The economics of a workshop change the moment you start fixing tools instead of replacing them. A drill, sander or grinder that costs £120 new can usually be brought back to working order for under £15 in parts. The list of faults that account for most failures is short, and almost every one of them is a bench job with a screwdriver and a multimeter.
The biggest single category is worn carbon brushes. Brushes are designed to wear; once they are below about a third of original length, the motor stops running cleanly. Symptoms are sparking, loss of power and intermittent running. Replacement is fifteen minutes on most tools and the direct-fit ranges are at Makita carbon brushes, DeWalt carbon brushes, Bosch carbon brushes and Black & Decker carbon brushes.
Second is bearings. A whining tool with a slight wobble at the chuck or disc has a bearing that has lost its grease. Ball bearings on power tools are standard sealed types specified by inner diameter, outer diameter and width — numbers usually printed on the steel seal. See bearings and mechanical components for direct equivalents. Third is the trigger switch. A drill that runs only when the trigger is squeezed just so, or that has lost its variable-speed feel, almost always has a worn switch contact rather than a battery or motor fault.
Fourth is drive belts on hand planers and sanders. A planer that loses its bite for no obvious reason has usually shed teeth from a worn belt; see planer drive belts for direct fitments. Fifth is filters and bags on dust extraction. A vacuum or extractor that has “lost suction” almost always just needs a fresh vacuum bag or filter.
The kit list for almost all of these is small: a set of Pozi and Torx drivers, a 17mm spanner, a multimeter for switch checks, and a pair of long-nose pliers for brush springs. The single best investment beyond that is a spare set of brushes for whichever tool you use most. The tool parts hub covers everything else.