Public Domain Differential

This 3D model is for educational purposes. Every math teacher should have one in the classroom. The source is a OpenSCAD script. Both the 3D model and the script are Public Domain. No additional parts are needed, but some glue is needed and maybe pliers for assembling.

The large gear (crown gear) is moved towards the center to make it as symmetrical as possible. The case with the four small gears is centered. The knob with the drive shaft is centered. The wheel axles have the same length.

Printing

I printed it with a layer height of 0.16 mm and a nozzle of 0.4 mm with PLA+ with three walls (perimeters).

The supplied 3mf file has separate parts, to allow to make a test print with only two parts. Most slicers can split the parts in the stl file as well. When the drive shaft with the knob and the drive gear are printed first, then the tolerance can be checked. The square tip of the axle should have a good fit in the gear.

The axles are standing straight up. To prevent that the nozzle knocks them of the build plate, use a infill pattern that prevents that the nozzle hits the infill. The infill pattern “Gyroid” and “3D Honeycomb” are good patterns.

If possible, try a scarf joint seam. That should make the axles more round.

Support is required:

Lubrication

A tiny amount of lubrication will improve the rotation of the gears. I put some transparent grease on a cotton swab. Then I wiped off all the grease from it. The tiny amount that was left on the cotton swab is enough for all the parts. I wiped it along the surfaces that slide along each other and on the gears.

Assembling

I suggest to start with the round case with the four small gears. Put the gears with the axles in the sides. Add the gears with the square holes to the sides. Check that they are aligned properly. Rotate the gears into the case. You might need pliers. Check that it rotates and that there is no misalignment.

The square top parts of the three axles have to be glued in the gears. Put a little glue on the sides of the square tip of the drive shaft with the knob. Put the drive gear in place and put the drive shaft through the support into the gear.

Place the crown gear with the case in place. Check which wheel is for the right side and which one for the left side. I prefer that the wheels have a forward motion according to the profile on the wheels, if the knob is turned clockwise . Add some glue to the square tip of the wheel axles and put the wheels in place.  

Using the script in OpenSCAD

OpenSCAD is free software: https://openscad.org/

The script will work in a 2021 version of OpenSCAD, but I recommend to use a 2024 version of OpenSCAD with the optimizations turned on.

To use OpenSCAD, unpack the file “Public Domain Differential.zip”. Keep those files together in a folder.

These files are included and are used by the script:

• The used font: “Cursive Sans - Bold”. 
  From: https://www.dafont.com/cursive-sans.font 
  License: OFL
• Gears for OpenSCAD: “Public Domain Involute Parameterized Gears”. 
  From: https://www.thingiverse.com/thing:6596095 
  License: CC0 (Public Domain).

The “Customizer” of OpenSCAD can be used to select the mode to show the 3D model. It can be shown assembled or as parts that can be printed. The Customizer can be used to tune the tolerances and adapt a few other things. The design is not fully customizable.

The mode to show the assembled differential in OpenSCAD is useful to study the design.

The mode for the separated parts is used to generate the 3mf and stl files for printing:

Version 1

This is version 1 and it is not perfect. The bevel gears are not real bevel gears, the tolerances have not been thoroughly tested for different settings of the slicer, and assembling the small gears in the case should be easier.

License

The license is set to CC0 (Public Domain) for both the script and the generated 3mf and stl files. You may print the 3D model and sell them.

If you create a new version of the script, please also change the text on the bottom that says “Version 1”. If you create a new version, please add some text to the version to identify that it is your alteration.