The web of a twist drill is the solid cylindrical core that runs down the center of the drill body between the flutes. It's not a cutting surface — it's structural material that gives the drill its rigidity. The chisel edge at the drill's tip is formed by the intersection of the two relief faces across the web, and this is where the web's thickness directly affects cutting behavior.
On a new twist drill, web thickness is typically designed as a proportion of drill diameter — usually 15–20% of diameter for standard geometry drills. A half-inch drill with a 0.10" web is operating at 20% — in the normal range. The same drill with a 0.18" web is at 36%, and the performance consequences of that thickened web are significant.
Twist drill flutes are helical grooves machined into a cylindrical blank. Because the drill tapers slightly from the body toward the tip, and because the web is a constant taper running through the drill body, the web is thinner at the point and thicker toward the shank end.
Every resharpening removes material from the cutting end of the drill. As the drill gets shorter, the point is being reground into a cross-section where the web is naturally thicker. A drill that starts with a 0.10" web at the point will have a 0.13" or 0.15" web at a point an inch further up the flute — and that's where the point ends up after multiple resharpenings.
This is a predictable and unavoidable geometric consequence of how twist drills are made. The question isn't whether the web thickens as the drill shortens — it does, always — but whether the web has been thinned to compensate.
The chisel edge doesn't cut — it scrapes and pushes material to the side rather than slicing it. A chisel edge that's 0.10" wide requires meaningfully less force to push through than one that's 0.20" wide. The difference in axial thrust (downward force required to advance the drill) scales directly with chisel edge width, which is determined by web thickness.
A thick web causes several compounding problems:
Increased thrust force. You need more downward pressure to advance the drill. On a drill press with limited quill travel force, a thick-web drill may require so much pressure that the operator pushes harder and harder, loading the cutting lips beyond their design intent. This causes rapid edge wear, work hardening of the surface being drilled, and potential breakage.
Work hardening under the chisel. In materials that work-harden under compressive stress — stainless steel, certain alloy steels, titanium — a thick chisel edge spending more time rubbing the center of the hole before the cutting lips engage can harden the surface. The drill then has to cut into harder material than you started with. This accelerates edge wear and creates a cycle of increasing thrust and increasing damage.
Reduced centering ability. A split-point geometry nearly eliminates the chisel edge and dramatically improves self-centering — the drill starts cutting rather than wandering. A thick web with conventional chisel geometry on a worn drill does the opposite: it pushes to one side, causes drill wander, and produces out-of-round or oversized holes even in materials that aren't particularly hard.
Web thinning is a secondary grinding operation that removes material from the chisel edge area to reduce its effective width. There are several approaches:
Notch or crankshaft web thinning grinds a small concave notch into the heel of each flute near the point, extending the cutting edge closer to center and shortening the chisel edge. This is the most common web thinning method on general-purpose drills.
X-thinning creates a small secondary cutting edge that runs through the center, essentially splitting the chisel and converting it into a short cutting edge. This is the geometry behind "split point" drills — the split point is an extreme form of web thinning.
Spiral or cam web thinning is used on production CNC grinding equipment and produces the cleanest geometry, but isn't typically done on manual regrind equipment.
In all cases, the goal is the same: reduce the width of material that has to be pushed rather than cut at the center of the hole.
Web thinning becomes important when any of these conditions apply:
The drill has been resharpened multiple times and the point is significantly shorter than original. As a rough rule, once a drill has been resharpened to 70% or less of its original length, ask for web thinning at the next resharpen. The web is predictably thicker at this length.
You're drilling hard or work-hardenable materials. Stainless steel, titanium, high-nickel alloys, and AR-grade wear plate all benefit from reduced chisel contact. Specify web thinning (or split-point geometry) when ordering resharpening of drills used in these materials.
You're having trouble starting holes without a center punch or pilot. If the drill consistently wanders on startup even in mild steel, a thick web is often the culprit — not the workpiece or the setup. A thinned web or split-point regrind fixes this.
You notice thrust force has increased noticeably compared to when the drill was new or freshly sharpened. Increased thrust at the same speed and feed is the most reliable sign that the web has thickened beyond the usable range.
Most professional resharpening services offer web thinning as a standard option, but it may need to be requested explicitly. When sending drills for resharpening, it's useful to note:
The application (mild steel general use vs. stainless vs. AR plate) — this tells the resharpener what geometry is appropriate and whether aggressive web thinning is beneficial or conservative web thinning is sufficient.
Whether you want split-point geometry — this is particularly valuable for drills ¼" and larger that will be used in hard materials or for high-precision work. Split-point geometry nearly eliminates the chisel edge and adds self-centering ability.
The intended material, especially if it's a work-hardenable grade — this allows the resharpener to set relief angle and web thinning to minimize chisel contact time, which directly affects how quickly the material work-hardens under the point.
A well-specified resharpening order — point angle, web thinning preference, material application — produces a drill that performs closer to new than a generic "resharpen" instruction. The geometry conversation is worth the extra thirty seconds per drill.
Mail in your worn drills with the application details — material, diameter, work-hardenable or not. We'll grind the right web geometry on the WinsloMatic for your next run.
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