Create a new design and plan 25mm cube building blocks.
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Parent note: This episode teaches engineering concepts like tolerances and press-fit design. These are real skills used by product designers!
Get started
1Log in to tinkercad.com, click Create → 3D Design
2Rename the project "My Magnetic Blocks"
3We're designing 25mm cubes, small enough to handle easily, big enough for 6mm magnets
4Drag a Box from the Shapes Panel onto the workplane
5Resize it to exactly 25mm x 25mm x 25mm, a perfect cube
All three numbers must match: 25 × 25 × 25. Click each white handle and type 25. About the size of a big dice, roomy enough inside for the little 6 mm magnets.
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25mm is about 1 inch, a great size for building blocks that are easy for kids to handle.
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Knowledge Check
+15 XP
Why is 25mm a good size for building blocks?
ASmall enough to handle, big enough to fit magnets inside
BIt's the only size TinkerCAD supports
CMagnets only come in 25mm sizes
2
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Round the Edges
Add a slight radius to make the blocks comfortable to hold
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Goal for this step
Round the cube edges with a 2mm radius for comfort and safety.
Round those edges
1Select the cube and look at the Shape Properties panel
2Find the "Radius" or "Bevel" option
3Set the radius to 2mm, the edges become slightly rounded
4This makes the blocks much more comfortable to hold and safer for small hands
5Rotate the view to check all edges are evenly rounded
6The block should look like a slightly rounded cube, not a sphere!
Find "Radius" in Shape Properties (the panel that appears when the box is selected) and type 2. The corners soften. If your cube turns into a blob, the radius is way too big, set it back to 2.
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True or False?
+15 XP
"Rounding edges makes blocks safer for small hands."
"A 2mm radius turns the cube into a sphere."
"The Radius setting is found in Shape Properties."
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Knowledge Check
+15 XP
What radius do we use for the block edges?
A10mm
B0mm
C2mm
3
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Add Magnet Holes
Create 6.2mm diameter holes for 6mm magnets on each face
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Goal for this step
Add a cylindrical hole on one face of the cube sized for a 6mm magnet.
Create the magnet holes
1Drag a Cylinder from the Shapes Panel onto the workplane
2Resize it to 6.2mm diameter (6.2mm x 6.2mm) and 3.2mm height
3Wait, why 6.2mm and not 6mm? We'll learn why in the next step!
4Change the cylinder to a "Hole" shape (click Hole in the top-right)
5Position the hole cylinder on the top face of the cube, centered
6Use Align (select both shapes, press L) to center the hole perfectly on the top face
7Make sure the hole goes 3.2mm into the cube, it should sit flush with the surface
The hole sits flush with the top, sinking 3.2 mm down into the cube. To sink it, select the hole cylinder and drag the little cone handle above it downward, or type the height position directly.
✅Checkpoint: orbit around the cube. The striped cylinder should be swallowed by the top face with nothing poking above the surface. If you can see stripes floating in the air, it has not been lowered into the cube.
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Fill in the Blanks
+15 XP
The magnet hole is mm in diameter and mm deep.
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Knowledge Check
+15 XP
How do you position the hole exactly in the center of a face?
AEstimate by eye
BUse the Align tool to center on both X and Y axes
CType coordinates manually
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Learn About Tolerances
Why 6.2mm not 6mm for a 6mm magnet?
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Goal for this step
Understand why 3D printed holes need to be slightly larger than the object they hold.
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Parent note: This is a real engineering concept! Tolerances are used in every manufactured product. This step teaches why precision matters.
Understanding tolerances
1Our magnets are exactly 6mm in diameter. So why did we make the hole 6.2mm?
23D printers aren't perfectly precise, each layer of plastic is slightly wider than designed
3This means a 6mm hole would actually print slightly smaller, maybe 5.8mm
4A 5.8mm hole is too tight for a 6mm magnet, it won't fit!
5By designing the hole at 6.2mm, it prints at about 6.0-6.1mm, a snug press-fit
6This 0.2mm extra is called a "tolerance", the small gap that accounts for printer imprecision
7Professional engineers use tolerances in everything, from car parts to phone cases!
Design the hole 0.2 mm bigger than the magnet. Printers squeeze holes slightly smaller than drawn, so a 6.2 mm hole prints at about the perfect snug size for a 6 mm magnet.
