Series: Why Drill Bits Fail | Article 9
Keywords: 135° split point, split point drill bit, drill point angle, 118° vs 135° point angle, self-centering drill point, chisel edge, drill bit centering geometry
The previous article in this series looked at why drill bits walk — the brief sideways skate at the moment of contact, and why it is a geometry problem rather than a material one. That article traced the cause back to the chisel edge: the section at the center of a standard twist drill point that does not cut cleanly, but extrudes and displaces material instead. It closed by noting that split point grinding shortens the chisel edge and reduces walking, and that this article would cover why in more depth — including why 135° is the angle most often specified alongside it.
This article looks at what a split point grind actually changes at the tip, why the 135° angle is paired with it so consistently, and what buyers should check before assuming the label on a spec sheet reflects what is actually ground into the steel.
The Chisel Edge Problem, Restated
A standard twist drill point is built from two cutting lips meeting at a chisel edge running through the center. The chisel edge does not shear material the way the lips do — it pushes material ahead of the point under compression. This is why a standard point needs a center punch or pilot hole to start cleanly on many jobs: the longer the chisel edge, the more the point relies on grinding symmetry alone to stay centered, and the more axial thrust it takes to get the drill to bite.
The chisel edge is not a flaw in the standard twist drill — it is a consequence of how a simple conical point is ground. But it is also the single largest lever available for improving how a drill starts a hole, which is exactly where split point geometry works.
What a Split Point Grind Actually Does
A split point adds a second grinding operation at the center of the point, cutting away part of the chisel edge and replacing it with two additional short cutting edges. The practical effect is straightforward:
• Shorter chisel edge — typically reduced to a fraction of its length on a standard point, leaving far less material that has to be extruded rather than sheared.
• Two extra cutting edges near the center — the point now cuts closer to the axis instead of relying on the outer lips alone to do all the shearing work.
• Lower axial thrust — less force is needed to get the drill to bite, which is why split point drills are often described as self-centering or self-starting.
• Reduced reliance on grinding symmetry alone — with a shorter chisel edge, small asymmetries between the two lips have less distance over which to act before the point engages properly, which directly reduces the walking tendency covered in the previous article.
Why 135°, Specifically
Point angle and split point grinding are two separate design decisions that are frequently specified together, and there is a mechanical reason for that pairing. A standard general-purpose point sits around 118°. A 135° point is flatter — closer to perpendicular to the drill axis — and this changes how the cutting load is distributed:
• Wider engagement at first contact — a flatter point angle spreads the initial cutting load across a broader section of the lips rather than concentrating it at a narrow tip, which lowers peak stress at the moment of entry.
• Better suited to harder and tougher materials — distributing load away from a narrow point matters most on stainless steels, higher-strength alloys, and other materials where a standard point concentrates heat and wear at the tip.
• Faster, cleaner starts without a pilot — the flatter angle combined with a split point grind lets the two center cutting edges engage the material sooner, which is why 135° split point drills are commonly used in portable and hand-held drilling where there is no fixture to hold the bit on center.
• Compounding benefit with split point grinding — the flatter angle gives the split point's center cutting edges more surface to work with, so the self-centering effect of the split point grind is more pronounced at 135° than it would be at a steeper angle.
In short: split point grinding changes what happens at the chisel edge, and the 135° angle changes how the load from that grind is distributed across the point. Neither is a substitute for the other — they are complementary parts of the same design intent.
What This Means for the Buyer
A 135° split point specification describes an intended geometry, not a guaranteed result. Whether that intent is delivered depends entirely on grinding precision, and the gap between the two shows up in predictable ways:
• Walking persists despite the split point label. If the secondary grind at the chisel edge is shallow, uneven, or off-center, the chisel edge is only nominally shortened, and the drill still exhibits the walking behavior split point grinding is meant to reduce.
• Inconsistent point angle across a batch. A batch specified as 135° but ranging several degrees either side will perform inconsistently from piece to piece — some bits starting cleanly, others behaving closer to a standard point.
• Asymmetric split point edges. If the two secondary cutting edges added by the split point grind are not equal in length or angle, the centering benefit is undermined in the same way an unequal standard lip grind causes walking, and the defect can be harder to see without close inspection of the tip.
None of this shows up by reading a spec sheet or a product title. It shows up in how the drill starts a hole — which is exactly what point geometry is meant to control in the first place.
How Buyers Can Verify It
A few checks separate a genuine 135° split point from a label applied to a standard grind:
• Ask for point angle inspection data, not just the nominal angle on the spec sheet. A projector or point-angle gauge reading across a sample batch will show whether the angle is held consistently, not just claimed.
• Check whether the drill still needs a center punch to start cleanly. A properly executed split point should start with markedly less hesitation than a standard point on the same material — if it does not, the secondary grind is not doing its job.
• Inspect the tip under magnification for symmetry. The two secondary cutting edges at the center should be visually and dimensionally even; unevenness here is the split point equivalent of an unequal lip grind.
As with the grinding symmetry discussed in the previous article, the design intent behind a 135° split point is sound engineering — the question for any buyer is always whether the grinding process behind the label actually delivers it, consistently, batch after batch.
About this series
Why Drill Bits Fail is a technical series written by our production team. Each article focuses on one specific factor in drill bit performance — from raw material to packaging. The goal is simple: help buyers understand what they are actually buying, and which questions to ask.
Post time: Jul-20-2026