Milling wellbore casing

1. A cutting insert for a milling tool, comprising:

a body including:

a back face configured to be coupled to a milling tool, the back face having a planar surface;

a contact face adjacent the back face, a cut-out portion of the body reducing a width of the contact face;

a cutting face opposite the back face and adjacent the contact face, a contact angle defined by the cutting face and the contact face being between 75° and 145°, the cut-out portion defining a cut-out width between the cutting face and a front face opposite the back face, the cut-out width being between 50% and 80% of a width of the body; and

a chip-breaking face adjacent the cutting face, a face angle defined by the cutting face and the chip-breaking face being between 75° and 130°.

2. The cutting insert of claim 1 , the cut-out portion being defined by a single cut-out in the body.

3. The cutting insert of claim 2 , the single cut-out being at least partially spherical.

4. The cutting insert of claim 1 , the cutting face and the chip-breaking face defining a continuous profile.

5. The cutting insert of claim 4 , the continuous profile having a variable radius of curvature.

6. The cutting insert of claim 1 , the contact angle being between 90° and 145°.

7. The cutting insert of claim 1 , the cut-out portion extending a partial height of the body, the cut-out portion forming a lip between the chip-breaking face and a top face.

8. The cutting insert of claim 1 , a line tangent to the cutting face being parallel to the back face, the cutting insert being continuous from the cutting face to the chip-breaking face, the cut-out portion defined by a single cut-out in the body located at the contact face, and the face angle being between 90° and 130°.

9. A cutting insert, comprising:

a body including an ultrahard material having a hardness greater than 80 Rockwell Hardness A (HRa) and including:

a back face; and

a cut-out portion, the cut-out portion defining:

a cutting face;

a chip-breaking face;

a cut-out width between the cutting face and a front face opposite the back face, the cut-out width being between 50% and 80% of a width of the cutting insert; and

a transition face between the cutting face and the chip-breaking face, the cutting face, chip-breaking face, and transition face defining a continuous partial elliptical or circular profile.

10. The cutting insert of claim 9 , the cutting face and the chip-breaking face being curved.

11. The cutting insert of claim 9 , the cutting face extending across an entire length of the body.

12. The cutting insert of claim 9 , the profile being elliptical along at least a portion of the length of the body.

13. The cutting insert of claim 9 , the cut-out portion further defining a cut-out height between a contact face and a top face opposite the cutting face, the cut-out height being between 50% and 80% of a height of the cutting insert.

14. The cutting insert of claim 9 , the cut-out portion forming a cutting edge where a contact face and the cutting face are joined.

15. A downhole milling tool, comprising:

a mill body configured to rotate within a wellbore;

a plurality of blades coupled to the mill body and which extend radially outwardly from the mill body;

one or more cutting inserts coupled to the plurality of blades, at least one of the one or more cutting inserts including a cutting insert body formed at least partially of an ultrahard material having a hardness greater than 80 Rockwell Hardness A (HRa), the cutting insert body including:

a cutting face having a cutting edge, the cutting face being oriented toward a direction of rotation of the plurality of blades;

a chip-breaking face, a least a portion of which is oriented at an angle between 75° and 130° relative to at least a portion of the cutting face, the chip-breaking face and the cutting face being defined by a cut-out in the cutting insert body, the cut-out extending a partial height and partial width of the cutting insert body, the cut-out reducing a thickness of the one or more cutting inserts at the contact face, the cut-out forming a lip between the chip-breaking face and a top face, the chip-breaking face and the cutting face being continuous;

a back face opposing the cutting face and coupled to at least one of the plurality of blades such that the cutting face is oriented toward a direction of rotation of the mill body and the chip-breaking face is oriented toward a downhole end portion of the mill body, the back face including a planar surface; and

a contact face adjacent the back face, a contact angle defined by the cutting face and the contact face being between 75° and 145°.

16. The downhole milling tool of claim 15 , the cutting face being at a relief angle of between 0° and 20° relative to a blade face of the blade.

17. The downhole milling tool of claim 15 , the at least one of the one or more cutting inserts being coupled to one of the plurality of blades at a negative rake angle.

18. The downhole milling tool of claim 15 , the cutting face extending axially upwardly from the cutting edge toward an uphole end portion of the mill body.

19. The downhole milling tool of claim 15 , the cut-out being defined by a single cut-out, and the single cut-out extending a height that is between 50% and 80% of the height of the cutting insert body.

20. The downhole milling tool of claim 15 , wherein:

the back face is rectangular;

the cutting insert body includes a single chip-breaking face;

the cut-out is defined by a single cut-out in the cutting insert body, the single cut-out having a width that is between 50% and 80% of a width of the cutting insert body and a height that is between 50% and 80% of a height of the cutting insert body;

the contact face is planar and has a width that is between 50% and 80% of the width of the cutting insert body; and

at an interface between the contact face and the cutting face, the cutting face is oriented at a negative rake angle that is between −2° and −20°.

21. The downhole milling tool of claim 15 , the cut-out defining a cut-out width between the cutting face and a front face opposite the back face, the cut-out width being between 50% and 80% of a width of the cutting insert.