Appearance
Stainless steel work-hardens at the point of cut faster than any other common engineering material. A 0.5-second dwell in a deep hole creates a localised hard spot that destroys the cutting edge on re-entry. In deep hole drilling, where the tool is buried 50–100× its diameter inside the workpiece, there is no room for recovery.
Stainless Steel Types and Their Machinability
Metallurgical Classification
| Type | Microstructure | Typical Grades | Machinability | Work-Hardening Rate | Chip Form |
|---|---|---|---|---|---|
| Austenitic | Face-centred cubic (FCC) | 304, 316, 321, 347 | Low (poor chip breakage) | Very high | Long, stringy |
| Martensitic | Body-centred tetragonal (BCT) | 410, 420, 431, 440C | Moderate | Moderate | Short to medium |
| Ferritic | Body-centred cubic (BCC) | 430, 409, 444 | Good | Low | Broken |
| Duplex | Mixed austenite + ferrite | 2205, 2507, 2304 | Low-moderate | High | Medium, tough |
| Precipitation-hardening (PH) | Martensitic/ semi-austenitic | 17-4 PH, 15-5 PH, 13-8 Mo | Moderate | Moderate | Medium |
Key Machinability Factors
| Factor | Carbon Steel (1045) | Austenitic SS (316) | Duplex SS (2205) | PH SS (17-4) |
|---|---|---|---|---|
| Thermal conductivity (W/m·K) | 50 | 15 | 14 | 18 |
| Work-hardening exponent (n) | 0.15–0.20 | 0.40–0.55 | 0.35–0.50 | 0.25–0.35 |
| Relative machinability (100% = 1212) | 55–65% | 30–40% | 20–30% | 35–45% |
| Chip type | Broken to medium | Long, stringy | Medium, tough | Short to medium |
Warning: Austenitic stainless steels (304, 316) and duplex grades (2205, 2507) are the most challenging for deep hole drilling. Their high work-hardening rates and low thermal conductivity concentrate heat at the cutting edge, accelerating flank wear and promoting built-up edge. Never attempt deep hole drilling in these grades without through-coolant tooling and continuous feed.
Gun Drilling Parameters
Cutting Speed and Feed
| Grade | Condition | Cutting Speed (m/min) | Cutting Speed (SFM) | Feed (mm/rev) | Feed (in/rev) |
|---|---|---|---|---|---|
| 304 / 316 (austenitic) | Annealed | 18–36 | 60–120 | 0.013–0.050 | 0.0005–0.002 |
| 321 / 347 (austenitic) | Annealed | 18–30 | 60–100 | 0.013–0.040 | 0.0005–0.0016 |
| 410 / 420 (martensitic) | Annealed | 24–42 | 80–140 | 0.013–0.050 | 0.0005–0.002 |
| 430 (ferritic) | Annealed | 30–50 | 100–165 | 0.020–0.060 | 0.0008–0.0024 |
| 2205 (duplex) | Solution treated | 15–25 | 50–80 | 0.010–0.035 | 0.0004–0.0014 |
| 2507 (super duplex) | Solution treated | 12–20 | 40–65 | 0.008–0.030 | 0.0003–0.0012 |
| 17-4 PH / 15-5 PH | H900–H1150 | 20–40 | 65–130 | 0.013–0.040 | 0.0005–0.0016 |
Coolant Pressure Requirements by Hole Diameter
| Drill Diameter (mm) | Minimum Coolant Pressure (bar) | Minimum Coolant Pressure (psi) |
|---|---|---|
| 1–3 | 100–150 | 1,500–2,200 |
| 3–6 | 80–120 | 1,200–1,750 |
| 6–10 | 60–100 | 870–1,450 |
| 10–20 | 50–80 | 725–1,160 |
| 20–40 | 40–70 | 580–1,015 |
Tip: The required coolant pressure for stainless steel gun drilling is approximately 1.5–2× that needed for carbon steel at the same diameter. If your coolant system delivers only enough pressure for carbon steel, reduce cutting speed by 30% and expect reduced tool life when switching to stainless.
