Series: Why Drill Bits Fail | Article 8
Keywords: drill bit walking, drill bit wandering, off-center drilling, drill point geometry, chisel edge centering, drill bit runout, oversized hole
The last three articles in this series dealt with heat treatment: why hardness matters, why drill bits chip or break instead of wearing down, and how buyers can evaluate heat treatment quality instead of taking a supplier's word for it. This article opens a new cluster in the series: geometry. Heat treatment decides what a drill bit can survive. Geometry decides where it actually goes.
Most machinists have seen this happen: the drill touches the surface, and instead of biting straight in at the mark, the tip skates sideways for an instant before it finally catches. In the shop, this is called walking — the bit wandering off its intended center before cutting properly begins. It usually lasts a fraction of a second. The position error it leaves behind does not correct itself.
This article looks at why walking happens, what it actually costs a buyer downstream, and how to tell whether the cause sits with the drill bit or with how it is being run.
What Walking Actually Is — and Why It's a Geometry Problem, Not a Material One
A standard twist drill point is not a true point. It is built from two cutting lips and a chisel edge running between them at the center. The chisel edge does not cut in the normal sense — it extrudes and displaces material ahead of the lips rather than shearing it cleanly. In a correctly ground point, the two lips are equal in length and angle, and the chisel edge sits exactly on the drill's axis. Under those conditions, the axial resistance on both lips is balanced, and the drill goes in straight.
Once that symmetry is disturbed — whether from grinding error or from a limitation in the point design itself — the two lips no longer meet equal resistance. The resulting force has a lateral component instead of a purely axial one, and the tip moves toward the side offering less resistance until the forces rebalance, or until the hole wall itself constrains the bit.
This is also why walking is confined almost entirely to the first moment of contact. Once the drill has entered the material far enough for the margins to engage the hole wall, the bit is mechanically constrained and lateral movement stops. Walking is not an ongoing problem through the cut — it is a problem with how the hole starts.
What Causes a Drill Bit to Walk
1. Unequal Lip Length or Angle
This is the most common cause, and the easiest one to overlook by eye. If the two cutting lips differ in length or angle — even by a small margin — the axial resistance on each side is no longer equal. The larger the discrepancy, the more pronounced the walking.
This kind of error tends to come from grinding equipment that isn't holding tight tolerances, or from a grinding process without a systematic inspection step behind it. Two lips can look identical to the eye and still measure outside an acceptable tolerance under a projector or point-angle gauge.
2. Off-Center Chisel Edge
If the chisel edge is not precisely centered on the drill axis, the point is effectively an eccentric structure. On contact, an off-center chisel edge causes a slight precession as the bit rotates, which shows up as a curved scuff mark on the surface rather than a clean entry.
3. No Self-Centering Geometry at the Point
A standard conical point has no built-in centering feature of its own — it relies entirely on grinding symmetry to enter straight. A split point, by contrast, adds a secondary grind at the chisel edge that turns what was a purely extruding surface into a short section of actual cutting edge, substantially shortening the chisel edge and reducing the drill's tendency to walk. We cover this in more depth in the next article in this cluster.
4. Workpiece Surface Condition
Even a perfectly symmetrical drill point can walk if the surface it meets is not. A sloped or curved surface, mill scale, an oxide layer, or an uneven coating all create unequal starting resistance between the two lips, independent of anything in the drill itself. This is an application condition, not a quality issue with the bit.
5. Speed and Rigidity Mismatch
Excessive spindle speed, feed rates set too low for the entry condition, or insufficient rigidity in the machine or workholding can all introduce slight radial vibration at the moment of contact. Vibration on its own rarely causes walking — but it amplifies an asymmetry that would otherwise have been minor, turning a small grinding tolerance into a visible, repeatable problem.
What Walking Costs the Buyer
Walking is easy to dismiss as cosmetic — a scuff mark, a moment of hesitation before the bit catches. In production, it has three concrete costs.
• Hole position error. Once a bit walks before catching, the finished hole is no longer where the print called for. On a single hole this may fall within tolerance. On a bolt-hole pattern, a jig, or a bracket that has to mate with a second part, the same small offset compounds across every hole in the pattern and can take an otherwise good part out of tolerance entirely.
• Oversized and out-of-round holes. A bit that walks before it catches does not cut a clean circular path from the first instant — it drifts, then corrects, then cuts. The finished hole ends up larger than the drill's nominal diameter and measurably out of round, which matters directly on any application with a fit tolerance: bushings, dowel pins, reamed holes, or anything going in with a press or slip fit.
• Assembly and rework costs. Fasteners that don't sit square, bolt patterns that don't line up between mating parts, and holes that need to be re-drilled oversized to correct position — all of these turn a drilling-stage problem into an assembly-stage cost, which is always more expensive to absorb than catching it at the drill.
On thin sheet metal and hand-held drilling in particular, walking also leaves a visible scar on the surface around the hole — a cosmetic defect on parts where appearance matters, on top of the dimensional one.
How Buyers Can Tell Which Cause They're Dealing With
Walking looks the same regardless of cause, but where it shows up — and under what conditions — usually points to where the problem actually sits.
• Multiple bits from the same batch walk, with no change in how they're being run. This points toward grinding symmetry or chisel-edge centering, and is worth raising with the supplier along with a request for point-angle inspection data.
• Walking only happens without a center punch mark or pilot hole. This is more likely an operating-condition issue than a drill defect. A standard twist drill point relies on grinding symmetry to center itself, and some walking without a guide is expected behavior, not a sign of a defective bit.
• Walking only occurs on angled, curved, or scaled surfaces, and not on flat, clean stock. This points to the workpiece surface, not the drill geometry.
• The same drill bit walks noticeably more at one spindle speed than another. This suggests vibration or rigidity is amplifying a small asymmetry, and is worth checking against workholding and speed settings before assuming the bit itself is at fault.
The distinction matters because the fix is different in each case. A grinding or centering issue is a manufacturing problem that calls for a conversation with the supplier. An operating-condition issue calls for a center punch, a pilot hole, or a speed adjustment — no amount of returned drill bits will fix it. Treating the two as the same problem usually means the actual cause never gets addressed.
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-13-2026