Fly Cutters Single Point Facing Tools for Manual and CNC Milling
A fly cutter is a single-point milling tool that sweeps a large diameter circle with one cutting tool bit, producing a flat surface with a characteristic arc finish pattern that is both functionally accurate and visually distinctive. Where a face mill uses multiple cutting inserts to generate a flat surface quickly, a fly cutter uses one bit to sweep the same area in a single rotation, creating a mirror-flat finish that is difficult to match with any other milling tool. In shops across the United States, fly cutters are the preferred facing tool for manual Bridgeport-style mills and CNC machining centers where the highest quality flat surface finish is the goal.
The core advantage of a fly cutter over a face mill is economy and finish quality. A fly cutter accepts a single HSS or brazed carbide tool bit that can be reground on a bench grinder when worn, eliminating the ongoing cost of replaceable inserts. The single point cutting action produces a long, flowing chip and a smooth arc pattern on the workpiece surface that machined inspection shows to be flatter and smoother than the overlapping insert patterns produced by multi-tooth face mills. For tool room work, surface plate lapping stock preparation, and any facing operation where finish quality takes priority over material removal rate, the fly cutter delivers results that justify its place in any milling machine shop.
How a Fly Cutter Works
The fly cutter body mounts in the mill spindle via an R8 shank, Morse taper, or straight shank arbor. A single tool bit is clamped in the body at a radial offset from the spindle centerline, and this offset determines the cutting circle diameter. Wider offsets produce larger cutting circles that cover more surface area per pass, reducing the number of passes needed to cover a workpiece but also increasing the cutting force at each revolution. A well-set fly cutter running at the correct spindle speed and a light depth of cut produces a surface that needs no further finishing for most engineering applications.
Buyohlic fly cutters are available in light-duty models for benchtop mills and small machining centers, and heavy-duty designs with rigid construction for full-size Bridgeport and VMC machines. Both types accept standard HSS tool bits for economical resharpening and brazed carbide bits for harder workpiece materials. The tool bit is clamped at the correct cutting angle in the body, and the radius of the cutter circle is adjusted by the position of the bit in the holder.
Fly Cutter vs Face Mill: When to Use Each
Choose a fly cutter when surface finish quality is the primary requirement, when you are working on a manual mill without the rigidity needed to run a multi-tooth face mill at production feeds and speeds, when you want to avoid the ongoing cost of carbide insert replacement, or when you need to produce a large flat area from a single tool that covers the full workpiece width in one setup. Choose a face mill when production rate is the priority, when you are surfacing hard materials at high speeds where carbide inserts outperform HSS, or when the machine rigidity and power support the higher cutting forces of a multi-insert tool.
For related milling tooling, see our milling tools and accessories collection and our milling cutters range. For workholding during fly-cutting operations, explore our clamping tools.
Frequently Asked Questions
A fly cutter is used to produce a flat, smooth surface across a metal workpiece on a milling machine. It sweeps a single cutting tool bit in a large circle as the spindle rotates, facing the workpiece surface in a smooth arc pattern. It is used for facing stock to establish a flat reference surface, surface-finishing previously roughed surfaces, and producing high-quality flat faces on precision parts where surface finish quality is critical.
RPM for a fly cutter is calculated based on the cutting circle diameter (not the spindle diameter), using the standard surface speed formula: RPM = (CS x 3.82) / Diameter, where CS is cutting speed in surface feet per minute. For a 3-inch diameter fly cutter on mild steel with an HSS bit, a cutting speed of 80 to 100 SFM gives approximately 100 to 125 RPM. For aluminum with carbide, 600 SFM gives approximately 750 RPM. Always start at the lower end and increase gradually.
Roughing passes with a fly cutter are typically 0.020 to 0.050 inch depth of cut, depending on machine rigidity and workpiece material. Finishing passes for the best surface finish use 0.002 to 0.005 inch depth of cut at a slow, steady feed rate. Taking too light a cut on a worn or dull tool bit produces burnishing rather than cutting, which produces a poor finish. Ensure the tool bit is sharp before the finishing pass.
No. A fly cutter generates significant radial (sideways) cutting forces that a drill press spindle and quill are not designed to withstand. Using a fly cutter on a drill press causes inaccurate results, excessive vibration, and risks permanent damage to the drill press spindle bearings. Fly cutters must be used on milling machines or machining centers with spindles designed for radial cutting loads.
High-speed steel (HSS) tool bits are the most common choice for fly cutters because they can be reground to the correct cutting geometry on a bench grinder when worn, and they perform well on mild steel, aluminum, brass, and cast iron at the lower speeds a fly cutter typically runs. Brazed carbide bits offer longer tool life on harder materials and can be run at higher speeds, but require diamond grinding to resharpen rather than a standard aluminum oxide grinding wheel.
