DOWNHOLE CUTTING TOOL

1 . A rotary tool for milling tubing in a borehole, the rotary tool comprising:

at least one cutter with a cutter body and a cutting surface on the body,

wherein the cutter is shaped and positioned on the rotary tool such that at least part of the cutting surface is back raked relative to a direction of rotation so that the cutting surface cuts furthest into the tubing at an edge which is a trailing edge of the cutting surface relative to the direction of rotation, and

wherein at least part of the back raked cutting surface extends from the edge with a rake angle between the cutting surface and a perpendicular to the direction of rotation which is in a range from 15 to 60 degrees and, at the edge of the cutting surface, has an angle greater than 90 degrees included between the cutting surface and the surface of the cutter body following the cutting surface.

2 . The rotary tool of claim 1 wherein the at least part of the back raked cutting surface extends from the edge with a rake angle which is in a range from 20 to 50 degrees.

3 . The rotary tool of claim 1 wherein, at the edge, the surface of the cutter body following the at least part of the back raked cutting surface diverges from the cutting surface at an angle of between 0 and 15 degrees to the direction of rotation.

4 . The rotary tool of claim 1 wherein the at least part of the back raked cutting surface extends from the edge with a rake angle which is in a range from 20 to 60 degrees and has an angle of at least 100 degrees included between the at least part of the back raked cutting surface and the surface of the cutter body following the cutting surface.

5 . The rotary tool of claim 1 wherein the at least one cutter comprises a cutter body of a hard material.

6 . The rotary tool of claim 5 wherein the hard material has a Knoop hardness of 1800 or more.

7 . The rotary tool of claim 1 , further comprising:

a central structure for insertion axially into the tubing, and

at least one element which carries the at least one cutter and which projects or is extensible from the central structure to bring the at least one cutter into contact with the tubing.

8 . The rotary tool of claim 7 wherein the at least one element is configured to bring the at least one cutter into contact with an internal surface of the tubing to cut radially outwardly into the tubing.

9 . The rotary tool of claim 7 wherein the at least one element has the at least one cutter with the cutter body partially embedded therein and partially exposed, such that the embedded portion of the cutter body is of greater volume than the exposed portion.

10 . The rotary tool of claim 9 wherein the at least one cutter has an exposed front and a partially embedded thickness following the exposed front with an extent which is at least half the length of any dimension across the cutter body, perpendicular to the partially embedded thickness.

11 . The rotary tool of claim 7 wherein the rotary tool has a plurality of elements which each carry at least one cutter formed of hard material, which project or are extensible from the tool body and which are distributed azimuthally around a longitudinal axis of the rotary tool.

12 . A method of removing a length of tubing in a borehole, the method comprising:

inserting into the tubing a rotary tool for milling tubing in a borehole, the rotary tool comprising at least one cutter as defined in claim 1 , and

advancing the rotary tool axially while rotating the rotary tool with the at least one cutter cutting into the tubing completely around the tubing.

13 . The method of claim 12 wherein the at least one cutter is carried by at least one element configured to bring the at least one cutter into contact with an internal surface of the tubing, whereby the at least one cutter cuts radially outwardly into the internal surface completely around the tubing.