Appearance
Gray cast iron is the ideal deep hole drilling material — its graphite flakes act as a built-in chip breaker and lubricant. Ductile iron, with its spherical graphite structure, is the opposite: it produces long, stringy chips and high cutting forces that punish tool edges. The same drilling parameters cannot be used for both, yet many shops try.
Material Fundamentals
Gray Cast Iron (GG)
Gray cast iron contains graphite in flake form. During machining:
| Property | Effect on Deep Hole Drilling |
|---|---|
| Graphite flakes | Act as chip breakers — chips fragment naturally |
| Free graphite | Provides lubricating layer at the tool-chip interface |
| Low ductility | Low cutting forces, excellent chip evacuation |
| Abrasive | Flake tips cause mild abrasive wear on tool |
Ductile (Nodular) Cast Iron (GGG)
Ductile iron contains graphite in spherical (nodular) form:
| Property | Effect on Deep Hole Drilling |
|---|---|
| Spherical graphite | No chip-breaking effect — long, continuous chips |
| Higher strength | 1.5–2× the cutting forces of gray iron |
| Higher toughness | Greater heat generation at cutting zone |
| Work-hardening tendency | Can form built-up edge on insert |
| Ferritic vs. pearlitic | Ferritic (130 HB) easier; pearlitic (250 HB) more abrasive |
| Parameter | Gray Iron (GG-25) | Ductile Iron (GGG-50) |
|---|---|---|
| Tensile strength | 250 MPa | 500 MPa |
| Hardness | 180–220 HB | 170–230 HB |
| Elongation | < 1% | 7–22% |
| Thermal conductivity | ~50 W/m·K | ~36 W/m·K |
| Machinability rating | Excellent (baseline) | 70–80% of gray iron |
Warning: The chip formation difference is the single most important factor in deep hole drilling. Gray iron produces short, broken chips that evacuate easily through gun drill flutes or BTA drill tubes. Ductile iron produces long helical chips that can pack and clog — requiring chip breaker geometries, higher coolant flow, and more frequent monitoring.
Gun Drilling Parameters
Mitsubishi MAS Gun Drills (Carbide)
| Drill Diameter | Gray Cast Iron | Ductile Cast Iron |
|---|---|---|
| Speed (SFM) | Feed (IPR) | |
| :---: | :----------: | :----------: |
| 3–6 mm (Ø0.125–0.234") | 130–260 | 0.004–0.008 |
| 6–10 mm (Ø0.250–0.391") | 195–360 | 0.004–0.012 |
| 10–20 mm (Ø0.406–0.781") | 230–425 | 0.008–0.016 |
Metric equivalents:
| Drill Diameter | Gray Cast Iron | Ductile Cast Iron |
|---|---|---|
| Speed (m/min) | Feed (mm/rev) | |
| :---: | :----------: | :-------------: |
| 3–6 mm | 40–80 | 0.10–0.20 |
| 6–10 mm | 60–110 | 0.10–0.30 |
| 10–20 mm | 70–130 | 0.20–0.40 |
Tungaloy Gun Drills (Carbide)
| Material | Cutting Speed (m/min) | Feed (mm/rev) by Drill Diameter |
|---|---|---|
| Ø3–5 mm | ||
| Gray iron (200 HB) | 20–50 | 0.05–0.15 |
| Ductile iron (300 HB) | 20–50 | 0.03–0.10 |
Tip: Tungaloy's conservative values are suitable for older machines or less rigid setups. Mitsubishi's ranges reflect modern CNC deep hole drilling machines with high-pressure coolant. For production environments with rigid setups and through-coolant, use the Mitsubishi ranges. For job-shop environments or variable conditions, use Tungaloy's ranges as a conservative starting point.
BTA Drilling Parameters
Cutting Speed and Feed
| Material | Condition | Hardness (HB) | Cutting Speed (m/min) | Feed (mm/rev) |
|---|---|---|---|---|
| Gray iron (GG) | Ferritic/pearlitic | 160–220 | 70–100 | 0.10–0.25 |
| Ductile iron (GGG) | Ferritic | 130 | 50–80 | 0.10–0.25 |
| Ductile iron (GGG) | Pearlitic | 250 | 50–80 | 0.10–0.38 |
| Malleable iron | Pearlitic | 230 | 50–80 | 0.10–0.25 |
Feed selection depends on drill diameter. As a general guideline:
| Drill Diameter (mm) | Feed Range (mm/rev) |
|---|---|
| 20–40 | 0.10–0.18 |
| 40–80 | 0.12–0.22 |
| 80–120 | 0.15–0.25 |
| 120–200 | 0.18–0.30 |
Insert Grade Selection (ISCAR FINEBEAM)
The ISCAR BTA drilling catalogue recommends:
| Material | Hardness (HB) | First Choice | For Fracture Resistance | For Wear Resistance |
|---|---|---|---|---|
| Gray iron (GG) | 160 | IC908 | IC806 | IC9025 |
| Nodular iron (GGG) | 250 | IC908 | IC806 | IC9025 |
| Malleable iron | 230 | IC908 | — | — |
Grade characteristics:
| Grade | Coating | Substrate | Application |
|---|---|---|---|
| IC908 | CVD Al₂O₃ + TiCN | Medium-hard | First choice for cast irons — balanced wear and toughness |
| IC806 | CVD multilayer | Tough | Interrupted cuts, casting skin, unstable conditions |
| IC9025 | CVD Al₂O₃ | Hard | High-speed finishing, abrasive wear conditions |
| IC520 | PVD TiAlN | Medium | General purpose |
| IC948 | PVD | Tough | Steel and cast iron in unstable conditions |
Chipbreaker selection:
| Chipbreaker | Feed Range | Application |
|---|---|---|
| G (General) | Standard | First choice for gray and ductile iron |
| HF (High Feed) | 1.5–2× standard | Higher productivity, ductile iron with good chip breaking |
Warning: For ductile iron, always use the HF (high feed) chipbreaker when feed rates exceed 0.20 mm/rev. Standard chipbreakers at high feed produce chips too thick to curl properly, leading to jamming in the drill tube.
