Cast iron is one of the most common materials in mechanical engineering, yet its behavior in deep hole drilling is fundamentally different from steel. The graphite content that makes cast iron valuable as an engineering material — its self-lubricating properties, vibration damping, and wear resistance — also creates distinct challenges for drilling: abrasive wear on cutting edges, fine graphite dust that packs in coolant passages, and wide variations in machinability between gray, ductile, and compacted graphite iron grades.
This guide covers the complete range of cast iron materials encountered in deep hole drilling, with manufacturer-sourced cutting data, tool selection criteria, and troubleshooting for the specific problems cast iron presents.
Cast Iron Material Grades and Properties
Gray Cast Iron (ASTM A48 / EN-GJL)
Gray iron is the most common cast iron grade. Its flake graphite structure provides excellent vibration damping and thermal conductivity but makes the material brittle with virtually no ductility.
| ASTM A48 Class | Min Tensile | Hardness (BHN) | EN-GJL Equivalent | Typical Applications |
|---|---|---|---|---|
| Class 20 | 20,000 psi (138 MPa) | 150–200 | EN-GJL-150 | Light-duty machine bases, low-stress components |
| Class 25 | 25,000 psi (172 MPa) | 170–229 | EN-GJL-200 | Medium-duty housings, pump bodies |
| Class 30 | 30,000 psi (207 MPa) | 187–241 | EN-GJL-250 | Engine blocks, brake drums, machine tool structures |
| Class 35 | 35,000 psi (241 MPa) | 207–255 | EN-GJL-300 | Heavy-duty machine bases, large dies |
| Class 40 | 40,000 psi (276 MPa) | 180–260 | EN-GJL-350 | High-strength castings, cylinder liners |
| Class 45–50 | 45,000–50,000 psi | 210–260 | — | Specialized high-strength applications |
Key characteristics for deep hole drilling:
- Graphite flakes act as chip breakers — chips are short and discontinuous
- Abrasive to cutting edges — graphite and hard carbide phases cause flank wear
- Cast surface skin can be harder than the interior — account for in entry conditions
- Classes 20–30 represent the majority of deep hole drilling work
Ductile (Nodular) Cast Iron (ASTM A536 / EN-GJS)
Ductile iron contains spheroidal graphite nodules rather than flakes, giving it measurable ductility and toughness. It machines differently from gray iron — the graphite nodules do not provide the same chip-breaking effect.
| ASTM A536 Grade | Tensile (ksi) | Yield (ksi) | Elongation | Hardness (BHN) | Typical Applications |
|---|---|---|---|---|---|
| 60-40-18 | 60 | 40 | 18% | 130–180 | Valves, pump housings, agricultural equipment |
| 65-45-12 | 65 | 45 | 12% | 160–210 | Gears, heavy machinery components |
| 80-55-06 | 80 | 55 | 6% | 170–230 | Crankshafts, heavy vehicle components |
| 100-70-03 | 100 | 70 | 3% | 190–270 | High-strength gears, rollers |
| 120-90-02 | 120 | 90 | 2% | 248–352 | Highly stressed components, military applications |
Key characteristics for deep hole drilling:
- More ductile chips than gray iron — larger, stringier chips require better chip breaking
- Less abrasive than gray iron at equivalent hardness — longer tool life potential
- Higher cutting forces than gray iron due to greater toughness
- Grades 60-40-18 and 80-55-06 are the most common for deep hole drilling work
Compacted Graphite Iron — CGI (ASTM A842 / EN-GJV)
CGI (also called vermicular graphite iron) has a graphite structure intermediate between gray and ductile iron — worm-like vermicular particles. It combines the strength and thermal conductivity of gray iron with the toughness approaching ductile iron. CGI is increasingly used in diesel engine blocks, brake discs, and heavy vehicle components.
