to rotate transverse to the drive shaft 5 the angle between the two shafts is expediently between 90 and 135 u00b0 110 u00b0 in the case shown the grinder shaft carries a bevel gear 9 whose teeth engage with those of the pinion 7 the grinder shaft 8 can in this way be driven in rotation by the drive shaft 5 it will be appreciated that this is just one example of the drive connection between the two shafts other gear types can be used for example a worm gear the gear itself or a cardan part connected to it can be configured with angle tolerance in order to permit adjustment of the angle between the shafts 5 and 8 a fixing means can be provided which allows the instrument to be locked in the chosen angle setting instead of this it is also possible to provide free angle movement a semispherical grinder tool 10 known per se and referred to in this description simply as a grinder is fitted onto the end of the grinder shaft 8 in a manner known per se and so as to be exchangeable to keep the size of the instrument head to a minimum the grinder is located close up to the housing 1 2 of the instrument head the distance of the center point 22 of the grinder or of the center point of its rear face from the intersection point 21 of the axes of the shafts 5 and 8 is smaller than the radius of the grinder preferably smaller than a quarter of the grinder diameter if the grinder is not semispherical in shape said diameter is replaced by the greatest diameter extending from the side of the instrument head 3.