The two formulas behind every chart
Every speeds-and-feeds chart, whatever its presentation, rests on two short formulas in US units. The first converts surface speed into spindle speed: RPM = (SFM x 3.82) / diameter in inches, where SFM is the surface feet per minute the tool edge should travel and the constant 3.82 is 12 divided by pi, rounded. The second converts chip load into table feed: feed rate = chip load per tooth x number of flutes x RPM, where chip load is the thickness of material each cutting edge should shave per engagement. That is the whole arithmetic. Charts feel authoritative because they print dozens of numbers, but each number is one of these two formulas wearing a material name - and each is a starting point to be adjusted to the machine, the setup and the part, which is the framing this site keeps everywhere speeds appear.
A worked example, clearly illustrative
Take a half-inch diameter end mill and a starting point of 500 SFM for the material family - a placeholder figure for the sake of the arithmetic, not a recommendation for any alloy. RPM = (500 x 3.82) / 0.5, which is 1,910 divided by 0.5, so the calculation starts near 3,800 RPM. The same 500 SFM on a quarter-inch tool starts near 7,600 RPM, because the constant divides by diameter: halve the tool and the spindle doubles. Now the feed side, still illustrative: suppose a shop's starting chip load for the setup is 0.002 inches per tooth on a three-flute tool running at the 3,820 RPM the first example produced. Feed = 0.002 x 3 x 3,820, which is about 23 inches per minute. Both numbers are arithmetic on placeholders; neither is advice. The deeper walk-through of the same math is in the note on the RPM formula with worked examples, and the basics sit in speeds and feeds basics.
Where starting points come from, qualitatively
The desk deliberately describes starting surface speeds by material family rather than printing a numeric table, because a table without your machine behind it is theater. The tendencies, though, are real and worth knowing cold: aluminum and soft non-ferrous materials run at the high side of the range, which is why aluminum work feels fast; low-carbon steels sit in the middle; alloy and tool steels pull the range down as hardness and abrasion rise; stainless holds a low-to-middle band with the work-hardening watch applying; titanium runs at the low end, because speed is the main dial on edge temperature in a material that holds heat. Coating shifts the bands somewhat - hard nitride coatings tolerate hotter edges - and operation matters too, with light finishing passes tolerating more speed than heavy roughing. The fuller qualitative table lives in the note on SFM starting points by material, and the application pages - aluminum through titanium - carry the same tendencies per material.
| Material family | Speed tendency | Why, in one line |
|---|---|---|
| Aluminum, soft non-ferrous | High side of the range | Soft material, low edge load, heat leaves with the chip |
| Low-carbon steels | Middle of the range | Manageable loads; gummy behavior argues for real chip loads |
| Alloy and tool steels | Middle to low | Hardness and abrasion raise edge temperature |
| Stainless steels | Low to middle | Work hardening punishes rubbing; keep the tool cutting |
| Titanium alloys | Low end | Heat stays at the edge; speed is the main temperature dial |
What the formulas do not know
The two formulas answer "how fast" and "how much feed" as if the machine were an ideal object. Real shops know better, and the adjustments are where competence lives. Machine rigidity: a lighter machine or a longer tool reduces what the setup can absorb, which usually means backing off from the starting point. Holder condition and runout: an edge carrying an uneven share of the chip load wears on the schedule of its worst moment, not its average. Depth and width of cut: a light radial engagement lets speeds rise where a full slot does not. Tool stickout, fixture chatter, coolant practice, work hardening on the previous pass - each moves the answer. This is why the site's repeated phrase is "starting-point reference, adjust to your machine and setup": the formulas get you onto the map, and only your setup says where on the map you stand. The note on chip load per tooth covers the feed side's adjustments in the same spirit.
Working in metric
The formulas above are US units, and metric tools slot into them with one conversion: divide the diameter in millimeters by 25.4 to get inches, then run the arithmetic as before. A six-millimeter tool is about 0.236 inches, which is why it spins slightly faster than a quarter-inch tool at the same surface speed. Shops that work mostly in metric often keep a cutting-speed conversion of their own; the desk takes no side in that, except to insist that the series on the spec sheet and the series in the calculation agree - the mixed-unit failure mode is described on the sizes page.
The honesty policy
Cutting-tool marketing loves a bold feed-rate promise, and this site declines the genre. Nothing here is a performance claim: no tool-life hours, no promised metal-removal rates, no chart pretending your machine is the chart's machine. What the desk offers instead is the standard engineering arithmetic, clearly labeled starting points, and programs whose geometry is confirmed in writing so that when you do tune parameters on your own spindle, the tool in the holder is the tool the sheet described. If a supplier quotes you speeds without asking about your machine, treat it the way you would treat a tool-life promise made before seeing the part - as a reason to keep reading specs. When you are ready to spec tools, the flute count guide and the geometry pages pair with this reference, and the quote form takes the diameter list from there.