Gun Drilling by Material: Parameters and Challenges
Every material presents unique challenges in gun drilling. What works well for low-carbon steel can cause chip packing in stainless steel, rapid tool wear in titanium, or poor surface finish in aluminum.
This guide covers the recommended parameters, common challenges, and best practices for gun drilling the most common engineering material groups.
Low-Carbon Steel (< 0.25% C)
Materials such as AISI 1010, 1018, 1020 are the easiest materials to gun drill. They produce consistent, well-broken chips and cause moderate tool wear.
| Parameter | Recommendation |
|---|---|
| Cutting speed | 120–180 m/min (400–600 SFM) |
| Feed rate (Ø6 mm) | 0.012–0.022 mm/rev |
| Feed rate (Ø12 mm) | 0.025–0.050 mm/rev |
| Feed rate (Ø20 mm) | 0.050–0.085 mm/rev |
| Coolant pressure | Standard for diameter |
| Nose grind | N-8, R1 relief, or facet grind |
Key challenges: Minimal. The main risk is pushing parameters too aggressively and causing chip packing.
Chip appearance target: Silver or light straw, short C-shaped segments.
Medium-Carbon and Alloy Steel (0.25–0.55% C)
Materials such as AISI 1045, 4140, 4340 (annealed) are the most common gun drilling materials in production environments.
| Parameter | Recommendation |
|---|---|
| Cutting speed | 100–140 m/min (330–460 SFM) |
| Feed rate (Ø6 mm) | 0.012–0.020 mm/rev |
| Feed rate (Ø12 mm) | 0.025–0.045 mm/rev |
| Feed rate (Ø20 mm) | 0.040–0.070 mm/rev |
| Coolant pressure | Standard for diameter |
| Nose grind | N-8, R1 relief, or facet grind |
Key challenges:
- Chip shape control: Alloy steels can produce long, stringy chips if feed is too low. Maintain feed at the middle to high end of the range.
- Work hardening: Some alloy grades (4140 PH) work-harden. Avoid letting the tool dwell in the cut.
- Surface finish: Guide pad burnishing is effective, but coolant concentration must be maintained.
For a dedicated deep dive on 4140 — the most common alloy in this group — see gun drilling 4140.
High-Alloy and Tool Steel
Materials such as H13, D2, A2, P20, and hardened die steels.
| Parameter | Recommendation |
|---|---|
| Cutting speed | 60–100 m/min (200–330 SFM) |
| Feed rate (Ø6 mm) | 0.007–0.015 mm/rev |
| Feed rate (Ø12 mm) | 0.018–0.035 mm/rev |
| Feed rate (Ø20 mm) | 0.030–0.060 mm/rev |
| Coolant pressure | Increase 15–25% above standard |
| Nose grind | N-8 with reinforced cutting edge |
Key challenges:
- Tool wear: Abrasive carbide structure accelerates edge wear. Use fine-grain carbide tools.
- Hardness variation: Heat-treated steels may have hardness gradients that cause the drill to wander.
- Heat generation: Reduce speed to control heat. At HRC 40+, cutting speeds drop to 15–30 m/min.
Stainless Steel (Austenitic: 304, 316, 321)
Austenitic stainless steels are among the most challenging materials for gun drilling due to work hardening, stringy chips, and poor thermal conductivity.
| Parameter | Recommendation |
|---|---|
| Cutting speed | 50–80 m/min (160–260 SFM) |
| Feed rate (Ø6 mm) | 0.008–0.015 mm/rev |
| Feed rate (Ø12 mm) | 0.020–0.040 mm/rev |
| Feed rate (Ø20 mm) | 0.030–0.055 mm/rev |
| Coolant pressure | Increase 20–30% above standard |
| Nose grind | N-8, R1 relief, with chipbreaker |
Key challenges:
- Work hardening: Stainless steel work-hardens rapidly if the feed is too low or if the tool dwells. Never let the tool stop while in contact with the workpiece.
- Stringy chips: Austenitic grades produce long, stringy chips that are prone to packing. Use chipbreaker geometries and maintain adequate feed.
- Heat buildup: Stainless steel retains heat. Ensure coolant flow is directed precisely at the cutting edge.
- Built-up edge: Common at lower speeds. Increase speed within the recommended range if BUE appears.
Stainless Steel (Martensitic/Ferritic: 410, 416, 430)
These grades are easier to gun drill than austenitic stainless steels.
| Parameter | Recommendation |
|---|---|
| Cutting speed | 60–100 m/min (200–330 SFM) |
| Feed rate | Similar to alloy steel for the same diameter |
| Coolant pressure | Standard or slightly elevated |
| Nose grind | N-8, R1 relief |
Key challenges: Less severe than austenitic grades. 416 (free-machining) drills very well.
Titanium Alloys (Ti-6Al-4V, Ti-6Al-4V ELI)
Titanium is challenging due to its low thermal conductivity, high strength at temperature, and chemical reactivity with tool materials.
| Parameter | Recommendation |
|---|---|
| Cutting speed | 15–25 m/min (50–80 SFM) |
| Feed rate (Ø6 mm) | 0.006–0.012 mm/rev |
| Feed rate (Ø12 mm) | 0.015–0.030 mm/rev |
| Feed rate (Ø20 mm) | 0.025–0.045 mm/rev |
| Coolant pressure | Increase 30–50% above standard |
| Nose grind | N-8 with sharp edge (no chamfer), polished flute |
Key challenges:
- Heat concentration: Titanium conducts heat poorly. Nearly all cutting heat stays in the tool. Use abundant coolant and keep speeds low.
