One of the more friendly materials for deep hole drilling — powdery chips are easy to evacuate, and gray iron's MnS inclusions self-lubricate the cutting zone. But the abrasive dust accelerates guide pad wear, and CGI (compacted graphite iron) is a different beast entirely: no MnS, up to 10× flank wear, and CBN tooling that fails in minutes.
| Type | Characteristics | Vc (m/min) | Feed (D=10mm) | Notes |
|---|---|---|---|---|
| Gray (GG25) | Powdery chips, MnS provides natural lubrication | 70–100 | 0.05–0.15 | Guide pad wear is the primary concern |
| Ductile (GGG40/50) | Nodular graphite, some toughness | 60–90 | 0.04–0.12 | Chip breaking matters — not fully powdery |
| CGI (compacted graphite) | High strength, low sulfur (<0.01%) | 50–80 | 0.03–0.10 | No MnS layer; 3–10× faster tool wear |
| Malleable iron | Annealed, some ductility | 60–85 | 0.04–0.12 | Behavior between gray and ductile |
| Method | Vc (m/min) | Feed | Coolant |
|---|---|---|---|
| Gundrill (gray / ductile) | 66–99 | 0.015–0.03 mm/rev | 2.5–6.0 MPa (25–60 bar) neat oil |
| Gundrill (CGI) | 40–70 | 0.01–0.025 mm/rev | 60–100 bar, sulfur-EP fluid |
| BTA (gray, ISCAR band) | 50–110 | 0.035–0.23 mm/rev | High-pressure external supply |
| BTA (CGI) | 45–80 | 0.05–0.15 mm/rev | 60–100 bar, EP additive |
| BTA Meehanite (machine spec) | — | 80–200 mm/min feed rate | — |
| Material | First choice | Why / notes |
|---|---|---|
| Gray iron | K-grade carbide, uncoated or thin PVD | High speeds OK; thick coatings peel on abrasive chips |
| Ductile iron | Coated K-grade carbide | Toughness of nodular graphite needs edge strength |
| CGI | Coated carbide at ~80–150 m/min | CBN fails fast on CGI — diffusion wear, ~1/20 of gray-iron life |
| CGI (production, high volume) | Cr-based PVD (AlCrN, TiSiN/AlCrN) or SiAlON ceramic | Cr-based best at 80 m/min; multilayers at 150 m/min |
Cast iron’s powdery chips make evacuation easy, but they are tiny abrasive particles that get dragged between guide pads and the bore wall. This is the dominant wear mode — not edge wear.
Avoid drilling into fractures, porosity, or chills — casting defects chip the cutting edge instantly.
Drill a flat-bottom pilot to drill diameter +0.02 mm, 1.5–3×D deep, or set the entry bushing.
Open coolant to full pressure before the tool touches — never start cutting with coolant off.
Engage at reduced speed/feed for the first 1–2×D, then ramp to machining parameters.
Small C-chips and steady coolant pressure = healthy process. Dusty blow-through = pad wear building.
Keep coolant flowing during retraction to flush residual dust; stop coolant only after the tool clears.