This last question is a good one. The answer comes down to the job quantity, machine setup and rigidity, and available budget. Let’s start with expense. As with anything that offers greater performance, carbide cutting tools cost significantly more. How much more depends on the manufacturer and tool geometry, but you can figure at least twice the price for substantially greater tool life and perhaps 10-20 times the metal removal per drill.

This vast offering of fully stocked tools includes cutter diameters as small as .001", and as large as 1/2".

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Regardless, metallurgists have continued adding various amounts and kinds of alloying elements to the tool steel crucible, among them tungsten, vanadium, chromium, and, most relevant to this discussion, cobalt. The result is six distinct groups of tool steel and many dozens of grades, a few of which we mentioned previously.

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Compared to regular M2 tool steel, M35 and M42 are indeed super. Where "regular" M2 HSS comes in at a hardness of around 62 HRC, M35 starts at 65 HRC and M42 measures 67 or so. That doesn't seem like much difference, but together with cobalt's greater heat resistance, it supports higher cutting speeds—depending on the brand and whether the drill is coated, this might mean 50% faster spindle RPM and commensurately higher feedrates. Either way, be sure to follow the cutting tool manufacturer's recommendations.

The M-series, for instance, gets its name from the primary alloying ingredient, molybdenum, a hard, heat-resistant element sitting at number 42 on the periodic table. M2 tool steel—the stuff of most drill bits—contains 5% molybdenum, but there are also M50, M7, and a few others, all of which provide differing degrees of flexibility, hardness, and toughness needed for holemaking operations.

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Machinability is a property of a material that is usually defined by four factors. These factors include (1) surface finish and integrity of the machined part, (2) obtained tool life or tool wear, (3) the magnitude of force and power required, and (4) ease of chip (excess material) production. Commonly this is achieved by alloying the material with sulfur or lead.

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This list is by no means complete, but we’d be remiss in not tagging a few members of the tool steel family. These include D2, A2, H13, S7, and other tough, heat-treatable, and wear-resistant metals used to make everything from plastic injection molds to stamping dies to high-speed steel (HSS) cutting tools.

Free machining is a manufacturing process utilizing steel, which has specially been designed to increase the machinability of a material during machining.

202287 — Pilot hole always works best for me. Using a small drill bit say 4mm then redrill/screw in with a bigger drill bit/screw.

At 82 HRC, give or take, tungsten carbide is much harder than cobalt or HSS. This is carbide’s only Achilles’ heel—where cobalt will flex a fair amount in the face of radial cutting forces, carbide will in many cases shatter. This means that proper alignment is critical for tool life and hole accuracy (which is true of any drilling operation). And when breaking through into intersecting holes as in the manifold example just given, the feedrate should be reduced slightly until the drill stabilizes.

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Since we’re comparing cobalt drills to those made of carbide, we won’t delve too deeply into the different types of carbide drills except to say that solid carbide is the closest equivalent from a size, length, and application perspective. After this come replaceable tip modular drills and indexable insert drills for larger holes (say anything above 1-1/2" in diameter).

Free machining steels are commonly produced by adding lead to molten steel in order to serve as a solid lubricant during the machining process. The process of free machining increases the machinability of the steels and consequently reduces the costs incurred by steel producers. The lubricating effect of lead is due to the lead particles shearing (breaking) off and building up on the chip-tool interface (contact point). As the tool machines the steel, the built up lead flows under the tool and along the leaded steel. Since lead has a low shear stress, it reduces the friction between the tool and the metal. As a result, the tool slides along the lead particles more efficiently. It is believed that the magnitudes of forces and power required to machine the steel is further reduced by the presence of lead in steel. This occurs due to the lead reducing the shear stress in the primary shear zone (focused area of machining). Leaded steels are are still the major type of free machining steels used today, inspite of lead's toxicity. Attempts to substitute other non-toxic metals with similar properties, such as bismuth and tin, have to-date met with only limited success in the steel industry.

As to the question “when should a cobalt drill bit be used," the answer is straightforward: anywhere you would use an HSS drill. Cobalt, however, will in most cases last longer than its less wear and heat-resistant cousin, and as we've seen, run much faster besides. The only caveat is that cobalt drills, due to their greater hardness, are more brittle than HSS. The user must therefore take extra care to align them properly and avoid the radial tool pressures that might occur when drilling into intersecting holes (hydraulic manifolds, for example) and angled surfaces.

A similar argument can be made for everything from high-end kitchen appliances to CNC machine tools, and if your shop has one of the latter, don’t short-change its potential by using low-cost drills and other cutting tools. Not only does carbide provide cutting speeds at least four to eight times that of cobalt, but far higher feedrates as well. And because carbide drills also support a feature unavailable with their cobalt cousins—coolant through-the-tool—there’s no need to peck. When coupled with high-pressure coolant (HPC), solid carbide blows the doors off traditional drills.

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Replace some of the iron found in M2 tool steel (and all steels, for that matter) with cobalt and you get “super high-speed steel.” The addition of 5% cobalt, for instance, gives us M35 tool steel, while 8% cobalt and some additional molybdenum make M42 tool steel—whenever anyone talks about cobalt drill bits, they are referring to one of these two.

This last point is important, since all cutting tools, whether they are HSS, cobalt, or tungsten carbide (more on this shortly), benefit from coating, be it TiN (titanium nitride), TiAlN (titanium aluminum nitride), or one of the other many tool coatings available on the market today. It’s also important to note that, despite what some websites suggest, there’s no such thing as a titanium drill bit (only drill bits coated with one or more of the thin film materials just mentioned).

Because it could cut metal much more quickly than the hardened carbon steel tools of the day, they dubbed it high-speed steel. As you will see, that moniker has since become a bit misleading, as tungsten carbide cutting tools—though a bit less forgiving than HSS—are far faster and therefore much more productive.

Adding sulfur to steels results in raised stresses within the primary shear zone of the steel. During machining, the chips that are produced are small and easily break up, increasing tool life and also increasing the ease of chip production. From this, the machinability of the material increases in the same manner as leaded steels; the cutting force and power required to complete the process are reduced.

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There’s neither the time nor space for a complete lesson on tool steels. Those interested can read more about it in this sample from ASM International’s Handbook, Volume 16: Machining Handbook. Otherwise, just know that so-called Mushet steel, which most consider the first true tool steel, left the furnace more than 150 years ago.

That brings us to tungsten carbide, more commonly referred to as carbide or sometimes cemented carbide. Think of it as cobalt on steroids. It's been around for nearly one hundred years but didn't come into widespread use as a cutting tool material until after World War II, when Kennametal founder Philip M. McKenna developed the first indexable carbide cutting tools. Since then, this extremely hard and wear-resistant material has consumed an enormous swath of the cutting tool market, drill bits included.

That’s not to say cobalt is obsolete, however. For prototype work and low-volume jobs, carbide’s higher cost probably doesn’t make sense. Nor is it appropriate for repair work or unstable machining conditions, where deflection can lead to disaster. Cobalt is also more suitable for softer materials like mild steel or aluminum, although here again, carbide's greater hole quality and straightness easily justify its use in higher-volume applications. At the end of the day, choosing the right tool means doing the math, asking questions, and being open to new technologies. Get drilling.

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It’s this last group, HSS, that we’re here to discuss today, along with that most widely used of all cutting tools, the drill bit. Despite their broad popularity, relatively low cost, and excellent mechanical properties, you’ll soon see why HSS and its cobalt counterparts aren’t always the best cutting tool materials for your drilling application. Read on.

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