Do I need impact-rated driver bits, and why do bits wear out so fast?

A driver bit is a consumable that most people treat as permanent, and the failure is invisible: the tip's sharp corners round over by a fraction of a millimetre, the contact area drops, and the pressure on the remaining metal rises until the screw head gives way. By the time a bit looks obviously damaged it has already ruined several screws.

Step by step

1. Understand what impact rating means.An ordinary bit is hardened all through so it resists wear and snaps under shock. An impact bit is deliberately a little softer, and often has a narrowed torsion zone in the shank that twists elastically under each hammer blow and returns.
2. Use impact bits in an impact driver, always.The tool delivers thousands of shock loads a minute. A standard bit fractures, often at the tip inside the screw head, and a snapped tip in a recess is a much worse problem than a stripped screw.
3. Know that the reverse is fine.Impact bits work perfectly well in an ordinary drill. If you own one set, make it the impact-rated one.
4. Recognise cam-out as the wear mechanism.Every time the bit climbs out of the recess under load, the corners of the tip abrade against hardened screw steel. Reducing cam-out is the same thing as extending bit life.
5. Push hard along the axis.Most cam-out is insufficient axial force. Firm pressure straight down the line of the screw keeps the bit fully seated, which spreads the load over the whole flank instead of the tips.
6. Keep the driver square.Any angle concentrates all the torque on one side of the recess. It rounds both the bit and the screw, and it is the most common cause of both.
7. Inspect the tip against a new one.Hold a used bit next to an unused one of the same type. The wear is at the corners and it is obvious in comparison and invisible in isolation. When the corners are visibly rounded, throw it away.
8. Match the size exactly.A PZ1 in a PZ2 screw, or a Torx one size down, destroys both. This is a size problem before it is a wear problem.
9. Buy the tip type you use in quantity.Bits cost very little compared with a rounded screw in finished work. Ten of the size you actually use beats a 100-piece set of sizes you never touch.

Tips

• The tip of a driver bit is the part that wears; the length is irrelevant. A 25mm bit and a 50mm bit wear at the same rate, so buy the length that suits the space.
• Bits with a diamond-dust or ribbed coating on the tip grip the recess better and slow cam-out significantly, at a modest premium. They are worth it for long screws.
• If you are stripping screw heads regularly and the bits are new, the problem is pilot holes, technique or the wrong screw for the material rather than the bit.

Common mistakes

• Using a cheap ordinary bit in an impact driver — It shatters. A tip broken off inside a screw recess means drilling the screw out, which is far more expensive than a proper bit.
• Carrying on with a bit that is starting to slip — The first slip damages the screw head; the second ruins it. Change the bit at the first slip, not the fifth.
• Assuming a magnetic bit holder makes a worn bit work — The magnet holds the screw on. It does nothing about the fit between the bit's tip and the recess.

Questions people ask