Why did my drill bit blow out the back of the wood?

Why the exit face always tears, and the three ways to stop it — backing board, drilling from both sides, and easing off at the end.

Difficulty: beginner
Time: 10 min
Read: 3 min

Short answer

The bit has nothing left to cut against as it exits, so it pushes the last fibres out instead of cutting them. Clamp a scrap board tightly behind the work, or drill until the point just breaks through and finish from the other side.

Breakout is not a technique failure so much as a physics one: at the exit face there is no material behind the fibres to hold them, so they lever out rather than shear.

Step by step

Clamp scrap timber tightly behind the work.The single most effective fix. The scrap supports the fibres so the bit keeps cutting rather than levering, and any tearing happens in the scrap.
Make sure the backing is in tight contact.A gap of even a millimetre lets the fibres flex and blow out anyway. Clamp it, do not just rest the work on it.
Or drill from both faces.Drill until the tip of the bit just shows through the back, stop, turn the work over and finish from that side using the small exit hole to centre the bit.
Ease off pressure near the end.Most breakout happens because full pressure is carried through the exit. Feel for the change and slow down.
Use a bit with a lead point.Brad point, Forstner and auger bits sever the rim fibres before removing the middle, which greatly reduces tearing. A standard twist bit is the worst offender in timber.
Reduce the speed for large holes.Big bits at high speed tear rather than cut, and the effect is worst at the exit.
Choose which face matters.If only one face is seen, put the good face up so any breakout happens on the hidden side — with a backing board you should get neither.

Tips

Any scrap works — MDF offcuts are ideal because they have no grain to tear.
In veneered board or laminate, always score the circle with a knife first as well as backing it.
A Forstner bit with a backing board leaves a hole clean enough on both faces to leave unfilled.

Common mistakes

Resting the work on a bench rather than clamping to scrap — The workpiece flexes away from the bench under drilling pressure, so the fibres are unsupported at exactly the moment they need support.
Using a spade bit at speed in thin material — A spade bit's flat cutters push rather than shear, so on thin stock it splits the exit face almost every time.

If it doesn't work

Large ragged chunk missing from the back face

Cause: No backing and full pressure at breakthrough — Fix: Cannot be undone. Fill and sand if it is hidden; otherwise remake the part with a backing board clamped on.

Splintering on both faces

Cause: Blunt bit or wrong bit type for timber — Fix: Use a brad point or Forstner bit, sharpen or replace, and reduce speed.

Laminate chipping around the hole

Cause: Fibres and surface film unsupported — Fix: Score the circle with a sharp knife first, apply masking tape over the area, back the work and drill slowly.

Backing board still allowed breakout

Cause: Gap between work and backing — Fix: Clamp them together firmly so the two surfaces are in solid contact.

Questions people ask

Does masking tape help?

A little, on laminates and veneers, by holding the surface film together. It does not stop true breakout in solid timber — only backing does that.

Why does it happen more in plywood?

The exit veneer is a thin cross-grained layer with nothing beneath it. It tears very readily, so plywood almost always needs a backing board.

Can I fix a blown-out hole?

Small tears can be glued back with the fragments if you kept them. Larger ones need filling and, if the face is visible, that repair will show under a clear finish.