Why radial engagement matters beyond just stepover
Radial width of cut as a raw number does not tell the whole story of what the tool actually experiences. Two cuts with the same width of cut but different tool diameters put very different loads on the cutter, since a small-diameter tool wraps much more of its circumference around a given width of cut than a large one does. Engagement angle captures that relationship directly.
How the numbers are calculated
Radial engagement as a percentage is simply the width of cut divided by the tool diameter. Engagement angle takes that same relationship and expresses it as the actual arc of the tool in contact with material, calculated as two times the arccosine of one minus two times the ratio of width of cut to diameter, giving a number in degrees that maps directly to how much of the tool circumference is cutting at any instant.
Practical tips
Low radial engagement, common in high-speed and trochoidal milling strategies, causes chip thinning, meaning the actual chip is thinner than the programmed chip load suggests, which is why those strategies typically run at higher feed rates to compensate. Full-width slotting sits at 100 percent engagement and 180 degrees, the maximum load case, and is usually where chatter and heat problems show up first when pushing depth or feed too aggressively.