Drill Bits for Steel and Metal: Bit Types, Speeds and Sizes (Charts)

Metal punishes guesswork. The same bit that sails through pine will skate across steel, glaze over, and be finished in thirty seconds. Two things decide the outcome: whether the bit is harder than what you are cutting, and whether the speed drops as the bit gets bigger. Most snapped bits are a speed problem wearing a bit problem’s clothes. Below are the speeds, the bit types, and what each one is actually for.

Drill Bit Speed Chart for Metal

Speed is the variable most people get wrong, and it is the one that decides whether a bit lasts a season or an afternoon. The rule is simple and counter-intuitive: as the bit gets bigger, the RPM must come down. A 1/2″ bit spinning at the speed suited to a 1/16″ bit is travelling eight times faster at its cutting edge, and that edge burns.

Figures are starting points for HSS in mild steel and aluminum, cobalt in stainless, hardened steel and cast iron. Derived from RPM = (SFM × 3.82) ÷ diameter using 90 SFM mild steel, 40 SFM stainless and hardened, 70 SFM cast iron, 250 SFM aluminum. Drop 30–50% for deep holes, no coolant, or a hand drill rather than a press.

Two figures in that table surprise people. Aluminum wants to be drilled fast — it is soft, and running slow lets it melt and weld itself into the flutes. Stainless wants to be drilled slowly and never allowed to stop, which is a different problem covered further down.

Reading Your Chips

You do not need a tachometer to know whether the speed is right. The chips tell you, and they tell you immediately:

• Silver, curling ribbons — correct. The bit is cutting, not rubbing.
• Straw or gold coloured — slightly hot. Drop the RPM about 10% or add fluid.
• Blue chips — too hot. Stop. In stainless this is the point where the surface work-hardens and the hole becomes much harder to finish.
• Black chips or smoke — the bit is being destroyed. Stop, let it cool, and reduce speed before continuing.
• Fine powder rather than chips — you are rubbing, not cutting. Increase feed pressure or reduce speed.

Colour change comes from heat in the chip, which is exactly the heat that would otherwise be going into the cutting edge. A straw-coloured chip is the bit telling you it is carrying the heat away successfully. A blue one means it is not.

Which Bit Material Cuts What

There are only two things happening here: a bit is either coated or it is alloyed. A coating reduces friction until it wears off, and once it is gone you have a plain HSS bit that cannot be resharpened without removing what is left of it. An alloy is the same material all the way through, so it survives heat and takes a new edge on a grinder as many times as you like.

The practical version: buy HSS or black oxide for mild steel and aluminum, and cobalt for anything that fights back. Titanium sits in an awkward middle — genuinely good while the coating lasts, then unremarkable. For a full breakdown of the metallurgy, heat limits and which grade to sharpen, see our titanium vs cobalt comparison.

One geometry detail matters more than the coating on the box. Look for a 135-degree split point. A traditional 118-degree point is steeper and walks across steel before it bites; a 135-degree split point sits flatter and starts cutting on contact, which is why it needs no pilot hole in thinner material and lands on your mark instead of somewhere near it.

Twist Bits, Step Bits and Sheet Metal

Bit material decides whether you can cut the metal. Bit shape decides whether you can cut it in the form it happens to be in. A cobalt twist bit is the right tool for a 1/2″ steel plate and completely the wrong tool for a 20-gauge electrical enclosure, and the failure in the second case is not gradual — the bit catches the far side and rips the panel out of your hands.

Twist and Jobber Bits

“Jobber” is not a brand or a grade — it is a length. A jobber-length bit has a flute length roughly 9 to 14 times its diameter, which is the standard proportion you picture when you think of a drill bit. It is the general-purpose choice, and it is what almost every index you buy contains.

The trade-off is stiffness. A long, thin bit flexes, and flex in steel means a wandering hole and a bit under bending stress it was not designed for. If you are drilling thick plate or working somewhere awkward, a screw machine (stub) bit in the same diameter is dramatically stiffer for the same money and will hold a line far better.

