Parametric Sprocket

• http://www.thingiverse.com/thing:87647/#files - has a good 15 tooth example - Bike Sprocket
• The above was derived from http://www.thingiverse.com/thing:7918

How to Design a Sprocket for Bulldozer Drive

• The sprocket generator in OpenSCAD creates a proportionally correct sprocket.
• How do we know how to scale it to match our specific case?
• We first design the track. We settled on pin spacing of 3", and XXH 1.5" pipe (see Pipe Chart) for the roller, with 1" pins. Thus: roller = 1.9", pitch = 3". Ratio of 3/1.9=1.58. Ratio of pitch to the roller is constant.
• Then we generate the sprocket:

• Number of teeth
• Roller (5 units)
• Pitch - 1.58x the roller size as determined by our track geometry (looks sensible, and 3" pitch was selected as a convenient distance, and 1.9" OD roller was selected as a readily-available stock material
• Thickness - that is the thickness of the plate material
• Tolerance - play in the distance between rollers (pitch)?

• To generate a correct sprocket, we can scale the resulting design in CAD - by superposing the track piece over the sprocket to make sure the track pieces fit in the valley-to-valley distance in the sprocket (which corresponds to the distance between rollers, or pitch)

• See the

Bulldozer 2015 Usage

Hint: The bulldozer from 2015 used an 8 tooth sprocket, generated with sprocket(8,5,7.5,2,0.2); in the last line of the Editor

Exporting DXFs from 3D Objects in OpenSCAD

• Exporting DXFs - [1] - essentially - load and flatten, then export DXF

• Note: stock Export DXF does not work for 3D files - they need to be projected first.

• To project - projection(cut=false) import("sprocket.stl"); STL file must exist in same directory

• Sample file for exercise in OpenSCAD - the sprocket STL and the .scad file that loads the STL and does a 2D projection. Note that the directory must be correct. File:Sprocketexample.zip
• Note: original srocket was generated using Parametric Sprocket Generator using sprocket(12,5,7.5,8,0.2);

Links

• Sprocket geometry for a correct sprocket pitch and roller diameter - example - File:Sprocketgeo.dxf

• Uses Sprocket Generator