Stab trim wheels – bicycle chain or timing belt

For the stab trim wheels my original plan was to use a similar construction with bicycle chains as in this project because it fits nicely in the narrow space between the TQ plates.
The derailleur jockey sprocket wheels that I intended to use for this had a problem however: they didn’t fit on the 10mm axes that are used (the hole inside the ball bearing is only 9mm)

I have tried to create a sprocket wheel with beveled teeth myself with the CNC router, based on a design I found on thingiverse.

This one comes with a script for an interesting free 3D design tool called OpenSCAD. It’s a bit different from other 3D tools because designs are constructed by combining primitive 3D objects like cylinders, cubes etc. via a scripting language. The result can be rendered and exported to several formats, a.o. STL for 3D designs and DXF or SVG for 2D designs
With some experimenting I modified the script to remove the top and bottom ring in the original design and added two extra 1/8 inch holes for connecting the wheel to the axis plates.

Rendered sprocket wheel in OpenSCAD

The STL file from OpenSCAD can be directly opened in Estlcam to generate G-code for a 3D object.
When checking the toolpaths that were generated from the STL file I noticed a problem: my plan to fix the object with two screws would not be possible because the toolpaths ran over them.

Toolpaths in Estlcam

I experimented a bit with the workspace masks that can be generated in Estlcam in free machining mode, but was not able to prevent this.
Also I realized that this symmetrical object could be routed more efficiently by routing consecutive deeper circles for the beveled teeth followed by a cut of the teeth outline. For this I wanted to use my standard tools flow with Inkscape and Estlcam.

To get the design to Inkscape I found a nice tutorial  how to make a 2D projection of a 3D object in OpenSCAD so that it can be exported to DXF.
An alternative would have been to replace the 3D object commands by a 2D command, but it was interesting to experiment a bit with projections as they can also be used to make different cross sections.

2D projection of sprocket design in OpenSCAD

So I created another projection of the wheel in OpenSCAD to check a cross section and drew the consecutive circles in Inkscape and Estlcam. I could even imagine that this can done with a standard G-code command (as it is similar to what is done with routing pockets), but I haven’t checked.

Cross section in OpenSCAD

Using this flow I created a sprocket wheel with beveled teeth, but I was not satisfied with the result.

2D design in Inkscape
Consecutive deeper circles in Estlcam



Time for a completely different approach.

On internet I had seen a solution with timing belts instead of bicycle chains before. Timing belts as used in inkjet and 3D printers.
But the timing belt pulleys that I found plus signal wheel or gear on the same axis looked to wide to fit between the TQ plates.
while searching I noticed that one of the standard bore diameters for timing belt pulleys is 10mm which perfectly matches the axes diameter.
The pulleys with the smallest width for 10mm bore diameter that I could find are 16 mm wide. These are for a belt of 6 mm width, which is similar to a bicycle chain.

After some fiddling around in my Inkscape design I found a solution to make the total of pulley plus signal wheel or gear fit between the plates: half of the pulley has a 16 mm diameter section where it is fixed to the axis with two screws.
By making a 16 mm hole in the signal wheel and gear and placing them over the 16mm diameter part of the pulley the total width remains 16mm.
Additional advantage is that I don’t need to create connection plates and slots to fix the signal wheel and gear to the axes anymore because the screws of the pulleys are now used for this.

Pulleys and belt have arrived and I’m currently working on a construction to keep the timing belt tight.

These ball bearings are used. In an earlier, much cheaper order from a different vendor all bearings appeared fitted askew in the flange, so don’t go for the cheapest solution.


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