Fly cutters produce good, flat surfaces in one pass — assuming that your machine is set true. Every garage machinist should have several different-sized fly cutters. Typically a single-point cutter uses a HSS or carbide tool bit.
When using a large diameter cutter or a fly cutter, if there is a noticeable back-cut as the cutter passes over the workpiece being machined, the spindle is probably out of true.
The depth of cut for a fly cutter will be small compared with that of other cutting tools so material removal will be slower, but the advantages far outweigh this drawback. Using fly cutters will allow you to keep your expensive cutters for flnishing work rather than general material removal. You can grind a form on the cutting edge of, say, a particular radius, then machine this radius into the workpiece.
One of the greatest advantages of fly cutters is the ease with which they can be re-sharpened on any off-hand grinder, unlike end mills and slot drills, which have to go off for specialist re-sharpening. When you are calculating the speeds for fly cutters, the diameter that the cutting edge sweeps should be used in the spindle speed equation.
Danger: When you are using your milling machine for fly cutting, only the body of the fly cutter may be visible at speed and the actual cutting edge, which is further out, may become invisible to the eye. Keep your fingers well away.