Speeds and feeds in one step
Every machining job, from drilling a hole on a drill press to milling a pocket on a CNC machine, starts with two numbers: how fast the tool spins and how fast it moves through the material. Run too slow and you waste time; run too fast and the tool overheats, dulls or breaks. The cutting speed of the material is the key that links them.
Cutting speed and spindle speed
The cutting speed is how fast the cutting edge moves across the material, in surface feet per minute (SFM) or meters per minute. The spindle speed then depends on the diameter:
RPM = SFM × 12 ÷ (π × D), with D in inches (often simplified to 3.82 × SFM ÷ D)
n = 1,000 × Vc ÷ (π × D), with Vc in m/min and D in mm
Typical starting speeds
| Material | HSS tool (SFM) | Carbide tool (SFM) |
|---|---|---|
| Aluminum | 300 | 800 |
| Mild steel | 100 | 400 |
| Stainless steel | 50 | 250 |
| Cast iron | 70 | 300 |
| Brass and bronze | 200 | 600 |
| Plastics | 300 | 800 |
These are conservative averages. Coated tools, flood coolant and rigid machines allow more; long overhangs, hobby machines and interrupted cuts call for less.
Feed rate
Feed rate = RPM × number of flutes × chip load
The chip load (feed per tooth) is the thickness of material each cutting edge removes per revolution. For small end mills in steel it is often 0.001 to 0.003 inch; for aluminum it can be twice that.
Example
A ½-inch, 2-flute HSS end mill in mild steel at about 98 SFM:
- RPM = 98 × 12 ÷ (π × 0.5) ≈ 750 RPM
- Feed = 750 × 2 × 0.002 ≈ 3.0 in/min
The same cutter in carbide could run about four times faster, near 3,000 RPM, with a proportionally higher feed.
Drilling and turning
- Drilling: use the drill diameter. Large drills need lower RPM; very small drills often need more RPM than a drill press can provide. Reduce speed by about a third for deep holes.
- Turning: use the workpiece diameter, which changes as you cut. Recalculate for each diameter, or use constant surface speed on a CNC lathe.
Signs you need to adjust
- Blue chips or burning smell in steel: too fast or too little coolant.
- Squealing or chatter: reduce speed, increase feed slightly, or shorten the tool overhang.
- Fine powdery chips and a rubbing sound: the feed is too light.
Always follow the tool manufacturer’s data when available, and wear eye protection.