The parts of a wood screw
A wood screw looks simple, but each part has a purpose. Understanding them explains why the way you drill matters so much.
Head. The head gives the driver something to turn and gives the screw something to bear against. A flat head is tapered so it can sit flush in a countersunk recess, covered in the article on countersinking screws. Pan, round and truss heads sit on the surface, and many have a flange or washer under the head to spread the load.
Drive recess. The shape cut into the head, such as Phillips, square or Torx, is what the bit grips. Different recess shapes are compared in the article on screw drive types.
Shank. The smooth, unthreaded section just beneath the head. Not every screw has one. Fully threaded screws have thread running up close to the head. A screw with a shank is designed to slide freely through the top piece, so the threads bite only into the bottom piece and the joint pulls tight.
Thread. The helical ridge that bites into the wood. A coarse, widely spaced thread grips softwood well and drives fast. A finer thread suits hardwood and thin or hard materials. The thread depth and spacing affect how much wood is engaged. This is related to, but different from, the pitch of a machine bolt thread, which is a separate subject.
Root or core. The solid cylinder at the center of the screw, beneath the thread. Its diameter is smaller than the outer thread diameter, and it is the part of the screw that carries the metal’s strength.
Point. The tip. Sharp points start quickly and can wedge wood fibers apart. Some screws have a self-drilling tip with a notch that cuts like a tiny drill bit, and some have a nibbed or cut thread that helps reduce splitting.
Screw sizes
Wood screws are sized by a gauge number for diameter, such as #6, #8 or #10, and a length in inches. A higher gauge number means a thicker screw. Lengths are measured differently depending on the head type: flat heads are measured from the top of the head to the tip, while round and pan heads are measured from under the head. Metric screws are labeled by diameter and length in millimeters.
A rough guideline for length is that the screw should extend into the receiving piece by about two-thirds of its length, or at least about the thickness of the top piece. For example, fastening a board that is 3/4 inch thick, using a 1-1/2 inch screw puts 3/4 inch of the screw into the base piece, which is half of the screw. A 2 inch screw puts 1-1/4 inches into the base. That is 62.5 percent, a bit nearer the two-thirds guide. These are hypothetical numbers for illustration, and the best length depends on the materials and the load.
Why wood splits
When a screw is driven into wood, the shank and threads must displace the wood fibers. The wood has nowhere to go, so it pushes sideways. If the board is narrow, dry, dense or close to an edge or end, it may split along the grain. Understanding wood grain helps explain why splits run along the length of the board and often start near the ends.
Hardwood is more prone to splitting, and also more likely to snap a screw. The difference between hardwood versus softwood is a major factor, though softwoods can split as well, especially thin pieces and ends.
What a pilot hole does
A pilot hole is a small hole drilled before the screw goes in. It removes some of the wood that would otherwise be pushed aside, so there is less pressure on the fibers. The screw threads still grip the surrounding wood, and they are guided straight.
Benefits include:
- Reduced splitting, especially near edges and ends.
- Less force needed, which means less chance of stripping the head or snapping the screw.
- Better alignment and more accurate placement.
- Easier driving in hardwood, with less strain on the driver or the battery.
Pilot hole versus clearance hole
Two different holes may be involved.
- The pilot hole goes into the piece that the screw threads will grip. It is sized so the threads can cut into the wood. A typical rule of thumb is to match the diameter of the screw’s inner core, or root. Softwood can take a slightly smaller hole than hardwood.
- The clearance hole goes through the top piece, so the screw can pass freely. It is sized about the same as the screw’s outer thread or shank diameter. With the top piece free to slide, the screw pulls the pieces together instead of holding them apart.
To check a pilot hole size, hold the drill bit up next to the screw. The bit should hide the core of the screw but let the thread show on either side. Test on a scrap of the same wood, because wood density varies. If the screw goes in with moderate effort and does not split, the hole is good. If it is hard to drive or the wood cracks, drill a bit larger. If the screw spins without gripping, the hole is too large.
Tapered drill bits made for specific screw sizes, which drill the pilot, clearance and countersink in one pass, are a convenience. The best bit for the job depends on hole size and the material, so see the article on drill bit types for more.
Steps for a clean screw joint
- Mark the position with a pencil or awl.
- Drill the clearance hole through the top piece, if used.
- Clamp the pieces together in position, so nothing shifts.
- Drill the pilot hole into the lower piece, to the depth of the screw or slightly less. A piece of tape on the bit marks the depth.
- Countersink if needed.
- Drive the screw with a driver that fits the head. Stop when the screw is snug. Overdriving strips the wood threads, which reduces holding power.
Soap or wax on the threads can ease driving in hardwood. Pilot holes for screws in the end grain of a board hold poorly, and screws near the end need extra care.
Safety
Clamp the workpiece, wear eye protection, and remove the battery or unplug the drill when changing bits. Dull bits overheat, so replace them. This is general information. Follow the guidance from your tool and screw manufacturer.