A twist drill starts as a round rod of high-speed steel — the same grade throughout the blank, whether M2, M42, or another HSS specification. The steel is produced as bar stock at a steel mill, typically in diameters close to the finished drill size for smaller drills or as larger stock that gets turned down for smaller diameters.
The exact steel composition matters. M2 — the most common drill grade — contains roughly 0.85% carbon, 4% chromium, 6% tungsten, 5% molybdenum, and 2% vanadium. M42 cobalt grade adds 8% cobalt to the base M2 formula. These aren't arbitrary numbers: each element serves a specific metallurgical role. Chromium provides hardenability and corrosion resistance. Tungsten and molybdenum provide hot hardness — the ability to hold a cutting edge at elevated temperatures. Vanadium refines grain structure and improves wear resistance. Cobalt in M42 further increases red hardness for elevated-temperature performance.
The steel is delivered to drill manufacturers in annealed (soft) condition — roughly 25 HRC — to allow machining. The hardening comes later in the process.
The most distinctive feature of a twist drill — the helical flutes that spiral down the shank — is ground into the blank rather than machined or rolled. On modern CNC flute grinding machines, a grinding wheel moves along the length of the blank while the blank simultaneously rotates on a precision spindle. The relationship between linear travel rate and rotational speed determines the helix angle of the finished flute.
Flute grinding is a material removal process: the grinding wheel cuts the flute relief into the steel blank, creating both the flute itself and the land between flutes. The depth, width, and cross-sectional profile of the flute determines the flute geometry — the relationship between flute width, core diameter (web), and the helix angle.
On a quality drill, this operation is done on CNC equipment with tight tolerances on helix angle consistency and flute-to-flute symmetry. On cheap import drills, variations in flute geometry produce unequal cutting lip lengths and asymmetric point geometry that cause drilling problems regardless of how well the point is ground.
Parabolic flute drills — designed for improved chip evacuation in deep holes — use a different flute profile that provides a larger chip channel and changes how chips curl and evacuate. This requires a different grinding wheel profile and setup.
After flute grinding, the drill is still relatively soft — HSS doesn't reach its intended hardness until heat treated. The heat treatment sequence is precise and involves multiple stages:
Preheat brings the drill up to an intermediate temperature slowly to prevent thermal shock to the tool. HSS is a poor thermal conductor and rapid heating can crack the material.
Austenitizing is the primary heating stage, bringing the drill to hardening temperature (typically 1,200–1,250°C for M2). At this temperature, carbon and alloying elements dissolve into the steel matrix in a form that will transform on quenching.
Quenching rapidly cools the drill, transforming the austenite to martensite — the hard, brittle phase that gives HSS its hardness. Most HSS drills are quenched in oil, salt bath, or air depending on the specific grade and manufacturer's process. Air quenching is common for HSS because the alloy content allows full hardening even with relatively slow cooling rates.
Tempering follows immediately — an important step that relieves internal stresses from the quench and converts some of the brittle martensite to tempered martensite, improving toughness without significantly sacrificing hardness. M42 typically gets triple-tempered — three cycles at 550–560°C — to ensure thorough stress relief and secondary hardening from carbide precipitation.
After heat treatment, HSS drills reach 62–65 HRC for M2 and 65–68 HRC for M42. This is the material condition in which they leave the factory and in which they are resharpened.
After heat treatment, the point geometry is ground on the hardened blank. This is the most precision-critical step in drill manufacturing — it determines the actual cutting performance of the finished tool.
Point grinding creates the two cutting lips, the chisel edge, and the clearance surfaces (flanks) behind the cutting lips. On a CNC drill point grinding machine, the drill blank is held at precise angles and fed against a grinding wheel under computer control. The wheel generates the conical flank surface that gives the drill its point angle and the clearance that allows the cutting edge to bite rather than rub.
Conventional grinding produces a standard conical point geometry. Split-point grinding uses a secondary grinding wheel pass or a different machine setup to create the notch that extends the cutting lips to center, nearly eliminating the chisel edge. This is a more complex operation and is why split-point drills typically cost more than conventional-point equivalents of the same grade.
The tolerances on point grinding are what separate quality drills from cheap ones. Lip length symmetry (both cutting lips the same length), chisel edge centering, and consistent relief angle behind both lips all determine whether a drill will run straight and produce a round hole of correct diameter. These tolerances are held to fractions of a thousandth on quality drills and are largely ignored on low-cost imports.
Understanding the manufacturing process explains why resharpening is a valid approach to tool life extension rather than a compromise.
The HSS material throughout the drill body is the same grade, the same heat treatment, and the same hardness as at the factory. The only thing that changes with use is the cutting edge geometry — the microscopic radius that develops as the sharp corner wears, the chisel edge that progressively loses its effectiveness, the relief angle that gradually degrades as the flank wears back.
Resharpening restores the geometry by grinding back into fresh, unworn material with the same hardness as the original cutting edge. A properly resharpened drill performs like a new drill — it has the same cutting angles, the same relief, the same edge quality. The metallurgy hasn't changed; only the geometry has been restored.
The limits on resharpening are geometric, not metallurgical. A drill can be resharpened until there isn't enough flute length remaining to clear chips effectively, or until the point is too close to the shank to hold securely. These are length limits, not material limits — the steel quality at the point after ten resharpens is identical to what it was new.
A resharpened HSS drill has the same M2 or M42 steel as the day it was made. Mail in your worn drills and we'll bring the cutting geometry back to factory spec on the WinsloMatic.
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