FIELD OF THE INVENTION

The present invention relates to a cutting tool and a cutting insert with rectangular shaped end surfaces having raised and lowered corners therefor, for use in metal cutting processes in general, and for milling operations in particular.

BACKGROUND OF THE INVENTION

Within the field of cutting tools used in milling operations, there are many examples of cutting inserts with rectangular shaped end surfaces having raised and lowered corners removably retained in an insert receiving pocket of a cutting body.

US 8,696,257 discloses a 'single-sided' cutting insert with rectangular shaped opposing end surfaces having an insert center line extending therethrough, with one end surface having diagonally opposed raised corners and diagonally opposed lowered corners, and both end surfaces having a concave shape in a cross-section taken in a plane containing the insert center line, as shown in Fig. 7 thereof.

US 8,876,440 discloses a 'double-sided' cutting insert with rectangular shaped opposing end surfaces, with both end surfaces having diagonally opposed raised corners and diagonally opposed lowered corners, and both end surfaces having a concave shape in a cross-section taken in a plane containing an imaginary axis extending through the end surfaces, as shown in Fig. 4 thereof.

US 9,186,732 discloses a 'double-sided' cutting insert with rectangular shaped opposing end surfaces, with both end surfaces having diagonally opposed raised corners and diagonally opposed lowered corners, and both end surfaces having a concave shape in a cross-section taken in a plane containing an imaginary axis extending through the end surfaces, as shown in Fig. 1G thereof. Also in a side view of the cutting insert, as shown in Fig. IB thereof, the two visible lowered corners are located on the same side of the said plane. It is an object of the present invention to provide an improved cutting insert. It is also an object of the present invention to provide an improved indexable cutting insert.

It is a further object of the present invention to provide an improved cutting insert which can be removably secured in a first insert receiving pocket of a cutting tool with a high level of stability.

It is yet a further object of the present invention to provide improved cutting insert which can be removably secured in a first insert receiving pocket of a milling tool, having an operative cutting edge configured to axially overlap with the operative cutting edge of axially adjacent second cutting insert.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a cutting insert comprising: non-identical, opposing front and back end surfaces, each of the front and back end surfaces having a rectangular shape in a respective end view thereof,

a peripheral side surface extending between the front and back end surfaces, the peripheral side surface including two opposing major side surfaces and two opposing minor side surfaces, a first axis intersecting the front and back end surfaces,

a first plane located between the two opposing minor side surfaces, the first plane containing the first axis,

a second axis perpendicular to the first plane and intersecting the two minor side surfaces, a second plane located between the two opposing major side surfaces, the second plane containing both the first axis and the second axis and being perpendicular to the first plane,

a median plane located between the front and back end surfaces, the median plane being perpendicular to the first axis, and

at least one front major cutting edge formed at the intersection of the front end surface and the two major side surfaces,

wherein:

the front end surface has two diagonally opposed front raised corners and two diagonally opposed front lowered corners with respect to the median plane, the back end surface has two diagonally opposed back raised corners and two diagonally opposed back lowered corners with respect to the median plane, and

in a cross-section taken in the first plane, the front end surface has a concave shape and the back end surface has a convex shape.

Also in accordance with the present invention, there is provided a cutting tool comprising a cutting body having a first insert receiving pocket provided with a seat surface and first and second support walls transverse to the seat surface; and

a cutting insert as described above removably secured in the first insert receiving pocket, wherein:

one of the insert's major side surfaces is in contact with the seat surface,

one of the insert's minor side surfaces is in contact with the first support wall, and the insert's back end surface is in contact with the second support wall.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, the invention will now be described, by way of example only, with reference to the accompanying drawings in which chain-dash lines represent cut-off boundaries for partial views of a member and in which: Fig. 1 is a front perspective view of a cutting insert in accordance with some

embodiments of the present invention;

Fig. 2 is a rear perspective view of the cutting insert shown in Fig. 1 ;

Fig. 3 is a major side view of the cutting insert shown in Fig. 1;

Fig. 4 is a minor side view of the cutting insert shown in Fig. 1 ;

Fig. 5 is a front end view of the cutting insert shown in Fig. 1 ;

Fig. 6 is a rear end view of the cutting insert shown in Fig. 1 ;

Fig. 7 is a cross-sectional view of the cutting insert shown in Fig. 6, taken along the line VII-VII;

