@T-slot cutters are commercially available, but I decided to make my own, which allows  great freedom of size and geometry.  Each cutter has eight cutting edges,  formed as   shown in the below drawing.  

My purpose was to cut t-slots in cast iron, which I have done and found easier than expected.  These cutters have held up well after repeated use.   

Hard tool steel is required for this application and  SKS-3 JIS (Japanese Industrial Standards) was chosen.

Machining of this material must proceed slowly.  Depth of cut was limited to 0.5 mm, with cutting speeds limited to 1,000 rpm.   The chips were hot and blue, necessitating care to avoid danger to the operator.

After basic machining, the cutters were hardened and the edges sharpened.  

@Shown below the cutting edges are machined while the t-slot cutter is held in the Simple Index Stand.  (See Article #15)