Set the speed too high and HSS edges burn out in minutes; set it too low and the job runs forever while the bit rubs and hardens the material. Because the correct value changes with material and diameter, many operators guess and then live with the consequences.
The good news is that two simple formulas replace guesswork.
Every drilling parameter comes from two ideas.
From these, spindle speed in rpm is:
rpm = (1000 x Vc) / (pi x D) where D is the drill diameter in mm.
Feed rate in mm per minute is simply rpm multiplied by feed per revolution.
In CNC, throughput depends on holding parameters consistently. Rigid holders, programmable peck cycles and coolant through a rigid setup let you push speeds to the safe limit without losing hole quality. Keep a parameter log per material and grade so proven values are reused instead of re-guessed.
| Material | Surface speed (m/min) | Feed (mm/rev) | Notes |
|---|---|---|---|
| Mild steel | 25 - 30 | D x 0.02 | Flood coolant |
| Alloy steel | 18 - 25 | D x 0.018 | Cobalt preferred |
| Stainless steel | 10 - 15 | D x 0.015 | M35 / M42, heavy feed |
| Cast iron | 20 - 25 | D x 0.02 | Dry or mist |
| Aluminum | 60 - 90 | D x 0.03 | High speed, polished flute |
| Brass | 40 - 60 | D x 0.025 | Watch for grabbing |
Reference starting values for HSS twist drills. Feed per revolution expressed as a fraction of diameter D.
| Condition | Adjustment |
|---|---|
| Deep hole (over 3xD) | Reduce speed 20 to 30 percent, peck |
| Hand / low-rigidity setup | Reduce speed and feed 20 percent |
| Coated / cobalt grade | Increase speed up to 30 percent |
Q: Which is the master number, speed or feed?
A: Speed is set from surface speed and diameter; feed follows from the load the bit can take.
Q: Why do small bits run so fast?
A: rpm rises as diameter falls, because a small circumference needs more turns to reach the same surface speed.
Q: Can I run HSS as fast as carbide?
A: No. Carbide tolerates several times the surface speed because it keeps hardness at much higher temperatures.
Q: Do I always need coolant?
A: Not for cast iron or many aluminum jobs, but steel and stainless benefit strongly from flood coolant or oil.
Speed and feed decide whether HSS drill bits cut efficiently or burn out. Dial in surface speed for the material, keep the feed positive, and HSS drill bits deliver consistent holes and long life. Treat HSS drill bits as part of a tuned process rather than a single variable, and CNC output rises with tool cost falling.
Choose a grade that matches your speed window. Browse HSS Drill Bits to compare cobalt series, coatings and lengths for CNC work.