CGI (Compacted Graphite Iron) Machining
Compacted graphite iron (CGI) has a vermicular graphite structure that falls between gray and ductile iron. It is increasingly used for diesel engine blocks, brake discs, and high-performance castings.
| Property | Gray Iron | CGI | Ductile Iron |
|---|---|---|---|
| Graphite form | Flake | Vermicular | Spherical |
| Relative machinability | 100% | ~83% | 70–80% |
| Tool wear mechanism | Abrasion | Adhesion + abrasion | Abrasion + deformation |
| Chip formation | Broken | Semi-broken | Continuous |
| Cooling strategy | Standard coolant | MQL preferred | Standard coolant |
CGI Deep Hole Drilling Parameters
Based on research using carbide drills in CGI-450 (420 MPa UTS, 210–265 HB):
| Parameter | Recommended Value |
|---|---|
| Cutting speed (carbide) | 80–100 m/min |
| Feed (per rev) | 0.10–0.20 mm/rev |
| Coolant strategy | MQL 5 mL/h or compressed air |
| Tool coating | Multi-layer PVD (TiAlN/AlCrN) |
| Expected tool life (4 mm drill) | 2,900+ holes (MQL) |
CGI tool life is strongly affected by coolant strategy:
| Coolant Condition | Tool Life (holes, 4 mm Ø) |
|---|---|
| Dry | 639 |
| Dry + compressed air | 2,969 |
| MQL 5 mL/h | 2,948 |
| MQL 20 mL/h | 2,685 |
Tip: For CGI deep hole drilling, MQL at low flow rates (5 mL/h) provides the best tool life. Flood coolant can reduce tool life in CGI by washing away lubricating graphite debris that would otherwise protect the cutting edge.
Coolant Parameters
BTA drilling of cast irons requires adequate coolant flow for chip evacuation.
Flow Rate Calculation
For BTA drilling: Q = 4.5 × D (L/min), where D = drill diameter in mm
| Drill Diameter (mm) | Coolant Flow (L/min) | Coolant Pressure (MPa) |
|---|---|---|
| 20 | 90 | 2–3 |
| 40 | 180 | 2–3 |
| 60 | 270 | 2–3 |
| 80 | 360 | 2–3 |
| 100 | 450 | 3–4 |
| 150 | 675 | 3–4 |
| 200 | 900 | 3–4 |
Coolant Type
| Coolant | Gray Iron | Ductile Iron | CGI |
|---|---|---|---|
| Soluble oil (5–8%) | ✓ Recommended | ✓ Recommended | ⚠ Flood can reduce life |
| Straight oil | ✓ Good finish | ✓ Good | ✓ Acceptable |
| MQL | — | — | ✓ Best for CGI |
| Compressed air | — | — | ✓ Acceptable |
Chip Control
Chip form directly determines success in deep hole drilling of cast irons.