- Chemical reactivity: Titanium welds to carbide at high temperatures. Keep the cut cool and use AlTiN-coated tools.
- Chip control: Titanium produces thin, serrated chips. Maintain adequate feed to avoid chip packing.
- Tool wear: Expect shorter tool life. Accept 300–500 linear inches between regrinds as normal.
- Springback: Titanium’s low modulus of elasticity causes the bore wall to spring back slightly after cutting. Account for this in guide pad interference.
Nickel-Based Superalloys (Inconel 718, Waspaloy, Hastelloy)
Superalloys are the most difficult materials to gun drill. Expect slow speeds, short tool life, and high cost per hole.
| Parameter | Recommendation |
|---|---|
| Cutting speed | 10–20 m/min (33–65 SFM) |
| Feed rate (Ø6 mm) | 0.005–0.010 mm/rev |
| Feed rate (Ø12 mm) | 0.010–0.025 mm/rev |
| Feed rate (Ø20 mm) | 0.020–0.040 mm/rev |
| Coolant pressure | Increase 50% above standard |
| Nose grind | N-8 with reinforced corner, polished surfaces |
Key challenges:
- Extreme work hardening: Inconel work-hardens aggressively. Maintain steady feed—never dwell.
- High cutting forces: Requires rigid machine setup. Use solid carbide or heavy-duty brazed tip tools.
- Short tool life: Expect 100–300 linear inches between regrinds. Accept this as normal.
- Heat management: Maximum coolant flow is essential. Consider through-tool coolant with supplementary external flood.
Aluminum (Wrought: 6061, 7075)
Aluminum is generally easy to gun drill, but its softness and ductility create unique challenges.
| Parameter | Recommendation |
|---|---|
| Cutting speed | 80–160 m/min (260–525 SFM) |
| Feed rate (Ø6 mm) | 0.010–0.080 mm/rev |
| Feed rate (Ø12 mm) | 0.025–0.175 mm/rev |
| Feed rate (Ø20 mm) | 0.050–0.210 mm/rev |
| Coolant pressure | Standard for diameter |
| Nose grind | N-4, R4 relief (polished) |
Key challenges:
- Built-up edge: Aluminum can weld to the carbide tip at lower speeds. Keep speed up and use polished tools.
- Chip control: Aluminum produces long, stringy chips at low feed. Use high feed rates to break chips.
- Oversize holes: Soft aluminum can “push” rather than cut, producing oversized holes. Reduce feed if diameter runs high.
- Gallium in 7075: Some aerospace grades contain gallium, which attacks carbide at high temperatures. Keep coolant flow high.
Cast Iron (Gray, Ductile)
Cast iron drills relatively easily but produces abrasive dust that can accelerate wear.
| Parameter | Recommendation |
|---|---|
| Cutting speed | 70–100 m/min (230–330 SFM) |
| Feed rate | Similar to alloy steel for same diameter |
| Coolant pressure | Standard for diameter |
| Nose grind | N-73 or N-8 with chipbreaker |
Key challenges:
- Abrasive wear: Graphite particles in cast iron are abrasive. Use fine-grain carbide for longer tool life.
- Dust: Cast iron dust is abrasive and can contaminate coolant systems. Use effective filtration.
- Interrupted cuts: Cast iron components often have cored holes or cavities. Reduce feed for interrupted cuts.
Material Selection Quick Reference
| Material | Speed (m/min) | Feed (mm/rev, Ø12 mm) | Coolant Pressure | Difficulty |
|---|---|---|---|---|
| Low-carbon steel | 120–180 | 0.025–0.050 | Standard | Easy |
| Alloy steel | 100–140 | 0.025–0.045 | Standard | Moderate |
| Tool steel | 60–100 | 0.018–0.035 | +15–25% | Moderate |
| Stainless (austenitic) | 50–80 | 0.020–0.040 | +20–30% | Difficult |
| Titanium | 15–25 | 0.015–0.030 | +30–50% | Very difficult |
| Superalloys (Inconel) | 10–20 | 0.010–0.025 | +50% | Extreme |
| Aluminum | 80–160 | 0.025–0.175 | Standard | Easy |
| Cast iron | 70–100 | 0.025–0.050 | Standard | Moderate |
| Brass | 80–150 | 0.020–0.140 | Standard | Easy |
| Hardened steel (HRC 40+) | 15–30 | 0.012–0.025 | +25–40% | Very difficult |
Summary
Selecting the right parameters for the material is the foundation of successful gun drilling. Low-carbon steel and aluminum are straightforward; titanium, superalloys, and austenitic stainless steel require careful parameter selection, robust coolant systems, and disciplined tool maintenance. Always start at the lower end of the speed range, monitor chip formation closely, and adjust based on what the chips tell you.
For general parameter selection including nose grind recommendations, see our gun drilling speeds and feeds guide. For troubleshooting material-specific problems, see common gun drilling problems and solutions. For a complete overview, visit the gun drilling guide.