Abstract: Where components are drilled by means of a robot a problem occurs when different size holes are needed to be drilled. The existing solution to this problem is to provide a drill unit (including drill bit, chuck and feed device) for each size of hole to be drilled and programme the robot to select the appropriate drill unit from the series of drill units supplied, perform its task, replace the drill unit in its store and select a further drill unit. Thus a complete drill unit is changed for every change in hole size. The presently described arrangement provides means to effect only drill bit change, the chuck and feed unit remaining on the robot at all times. The arrangement also provides means to effect such drill bit change while the chuck is rotating thereby obviating the need to stop and restart every time the drill bit is changed.