GB2270097A - Cutting element for use on a mill - Google Patents

Abstract

A mill, for cutting casing or tube down-hole comprises a body 1 which is to be incorporated into a drilling string. The body has radially outwardly directed blades 6. The leading face of each blade is dressed with cutting elements 7 of hard material. Each culling element has a rear face 8 secured to the blade, a lower face 9 and an upper face 10, the lower face and upper face being parallel and being inclined downwardly relative to the rear face. The culling element defines a front face 13 of recessed curved form, at least the lower part of the curve being such that the tangent to the curve makes an acute angle with the lower face of the insert. <IMAGE>

Description

DESCRIPTION OF INVENTION "IMPROVEMENTS IN OR RELATING TO A MILL AND TO A CUTTING ELEMENT FOR USE ON A MILL" THE PRESENT INVENTION relates to a mill and to a cutting element for use on a mill. More particularly, the present invention relates to a mill for use in cutting casing or tubing down-hole, for example in an oil-well or gas-well.

It has been proposed previously to provide many different -types of mill for use in milling, casing and tubing down-hole. A typical mill consists of a generally cylindrical body adapted to be mounted in a drilling string, the mill having a plurality of substantially radially outwardly directed blades. The blades may be vertical or may be inclined to the vertical. The lower edges of the blades are adapted to rest on top of a casing or tube which is to be milled. The mill is adapted to be rotated, typically in a clockwise direction as viewed from above. The leading face of each blade is provided with a "dressing" of a cutting material. Typically the cutting material is tungsten carbide. As the mill is rotated the casing or tube engaged by the blades is milled away - and the blades themselves also become worn away.

It has been proposed that tungsten carbide elements may be mounted in a matrix, and also it has been proposed that tungsten carbide "buttons" or "inserts" may be secured, for example, by brazing on to the leading face of each blade. Such carbide "inserts" or "buttons" are typically substantially circular discs of tungsten carbide material, sometimes having the front face of each disc dished to provide a "chip-breaking" effect.

When the mill is run down-hole, as the radially outwardly directed blades run across the top of a casing or tube, the tungsten carbide cutting elements cut into the upper edge of the casing or tubing, cutting off a strip or strand of the metal. This strip or strand is bent by the dished i shed shape present on the cutting elements and thus breaks off in short lengths.

The present invention seeks to provide an improved mill which has an improved milling performance. The invention also seeks to provide an improved cutting element for use on a mill.

According to one aspect of this invention there is provided a mill, for cutting casing or tube down-hole, the mill comprising a body having means to enable the body to be incorporated into a drilling string, the body presenting a plurality of substantially radially outwardly directed blades, the leading face of each blade being dressed with cutting elements of hard material, each cutting element having a rear face secured to the blade, a lower face and an upper face, the lower face and upper face being substantially parallel with each other, and being inclined downwardly relative to the perpendicular to the rear face, the cutting element defining a front face of recessed curved form, at least the lower part of the curve being such that the tangent to the curve makes an acute angle with the lower face of the cutting insert.

According to another aspect of this invention there is provided a cutting element adapted to be mounted on a mill, the cutting element having a rear face adapted to be secured to a blade, a lower face and an upper face, the lower face and upper face being substantially parallel with each other, and being inclined downwardly relative to the perpendicular to the rear face, the cutting element defining a front face of recessed curved form, at least the lower part of the curve being such that the tangent to the curve makes an acute angle with the lower face of the cutting insert.

Preferably the lower part of the front face extends at least substantially one-half of the distance between the lower face and the upper face.

In an alternative embodiment of the invention the lower part of the front face extends at least substantially two-thirds of the distance between the lower face and the upper face.

Preferably the upper part of the curve of the front face is such as the tangent thereto defines an acute angle with the upper face of the cutting element.

Conveniently the extent of the cutting element from the rear face of the cutting element is greater in the region of the lower face than in the region of the upper face.~~ .! Advantageously the cutting elements are mounted on the blade such that the upper face of the lower-most cutting element firmly abuts the lower face of the next adjacent cutting element and so on.

Preferably each cutting element is formed of a tungsten carbide material.

Advantageously the blades of the mill have a negative rake.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described, by way of example, with reference to the accompanying drawings in which FIGURE 1 is a side elevational view of a mill for cutting casing on tube down-hole provided with cutting elements in accordance with the invention, FIGURE 2 is an enlarged side view of part of the blade of Figure 1, showing some of the cutting elements, and FIGURE 3 is an enlarged side view illustrating a blade in a modified embodiment of the invention.