BTA Drilling Parameters
Recommended Starting Parameters
| Grade | Cutting Speed (m/min) | Cutting Speed (SFM) | Feed (mm/rev) | Feed (in/rev) |
|---|---|---|---|---|
| 304 / 316 (austenitic) | 60–90 | 200–295 | 0.05–0.15 | 0.002–0.006 |
| 410 / 420 (martensitic) | 75–105 | 245–345 | 0.08–0.20 | 0.003–0.008 |
| 430 (ferritic) | 80–110 | 260–360 | 0.10–0.22 | 0.004–0.009 |
| 2205 / 2507 (duplex) | 45–75 | 145–245 | 0.05–0.13 | 0.002–0.005 |
| 17-4 PH | 60–100 | 195–330 | 0.08–0.18 | 0.003–0.007 |
Feed by Drill Diameter
| Drill Diameter (mm) | Feed Range — Austenitic (mm/rev) | Feed Range — Duplex (mm/rev) |
|---|---|---|
| 20–40 | 0.05–0.10 | 0.05–0.08 |
| 40–80 | 0.08–0.13 | 0.06–0.10 |
| 80–120 | 0.10–0.15 | 0.08–0.13 |
| 120–200 | 0.12–0.18 | 0.10–0.15 |
Insert Grade Selection
ISCAR FINEBEAM Recommendations
| Material Grade | First Choice | For Fracture Resistance | For Wear Resistance |
|---|---|---|---|
| Austenitic (304, 316) | IC908 | IC806 | IC9025 |
| Martensitic (410, 420) | IC908 | IC806 | IC9025 |
| Duplex (2205, 2507) | IC806 | IC806 | IC908 |
| PH (17-4, 15-5) | IC908 | IC806 | IC9025 |
Grade characteristics:
| Grade | Coating | Substrate | Best For |
|---|---|---|---|
| IC908 | CVD Al₂O₃ + TiCN | Medium-hard | First choice for stainless — balanced wear and toughness |
| IC806 | CVD multilayer | Tough | High feed, interrupted cuts, unstable conditions in stainless |
| IC9025 | CVD Al₂O₃ | Hard | High-speed finishing, abrasive wear conditions |
Recommended Insert Geometry
| Feature | Recommendation | Why |
|---|---|---|
| Chipbreaker | Sharp, positive rake | Reduces cutting forces, promotes chip breakage |
| Edge preparation | Small edge hone (0.02–0.05 mm) | Prevents micro-chipping without excessive force |
| Rake angle | Positive (8–15°) | Minimises heat generation and BUE formation |
| Clearance angle | 7–10° | Adequate to avoid rubbing on work-hardened surface |
Coolant Requirements
Coolant Type Selection
| Coolant Type | Suitability | Advantages | Limitations |
|---|---|---|---|
| Sulfurized cutting oil | Excellent | Best EP lubrication for stainless | Restricted in some plants; odour |
| EP additive oil (P-based) | Good | Lower odour than sulfurized | Less effective at extreme pressure |
| High-performance soluble oil (8–12%) | Moderate for BTA, poor for gun drilling | Lower cost, water-based | Limited lubricity for gun drilling |
| Straight oil with Cl-free EP | Good | Suitable for duplex grades | Higher cost |
Pressure and Flow
| Process | Pressure (bar) | Pressure (psi) | Flow Rate |
|---|---|---|---|
| Gun drilling (small Ø < 6 mm) | 100–150 | 1,500–2,200 | 10–40 L/min |
| Gun drilling (medium Ø 6–20 mm) | 50–100 | 725–1,450 | 40–150 L/min |
| BTA drilling (austenitic) | 30–60 | 435–870 | 200–600 L/min |
| BTA drilling (duplex) | 35–70 | 510–1,015 | 200–600 L/min |
Filtration
| Requirement | Recommendation |
|---|---|
| Maximum particle size | ≤ 25 μm |
| Target particle size | ≤ 10 μm for gun drilling |
| Filter type | Paper band or cartridge filter |
| Magnetic separator | Recommended for martensitic grades |
| Coolant temperature | ≤ 50°C (monitor with thermocouple) |
Challenges and Mitigation
Work-Hardening
Problem: Stainless steel work-hardens at the shear zone to depths of 0.05–0.10 mm. The hardened layer (up to 500–600 HV vs. 180–200 HV bulk) makes subsequent cuts extremely difficult.
