Reaming tool as well as a head and a cutting insert therefor

A reaming insert for a reaming tool, comprising:

• four clearance surfaces
• wherein the reaming insert has a shape of a mirror-symmetrical polyhedron comprising six limiting surfaces, two opposite and mutually parallel ones of which are equidistantly separated from a central symmetry plane (SP1) and serve as chip surfaces that connect to limiting surfaces serving as clearance surfaces via alternately usable cutting edges,
• wherein the reaming insert has a rhombic cross-sectional shape in the symmetry plane (SP1),
• wherein the four clearance surfaces meet each other in pairs at obtuse angles (β) in two opposite corners, and which, along each individual chip surface, together with the same, form two cutting edges alternately usable by the indexing of the cutting insert, and
• wherein, in each one of the clearance surfaces, a cavity is countersunk that comprises an axial contact surface.

A rotationally symmetrical tool head for a reaming tool comprising:

• front and rear end surfaces;
• an envelope surface concentric with a centre axis (C);
• and
• a seat for the receipt of a cutting insert, the seat countersunk in the envelope surface and includes three support surfaces, one of which is a radial support surface and another one a tangential support surface,
• wherein the seat, in addition to said radial and tangential support surfaces, includes a slope surface that forms an obtuse angle with the radial support surface, and
• wherein the tool head includes axially extending evacuation channels in the envelop surface, and wherein the radial support surface transforms into one of the evacuation channels at a boundary line, the boundary line situated at a distance from the tangential support surface and running approximately parallel to the tangential support.