If you've ever watched a large drill bit struggle to start, requiring heavy feed pressure before it begins cutting, you've seen the chisel edge at work — or rather, at odds with you. The chisel edge is a fundamental limitation of conventional twist drill geometry. Point thinning is how machinists and resharpening operations reduce or eliminate that limitation. Here's what's actually happening and why it matters.
What the Chisel Edge Is
A standard twist drill has two cutting lips — the main cutting edges that do the majority of the work. Where those lips meet at the center of the bit is the web, the solid core of steel that runs the length of the flute. The very tip of the web, where it intersects the drill point, creates a short, transverse cutting edge called the chisel edge.
The chisel edge doesn't really cut. It scrapes. Because it's located at the center of the drill where there's no peripheral velocity, it can't slice material the way the outer lips do. Instead, it deforms and pushes material outward from the center toward the cutting lips. This requires significant axial force — downward feed pressure — to advance the bit. On small drills in a press, it's not a problem. On large bits, in hard materials, or in handheld applications, the chisel edge becomes the limiting factor in how much thrust force you need to start and advance the cut.
What Point Thinning Does
Point thinning — also called web thinning — is the process of grinding away material at the drill tip to reduce the chisel edge length. The goal is to reduce the non-cutting center portion of the drill point, which decreases the thrust force required, improves self-centering, and often produces better hole quality.
The effect is most pronounced on larger drill sizes because the web-to-diameter ratio increases as drills get larger. A 1/8" bit has a tiny chisel edge relative to its diameter. A 1" bit has a chisel edge that can be nearly 1/4" long — a substantial non-cutting zone that requires real force to push through.
Types of Point Thinning
There are several standard approaches to point thinning, each producing a different geometry:
Notch Thinning (X-Notch)
A pair of small grinding notches cut into the web on each side of the chisel edge. The notches don't eliminate the chisel edge entirely but reduce its effective length by creating small cutting edges at the center. Quick to grind, effective for moderate reductions in thrust force. Common on mid-size bits in general production.
Crankshaft Thinning
The most aggressive web thinning approach — the web is ground to near-zero at the tip, creating a point that resembles the split-point on a factory-made bit. Provides the greatest reduction in thrust force and the best self-centering behavior. More metal is removed, so the tip is more fragile. Used for drilling in hard or difficult materials where feed force is the primary constraint.
Split Point (Factory Method)
The split-point geometry found on premium factory bits is a form of point thinning done during original manufacture. The chisel edge is ground away entirely and replaced with two additional small cutting lips that extend from the main lips to the center. The result: the bit cuts all the way to center, requires minimal thrust force to start, and self-centers without a center punch in most applications. Split-point geometry is the reason many machinists prefer those bits for handheld drilling.
When Point Thinning Is Necessary
Not every drill benefits from point thinning. The cases where it's specifically useful:
- Large diameter bits (1/2" and up): The chisel edge grows with drill size. Above 1/2", thrust force without thinning becomes significant, especially in steel.
- Hard or work-hardening materials: Stainless steel, titanium, and nickel alloys work-harden under the scraping action of the chisel edge. Reducing or eliminating the chisel edge reduces work-hardening at the hole center.
- Thin-wall drilling: Breakthrough burr and sheet deformation are caused partly by the chisel edge pushing material at the point of breakthrough. Thinned tips break through more cleanly.
- Handheld drilling where wandering is an issue: Split-point or thinned bits self-center better on entry, reducing the need for pilot holes.
- CNC/automated drilling with feed rate concerns: Reducing required thrust force allows higher programmed feed rates without stressing the spindle.
Point Thinning and Resharpening
When a thinned or split-point drill is resharpened, the question is whether the thinning is restored along with the main cutting geometry. A standard resharpening restores the two main lips and clearance angles but does not automatically recreate split-point or notch geometry unless specifically requested.
For most production applications, standard resharpening is correct — the bit goes back to conventional point geometry, which works fine on a press with adequate feed force. For bits that were specifically purchased as split-point for a reason (handheld use, difficult materials, automated feed applications), the point thinning should be specified when ordering a resharpen.
How to Know If Your Bit Needs Thinning
The signs that point thinning would help:
- You're using excessive feed pressure to start a hole in steel with a bit over 3/8"
- The bit "pecks" on entry rather than starting smoothly
- You're drilling stainless or high-alloy steel and the bit glazes the hole center before cutting
- Large-diameter bits are causing spindle load spikes at the start of each hole in CNC applications
For general drilling in mild steel and aluminum with typical sizes (under 3/8"), standard geometry is fine. The chisel edge does its job without requiring extraordinary thrust force. Point thinning is a solution to a specific geometry limitation — and a valuable one when that limitation is causing real problems.
Need a Resharpen With Web Thinning?
MachinistPost resharpens HSS drill bits by mail. Standard geometry restored on every order. Ask about split-point or notch thinning for bits that need it.
Get a Quote →The chisel edge is a design compromise in conventional twist drill geometry. Point thinning is the correction for that compromise when it limits cutting performance. Understand what it does and when to use it, and you'll get more out of your large-diameter and specialty bits — resharpened or new.