diameter corresponding substantially to the diameter d of a pre hole already resulting from a first operation the cylindrical portion 1f assures the axial guidance of the tool which by its upper truncated conical working portion le enlarges the hole to the final diameter a in the embodiment of the invention shown in fig7 tool 15 comprises a first front centering conical head 15a which is adapted to assure the centering of the tool in a conical pre hole formed in element 2 above this front centering head 15a tool 15 comprises an annular neck 15b containing a lubricating mass 16 for example a glass crown and above the neck 15b a working portion 15c of a truncated shape and whose major rear base has a diameter a corresponding to the final diameter which one desires to obtain for the hole at the working portion 15c a cylindrical portion 15d of a diameter a follows whose role is to avoid deviation of the hole in the course of its being made in the embodiment of the invention shown in fig8 tool 16 comprises a cylindrical head 16a followed by a cylindrical portion 16b of a smaller diameter defining a neck containing a lubricating mass 17 such as melted glass the head 16a preferably has in its frontal surface an annular cavity 16d centered on the axis and containing a small quantity of lubricant at the rear of the cylindrical portion 16b extends the truncated conical working portion 16c whose front minor base is connected to the cylindrical portion 16b and whose rear major base defines the final diameter a of the bored hole in the embodiment of the.