AU1073397A - A method of breaking chips and apparatus therefor - Google Patents

Description

A METHOD OF BREAKING CHIPS AND APPARATUS THEREFOR

FIELD OF THE INVENTION

This invention relates to a method of breaking chips in metal cutting and a chip breaker for

use in the method.

BACKGROUND OF THE INVENTION

In metal cutting, particularly in lathing of ductile materials, continuous chips are usually

generated. Such continuous chips cause serious difficulties in operational safety, chip disposal and quality control of the machined workpiece surface. More seriously, continuous chips can cause unpredictable jams in the cutting process, which prevent

automation of machining systems. Therefore, in metal cutting it is strongly desired to

have the chips generated in discontinuous forms. Chip breakers for metal cutting processes are devices used for breaking chips into small pieces. Two types of chip breakers have been proposed in the prior art - obstruction devices separate from a tool insert, and

combinations of grooves and obstacles formed on the rake face of the tool insert. During

metal cutting processes, such chip breakers work to bend the chip after it has been generated. These chip breakers do not make sure that the chip is broken during cutting and chip breaking can be achieved only in some machining conditions. It is very difficult to predict whether the chip will be broken by these chip breakers from a set of given machining conditions, that is, when these chip breakers are used in metal cutting, it is

uncertain that the chip will break. Further, when chips are broken by such chip breakers, the chips usually fly in many directions and these still cause difficulties in operational safety, control of machined workpiece surface quality and chip disposal. Furthermore,

some of these chip breakers weaken the tool cutting edge and consequently shorten the tool

life. Typical examples of existing chip breakers may be seen in patent specifications numbers US 5193947, SU 1704939, WO 9427769 and WO 951 1 102.

OBJECT OF THE INVENTION

It is the primary object of the invention to provide an improved method of chip breaking in metal cutting and apparatus for performing the method.

SUMMARY OF THE INVENTION

According to the invention in the first aspect there is provided a method of breaking chips

in a metal cutting process comprising the step of increasing shear strain at the shear plane of chip formation until fracture of a said chip occurs at the shear plane irrespective of machining conditions.

According to the invention in a second aspect, there is provided a method of breaking chips in a metal cutting process in which a chip is parted from a workpiece at a shear plane of

chip formation, the method comprising the step of forming a chip into a lever for exerting a

force to increase shear strain at the shear plane until fracture occurs at the shear plane.

In third and fourth aspects of the invention, apparatus for breaking chips in a metal cutting process is provided for performing the methods of the first and second aspects.

Further features of the invention may be found in the appended claims.

In the described embodiments of the invention, a shear plane fracturing method for chip

breaking in metal cutting is disclosed. In this method, the chips generated in metal cutting

are broken by increasing the shear stress and thus strain at the shear plane of chip formation until fracture occurs at the shear plane. A leverage chip breaking approach is also disclosed in which the chip is forced to break into small pieces by a process which forms the chip as soon as it has been generated from the shear plane into a lever of curled

form, in which the head of the chip is the load-lifting point receiving force from a fixed body, the contact between the chip and the pivoting plane of the chip breaker is the

fulcrum, and the root of the chip is the force-exerting point exerting a force acting along the shear plane to increase the shear strain until fracture along the shear plane occurs. A

leverage chip breaker is further disclosed which has a chip breaking body having a pivoting plane, a side-flow restricting plane, an up-curl guiding plane and a supplementary pivoting

plane.

The chip breaker of the described embodiments of the present invention breaks the chips generated during metal cutting processes including turning, facing, boring, parting and other metal cutting processes in which chip breaking is required, at most combinations of

machining conditions including the workpiece material properties, cooling conditions,

cutting speed, feed rate and depth of cut, into small pieces in forms of less than one circle of small radius. The chip breaker guides the broken chips to drop gently from the tool main flank to the container of the machine, such that the operational safety, machined workpiece surface quality and chip disposal efficiency are ensured. The chip breaker reduces the force

acting on the tool cutting edge by sharing the force acting on the tool rake face, reduces

temperature at the tool cutting region by reducing cutting force at the tool rake face and by

breaking the chips frequently, and consequently reduces the tool wear rate and increases the tool life. The chip breaker works with inserts of all kinds of shapes and obviates the need for geometries constructed on the tool inserts for chip breaking purposes, such that the cost

on tool inserts can be substantially reduced by using flat rake face inserts rather than

inserts with complicated geometry at the rake face. The breaker may be constructed with the tool insert, in which the chip breaker body is formed on the tool rake face.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example, with reference to

the accompanying drawings, in which:

