What happens around here is the groundies tend to "shovel" up derbis and toss it in to the chipper- rather than emptying shovel directly in to truck hence the blades come in sometimes with 1/4" or deeper knock outs on the edge. Not practical for me to do .001 grinds for hours on end to clean up that kind of damage. Single edge 40 degree blades are relatively quick to grind vs the double edged which have to be matched length from the center line on both sides X 2 , 4, or 6 blades in a set. They all have to match. I can run up to 4 -12" knives as a set. On single edged knives you have a contact area of apx 1/2" or less by the width of the wheel face - double edge at 30 degrees apx you are looking at an 1" or more of contact area length wise- On my machine the wheel face is 2" wide x10" dia. got to have coolant on that.