Fig. 8 is a cross-sectional view of the cutting insert shown in Fig. 6, taken along the line VIII-VIII; Fig. 9 is a detailed partially exploded view of a cutting tool in accordance with some embodiments of the present invention;

Fig. 10 is a detailed side view of the cutting tool shown Fig. 9;

Fig. 11 is a detailed side view of the cutting tool shown Fig. 9, taken perpendicular a seat surface of a first insert receiving pocket, with the cutting insert and its clamping screw removed; and

Fig. 12 is a cross-sectional view of the cutting tool shown in Fig. 10, taken along the line XII-XII. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a cutting insert 20. In some embodiments of the present invention, the cutting insert 20 may preferably be manufactured by form pressing and sintering a cemented carbide, such as tungsten carbide, and may be coated or uncoated.

As seen in Figs. 1 to 8, the cutting insert 20 has non-identical, opposing front and back end surfaces 22, 24 with a peripheral side surface 26 extending therebetween.

The various features of the cutting insert 20 are shown in proportion to one another in Figs. 1 to 8, except for unavoidable inaccuracies due to copying, etc.

The peripheral side surface 26 includes two opposing major side surfaces 28a, 28b and two opposing minor side surfaces 30a, 30b, and as shown in Figs. 5 and 6, each of the front and back end surfaces 22, 24 has a rectangular shape in a respective end view thereof.

A first axis Al intersects the front and back end surfaces 22, 24. As seen in Fig. 3, a first plane PI located between the opposing minor side surfaces 30a, 30b contains the first axis Al and also intersects the front and back end surfaces 22, 24.

As seen in Figs. 5 and 6, in views along the first axis Al, the front end surface 22 may have a larger surface area than the back end surface 24.

A second axis A2, perpendicular to the first plane PI, intersects the two minor side surfaces 30a, 30b. A second plane P2 located between the two opposing major side surfaces 28a, 28b is perpendicular to first plane PI and contains the first and second axes Al, A2.

As seen in Fig. 3, a median plane M, perpendicular to the first axis Al is located between the front and back end surfaces 22, 24. The median plane M intersects the peripheral side surface 26 and is perpendicular to the first and second planes PI, P2.

In some embodiments of the present invention, the first axis Al may intersect the front and back end surfaces 22, 24 at front and back end points Nl, N2, respectively, and the median plane M may be located midway between the front and back end points Nl, N2.

As shown in Figs. 1, 2, 3 and 7, a through bore 54 may extend between and open out to the two major side surfaces 28a, 28b.

In some embodiments of the present invention, the through bore 54 may extend along a bore axis AB. The bore axis AB may be contained in the first plane PI.

Also in some embodiments of the present invention, the median plane M may contain the bore axis AB.

As shown in Figs. 1, 3 and 5, at least one front major cutting edge 32 is formed at the intersection of the front end surface 22 and the two major side surfaces 28a, 28b.

In some embodiments of the present invention, as shown in Fig. 3, the at least one front major cutting edge 32 may be straight in a side view of its associated major side surface 28a, 28b.

As shown in Fig. 5, the front end surface 22 may exhibit rotational symmetry about the first axis Al.

In some embodiments of the present invention, two spaced apart front major cutting edges 32 may be formed at the intersection of the front end surface 22 and the two major side surfaces 28a, 28b.

Also in some embodiments of the present invention, the cutting insert 20 may exhibit rotational symmetry about the first axis Al.

Further in some embodiments of the present invention, the cutting insert 20 may lack rotational symmetry about both the second axis A2 and the bore axis AB.

Yet further in some embodiments of the present invention, the cutting insert 20 may lack mirror symmetry about all three of the first plane PI, the second plane P2 and the median plane M.

It should be appreciated that in some embodiments of the present invention, the cutting insert 20 may be indexable about the first axis Al. In some embodiments of the present invention, two spaced apart front minor cutting edges 34 may be formed at the intersection of the front end surface 22 and the two minor side surfaces 30a, 30b, and the two front minor cutting edges 34 may alternate with the two front major cutting edges 32.

Also in some embodiments of the present invention, as shown in Fig. 4, each front minor cutting edge 34 may be straight in a side view of its associated minor side surface 30a, 30b.

