Whatever the angle between the cutting flutes is, it's important that both flutes cut evenly. Otherwise the frill tends to "walk" in one direction as the hole progresses, and the other cutting edge is just along for the ride- - - -it's not doing much cutting. Normally the included angle between the flutes is 110 degrees- - - -55 degrees per side. Soft material such as aluminum can use a sharper angle, and hard stuff such as annealed tool steel needs a flatter cutting angle, a lower speed, and lots of coolant.
Jerry