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The standard tolerance for press-fit holes in 3D printing is 0.1-0.3mm larger than the object. 0.2mm works well for most printers.
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True or False?
+15 XP
"A 3D printed hole usually comes out slightly smaller than designed."
"Tolerance means making things exactly the right size."
"0.2mm tolerance means making the hole 0.2mm larger than the magnet."
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Knowledge Check
+15 XP
Why do we design the hole at 6.2mm for a 6mm magnet?
ABecause 6.2 is a rounder number
BBecause magnets expand when warm
CBecause 3D printers aren't perfectly precise, so the hole will shrink slightly when printed
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Align Holes on Each Face
Use the Align tool to center holes perfectly
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Goal for this step
Add magnet holes to multiple faces of the cube using Align.
Add holes to more faces
1We currently have one hole on the top face. Let's add holes to more faces
2Duplicate the hole cylinder (Ctrl+D)
3Rotate the duplicate 90 degrees so it faces a different direction (use the rotation handles)
4Use Align to center it on the front face of the cube
5Repeat: Ctrl+D, rotate 90°, Align to center on the right face
6For a basic set, add holes on the top, front, and right faces (3 holes total)
7This lets blocks connect on 3 of their 6 faces
Duplicate, rotate 90°, Align, repeat. Each copy of the hole cylinder gets rotated to face out of a different side, then centred on that face with Align. Three holes lets blocks connect three ways.
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For advanced builders, you can add holes on all 6 faces. But 3 faces is plenty to start with, it keeps the block strong.
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Put It In Order
+15 XP
Click these steps in the correct order to add holes to multiple faces:
Rotate the duplicate 90 degrees
Duplicate the hole cylinder with Ctrl+D
Use Align to center on the face
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Knowledge Check
+15 XP
Why do we only put magnet holes on 3 faces instead of all 6?
ATinkerCAD only allows 3 holes
BIt keeps the block structurally strong while still allowing connections
CMagnets don't work on all faces
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Duplicate & Mirror the Block
Create a matching block with opposite magnet polarity
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Goal for this step
Duplicate the block and understand why magnets need opposite polarity to attract.
Duplicate and understand polarity
1Before grouping, let's understand polarity: magnets have North and South poles
2North attracts South, but North repels North. So adjacent blocks need opposite orientations
3Select all shapes of your first block (cube + holes). Group them (Ctrl+G)
4Press Ctrl+D to duplicate the entire grouped block
5Move the copy to the right, leaving space between them
6The hole positions are identical, when you insert magnets, you'll put them in with opposite poles facing out
7Label your blocks mentally: Block A has North poles facing out, Block B has South poles facing out
The 3D design of A and B is identical. The difference happens after printing: push the magnets into Block A with North facing out, and into Block B with South facing out. Opposite poles pull together, so A sticks to B.
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Parent note: When actually inserting the magnets after printing, use a marker to mark which end is North. A simple test: if two magnets stick together, the facing sides are opposite poles.
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Knowledge Check
+15 XP
Why do adjacent blocks need magnets with opposite polarity?
ABecause opposite poles (North-South) attract each other
BBecause same poles are stronger
CBecause the magnets are different sizes
7
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Create Connector Pegs & Sockets
Add interlocking pegs for stacking without magnets
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Goal for this step
Add a peg on top and a socket on the bottom for mechanical connection.
Build pegs and sockets
1Create a new Cylinder: 5mm diameter, 4mm tall, this is the connector peg
2Position it on the top face of Block A, centered (use Align)
3Now create the matching socket: another Cylinder, 5.2mm diameter (tolerance!), 4.2mm deep
4Change the socket cylinder to a Hole shape
5Position the socket hole on the bottom face of Block B, centered
6When Block A sits on top of Block B, the peg fits snugly into the socket
7Group each block with its peg or socket (select block + connector, Ctrl+G)
Peg on top of A, socket in the bottom of B. Same tolerance trick as the magnets: the socket is 0.2 mm wider than the peg so they click together instead of jamming.
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Fill in the Blanks
+15 XP
The peg is mm in diameter, and the socket is mm, the extra 0.2mm is the .
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Knowledge Check
+15 XP
What is the purpose of the connector peg and socket?
