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The best drill bits for hardened steel are chosen by actual workpiece hardness, bit material, point geometry, machine rigidity, diameter, feed, and heat control. Cobalt HSS is a practical starting point for many hard steels that are still machinable with twist drills, but very hard heat-treated parts can exceed what a handheld drill and cobalt bit can reasonably handle.
For a compact M42 set, the Bosch CO14B is the strongest general starting point. For broad fractional coverage, the Drill America D/A29J-CO-PC, IRWIN 3018002, and Chicago-Latrobe 57850 are better shop choices. If a known sharp cobalt bit cannot establish a cut under correct conditions, stop rather than adding force; carbide or another machining process may be required.
We did not perform original drilling tests. No hole-count, drilling-speed, temperature, or bit-life scores are invented.
Quick Picks
- Best compact M42 set: Bosch CO14B.
- Best larger Bosch M42 set: CO21B.
- Best mainstream compact DEWALT: DWA1240.
- Best broad DEWALT fractional set: DWA1269.
- Best traditional M35 29-piece set: IRWIN 3018002.
- Best Milwaukee 29-piece set: 48-89-2332 RED HELIX.
- Best M42 industrial value: Drill America D/A29J-CO-PC.
- Best compact Milwaukee set: 48-89-2331.
- Best very large M42 index: Viking/Norseman 69280.
- Best premium 29-piece industrial set: Chicago-Latrobe 57850.
How We Chose Bits for Hardened Steel
- Verified cobalt construction: M35 or M42 where documented.
- Point geometry: split-point/Pilot Point-style designs help start on hard surfaces.
- Heat resistance: hard steel quickly punishes dull or overheated edges.
- Size coverage: full fractional indexes are useful when exact diameters vary.
- Machine fit: rigid drill-press or machine setups are preferred for demanding work.
- Application limit: cobalt twist drills are not assumed capable of every hardened part.
Quick Comparison
| Product | Price | Type | Exact ASIN | Best Fit |
|---|---|---|---|---|
| Bosch CO14B | — | M42 cobalt / 14 pc | B07Q9HNTBH | Compact hard-metal work |
| Bosch CO21B | — | M42 cobalt / 21 pc | B07Q64LTFB | Broader M42 coverage |
| DEWALT DWA1240 | — | Cobalt / 14 pc | B015J5HU4Y | Mainstream hard-metal set |
| DEWALT DWA1269 | — | Cobalt / 29 pc | B015J5HVAC | Broad fractional index |
| IRWIN 3018002 | — | M35 cobalt / 29 pc | B000BO9JMK | Traditional shop use |
| Milwaukee 48-89-2332 | — | RED HELIX cobalt / 29 pc | B073R8T8QH | Hard-metal shop work |
| Drill America D/A29J-CO-PC | — | M42 cobalt / 29 pc | B00B0326OE | Industrial M42 value |
| Milwaukee 48-89-2331 | — | RED HELIX cobalt / 15 pc | B017POPUTE | Compact hard-metal kit |
| Viking/Norseman 69280 | — | M42 cobalt / 115 pc | B01CHJEVSI | Large professional index |
| Chicago-Latrobe 57850 | — | Cobalt / 29 pc | B000LDIR5Q | Premium controlled-shop work |
1. Bosch CO14B — Best Compact M42 Set
Bosch identifies the CO14B as an M42 cobalt set for hard, abrasive materials. Its 14-piece size is practical when common fractional diameters are enough.
- Best for: Best Compact M42 Set where actual workpiece hardness, cobalt grade, point geometry, diameter, and machine rigidity are matched.
- Why we picked it: Bosch CO14B fills a distinct hard-metal role based on M35/M42 alloy, point geometry, set coverage, and suitability for controlled chip formation in machinable hardened steel.
- Key strengths: heat-resistant cobalt construction, controlled starting geometry, and practical size coverage for hard-metal drilling under the correct speed/feed.
