What's inside
As an Amazon Associate I earn from qualifying purchases. This post may contain affiliate links at no extra cost to you.
What we cover
Choose the Right Bit for Thick Stainless Steel
Thick stainless steel is hard on drill bits because it work-hardens. If a bit rubs without cutting, the metal underneath becomes harder than it was originally. A dull bit, excessive speed, or a pause in the feed can turn a straightforward hole into a blue, polished crater that ordinary bits will not penetrate.
For most holes, the best starting point is a cobalt drill bit set with a 135-degree split point. Look for M35 cobalt for general work and M42 cobalt when you regularly drill thick, tough stainless. These are high-speed steel bits with cobalt blended into the steel, not bits merely coated with cobalt. A true cobalt bit tolerates more heat and keeps its edge longer.
As an Amazon Associate we earn from qualifying purchases at no extra cost to you.
| Bit type | Best use | Main limitation |
|---|---|---|
| M35 cobalt HSS | General holes through stainless plate and tubing | Needs cutting oil and controlled speed |
| M42 cobalt HSS | Frequent drilling or very tough stainless | Costs more and can chip if abused |
| Carbide-tipped bit | Large holes, hardened material, or production work | Brittle; poor choice in a handheld drill if the setup moves |
| Bi-metal hole saw | Large holes for pipes, cables, and fittings | Slow in thick plate and prone to overheating |
| Titanium-coated HSS | Thin stainless and occasional light work | Coating does not make it a high-heat bit |
Which Bit Style Makes Sense?
Standard twist bits with a 118-degree point can work, but a 135-degree split point starts more easily and is less likely to wander. A split point also reduces the need to use a center punch, though punching a starting mark is still useful on a vertical surface.
For holes larger than about 1/2 inch, consider a step drill only when the stainless is relatively thin. Step bits are convenient for sheet metal, but each step has a long cutting edge in contact with the work. In thick stainless plate, they heat quickly and wear out at the larger diameters. A cobalt jobber bit, followed by a larger cobalt bit, is usually the better choice.
For a hole around 3/4 inch or larger, a stainless-rated bi-metal hole saw can be economical if you only need a few holes. Use a pilot bit made for metal and keep the saw square. For repeated holes in thick plate, a carbide-tipped annular cutter or magnetic drill is faster and cleaner, but it requires a rigid setup. Do not use an annular cutter in a loose handheld drill.
Speed, Pressure, and Cutting Fluid
Run slower than you would in mild steel. A useful starting point for cobalt bits in 304 or 316 stainless is about 600 rpm for a 1/4-inch bit, 300 rpm for a 1/2-inch bit, and roughly 150 to 200 rpm for a 3/4-inch bit. These are starting values, not absolutes. A cordless drill with a low-speed gear is preferable; a drill press gives better control.
Use firm, steady feed pressure. The bit should produce chips, not dust or squealing. If you lighten pressure while the bit is cutting, it may rub and work-harden the hole. If the bit turns blue, stop immediately. The temper is already damaged, and continuing usually makes the bit dull beyond practical sharpening.
Apply a sulfurized cutting oil or a dedicated metal-cutting fluid. Ordinary penetrating oil is better than drilling dry but is not as effective under heavy heat. Add oil frequently, especially in holes deeper than the bit diameter. Withdraw the bit every few seconds to clear chips and refresh the lubricant, but do not let the cutting edges spin against the hole without pressure.
Drill and Workholding Requirements
A high-torque low-speed high-torque cordless drill is suitable for many jobs, provided it has a metal chuck and a controllable trigger. Set the clutch to drill mode, use the lowest gear, and keep both hands on the tool. An impact driver is a poor choice for large stainless holes: its hammering action can chip bits, enlarge the hole, and make control difficult. It can drive small hex-shank cobalt bits in a pinch, but a conventional drill is safer and more predictable.
Clamp the work securely. Stainless plate can grab the bit when it breaks through, and a loose piece can spin violently. Put scrap wood behind thin material to support the exit side. On thick plate, mark the center clearly and check that the drill is square before starting. A small pilot hole, typically 1/8 inch, helps guide the larger bit, but do not use a tiny pilot at high speed and then leave the work unattended.
A Reliable Drilling Procedure
Mark the hole and make a shallow center punch mark. Start the cobalt bit at low speed with cutting oil. Once both cutting lips are engaged, increase pressure enough to form continuous chips. Keep the bit straight and use short pecking cycles to remove chips from a deep hole.
For a hole above 3/8 inch, drilling in two or three size increases is usually easier on the tool than jumping directly to the final diameter. Do not make every pilot hole excessively small; a 1/4-inch pilot is often more useful before a 1/2-inch finish hole than a 1/8-inch pilot. On very thick stock, a pilot roughly one-third to one-half of the final diameter leaves enough material for the finishing bit to cut properly.
When the bit is close to breaking through, reduce pressure slightly while maintaining rotation and lubrication. This limits breakout and prevents the bit from grabbing the far edge. Deburr both sides with a countersink or a fine file. For large holes, a bi-metal hole saw for stainless steel is fine for occasional work, but stop often to clear the slug and cool the saw.
When a Cheaper Bit Is Fine
HSS or titanium-coated bits are acceptable for a single small hole in thin stainless sheet, particularly if the material is under 1/16 inch thick. They are not a good bargain for thick plate: one overheated bit can cost as much as a cobalt bit, while leaving a hardened hole that complicates the next attempt.
For thick stainless, buy fewer good bits rather than a large assortment of inexpensive coated bits. A small set of cobalt bits from 1/8 to 1/2 inch covers common fastening work. Replace a bit when it needs excessive pressure, produces fine powder instead of chips, or leaves a mirror-polished surface. Those signs usually mean the edge is dull, and more speed will only make the problem worse.