AJust decoration
BTo hold the magnets in place
CTo physically interlock blocks for stacking, even without magnets
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Test the Fit
Place blocks next to each other to check alignment
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Goal for this step
Position blocks adjacent and stacked to verify the design works.
Test your design
1Move Block A next to Block B so their magnet-hole faces touch
2Check that the holes line up, the magnets would sit face-to-face
3Now stack Block A on top of Block B
4Check that the peg from Block A lines up with the socket in Block B
5Use the Align tool to verify the peg is centered over the socket
6If anything is misaligned, ungroup (Ctrl+Shift+G), adjust, and regroup
Two quick checks before exporting: put the blocks side by side (magnet holes should sit on one straight line), then stack them (the peg should be exactly over the socket). Fix any misalignment with Align.
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In TinkerCAD, you can't actually test magnetic force, but visually confirming alignment ensures your printed blocks will connect properly.
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Knowledge Check
+15 XP
How do you ungroup shapes that have been grouped?
APress Delete
BPress Ctrl+Shift+G
CClick the Hole button
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Add Colour Coding
Use different colours for different block types
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Goal for this step
Apply colours to distinguish block types visually.
Colour your blocks
1Select Block A. In the Shape Properties panel, click the colour swatch
2Choose a bright colour, red, blue, green, or yellow work great
3Select Block B and choose a different colour
4You could create a colour system: red = 2 magnets, blue = 3 magnets, green = peg blocks
5Colour coding helps builders know which blocks connect where
6Try creating a third block in a new colour with a different configuration
Colours are a code, not just decoration: anyone building with your set instantly knows red sticks to blue, and green stacks. Pick your colours in Shape Properties.
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Real building block sets like LEGO use colour coding too. Design your own colour system!
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Knowledge Check
+15 XP
Why use different colours for different block types?
ABecause TinkerCAD requires it
BTo make the blocks look nicer
CTo help builders quickly identify which blocks have which connection types
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Export Individual Blocks
Learn about print orientation and export each block
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Goal for this step
Export each block as a separate STL and understand print orientation.
Export and print settings
1Important: export each block as a separate STL file, not all together
2Select Block A only. Click Export → STL. Save as "block-A.stl"
3Select Block B only. Click Export → STL. Save as "block-B.stl"
4Print orientation matters: lay each block on its largest flat face
5This means the magnet holes on top/bottom faces print vertically, they'll be the most accurate
6Recommended settings: 0.2mm layer height, 40% infill (blocks need to be sturdy!), PLA material
7After printing, press 6mm magnets into the 6.2mm holes, they should click in with a satisfying snap!
Select just one block, then Export → .STL. Repeat per block. Printing flat-side-down keeps the vertical magnet holes crisp, and the magnets press in with a satisfying click afterwards.
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Parent note: You'll need 6mm x 3mm neodymium disc magnets (widely available online). Supervise magnet insertion as small magnets can be a swallowing hazard for young children.
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True or False?
+15 XP
"Each block should be exported as a separate STL file."
"Print orientation doesn't matter for 3D printing."
"40% infill makes the blocks sturdy enough for building."
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Final Knowledge Check
+15 XP
Why do we export each block as a separate STL?
ASo each block can be positioned optimally in the slicer for best print quality
BBecause TinkerCAD can only export one shape at a time
CBecause the printer can only print one colour
🛠️ Printed it and something's not right?
The magnets won't go into the holes. The hole printed under 6mm. Check you designed 6.2mm (the tolerance from step 4), and clean any stray plastic out of the hole with a toothpick before pressing the magnet in.
The magnets fall out. The press-fit is too loose for your printer. A tiny dot of PVA or super glue (adult job!) in the bottom of the hole locks them in permanently.
My blocks push each other away instead of sticking. Classic polarity mix-up: two North poles are facing each other. Pop the magnet out of one face, flip it over, and push it back in (step 6's plan avoids this).
The peg won't fit the socket. Same tolerance rule as the magnets: the socket must be 0.2mm wider than the peg. Sand the peg lightly or enlarge the socket to 5.4mm and re-print block B.
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You Designed Magnetic Building Blocks!
Incredible, you've learned tolerances, alignment, and multi-part design. Your blocks snap together with magnets!