| Typical Grade | Tensile (MPa) | Hardness (BHN) | Applications |
|---|---|---|---|
| GJV-300 | 300 min | 170–230 | Engine blocks, cylinder heads |
| GJV-350 | 350 min | 190–250 | Heavy-duty engine blocks, brake discs |
| GJV-400 | 400 min | 210–270 | High-stress diesel components |
| GJV-450 | 450 min | 230–280 | High-performance engine blocks |
Key characteristics for deep hole drilling:
- Significantly harder to machine than gray iron at equivalent hardness — tool life can be 50–80% lower
- Requires tougher carbide grades and specialized coatings (TiAlN/AlTiN)
- Chip evacuation is more critical — CGI produces larger, more cohesive chips than gray iron
- The most challenging of the three cast iron types for deep hole drilling
Comparison Summary
| Property | Gray Iron | Ductile Iron | CGI |
|---|---|---|---|
| Graphite form | Flakes | Spherical nodules | Vermicular (worm-like) |
| Relative machinability | Best (baseline) | Good (80–90% of gray) | Moderate (50–70% of gray) |
| Chip type | Short, powdery | Ductile, stringy | Mixed — shorter than ductile |
| Abrasiveness | High (graphite + carbides) | Moderate | Moderate to high |
| Typical cutting speed (HSS) | 60–100 SFM | 40–70 SFM | 30–50 SFM |
| Typical cutting speed (carbide) | 200–500 SFM | 150–375 SFM | 100–150 SFM |
Cutting Parameters for Gun Drilling and BTA Drilling
The following values are compiled from published data by Allied Machine, ISCAR, Mitsubishi Carbide, and other tool manufacturers. Starting values are at the lower end of each range — increase after confirming stable process conditions.
Gray Cast Iron — Gun Drilling with Carbide Tooling
By Hardness and Series (Allied Machine Data)
| Hardness (BHN) | Speed (SFM) | Feed (IPR) by Series (Diameter Range) |
|---|---|---|
| 120–150 BHN | 460 | Y/Z: 0.006; 0: 0.009; 1: 0.011; 2: 0.015; 3: 0.020 |
| 150–200 BHN | 400 | Y/Z: 0.005; 0: 0.008; 1: 0.010; 2: 0.014; 3: 0.018 |
| 200–220 BHN | 360 | Y/Z: 0.005; 0: 0.007; 1: 0.008; 2: 0.012; 3: 0.015 |
| 220–260 BHN | 310 | Y/Z: 0.004; 0: 0.006; 1: 0.007; 2: 0.010; 3: 0.013 |
| 260–320 BHN | 270 | Y/Z: 0.004; 0: 0.005; 1: 0.006; 2: 0.008; 3: 0.011 |
Series Y/Z: 0.374–0.500“ (9.5–12.7 mm); Series 0: 0.500–0.695“ (12.7–17.7 mm); Series 1: 0.695–0.960“ (17.7–24.4 mm); Series 2: 0.960–1.380“ (24.4–35.1 mm); Series 3: 1.380–1.882“ (35.1–47.8 mm)
By Drill Diameter (General Guidelines for Cast Iron — Carbide Gun Drills)
| Diameter (mm) | Speed (SFM) | Feed (mm/rev) |
|---|---|---|
| 3–5 | 350–500 | 0.01–0.04 |
| 5–10 | 300–450 | 0.03–0.10 |
| 10–15 | 280–400 | 0.05–0.14 |
| 15–20 | 250–380 | 0.07–0.18 |
| 20–30 | 220–350 | 0.10–0.22 |
| 30–50 | 200–300 | 0.14–0.26 |
Gray iron can be drilled at higher speeds than almost any other ferrous material due to its chip-breaking characteristics. The limiting factor is abrasive wear, not heat.
Gray Cast Iron — BTA Drilling with Carbide Tooling
| Diameter Range | Speed (SFM) | Feed (IPR) |
|---|---|---|
| 3/8“–1/2“ (9.5–12.7 mm) | 295–525 | 0.004–0.006 |
| 33/64“–11/16“ (13–17.5 mm) | 295–525 | 0.007–0.009 |
| 23/32“–1“ (18–25.4 mm) | 260–460 | 0.008–0.011 |
| 1“–1-3/8“ (25–35 mm) | 260–460 | 0.010–0.013 |
| 1-13/32“–1-7/8“ (36–48 mm) | 260–460 | 0.012–0.015 |
Gray iron BTA drilling can use the upper end of the feed range because chips are naturally short. Higher feed rates also help manage the abrasive wear by engaging fresh cutting edge more frequently.
