Design Guide

Organic Modeling

After you have an idea for your Digifidget, the next step is to develop the initial organic shape, meaning the exterior surfaces that would be difficult to design in CAD. The organic shape is usually a mesh model.

You can develop a mesh model manually using Blender, ZBrush or another mesh modeling software. You can also 3D-scan a physical object, like we did for the Rubber Duck.

If you're into AI tools, we recommend Meshy.ai or similar tools like Tripo3D.ai or Hi3D.ai. With a detailed description, you can have a usable model in just minutes.

Even with an AI-generated model, you not only need to make sure the figure has realistic features and the correct number of hands and fingers, but you need to evaluate its potential use for 3D printing and making a Digifidget.

For Digifidgets, you'll want to consider 1) what part is going to move and what will be fixed, 2) where will be the axis of rotation for the moving part, 3) where will you create the split line between the moving and fixed portions, 4) how you will cut the model to create enough space for the moving part to rotate +/- 15 degrees without hitting the fixed portion, 5) make sure there is enough volume inside the figure for the internal parts tied to the moving portion, and 6) consider where the rest of the circuit kit components will be located, especially the solar cell.

Considering these questions now will limit the number of design changes later. Ideally, everything will fit inside of the model, but if necessary, you can add a pedestal like we did on the Headbanger.

Mechanical Modeling

Use CAD software such as Fusion or SolidWorks to add the mechanical features necessary for movement and component mounting.

1

Start by converting the mesh surface to a mesh solid body.
2

Align the model with x, y, and z axes.
3

Ensure or create a flat base for the figure to sit/print on.
4

Define the axis of rotation for moving parts. We use +/- 15 degrees movement range of motion. This needs to be considered in the design there will need to be no interferences throughout the range of motion without affecting the mesh model outer surfaces or overall appearance.
5

Create subtraction solids to create the gaps between the fixed and moving bodies. Convert to mesh bodies, then subtract from the main body to create the fixed and moving bodies.
6

Build moving interfaces on the fixed body and moving parts using a standard wedge-and-cylinder pivot design (see below). Gaps of 0.5mm are recommended between moving surfaces.
7

Verify fits and clearances through the full range of motion, typically +/- 15 degrees. A minimum clearance of 0.5mm is recommended between potential contact surfaces.
8

Save a copy each for the fixed and moving portions to maintain the same coordinate system, so the cut lines match and gaps are maintained.
9

Open each fixed and moving body and delete the irrelevant portions.
10

Align the fixed and moving portions in an assembly model to develop their interfaces.
11

Create subtraction solids to hollow out the main body, creating space for the moving parts.
12

Create a base opening and cover plate to close out the figure using a standard snap-fitting design.
13

Create the bolt mounting interface on the moving body. When the assembly is at rest (neutral position), the bolt and magnet should be centered over one of the sectors of the coil, not the center of the coil. That way, the magnet will move, regardless of the polarity of the magnet or coil.
14

Create a coil mounting surface on the fixed body or cover plate.
15

Create the solar cell mounting fitting using a standard snap-fit housing design on the fixed body. See standard models library.
16

Identify the location for mounting the PCB assembly on the fixed body. Consider where the wires between the PCB and the coil will run.
17

Double-check fits and clearances with all components in the model. The bottom of the magnet should sit between 0.5 and 1.0mm above the surface of the coil in the neutral position, and should not contact the coil through the swing radius. The neutral position of the bolt and magnet should be centered over a sector of the coil, not the center.

Prototype, Test & Iterate

1

Prepare for prototype printing

Set all parts to neutral position, hide all non-plastic parts, and create an STL file for prototype printing and testing. Ideally, all plastic parts can be printed as an assembly, but some designs may require printing bodies separately. Identify and support material that may be hidden within the print if difficult to remove — or consider printing without that support material. Identify any areas of overhang that may yield undesirable printing results and consider if they should be redesigned or removed.

2

Populate the test print

Populate the test print with the ballast bolt, magnet, solar cell, coil and PCB — typically in that order. See if the test print operates as expected. Look for areas of binding or parts fused together. Consider design edits, increased gaps, or changes in part configuration to reduce friction and improve part motion.

3

Troubleshoot and iterate

Troubleshoot and iterate the design from lessons learned during testing.

Verify all parts print as intended

Supports only where desired/expected
No hidden overhangs creating loose or dangling filament that increases friction between parts
No external overhangs creating loose or dangling filament that affects appearance

Verify all moving parts do so freely with proper clearances

IMPORTANT: The assembly should be well-balanced with friction low enough that the moving part swings 10+ times freely with a gentle touch (with the ballast bolt installed).
The simplest designs for moving parts are usually the best Any friction from an axle or linkages reduces motion and would require a larger solar cell to provide more power to overcome the friction.

Pivot Design

You might think that an axle would work best, but with the imperfections of 3D printing it actually has too much friction, and it's expensive and impractical to add bearings.

The best pivot design we've found is a blade or wedge, balanced on its edge, inside a cylindrical receiver surface. The blade should be long enough (>2mm) to constrain the motion to the axis parallel to the blade edge. Having two pivots per design provides more stable motion.

The image on the right shows the pivot design and dimensions we use in SolildWorks. Dimensions are in millimeters. The 0.20 edge width is intended to match the single layer width of your printer. The 0.10 gap is important so that the printer does not fuse the moving and fixed portions together, if you choose to print everything in one model.

The 15 degree angle shows the intended range of rotation on each side. You can see a representation of the blade rotated left in phantom. The other dimensions and angles can vary somewhat to meet your needs.

Pendulum Weight and Balance

The motion of the fidget works best when there is weight (usually a steel ballast bolt) hanging below the pivot point, with the magnet and coil interface at the bottom of the bolt. The system then swings back and forth like a pendulum.

The mass below the horizontal line needs to be more than the mass above it, otherwise the head would just fall over. Using a steel ballast bolt provides significant weight and the steel allows the magnet to easily attach to it.

It is also important that the mass on the left of the vertical line and the mass on the right of the vertical line are approximately equal (for the moving portion). Otherwise, when the neutral position of the bolt and magnet will not settle in the correct position over the coil, and the range of motion of the moving part will be reduced in the direction it is already tilting towards.

On the Rubber Duck, we addressed the off-center nature of the duck's bill by adding a void inside the head near the bill to reduce the weight on that side. We added small internal features (an array of slots) inside the rear of the head to force the printer to replace the standard infill pattern with more filament for the small features.

Other approaches for balancing the moving portion may also work. Getting this part right usually requires an iterative effort of designing, printing, testing and revising until the balance is just right, but the effort is essential to a well performing fidget.

Resources

Design Guide Videos

Step-by-step walkthroughs covering mesh splitting, interface design and test-print troubleshooting.

Design Guide: Highland Cow

Design Standard Models

Download STL files of standard pivot geometry, coil mount, solar cell mount and PCB mount reference models to use in your own CAD assembly. Also includes STL models of solar cells, magnets and PCBs so you can include them in your design.

Standard Models Page