Choose a Cobalt Drill Bit for Metal Without Overbuying

A cobalt drill bit is a practical upgrade for stainless steel and other metal work that generates enough heat to shorten the life of ordinary high-speed steel. It is not automatically the best bit for every hole: standard high-speed steel is usually adequate for occasional drilling in mild steel or aluminum if the edge is sharp and the setup is correct. Even stainless can be drilled with standard HSS, according to the British Stainless Steel Association, although cobalt HSS may provide more useful wear and heat resistance.

“Cobalt” should identify cobalt added to the high-speed-steel substrate, not merely a bronze-colored surface. Bosch, for example, describes its HSS-Co bits as high-speed steel reinforced with cobalt, while identifying the finish and geometry separately (Bosch HSS-Co specification). Look for HSS-Co, M35, M42 or an explicit cobalt percentage in the specification. Color and phrases such as “for metal” do not establish the alloy.

M35 or M42?

M35 is commonly sold as 5% cobalt HSS, while M42 is commonly sold at about 8%. These labels identify the tool material, not its point angle, length, coating or quality of manufacture. Dormer Pramet lists an HSS-E drill with 5% cobalt, and Bosch specifies 8% cobalt for its M42 range.

Choice	Sensible use	Buying check
M35 / typically 5% cobalt	Intermittent holes in stainless and tougher steels, including many handheld jobs	Confirm that the exact bit is rated for the metal, diameter and drilling method
M42 / about 8% cobalt	Repeated holes or demanding cuts where added hot-hardness and wear performance may justify the premium	Do not assume the grade compensates for poor rigidity, wrong speed or a dull edge
Standard HSS	Occasional holes in mild steel, aluminum and other easier metals	A sharp, correctly ground bit may be the better-value purchase
Carbide	Rigid production work or a specialized hard-metal application	Verify workpiece hardness, machine rigidity and the bit’s explicit material rating

Do not turn this into a universal “M35 for hand drills, M42 for drill presses” rule. DeWalt does describe its own 5% cobalt Pilot Point line as intended for hand drilling and its 8% industrial line as having better wear properties and its best drill-press performance, but that is guidance for those DeWalt product lines, not every M35 and M42 bit.

For intermittent DIY or repair holes, start by comparing well-specified M35 products rated for the workpiece. Consider M42 when drilling is repetitive, cutting heat is a persistent limitation or the bit maker recommends it for the exact alloy and setup. In either grade, geometry, heat treatment, grinding and sharpness can matter more than the cobalt number alone.

Cobalt HSS is also not an automatic answer for fully hardened steel. If the workpiece hardness is known to be high, select a tool specifically rated for that condition rather than trusting a broad “cobalt” claim. Our guide to carbide bits for hardened steel covers that separate decision.

Check the point, length and shank

• Point: A 135-degree point is a useful default for stainless and many harder metals. BSSA recommends a cutting angle around 135 degrees for stainless and stresses that both cutting edges must be equal and sharp. A split point is a separate feature that can improve starting; do not assume every cobalt bit has one.
• Length: Choose the shortest bit that reaches the required depth. BSSA specifically recommends shorter HSS drills for stainless to reduce deflection and breakage. Stub length improves rigidity; jobber length provides more reach.
• Shank: Confirm the chuck type and capacity. A cylindrical shank fits a standard three-jaw chuck when its diameter is within the chuck’s range; Bosch states that compatibility for its cylindrical-shank cobalt range.
• Sizes: Buy the diameters the work requires and check whether frequently used sizes are sold individually. A large set is poor value if worn or broken sizes cannot be replaced economically.
• Full specification: The grade does not tell you the working length, point, flute geometry or finish. Compare those details as well as the alloy.

For relatively thin sheet and several possible finished diameters, a step drill bit may be more convenient than a large twist drill.

Set RPM by diameter and metal

“Slow” is not one trigger position. Norseman publishes a general HSS range of 30–50 surface feet per minute (SFM) for stainless and high-nickel steel, using this formula:

RPM = 3.8197 × SFM ÷ bit diameter in inches

That range produces these calculated starting points:

Bit diameter	Approximate RPM at 30–50 SFM
1/8 in.	920–1,530
1/4 in.	460–760
3/8 in.	305–510
1/2 in.	230–380

These are generic HSS starting values from the Norseman feed-and-speed table, not fixed limits for every cobalt bit or stainless grade. Use the bit manufacturer’s data when it identifies the exact tool and workpiece. Tool material, coating, diameter, workpiece and setup stability all affect the correct cutting data (Dormer Pramet guidance).

A large bit, uncertain alloy or flexible handheld setup generally favors the lower part of an applicable range. Do not simply choose the lowest possible speed, however. BSSA warns that both excessive and insufficient speed can contribute to breakage and that small bits should not run too slowly. The edge must keep cutting rather than rubbing.

Keep the edge cutting

1. Secure the work and protect yourself. Clamp sheet, round stock and small parts rather than holding them. Wear eye protection, secure loose clothing and hair, and stop the tool before clearing chips. LSU’s drill-press safety rules also call for clamping, suitable cutting fluid and a brush rather than bare hands for chip removal.
2. Mark stainless carefully. On austenitic grades such as 304 and 316, a heavy conventional center-punch mark can locally work-harden the entry. BSSA recommends a light mark with a three-cornered pyramid-tip punch when a mark is needed.
3. Use an appropriate cutting fluid. Apply it at the cut unless the bit or workpiece instructions specify dry drilling.
4. Feed steadily. Maintain enough pressure to form chips without stalling or bending the bit. Austenitic stainless work-hardens as it is cold-worked, so dwelling and rubbing can leave a glazed area that is harder to restart. See the complete procedure in drilling stainless steel.
5. Clear deep holes. Back the bit out periodically to remove chips and renew lubricant. Re-enter promptly at cutting speed and feed rather than dwelling against the work-hardened surface.
6. Ease pressure near breakthrough. The remaining metal is thin and more likely to grab the cutting lips. Reduced feed at this stage lowers the chance of the work spinning or the bit breaking.

For most buyers, the practical order is: match the bit to the exact metal, choose the required diameter and shortest workable length, confirm the point and chuck fit, and only then pay extra for M42 if the workload gives its higher-cobalt grade a job to do.