the form of balls 13 that locate permanently in the recesses 10 and engage the surface of an annular holder 14 maintained in contact with the face 15 of a mode change control member 16 of generally tubular form by a helical spring 17 with the above described components in the positions shown in fig1 with the bit being pressed against a work piece the beat price 7 is in use cyclically impacted by the nose 18 of the ram 5 as the latter is reciprocated in the hollow piston 6 and those impacts are transferred by the beat piece 7 to the bit at the same time the sleeve 1 is rotated about its longitudinal axis by the drive transmitted to gear 3 and this rotation is transferred to the bit by the driving elements 9 the hollow piston 6 is driven by a crank 20 rotated via gearing by the motor 4 the piston 6 has a stepped bore the inner portion 21 see particularly fig2 a b and c of the bore being narrower than the outer portion 22 and the rear end of the ram 5 has a seal 23 which seals against the inner portion 21 of the bore but not against the outer portion 22 in the hammer mode of the drill even when the ram is in its most forward position the position shown in.