Overview
This lecture explains the key differences between screw types and features, and provides practical advice on selecting and using screws effectively in woodworking projects.
Anatomy of a Screw
- A screw has four parts: tip, thread, shank, and head.
- The tip initiates cutting into the material and can influence splitting.
- Threads provide grip and holding power.
- The shank connects the threaded part to the head.
- The head allows the screw to be driven and provides a stopping point.
Tips and Splitting Prevention
- Standard screw tips risk splitting wood, especially in hardwoods.
- Self-drilling or serrated tips help reduce splitting and may eliminate pre-drilling in softwoods.
- Pre-drilling is still recommended, especially in hardwoods or for critical joints.
- Golden Rule: When in doubt, drill a pilot hole to prevent splitting.
Threads and Material Matching
- Coarse threads are best for softwoods; they grip spongy fibers well.
- Fine threads are ideal for hardwoods; they offer precision with less risk of splitting.
- Fine-thread wood screws are uncommon in stores; advanced coarse-thread screws with cutting tips are an alternative.
- Regardless of screw type, pilot holes provide consistent results.
Shank and Jacking Prevention
- Fully threaded screws can cause "jacking," keeping boards apart if not clamped.
- Partially threaded screws pull boards together tightly.
- "Ribs" on screws or pre-drilling a clearance hole in the top board help prevent jacking.
Head Types and Finishing
- Flathead screws sit flush with the wood surface for a clean look.
- Metal hat (truss head) screws are for attaching hardware or non-wood materials.
- Create a countersink with a bit for flush fit; some screws have built-in countersink ribs.
Drive Types
- Slotted: oldest, problematic with power tools.
- Phillips: improved but prone to cam-out (slipping).
- Square drive (Robertson): rarely slips, ideal for pocket holes.
- Star drive: best grip, rarely slips, preferred for most woodworking.
Screw Types and Uses
- Construction screws: strong, general purpose.
- Wood screws: softer metal, usually Phillips, less preferred.
- Cabinet screws: wide head for cabinets.
- Trim screws: small heads, easier to hide, may have reverse threads.
- Drywall screws: cheap but brittle—not recommended for structural woodworking.
- Gauge determines thickness: #6 (fine jobs), #8 (general), #10 (heavy-duty).
- Screw length should be at least 2/3 the thickness of the bottom board.
Key Terms & Definitions
- Splitting — cracking of wood caused by screw insertion.
- Jacking — boards being pushed apart by fully threaded screws.
- Pilot hole — a small pre-drilled hole for screw threads.
- Clearance hole — larger hole in top board for screw shank to pass through.
- Countersink — a conical recess for the screw head to sit flush.
- Cam-out — when a screwdriver slips out of the screw head.
- Gauge — the thickness of a screw.
Action Items / Next Steps
- Drill pilot holes in hardwoods or for critical joints to prevent splitting.
- Use a countersink bit or screws with built-in ribs for a flush finish.
- Select screws based on wood type, thread design, and appropriate gauge/length.
- Review pilot hole size charts as needed.