Further in some embodiments of the present invention, a curved corner cutting edge 36 may extend between each front major cutting edge 32 and one of the front minor cutting edges 34.

According to the present invention, as shown in Figs. 1, 3, 4 and 5, the front end surface 22 has two diagonally opposed front raised corners Rr and two diagonally opposed front lowered corners Lr with respect to the median plane M.

It should be appreciated that the two front raised corners Rr are located further from the median plane M than the two front lowered corners Lr.

It should be appreciated that for embodiments of the present invention having two front major cutting edges 32, two front minor cutting edges 34 and two curved corner cutting edges 36, each front raised corner Rr may be disposed on one of the curved corner cutting edges 36.

As shown in Figs. 1, 3, 4 and 5, a continuous front peripheral edge 38 may be formed at the intersection of the front end surface 22 and the peripheral side surface 26, and a front raised plane PRr parallel to the median plane M may intersect the two front raised corners Rr.

In some embodiments of the present invention, no point along the front peripheral edge 38 may be located further from the median plane M than the front raised plane PRr.

Also in some embodiments of the present invention, no point on the front end surface 22 may be located further from the median plane M than the front raised plane PRr.

According to the present invention, as shown in Figs. 2, 3, 4 and 6, the back end surface 24 has two diagonally opposed back raised corners RB and two diagonally opposed back lowered corners LB with respect to the median plane M.

It should be appreciated that the two back lowered corners LB are located closer to the median plane M than the two back raised corners RB. As shown in Figs. 2, 3, 4 and 6, a continuous back peripheral edge 40 may be formed at the intersection of the back end surface 24 and the peripheral side surface 26, and a back lowered plane PBL parallel to the median plane M may intersect the two back lowered corners LB.

In some embodiments of the present invention, no point along the back peripheral edge 40 may be located closer to the median plane M than the back lowered plane PBL.

Also in some embodiments of the present invention, no point on the back end surface 24 may be located closer to the median plane M than the back lowered plane PBL.

As shown in Fig. 3, the cutting insert 20 has a length dimension measured along the second axis A2. A minimum length LMIN is defined as the shortest distance between the two minor side surfaces 30a, 30b, while a maximum length LMAX is defined as the longest distance between the two minor side surfaces 30a, 30b.

In some embodiments of the present invention, the maximum length LMAX may be greater than the minimum length LMIN.

Also in some embodiments of the present invention, a ratio Rl of the maximum length LMAX to the minimum length LMIN may be greater than or equal to 1.1. (Rl = LMAX / LMIN > 1.1 ).

As shown in Fig. 4, the cutting insert 20 has a width dimension measured in a direction perpendicular to the second plane P2 (e.g., along the bore axis AB). The insert 20 has a maximum width WMAX defined as the greatest distance between the two major side surfaces 28a, 28b.

In some embodiments of the present invention, the minimum length LMIN may be greater than the maximum width WMAX.

Also in some embodiments of the present invention, as shown in Fig. 3, the minimum length LMIN may be measured on either side of the median plane M.

Further in some embodiments of the present invention, the back end surface 24 may be devoid of a cutting edge, and the cutting insert 20 may be termed as 'single-sided'.

According to the present invention, as shown in Fig. 7, in a cross-section taken in the first plane PI, the back end surface 24 has a convex shape.

It should be appreciated that the term "convex", used herein, refers to at least part of the cross-sectional profile of the back end surface 24 having two side portions spaced apart by a mid- portion, where the mid-portion is raised or bulges in a direction away from the median plane M, with respect to the two adjacent side portions. In some embodiments of the present invention, as also seen in Fig. 7, in the cross-section taken in the first plane PI, the second plane P2 may intersect the convex profile of the back end surface 24 at its raised mid-portion.

Also in some embodiments of the present invention, in the cross-section taken in the first plane PI, as shown in Fig. 7, the convex profile of the back end surface 24 may span the entire extent of the cutting insert 20 between the two opposing major side surfaces 28a, 28b.

Further in some embodiments of the present invention, no point on the back end surface 24 may be located further from the median plane M than the back end point N2.

Yet further in some embodiments of the present invention, in a cross-section taken in any plane parallel to the first plane PI and intersecting the front and back end surfaces 22, 24, the back end surface 24 may have a convex shape.

As shown in Fig. 3, the first plane PI may intersect the two major side surfaces 28a, 28b and the at least one front major cutting edge 32.