Referring initially to the drawings, a mill for cutting casing or tube down-hole comprises a substantially cylindrical body 1 which is provided at its upper end 2 with means to mount the mill on a drilling string. At its lower end 3 further connecting means may be provided for connecting other elements beneath the mill in the drilling string'c- The central body 1 of the mill provides an axially extending bore 4, as is conventional, to carry drilling mud or other hydraulic fluid.

On the exterior of the body 1, adjacent the lower end 3 thereof, is provided a plurality of stabiliser protrusions 5, each of part helical form.

At an intermediate position on the body 1 are a plurality of radially outwardly directed cutter blades 6.

In the illustrated embodiment of the invention four such cutter blades are provided. However, in alternative embodiments of the invention more blade may be provided.

It can be observed that the cutter blades are mounted on the body 1 in such a way that the plane of each cutter blade-makes an angle with the axis of the body 1. In the illustrated embodiment the angle is such that the blades have a so-called negative rake. Thus, the upper part of each blade lies in a position forward of the lower part of each blade in the direction of rotation of the mill.

The front face of each blade 6 is provided with a dressing of cutting elements 7 formed of tungsten carbide.

As thus far described the mill is conventional.

However, the cutting elements 7 of the described embodiment are of a very different configuration to the cutting elements utilised previously.

Referring now to Figure 2 the lower part of a blade 6 is illustrated, as from the side, and a plurality of the cutting elements 7 secured to the leading face of the blade 6 can be seen. Each cutting element is formed of a tungsten carbide material and presents a substantially planar rear face 8 adapted to be brazed to the front face of the blade 6. The cutting element 7 presents a lower face 9 and an upper face 10. The lower face and the upper face are substantially parallel to each other. However, the lower face makes an angle of approximately 950 with the rear face of the block and the upper face 10 makes an angle of approximately 850 with the rear face of the block.

Consequently, the upper and lower face do not project perpendicularly to the front face of the blade 6, but instead slightly project downwardly at an angle of approximately 50 from the perpendicular.

The lower face 9 terminates at a rounded forward nose 11 and the upper face 10 terminates at a rounded upper nose 12. The noses 11 and 12 are substantially equi-spaced from the rear face of the cutting element 7, with the lower nose 11 slightly forward of the upper nose 12. The front face 13 of the cutting element extends between the lower nose 11 and the upper nose 12 and is of a complex curved form. The face 13 comprises a lower portion, the target of which makes an angle of substantially 450 with the lower face 9 and an upper portion, the target of which makes an angle of approximately 300 with the upper face 10. The curve is of complex form and does not have a single centre of curvature. Thus the curve can be considered to be built up from a blend of curves which typically form a paraboloid or ellipsoidal shape.

The curve is thus of a form such that the tangent at any point on the lower part of the curve makes an acute angle with the lower face 9 of the cutting element. The lower part of the curve may advantageously extend over at least half of the height of the cutting element and preferably over approximately two-thirds of the height of the cutting element. The curve is also of such a form that the target at any point on the upper part of the curve defines an acute angle with the upper face of the cutting element.

Each cutting element 7 may be of any appropriate size, but in the preferred embodiment of the invention the vertical distance between the lower face 9 and the upper face 10 is of the order of 9.0 mm (0.353 inches) and the distance between the rear face and the lower nose 11, measured perpendicularly to the rear face, is of the order of 7.8 mm (0.308 inches). The upper nose 12 is spaced by approximately 7.5 mm (0.297 inches), measured perpendicularly, from the rear face 8.

It is to be appreciated that effectively the crosssection of the cutting element 7 has been described above, but the element may have any appropriate length. Thus, the length of the element may be chosen to extend across the entire width of the blade, but it is preferred that two or more elements are dimensioned to extend together across the width of the blade.

The cutting elements 7 are brazed or secured to the blade so that the upper face 10 of the lower-most cutting element firmly abuts the lower face 9 of the next adjacent cutting element, and so on.

