What do the numbers on a drill clutch actually mean?

Almost every drill owner has spun the collar and concluded the numbers mean power. They do not. Behind the ring is a spring pressing a ball into a detent in the drive train, and the number sets how hard that spring pushes. When the resistance at the bit exceeds it, the ball jumps the detent, the chuck stops turning and you hear the clutch chatter. The diagram shows the mechanism and the two symbol positions beyond the numbers.

Step by step

1. Read the collar from low to high.The scale runs from the lowest setting round to the highest, then past the numbers to one or two symbols. More numbers on a drill does not mean more torque — it means finer steps across the same range.
2. Know what the drill symbol does.The position marked with a plain drill bit locks the clutch out entirely. The drill will apply everything the motor has and will never slip. Use it for drilling, and only for drilling.
3. Know what the hammer symbol does.It locks the clutch out as the drill symbol does and additionally engages the percussion mechanism. It is for masonry, and it is not a stronger version of the drill setting.
4. Accept that the numbers are arbitrary.Setting 8 on one drill is not setting 8 on another, and there is no unit behind them. Any chart telling you which number to use for which screw is guessing on your behalf.
5. Find the setting by testing, not by reading.Pick a middle-low number, drive a screw into an offcut of the same material, and see what happens. If the clutch slips before the head seats, go up one. If the screw disappears below the surface, come down two.
6. Reset it when the material changes.The right setting for softwood is wrong for chipboard, wrong for hardwood and badly wrong for plasterboard. The collar is meant to be adjusted constantly, which is why it is where your fingers already are.
7. Use a low setting for anything delicate.Hinges, handles, electrical accessories, brass screws, plastic fittings and anything into a small pilot hole. It is far easier to go up a number than to repair a stripped hole.
8. Go back to the numbers after drilling.The commonest sequence in the world is drill a pilot hole, leave the collar on the drill symbol, drive the screw, and bury it. Turning the collar back is a half-second habit worth building.

Tips

• The clutch is protecting the screw, the workpiece and your wrist, not the drill. A drill in the drill setting will happily twist hard enough to hurt you if a big bit grabs.
• If you have an impact driver, its lack of a clutch is why it strips small screws so easily. For anything delicate, the drill with its clutch set low is the better tool.
• The clutch does nothing when you are drilling, because a bit does not generate the sudden torque spike that a seating screw head does. That is why the drill setting exists at all.

Common mistakes

• Treating high numbers as more power for hard drilling — Above the top number the clutch is out entirely, so raising the number changes nothing about drilling. If a hole is going badly the problem is the bit, the speed or the gear.
• Driving every screw in the drill symbol position — Nothing stops the drill when the head seats, so it keeps turning: heads round out, screws snap, plasterboard tears and hinges strip.

Questions people ask