30-Second Summary: Carbon and alloy steels are the most common materials for deep hole drilling. Lower carbon content means better machinability; increasing alloy content requires reducing Vc and using tougher tool grades. AlTiN+ coatings permit 10-15% higher cutting speeds. Coolant pressure of 50-100 bar is standard, with the higher end required for deeper holes and higher alloy content.
Material Properties
| Material | Hardness HB | Characteristics | Recommended Vc m/min |
|---|---|---|---|
| Low-carbon steel (< 0.25%C) | ~125 | Good machinability, continuous chips | 120-180 |
| Medium-carbon steel (0.25-0.45%C) | ~180 | Balanced properties | 100-150 |
| High-carbon steel (> 0.45%C) | ~220 | Higher hardness, control Vc | 80-120 |
| Low-alloy steel (annealed) | ~200 | High toughness, pay attention to chip breaking | 60-80 |
| Alloy steel (quenched & tempered) | ~300 | High hardness, reduce Vc | 40-60 |
Recommended Feed Rates by Drill Diameter
Feed rate is highly dependent on drill diameter. Below are typical starting ranges for deep hole drilling of carbon and alloy steels:
| Drill Diameter (mm) | Feed Rate (mm/rev) — Carbon Steel | Feed Rate (mm/rev) — Alloy Steel |
|---|---|---|
| 3 - 5 | 0.006 - 0.037 | 0.005 - 0.030 |
| 6 - 10 | 0.010 - 0.109 | 0.008 - 0.090 |
| 10 - 12 | 0.025 - 0.174 | 0.020 - 0.140 |
| 16 - 20 | 0.050 - 0.209 | 0.040 - 0.170 |
| 20 - 24 | 0.060 - 0.254 | 0.050 - 0.200 |
| 32 - 40 | 0.096 - 0.455 | 0.080 - 0.360 |
Start from the lower bound and increase based on chip shape and surface finish.
Recommended Parameters
- Tooling: Standard carbide gundrill or BTA drill head. TiAlN coating for general use; AlTiN+ coating permits 10-15% higher cutting speed. For alloy steels, tougher substrate grades are recommended.
- Coolant: Oil-based coolant, pressure 50-100 bar (higher end for depth-to-diameter ratios above 20:1). For low-carbon steel, 50-70 bar is typically sufficient.
- Chip breaking: Chip breaker must be effective for medium-carbon steel and above. Monitor chip shape — tight, curled chips indicate good breaking; long ribbons indicate insufficient feed or worn chip breaker geometry.
⚠️ Note: Hardness fluctuations in quenched and tempered steels can cause unstable cutting forces. Verify material hardness uniformity before machining. When transitioning between case-hardened and core material, reduce feed by 20-30% through the transition zone to avoid edge chipping.
💡 Practical tip: For alloy steels over 250 HB, consider using a pilot hole at 80-90% of full diameter to reduce axial forces and improve hole straightness. This is especially beneficial for depth-to-diameter ratios exceeding 30:1.