Ductile (Nodular) Cast Iron — Gun Drilling with Carbide Tooling
Ductile iron requires approximately 15–25% lower cutting speeds than gray iron at equivalent hardness due to the tougher chip formation and higher cutting forces.
| Hardness (BHN) | Speed (SFM) | Feed (IPR) by Series (YG1/T-A System) |
|---|---|---|
| 120–150 BHN | 375–460 | Y/Z: 0.006; 0: 0.009; 1: 0.011; 2: 0.015; 3: 0.020 |
| 150–200 BHN | 325–400 | Y/Z: 0.005; 0: 0.008; 1: 0.010; 2: 0.014; 3: 0.018 |
| 200–220 BHN | 260–325 | Y/Z: 0.005; 0: 0.007; 1: 0.008; 2: 0.012; 3: 0.015 |
| 220–260 BHN | 195–260 | Y/Z: 0.004; 0: 0.006; 1: 0.007; 2: 0.010; 3: 0.013 |
Ductile Cast Iron — BTA Drilling
| Diameter (mm) | Speed (SFM) | Feed (mm/rev) |
|---|---|---|
| 10–15 | 260–330 | 0.10–0.20 |
| 15–25 | 230–300 | 0.15–0.28 |
| 25–35 | 195–260 | 0.18–0.32 |
| 35–50 | 165–230 | 0.20–0.35 |
CGI — Deep Hole Drilling Parameters
CGI is significantly more difficult to machine than gray iron. Research data from the University of Michigan and multiple tool manufacturers supports the following parameters for carbide tooling:
| Parameter | Gray Iron (baseline) | CGI | Ductile Iron |
|---|---|---|---|
| Cutting speed (m/min) | 100–160 | 80–150 | 60–100 |
| Cutting speed (SFM) | 330–525 | 260–490 | 200–330 |
| Feed rate (mm/rev) | 0.10–0.30 | 0.10–0.25 | 0.10–0.30 |
| Typical tool life vs gray iron | 100% | 25–50% | 60–80% |
Specific findings from CGI deep hole drilling research:
- Optimum cutting speed for deep holes (L/D > 6): 100 m/min (330 SFM) with compressed air through the drill. This speed balances productivity against the accelerated tool wear at higher speeds.
- Feed rate: 0.15–0.25 mm/rev is the typical range. Higher feed rates improve chip cross-section and heat dissipation but increase torque requirements.
- Tool life at 100 m/min, 0.2 mm/rev, with compressed air: 2,900–3,150 holes in a 4 mm diameter deep hole application before failure.
- Without compressed air: Tool life dropped to 640–1,742 holes — a 50–80% reduction.
- MQL (minimum quantity lubrication) with compressed air: Tool life of 2,570–2,950 holes — comparable to dry with compressed air alone.
Coolant Strategy for Cast Iron
Gray and Ductile Iron
Gray iron can be drilled dry at moderate depths (under 5×D) because the graphite acts as a natural lubricant. For deep hole drilling above 5×D, coolant is required for chip evacuation.
- Emulsion or cutting oil: Low-viscosity cutting oil is preferred for deep hole drilling. Emulsions at 5–8% concentration can be used but may cause graphite dust to form a paste in the coolant.
- Coolant pressure: 300–800 PSI for gun drilling gray iron (lower end of the range compared to steel). BTA gray iron requires 200–600 PSI depending on diameter.
- Filtration: Essential — graphite dust and fine carbide particles must be filtered to below 10 µm. Unfiltered graphite dust recirculates through the coolant passages and accelerates tool wear.
CGI Coolant Requirements
Compressed air through the drill is the most effective cooling method for deep hole drilling of CGI, based on published research. Dry drilling with compressed air outperformed both MQL and flood coolant in deep hole applications at L/D ratios above 6:1.
If coolant is used, high-pressure through-tool delivery (minimum 1,000 PSI) is required to prevent chip packing in the flutes.
