A timber-working device (8) includes a frame (200), including a feed axis (212) having first and second sides. A drive system independently feeds stems along the feed axis, including a first rotary drive (316a) having a first wheel (206a) acting against a first stem on the first side, and a second rotary drive (316b) having a second wheel (206b) acting against a second stem on the second side. The device (8) includes at least two distance measurement devices (320a, 320b, 324), each side of the feed axis having an associated distance measurement device. Each generates measurement signals indicative of the length of stems fed along the feed axis, at least one of them indicative of rotation of one of the rotary drives. A processor (302, 304) receives the respective measurement signals, and for each of the first and second stems, determines the length of at least a portion of the stem fed by the drive system based at least in part on the respective measurement signals.
