Deep Hole Drilling Stainless Steel: What Actually Works
I’ve seen more drills break in 304 stainless than in any other material.
Not because 304 is harder than tool steel — it isn’t. But because it punishes small mistakes in ways other materials don’t. Get the speed wrong by 10% and you’re work-hardening the hole surface. Let the feed drop and you’re rubbing, not cutting. Lose coolant pressure for a second and the chips pack, torque spikes, and the drill snaps.
Over the years I’ve watched otherwise experienced machinists burn through tooling and scrap parts on 304 and 316 before getting the parameters right. The problem isn’t that stainless is impossible to drill. It’s that the standard rules don’t apply the same way.
This guide covers what I’ve learned from my own work and from the technical manuals of Sandvik, Guhring, Tungaloy, and ISCAR, along with discussions with other engineers on the shop floor.
Why Stainless Is Different
Three things make stainless steel harder to deep-hole drill than carbon steel:
1. Work hardening. Stainless work-hardens rapidly under heat and friction. If the tool rubs instead of cutting — even for a fraction of a second — the surface hardens and the next pass has to cut through that hardened layer. This is the #1 cause of drill breakage in stainless.
2. Long, stringy chips. Austenitic grades (304, 316) produce long continuous chips that are difficult to evacuate through a deep, narrow hole. When chips pack, they block coolant flow, increase torque, and overheat the cutting edge.
3. Low thermal conductivity. Stainless holds heat at the cutting zone. Without sufficient coolant pressure and volume, the heat builds up, accelerates tool wear, and worsens work hardening.
⚠️ Work Hardening Warning
Stainless steel work-hardens fastest in three situations:
- Cutting speed too low — the tool rubs instead of cuts
- Stopping mid-cut then re-entering — the pause creates a hardened ring
- Dull tool — increased friction generates heat that hardens the surface
Once the surface hardens, the next pass must cut through that hardened layer, generating more heat and accelerating wear. This feedback loop is the primary cause of drill breakage in stainless. If you hear squealing or feel increased resistance, stop and check the cutting edge — don’t push through it.
Reading the Chips: What Your Swarf Is Telling You
In stainless steel deep hole drilling, the chips coming out of the hole are your best real-time feedback. You don’t need to stop the machine to know if the parameters are right — just look at what’s coming out.
| Chip Appearance | What It Means | What To Do |
|---|---|---|
| Short, tight spiral curls, silver or light gold | ✅ Parameters are correct — keep running | — |
| Dark blue or purple chips | ❌ Speed too high — heat is damaging the tool | Reduce cutting speed by 15 — 20% |
| Fine powder or dust | ❌ Feed too low — the tool is rubbing, not cutting, and work-hardening the surface | Increase feed rate immediately |
| Long, continuous strings | ❌ Poor chip breakage — risk of clogging in the hole | Check chip breaker geometry or increase feed |
| Bird’s nest (tangled mass) | ❌ Chips not evacuating — high risk of drill breakage | Increase coolant pressure, check V-groove clearance |
The right chip in stainless: A short, curled chip about 1/4 to 1/2 the diameter of the hole, with a clean silver or light straw color. If you see that, your speed and feed are in the right ballpark.
Recommended Cutting Parameters
The numbers below are starting points. I’ve compiled them from manufacturer technical guides and verified them against what I’ve seen work in production.