| Chip Type | Gray Iron | Ductile Iron | CGI |
|---|---|---|---|
| Natural form | Short broken chips | Long helical chips | Semi-broken chips |
| Target chip type | C-type (broken) | C-type or short spiral | C-type |
| Chip breaker needed | Standard | HF or aggressive | Standard to HF |
| Evacuation difficulty | Low | High | Medium |
For ductile iron chip control:
- Increase feed rate to promote chip breakage (0.15–0.30 mm/rev minimum)
- Use chip breaker geometry with positive rake
- Maintain coolant pressure at or above 2.5 MPa
- Reduce feed if chips exceed 1/3 of tube cross-section
Problem Solving
| Problem | Likely Cause | Solution |
|---|---|---|
| Rapid flank wear (gray iron) | Speed too high, abrasive graphite | Reduce speed, switch to IC9025 |
| Built-up edge (ductile iron) | Low speed, no coating | Increase speed, use PVD-coated grade |
| Chip packing (ductile iron) | Inadequate chip breaking | Increase feed, use HF chipbreaker |
| Oversize bore | Insert wear or vibration | Check insert condition, adjust speed |
| Poor surface finish | Worn guide pads | Replace or index guide pads |
| Tool chatter | Feed too low for material | Increase feed rate |
| Broken inserts (ductile iron) | Interrupted cut or hard spot | Switch to IC806 (tougher grade) |
| Coolant pressure drop | Clogged filter or chip blockage | Check filter, clear drill tube |
Application Guide
| Application | Recommended Process | Key Parameters |
|---|---|---|
| Small holes < 20 mm Ø, gray iron | Gun drilling | 70–130 m/min, 0.10–0.40 mm/rev |
| Small holes < 20 mm Ø, ductile iron | Gun drilling | 50–100 m/min, 0.07–0.30 mm/rev |
| Medium holes 20–80 mm Ø, gray iron | BTA drilling | 70–100 m/min, 0.12–0.22 mm/rev |
| Medium holes 20–80 mm Ø, ductile iron | BTA drilling | 50–80 m/min, 0.12–0.25 mm/rev |
| Large holes > 80 mm Ø, gray iron | BTA drilling | 60–90 m/min, 0.15–0.25 mm/rev |
| Large holes > 80 mm Ø, ductile iron | BTA drilling | 50–70 m/min, 0.15–0.30 mm/rev |
| CGI (any diameter) | Gun or BTA with MQL | 80–100 m/min, 0.10–0.20 mm/rev |
| Casting with skin/hard spots | BTA with IC806 | Reduce speed 20%, increase feed 10% |
FAQ
What is the main difference between drilling gray iron and ductile iron?
Gray iron produces short, broken chips due to its flake graphite structure, making deep hole drilling relatively straightforward. Ductile iron produces long, continuous chips that require aggressive chip breakers, higher coolant flow, and tougher insert grades. Cutting speeds for ductile iron should be 30–40% lower than for gray iron.
What cutting speed should be used for gun drilling gray iron?
60–130 m/min depending on drill diameter, with larger diameters allowing higher speeds. For conservative starting conditions, begin at 60–80 m/min and increase based on tool wear observation.
What insert grade is recommended for BTA drilling of ductile iron?
ISCAR IC908 is the first choice for nodular/ductile iron (GGG). For interrupted cuts or casting skin, switch to IC806 (tougher). For high-speed finishing with abrasive wear, IC9025 is preferred.
How is coolant flow calculated for BTA drilling of cast iron?
Q = 4.5 × D (L/min), where D is the drill diameter in mm. For a 100 mm diameter BTA drill, minimum coolant flow is 450 L/min at 3–4 MPa.
Why is CGI harder to machine than gray iron?
CGI (compacted graphite iron) has vermicular graphite that does not provide the same chip-breaking or lubricating effect as gray iron's flake graphite. It also lacks the MnS layers that form on the cutting edge during gray iron machining, resulting in adhesion wear and higher cutting forces.
What is the best coolant strategy for CGI deep hole drilling?
MQL at low flow rates (5 mL/h) provides the best tool life in CGI — up to 2,948 holes in tests, versus 639 holes dry. Flood coolant can actually reduce tool life in CGI by washing away protective graphite debris.
What chip form should be targeted in ductile iron deep hole drilling?
C-type (broken) chips are ideal. If chips are long and stringy, increase feed rate or switch to an HF (high feed) chipbreaker geometry. Chips should not exceed 1/3 of the drill tube cross-section to ensure reliable evacuation.
What causes oversize bores in cast iron BTA drilling?
The most common causes are worn or chipped peripheral inserts, worn guide pads, or excessive vibration. Check insert condition first — a worn peripheral insert increases the effective cutting diameter.
Can gun drilling be used for CGI?
Yes, but with reduced parameters. Start at 80–100 m/min with 0.10–0.20 mm/rev feed. Use multi-layer PVD coated carbide tools. MQL at low flow rates significantly improves tool life compared to flood coolant.
What ISO standards govern cast iron machining?
ISO 513 classifies tungsten carbide grades for cast iron (K-group). ISO 185 covers gray iron grades (GG-15 to GG-40). ISO 1083 covers ductile iron grades (GGG-35 to GGG-80). ISO 513:2012 specifies the classification system for hard cutting materials including carbides for cast iron machining.
Conclusion
Gray iron and ductile iron require fundamentally different approaches in deep hole drilling. Gray iron's flake graphite structure makes it one of the most forgiving materials for gun drilling and BTA — natural chip breaking, low cutting forces, and good surface finish at speeds up to 130 m/min. Ductile iron's spherical graphite structure demands tougher insert grades (ISCAR IC908 first choice), aggressive chip breaker geometries (HF for feeds above 0.20 mm/rev), cutting speeds reduced by 30–40%, and higher coolant flow for reliable chip evacuation. CGI occupies a middle ground with machinability approximately 83% of gray iron, best addressed with MQL coolant strategy at low flow rates. Across all three cast iron types, the fundamental principle remains the same: chip control is the critical success factor, and the insert grade, chip breaker geometry, and coolant parameters must be selected to produce broken, easily evacuated chips.