How do I drill a deep hole without the bit binding or wandering?

Why long holes jam and curve, and the peck-drilling, pilot and alignment routine that keeps them straight.

Difficulty: intermediate
Time: 20 min
Read: 3 min

Short answer

Drill in short bites, withdrawing fully each time to clear the flutes. Waste that cannot escape is what binds a deep hole, and a bit under load flexes, which is what curves it. A pilot hole and a guide block fix the direction; pecking fixes the binding.

Every drill bit is designed to lift waste up its flutes and out. Once the hole is deeper than about five times the bit diameter, that stops working reliably and the hole begins to fight you.

Step by step

Choose a bit long enough with margin.A bit used at its absolute maximum depth has no flute left outside the hole to eject waste. Allow extra length.
Mark the depth on the bit.Masking tape wrapped round the bit is the simplest depth stop and tells you at a glance how far in you are.
Start with a short pilot.A shorter, stiffer bit of the same or smaller size establishes the direction accurately. A long bit started freehand wanders immediately and then follows its own error.
Get the alignment right in the first centimetre.Sight the drill against a square from two directions ninety degrees apart. Once the bit is in, the angle is fixed.
Peck-drill.In and out in short bites — perhaps a bit-diameter at a time in metal, more in softwood. Withdraw fully so the flutes empty rather than repack.
Reduce pressure as you go deeper.Long bits flex under load and a flexing bit cuts a curve. Steady light pressure and let the flutes clear.
Watch the waste.Fine powder instead of chips, or nothing coming out, means the flutes are packed. Withdraw and clear before continuing.
In masonry, blow or vacuum the hole periodically.Deep masonry holes fill with dust that cushions the hammer blow. Clearing it restores the cutting rate immediately.
Cool the bit on long runs.In metal, oil and pauses. In timber, resin and heat will glaze a bit and start scorching the hole walls.
Drill from both ends where accuracy matters.For a through hole in thick stock, mark both faces accurately and drill halfway from each side. Any wander shows as a small step in the middle rather than a hole in the wrong place.

Tips

Masking tape on the bit is a free depth gauge and a reminder to peck.
A short pilot bit sets the direction far better than a long bit ever will.
If a deep hole starts screeching, it is the flutes rubbing the walls, not the tip cutting. Withdraw and clear.

Common mistakes

Drilling straight to depth in one push — Waste packs into the flutes, friction rises, the bit heats and binds, and in metal it will seize and snap.
Correcting direction partway down — You cannot steer a bit once it is committed; trying to lever it straight only bends the bit and enlarges the mouth of the hole.

If it doesn't work

Bit seized in the hole

Cause: Packed flutes or a snatch at depth — Fix: Reverse it out gently if the drill has reverse. Never lever sideways — that snaps bits, and a snapped bit in a deep hole is very hard to recover.

Hole exits in the wrong place

Cause: Angle set at the start, then amplified by depth — Fix: Use a pilot, a guide block and two-direction sighting. On thick stock, drill halfway from each face.

Scorched hole walls in timber

Cause: Heat from a blunt bit or no waste clearance — Fix: Sharp bit, peck-drill, lower speed on large diameters.

Masonry hole stops progressing at depth

Cause: Dust packed at the bottom absorbing the hammer blow — Fix: Withdraw fully, vacuum or blow the hole out, then continue.

Questions people ask

What is an auger bit for?

Deep holes in timber. The screw point pulls the bit in and the single wide flute lifts waste efficiently, which is exactly what a long hole needs. Run it slowly.

How deep is too deep for a normal twist bit?

Roughly five times the diameter before waste clearance becomes the limiting factor. Beyond that, peck-drill or use a bit designed for depth.

Can I extend a bit?

Bit extensions exist and work, but every joint adds runout, so accuracy suffers. Use one where the hole is oversized anyway, not where it has to be precise.