How to correctly drill a pilot hole?
What is a Pilot Hole?
A pilot hole is a small hole drilled before screwing a screw into wood. It helps prevent surface lacquer cracking on the guitar and reduces the risk of wood splitting, especially with hard wood. A bonus is that the screw will be installed directly, following the path of the pilot hole.
What Diameter Should the Pilot Hole Have?
The goal is to have the screw threads embedded in the wood, outside the hole, see the attached image.
We recommend measuring the diameter of the screw's threaded part and using a drill bit for metal with a diameter ranging from 50% to 60% of the measured value.
For example, if the screw has a diameter of 2.9mm, the drill bit should have a diameter from 1.45mm to 1.74mm. We must consider the sizes of available drill bits, which in this case is a 1.5mm diameter drill bit. If we have a choice, always opt for a slightly smaller drill bit.
The most common diameters of needed drill bits for pilot holes in guitar making are 1mm, 1.2mm, 1.5mm, 2mm, and 3mm.
How Deep Should the Pilot Hole Be?
The recommended depth of the pilot hole is the length of the screw. We can mark the drill bit with adhesive tape, as illustrated in the picture, to avoid drilling unnecessarily deep.
Of course, we cannot have a screw that is too long, longer than the thickness of the wood. Ideally, the hole's depth should be at least 8-10mm shorter than the thickness of the wood. This will be a challenge with the head of the guitar, where the thickness of the wood is only 14 to 16mm. In this case, we need to adapt and drill deeper; the depth of the hole will be only 5mm shorter than the thickness of the wood.
Procedure for Drilling a Pilot Hole
- Use a pencil or a thin marker to mark the spot where you plan to place the screw.
- With a center punch (or at least a nail in an emergency), create a small indentation at the marked point. Without this, the drill bit would slide on the surface, and we would miss the center of the hole.
- Properly selected, mark the drill bit with adhesive tape to indicate the depth of the hole and secure it in the drill.
- Switch the drill to a higher speed but do not turn it on yet.
- Place the drill bit's tip into the indented mark on the wood.
- Make sure, by looking from the side, that you hold the drill vertically or at the desired angle for screw positioning.
- With a smooth motion, drill the pilot hole. Without stopping the drill, gently withdraw the drill bit.
Screwing Without Damaging the Screw Head
For small screws, it is better to use a manual precision screwdriver. It is crucial to use a high-quality screwdriver of the exact shape and size for the screw used. The screwdriver must have an undamaged tip; otherwise, there is a risk of damaging the appearance of the screw head or its surface finish, which is a significant issue with guitars.
To protect the surface, it is also recommended to place a thin cloth between the screwdriver and the screw head.