A device for forming an undercut in a drill hole includes a drilling tool having a shank receivable in a chuck of a drilling machine and having a convex collar supported on a corresponding concave bearing surface of a bearing element of the device. The bearing element has a bore that communicates the bearing surface with an opening which communicates with a source of a lubricant liquid. The collar has at least one groove for conducting the liquid into the drill hole to the drill bit head of the tool.