Stainless steel is one of the most demanding materials to drill. Unlike mild steel or aluminium, it punishes poor technique and wrong tooling quickly and visibly — through broken bits, oversized holes, and rough surface finishes. This guide explains why stainless steel behaves the way it does, and what that means for choosing the right drill bit, setting the right parameters, and keeping quality consistent across a production run.
Why stainless steel is difficult to drill
Understanding the problem starts with the material itself. Stainless steel — particularly austenitic grades like 304 and 316 — has two properties that make drilling significantly harder than most other metals.
Work hardening
When stainless steel is subjected to mechanical stress — including the cutting forces from a drill — the material at the cutting zone hardens rapidly. This is called work hardening, and it happens faster in austenitic stainless grades than in almost any other common engineering material.
In practice this means: if a drill bit pauses, slows, or rubs instead of cutting cleanly, the surface below the bit hardens within seconds. The next cutting pass then encounters a harder layer than the one before it. This accelerates tool wear dramatically and, if left uncorrected, leads to work hardening deep into the hole — making the job progressively more difficult as it progresses.
Low thermal conductivity
Stainless steel conducts heat poorly compared to mild steel or aluminium. During drilling, cutting generates significant heat. In a material with good thermal conductivity, much of that heat is carried away by the workpiece and the chips. In stainless steel, heat concentrates at the cutting edge instead.
The combination of work hardening and heat concentration creates a compounding problem: as the drill bit heats up, its hardness at the cutting edge begins to drop. A softer edge cuts less efficiently, generates more friction, creates more heat — and the cycle continues until the bit fails.
Choosing the right drill bit: M2 vs M35
The single most important variable in drilling stainless steel is the drill bit material. The two most commonly used options in industrial settings are M2 and M35 high-speed steel. Understanding the difference between them matters far more than most buyers realise.
M2 high-speed steel
M2 is the standard grade of high-speed steel and the most widely produced drill bit material in the world. Its composition — approximately 6% tungsten, 5% molybdenum, 4% chromium, and 2% vanadium — gives it good hardness, reasonable toughness, and adequate performance across a wide range of common materials.
The limitation of M2 for stainless steel work is its hot hardness, also called red hardness. This refers to a material's ability to retain its hardness at elevated temperatures. M2 begins to lose hardness meaningfully at around 550–600°C. In stainless steel drilling — particularly in production environments or when cooling is insufficient — cutting edge temperatures regularly exceed this range.
When the edge softens, it stops cutting and starts rubbing. Friction increases, heat increases further, the work-hardened layer becomes harder to penetrate, and tool life drops sharply. The failure is not always sudden — it typically manifests as progressively heavier feed feel, increased burring around the hole exit, and deteriorating hole diameter consistency before the bit finally fails.
M35 cobalt high-speed steel
M35 uses the same base composition as M2 with the addition of approximately 5% cobalt. Cobalt does not directly participate in cutting — it does not form carbides or add hardness at room temperature in any meaningful way. Its role is specifically to stabilise the steel matrix at high temperatures, raising the point at which hardness begins to degrade.
M35 retains useful hardness up to approximately 630–650°C. The difference of 50–80°C compared to M2 may seem modest, but tool life is highly sensitive to temperature in this range. In controlled comparative tests on austenitic stainless steel, M35 typically produces two to three times as many holes per bit as M2 under equivalent conditions — and often significantly more when the work involves interrupted cuts, variable feed rates, or imperfect cooling.
For buyers, the economic comparison is straightforward: M35 bits cost more per unit, but the cost per hole drilled — accounting for replacement frequency, machine downtime, and reject rates from poor hole quality — is generally lower in any production context involving stainless steel.
When M2 is still acceptable
M2 is not categorically unsuitable for stainless steel. Under the following conditions, it can perform adequately:
• Low cutting speeds with generous flood coolant
• Intermittent, low-volume drilling where the bit has time to cool between holes
• Thinner material (≤3mm) where the drill is in contact with the workpiece for a short time
• When cost constraints make M35 impractical and hole quality tolerance is wide
In high-volume production, on thicker material, or where dimensional accuracy is critical, M35 is the more reliable choice.
Cutting speed and feed rate
Even with the correct bit material, wrong cutting parameters will cause premature failure. The two variables to control are cutting speed (expressed as surface speed in m/min, or converted to RPM for a given diameter) and feed rate (how fast the drill advances per revolution).