| Cause | Effect | Mitigation |
|---|---|---|
| Dwell or feed interruption | Hard ring at interruption point | Never stop feed; retract tool if interruption unavoidable |
| Light depth of cut | Tool rubs instead of cuts | Maintain minimum chip load of 0.05 mm/rev |
| Worn insert | Increased cutting forces compress and harden material | Replace insert at first sign of wear |
| Incorrect feed-speed balance | Excessive heat builds up at cutting zone | Maintain ratio: higher feed, moderate speed |
Danger: If feed must be interrupted during BTA drilling of austenitic stainless: retract the tool completely, clear chips, and re-enter with a new insert. Re-entering a partially worn insert into a work-hardened ring will almost certainly break the insert or the drill tube.
Heat Management
| Cause | Effect | Mitigation |
|---|---|---|
| Low thermal conductivity (15 W/m·K) | 80% of cutting heat goes into the tool | High-pressure coolant directed at cutting edge |
| High friction at chip-tool interface | Crater wear, BUE | EP lubricants, coated carbide, positive rake |
| Chip packing restricting coolant | Localised overheating | Maintain chip evacuation, monitor coolant return flow |
| Excessive speed | Thermal cracking of insert | Reduce speed 20–30% if cracking observed |
Chip Control
| Chip Type | Cause | Correction |
|---|---|---|
| Long, stringy chips | Feed too low, chipbreaker too mild | Increase feed 10–15%, use aggressive chipbreaker |
| Ribbon chips packing drill tube | Chipbreaker ineffective | Switch to IC908 with HF chipbreaker geometry |
| Short, broken chips (ideal) | Correct parameters | Maintain current settings |
| Powder/chip dust | Feed too high or speed too low | Reduce feed or increase speed |
| Built-up edge on chipbreaker | Adhesion at low speed | Increase speed 10–15%, check coolant EP concentration |
Production Example
TechniDrill Systems — BTA drilling of 15-5 PH stainless (36 HRC):
| Parameter | Value |
|---|---|
| Hole diameter | 1.600 inches (40.6 mm) |
| Material | 15-5 PH stainless, 36 HRC |
| Penetration rate | 7.8 inches per minute (198 mm/min) |
| Coolant flow | 80 GPM (303 L/min) |
| Coolant pressure | 1,000 psi (69 bar) |
| Rotation mode | Counter-rotation |
| Straightness | 0.0005 inches per inch of depth |
| Surface finish | Ra 1.6–3.2 μm |
This demonstrates that BTA drilling of stainless steel, with proper parameters and counter-rotation, can achieve productivity approaching that of carbon steel — provided the coolant system, tooling, and machine alignment are adequate.
Troubleshooting
| Symptom | Likely Cause | Solution |
|---|---|---|
| Rapid flank wear | Speed too high, coolant insufficient | Reduce speed 15–20%, increase coolant pressure |
| Built-up edge | Speed too low, EP additives depleted | Increase speed 10%, check oil EP concentration |
| Oversize bore | Worn peripheral insert or guide pad | Replace insert, check pad wear |
| Poor surface finish | Worn guide pads, chip re-cutting | Replace pads, increase coolant flow |
| Tool breakage | Chip clogging, feed interruption | Check chipbreaker, ensure continuous feed |
| Chatter marks | Vibration at resonance, insufficient support | Adjust RPM, add steady rest, check guide bushing |
| Hole spiralling | Feed too low for material hardness | Increase feed 20%, check machine alignment |
| Coolant by-pass (BTA) | Worn guide bushing or drill tube seal | Replace bushing, inspect tube surface |
Application Guide
| Application | Recommended Process | Key Considerations |
|---|---|---|
| Cooling channels in 316L molds | Gun drilling Ø3–10 mm | High pressure coolant, PVD-coated carbide |
| Hydraulic cylinder barrels (304/316) | BTA drilling Ø40–120 mm | Counter-rotation for straightness, EP oil |
| Valve stems (17-4 PH) | Gun drilling Ø6–20 mm | Positive rake, sharp edge prep |
| Heat exchanger tube sheets (duplex) | BTA drilling Ø20–50 mm | IC806 grade for toughness, controlled feed |
| Food processing shafts (316L) | Gun drilling Ø10–30 mm | Chlorine-free coolant, Ra ≤ 0.8 μm finish |
| Offshore components (super duplex) | BTA drilling Ø50–200 mm | Reduced speed (12–20 m/min), IC806 inserts |
| Medical implants (316LVM) | Gun drilling Ø2–8 mm | 150+ bar coolant, micro-peck strategy |
FAQ
Why is stainless steel harder to deep-hole drill than carbon steel?
