The correct dimensions of Hitec C1 spline

The C1 spline of 24 teeth is the standard shaft size for Hitec's servo motors, which are quite popular in maker community. Recently, I needed to design some parts that fits with it. However, finding it's accurate inner (basic) and outer (tip or addendum) diameters is very difficult. I don't know why this company does not simply release a drawing of it.

Some info floating on the Internet is not really accurate, such as this one: www.robotshop.com/blog/en/hobby-robot-industry-standards-hubs-patterns-18681

In the end, I reverse engineered from a design shared on GrabCAD and get the dimensions that work: https://grabcad.com/library/servo-horn-with-c1-spline-1

Here we go:
  • Inner radius: 2.520 mm
  • Outer radius: 2.845 mm. 
If you wanna 3D print an inner gear (a.k.a., annular gear) that fits with C1 spline, be sure to take the error or expansion that 3D printing can produce into consideration. In my case, I increased both inner and outer radii by 0.15 mm to 2.670 and 2.995mm respectively. 

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