Drill and driver

Two holes, not one

A screwed joint in wood wants two different holes, and this is the thing that separates a joint that pulls tight from one that never does. The far board gets a pilot hole, narrower than the threads, so the screw cuts into it and grips. The near board gets a clearance hole, wide enough that the shank passes through freely and the threads bite nothing at all.

If the near board is pilot-drilled too, its threads hold it at whatever distance they first engaged, and the screw jacks the two boards apart instead of clamping them together. The gap is usually small enough to be invisible and large enough to matter.

How long a screw

The rule is that the screw should bury about two thirds of the receiving board's thickness — deep enough that the threads have real wood to hold, shallow enough that the tip is not about to break the far face. The length to buy is the near board's full thickness, which the threads do no work in, plus that two thirds.

For the joint a cabinet shop makes a hundred times a day — three quarter material into three quarter material — that comes out at a number worth memorising.

What it will hold

The wood matters far more than the screw. Withdrawal strength goes up with the square of the density of what the screw is in, so the same fastener is worth several times as much in oak as in pine — and no amount of choosing a better screw closes that gap.

These are ultimate loads from a test machine, on clear straight-grained wood, and they are not loads to build to. Design practice divides by five for a static load and by more for anything sustained or overhead.

Tapping, for jigs and fixtures

The moment a shop fixture needs to come apart repeatedly — a router plate, a bench dog, a T-track bolt — the hole sizes stop coming off the wood screw chart and start coming off a machinist's. The tap drill is the thread's major diameter less one pitch, and the clearance drill is the next sixty-fourth up.

The same two-hole rule applies and is even less forgiving: tap the far piece, clearance-drill the near one. For anything that gets adjusted often, a threaded insert outlasts a thread cut straight into wood, which is fibres and wears.

What this page does not tell you

• —The pilot hole table is a widely reprinted shop reference rather than a spec for one brand's thread form. Treat it as a starting point for the drill index, and open the hole up if a screw is fighting you in dense hardwood.
• —Withdrawal figures are for side grain only. End grain holds around three quarters of it at best and unreliably enough that the Wood Handbook advises against designing for it at all.
• —Nothing here covers a structural connection. That is the NDS, the fastener manufacturer's rated capacity, and an engineer.