Three factors combine: work-hardening (austenitic grades form a 500+ HV layer at the cut), low thermal conductivity (15 W/m·K traps heat at the cutting edge), and stringy chip formation (long chips clog flutes and drill tubes). Any one of these would demand adjusted parameters; all three together make stainless one of the most challenging materials for deep hole drilling.
What is the most critical rule for deep hole drilling stainless steel?
Never let the tool dwell. A feed interruption of even 0.5 seconds creates a work-hardened ring that can destroy the cutting edge on re-entry. If the tool must stop, retract it completely, replace the insert, and re-enter with a fresh cut.
What cutting speed should be used for gun drilling 304 stainless steel?
18–36 m/min (60–120 SFM), depending on hole diameter and coolant pressure. Smaller diameters require lower speeds. Coolant pressure must be at least 800 psi (55 bar) at the cutting zone.
What is the best carbide grade for BTA drilling of duplex stainless steel?
ISCAR IC806 is the first choice for duplex (2205, 2507) because of its toughness — duplex's high strength and work-hardening tendency demand fracture-resistant tooling. IC908 is preferred for austenitic grades where wear resistance is more important.
What coolant type is recommended for stainless steel deep hole drilling?
Sulfurized cutting oil provides the best extreme-pressure lubrication for stainless. If sulfurized oil is restricted (environmental or health regulations), use phosphorus-based EP additive oils. For gun drilling, soluble oils are generally inadequate — straight oil is required.
What coolant pressure is needed for BTA drilling of stainless?
30–70 bar (435–1,015 psi) depending on grade and diameter. Duplex grades require higher pressure (up to 70 bar) than austenitic (30–60 bar). Pressure must be maintained at the cutting zone — monitor the BOZA gauge, not the pump outlet.
How can chip packing be prevented in BTA drilling of stainless?
Use a chipbreaker geometry designed for long-chipping materials (ISCAR HF chipbreaker), maintain minimum feed of 0.05 mm/rev, ensure coolant flow meets Q = 4.5 × D (L/min), and monitor chip form at the return port. If chips fill more than 1/3 of the drill tube cross-section, stop and clear.
Can counter-rotation improve results for stainless steel BTA drilling?
Yes. Counter-rotation cancels drift forces and improves straightness. TechniDrill Systems achieved 7.8 IPM in 15-5 PH stainless (36 HRC) with 0.0005-inch per inch straightness using counter-rotation with BTA.
What is the expected tool life when gun drilling stainless?
Tool life in stainless is typically 30–50% of that in carbon steel. In austenitic 316, expect 5–15 metres of drilled length per regrind for gun drills (vs. 15–40 m in 1045 steel). BTA insert life: 100–500 holes depending on diameter, depth, and grade.
Is MQL feasible for deep hole drilling of stainless?
No. The EP lubrication and cooling requirements of stainless deep hole drilling exceed what MQL can deliver. Through-tool high-pressure coolant delivery is mandatory for any hole exceeding 10× diameter in stainless.
Conclusion
Successful deep hole drilling of stainless steel requires recognition that it is not a single material but a family with widely varying behaviours. Austenitic grades (304, 316) demand the most conservative parameters — low speeds (18–36 m/min gun drilling, 60–90 m/min BTA), continuous feed without interruption, aggressive chipbreaker geometry, and high-pressure coolant (50–150 bar). Duplex grades (2205, 2507) require tougher insert grades (IC806) and reduced speed. Martensitic and PH grades (17-4, 15-5) are more forgiving but still require positive rake and adequate coolant. The single unifying rule across all stainless types: the tool must always be cutting — never dwelling, never rubbing, never re-entering a work-hardened surface without a fresh cutting edge.