In some embodiments of the present invention, as shown in Fig. 3, the first plane PI may be located midway between the two minor side surfaces 30a, 30b, and preferably bisect the minimum length LMIN dimension.

As shown in Fig. 7, in the cross-section taken in the first plane PI, the front end surface 22 has a concave shape.

It should be appreciated that the term "concave", used herein, refers to at least part of the cross-sectional profile of the front end surface 22 having two side portions spaced apart by a mid- portion, where the mid-portion is recessed or sunken in a direction towards the median plane M, with respect to the two adjacent side portions.

In some embodiments of the present invention, in the cross-section taken in the first plane PI, the second plane P2 may intersect the concave profile of the front end surface 22 at its recessed mid-portion.

Also in some embodiments of the present invention, in the cross-section taken in the first plane PI, the concave profile of the front end surface 22 may span the entire extent of the cutting insert 20 between the two opposing major side surfaces 28a, 28b.

Further in some embodiments of the present invention, no point on the front end surface 22 may be located closer to the median plane M than the first end point Nl. As shown in Fig. 3, the minimum length LMIN may be measured perpendicular to the first plane PI.

In some embodiments of the present invention, each of the at least one front major cutting edge 32 may extend further than half the minimum length LMIN from the first plane PI in at least one direction therefrom.

Also in some embodiments of the present invention, each of the at least one front major cutting edge 32 may extend further than half the minimum length LMIN from the first plane PI in two opposing directions therefrom.

As shown in Fig. 4, the second plane P2 may intersect the two minor side surfaces 30a, 30b and the two front minor cutting edges 34.

In some embodiments of the present invention, as shown in Fig. 4, the second plane P2 may be located midway between the two major side surfaces 28a, 28b, and preferably bisect the maximum width WMAX dimension.

As shown in Figs. 6 and 7, each of the major side surfaces 28a, 28b may include a planar major sub-surface 42a, 42b, and each of the major sub-surfaces 42a, 42b may be perpendicular to the median plane M and the first plane PI.

In some embodiments of the present invention, each of the major sub-surfaces 42a, 42b may be contiguous with the back end surface 24.

Also in some embodiments of the present invention, as shown in Fig. 7, the maximum width WMAX may be measured between the two major sub-surfaces 42a, 42b.

As shown in Figs. 3 and 6, each of the minor side surfaces 30a, 30b may include a planar minor sub-surface 44a, 44b, and the first plane PI may be located midway between and parallel to the two minor sub-surfaces 44a, 44b.

In some embodiments of the present invention, the median plane M may intersect each of the minor sub-surfaces 44a, 44b.

Also in some embodiments of the present invention, each of the minor sub-surfaces 44a, 44b may be contiguous with the back end surface 24.

As shown in Figs. 3, the minimum length LMIN may be measured between the two minor sub-surfaces 44a, 44b, on either side of the median plane M. In some embodiments of the present invention, each of the front lowered corners Lr may be located further from the first plane PI than each of the minor sub-surfaces 44a, 44b.

Also in some embodiments of the present invention, each of the front lowered corners Lr may be located further from the first plane PI than each of the back raised and lowered corners RB, LB.

Further in some embodiments of the present invention, each of the front raised and lowered corners Rr, Lr may be located further from the first plane PI than each of the minor subsurfaces 44a, 44b.

Yet further in some embodiments of the present invention, each of the front raised and lowered corners Rr, Lr may be located further from the first plane PI than each of the back raised and lowered corners RB, LB.

As shown in Fig. 3, in a side view of each major side surface 28a, 28b, the associated front lowered corner Lr and back lowered corner LB may be located on opposite sides of the first plane PI.

As shown in Figs. 2 and 6, the back end surface 24 may have two back abutment surfaces

46a, 46b.

In some embodiments of the present invention, each back abutment surface 46a, 46b may be planar.

Also in some embodiments of the present invention, the two back abutment surfaces 46a, 46b may be spaced apart by an intermediate surface 48.

As shown in Fig. 7, in the cross-section taken in the first plane PI, the two back abutment surfaces 46a, 46b may form an internal obtuse first insert angle γΐ.

In some embodiments of the present invention, the first insert angle γΐ may have a range from 130 to 170 degrees.

