Drilling Clean Holes in Wood: Bits, Backers, Pilots

Most holes in woodworking are drilled with whatever bit was in the case. That works until the hole shows, or has to be flat-bottomed, or comes out the back of a visible face with a ring of torn fibres around it. Bit choice is not fussiness. Each type solves a different problem and none of them solves all of them.

Which Bit for Which Hole

The brad point is the default for woodworking and the twist bit is the default in most homes, which is why so many holes have ragged edges. The centre spur locates the hole before cutting begins, and the two raised outer spurs score a circle through the surface fibres before the main cutters lift them out. That scoring is what separates a clean rim from a torn one.

Forstner bits are guided by their rim, not by a point, so they can start on an edge, overlap an existing hole, or enter at an angle. That same rim needs support, which is why a Forstner bit belongs in a drill press or a guide rather than in a freehand drill where it can tip and grab.

Stopping Blowout

Wood fibres at the exit of a hole have nothing behind them. The bit pushes them out rather than cutting them, and they break away in a ring.

The fix is support. Clamp a flat offcut hard against the exit face and drill straight through into it. The fibres are held by the backer and cut cleanly. The backer is consumable, so keep a piece near the drill press and replace it when it becomes a colander.

Where a backer is impossible, drill from the show face until the tip of the bit just breaks the back, then stop, flip the board, and complete the hole from the other side using the small exit as the centre mark. Both faces come out clean and the join happens inside the board.

A brad point, a Forstner or a sharp auger all exit more cleanly than a spade bit, which relies on the point emerging first and then tears its way out.

Pilot Holes

A pilot hole does two jobs at once: it gives the thread wood to bite into and it stops the shank from acting as a wedge.

Size the pilot to the screw’s shank diameter, measured across the solid core rather than across the threads. The thread then cuts into the surrounding wood and the shank passes freely. A pilot that matches the thread diameter leaves nothing for the thread to hold. No pilot at all means the shank splits the wood along the grain, which is exactly what happens near an end.

Hardwood needs a pilot the full depth of the screw. Softwood often only needs one through the top piece. Near the end of a board, where splitting is likeliest, drill the pilot regardless of species.

Where two pieces are being joined, drill a clearance hole through the top piece at the full screw diameter, so the screw passes through without gripping. Without it the screw threads into both pieces and holds them apart rather than pulling them together.

Countersink so the head seats flush. A flat head driven hard into an un-countersunk surface compresses the wood around it and can split a thin part.

Depth and Clearing

Deep holes need the bit withdrawn periodically to clear the flutes. Chips that cannot escape pack into the flutes, rub against the walls of the hole, and burn. Burnished chips also make the bit work harder and heat the cutting edges.

On a drill press, use a depth stop for repeated depths and set it from a test hole in scrap. A stop set from the chuck rather than from a cut is usually wrong by the amount the bit has crept.

For blind holes with a flat bottom, a Forstner bit is the only bit that produces a genuinely flat floor. A spade bit leaves a point, and a twist bit leaves a cone.

Drilling at an Angle

An angled hole wants to skate on entry because the bit contacts one side first. Start it perpendicular to the surface until the cutters are engaged, then tilt to the intended angle. On a drill press, tilt the table and keep the bit vertical instead.

A drilling guide block, made by boring a hole at the desired angle through a piece of scrap, is more reliable than any freehand attempt and can be made in a minute.

Speed

Large diameters need slower speeds. The outside edge of a wide bit travels much further per revolution than the edge of a small one, so at the same spindle speed it is moving faster and generating more heat. Burnt walls and a scorched bit are the visible result.

Small twist bits tolerate high speed. Large Forstner bits and hole saws do not. Where the drill press has a speed chart, it is worth reading once and remembering the shape of it: bigger means slower.

What People Get Wrong About This

The commonest mistake is using a twist bit for everything. It is the bit that comes in the set, and it is ground for steel. In wood it wanders on entry and tears on exit.

The second is drilling a pilot sized to the thread instead of the shank. The screw spins without pulling, then strips, and the joint is worse than if there were no pilot at all.

A third is omitting the clearance hole through the top workpiece. Two pieces held apart by a screw that has threaded into both is a joint with a gap, and no amount of extra tightening closes it.

A fourth is drilling into an unsupported exit face and blaming the bit for the torn ring around the hole. Any bit does that without a backer.

And the last is running a large bit at full speed because the drill has one trigger position that feels normal. Heat dulls carbide and scorches wood, and neither is recoverable.

Keeping Bits Cutting

A dull bit needs more feed pressure, which generates more heat, which dulls it faster. Brad point and Forstner bits can be touched up on their cutting edges with a fine file or slipstone, keeping the original angles.

Store bits so their edges do not knock together loose in a drawer. The spurs on a brad point are fine and a chipped spur leaves a torn rim from then on.

Where Holes Meet Joinery

Dowel joints live entirely on hole accuracy: two holes that are not aligned pull a joint out of square no matter how well the parts were milled. Pocket hole joinery is a drilling technique before it is a joint, and its jig exists to control exactly the angle and depth discussed here. For fastener selection once the holes are drilled, see screws and fasteners for wood.