FIG.l is an illustration showing an embodiment of the shear plane fracturing method for chip breaking of the present invention.

FIG.2 is a perspective view of one embodiment of the chip breaker constructed according to the present invention showing the structure of the chip breaker for turning with a

rectangular tool insert.

FIG.3 is a perspective view of a second embodiment of the chip breaker constructed according to the present invention showing the structure of the chip breaker for turning with

a non-rectangular tool insert.

FIG.4 is a perspective view of a third embodiment of the chip breaker constructed according to the present invention showing the structure of the chip breaker for facing or

boring with a rectangular tool insert.

FIG.5 is a perspective view of a fourth embodiment of the chip breaker constructed according to the present invention showing the structure of the chip breaker for parting with

a parting tool insert.

FIG.6 is a perspective view of a fifth embodiment of the chip breaker similar to that shown in FIG.2, with an adjustable chip breaking body.

FIG.7 is a perspective view of a sixth embodiment of the chip breaker constructed

according to the present invention showing the structure of a chip breaker tool insert of the present invention.

FIG.8 is a perspective view of a seventh embodiment of the chip breaker constructed

according to the present invention showing a variation of the chip breaking body of the

chip breaker as shown in FIG.2.

FIG.9 is a perspective view of a eighth embodiment of the chip breaker constructed according to the present invention showing another variation of the chip breaking body of the chip breaker as shown in FIG.2.

FIG.10 is a perspective view of a ninth embodiment of the chip breaker constructed

according to the present invention showing another variation of the chip breaking body of the chip breaker as shown in FIG.2.

FIG. l 1 is a perspective view of a tenth embodiment of the chip breaker constructed

according to the present invention showing another variation of the chip breaking body of the chip breaker as shown in FIG.2.

FIG.12 is a perspective view of an eleventh embodiment of the chip breaker of the present invention showing another variation of the chip breaking body of the chip breaker as shown in FIG.2.

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

During a metal cutting process, as shown in FIG. l , a chip 1 is formed from cutting off a

layer of material 2 from the workpiece 3 by the cutting tool 4. The chip formation is a

process of material velocity discontinuation, in which the layer of the work material to be cut is forced to change its moving velocity from a velocity indicated by vector 5 to a velocity indicated by vector 6 within a narrow deformation zone 7, the so called "shear

plane". At the shear plane the workpiece material is shear stressed and strained, and is

formed into the chip flowing along the tool rake face 10. In a metal cutting process, the shear strain at the shear plane is a constant when the tool rake angle and the shear angle (the angle between the cutting direction and the shear plane)

are unchanged. Different materials of the workpiece respond differently to the same value

of shear strain at the shear plane. A brittle material tends to fracture at the shear plane

after undergoing only a small shear strain and consequently forms discontinuous chips.

Therefore, in cutting of a brittle material, such as a cast iron, discontinuous chips are usually generated and chip breaking is not a problem. A ductile material tends to fracture

at the shear plane only after it has undergone a large enough shear strain. How large a

strain is enough for the fracture to occur depends on the ductility of the material, which is a

function of temperature and strain-rate in the cutting region. The ductility decreases linearly with the temperature and increases linearly with the logarithm of the strain-rate.

