Method performed by control device for changing drill bit in rock drilling unit, control device for rock drilling unit, rock drilling unit and rock drilling machine

The invention relates to a method performed by a control device (100) for replacing a drill bit (3) in a rock drilling unit (8), the drill bit (3) being configured to be connected to at least one drill rod (7) by a first threaded connection (40), the rock drilling unit (8) comprising: first feeding means (42) for feeding the at least one drill rod (7) and the drill bit (3) in an axial direction; a rotating device (44) configured to generate a rotational movement of the at least one drill rod (7) and the drill bit (3); percussion means (46) for providing percussion pulses on the at least one drill rod (7) and the drill bit (3); and a drill bit changer (1) for changing a drill bit (3) on at least one drill pipe (7), the method comprising: receiving (s101) information about changing the drill bit (3); controlling (s102) the percussion device (46) to cause the drill bit (3) to punch the bottom (48) of the formed bore (50) so as to release the first threaded connection (40) between the drill bit (3) and the at lea