Screw Drive Types, and Why Some Slip

A screw drive is the shaped recess in the head that the bit engages, and the difference between them is the difference between a driver that stays put and one that spins out and destroys the head halfway in.

Most people treat a chewed screw head as their own fault. Frequently it is the drive doing exactly what it was designed to do.

Cam-out is a feature, not a defect

The Phillips cross has tapered, angled flanks. Past a certain torque, the driver rides up and out of the recess rather than continuing to turn — the effect called cam-out.

This was deliberate. On assembly lines with early powered drivers and no reliable clutch, a drive that ejected itself at a roughly consistent torque was a crude way of preventing overdriving and snapped screws. The self-limiting behaviour was the point.

Modern drivers have adjustable clutches, so the problem cam-out solved no longer exists — but the drive is still everywhere, and now it simply damages heads.

What each screw drive is actually for

Slotted is the oldest and the worst for driven work. The bit slides out sideways along the slot. It survives on visible hardware where it looks right, and nowhere else.

Phillips self-centres, which made it fast on a production line, and cams out as described. Still the default on general hardware.

Pozidriv looks like a Phillips with extra small ticks between the arms. The flanks are parallel rather than tapered, so it does not cam out. Crucially, Phillips and Pozidriv bits are not interchangeable — using one on the other engages only partially and destroys both. The tick marks on the head are how you tell.

Square grips well, holds the screw on the bit without a magnet, and resists cam-out completely. Common in North American cabinetry and deck screws.

Torx uses six lobes with parallel walls, spreading load over more bearing surface than any of the above. It transmits the most torque with the least wear on both bit and head, which is why it dominates structural screws, decking screws and anything long enough to need real driving force.

Combination heads accept more than one bit type. Convenient, and typically a compromise on each individual drive.

Security and tamper-resistant patterns — pins in the centre of a Torx recess, three-lobed and five-lobed shapes — exist to require a specific bit rather than to transmit more torque. They turn up on public fixtures, appliances and electronics, and the only thing to know is which one you are looking at before buying a set.

Worth noting that head shape and screw drive are independent choices. A flat, round, pan or washer head describes how the screw seats against the surface; the drive describes how torque gets into it. Any combination is possible, and confusing the two is why people ask for "a Torx screw" when what they need is a particular head profile that happens to be sold with one.

What actually destroys a head

Cam-out gets the blame and is only part of it.

The wrong bit size. A slightly undersized bit contacts only the bottom of the recess and rounds it out immediately. Bit sizes are numbered for a reason, and matching the size matters more than matching the type.

A worn bit. Bits are consumables. A rounded bit damages every screw it touches, and people persist with them far longer than they should because the failure looks like a screw problem.

Not enough axial pressure. Cam-out is resisted by pushing the driver into the screw. Most stripped Phillips heads come from someone letting the driver float while the trigger is down.

Too much clutch or too much speed. Driving fast into hard timber generates torque faster than the recess can transmit it. Slower and firmer strips fewer heads.

Skipping the pilot hole. Most excessive torque is the screw fighting timber that should have been drilled — the pilot hole guide covers when that is genuinely required.

What to buy

Prefer Torx or square for anything structural or long. Deck screws, structural screws and cabinet screws are available in both, and the difference in frustration over a large project is substantial. This matters most on work with high fastener counts, like a deck or a stud wall.

Buy bits in quantity and replace them early. They cost little relative to the time lost to a chewed head, and an impact-rated bit lasts considerably longer in an impact driver.

Match bit to recess exactly, including telling Phillips from Pozidriv before driving.

Use the clutch. Set it to stop at the depth you want rather than driving by feel, which also prevents overdriving — the failure the nails vs screws guide covers, where sinking the head too deep destroys the fibres doing the holding.

Keep a screw extractor for the ones that go wrong anyway.

Getting a stripped one out

When a head is already rounded, in rough order of how much damage each does:

Press a slightly larger bit in hard and try once more. Put a wide rubber band between bit and head to fill the gap. Cut a slot across the head with a rotary tool and use a flat driver. Use a purpose-made extractor bit, which cuts a reverse thread and backs the screw out. As a last resort, drill the head off, remove the component, and grip the exposed shank with pliers.

The general lesson is that most of this is avoidable upstream. A correctly sized, unworn bit in a drive that was never designed to eject it, driven with firm pressure into a pilot hole, does not strip — and choosing the drive type deliberately rather than accepting whatever came in the box removes most of the problem before it starts.