Creating an internal gear for servo horns

[djb-systems]

Just to make sure I am on the correct track, is this how you would cut a tool for an internal spline.

 

[Huub Buis]

Yes, this form is for broaching an internal spline. The tip is the cutting edge. You could chamfer the tip a few degrees to create some clearance.
See this link for more information about Servo Splines
This is a "flat" spline (serration) but "involute" splines are also common. The root can be a flat or a filet.
More spline info

 

[Alexander]

What is your choice of materials?

 

[epanzella]

Forget about the splines. Use aluminum. Bore or ream the hub for a tight slip fit on the servo shaft. Slit the hub and tap it for a pinch bolt that tightens the arm onto the servo output shaft.