This nature of the material properties means that at low cutting speeds the ductility decreases with increasing the speed (due to the domination of strain-rate effect), and at high

cutting speeds the ductility increases with increasing the speed (due to the domination of the temperature effect). That is, the value of shear strain leading to fracture at the shear

plane varies with workpiece material ductility which is a function of cutting speed. For the same material, the value of shear strain leading to fracture at the shear plane is lower in cutting with a coolant compared to in cutting without a coolant, because the cooling

reduces the ductility of workpiece material in the cutting region. In general, in cutting of ductile materials, the shear strain at the shear plane due to chip formation is not large enough to cause fracture, and therefore continuous chips are usually generated, although in

cutting of some hardened steels with cooling or at low cutting speeds, fracture may occur at the shear plane and discontinuous chips may be generated. The shear plane fracturing method for chip breaking to be described aims to break the chips

generated during metal cutting processes at all machining conditions, regardless of differences in the work material properties, cooling conditions and cutting conditions, by

increasing, in addition to the shear strain due to chip formation for work piece material removal, the shear strain at the shear plane to whatever a value is necessary to lead to fracture at the shear plane.

The increasing of the shear strain can be provided by the leverage chip breaking approach of the described embodiments of the present invention. The approach is as shown in FIG. l . The chip 1 is firstly generated from the primary deformation zone (the shear plane 7) and is

secondly deformed along its back in contact with the tool rake face 10 by friction at the tool-chip interface. The secondary deformation causes the chip to curl up ( because of surface expansion at the back) and flow away from the rake face. Driven by the chip

generation at the shear plane 7, the chip flows and then curls further when its head 1 1 reaches the first fixed (on the tool) plane 12, caused by the force from the shear plane 7

acting on the root 8 of the chip and the reaction force from plane 12 acting on the head 1 1 of the chip. The chip then flows up until its head 1 1 reaches the second fixed (on the tool)

plane 14. As soon as the head of the chip reaches plane 14, a reaction force from plane

14 acting on the head 1 1 of the chip will work together with the force from the shear plane acting on the root 8 of the chip to form the chip into a curled form of small radius. The

chip then flows along an arc, with slight elastic spring back in its front portion and with its

side-flow (caused by the side-curl of the chip) being restricted by the third fixed (on the

tool) plane 13, until its head 1 1 is stopped by the fourth fixed (on the tool) plane 9. As soon as the chip 1 reaches plane 9 with its head 1 1 , it acts as a lever of curled form, in which the head 1 1 is the load-lifting point receiving a force from plane 9, the contact

between the chip 1 and plane 12 is the fulcrum, and the root 8 of the chip, joining the workpiece 3 at the shear plane 7, is the force-exerting point exerting a force additional to

the shear force at the shear plane for chip formation until fracture occurs at the shear plane

Detailed description of the embodiments of the chip breaker constructed according to the

present invention are as follows.

FIG.2 shows a first embodiment of the chip breaker for turning processes. The chip breaker 22 is clamped together with the tool insert 18 on a tool holder. The bottom face 16 of the chip breaker is in contact with the rake face 21 of the tool insert. The clamping face

17 of the chip breaker is in contact with the clamp of the tool holder (not shown). The

chip breaking body of the chip breaker is adjacent to the main cutting edge 19, and comprises a pivoting plane 12 which is inclined to the tool rake face 21 , a side-flow restricting plane 13 which is approximately perpendicular to cutting edge 19, an up-curl guiding plane 14 and a supplementary reaction plane 15. During cutting, the chip grows from the shear plane and flows along the tool rake face 21 towards plane 12 of the chip

breaker, with an up-curl caused by friction and thermal expansion at the tool-chip interface and a side-curl caused by differences in the material flow velocities along the main cutting

edge 19 and tool nose 20. Once the head of the chip reaches plane 12, the body of the chip is bent, and the chip is forced to curl further up when it flows over plane 12, with compressed elastic and plastic deformation on the inner-side of the curl and tensile elastic and plastic deformation on the outer-side of the curl. The larger the feed rate of cutting, the bigger the plastic deformation on the two sides of the chip. The plastic deformation will prevent the curled shape of the chip from opening. With the cutting continuing, due to

the side-curling of the chip formation process, the chip is pushed such that a side of the

chip reaches plane 13, and the side-flow of the chip is then restricted by plane 13. The curled chip will then flow outwards from the chip breaker 22 and then flows down towards

the tool main flank. As soon as the head of the chip has reached the tool main flank face

