Stainless Steel Deep Hole Drilling: What I've Learned About 304, 316, and 17-4

Why Stainless Steel Is Different

Stainless steel is the material that causes the most tool breakage in my experience — not because it’s the hardest, but because it work-hardens and grabs the tool if you hesitate. I’ve watched operators who can drill aluminum all day snap a carbide gun drill inside the first inch of 304. It’s not a material you muscle through.

Over the years I’ve drilled thousands of holes in three stainless grades: 304, 316, and 17-4 PH. They look similar on paper but behave completely differently under the cutting edge. This article is what I wish someone had told me before I broke my first $400 tool.

The Work Hardening Mechanism

Here’s what happens at the microscopic level. Stainless steel has a high nickel and chromium content. When the cutting edge pushes rather than shears, the material’s austenitic structure deforms and transforms into a hard, abrasion-resistant martensitic layer right beneath the cut surface. That layer can reach 45–50 HRC in 304 and 316, and substantially higher in 17-4 PH if it’s been aged.

The problem is recursive. Once that hardened layer forms, the next revolution of the tool has to cut through it, which generates more heat and more deformation, which hardens it further. The tool starts rubbing instead of cutting. Rubbing creates heat. Heat expands the chip and jams the flute. Then you get a twist-off.

The only way to stop this loop is to maintain a constant chip load. Every engagement of the cutting edge needs to remove material above the work-hardened depth. If you dwell — even for a fraction of a second — you lose.

Chip Breaking in Stainless: The Real Bottleneck

Chip breaking is the hardest part of deep hole drilling in stainless. In mild steel, chips curl and break naturally. In stainless, the chip is both tough and gummy. It wants to form a long, continuous ribbon that wraps around the tool shank.

I run chip breakers on every stainless job. For 304 and 316, a modified parabolic flute profile with a notch-style breaker at 0.012–0.018 inches per revolution gives me consistent segmentation. For 17-4 PH, especially in the H900 condition, I back off to 0.008–0.012 IPR. The material is harder, so the chip naturally fractures earlier — pushing harder just wears the tool faster.

Coolant pressure is critical here. You need enough velocity to flush each segment past the drill head before the next one forms. If your coolant pump can’t deliver continuous pressure at the drill tip, chips pack, and the tool seizes.

304 vs 316 vs 17-4 PH: The Numbers

Here are the parameters I use as a baseline. Every setup is different, but these numbers have held up across multiple machines and part geometries.

A few notes on these numbers. 316 runs about 15% slower than 304 because of its higher molybdenum content, which increases gumminess. 17-4 in the H900 condition is harder than both, so the tool life drops, but the surface finish is consistently better — the material doesn’t smear the way 316 does.

I also raise coolant pressure by about 200 psi for any hole deeper than 30 diameters (30xD). Chip evacuation distance matters more than the grade at that point.

Tool Coatings: AlCrN vs TiAlN

I’ve tested both coatings extensively in stainless. Here’s what I’ve found.

AlCrN (aluminum chromium nitride) handles heat better in interrupted cuts and at higher speeds. It has a higher oxidation temperature — around 1,650°F compared to TiAlN’s 1,470°F. For 304, where I can push speed higher, AlCrN consistently gives me 20–30% more tool life.

TiAlN (titanium aluminum nitride) performs better in 316 and 17-4. The coating is tougher and adheres better to the carbide substrate. In 316, where chip flow is the dominant wear mechanism, TiAlN resists the micro-abrasion from the gummy chip better. In 17-4 PH, TiAlN’s lower coefficient of friction reduces edge buildup.

That said, I don’t use uncoated carbide in stainless at all. I tried it once on a small 316 run and got six inches of tool life. The coating is not optional for stainless.

For more on tool selection across difficult materials, see my article on Deep Hole Drilling in Difficult Materials.

Coolant Pressure: The Three Things That Matter

Coolant pressure does three separate jobs in a stainless deep hole drilling operation.

