Appearance
A chemical process equipment manufacturer needs to deep-drill 2,000 components per year from 316L stainless steel, with 15 mm diameter × 600 mm deep holes. Production using standard steel parameters results in severe work hardening of the drilled surface, stringy chip formation that clogs evacuation passages, and rapid notching wear at the drill margin. Tool life averages only 15 holes per gun drill. The process team develops 316L-specific parameters: carbide gun drills at 60–85 m/min, feed rate 0.03–0.06 mm/rev, coolant pressure 70–100 bar with high-EP oil, point angle 118–130°, and a specialised chipbreaker geometry to produce short, segmented chips. Tool life increases to 80–120 holes per regrind, work hardening is eliminated, and surface finish achieves Ra 1.6 µm.
Stainless Steel Properties Relevant to Deep Hole Drilling
| Property | 304 / 304L | 316 / 316L | Impact on Drilling |
|---|---|---|---|
| Hardness (HB) | 150–200 | 150–200 | Moderate but work-hardens to 350+ HB |
| Tensile strength (MPa) | 515–750 | 485–690 | High cutting forces required |
| Elongation (%) | 40–60 | 40–60 | High — produces stringy, tough chips |
| Thermal conductivity (W/m·K) | 16 | 16 | Low — 1/3 of carbon steel, heat concentrates |
| Work hardening rate | Very high | Very high | Surface hardens rapidly under cutting action |
| Machinability rating | 45% | 45% | Poor — stringy chips, high cutting forces |
Cutting Parameter Recommendations
| Parameter | 304 / 304L | 316 / 316L |
|---|---|---|
| Cutting speed — carbide gun drill (m/min) | 60–90 | 60–85 |
| Cutting speed — HSS gun drill (m/min) | 20–35 | 20–30 |
| Feed rate (mm/rev) for 6 mm diameter | 0.025–0.050 | 0.025–0.045 |
| Feed rate (mm/rev) for 10 mm diameter | 0.035–0.065 | 0.030–0.060 |
| Feed rate (mm/rev) for 15 mm diameter | 0.040–0.080 | 0.035–0.070 |
| Feed rate (mm/rev) for 20 mm diameter | 0.050–0.100 | 0.045–0.090 |
| Coolant pressure (bar) | 70–100 | 70–100 |
| Point angle (°) | 118–130 | 118–130 |
| Helix angle (°) | 25–30 | 25–30 |
| Expected tool life (holes per regrind) | 80–150 | 60–120 |
Feed Rate by Drill Diameter
| Drill Diameter (mm) | Feed Range (mm/rev) 304 | Feed Range (mm/rev) 316 | RPM at 75 m/min | Penetration (mm/min) |
|---|---|---|---|---|
| 4 | 0.015–0.030 | 0.012–0.025 | 5,970 | 90–180 |
| 6 | 0.025–0.050 | 0.025–0.045 | 3,980 | 100–200 |
| 8 | 0.030–0.060 | 0.028–0.055 | 2,985 | 90–180 |
| 10 | 0.035–0.065 | 0.030–0.060 | 2,390 | 70–155 |
| 12 | 0.040–0.075 | 0.035–0.065 | 1,990 | 70–150 |
| 15 | 0.040–0.080 | 0.035–0.070 | 1,590 | 55–125 |
| 20 | 0.050–0.100 | 0.045–0.090 | 1,195 | 55–120 |
| 25 | 0.055–0.110 | 0.050–0.100 | 955 | 50–105 |
Tool Geometry for Stainless Steel
| Geometry Parameter | Recommended Value | Rationale |
|---|---|---|
| Point angle | 118–130° | Standard range; 130° reduces work hardening tendency |
| Rake angle | 6–10° positive | Sharp edge reduces cutting pressure and work hardening |
| Relief / clearance angle | 8–12° | Adequate clearance prevents rubbing on springback surface |
| Chipbreaker geometry | Required | Essential for breaking stringy austenitic chips |
| Coating | AlTiN, TiAlN or AlCrN | High hot hardness, thermal barrier, oxidation resistance |
| Carbide grade | Micrograin (0.5–1.0 µm) | Toughness resists notching wear at margin |
| Edge preparation | Sharp with light T-land | Sharp edge cuts cleanly; T-land prevents micro-chipping |
| Guide bushing tolerance | G6 | Adequate guidance without binding |
DANGER
Work hardening is the most serious risk when deep hole drilling austenitic stainless steels 304 and 316. The material work-hardens at an extremely high rate when rubbed rather than cut — a dull tool, dwell at the hole bottom, or insufficient feed rate can instantly create a hard surface layer (up to 350–400 HB) that destroys the cutting edge on the next pass. Three rules to prevent work hardening: (1) always maintain feed above 0.025 mm/rev — never let the tool rub; (2) never dwell while the tool is in contact with the workpiece — reduce spindle speed before stopping feed at hole bottom; (3) replace tools at the first sign of flank wear (> 0.20 mm VB) — a worn tool rubs rather than cuts, accelerating work hardening exponentially. If the drill stops cutting and begins producing a burnished, shiny hole surface, work hardening has already occurred — stop and replace the tool immediately.