- Important limitations: cobalt is not guaranteed to cut every heat-treated component; case-hardened or very hard parts may require carbide, grinding, EDM, annealing, or another machining process.
- What to check before buying: workpiece hardness, M35 vs M42, drill diameter, point condition, spindle/chuck rigidity, cutting speed, feed, lubricant, and whether carbide/machine tooling is required.
- Who should buy it: users drilling hard but still machinable steels with a rigid setup and controlled cutting parameters.
- Who should skip it: users trying to force a handheld cobalt bit through very hard bearing/tool steel after the edge cannot establish a chip.
| ✅ Pros | ❌ Cons |
|---|---|
| • M42 cobalt • compact set • split-point geometry | • limited size coverage versus 29-piece indexes |
Manufacturer specs | Amazon — CO14B
2. Bosch CO21B — Best Larger Bosch M42 Set
The CO21B expands Bosch’s M42 line to 21 pieces, giving more intermediate sizes while keeping the same hard-metal focus.
- Best for: Best Larger Bosch M42 Set where actual workpiece hardness, cobalt grade, point geometry, diameter, and machine rigidity are matched.
- Why we picked it: Bosch CO21B fills a distinct hard-metal role based on M35/M42 alloy, point geometry, set coverage, and suitability for controlled chip formation in machinable hardened steel.
- Key strengths: heat-resistant cobalt construction, controlled starting geometry, and practical size coverage for hard-metal drilling under the correct speed/feed.
- Important limitations: cobalt is not guaranteed to cut every heat-treated component; case-hardened or very hard parts may require carbide, grinding, EDM, annealing, or another machining process.
- What to check before buying: workpiece hardness, M35 vs M42, drill diameter, point condition, spindle/chuck rigidity, cutting speed, feed, lubricant, and whether carbide/machine tooling is required.
- Who should buy it: users drilling hard but still machinable steels with a rigid setup and controlled cutting parameters.
- Who should skip it: users trying to force a handheld cobalt bit through very hard bearing/tool steel after the edge cannot establish a chip.
| ✅ Pros | ❌ Cons |
|---|---|
| • M42 • broader coverage | • not a full 29-piece 1/64 index |
Manufacturer specs | Amazon — CO21B
3. DEWALT DWA1240 — Best Mainstream Compact DEWALT Set
DEWALT’s DWA1240 uses cobalt alloy steel and PILOT POINT geometry and is designed for stainless steel and other hard metals.
- Best for: Best Mainstream Compact DEWALT Set where actual workpiece hardness, cobalt grade, point geometry, diameter, and machine rigidity are matched.
- Why we picked it: DEWALT DWA1240 fills a distinct hard-metal role based on M35/M42 alloy, point geometry, set coverage, and suitability for controlled chip formation in machinable hardened steel.
- Key strengths: heat-resistant cobalt construction, controlled starting geometry, and practical size coverage for hard-metal drilling under the correct speed/feed.
- Important limitations: cobalt is not guaranteed to cut every heat-treated component; case-hardened or very hard parts may require carbide, grinding, EDM, annealing, or another machining process.
- What to check before buying: workpiece hardness, M35 vs M42, drill diameter, point condition, spindle/chuck rigidity, cutting speed, feed, lubricant, and whether carbide/machine tooling is required.
- Who should buy it: users drilling hard but still machinable steels with a rigid setup and controlled cutting parameters.
- Who should skip it: users trying to force a handheld cobalt bit through very hard bearing/tool steel after the edge cannot establish a chip.
| ✅ Pros | ❌ Cons |
|---|---|
| • compact 14-piece kit • cobalt alloy • controlled starting | • fewer sizes than full indexes |
Manufacturer specs | Amazon — DWA1240
4. DEWALT DWA1269 — Best Broad DEWALT Fractional Set
The DWA1269 is a 29-piece cobalt fractional set aimed at hard metals. DEWALT now lists this exact model as discontinued, so availability may vary even though the ASIN remains identifiable.
