Device for drilling borehole undercuts

Device for drilling an undercut section in a borehole in a target material comprising a housing (10, 12) arranged to abut an outer surface of a target material containing the borehole, an axially extending bore extending through said housing, a guide sleeve (13) mounted in said bore in said housing and having an eccentric bore in axially parallel relation with said housing bore, an eccentric bushing (14) positioned within said eccentric bore in said guide sleeve and rotationally displaceable through an angle of approximately 180.degree. relative to said guide sleeve, a drive shaft (15) having a cutter head (28) thereon rotatably supported in said eccentric bushing, a driving unit coupled to said drive shaft, wherein the improvement comprises blocking means (25) disposed between said housing and said guide sleeve for permitting rotation of said guide sleeve relative to said housing in only one rotational direction.