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How to Use a Fly Cutter
Guide · Updated October 28, 2026
A fly cutter is a single-point tool that swings a wide circle, leaving a very flat, fine finish on a face for little cost. It shows every tramming error and every bit of looseness, so setup matters more than the tool.
Prove the face is flat
Check the finished face with an indicator and gage blocks; the free Gage Block Calculator builds the stacks.
Step by step
- Tram the head: Tram the head in X and Y first; out-of-tram shows as a dished or crosshatched face.
- Set the tool: Use a sharp HSS or carbide tool with a small radius, short stickout in the body, locked tight.
- Clamp the part: Clamp the part flat on parallels in the vise or to the table with no rocking.
- Pick the RPM: Use the swing diameter to figure RPM; a 3 in swing in aluminum may run 1000–1500 rpm, steel far slower.
- Touch off and set depth: Touch off, then take light cuts: 0.005–0.020 in finish, a little more roughing.
- Feed across: Feed steadily the full width with gibs snug on axes you are not moving.
- Inspect: Check the pattern: an even crosshatch means good tram; one-way swirls are normal with a slight tilt.
Worked example
Aluminum plate, 3 in swing, carbide tool, 600 SFM: RPM ≈ 600 × 3.82 / 3 ≈ 760 rpm. Feed 0.004–0.006 in per rev = about 3–4.5 in/min. Finish pass 0.005 in deep.
Reading the finish
A slight dish means the head is out of tram across the swing. Scallops mean too much feed per rev or a dull tool. Chatter means too much stickout, too much speed or a loose gib.
Safety
Fly cutters throw chips wide and a loose tool is dangerous. Keep the tool short and tight, guard around the mill and never reach in while it spins.
Tips
- Tram before you fly cut; see how to tram a mill.
- Use milling speeds and feeds with swing diameter as the cutter size.
- Check flatness after with this method.
- Grade finish with surface finish basics.
FAQ
What RPM should I run a fly cutter?
Figure RPM from surface speed using the swing diameter as the cutter diameter, then run slower if it chatters.
Why does my fly cut leave a crosshatch?
A crosshatch means the cutter cuts on both the front and back of the swing, which usually means the head is well trammed; a single swirl means a small tilt.
How deep can you cut with a fly cutter?
Light, usually 0.005 to 0.030 in depending on rigidity; it is a finishing tool, not a roughing tool.
Is a fly cutter better than a face mill?
It gives a finer finish at low cost but cuts slower; a face mill removes material faster.
What tool goes in a fly cutter?
A ground HSS bit or a carbide insert or brazed tool with a small nose radius.
Related guides and calculators
- How to Tram a Mill
- Milling Speeds and Feeds
- Surface Finish Basics
- Flatness and Parallelism Checks with Gage Blocks
- How to Face Mill
- Parallels and Setup Blocks Basics
- How to Indicate a Vise True
- Cutting Fluid and Coolant Basics
- How to Use a Ball End Mill
- All free machinist calculators
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General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.