What is a Fly Cutter?

Lori Kilchermann

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 is used for milling metals, and allows machinists to make circular cuts and notches in, as well as plain down, metals such as aluminum.
The fly cutter is used for milling metals, and allows machinists to make circular cuts and notches in, as well as plain down, metals such as aluminum.

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.

You might also Like

Readers Also Love

Discussion Comments


@miriam98 - I don’t have the patience for this kind of work, personally, especially if we are talking about attaching the fly cutter to a manual milling machine.

Moving ever so slowly to get the right cut is painstaking work indeed; it’s kind of like delicate woodworking.

However, in our area, there are lots of jobs available for people who want to work in metal fabrication. I don’t know if they are all CNC positions, but they are trade positions which allow you to learn on the job, if this is the kind of thing you like to do.


@MrMoody - Actually the article doesn’t say, but I think either machine would work. Traditional milling machines could do the job (with a lot of manual human intervention to work within expected tolerances), or CNC machines could too.

The CNC machines have the computer control pads which make everything easier to do in my opinion, but they are bulkier, and more expensive. I think it depends on the application.


It seems that there are a lot of variables involved in getting that fly cutter to cut that piece of metal accurately. I assume that there would be some programmable machine, like a CNC machine or something like that, used for the job.

I know CNC machines are used a lot in the metal fabrication industry because the machines can be programmed with precise instructions. I think that for automotive applications where you must have exacting tolerances for pistons, as the article talks about, programmed applications would be a must.

I believe that this would be especially true considering the wide variety of engine blocks and sizes with different makes and models of cars.

Post your comments
Forgot password?