ISO 2541:2016 specifies the dimensions of centre drills for centre holes with radius ? Type R. It covers only metric dimensions, regarded as the only recommended dimensions in the future for this type of drills. The flutes can be straight or spiral at the option of manufacturer. Unless otherwise stated, these drills are right-hand cutting. Annex A gives the recommended dimensions for the centre holes, Type R, which can be obtained by a rational use of the centre drills listed in this International Standard.