A fly cutter is a metal cutting tool primarily used in milling machines. The fly cutter is used to make circular cuts in aluminum flat stock. The most prevalent job for a fly cutter is to make clearance notches in automobile pistons as well as planing a piece of stock square. In high-performance applications, the pistons must be notched to give clearance for the intake and most importantly the exhaust valves. Without this added clearance, the valves would make contact with the piston, causing catastrophic damage.
The fly cutter tool actually resembles a metal lathe cutting tool. The fly cutter is mounted in an arbor that is inserted into the milling machine's chuck. The milling machine can be adjusted to the proper depth and angle to cut the desired clearance into the top of a piston or any other flat aluminum stock that the machinist wishes to mill. The milling machine is operated very slowly to ensure a smooth and exact cut.
Cutting the soft aluminum is an easy task for the milling machine and the fly cutter. The operator must not attempt to rush the job and must use patience when milling the soft material. A cut that is forced or completed too fast will result in an uneven and rough finish. On a piston, this will create hot spots which can eventually lead to the destruction of the piston and possibly the entire engine.
When using the fly cutter to plane or machine a piece of stock square, it is imperative that the tool not be operated to fast. Slow and steady cutting will produce the flattest surface possible. When reaching the edge of the surface that is being machined, allow the cutting bit to proceed past the edge completely before beginning the return cut. Most cutters will remove a slight bit of material on the back side of the cutting radius regardless of how square the work piece is clamped into the machine.
When using the fly cutter to machine a flat surface, allowing the cutting head to overlap each cut slightly can help ensure a seamless and smooth surface on the finished piece. When operating the fly cutter at the proper speed, the chips coming off of the work piece will be curly in nature. While some cutters use steel cutting bits, a carbide cutting tip will produce the best results in most cases. As with any cutting tool, eye protection and protective clothing should be worn at all times.
