Fly cutters are commercially available in a number of styles and sizes. As has happened before, I wanted to gain experience and add to my tooling, so a made a series of fly cutters as shown below. 

See Article Number 36 for a fly cutter design using carbide inserts.  

Below are two versions with different bit angles.  Both are made from free-cutting steel and have a shank diameter of 12 mm.  The bits are HSS,  6 mm diameter.  

The below fly cutter uses a HSS tool.  The shaft size is 12 mm and the material is free-cutting steel.  

The below fly cutter holds three round HSS bits. This design has a narrower  range of application -- compared to single-point designs -- because of the difficulty of  setting the cutters to a common height.