A drill that refuses to spin, runs intermittently, or has lost its old bite is rarely beyond saving. In our experience, almost every dead drill that comes in for diagnosis fails in one of five predictable ways, and four of those are jobs you can do yourself at the bench.
The most common culprit by a wide margin is worn carbon brushes. A drill that powers up briefly then stops, runs only when held at a certain angle, or sparks heavily through the vents has brushes at the end of their working life. Replacement is fifteen minutes and a few pounds; left unfixed, the worn brush eventually scores the commutator and turns a small job into a motor rebuild. Direct fitments for the major brands sit under Makita, DeWalt, Bosch, Hitachi and HiKOKI and Black & Decker.
The second most common is a worn or jammed chuck. If the drill spins fine but won't grip the bit, the chuck jaws have either filled with debris or worn smooth. A few minutes of cleaning with compressed air and a drop of light oil restores most chucks; the rest can be replaced as a unit. The third is a failed switch — cordless drills with intermittent triggers usually have a worn switch rather than a battery fault. The fourth is bearings, which announce themselves with a rising whine and a slight wobble at the spindle; replacement bearings sit under bearings and mechanical components. The fifth, and only one usually worth a workshop visit, is a damaged armature, which normally follows brushes that were left too long.
Diagnosis is faster if you start by listening. A drill that hums but doesn't spin is usually electrical — brushes, switch or armature. A drill that runs but feels weak is mechanical — chuck or gear train. A drill that runs but whines is bearings.
Before ordering any spare, find the model number on the rating plate — brushes and bearings vary by a millimetre or two between models that look identical from the outside. With the model number in hand, the right part is usually a single click away from the tool parts hub.