Also in some embodiments of the present invention, in a cross-section taken in any plane parallel to the first plane PI and intersecting the front and back end surfaces 22, 24, the two back abutment surfaces 46a, 46b may form an internal obtuse angle equal to the first insert angle γΐ.

It should also be appreciated that use of the terms "internal angle" and "external angle" throughout the description and claims refers to an angle between two surface components of a pair of surfaces as measured internal and external to the member on which the two surface components are formed, respectively.

As shown in Fig. 8, in a cross-section taken in the second plane P2, the back end surface 24 may have a convex shape.

In some embodiments of the present invention, in the cross-section taken in the second plane P2, the first plane PI may intersect the convex profile of the back end surface 24 at its raised mid-portion.

Also in some embodiments of the present invention, in the cross-section taken in the second plane P2, the convex profile of the back end surface 24 may span the entire extent of the cutting insert 20 between the two opposing minor side surfaces 30a, 30b.

As shown in Fig. 8, in the cross-section taken in the second plane P2, the two back abutment surfaces 46a, 46b may form an internal obtuse second insert angle y2.

In some embodiments of the present invention, the second insert angle y2 may be greater than the first insert angle γΐ.

Also in some embodiments of the present invention, the second insert angle y2 may have a range from 150 to 175 degrees.

As shown in Figs. 2, 3, 6 and 7, each back abutment surface 46a, 46b may be contiguous with one of the major side surfaces 28a, 28b, and a major back edge 50 may be formed between each back abutment surface 46a, 46b and its associated major side surface 28a, 28b.

It should be appreciated that the major back edge 50 gets closer to the median plane M as it extends towards the back lowered corner LB.

In some embodiments of the present invention, each major back edge 50 may be straight and form an acute first slant angle 51 of less than 15 degrees with the median plane M.

As shown in Figs. 2, 4, 6 and 8, each back abutment surface 46a, 46b may be contiguous with at least one of the minor side surfaces 30a, 30b, and a minor back edge 52 may be formed between each back abutment surface 46a, 46b and one of the associated minor side surfaces 30a, 30b.

It should be appreciated that the minor back edge 52 gets closer to the median plane M as it extends towards the back lowered corner LB. In some embodiments of the present invention, each minor back edge 52 may be straight and form an acute second slant angle 52 of less than 25 degrees with the median plane M.

Also in some embodiments of the present invention, each back abutment surface 46a, 46b may be contiguous with both of the minor side surfaces 30a, 30b.

As shown in Figs. 9 to 12, the present invention also relates to a cutting tool 56 comprising a cutting body 58 having a first insert receiving pocket 60 with the cutting insert 20 removably secured therein, the first insert receiving pocket 60 having a seat surface 62 with first and second support walls 64, 66 transverse thereto.

In an assembled state of the cutting tool 56:

one of the insert's major side surfaces 28a, 28b may be in contact with the seat surface 62, one of the insert's minor side surfaces 30a, 30b may be in contact with the first support wall

64, and

the insert's back end surface 24 may be in contact with the second support wall 66.

In some embodiments of the present invention, a clamping screw 68 may extend through the through bore 54 and threadingly engage a screw bore 70 in the seat surface 62.

Also in some embodiments of the present invention, the clamping screw 68 may have a head portion 72 in clamping contact with the through bore 54 and a threaded shank portion 74 extending therefrom.

As shown in Fig. 11, in a side view of the cutting body 58, the first and second support walls 64, 66 may form an external acute first pocket angle al.

It should be appreciated that with respect to the first insert receiving pocket 60, Fig. 11 represents a side view of the cutting body 58, in view of the cutting insert 20 and the clamping screw 68 being removed.

It should also be appreciated that the first pocket angle al may be approximately 90 degrees minus the first slant angle 61. This helps ensure adequate contact between one of the two back abutment surfaces 46a, 46b of the convex shaped back end surface 24, and the second support wall 66, in each index position of the cutting insert 20.

As shown in Fig. 12, in a cross-section taken in a tool plane PT intersecting the seat surface 62 and the second support wall 66, the seat surface 62 and the second support wall 66 may form an external acute second pocket angle a2. In some embodiments of the present invention, the tool plane PT may intersect the at least one front major cutting edge 32.