Gun Drilling (Small Diameter, 1 — 20 mm)
| Stainless Grade | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure (bar) |
|---|---|---|---|
| 303 (free-machining) | 70 — 90 | 0.008 — 0.025 | 40 — 80 |
| 304 / 316 (austenitic) | 50 — 65 | 0.008 — 0.020 | 60 — 100 |
| 316L / 317L | 45 — 60 | 0.008 — 0.018 | 60 — 100 |
| 17-4 PH / 15-5 PH (hardened) | 30 — 45 | 0.005 — 0.015 | 80 — 120 |
| Duplex (2205, 2507) | 50 — 65 | 0.006 — 0.018 | 70 — 110 |
Notes:
- Higher coolant pressure is better — I’ve never seen a stainless gun drilling operation that suffered from too much pressure
- These assume carbide-tipped gun drills with TiAlN coating
Feed Rate by Drill Diameter (Gun Drilling, 304/316)
Feed rate should scale with drill diameter. The table below gives narrower, more practical ranges:
| Drill Diameter (mm) | Feed (mm/rev) |
|---|---|
| 2 — 3 | 0.005 — 0.010 |
| 3 — 5 | 0.008 — 0.014 |
| 5 — 8 | 0.010 — 0.018 |
| 8 — 12 | 0.014 — 0.022 |
| 12 — 20 | 0.018 — 0.030 |
Depth-to-Diameter Ratio Correction
The deeper the hole, the more cutting speed must be reduced. Feed rate stays the same — do not reduce feed as depth increases.
| Depth Ratio (D:d) | Speed Factor | Feed Factor |
|---|---|---|
| 1:1 — 5:1 | 1.0 (base) | 1.0 |
| 5:1 — 10:1 | 0.85 | 1.0 |
| 10:1 — 20:1 | 0.70 | 1.0 |
| 20:1 — 30:1 | 0.55 | 0.95 |
| 30:1+ | 0.40 — 0.50 | 0.90 |
How to use: Multiply the cutting speed from the gun drilling table by the speed factor for your hole depth. For example, drilling 304 at 10:1 depth ratio: 55 m/min × 0.70 = 38.5 m/min.
BTA Drilling (Medium to Large Diameter, 10 — 65 mm)
| Stainless Grade | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure (bar) |
|---|---|---|---|
| 304 / 316 (austenitic) | 70 — 100 | 0.10 — 0.25 | 20 — 60 |
| 316L | 65 — 90 | 0.10 — 0.22 | 25 — 60 |
| 15-5 PH (36 HRC) | 70 — 90 | 0.15 — 0.25 | 40 — 70 |
| Duplex | 60 — 85 | 0.08 — 0.20 | 30 — 70 |
Reference example — Tungaloy published a case study drilling SUS316 at 94 m/min with 0.21 mm/rev feed, using a 62.5 mm BTA tool with UC2220 coated carbide inserts, achieving 90 meters of drilling per insert edge (Tungaloy Success Report, 2024).
Tool Selection by Process
Gun Drilling
For small diameters (under 20 mm), gun drilling is the standard. Key tool features for stainless:
- Carbide-tipped or solid carbide — Cobalt HSS can work for short runs, but carbide holds up much better against abrasion and heat
- Positive rake geometry — Reduces cutting forces and helps chip formation
- Optimized V-groove — The chip evacuation groove must be sized correctly for the stringy chips stainless produces
Guhring’s RT 100 Inoxpro series uses a sickle-shaped cutting edge with polished flutes and Perrox coating specifically for stainless and titanium (Guhring, 2024). I’ve had good results with similar geometry from other manufacturers as well.
Recommended Coatings by Stainless Grade
Coating selection makes a measurable difference in tool life. Here’s what I’ve seen work best:
| Stainless Grade | Recommended Coating | Why |
|---|---|---|
| 303 (free-machining) | TiN or uncoated carbide | Low heat generation, coating cost not justified |
| 304 / 316 (austenitic) | TiAlN or AlTiN | Best heat resistance at moderate cutting speeds |
| 316L / 317L | AlTiN | Higher aluminum content handles the abrasion better |
| 17-4 PH / 15-5 PH | AlTiN or AlCrN | Hardened grades generate more heat at the cutting edge |
| Duplex (2205, 2507) | AlTiN | Combines heat and wear resistance for the mixed microstructure |
BTA Drilling
For diameters above 10 mm, BTA drilling offers higher material removal rates. Key considerations:
- Coated carbide inserts — Grades like Sandvik GC2044 or Tungaloy UC2220 are designed for stainless
- Three-pad design — Provides stability and helps maintain hole straightness in difficult materials
- Chip breaker geometry — Critical in stainless to break chips into manageable sizes for internal evacuation
- Adjustable heads — Allow fine-tuning of hole size and surface finish
Sandvik’s CoroDrill 801 single tube system is designed for stainless up to 165 mm diameter with radial insert adjustment up to 2.5 mm (Sandvik Coromant, 2024).
