Rod factories are included in the script and STL files as well. You may need to adjust clearances depending on your printer and materials. (Note that I've only tested parameters that match my cardboard cores.)

Build things

Background

I used AI, in particular gpt-5-pro, to create a Python script to generate hubs and rods in STL format. It took 14 minutes and $2.26. This formed the basis of this project. For those interested the prompt was:

model: gpt-5-pro
reasoning: high
max output tokens: 40k
timeout: 60 min
mode: streaming (incl. reasoning)

cost $2.26 USD, run time 14:01

system:
You are an expert in CAD code. You understand the CAD design requirements for 3D filament printing.

user:
We\u2019re making a kids\u2019 polyhedron construction set using thousands of cardboard cores (102 mm long, OD 30 mm, ID 25 mm). These are the rods.

Goal: Design \u226412 hubs that maximise the range of buildable 3D shapes (cubes, tetra-, octa-, icosa-, dodecahedra, pyramids, and compounds). You choose the joiner set, socket count/orientations, angles, and any offsets needed for versatility.

Deliverable:
- one file: geocode_v1.py \u2014 CadQuery 2.x, mm units.
- Factories: one function per hub, returning its solid.
- Export: write all hubs to ./stl/ as STL files.
- Printability: every part must be a manifold and FDM-printable.

Non-negotiables:
Params: top-level press_fit_clearance=0.3 mm (default) and insertion_depth \u2014 no hard-coding.
Geometry: round sharp tips; min wall \u2265 2 mm; avoid support-heavy features.
Log: at the end, print a short list of the hubs produced.

More of my builds at makies.au

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GeoCore

Overview

Hubs

1. straight_2.stl \u2014 Inline Coupler (top 1, pink)

2. elbow_90_2.stl \u2014 Right-Angle Elbow (top 2, yellow)

3. corner_cube_3.stl \u2014 3-Way Orthogonal Corner (top 3, green)

4. tetra_3.stl \u2014 3-Way ~60\u00b0 Connector (top 4)

5. octa_4.stl \u2014 4-Way Orthogonal Pair Connector (mid 1, orange)

6. icosa_5.stl \u2014 5-Way Icosahedral Vertex (mid 2, teal)

7. dodeca_3.stl \u2014 3-Way (Dodeca) (mid 3, light purple)

8. cubic_6.stl \u2014 6-Way Cartesian Connector (center, blue)

9. trigonal_planar_3.stl \u2014 3-Way Planar (bottom 1, yellow-green)

10. hex_planar_6.stl \u2014 6-Way Planar (bottom 2, blue)

11. tetrahedral_4.stl \u2014 4-Way Regular Tetrahedral (bottom 3, green)

GeoCore: Kids 3D Construction Set

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