Tool Selection for Cast Iron Deep Hole Drilling
Carbide Grade Selection
| Cast Iron Type | ISO Grade | Coating | Edge Preparation |
|---|---|---|---|
| Gray iron (roughing) | K20–K30 | CVD TiCN/Al₂O₃/TiN | Honed edge, 0.03–0.08 mm radius |
| Gray iron (finishing) | K10–K20 | CVD or PVD TiAlN | Sharp edge with light hone |
| Ductile iron | K15–K25 | PVD TiAlN or AlTiN | Honed edge, 0.02–0.05 mm radius |
| CGI | K20–K30 | PVD AlTiN or TiAlSiN | Honed edge, 0.03–0.08 mm radius |
| CGI (high-speed) | K10–K15 | CVD Al₂O₃ multilayer | Light hone |
The Signum (TiAlSiN) coating has demonstrated the lowest wear rate in CGI drilling at higher cutting speeds, attributed to its nanohardness of 3,546 HV. At lower speeds (80 m/min and below), chromium-based coatings such as AlCrN and TiSiN/AlCrN provided better tool life.
Gun Drill Head Profile for Cast Iron
- Profile A — specifically designed for cast iron and aluminum alloys. Features large coolant gaps between pads for improved chip flow. Usually coated. Suitable for cross drilling, angular entry, and interrupted cuts.
- Profile D — suitable for cast iron only. Very effective in gray cast iron, usually coated.
- Profile H — recommended for nonferrous and cast iron materials up to 5 mm diameter.
For ductile iron, Profile A or G (universal) is appropriate. For CGI, Profile A with TiAlN coating is recommended.
BTA Drill Head Selection
For cast iron BTA drilling, indexable insert heads are the standard choice for diameters above 20 mm. Brazed heads are preferred for smaller diameters where precision is critical.
Carbide grade recommendation for cast iron BTA heads: C2 (ISO K20) grade for general cast iron, with CVD TiCN/Al₂O₃ coating for abrasive wear resistance.
Common Problems in Cast Iron Deep Hole Drilling
Abrasive Flank Wear
Symptom: Rapid increase in cutting forces, deteriorating surface finish, visible wear land on the flank face of cutting edges.
Cause: Gray iron contains hard iron carbides and graphite particles that are abrasive to carbide tooling. The wear rate accelerates as the coating is penetrated.
Solution:
- Use CVD-coated grades with Al₂O₃ top layer for maximum abrasive wear resistance
- Maintain cutting speeds within the recommended range — higher speeds accelerate wear disproportionately
- Ensure adequate coolant flow to remove graphite dust from the cutting zone
- Monitor tool wear and establish a regrind schedule before wear becomes excessive — typically at 0.15–0.20 mm flank wear
Graphite Dust Packing in Coolant Passages
Symptom: Coolant pressure drops, tool overheating, poor chip evacuation.
Cause: Fine graphite dust from gray iron drilling accumulates in coolant passages and filter media, restricting flow. In water-based coolants, graphite dust forms a paste that is difficult to remove.
Solution:
- Use oil-based coolant where possible — graphite does not form paste in oil
- Maintain filtration to 5–10 µm — magnetic separators are ineffective for non-magnetic graphite particles
- Implement a coolant settling tank with sufficient residence time for graphite particles to settle
- Clean coolant passages in the tool during regrinding
Casting Skin Hardness Variations
Symptom: Difficult entry, tool chipping at the start of the hole, inconsistent cutting forces.
Cause: Cast iron castings develop a hard surface layer (chill) during solidification. This skin can be 20–50 points harder on the BHN scale than the interior material.
Solution:
- Spot face the entry surface before deep hole drilling to remove the casting skin
- If spot facing is not practical, reduce entry feed rate by 50% for the first 2–3 mm of penetration
- Ensure the pilot hole is deep enough to guide the drill past the skin layer before engaging full feed
Ductile Iron Chip Breaking
Symptom: Long stringy chips that wrap around the tool, chip packing in the evacuation tube.
Cause: Ductile iron’s spheroidal graphite structure does not provide the same chip-breaking effect as gray iron’s flake graphite. At the low feed rates sometimes used for deep hole drilling, ductile iron produces continuous chips.
Solution:
- Increase feed rate until chips break into segments — 0.15 mm/rev is a practical minimum for ductile iron
- Use gun drill head Profile G or A, which provide positive chip breaking geometry
- Ensure coolant pressure and flow are at the upper end of the recommended range for the diameter
CGI Tool Life Management
Symptom: Tool life 50–75% shorter than gray iron at the same cutting speed.
Cause: CGI’s vermicular graphite structure combines the abrasiveness of gray iron with the toughness of ductile iron, creating a more demanding cutting environment than either material individually.