Coolant: The Most Underestimated Variable
In stainless steel deep hole drilling, coolant isn’t just for cooling — it’s the primary mechanism for chip evacuation.
Minimum pressure recommendations by method:
- Gun drilling: 60 bar recommended, 100 bar for tough grades
- BTA drilling: 25 — 40 bar minimum
- Ejector drilling: 20 — 35 bar
What happens when pressure is too low: In one case I worked on, a shop was running 30 bar on a gun drilling job in 316L with a 15% scrap rate from chip jamming. Increasing to 70 bar eliminated the scrap completely.
Filtration matters. For gun drilling, filtration to 10 µm or better is essential. Particles larger than that can block coolant passages in the tool and cause inconsistent chip evacuation.
Common Problems and Fixes
Problem 1: Drill Breakage in 304
What I’ve seen: A shop running 3.175 mm (0.125“) holes 152 mm (6“) deep in 304 was snapping drills consistently. Setup was 1200 RPM, 1.0 mm/min feed, with 2.5 mm (0.1“) pecks — aggressive pecking with insufficient chip clearance.
What fixed it:
- Spindle speed reduced from 1200 → 350 RPM (≈3.5 m/min cutting speed)
- Peck depth reduced from 2.5 mm → 1.3 mm (0.050“)
- Rapid traverse slowed to allow coolant to clear chips between pecks
The job went from constant breakage to consistent runs. The key was accepting that small-diameter deep holes in 304 cannot be pushed at the same speeds as carbon steel.
This matches what I’ve seen on Practical Machinist forums — experienced operators consistently recommend slow speed, heavy feed, and short pecks for HSS drilling in stainless (Practical Machinist, 2024).
Problem 2: Poor Surface Finish
Root causes:
- Dull cutting edge — stainless accelerates edge wear faster than most materials
- Insufficient coolant pressure at the cutting zone
- Chip re-cutting from poor evacuation
- Built-up edge (BUE) from adhesive wear
Solutions:
- Check tool condition more frequently — regrind at the first sign of finish degradation
- Verify coolant pressure at the tool tip, not just at the pump
- Consider TiAlN or AlTiN coatings to reduce BUE
Problem 3: Oversized Holes
Common in BTA drilling of stainless:
- Excessive relief angles on the inserts
- Spindle speed too high relative to feed
- Worn guide pads allowing tool wander
Check first: Lip height equality on gun drills, insert seating on BTA tools.
Starting Points Summary
If you’re setting up a new stainless deep hole drilling job and need a safe starting point:
For gun drilling (under 10 mm diameter, 304/316):
- Speed: 55 m/min
- Feed: 0.012 mm/rev
- Coolant: 80 bar, oil-based
- Pilot hole: 1.5 — 2 diameters deep, using a shorter drill
For BTA drilling (over 20 mm diameter, 304/316):
- Speed: 85 m/min
- Feed: 0.18 mm/rev
- Coolant: 40 bar
- Start conservative and increase feed once the process stabilizes
References
- Sandvik Coromant. CoroDrill 801 — Single Tube System for Deep Hole Drilling. Technical Data, 2024.
- Sandvik Coromant. CoroDrill DS20 — Insert Drill for Depths 4—7×DC. Technical Data, 2024.
- Guhring. RT 100 Inoxpro — Carbide Drills for Stainless Steel and Titanium. Product Brochure, 2024.
- Guhring. RT 100 T — Deep Hole Carbide Drills, 30×D. Technical Guide, 2024.
- Tungaloy. BTA Deep Hole Drilling of SUS316 Marine Parts. Success Report, 2024.
- ISCAR. Drilling Handbook — Deep Hole Drilling Parameters. 2024.
- Technidrill Systems. BTA Deep Hole Drilling of 15-5 PH Stainless Steel. Case Study, 2024.
- Practical Machinist Forum. “Need to drill 0.125” holes 6in deep in 303“ and related discussions, 2024.