Why will my screw not pull the two pieces of wood together?

This is the single most common woodworking frustration there is, and it is almost never a matter of not tightening hard enough. Tightening harder into a jacked joint only strips the thread or snaps the screw. The diagram shows the mechanism, which is worth understanding once because it then explains a dozen other things — why partially threaded screws exist, why pocket screws are shaped as they are, and why hinges sometimes will not pull up.

What you'll need

• A drill bit matching the screw's outer thread diameter
• A clamp or a cramp
• Partially threaded screws, if you have them (optional)

Step by step

1. Confirm the cause by looking at the screw.Pull one out. If the thread runs all the way to the head, that is your answer. If there is a plain shank under the head, look at whether it is long enough to pass right through the top piece — a short plain section on a thick top board behaves exactly like a fully threaded screw.
2. Back the screw out fully.Do not try to force it through. The joint cannot close while thread is engaged in both pieces.
3. Drill a clearance hole through the top piece only.Use a bit the size of the screw's outer thread diameter, so the thread passes through without touching. Drill through the top piece and stop at the joint line.
4. Clamp the two pieces together before you drive the screw.The screw's job is then to hold a joint that is already closed, which is a far smaller task than closing one. This alone fixes many stubborn joints.
5. Drive the screw again and it will pull up.The head now bears on the top piece and the thread grips only the bottom one, which is the whole mechanism.
6. Check the hole is not blocked if it still will not close.A pilot hole full of shavings, or a screw bottoming out on solid wood at the end of a too-shallow pilot, gives exactly the same symptom for a different reason. Drill the pilot deeper.
7. Check for something trapped between the faces.A shaving, a proud dowel, a blob of dried glue, a lump of filler. If the joint feels solid rather than springy as you tighten, something is holding it apart mechanically.

Tips

• Buy partially threaded screws for anything where two pieces have to come together, and fully threaded ones for fixing sheet material down where clamping does not apply.
• In hardwood you need both holes: a clearance hole through the top piece and a pilot in the bottom one. In softwood you can often get away with the clearance hole alone.
• Pocket-hole screws work on the same principle — a long plain shank and a wide head — which is why they close a joint so decisively.

Common mistakes

• Tightening harder to close the gap — The forces are working against each other, so extra torque goes into stripping the hole or shearing the screw rather than into closing anything.
• Adding a second screw next to the first — It has the same problem and adds a second jacking point. The joint gets worse, not better.

If it doesn't work