Whatever the length, the point matters more: 135-degree split point for steel. It sits flat, bites on contact, and does not need a centre punch in thin material. A 118-degree point walks.

Step Bits and Unibits

A step bit is a cone of progressively larger cutting diameters. You drill until the hole is the size you want and stop. Its advantages in sheet are real and not marketing:

• It cannot grab. Each step engages gradually, so there is no moment where the full cutting edge breaks through at once.
• It deburrs as it goes. The next step up chamfers the hole the previous one cut.
• It replaces an index. One bit covers ten or more sizes, which is why electricians and HVAC installers carry one instead of a case.
• It gives round holes in thin stock. A twist bit in 22-gauge tends to produce a triangle.

The limitation is depth. A step bit can only cut material thinner than the height of a single step — roughly 1/8″ on most. It is not a plate bit and never will be.

Drilling Sheet Metal Without Tearing It

Even with the right bit, thin metal has its own rules:

• Back it with scrap wood. Clamping the sheet to a sacrificial board removes the unsupported far side entirely, which is where tearing starts.
• Clamp the workpiece, always. A sheet that spins is a laceration. It will not be your hand that wins.
• Ease off at breakthrough. The last fraction of a millimetre is where a twist bit grabs. Reduce pressure as you feel it start to give.
• Deburr both sides. The underside burr on drilled sheet is sharp enough to cut through a glove.

Tap and Clearance Hole Sizes

Two different holes get drilled for the same bolt, and mixing them up is one of the most common metalworking errors. A tap drill hole is undersized so a tap can cut threads into it. A clearance hole is oversized so the bolt passes straight through. Get them backwards and you either strip the thread or thread the wrong piece.

Tap drill sizes are for approximately 75% thread engagement per ASME B1.1 — the standard compromise between thread strength and tapping torque. Going for 100% engagement raises holding power marginally and snaps taps often.

For metric, the shortcut is arithmetic rather than a table: tap drill = major diameter minus thread pitch. An M8×1.25 needs 8.0 − 1.25 = 6.75 mm, so you round up to a 6.8 mm bit. That single rule replaces most of a metric tap chart.

Technique That Keeps the Bit Alive

Bit choice and speed get you most of the way. These are the habits that decide whether a set lasts years or months.

• Centre punch first. Even with a split point, a punched divot anchors the tip exactly on your mark. On anything thicker than sheet it is the difference between a hole where you wanted it and a hole near where you wanted it.
• Pilot then step up above 3/8″. Drill 1/8″ or 3/16″ first. The larger bit then removes far less material, which cuts both the torque your drill has to supply and the heat going into the edge.
• Feed hard enough to cut. The instinct is to go gently. It is wrong. Light pressure at high speed makes the bit rub, and rubbing generates heat without removing metal.
• Use low gear. Gear 1 on a two-speed drill trades RPM for torque, which is exactly the trade you want in steel.
• Let it cool. A bit too hot to touch has already lost some hardness. Pause rather than push through.

Work Hardening: Why Stainless Fights Back

Austenitic stainless — 304, 316, the common grades — hardens under friction. Rub it instead of cutting it, or stop mid-hole and let the bit spin without advancing, and the surface layer becomes measurably harder than the material underneath. Now even a cobalt bit struggles, and the usual response, more speed, makes it worse.

The prevention is constant feed pressure and a speed low enough to cut. Once a spot has work-hardened, starting a fresh hole a few millimetres away is usually faster than trying to win the one you started. This is the same failure mode behind most burnt bits — more on that in why drill bits burn or break.

Cutting Fluid, and the Cast Iron Exception

Cutting fluid cools the cutting edge and flushes chips out of the flutes. On steel and stainless it routinely multiplies bit life several times over, which makes a small bottle the cheapest upgrade available to anyone drilling metal.