- Best for: Best Broad DEWALT Fractional Set where actual workpiece hardness, cobalt grade, point geometry, diameter, and machine rigidity are matched.
- Why we picked it: DEWALT DWA1269 fills a distinct hard-metal role based on M35/M42 alloy, point geometry, set coverage, and suitability for controlled chip formation in machinable hardened steel.
- Key strengths: heat-resistant cobalt construction, controlled starting geometry, and practical size coverage for hard-metal drilling under the correct speed/feed.
- Important limitations: cobalt is not guaranteed to cut every heat-treated component; case-hardened or very hard parts may require carbide, grinding, EDM, annealing, or another machining process.
- What to check before buying: workpiece hardness, M35 vs M42, drill diameter, point condition, spindle/chuck rigidity, cutting speed, feed, lubricant, and whether carbide/machine tooling is required.
- Who should buy it: users drilling hard but still machinable steels with a rigid setup and controlled cutting parameters.
- Who should skip it: users trying to force a handheld cobalt bit through very hard bearing/tool steel after the edge cannot establish a chip.
| ✅ Pros | ❌ Cons |
|---|---|
| • broad 29-piece coverage • cobalt construction | • discontinued model |
5. IRWIN 3018002 — Best Traditional M35 29-Piece Set
The 3018002 is a 29-piece M35 5% cobalt set covering 1/16–1/2 in. in 1/64 in. increments with 135° split points.
- Best for: Best Traditional M35 29-Piece Set where actual workpiece hardness, cobalt grade, point geometry, diameter, and machine rigidity are matched.
- Why we picked it: IRWIN 3018002 fills a distinct hard-metal role based on M35/M42 alloy, point geometry, set coverage, and suitability for controlled chip formation in machinable hardened steel.
- Key strengths: heat-resistant cobalt construction, controlled starting geometry, and practical size coverage for hard-metal drilling under the correct speed/feed.
- Important limitations: cobalt is not guaranteed to cut every heat-treated component; case-hardened or very hard parts may require carbide, grinding, EDM, annealing, or another machining process.
- What to check before buying: workpiece hardness, M35 vs M42, drill diameter, point condition, spindle/chuck rigidity, cutting speed, feed, lubricant, and whether carbide/machine tooling is required.
- Who should buy it: users drilling hard but still machinable steels with a rigid setup and controlled cutting parameters.
- Who should skip it: users trying to force a handheld cobalt bit through very hard bearing/tool steel after the edge cannot establish a chip.
| ✅ Pros | ❌ Cons |
|---|---|
| • full fractional coverage • industrial case • M35 cobalt | • larger set than occasional users need |
6. Milwaukee 48-89-2332 — Best Milwaukee 29-Piece Set
The 48-89-2332 is a 29-piece RED HELIX cobalt set intended for demanding metal drilling with chip-removal-focused flute geometry.
- Best for: Best Milwaukee 29-Piece Set where actual workpiece hardness, cobalt grade, point geometry, diameter, and machine rigidity are matched.
- Why we picked it: Milwaukee 48-89-2332 fills a distinct hard-metal role based on M35/M42 alloy, point geometry, set coverage, and suitability for controlled chip formation in machinable hardened steel.
- Key strengths: heat-resistant cobalt construction, controlled starting geometry, and practical size coverage for hard-metal drilling under the correct speed/feed.
- Important limitations: cobalt is not guaranteed to cut every heat-treated component; case-hardened or very hard parts may require carbide, grinding, EDM, annealing, or another machining process.
- What to check before buying: workpiece hardness, M35 vs M42, drill diameter, point condition, spindle/chuck rigidity, cutting speed, feed, lubricant, and whether carbide/machine tooling is required.
- Who should buy it: users drilling hard but still machinable steels with a rigid setup and controlled cutting parameters.
