A method for joining or machining, in particular a friction stir welding method, using an apparatus, in which a tool, in particular a friction stir welding tool, is driven via a spindle with a spindle axis. To achieve high process reliability, even when the method is applied in poorly accessible positions, an adapter, in particular an angle head, is arranged between the spindle and the tool so that the tool axis about which the tool rotates does not coincide with the spindle axis. A force acting on the spindle is measured and the method is controlled depending on the force measured at the spindle. The apparatus for joining or machining, in particular for friction stir welding, includes a headstock with a spindle, which can be rotated about a spindle axis and a tool, in particular a friction stir welding tool, which can be driven about a tool axis by the spindle.