23, it exerts a force on the flank face 23 and at the same time receives a reactive force

from the flank face 23. The chip is then acting as a lever of curled form, in which the head of the chip is the load-lifting point receiving a force from the tool flank face 23, the contact between the chip and plane 12 is the fulcrum, and the root of the chip, which joins the workpiece at the shear plane, is the force-exerting point exerting a force additional to the shear force at the shear plane. Since the distance from the fulcrum to the head of the chip is much larger than the distance from the fulcrum to the root of the chip, the force exerted at the shear plane for the additional strain leading to fracture at the shear plane requires

only a small load-lifting force applied on the head of the chip. At this stage, if the work material is of low ductility or the feed rate is large, the load-lifting force is not large enough to open up the curled form of the chip before the chip acts as a lever to cause

fracture at the shear plane. If the workpiece material is of high ductility or the feed rate is small, however, the chip once has been generated from the shear plane and bent by plane 12 of the chip breaker, is restricted by both planes 13 and 14 to flow up then down towards

the tool flank. As soon as the chip head has reached the flank face, it acts as a lever

exerting a force at the shear plane as described in the above. In cases like this, there are two possibilities of breaking the chip at the shear plane. If at the head of the chip the load- lifting force required is not large enough to open up the curled chip before the fracture occurs, the chip will be broken as described in the above cases with low ductility of

workpiece material or large feed rate. Otherwise, the curled chip will be opened up until the opening is stopped by plane 15. As soon as the body of the chip, which is on the load-

lifting side of the lever, has reached plane 15, there will be a frictional force between the chip and plane 15, which is a further load-lifting force of the lever. The lever will then exert a further force from the root of the chip to the shear plane until fracture occurs at the

shear plane. As soon as the fracture has occurred, the chip will break at its root, the forces

acting on the chip will disappear and the next cycle of chip generating and breaking will start. The broken chip will then be pushed at its root by the head of the next chip in generation, towards the tool flank, and will drop downwards from the tool flank, before the

head of the next chip reaches the tool main flank face 23.

The broken chips generated in this way are in the shape of a "9". If the cutting feed rate is large, there is a possibility that as soon as the fracture has occurred, the fracturing zone which binds the broken chip and the new chip growing from the shear plane will be pushed

out by the new chip and broken on its way towards to the tool flank when it is bent by the opening of the arc formed by the broken chip and the new ship. The opening is caused by the reactive force on the head of the broken chip. At the mean time for the opening, the

arc formed by the broken chip and new chip may work as a lever to fracture the new chip at its root (the shear plane). The new fracturing zone will then be pushed out to start

another chip breaking cycle, while the old fracturing zone is bent and broken. The broken chips generated in this way are in the shape of a "c".

FIG. 3 shows a second embodiment of the chip breaker of the present invention, for turning with a triangular tool insert 30. The bottom face 28 of the chip breaker 32 is in contact

with the tool rake face 33. The clamping face 29 of the chip breaker is in contact with the clamp of the tool holder (not shown). The chip breaking body of the chip breaker is the

same as that of the embodiment shown in FIG 2, which comprises a pivoting plane 24, a

side-flow restricting plane 25, an up-curl guiding plane 26 and a supplementary pivoting

plane 27. The chip breaking mechanism of this embodiment is the same as the one shown in FIG. 2.

Compared the embodiments of the chip breaker shown in FIGs. 2 and 3, it can be seen that

embodiments in different shapes of the chip breaker can be constructed according to the present invention, which suit the shape of the tool inserts used but share the same chip

breaking body.

FIG. 4 shows another embodiment of the chip breaker of the present invention, for facing

or boring processes with a rectangular insert 40. The bottom face 38 of the chip breaker 42 is in contact with the tool rake face 41. The clamping face 39 of the chip breaker is in contact with the clamp of the tool holder (not shown). The chip breaking body of the chip breaker is the same as the one in the embodiment shown in FIG 2, which comprises a

pivoting plane 34, a side-flow restricting plane 35, an up-curl guiding plane 36 and a

supplementary pivoting plane 37. The chip breaking mechanism of this embodiment is the

same as the one shown in FIG. 2.