- Who should skip it: users trying to force a handheld cobalt bit through very hard bearing/tool steel after the edge cannot establish a chip.
| ✅ Pros | ❌ Cons |
|---|---|
| • 29 sizes • cobalt construction • hard-metal focus | • large kit for limited-diameter work |
7. Drill America D/A29J-CO-PC — Best M42 Industrial Value
This 29-piece M42 cobalt jobber set spans 1/16–1/2 in. by 1/64 increments and uses 135° split points.
- Best for: Best M42 Industrial Value where actual workpiece hardness, cobalt grade, point geometry, diameter, and machine rigidity are matched.
- Why we picked it: Drill America D/A29J-CO-PC fills a distinct hard-metal role based on M35/M42 alloy, point geometry, set coverage, and suitability for controlled chip formation in machinable hardened steel.
- Key strengths: heat-resistant cobalt construction, controlled starting geometry, and practical size coverage for hard-metal drilling under the correct speed/feed.
- Important limitations: cobalt is not guaranteed to cut every heat-treated component; case-hardened or very hard parts may require carbide, grinding, EDM, annealing, or another machining process.
- What to check before buying: workpiece hardness, M35 vs M42, drill diameter, point condition, spindle/chuck rigidity, cutting speed, feed, lubricant, and whether carbide/machine tooling is required.
- Who should buy it: users drilling hard but still machinable steels with a rigid setup and controlled cutting parameters.
- Who should skip it: users trying to force a handheld cobalt bit through very hard bearing/tool steel after the edge cannot establish a chip.
| ✅ Pros | ❌ Cons |
|---|---|
| • M42 cobalt • full fractional index • industrial focus | • larger round case |
8. Milwaukee 48-89-2331 — Best Compact Milwaukee Set
The 48-89-2331 is a 15-piece RED HELIX cobalt kit. Milwaukee lists this exact model as no longer current, so availability can vary.
- Best for: Best Compact Milwaukee Set where actual workpiece hardness, cobalt grade, point geometry, diameter, and machine rigidity are matched.
- Why we picked it: Milwaukee 48-89-2331 fills a distinct hard-metal role based on M35/M42 alloy, point geometry, set coverage, and suitability for controlled chip formation in machinable hardened steel.
- Key strengths: heat-resistant cobalt construction, controlled starting geometry, and practical size coverage for hard-metal drilling under the correct speed/feed.
- Important limitations: cobalt is not guaranteed to cut every heat-treated component; case-hardened or very hard parts may require carbide, grinding, EDM, annealing, or another machining process.
- What to check before buying: workpiece hardness, M35 vs M42, drill diameter, point condition, spindle/chuck rigidity, cutting speed, feed, lubricant, and whether carbide/machine tooling is required.
- Who should buy it: users drilling hard but still machinable steels with a rigid setup and controlled cutting parameters.
- Who should skip it: users trying to force a handheld cobalt bit through very hard bearing/tool steel after the edge cannot establish a chip.
| ✅ Pros | ❌ Cons |
|---|---|
| • compact • cobalt • hard-metal geometry | • discontinued exact model |
9. Viking/Norseman 69280 — Best Large M42 Index
Norseman’s 69280 D-115 is a 115-piece M42 cobalt set covering fractional, letter, and numbered sizes for professional shops.
- Best for: Best Large M42 Index where actual workpiece hardness, cobalt grade, point geometry, diameter, and machine rigidity are matched.
- Why we picked it: Viking/Norseman 69280 fills a distinct hard-metal role based on M35/M42 alloy, point geometry, set coverage, and suitability for controlled chip formation in machinable hardened steel.
- Key strengths: heat-resistant cobalt construction, controlled starting geometry, and practical size coverage for hard-metal drilling under the correct speed/feed.
- Important limitations: cobalt is not guaranteed to cut every heat-treated component; case-hardened or very hard parts may require carbide, grinding, EDM, annealing, or another machining process.
- What to check before buying: workpiece hardness, M35 vs M42, drill diameter, point condition, spindle/chuck rigidity, cutting speed, feed, lubricant, and whether carbide/machine tooling is required.
