Internal-coolant (through-tool) carbide drills are now standard for production drilling above 4×D depth. Here is when each makes sense.
Internal Coolant (Through-Tool)
Coolant flows through the drill body and exits at the cutting tip:
- Reaches the cutting edge directly
- Flushes chips out of the hole
- Allows 30-50% faster cutting speeds
- Required for depths > 4×D
- Required for difficult materials (stainless, titanium, Inconel)
External Coolant Only
- Lower-cost drill (no internal channel)
- OK for shallow holes (< 3×D)
- OK for cast iron (often dry anyway)
- Adequate for soft aluminum
Machine Requirements
Internal-coolant drills require through-spindle coolant (TSC) at the machine:
- Spindle must have TSC capability (newer machines do; older retrofits possible)
- Coolant pump pressure: 70 bar (1000 PSI) typical, 150 bar (2200 PSI) for premium HP coolant
- Tool holder must pass coolant (BT/CAT/HSK with TSC channel)
Drill Selection
| Hole Specs | Drill Type |
|---|---|
| Steel, depth ≤ 3×D | External-coolant carbide OK |
| Steel, depth 3-5×D | Internal-coolant carbide preferred |
| Steel, depth > 5×D | Internal-coolant mandatory |
| Stainless any depth | Internal-coolant mandatory |
| Aluminum shallow | External or polished uncoated |
| Aluminum deep | Internal-coolant for chip evacuation |
| Titanium / Inconel | Internal-coolant + HP (1000+ PSI) |
| Cast iron (gray) | External or dry (no liquid coolant) |
Premium Brands
- Sandvik CoroDrill 460 (steel/stainless)
- Walter A3782/A3792 (general-purpose)
- OSG ADO series (production)
- YG-1 D-Power (hardened)
- Mitsubishi MWS series (multi-material)
Tool Life Comparison
Same 12mm carbide drill, 1045 steel, 4×D depth, 80 m/min Vc:
- External coolant only: 200-400 holes
- Internal coolant @ 70 bar: 800-1500 holes (4× longer)
- Internal coolant @ 150 bar HP: 1500-3000 holes (5-7× longer)
See our drill selection guide.
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