Solution:
- Reduce cutting speed by 30–40% from gray iron values
- Use PVD AlTiN or TiAlSiN coatings (Signum-type) for best wear resistance
- Maximize feed rate within tool edge strength limits to reduce specific cutting energy
- Use compressed air coolant delivery for deep holes — it is the most effective method for CGI tool life
Surface Finish Expectations
| Cast Iron Type | Gun Drilling (Ra) | BTA Drilling (Ra) |
|---|---|---|
| Gray iron (Class 20–30) | 0.4–1.6 µm | 0.8–3.0 µm |
| Ductile iron (60-40-18) | 0.4–1.8 µm | 1.0–3.2 µm |
| CGI | 0.8–2.0 µm | 1.2–3.5 µm |
Gray iron achieves the best surface finish at moderate feed rates. The graphite flakes in gray iron act as natural chip breakers that prevent built-up edge formation, maintaining a stable cutting condition. Ductile iron surface finish is more sensitive to feed rate — excessive feed produces torn surface from the ductile chip formation.
Application Examples
Automotive Brake Disc / Drum Drilling
Gray cast iron (Class 30–35) is the standard material for brake discs and drums. Deep holes are typically cooling passages or mounting bolt holes:
- Typical diameter: 8–20 mm
- Depth: 100–300 mm (L/D 10:1 to 20:1)
- Method: Gun drilling with carbide-tipped tool
- Speed: 300–400 SFM
- Feed: 0.05–0.15 mm/rev
- Coolant: Oil or high-performance emulsion at 500+ PSI
Hydraulic Valve Body — Ductile Iron
Ductile iron (80-55-06) is used for hydraulic valve bodies requiring pressure tightness:
- Typical diameter: 6–25 mm
- Depth: 50–300 mm
- Method: Gun drilling
- Speed: 200–300 SFM
- Feed: 0.03–0.12 mm/rev (diameter-dependent)
- Coolant: Low-viscosity oil recommended for surface finish
- Note: Spot face all entry surfaces to remove casting skin before drilling
Diesel Engine Block — CGI
CGI (GJV-350 or GJV-400) is used in heavy-duty diesel engine blocks for improved strength and thermal management:
- Typical diameter: 4–12 mm (oil galleries, coolant passages)
- Depth: 200–600 mm
- Method: Gun drilling with TiAlSiN-coated carbide
- Speed: 80–120 m/min (260–395 SFM)
- Feed: 0.08–0.20 mm/rev
- Coolant: Compressed air through the drill preferred; high-pressure oil minimum 1,000 PSI if using liquid coolant
- Tool life expectation: Generally 50–60% of gray iron at equivalent hole count
Depth Adjustment Factors
When drilling cast iron at depths exceeding 7× diameter, apply the following adjustments based on Allied Machine guidelines:
| Drilling Depth | Speed Factor | Feed Factor |
|---|---|---|
| < 7×D | 1.0 | 1.0 |
| 7×D to 12×D | 0.9 | 1.0 |
| 12×D to 20×D | 0.8 | 0.9 |
| > 20×D | 0.7 | 0.8 |
Summary
Cast iron deep hole drilling is primarily a battle against abrasive wear, not heat or chip control. Key points to remember:
- Gray iron can be drilled at higher speeds than most ferrous materials — up to 500 SFM with carbide. Abrasive wear is the limiting factor, not heat.
- Ductile iron requires 15–25% lower speeds than gray iron at equivalent hardness. Chip breaking is a greater concern due to the tougher chip formation.
- CGI is the most challenging — reduce speeds by 30–40% from gray iron and use specialized PVD coatings (TiAlSiN).
- CVD-coated carbide with an Al₂O₃ top layer provides the best abrasive wear resistance for gray iron. PVD AlTiN coatings are preferred for ductile iron and CGI.
- Coolant filtration to below 10 µm is essential — graphite dust recirculation accelerates tool wear.
- Compressed air through the drill is the most effective coolant method for deep holes in CGI, outperforming flood coolant and MQL.
For related reading, see the Material Machinability Ratings for Deep Hole Drilling, the Speeds and Feeds Reference for Deep Hole Drilling, and the Deep Hole Drilling Troubleshooting Guide.