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Stainless steel is where ordinary drill bits expose their weaknesses. It is tough, work-hardens quickly, and does not carry heat away as readily as mild steel. A bit that cuts cleanly through wood or aluminum can become dull after one stainless hole, especially if the drill is run too fast or allowed to rub without cutting.
For repeated stainless steel drilling, cobalt high-speed steel (HSS) bits are the practical upgrade. They tolerate more heat than standard HSS and keep a cutting edge longer. The best choice depends on hole size, material thickness, how many holes you need to make, and whether you value fast replacement over maximum service life.
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What makes cobalt bits better for stainless steel
Cobalt bits are usually made from an alloy such as M35 HSS, which contains about 5% cobalt, or M42, which contains about 8%. The cobalt is alloyed into the steel; it is not simply a coating. This improves hot hardness, so the cutting edges resist softening when drilling generates heat.
M35 bits are the common choice for general stainless work. M42 bits cost more and can last longer in demanding production use, but they are also harder and more brittle. That makes them less forgiving if the bit is twisted, dropped, or forced sideways in a handheld drill.
Look for a true cobalt alloy designation rather than marketing that only says “cobalt coated.” A titanium or black-oxide coating can help with wear and corrosion, but it does not turn ordinary HSS into a cobalt bit. For occasional household work, a quality coated HSS set may be sufficient. For repeated stainless holes, start with an M35 cobalt drill bit set.
The best cobalt bit types to consider
| Bit type | Best use | Main advantage | Trade-off |
|---|---|---|---|
| Standard twist, 118-degree point | General stainless drilling with accurate setup | Widely available and easy to sharpen | More prone to walking on thin sheet |
| 135-degree split point | Handheld drilling and larger batches | Starts more accurately and needs less pressure | Usually costs more |
| Reduced-shank set | Large holes with a standard 3/8-inch drill chuck | Allows larger diameters without a larger drill | Shank can slip if the chuck is not tightened well |
| Step drill, cobalt alloy | Thin stainless sheet and electrical knockouts | One tool makes several diameters and leaves a round hole | Not suitable for thick plate or deep holes |
A 135-degree split point is usually the easiest style for handheld work. It reduces walking and starts cutting with less pressure. A 118-degree point can cut well in a drill press, where the bit is held square and the workpiece is clamped.
For thin sheet, consider a cobalt step drill bit set. Step bits are particularly useful for holes from roughly 1/4 to 1-1/2 inches in sheet around 18 gauge through 1/8 inch. They are not a substitute for twist bits when drilling thick brackets, tubing walls, or stainless plate.
How to choose sizes and sets
Buy individual bits if you repeatedly make one or two hole sizes. A 1/4-inch or 3/8-inch bit used for mounting hardware is cheaper to replace individually than an entire set. Choose a set when you need clearance holes for different fasteners, or when a damaged bit would stop the job.
A useful general-purpose set runs from 1/16 to 1/2 inch in 1/64-inch steps, though many inexpensive sets skip sizes or stop at 3/8 inch. Check the actual size list before buying. A case with a full range is more useful than a large-looking set containing duplicate sizes.
For larger holes, use a reduced-shank set only if your drill has enough torque and a solid chuck. A cordless drill can handle many stainless holes up to 1/2 inch, but it may struggle with a 3/4-inch bit. In that situation, a drill press or a slower, higher-torque corded drill is easier to control. Do not use an impact driver with a regular round-shank bit unless the bit is specifically rated for an impact hex holder.
For a budget-conscious occasional job, a black-oxide HSS drill bit set can be fine if you are drilling only a few thin holes and can maintain low speed. The cheaper option becomes false economy when overheating destroys several bits in one project.
How to drill stainless without ruining the bit
Mark the hole with a center punch, clamp the work securely, and use cutting oil. Start slowly with firm, steady pressure. The bit should produce a continuous spiral chip. If it only polishes the surface or squeals, increase pressure slightly or reduce speed; do not continue rubbing.
As a starting point, use about 25 to 35 surface feet per minute for cobalt bits in stainless. Approximate drill speeds are:
- 1/8 inch: 750 to 1,000 rpm
- 1/4 inch: 375 to 500 rpm
- 3/8 inch: 250 to 330 rpm
- 1/2 inch: 190 to 265 rpm
These are starting ranges, not guarantees. Harder stainless, a dull bit, poor lubrication, or a handheld drill may require slower operation. Use the drill’s low-speed range for larger bits. A brief pecking action helps clear chips in thick material, but do not lift the bit so often that it loses cutting pressure.
Back thin sheet with scrap wood to reduce burrs and prevent the sheet from catching the bit. For a hole larger than about 3/8 inch, drill a pilot hole only slightly smaller than the final bit. An oversized pilot hole removes the cutting support needed at the larger bit’s chisel point.
Common failure modes and maintenance
The most common failure is a blue or straw-colored cutting edge. That indicates excessive heat and reduced hardness. A bit that turns blue may still cut, but its service life is usually much shorter. Excessive pressure can also chip the point, while side loading can snap a small bit.
Keep bits in a case so the edges do not knock against one another. Do not quench a red-hot bit in water; sudden cooling can damage the steel. Let it cool naturally, then wipe away cutting oil and chips. Small cobalt bits can be touched up with a proper drill-bit grinder, but badly overheated or chipped bits are generally not worth rescuing.
If you drill stainless every week, prioritize a sharp split-point M35 or M42 set, a slow-speed drill with a reliable clutch, and cutting fluid. If you drill six holes once a year, buy a modest M35 set or quality HSS set, use the correct speed, and replace individual dull bits rather than paying for a premium assortment you will rarely use.