It should be appreciated that the second pocket angle a2 may be approximately 90 degrees minus the second slant angle 52, and approximately half of the first insert angle γΐ. This helps ensure adequate contact between one of the two back abutment surfaces 46a, 46b of the convex shaped back end surface 24, and the second support wall 66, in each index position of the cutting insert 20.

As shown in Figs. 9 and 10, the cutting tool 56 may be rotatable about a tool axis AT defining a forward-rearward direction Dr, DR, and the first insert receiving pocket 60 may open out at a forward end surface 76 of the cutting body 58.

In some embodiments of the present invention, the tool plane PT may be perpendicular to the tool axis AT.

As shown in Figs. 9 and 10, the cutting tool 56 may have a direction of rotation R about the tool axis AT, and the second support wall 66 may face the direction of rotation R.

In some embodiments of the present invention, the seat surface 62 may provide radial support for the cutting insert 20, the first support wall 64 may provide axial support for the cutting insert 20, and the second support wall 66 may provide tangential support for the cutting insert 20.

It should be appreciated that configuring the first and second support walls 64, 66 to form an external acute first pocket angle al results in axial cutting forces acting on the first support wall 64 being partially directed towards the second support wall 66, and tangential cutting forces acting on the second support wall 66 being partially directed towards the first support wall 64, which advantageously results in a stable clamping arrangement and reduced axial and tangential cutting forces acting on the clamping screw 68 during machining operations.

It should also be appreciated that configuring the seat surface 62 and the second support wall 66 to form an external acute second pocket angle a2 results in tangential cutting forces acting on the second support wall 66 being partially directed towards the seat surface 62, which advantageously results in a stable clamping arrangement and reduced tangential cutting forces acting on the clamping screw 68 during machining operations.

As shown in Fig. 10, one of the minor sub-surfaces 44a, 44b may be in contact with the first support wall 64. In some embodiments of the present invention, one of the minor sub-surfaces 44a, 44b may be in contact with the entire first support wall 64.

As shown in Fig. 11 , the median plane M may intersect the first support wall 64.

It should be appreciated in Fig. 11 , that although the cutting insert 20 and the clamping screw 68 have been removed, the median plane M is shown.

It should also be appreciated that for embodiments of the present invention in which one of the minor sub-surfaces 44a, 44b is in contact with the entire first support wall 64, and the median plane M is located midway between the front and back end points Nl, N2, configuring the median plane M to intersect the first support wall 64 provides central and stable support for the cutting insert 20.

As shown in Fig. 9, the cutting body 58 may have a cylindrical outer peripheral surface 78 extending axially rearwardly from the forward end surface 76, and the second support wall 66 may intersect the outer peripheral surface 78.

In some embodiments of the present invention, only one of the two back abutment surfaces 46a, 46b may be in contact with the second support wall 66, with contact occurring at the radially outermost portion of the second support wall 66.

It should be appreciated that apart from only one of the two back abutment surfaces 46a,

46b being in contact with the second support wall 66, no other portion of the back end surface 24 is in contact with the first insert receiving pocket 60.

It should also be appreciated that configuring the radially outermost portion of the second support wall 66 to be in contact with the back end surface 24 results in tangential support for the cutting insert 20 being provided radially outward of the region of clamping contact between the head portion 72 and the through bore 54, which advantageously results in a stable clamping arrangement and reduced tangential cutting forces acting on the clamping screw 68 during machining operations.

As shown in Fig. 10, one of the at least one front major cutting edge 32 is operative, having a positive axial rake angle βΐ.

Also, as shown in Fig. 10, the cutting tool 56 may include a second insert receiving pocket

80 located axially rearward of the first insert receiving pocket 60, with a second cutting insert 82 removably secured therein having an operative second insert front major cutting edge 84. As seen in Figs. 9 to 12, the first cutting insert 20 and the second cutting insert 82 are dissimilar.

It should be appreciated that for embodiments of the present invention in which one of the minor sub-surfaces 44a, 44b is in contact with the first support wall 64, and each front major cutting edge 32 extends further than half the minimum length LMIN from the first plane PI, the operative front major cutting edge 32 and second insert front major cutting edge 84 may axially overlap, to advantageously provide a continuous effective cutting edge.

Although the present invention has been described to a certain degree of particularity, it should be understood that various alterations and modifications could be made without departing from the spirit or scope of the invention as hereinafter claimed.