Chip Control in Stainless Steel
| Chip Type | Appearance | Cause | Risk Level | Corrective Action |
|---|---|---|---|---|
| Short helical / washer (ideal) | Tightly curled segments | Correct chipbreaker engagement, adequate feed | Low | Maintain parameters |
| Long stringy ribbon | Continuous chip > 100 mm | Feed too low, chipbreaker not engaging | Critical | Increase feed 15–25%, check chipbreaker geometry |
| Bird nest | Tangled mass at drill entry | Chip evacuation failure | Critical | Stop immediately, clear chips, increase coolant pressure |
| Segmental with torn edges | Rough fractured segments | Feed too high | Medium | Reduce feed 10–15% |
| Powder / dust | Fine particles | Tool rubbing / work hardening | Critical | Replace tool immediately |
| Blue / burned | Heat tint | Speed too high, coolant failure | Critical | Reduce speed, check coolant system |
Surface Finish Expectations
| Condition | Ra (µm) | Rz (µm) |
|---|---|---|
| Optimised carbide gun drill, new tool | 1.0–1.6 | 6–12 |
| Production drilling, mid-tool-life | 1.6–3.2 | 10–20 |
| Worn tool or work hardening present | 3.2–6.3 | 20–40 |
| BTA drilling, sharp head | 3.2–6.3 | 20–40 |
| With BUE or notching | > 6.3 | > 40 |
Troubleshooting
| Symptom | Likely Cause | Solution |
|---|---|---|
| Rapid notching wear at margin | Work hardening, speed too high, feed too low | Increase feed, reduce speed, check chipbreaker |
| Work hardening of drilled surface | Dwell, dull tool, insufficient feed | Eliminate dwell, replace tool, increase feed above 0.025 mm/rev |
| Stringy chip clogging | Feed too low, chipbreaker worn | Increase feed, inspect chipbreaker geometry |
| Short tool life (< 30 holes) | Speed too high, coolant pressure insufficient | Reduce speed 20%, increase coolant pressure to 100 bar |
| Poor surface finish (Ra > 3.2) | Tool wear, work hardening, BUE | Replace tool, verify feed is adequate, check coolant EP level |
| Hole oversize (+0.1 mm+) | Tool wear, guide pad wear, misalignment | Replace tool, inspect guide pads, check spindle alignment |
| Tool breakage | Chip packing, coolant interruption | Verify coolant flow before cycle, implement peck cycle |
| Coolant pressure drop | Filter clogging, pump wear, leak | Change filter, check pump, inspect coolant line seals |
| Chatter marks | Vibration, excessive tool overhang | Reduce overhang, increase feed, check guide bushing fit |
| Exit burr excessive | Dull tool, feed too high at exit | Reduce feed for last 3 mm, replace tool |
FAQ
What cutting speed is recommended for gun drilling 304 stainless steel?
For carbide gun drills in 304/304L, recommended cutting speed is 60–90 m/min (200–300 SFM). For 316/316L, reduce to 60–85 m/min due to slightly higher work hardening tendency. HSS gun drills should run at 20–35 m/min. The speed should be on the lower end for larger diameters (> 15 mm) and the higher end for smaller diameters (< 8 mm). Running above 100 m/min with carbide causes rapid thermal notching at the drill margin — the most common tool failure mode in stainless steel deep hole drilling.
What feed rate is needed to prevent work hardening in 316L?
Minimum feed rate to prevent work hardening in 316L deep hole drilling is 0.025 mm/rev. Below this threshold, the tool rubs rather than cuts, inducing severe plastic deformation that hardens the surface layer. For 10 mm diameter drilling, use 0.030–0.060 mm/rev. For 20 mm, use 0.045–0.090 mm/rev. If the drilled surface has a shiny, burnished appearance instead of a matte machined finish, the feed is too low and work hardening has occurred. The general guideline is feed per revolution = D/200 to D/350 where D is drill diameter in mm.
What coolant pressure is required for deep hole drilling stainless steel?
Minimum 70 bar coolant pressure is required for deep hole drilling 304/316 stainless steel. Production operations should target 80–100 bar. The high pressure is needed to: (1) evacuate tough, stringy austenitic chips that resist breakage; (2) provide adequate cooling to the cutting edge — stainless steel's low thermal conductivity concentrates heat; (3) lubricate guide pads to prevent galling. Coolant should be oil-based with extreme pressure (EP) additives. Filtration to 15 µm is essential.
What tool coating works best for stainless steel deep hole drilling?
