A screw holds by shearing material — so "stripped" can mean either side gave way
A screw does not grip by friction or by glue. It cuts a thread into the material, and it holds because a thin helical slice of that material sits on top of each thread flank and resists being torn off. Two things can break that arrangement, and they break on opposite sides of the joint. The recess in the head can wear round, so the driver no longer transmits torque — that is a failure of the tool interface. Or the thread in the material can be torn smooth, so the screw turns freely and carries nothing — that is a failure of the material.
The distinction matters because the repairs run in opposite directions. A worn head has to be persuaded outwards: more friction, a fresh slot, a reverse-threaded extractor. A stripped hole has to be rebuilt inwards: new material packed in, then a screw driven back into it. People who treat these as one problem reach for a bigger screw, which does nothing for a rounded head and actively destroys the remaining hole wall on a stripped one.
Why the head wears round in the first place
The correct ratio when driving a screw is roughly seven parts push to three parts turn. Most people invert it — light pressure, heavy rotation — and the bit climbs out of the recess instead of turning the screw. Each time it climbs, it shaves a little metal off the corners. That is why a screw that was fine on the way in refuses to come out a year later: nothing about it changed, except that the recess was quietly being rounded every time it was driven.
Three contributing causes, in order of how often they turn out to be the answer. The wrong bit size. Phillips comes in No.1, No.2 and No.3; household screws are overwhelmingly No.2, and using a No.2 bit on a No.1 screw is the single most common way heads get destroyed. Test it by seating the bit and rocking it sideways — any wobble means the size is wrong. Too little downward pressure, covered above. Too much speed or a crooked angle, which concentrates the wear on one corner and is easy to do with a clutchless drill.
The material decides whether a hole can be rebuilt at all
- Solid wood and plywood can be rebuilt. Pack the hole with toothpicks, matches or a glued-in dowel, wait for the glue to cure, and the screw has fresh fibres to cut into. Three things go wrong: filling the hole completely (the screw then has nowhere to enter and pushes the packing back out), driving it before the glue has cured (30 to 60 minutes minimum, and the glue fails silently), and packing into a hole still full of sawdust and old glue, which is what the new glue ends up sticking to.
- Particleboard and MDF barely can. Their internal bond is low by design, which is why flat-pack fittings strip so easily and why every extra turn makes it worse. Epoxy putty or a dowel helps a little; adding a metal corner bracket to carry the load helps a lot more.
- Plastic and metal cannot — not with wood methods. There are no fibres to bite. This is where the toothpick trick, which is genuinely excellent on timber, becomes actively harmful: it fills the hole with something softer than the material that just failed. Plastic takes a coarser screw one millimetre up in diameter, or epoxy putty drilled fresh. Metal takes a threaded insert, or a bolt right through with a nut on the far side.
The numbers worth remembering
Thread engagement should be at least two-thirds of the screw's length in the receiving piece — three-quarters in softwood — and only the thread actually sitting in that piece counts. Past about six to eight times the screw diameter, extra length adds almost nothing, because the load concentrates in the top few threads. Pilot holes are the other half: for a No.8 screw, roughly 2.0 mm in softwood and 2.4 mm in hardwood, and hardwood without a pilot hole will split rather than grip.