HSS E versus Cobalt Drills for Metalwork
HSS E versus cobalt drills is not always a comparison between two different drill materials. In most industrial catalogues, HSS-E denotes cobalt-alloyed high-speed steel, while a ‘cobalt drill’ is commonly another name for an HSS-Co drill. The useful comparison is therefore between the stated HSS grade, drill geometry, finish and the material being machined - not simply the word cobalt on the label.
For a production job in stainless steel, alloy steel or nickel alloy, that distinction affects tool life, hole position, cycle time and whether the drill survives the first component. Check the grade before selecting the drill.
What HSS-E and cobalt actually mean
HSS is high-speed steel: the conventional substrate for jobber drills, taper shank drills, reduced-shank drills and many machine drills. It offers a practical balance of hardness, toughness and regrindability. Standard HSS grades remain appropriate for aluminium, brass, cast iron, low-carbon steel and general-purpose workshop work where cutting temperatures are controlled.
HSS-E is high-speed steel alloyed with cobalt. The added cobalt improves red hardness, meaning the cutting edge holds its hardness better at elevated temperature. This is the reason cobalt-bearing drills are specified for work-hardening stainless grades, higher-tensile alloy steels and operations where heat cannot be avoided.
The term does not identify one fixed chemical composition. Two common grades are M35, generally containing about 5% cobalt, and M42, generally containing about 8% cobalt. Both are cobalt-bearing HSS, but M42 is usually selected where greater hot hardness and wear resistance are needed. That comes with a trade-off: as cobalt content and hardness rise, the drill can become less forgiving of poor alignment, vibration and interrupted entry.
A drill described only as ‘cobalt’ may be M35, M42 or a supplier-specific cobalt HSS grade. It should not be assumed to be M42. Equally, HSS-E should not be treated as an inferior alternative to a cobalt drill without reading the specification. In many cases, they are describing the same material family.
HSS E versus cobalt drills: the decision points
For general mild steel and short-run work, standard HSS is often the economical choice. Moving to M35 HSS-E makes sense when drilling austenitic stainless steel, tougher alloy steels or repeated batches where edge wear is limiting consistency. M42 is more defensible on difficult, heat-intensive work where the machine, holder and setup can support it.
The application should determine the grade.
| Requirement | Suitable starting point | What to verify |
|---|---|---|
| Mild steel, aluminium, brass | Standard HSS or HSS-E | Point angle and flute form for the material |
| 304 and 316 stainless steel | M35 HSS-E as a practical baseline | Split point, rigid setup and sufficient feed |
| Higher-strength alloy steel | M35 or M42, depending on hardness and batch size | Workpiece hardness, coolant access and runout |
| Heat-resistant or hard-to-machine alloys | M42 HSS-E where applicable | Manufacturer material recommendation and stable cutting conditions |
| Deep holes or poor chip evacuation | Application-specific HSS-E geometry | Flute design, coolant delivery and cycle strategy |
This is not a rule that M42 must replace M35 wherever possible. If the drill is running in a light machine, in a long-reach holder or on a part with an uneven entry surface, the tougher and more forgiving option can produce better results. The most wear-resistant drill is not automatically the most productive drill.
Match the point geometry before upgrading the grade
A 135-degree split point is commonly preferable for stainless and alloy steels because it reduces thrust at the chisel edge and starts more positively than a conventional 118-degree point. It is particularly useful on CNC machines where repeatable hole location matters. A 118-degree point still has a place in general-purpose drilling and softer materials, but it is not the first choice for every steel job.
For thin material, a drill can grab as it breaks through regardless of whether it is M35 or M42. For cross-holes, angled surfaces and interrupted entry, a short, rigid drill length and controlled feed are more valuable than a nominally higher cobalt content. Spot drilling may be justified for position and surface condition, but an excessive pilot hole can remove the guidance provided by the twist drill’s chisel edge. Size the preparation around the drill geometry and the hole requirement rather than applying a default pilot operation.
Flute design also matters. Standard flute geometry is suitable for many steels, while parabolic flute forms can improve chip transport in deeper holes. In gummy stainless steel, poor chip control causes heat at the margins and lands long before the substrate reaches its limit.
Coating is a separate choice from drill grade
A coating does not turn a standard HSS drill into an HSS-E drill, and it cannot recover a drill that is rubbing through lack of feed. It is an additional variable.
Bright-finish drills are often suitable where coolant and chip evacuation are good, especially in aluminium and general machining. Steam-tempered or black-oxide finishes can assist lubricant retention and reduce friction in routine steel drilling. TiN-coated HSS drills are widely used for steel work, but their benefit depends on maintaining the coating at the cutting edge. Once a drill has been reground, that original edge coating is removed unless the tool is recoated.
TiAlN and related aluminium-rich coatings have a place in hotter dry or minimum-quantity-lubrication operations, but they need the correct application and cutting conditions. In a flooded-coolant stainless operation, a well-ground M35 or M42 drill with the right geometry may be the more relevant improvement. Select the coating from the actual coolant strategy, workpiece material and production requirement, not from colour.
Where cobalt drills fail in practice
Most premature failures attributed to ‘the wrong drill grade’ are process failures. Stainless steel is the usual example. If the feed is too low, the drill rubs rather than shears, generating heat and work-hardening the surface ahead of the cutting edge. Reducing the feed further after the drill starts squealing compounds the problem.
Use a stable workholding arrangement, keep drill overhang to the minimum needed, and check spindle runout at the drill rather than assuming the holder is accurate. Unequal lip loading from runout shortens tool life and can oversize the hole. On a CNC machine, avoid dwell at the bottom of a peck unless the cycle and application require it. The drill should continue cutting, with pecks used to manage chips and coolant access rather than to mask an unsuitable feed rate.
Coolant delivery deserves the same attention. External flood coolant is often sufficient for shallow holes when access is clear. As depth rises, chip packing and heat retention become more likely. Through-coolant drill designs, where available and appropriate, direct coolant to the cutting zone and help move chips out of the flutes. That is a process advantage, not just a premium feature.
Cobalt HSS also has limits. It is not a substitute for solid carbide where high-volume drilling, hardened materials, close positional control or demanding depth-to-diameter ratios justify carbide geometry and rigidity. Conversely, carbide is not automatically the right answer for a manual operation, variable setup or machine with insufficient rigidity. HSS-E remains valuable because it tolerates real workshop conditions better than brittle alternatives in many jobs.
Buying the right drill specification
When specifying a drill, start with the workpiece material and hardness, then define the hole diameter, tolerance, depth, entry condition, machine type and coolant method. From there, choose the substrate grade, point form, flute geometry, shank style and coating. A metric straight-shank jobber drill is not interchangeable with a Morse taper drill simply because both are M35, and a reduced-shank drill may be necessary when the required diameter exceeds the machine chuck capacity.
For procurement, record the full description rather than ordering from a shorthand such as ‘cobalt drill’. Include M35 or M42 where stated, diameter, overall and flute length, point angle, coating, DIN pattern if relevant, and the intended material group. That makes repeat ordering simpler and prevents a general-purpose HSS substitute appearing on a stainless production job.
Protool Precision Tools can help match the drill grade and geometry to the job, particularly where a drawing calls for a difficult material, a tight hole position or an established cycle is giving inconsistent life. The best next step is usually to bring the failed drill, material grade and cutting conditions into the same conversation. That is where the real difference between HSS-E and cobalt-labelled drills becomes clear.