AlTiN (aluminium titanium nitride) provides the best performance for carbide gun drills in austenitic stainless steel due to its high oxidation temperature (900°C+) and hot hardness. TiAlN and AlCrN are also effective. The coating acts as a thermal barrier, reducing heat transfer to the carbide substrate and protecting against diffusion wear. For HSS tools, TiN or TiCN coatings provide adequate performance but tool life is significantly shorter than carbide. Uncoated carbide is not recommended — it fails rapidly due to adhesion and notching wear.
How do you prevent stringy chips in 304/316 deep hole drilling?
Stringy chips in austenitic stainless steel are prevented by: (1) ensuring the gun drill or BTA head has an effective chipbreaker geometry on the cutting edge — without a chipbreaker, 304/316 produces continuous ribbons that clog evacuation passages; (2) maintaining adequate feed rate (above 0.025 mm/rev minimum) to engage the chipbreaker; (3) using a positive rake angle (6–10°) to shear material cleanly; (4) running coolant pressure above 70 bar for hydraulic chip transport. If chips are longer than 30 mm, increase feed by 15–20% or modify the chipbreaker width and depth.
What gun drill geometry is recommended for 316L?
For 316L stainless steel, use a gun drill with: point angle 118–130°, positive rake angle 6–10°, relief angle 8–12°, AlTiN coating, and integrated chipbreaker geometry. The carbide grade should be micrograin (0.5–1.0 µm grain size) for toughness. Tip displacement of 0.22–0.25 × D is standard. A sharp cutting edge with a light T-land (0.02–0.05 mm) provides the best balance of cutting sharpness and edge strength. Guide bushing tolerance should be G6.
Can BTA drilling be used for 304/316 stainless steel?
Yes, BTA drilling works well for 304/316 at diameters above 12 mm. Recommended parameters: cutting speed 55–80 m/min, feed rate 0.04–0.12 mm/rev, coolant pressure 60–100 bar. BTA offers advantages in chip evacuation — chips are transported internally through the drill tube, which is beneficial for the stringy chips produced by austenitic stainless steel. BTA indexable insert heads with chipbreaker geometries are widely available for stainless steel grades. Surface finish is typically Ra 3.2–6.3 µm.
How do you detect and prevent work hardening during deep hole drilling?
Work hardening is detected by: (1) visual — a shiny, polished appearance on the hole wall instead of a matte machined finish; (2) torque — increasing torque trend indicates the tool is rubbing against a hardened surface; (3) tool wear — rapid notching at the drill margin; (4) surface hardness testing — a hardened layer of 300–400 HB compared to 150–200 HB base material. Prevention requires: adequate feed rate (minimum 0.025 mm/rev), sharp tools replaced at VB ≥ 0.20 mm, no dwell at hole bottom, and consistent coolant flow. Once work hardening has occurred, the hardened layer must be removed with a carbide or CBN boring tool before continuing — attempting to re-cut a work-hardened surface with the same drill causes immediate tool failure.
What surface finish can be expected when gun drilling 304/316?
With an optimised carbide gun drill in good condition, surface finish of Ra 1.0–1.6 µm is achievable in 304/316 stainless steel. Production runs typically achieve Ra 1.6–3.2 µm through the tool life. This is slightly rougher than carbon steel gun drilling due to the material's higher ductility and work hardening characteristics. BTA drilling produces Ra 3.2–6.3 µm. For sealing surface applications, a secondary finishing operation (roller burnishing, honing) may be required to achieve Ra < 0.8 µm.
What is the most common mistake in deep hole drilling 304/316?
The most common mistake is using the same parameters as carbon steel drilling. Stainless steel 304/316 requires 40–50% lower cutting speed, 20–30% higher coolant pressure, and mandatory chipbreaker geometry compared to carbon steel. The second most common mistake is allowing the tool to dwell at the hole bottom while still rotating — this instantly work-hardens the surface and destroys the tool on retraction or the next pass. Always reduce spindle speed to 200–300 RPM before stopping feed at the bottom of a blind hole, or use a through-hole strategy to exit completely before stopping feed.
Summary
Deep hole drilling of austenitic stainless steels 304 and 316 requires addressing three primary challenges: work hardening, stringy chip formation, and heat concentration at the cutting edge. Work hardening is prevented by maintaining feed rate above 0.025 mm/rev, using sharp tools replaced at VB ≥ 0.20 mm, and eliminating dwell while the tool contacts the workpiece. Stringy chips are controlled with chipbreaker geometry on the cutting edge and adequate feed rate to engage the chipbreaker. Heat concentration is managed with cutting speed 60–90 m/min (carbide), coolant pressure 70–100 bar, and AlTiN-coated micrograin carbide tools. Surface finish of Ra 1.0–1.6 µm is achievable with optimised gun drilling parameters. The most critical process rule is: never let the tool rub — stainless steel work-hardens instantly under a dull or inadequately fed tool, causing a cascading failure of edge chipping, notching, and tool breakage. With correct parameters and process discipline, tool life of 80–150 holes per regrind is achievable in production stainless steel deep hole drilling operations.