Speeds and feeds is the math that gets a tool through material without burning it up, breaking it, or producing a finish that looks chewed. The formulas are simple. The judgment isn't — but you have to start with the math.

The four numbers

• SFM — surface feet per minute (material × tool grade)
• RPM — spindle speed
• IPM — inches per minute (feed rate)
• MRR — material removal rate (cubic inches per minute)

The formulas

RPM   = (SFM × 12) / (π × D)
IPM   = RPM × Z × Fz
MRR   = IPM × DOC × WOC

D    = tool diameter (inches)
Z    = number of flutes
Fz   = chip load per tooth (in/tooth)
DOC  = depth of cut (inches)
WOC  = width of cut (inches)

SFM ranges to start with

• Aluminum 6061 — 600–1500 SFM
• Mild steel 1018 — 80–200 SFM
• Alloy steel 4140 — 60–150 SFM
• Stainless 304 — 60–130 SFM
• Titanium Gr5 — 50–120 SFM
• Brass 360 — 200–600 SFM

Carbide handles the high end of those ranges. HSS handles the low end. Coatings (TiN, TiAlN, AlTiN) push the high end further if you have rigidity and coolant.

Chip load — the easy way

• Tool ≤ 1/8" — start at 0.001 in/tooth
• 1/8" – 1/4" — start at 0.002 in/tooth
• 1/4" – 1/2" — start at 0.003 in/tooth
• 1/2" – 3/4" — start at 0.004 in/tooth
• Above 3/4" — start at 0.005+ in/tooth

Worked example: 1/2" 3-flute end mill in aluminum

Material: 6061 → SFM ≈ 1000
Tool:     1/2" Ø, 3 flutes
Chip:     0.003 in/tooth

RPM = (1000 × 12) / (π × 0.5)
    = 12000 / 1.5708
    ≈ 7,640 RPM

IPM = 7640 × 3 × 0.003
    ≈ 68.8 IPM

For DOC = 0.25, WOC = 0.5:
MRR = 68.8 × 0.25 × 0.5
    = 8.6 in³/min

Run it on your phone

The ShopCalc Android app (full name: ShopCalc: Machinist Calculator) runs all of this in your pocket — material presets, chip-thinning math, ±50% Surface Speed and Chip Load sliders, imperial/metric. 100% offline, no ads, no accounts. Free on Google Play.

Related

• ShopCalc: Machinist Calculator — free on Google Play
• Ace & Alan Tool — carbide and end mills

Note: Use as a starting point. Always validate with your machine, fixture, and tooling.