Compared the embodiments of the chip breaker shown in FIGs. 2 and 4, it can be seen that

embodiments of the chip breaker for cutting processes other than turning can be constructed according the present invention, which share the same chip breaking body but locate it

differently.

FIG. 5 shows another embodiment of the chip breaker of the present invention, for use with a parting tool insert. The bottom face 46 of the chip breaker 51 is in contact with the rake

face 50 of the tool insert 48. The clamping face 47 of the chip breaker is in contact with

the clamp of the tool holder (not shown). The chip breaking body of the chip breaker is

adjacent the main cutting edge 49, and comprises a pivoting plane 44 and a supplementary reaction plane 45. During cutting, the chip grows from the shear plane and flows along the tool rake face 50 towards pivoting plane 44 of the chip breaker, with up-curling caused by

friction and thermal expansion at the tool-chip interface. As soon as the head of the chip

has reached plane 44, the body of the chip will be bent to curl up, with compressed elastic and plastic deformation on the inner-side of the curl and tensile elastic and plastic deformation on the outer-side of the curl. The larger the feed rate of cutting, the bigger the

plastic deformation on the two sides of the chip. The plastic deformation will prevent the curled shape of the chip from opening. With the chip being continuously generated from the shear plane and bent to curl up when it flows over plane 44, the head of the chip will

reach the workpiece surface to be machined. As soon as the head has reached the workpiece surface, it will exert a force on the workpiece and at the same time receive a force from the workpiece. The chip body is then formed into a lever of curled form, in

which the head of the chip is the load-lifting point receiving a force from the workpiece, the contact between the chip and plane 44 is the fulcrum, and the root of the chip is the

force-exerting point exerting a force additional to the shear force at the shear plane. Since the distance from the fulcrum to the head is much larger than the distance from the fulcrum to the root, the force exerted to the shear plane for the additional strain leading to fracture

at the shear plane requires only a small load-lifting force applied on the head of the chip.

If the load-lifting force is not large enough to open up the curled form of the chip before

the chip acts as a lever to cause fracture at the shear plane, the chip will be broken and the

next cycle of chip breaking will start. Otherwise, the curled chip will be opened up by the load-lifting force until the opening is stopped by plane 45. As soon as the body of the chip, which is on the load-lifting side of the lever, has reached plane 45, there will be a frictional force between the chip and plane 45, which is a further load-lifting force on the

lever. The lever will then exert a further force from the root of the chip to the shear plane

until fracture occurs at the shear plane. As soon as the fracture has occurred, the chip will break at its root, forces acting on the chip will disappear, and the next cycle of chip generating and breaking will start. The broken chip will then be pushed at its root by the head of the next chip in generation towards the workpiece surface to be machined and brought by the workpiece to drop downwards to the chip container of the machine, before the head of the next chip reaches the workpiece surface to be machined.

From the above descriptions of embodiments of the chip breaker, it can be seen that the dimensional values of the chip breaking body in the chip breaker can be varied within a

range without losing effectiveness in chip breaking. However, the shape of the broken chip

might vary with the ratio of the depth of cut to the distance from the tool nose 20 to the side-flow restriciting plane 13 as shown in FIG.2, and the radius of the chip curl might vary with the ratio of the feed rate to the distance from the tool main cutting edge 19 to the

pivoting plane 12 as shown in FIG.2. Therefore, in one preferred form, the dimensional

values of the chip breaking body may be adjustable. FIG. 6 shows a fifth embodiment of the chip breaker of the present invention, with the chip breaking body being adjustable.