- Who should buy it: users drilling hard but still machinable steels with a rigid setup and controlled cutting parameters.
- Who should skip it: users trying to force a handheld cobalt bit through very hard bearing/tool steel after the edge cannot establish a chip.
| ✅ Pros | ❌ Cons |
|---|---|
| • huge size coverage • M42 | • expensive and excessive for occasional work |
Amazon — Viking/Norseman 69280
- Best for: Best Premium 29-Piece Industrial Set where actual workpiece hardness, cobalt grade, point geometry, diameter, and machine rigidity are matched.
- Why we picked it: Chicago-Latrobe 57850 fills a distinct hard-metal role based on M35/M42 alloy, point geometry, set coverage, and suitability for controlled chip formation in machinable hardened steel.
- Key strengths: heat-resistant cobalt construction, controlled starting geometry, and practical size coverage for hard-metal drilling under the correct speed/feed.
- Important limitations: cobalt is not guaranteed to cut every heat-treated component; case-hardened or very hard parts may require carbide, grinding, EDM, annealing, or another machining process.
- What to check before buying: workpiece hardness, M35 vs M42, drill diameter, point condition, spindle/chuck rigidity, cutting speed, feed, lubricant, and whether carbide/machine tooling is required.
- Who should buy it: users drilling hard but still machinable steels with a rigid setup and controlled cutting parameters.
- Who should skip it: users trying to force a handheld cobalt bit through very hard bearing/tool steel after the edge cannot establish a chip.
10. Chicago-Latrobe 57850 — Best Premium 29-Piece Industrial Set
The 57850 is a 29-piece cobalt jobber set from 1/16–1/2 in. by 1/64 increments with 135° split points.
| ✅ Pros | ❌ Cons |
|---|---|
| • premium industrial format • broad fractional range | • premium price |
Amazon — Chicago-Latrobe 57850
M35 vs. M42 Cobalt
M42 generally offers higher hot hardness, while M35 is widely used for demanding metal drilling. Alloy grade alone does not guarantee success; geometry, sharpness, machine rigidity, and workpiece hardness still matter.
When Cobalt Is Not Enough
If the edge cannot establish a chip under correct speed/feed and rigid workholding, adding more force is not the answer. Very hard tool steels, bearings, or case-hardened surfaces may require carbide tooling, annealing where appropriate, or another machining process.
Speed, Feed, and Heat
Use the bit manufacturer’s cutting recommendations for the exact diameter and material. Excess speed overheats the edge; inadequate feed can cause rubbing rather than cutting.
Manufacturer References
- Bosch Tools, DEWALT, and Milwaukee Tool — cobalt alloy, point geometry, set contents, and hard-metal application specifications.
- IRWIN, Drill America, and Norseman — industrial cobalt jobber/index specifications.
FAQ
Can cobalt drill fully hardened steel?
Sometimes, depending on hardness and condition. Extremely hard parts may require carbide or a machine-tool process.
Is M42 always better than M35?
No. M42 has higher hot-hardness potential, but the job, geometry, quality, and available setup determine suitability.
Can I use a handheld drill?
For some holes, yes. A rigid drill press or machine setup is preferable for difficult hardened steel.
Full-Depth Product Decision Notes
Hardened steel varies widely—from moderately hard tool steel to case-hardened surfaces and fully hardened components that may be impractical for ordinary twist drills. These products should therefore be viewed as increasingly capable cobalt options, not guarantees against every hardened part.
Bosch CO14B — Detailed Fit
Material: documented M42 cobalt. Pros: compact set, high hot-hardness potential, split-point control. Cons: limited size range. Buy if: you need common fractional sizes in a small case. Skip if: you routinely need a full 1/64 in. index.
Bosch CO21B — Detailed Fit
Pros: broader M42 coverage than CO14B. Cons: still not a full 29-piece index. Buy if: you want more Bosch M42 sizes without going to a full industrial set.