First, it lubricates the cutting zone. The high pressure forces coolant past the cutting edge, reducing friction and heat at the point of shear. Without enough pressure, the edge temperature spikes and the work hardening cycle starts.

Second, it breaks and flushes chips. Every chip segment needs to travel the full length of the flute back to the exit. In stainless, the chip is heavy and sticky. Low pressure lets it settle against the wall of the hole, which creates a packed flute in under a second.

Third, it stabilizes the drill. In deep holes, the coolant film between the drill head and the hole wall acts as a hydrodynamic bearing. Drop below 1,000 psi in a 40xD 316 hole and you’ll see chatter marks on the wall. The tool vibrates, the edge chips, and you scrap the part.

I use a minimum of 1,200 psi for any stainless job, and I verify pressure at the drill tip — not at the pump. A 50-foot hose with two swivel fittings can drop 300 psi before it reaches the tool.

Surface Finish: What You Can Expect

Stainless can produce an excellent surface finish if the process is stable. Here’s what I get in production.

304 with sharp tools and good coolant pressure consistently finishes at 32 Ra. I’ve hit 16 Ra with a wiper flat on the drill head and a reduced feed of 0.008 IPR at the final 0.050 inches of depth. The trade-off is risk — that reduced feed at the exit can trigger work hardening if the tool dwells.

316 finishes rougher by default, typically 63 Ra. The gummy chip tears rather than shears cleanly at the margin. I improve this by increasing coolant pressure rather than reducing feed. At 1,800 psi, the hydraulic force keeps the chip from smearing against the finished wall.

17-4 PH finishes the best of the three. In the H900 condition, with TiAlN-coated tools at 140 SFM and 0.010 IPR, I regularly get 16–20 Ra. The material’s higher hardness means the cutting edge doesn’t push the material aside — it shears cleanly.

Troubleshooting Common Problems

Built-up edge is the most common issue I see in 316. The gummy chip welds to the cutting edge, changing the effective geometry of the drill. The fix is usually a combination of higher coolant pressure, a sharper edge hone, and TiAlN coating. I have a dedicated guide on this: Built-Up Edge in Gun Drilling.

Chatter in 304 is almost always a feed problem. Operators slow down the feed to reduce load, which drops the chip below the minimum thickness and creates rubbing. The fix is counterintuitive: increase feed. A thicker chip absorbs more energy and dampens vibration.

Tool breakage at the exit is the classic stainless problem. The drill breaks through the far side, the load drops instantly, and the operator hesitates on the feed override. That 0.1-second dwell work-hardens the remaining material — when the feed resumes, the tool sees a hard spot and snaps. I now program a controlled breakthrough dwell of 0.5 seconds before feed-out, with no manual override allowed.

Flute packing shows up as erratic coolant return. If your return coolant temperature is climbing but the pressure gauge is steady, you have chips stuck in the flute. Stop the spindle immediately. Do not reverse the spindle — that can wedge the chip deeper. Withdraw the tool manually and clear the flute.

Key Takeaways

• Stainless steel work-hardens instantly when the cutting edge dwells. Never reduce feed below the minimum chip thickness.
• 304 tolerates higher speeds but requires rigid setup and sharp tools. AlCrN coating is my first choice.
• 316 is the toughest to drill because of chip gumminess. TiAlN coating and high coolant pressure are non-negotiable.
• 17-4 PH gives the best surface finish but the shortest tool life, especially in the H900 condition.
• Coolant pressure below 1,200 psi at the tool tip invites built-up edge, chatter, and tool seizure. Verify pressure at the drill head, not the pump.
• Coatings are not optional for stainless. Uncoated carbide will fail inside a few inches.
• Expect 32–63 Ra for 304 and 316, and 16–32 Ra for 17-4 PH, with appropriate parameters.
• Always program a controlled exit strategy. The hole exit is where most tools break in stainless.