Keep a variety of fly cutters on hand to tackle different sized workpieces. The fly cutters in these sets hold a lathe tool (sold separately) to shave a layer off your workpiece with each pass, creating a smooth, even finish. Use them with milling machines. Compared to end mills, these make a wider cut per pass. They also generate less heat, so you can run them at higher speeds. In the long run, these cutters will save you the expense of replacing entire end mills once they dull—you can either sharpen your worn lathe tool or just replace that one part.