Drills wander, and a punch stops them

Why a bit walks across a surface, and the three ways to stop it.

A spinning drill bit meeting a flat or curved surface has nothing constraining its pointed tip from skating sideways before it manages to bite in, and a small dimple made in advance with a centre punch gives that tip a wall to push against instead of an open surface to slide across, which is why a punch mark stops a drill wandering in a way that careful aiming alone cannot.

What is actually happening

The tip of a standard twist drill bit is shaped like a shallow cone rather than a true point, and on a flat, smooth or slightly curved surface, that cone has nothing to grip until it has already cut its way in a short distance, since the material directly beneath it has not yet been removed enough to constrain the bit's position. Any small sideways force during that brief unconstrained moment, a slightly angled approach, a little flex in the bit, an uneven patch on the surface, pushes the spinning tip sideways rather than straight down, and because the tip is spinning fast and nothing is stopping the drift, that small sideways nudge can carry the whole hole noticeably away from where it was meant to start before the bit finally engages properly. Once the bit has cut deep enough to be surrounded by material on all sides, the wandering stops on its own, but by then the damage to the hole's position is already done. A hard, smooth surface makes the whole problem worse, since the tip needs to remove at least a little material to gain any purchase at all, and a surface that resists being cut in the first place gives the tip that much longer to skate around before it manages to start.

The dimple comparison

A marble or a ball bearing set down on a flat table rolls away readily at the lightest nudge, since nothing about a flat surface resists it moving in any direction. Set the same marble into a shallow dimple or saucer pressed into the same table, and the same light nudge barely moves it, because the marble would now have to climb the dimple's own wall to escape, and a small nudge simply is not enough force to do that. A centre punch mark does exactly the job of that dimple for a drill bit, giving its tip a small conical wall to push against from the very first instant it touches the surface, so any sideways nudge that would otherwise send the tip skating instead just presses it more firmly into the dimple already waiting for it.

The one number worth remembering

A punch mark only needs to be a shallow fraction of a millimetre deep to do its job, since the goal is never to remove material the way the drill itself will, it is only to give the spinning tip a wall to seat against instead of a flat, unconstrained surface to wander across, and even a light tap with a punch is usually enough to leave a mark that deep in most materials worked on a bench.

What follows from this

Three simple habits address the wandering problem from different angles. Making a punch mark before drilling gives the bit a starting point it physically cannot slide away from, which is the most reliable fix on its own. Starting the drill at a low speed and light pressure, only bringing it up to full speed once the tip has visibly seated into the material, gives the bit time to engage properly before it is spinning fast enough for a sideways nudge to carry it far. Starting with a smaller pilot bit and following up with progressively larger ones keeps the tip well constrained throughout, since a smaller bit wanders over a smaller distance to begin with, and each following bit is guided by the hole the previous one already cut rather than starting from an open surface at all. Used together, these three habits address wandering at every stage it could occur, the punch mark stopping it before drilling even starts, the slow initial speed stopping it in the first instant of contact, and the pilot hole stopping it from ever facing an unconstrained surface in the first place.