Set the spindle speed for a fly cutter on a milling machine. Enter the swing diameter of the tool tip and a surface speed for your material and tool; add a chipload if you want a table-feed starting point.
A fly cutter is a single tool bit swung on a large radius. It leaves a flatter, finer face than most small face mills on a light manual machine, but its large diameter means the tip is moving very fast even at modest spindle speeds. Speed that suits a ½ in end mill would ruin a fly cutter bit in seconds.
The calculator converts a surface speed (feet per minute at the tip) into RPM for the swing you set, and optionally turns a chipload into a feed rate.
RPM comes from the same identity as every rotating cutter. Because there is only one cutting edge, feed per revolution and chipload are the same thing.
RPM = SFM × 3.82 ÷ D · Feed (in/min) = RPM × chipload
D is the diameter of the circle the tool tip sweeps — measure tip to tip across the swing, or double the radius from spindle center to tip.
Typical starting surface speeds for the four columns are shown in the headers. They are conservative shop starting points; your tool grind, rigidity and coolant move them.
| Swing D | 90 SFM Mild steel, HSS | 150 SFM Brass, HSS | 300 SFM Aluminum, HSS | 400 SFM Mild steel, carbide |
|---|---|---|---|---|
| 1.5 in | 229 | 382 | 764 | 1,019 |
| 2 in | 172 | 287 | 573 | 764 |
| 2.5 in | 138 | 229 | 458 | 611 |
| 3 in | 115 | 191 | 382 | 509 |
| 4 in | 86 | 143 | 287 | 382 |
| 5 in | 69 | 115 | 229 | 306 |
| 6 in | 57 | 96 | 191 | 255 |
Facing a mild-steel plate. 3 in swing, HSS tool, 90 SFM: 90 × 3.82 ÷ 3 = 114.6 RPM. With a 0.002 in chipload the feed is 114.6 × 0.002 = 0.2292 in/min — slow, which is normal for a single point.
Skimming an aluminum fixture plate. 4 in swing, HSS, 300 SFM: 286.5 RPM. At 0.003 in per rev the feed is 0.8595 in/min.
When the machine cannot go slow enough. A 6 in swing at 90 SFM needs 57.3 RPM. If your lowest speed is higher, shorten the swing or use a carbide tool rated for the higher surface speed — do not just run fast.
Stand clear of the swing and keep the spindle guard in place if the machine has one. Check that the tool bit is locked tightly in the holder before starting; a bit that loosens at speed leaves the holder. Wear eye protection — single-point cuts throw hot chips in a wide arc. Never reach across a turning fly cutter to brush chips away.
Use the figure for the tool you actually have. Carbide tolerates higher surface speeds but needs a rigid setup; on light machines HSS often gives the better finish.
The trailing side of the cutter is recutting the surface, usually because the head is slightly out of tram. Re-tram or reduce the swing.
The RPM formula is the same, but feed needs to be multiplied by the number of inserts. This page assumes one cutting edge.
It depends on machine rigidity. On a small mill, 0.010–0.020 in roughing passes and 0.002–0.005 in finish passes are common starting points.