Series: Why Drill Bits Fail | Article 6
Keywords: drill bit chipping, drill bit breakage, HSS drill bit failure, drill bit toughness, drill bit heat treatment quality, drill bit snapping, drill perpendicularity
In our previous article, we looked at why hardness (HRC) matters for HSS drill bit quality — and why higher hardness is not always better. A drill bit with high hardness but insufficient toughness doesn’t simply wear out faster. It tends to fail a different way entirely: by chipping or breaking, rather than by gradual, predictable wear.
This article looks at that failure pattern directly. Why do drill bits chip or break instead of wearing down the way they’re supposed to? It’s one of the most overlooked questions when buyers try to work out what actually went wrong — and who is responsible for fixing it.
Two Very Different Ways a Drill Bit Can Fail
Drill bit failure falls into two categories, and they come from two different mechanical processes.
Normal Wear
This is the failure mode every buyer wants to see. The cutting edge gradually dulls as material is worn away, evenly, over the course of use. It is progressive and predictable — buyers can estimate roughly how long a bit will last and plan tool changes accordingly. There are no surprises.
Chipping or Fracture
This is the failure mode every buyer wants to avoid. A small section of the cutting edge suddenly breaks away, or the entire drill bit snaps. This is not material being worn off — it is material failing suddenly once the stress placed on it exceeds what it can withstand.
This kind of failure usually gives no warning. A drill bit can be cutting normally one moment and be scrap the next — sometimes taking the workpiece down with it.
Understanding the difference matters because chipping and breakage are rarely caused by the same things that make normal wear happen faster. They need to be diagnosed separately.
Three Root Causes Behind Chipping and Breakage
1. Heat Treatment Defects: A Drill Bit That Is Already Brittle
A drill bit’s toughness comes almost entirely from heat treatment, not from the raw steel itself.
After quenching, HSS forms a martensitic structure that is very hard but also very brittle. If tempering is insufficient — or skipped, or poorly controlled — a drill bit can measure an impressive hardness number while still carrying the brittleness of the as-quenched state, with almost no ability to absorb shock. A drill bit in this condition will chip under even mild impact or interrupted cutting.
A related issue is poor control of quenching temperature. If the austenitizing temperature runs too high, it coarsens the grain structure and leaves unstable retained austenite in the microstructure. Both effects reduce toughness and raise the risk of cracking, both during quenching and later in service.
There is also a failure mode buyers often miss entirely: a drill bit hardened to the same number from tip to shank.
A correctly heat-treated HSS twist drill does not carry uniform hardness along its length. The cutting point needs to be hard enough to hold an edge and resist wear. The shank needs to retain enough toughness to survive the clamping force and torsional shock of the chuck. When a supplier hardens the entire body to one high number, it can look like a premium product at first glance — hard everywhere. In practice, the shank has lost the toughness it needs, and the drill bit becomes prone to sudden, brittle fracture at the shank under heavy load. This is the same principle from our previous article, taken one step further: higher hardness, applied in the wrong place, does not make a better drill bit.
2. Cutting Conditions That Overload the Edge
Even when material and heat treatment are both sound, the way a drill bit is used can still cause chipping. Common conditions include:
• Interrupted cutting — when a drill bit enters or exits an angled surface, a cross-hole, or a weld seam, the load on the cutting edge becomes momentarily unbalanced, producing a shock load well above normal cutting forces.
• Feed rate too low on work-hardening materials — on materials like stainless steel, a feed that is too slow lets the edge rub across an already work-hardened surface instead of cutting into fresh material below it. The next pass then meets material harder than the tool itself.
• Poor chip evacuation — chips that cannot clear the flutes get recut by the cutting edge, adding extra mechanical shock with every pass.
• Insufficient machine or workholding rigidity — vibration repeatedly hammers the cutting edge with shock loading, accelerating local chipping.
• Breakthrough at the back face of the workpiece — as the drill bit nears full penetration, resistance drops suddenly and the bit can plunge forward, putting a sharp spike of load on the edge at the worst possible moment.
None of these conditions are caused by the drill bit’s material or heat treatment. They are caused by parameters and setup — and they will chip a perfectly good drill bit just as easily as a defective one.
3. Off-Axis Drilling: When the Bit Gets Snapped, Not Worn Down
There is a third failure pattern that is common and frequently mistaken for a quality defect: the drill bit is not perpendicular to the surface it is cutting, and it gets bent — and snapped — by side load.
A twist drill is a long, slender rotating tool. Its geometry is built to handle axial cutting force and torque, not bending load. When the drill bit is not aligned perpendicular to the hole axis — because the workpiece surface itself is angled, the operator is holding a hand drill off-angle, the drill press spindle and workpiece are not properly aligned, or the bit deflects on entry — it ends up carrying both cutting force and a side-bending force at the same time.
A slender shaft is poorly suited to carry that kind of side load. Even a drill bit made from sound material with correct heat treatment will snap once the bending stress exceeds what its cross-section can take. This kind of failure tends to happen fast, with a clean-looking break, and it shows up more often on smaller-diameter, longer drill bits — the higher the length-to-diameter ratio, the larger the bending moment produced by the same small angle of misalignment, and the weaker the bit’s resistance to it.
This case is different from the first two: it is not a material or process issue at all — it is a geometry and setup issue.
Put differently, even the best drill bit on the market will eventually snap if it is consistently run off-perpendicular. This is exactly why experienced machinists pay close attention to alignment and centering — particularly with hand-held tools, thin sheet, and angled surfaces, where perpendicularity is easy to overlook but affects tool life just as directly as speed or feed.
How Buyers Can Tell Which Cause They’re Looking At
The way a drill bit fails often points toward where the problem actually sits:
• Chipping on new drill bits, with cutting parameters unchanged from before — this points toward the material or heat treatment, not a sudden change in how the bit is being used.
• Chipping that only shows up under specific conditions (interrupted cuts, deep holes, stainless steel) — this points toward cutting parameters or application, not the drill bit itself.
• A clean break at the shank, with little visible deformation — worth questioning whether the bit was through-hardened, leaving the shank without the toughness it needed.
• A bent-looking break on angled surfaces, thin sheet, or poorly aligned setups — check perpendicularity and alignment before assuming the drill bit is at fault.
These causes get lumped together in conversation, but they sit on completely different paths: a material or heat treatment issue calls for a conversation with the supplier about process and verification data; a cutting-condition issue calls for adjusting parameters; a perpendicularity issue calls for a look at setup and alignment. Knowing which one you’re dealing with is what actually solves the problem — swapping in a new batch of drill bits will not fix a setup issue, and adjusting feed rate will not fix a heat treatment defect.
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: Jun-29-2026