The clamping surface 57 of the chip breaker 64 is in contact with the clamp of the too!

holder. The chip breaking body is formed by an adjustable body 68 comprising a first

surface forming a pivoting plane 52 and a second surface forming an up-curl guiding plane

54, an adjustable side-flow restricting body 61 having a surface forming a side-flow

restricting plane 53, and the body of the chip breaker 64 having a surface forming a supplementary reaction plane 55. The distance from the tool cutting edge 59 to the pivoting

plane 52 can be adjusted by adjusting the location of the saw-shaped teeth 65 of the chip bending body 68, over the saw-shaped teeth 66 of the body of the chip breaker. Similarly, the distance between the tool nose 69 to the side-flow restricting plane 53 can be adjusted

by adjusting the location of the saw-shaped teeth 62 of the side-flow restricting body over

the saw-shaped teeth 67 of the body of the chip breaker. The position of supplementary reaction plane 55 may be varied by adjusting the relative position of chip breaker body 64 and tool insert 58. Besides the structure as shown in FIG.6, other methods of fixing and adjusting can also be used to construct the adjustable chip breaker body of the chip breaker

according to the present invention. The same adjustable chip breaking body can also be constructed on other embodiments of the chip breaker shown in Figs 3, 4 and 5.

FIG.7 shows a sixth embodiment of the chip breaker of the present invention, in which four chip breaking bodies of the present invention are constructed on the top of a tool insert to form a chip breaker insert 71 . The chip breaker insert has four cutting edges 72, 73, 74

and 75 to be in turn used as a main cutting edge in cutting processes. A chip breaking

body as described in FIG.2, comprising planes 76, 77, 78 and 79, is constructed to work

with the cutting edge 72. Another chip breaking body as described in FIG.2, comprising planes 80, 81 , 82 and 83, is constructed to work with the cutting edge 73. Another chip breaking body as described in FIG.2, comprising planes 84, 85, 86 and 87, is constructed to

work with the cutting edge 74. Another chip breaking body as described in FIG.2,

comprising planes 88, 89, 90 and 91 , is constructed to work with the cutting edge 75. The clamping surface 92 is in contact with the tool holder clamp (not shown).

The chip breaking body of the chip breaker as described in FIG.2 can be constructed, by

varying its geometrical parameters, in many forms to suit the needs of practical application of the chip breaker or to meet production requirements. The geometrical parameters of the chip breaking body include (see FIG. 2):

1 ) the angle between the tool rake face 21 and the pivoting plane 12 of the chip breaking

body, α,

2) the distance from the tool main cutting edge to the pivoting plane 12 of the chip breaking body, _>,

3) the width of the pivoting plane 12, w,

4) the angle between the pivoting plane 12 and the up-curl guiding plane 14, β,

5) the length of the upper-side of the pivoting plane which joins with the lower-side of the up-curl guiding plane, /, and

6) the angle between the side-flow restricting plane 13 of the chip breaking body and the

tool main cutting edge 19, Ψ.

In the chip breaking body of the chip breaker as shown in FIG.2, for simplicity, the angle Ψ is designed to be approximately perpendicular to the tool main cutting edge. An

optimum value for this angle should be determined according to the chip side-flow angle η which is commonly defined as the angle between the flow direction of the chip at the cutting region and the normal to the cutting edge in the rake face of the tool (see FIG. 2).

The value of Ψ should be determined such that, during the chip formation and breaking

process, the chip can flow out from the chamber of the chip breaking body and on the other hand its side-flow is restricted by the side-flow restricting plane 13 so that the leverage chip breaking process of the present invention can be achieved. Therefore, an

optimum design for the angle Ψ should follow the equation

ψ = 90° - η (1 )

In view of the fact that the chip side-flow angle η varies with machining conditions,

especially with the combination of the radius of the tool corner, feed rate and depth of cut (when the radius of the tool corner varies from 0.4 to 1.2, feed rate varies from 0.1 to 0.3 and depth of cut varies from 0.3 to 4, the chip side-flow angle varies within the range from 5 degrees to 70 degrees), for simplicity, the angle Ψ can be assigned with a fixed value

which is calculated from equation (1 ) using a usual value of the chip side-flow angle.

FIG.8 shows a seventh embodiment of the chip breaker of the present invention similar to

Fig. 2, having an angle Ψ different from 90 degrees. In this embodiment the side-flow

restricting plane 93 of the chip breaking body in the chip breaker 22 is perpendicular to the tool rake face 21 , and the angle between the plane 93 and the tool main cutting edge 19 is less than 90 degrees measured clockwise from the tool cutting edge 19.