It is to be appreciated that the recessed curved form of the front face 13 of each cutting element defines a leading edge adjacent the nose 11 which will attack the surface of the casing or tube to be milled and which will cut aggressively. The curved form also provides a chipbreaker effect. It is to be appreciated that as the mill is used, the lower part of the supporting blade 6, and the lower part of the lower-most tungsten cutting element will be worn away. During this procedure the successive points of the lower part of the recessed curved front face of the lower-most cutting element successively become the leading edge. Since the tangent to the curve, at all points in the lower part of the curve, makes an acute angle with the original lower face 9 of the cutting element, the tungsten carbide element presents an appropriately angled leading edge to maintain an aggressive cut.It is to be appreciated that, due to the angular inclination of the blade and due to the fact that the lower surface 9 and the upper surface 10 are inclined downwardly relative to the perpendicular to the blade, the nose 11 of the next adjacent cutting element 7 will start to attack the surface of the casing before the upper part of the curved front face of the lowest cutting element is fully worn away.

Thus, a significantly aggressive cut is maintained at all times.

It is intended that the nature of the curved front face 13 is such that the cuttings, instead of being relatively short and straight (this being the typical form of a cutting produced by a prior art mill with a "chipbreaker") will be shorter and will be curled or rolled. It is to be appreciated that a spiral or twisted cutting is not desirable since a number of cuttings of this type can tangle themselves together and form a so-called "birds nest".

Referring now to Figure 3 of the drawings, a blade 6' of a modified mill is illustrated, again provided with cutting insert 7 as described above, the blade 6' being aligned to be parallel with the vertical axis of the mill. It is preferred to utilise the blade with a negative rake as illustrated in Figure 2, since this provides an enhanced attack on the metal of the casing or tube, but a mill~fi-aving axially oriented blades, as shown in Figure 3, may still prove to be satisfactory.

Claims (18)

CLAIMS:

1. A mill, for cutting casing or tube down-hole, the mill comprising a body having means to enable the body to be incorporated into a drilling string, the body presenting a plurality of substantially radially outwardly directed blades, the leading face of each blade being dressed with cutting elements of hard material, each cutting element having a rear face secured to the blade, a lower face and an upper face, the lower face and upper face being substantially parallel with each other, and being inclined downwardly relative to the perpendicular to the rear face, the cutting element defining a front face of recessed curved form, at least the lower part of the curve being such that the tangent to the curve makes an acute angle with the lower face of the cutting insert.

2. A mill according to Claim 1 wherein the lower part of the front face extends at least substantially one-half of the distance between the lower face and the upper face.

3. A mill according to Claim 1 wherein the lower part of the front face extends at least substantially two-thirds of the distance between the lower face and the upper face.

4. -A mill according to any one of the preceding Claims wherein the upper part of the curve of the front face is such as the tangent thereto defines an acute angle with the upper face of the cutting element.

5. A mill according to any one of the preceding Claims wherein the extent of the cutting element from the rear face of the cutting element is greater in the region of the lower face than in the region of the upper face.

6. A mill according to any one of the preceding Claims wherein the cutting elements are mounted on the blade such that the upper face of the lower-most cutting element firmly abuts the lower face of the next adjacent cutting element and so on.

7. A mill according to any one of the preceding Claims wherein each cutting element is formed of a tungsten carbide material.

8. A mill according to any one of the preceding Claims wherein the blades of the mill have a negative rake.

9. A cutting element adapted to be mounted on a mill, the cutting element having a rear face adapted to be secured to a blade, a lower face and an upper face, the lower face and upper face being substantially parallel with each other, and being inclined downwardly relative to the perpendicular to the rear face, the cutting element defining a front face of recessed curved form, at least the lower part of the curve being such that the tangent to the curve makes an acute angle with the lower face of the cutting insert.

10. An element according to Claim 9 wherein the lower part of the front face extends at least substantially one half-öf the distance between the lower face and the upper face.

11. An element according to Claim 9 wherein the lower part of the front face extends at least substantially twothirds of the distance between the lower face and the upper face.

12. An element according to Claim 9, 10 or 11 wherein the upper part of the curve of the front face is such as the tangent thereto defines an acute angle with the upper ace of the cutting element.

13. An element according to any one of the Claims 9 to 12 wherein the extent of the cutting element from the rear face of the cutting element is greater in the region of the lower face than i the region of the upper face.

14. An element according to any one of Claims 9 to 13 wherein the element is formed of a tungsten carbide material.

15. A mill substantially as herein described with reference to and as shown in Figures 1 and 2 of the accompanying drawings.

16. A mill substantially as herein described with reference to and as shown in Figures 1 and 2 as modified by Figure 3 of the accompanying drawings.

17. A cutting element substantially as herein described with reference to Figures 1 to 3 of the accompanying drawings.

18. --- Any novel feature or combination of features disclosed herein.