Perfect Pilot Holes – How to Match Drill Bit Size to Woodscrew

Most of the problems related to driving screws have nothing to do with the screw itself, but everything to do with the pilot hole – or in some cases, the complete lack of a pilot hole. As tempting as it might be to muscle a woodscrew into a bare board, just remember that most wood screws are too large to be forced into a board without some type of path to lead the way. I know this extra step is tempting to ignore, but drilling a properly-sized pilot hole first can mean the difference between your wood project being a success—or a project disaster.

What Size Pilot Hole Should I Drill?

As a general rule, a pilot hole should be the same diameter as the root of the screw (the center core just below the threads). This allows the bulk of a screw to enter a board without splitting the grain, yet still allow the threads to do their work of pulling two boards together to form a joint. The most common mistake people make in drilling pilot holes is to make the hole too small, believing that the more snug the woodscrew, the stronger the joint. Not so, necessarily. Keep in mind that the real job of a woodscrew is to pull boards together just long enough to complete whatever joinery technique you’re using for that project. If we’re talking about a glued joint, that means the primary job of a woodscrew is to act like a clamp — bringing two boards together just long enough to make the surfaces bond to each other and the glue to dry. For joints without glue, the job of a woodscrew is to align the project pieces in their rightful place, allowing the joint to work in tandem with other joints to support the overall structure. Interestingly enough, the woodscrew itself offers little value to the strength of a joint.

Countersink for the Perfect Fit

A pilot hole by itself might be fine for the threaded portion of the screw, but it doesn’t provide room for the head, which is quite a bit larger than the rest of the screw. A simple solution to this problem is to use a countersink bit, which creates both a pilot hole for the threads, and a larger hole for the head of the screw.

Straight or Tapered?

When shopping for a countersink bit, you’ll discover that most stores carry two basic styles: straight and tapered. The straight style works best with wood screws that have a narrow, straight shank (usually the same diameter as the threaded portion of the screw). The tapered style works best with woodscrews that have a wide, tapered shank (usually larger than the threaded portion of the screw).

Top 3 Problems Driving Screws

1.Stalled Screw

You’re probably already familiar with this frustrating scenario — the screw seems to be going in, but suddenly it stops short about halfway into the board. You try pushing harder, but end up stripping the head. Worse yet, you somehow manage to muscle the woodscrew into the wood, but you end up splitting the board.

What’s the Problem?
–No pilot hole. Most wood screws are just too large in diameter to drive into a board without some type of path for it to follow.

–Pilot hole too small. This is the most common mistake made in drilling pilot holes. Fortunately, it’s an easy fix.

The EZ Fix

First, put your drill in reverse and carefully back the screw out of the board. In most cases, it’s the shank of the screw that’s being stubborn—which can easily fixed by using a tapered countersink bit to make a new, larger-size pilot hole. However, test the new pilot hole on a piece of scrap wood first, just to make sure it’s the right fit for the woodscrew.

2. Over-Drive

This is an easy mistake to make with soft wood like pine. Even with a properly-sized pilot hole, it’s still possible to make a mess of your project by sending a wood screw so far into the board that it breaks through the other side.

What’s the Problem?

–Pilot Hole Too Large. Too much room in a pilot hole can easily send your woodscrew careening through the board and breaking through to the other side.

–Drill Torque Set Too High. Even with a properly-sized pilot hole, your can still send a woodscrew too far into a board by unleashing too much power from your drill/driver

The EZ Fix

Aside from test-drilling pilot holes in scrap wood first (which I always recommend), the best way to avoid ruining a project from overdriving screw is to take advantage of the torque adjustment ring on your drill/driver. It’s one of the most useful (yet most ignored) features of the tool, and can precisely control just how much force you want to put behind the woodscrew. When set correctly, the torque adjustment allows you to carefully set screws just below the surface of the board – and no further.

3. Bridging

Sometimes a wood screw can end up forcing apart the two board surfaces you are trying to join. What’s worse, you might not even notice the problem till you’ve moved on to another part of the project. Don’t blame the woodscrew, though. The problem usually starts with the wood, not the hardware.

What’s the Problem?

–Boards are warped: Warped lumber can leave a gap in the joint, preventing the flush union we need for a solid joint between two boards. Woodscrews are strong, but they may not be strong enough to bring warped boards together.

–Pilot hole splinters block the joint: This is a very common problem that can obviously interfere with a solid, flush joint between two boards.

The EZ Fix

Depending on the severity of the warp, you might be able to first pull the boards together with a wood clamp, and then drive the screws. However, if your boards are noticeably warped to begin with, don’t use them for the parts of your project that involve joinery.

Pilot hole splinters can be cured by drilling countersink holes on the inside of the adjoining boards—which will effectively keep any wood splinters from blocking the joint.

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