Cutting speed
For stainless steel, recommended surface speeds for HSS drill bits are significantly lower than for mild steel. General guidance for austenitic grades (304, 316):
• M2 HSS: 10–15 m/min surface speed
• M35 cobalt HSS: 15–25 m/min surface speed
These convert to RPM as: RPM = (surface speed × 1000) ÷ (π × drill diameter in mm). A 10mm M35 bit in 316 stainless steel, targeting 20 m/min, would run at approximately 637 RPM.
Running too fast is the most common mistake when drilling stainless steel. High speed generates heat faster than coolant can remove it and faster than the chip can carry it away. The bit overheats, loses edge hardness, and fails early. Operators who are accustomed to drilling mild steel or aluminium often run stainless at speeds that are two to three times too high.
Feed rate
Feed rate must be high enough that the bit is cutting continuously rather than rubbing. A feed that is too light allows the bit to skate across the work-hardened surface without penetrating — generating heat without removing material. This is the primary cause of premature work hardening in the hole.
Recommended feed rates for stainless steel (per revolution): 0.05–0.10 mm for bits up to 5mm diameter; 0.10–0.20 mm for 5–15mm; 0.20–0.35 mm above 15mm. These are starting points — adjust based on observed chip formation, cutting sound, and temperature at the drill tip.
The most reliable indicator that parameters are correct is the chip. A thin, tightly curled chip indicates correct cutting. Long, stringy chips suggest feed is too light. Powdery or discoloured chips indicate excessive heat.
Coolant and lubrication
Coolant is not optional when drilling stainless steel. Its role is twofold: to remove heat from the cutting zone, and to lubricate the interface between the bit flutes and the hole wall to reduce friction-generated heat.
Flood coolant (soluble oil mixed at approximately 8–10%) is the most effective method in machine-tool settings. For hand drilling or drill press operations without flood coolant, a cutting fluid applied directly to the bit and the entry point before and during drilling is essential. Dry drilling of stainless steel should be avoided in any context where hole quality or tool life matters.
The coolant supply must reach the cutting zone. External flood coolant on thick workpieces (above 15–20mm) may not adequately cool the bit tip — in these cases, through-coolant tooling or periodic bit retraction to clear chips and allow cooling is necessary.
Geometry and point angle
Standard 118° point angle drill bits are designed for general-purpose use. For stainless steel, a 135° split-point geometry is significantly more effective for two reasons.
First, the split point eliminates the chisel edge present on standard ground bits. The chisel edge does not cut — it pushes and scrapes, generating heat without chip removal. Removing it means the bit begins cutting immediately upon contact, reducing the initial heat spike and the tendency to skate.
Second, the shallower included angle of 135° distributes cutting forces more evenly across the cutting lips, reducing the load per unit area and improving bit durability in hard or work-hardening materials.
For most stainless steel drilling applications, a 135° split-point M35 cobalt bit is the correct starting specification.
Setup and workholding
Rigidity in the setup matters more with stainless steel than with softer materials. Any vibration or deflection between the bit and the workpiece causes the bit to rub intermittently rather than cut continuously, promoting work hardening and edge chipping.
Workpieces should be firmly clamped, not hand-held. The drill bit should be checked for runout before a production run — even small amounts of runout accelerate wear unevenly across the cutting lips. Short bit extensions are preferable to long ones; longer reach increases deflection under load.
For hole entry in thin stainless sheet, a centre punch mark or a pilot hole reduces skating at the start of the cut. For hole exit, a backing plate of scrap mild steel or aluminium prevents the burr and material tear-out that commonly occurs as the bit breaks through.
Summary: the key variables to control
• Bit material: use M35 cobalt HSS for any production drilling in stainless steel. M2 is a compromise.
• Cutting speed: run slower than you would for mild steel. Heat is the primary failure mechanism.
• Feed rate: maintain positive feed at all times. Light rubbing triggers work hardening.
• Coolant: apply continuously to the cutting zone. Dry drilling significantly shortens tool life.
• Geometry: 135° split-point is preferable to 118° standard for stainless.
• Rigidity: clamp firmly, minimise runout, avoid long extensions.
Stainless steel drilling is not inherently difficult — it becomes difficult when the tooling or parameters are chosen for a different material. With the right bit specification, appropriate speeds and feeds, and consistent coolant application, it is entirely controllable at production scale.
Post time: May-06-2026