Tutorial: How to model involute gears in SolidWorks and show design intent.
In this tutorial I am going to show how to design involute gears from scratch without using fancy mathematics or built-in functionality. I use SolidWorks but you should be able to use just about any CAD program.
The model from this tutorial is found here:
Involute Gear Design from Scratch
Answered with a tutorial: https://grabcad.com/tutorials/tutorial-how-to-model-involute-gears-in-solidworks-and-show-design-intent
Another good question. Sorry for not having spotted it until now. I had to dig deep to come up with the explanation. The value is obviously correct, since the gears that you design with the method mesh perfectly. If you had used a different value, the cogs would be too thin or too thick.
The explanation is that in order to mesh perfectly, the gap between two teeth need to be identical to the width of a tooth: Think of gear racks that can be considered gears with infinite radius. When two identical gear racks are held against each other all teeth will contact at the same time (in theory anyway). So the angle between two teeth are double the angle between the contact points on the pitch circle and the centerline for a tooth is exactly in the middle because of symmetry, which is half of the angle between the contact points or a quarter of the angle of a cog. Q.E.D.
Hope this makes sense?
It is a detailed tutorial but I'm finding that the model it produces is not robust enough to change the size later. The geometry fails and SW reinstates the previous values. I'm going to start again from the beginning creating each step as a separate sketch, and if I can get it to behave more parametrically I will then maybe set up some variables to change in the part properties. It is a pity there isnt a built-in gear tool in SW as this is taking too much time to create a basic part.
The attached shows how to model a helical involute gear in SolidWorks.
In real manufacturing modeling requires special software because gears are often not pure involute: involute has modifications and the root surface is complex. There is special software to calculate these gear tooth surfaces offered by spiralbevel.com