DEWALT DWA1240 — Detailed Fit
Pros: compact cobalt alloy kit and controlled starting geometry. Cons: fewer diameters than fabrication indexes. Buy if: common shop sizes are enough. Skip if: precise incremental sizing is routine.
DEWALT DWA1269 — Detailed Fit
Pros: 29-piece fractional coverage. Cons: discontinued model means future availability may be inconsistent. Buy if: you find verified current stock at a sensible price. Skip if: you want a current supported product line.
IRWIN 3018002 — Detailed Fit
Material: M35 5% cobalt, 29-piece 1/16–1/2 by 1/64 in. index. Pros: excellent fractional coverage and traditional metal index. Cons: bulkier and more expensive than compact kits. Buy if: fabrication-size coverage matters.
Milwaukee 48-89-2332 — Detailed Fit
Pros: 29-piece cobalt set with RED HELIX geometry aimed at hard-metal drilling. Cons: large kit for occasional users. Buy if: you want broad coverage in the Milwaukee consumable ecosystem.
Drill America D/A29J-CO-PC — Detailed Fit
Material: M42 cobalt, full fractional 29-piece set, 135° split points. Pros: high-spec alloy and full size coverage. Cons: industrial round case is less compact. Buy if: hard-metal fabrication is regular work.
Milwaukee 48-89-2331 — Detailed Fit
Pros: compact 15-piece cobalt kit. Cons: discontinued exact model. Buy if: verified stock is available and the included sizes match your work. Skip if: long-term replenishment consistency matters.
Viking/Norseman 69280 — Detailed Fit
Material: M42 cobalt, 115-piece fractional/letter/number index. Pros: enormous size coverage. Cons: expensive and unnecessary for most users. Buy if: this is for a professional machine/fabrication shop.
Chicago-Latrobe 57850 — Detailed Fit
Pros: premium 29-piece cobalt jobber set and full fractional coverage. Cons: premium cost. Buy if: controlled shop drilling and industrial quality justify the spend.
Included Contents and Replacement Strategy
Set count is not the same as useful coverage. Check the exact diameter range and increments, and consider whether individual replacement bits are readily available. Shops often wear out a few common sizes long before the rest of the index.
Performance: Hardness, Speed, Feed, and Rigidity
Hard steel punishes rubbing. Use a sharp bit, correct cutting speed, steady feed, rigid clamping, and suitable cutting fluid where recommended. If the bit cannot establish a chip, adding pressure can destroy the edge and harden the work locally.
M35 vs. M42 vs. Carbide
M42 generally offers higher hot-hardness than M35, but carbide can be appropriate when the workpiece hardness exceeds practical cobalt capability. Carbide also demands greater rigidity and is less tolerant of shock.
Who Should Buy Which Type?
- Compact maintenance kit: Bosch CO14B/CO21B or DEWALT DWA1240.
- 29-piece fabrication index: IRWIN 3018002, Drill America D/A29J-CO-PC, Milwaukee 48-89-2332, or Chicago-Latrobe 57850.
- Large professional index: Viking/Norseman 69280.
Warranty, Value, and Availability
Drill-bit warranties and replacement policies differ from power-tool warranties and may exclude normal wear. Compare price per useful size, replaceability, current availability, and whether the manufacturer still supports the exact set.
Alternative Methods
For very hard heat-treated parts, consider carbide, an annular cutter in a suitable machine, EDM, grinding, or professional machining rather than forcing a handheld cobalt twist drill.
Related Tool Engineers Guides
For alloy/tooling context, see Cobalt vs Titanium Drill Bit, What Are Cobalt Drill Bits Used For?, and Types of Drill Bits.
Final Verdict
The Bosch CO14B is the strongest compact M42 starting point, while the Drill America D/A29J-CO-PC, IRWIN 3018002, and Chicago-Latrobe 57850 are better for broad shop coverage. This page stays focused on hardness and machinability, separate from the stainless-steel guide’s work-hardening emphasis.