Cast iron is the exception: drill it dry. Its graphite content is self-lubricating, and flood coolant on hot cast iron causes thermal shock that can crack the workpiece. Dry, slow, and let the dust clear.

Why 135° Beats 118° on Steel

The number on the packet describes the included angle at the tip. It sounds like a detail and it changes the behaviour of the bit completely.

A 118-degree point is steeper, so its very tip touches first and has almost no lateral support — on a hard, smooth surface it skates before it bites. A 135-degree point is flatter, and the split grinds the chisel edge into two cutting lips that start removing material on contact. That is why split-point bits do not need a pilot hole in thin material and land where you aimed them.

Common Mistakes When Drilling Metal

Quick Reference Summary

Everything above, condensed. RPM figures are 1/4″ / 1/2″.

Frequently Asked Questions

What is the best drill bit for drilling through steel?

For mild steel, an HSS or black oxide bit with a 135-degree split point is enough. For stainless steel, hardened steel and cast iron, use cobalt (M35 or M42), which is alloyed all the way through rather than coated and survives the heat. Carbide-tipped bits handle hardened material that defeats cobalt, but they are brittle and shatter if flexed.

What drill speed should I use for metal?

Speed drops as the bit gets bigger. In mild steel, roughly 1,375 RPM at 1/4 inch and 690 RPM at 1/2 inch. In stainless, roughly 610 RPM at 1/4 inch and 305 RPM at 1/2 inch. The formula is RPM = (SFM x 3.82) divided by bit diameter in inches, using about 90 SFM for mild steel and 40 SFM for stainless. Reduce by 30 to 50 percent for a hand drill or a deep hole.

Do I need cutting oil when drilling metal?

Yes for steel, stainless, aluminum and brass - cutting fluid cools the edge and clears chips, and it commonly extends bit life several times over. The exception is cast iron, which should be drilled dry: coolant causes thermal shock that can crack the material.

Why do my drill bits break when drilling metal?

Usually speed, pressure or angle. Running too fast burns the edge; too little feed pressure makes the bit rub rather than cut, which work-hardens the surface; and tilting the drill off-axis snaps small bits, especially cobalt, which is harder and therefore more brittle. Centre punch first, keep the drill perpendicular, and let the bit cut.

What is a jobber drill bit?

Jobber is a length, not a grade or a brand. A jobber-length bit has a flute length roughly 9 to 14 times its diameter, which is the standard proportion in almost every drill index. Shorter screw machine or stub bits in the same diameter are considerably stiffer and wander less in thick steel.

When should I use a step bit instead of a twist bit?

In sheet metal thinner than about 1/8 inch. A twist bit catches the trailing edge as it breaks through and can spin the workpiece; a step bit cuts progressively, deburrs as it steps up, and leaves a round hole in thin stock. A step bit cannot cut material thicker than one step height, so it is not a substitute for a twist bit in plate.

What is the difference between a tap drill and a clearance hole?

A tap drill hole is undersized so a tap can cut threads into it - for 1/4-20 that is a number 7 bit at 0.201 inches. A clearance hole is oversized so the bolt passes straight through without threading, which for 1/4-20 is 17/64 inch. Drilling one when you needed the other either strips the thread or threads the wrong piece.

Why does stainless steel get harder while I drill it?

Austenitic stainless work-hardens under friction. If the bit rubs instead of cutting - too fast, too little pressure, or dwelling in one spot - the surface layer hardens and becomes much more difficult to penetrate, even with cobalt. Constant feed pressure and lower RPM prevent it. Once a spot has hardened, moving to a fresh location is often faster than fighting it.

Related Guides

• Titanium vs Cobalt Drill Bits: Which Is Actually Better? — the metallurgy, heat limits and sharpening in depth
• Why Drill Bits Burn or Break (And How to Stop It Happening)
• Drill Bit Speed Guide: RPM by Material
• Best Drill Bit for Every Material: Complete Guide
• Drill Bit Size Chart & Selector — enter your material and get the exact bit