A design for the chip breaking body of the present invention without having the supplementary reaction plane 15 as shown in FIG.2 can be constructed by properly determining the geometrical parameters α, b, w, / and β of the chip breaking body, using

the correlations between the geometrical parameters of the chip breaking body, machining conditions and radius of the broken chips, such that the chip, when it flows out from the

chamber formed by the pivoting plane 12, side-flow restricting plane 13 and up-curl

guiding plane 14, is highly strain-hardened in curled form of small radius, thus the supplementary reaction plane 15 is not needed in the chip breaking process because the chip under such condition will not be opened up to touch the supplementary plane 15

before fracture caused by the leverage chip breaking process occurs at the shear plane.

Expressions for the correlation between the geometrical parameters of the chip breaking body, machining conditions and radius of the broken chips can be derived from the

geometry of the chip breaking body at the cutting region. By neglecting the elastic spring back of the chip, the correlation between the radius of a chip broken by the chip breaker, r, and the parameters b and α can be represented by the equation

b-h tan—

(2)

where h is the tool-chip contact length (see FIG. 2) resulted from machining conditions,

namely the feed rate, tool rake angle and workpiece material flow stress properties which are functions of strain-rate and temperature. The smaller the value for b, the smaller the

radius of the chip. However, b must be larger than h to allow the chip to be generated

naturally from the cutting region. An increase in the angle α will decrease the radius of the chip, but on the other hand it will also, during the leverage chip breaking process,

decrease the force component exerting from the root of the chip to the shear plane, which

is to be avoided.

The smaller the values for W and β, the smaller the radius of the chip and also the smaller

the size of the chip breaking body. However, W and β have to be large enough to allow the chip flow out from the chamber of the chip breaking body. The inventor has found that

b < w ≤ 5b (3)

and

90° < β < 130° ( )

The length / has to be determined such that the chip side-flow is restricted by the side-flow

restricting plane 13 (see FIG.2) to be within the limit required by the leverage chip breaking process, and on the other hand the chip can flow out from the chip breaking body.

The length 1 should fall in the range:

d≤l≤lOd (5)

where d is the width of cut (see FIG.2).

FIG.9 shows a eighth embodiment of the chip breaker of the present invention, showing another design of the chip breaking body in the chip breaker as shown in FIG.2. In this

embodiment the chip breaking body in the chip breaker 22 is formed by the pivoting plane

12, the side-flow restricting plane 93 and the up-curl guiding plane 14. The supplementary reaction plane 15 as shown in FIG.2 is not included in the chip breaking body. In such a

design, the geometrical parameters of the chip breaking body, α, b, w, / and β, are

determined, using expressions (2), (3), (4) and (5), such that the chip, when flow out from the chip breaking body, is highly strain-hardened in curled form of small radius and with its side-slow being restricted by the side-flow restricted plane 93. Therefore, the

supplementary reaction plane 15 as shown in FIG.2 is not needed in this design.

In the design of the chip breaking body as shown in FIG.9, the side-flow restricting plane 93 is not needed in cutting under certain ranges of machining conditions, in which the chip, when it flows out from the corner formed by the pivoting plane 12 and up-curl guiding plane 14, is highly strain-hardened in curled form of small radius with its side-flow being naturally (without being restricted by the side-flow restricting plane) under the limit required by the leverage chip breaking process. One of the examples under the certain

ranges of machining conditions stated above is in turning of low carbon steels at the feed rate equal to and larger than 0.15 mm/rev when the ratio of the radius of the tool corner to the width of cut is less than or equal to 0.5. In such cutting cases, the side-flow restricting

plane 93 as shown in FIG.9 is not needed for chip breaking, and it can be removed from

the design of the chip breaking body to simplify the structure of the chip breaker. This is

illustrated in FIG.10 which shows a ninth embodiment of the chip breaker of the present invention, showing another modification of the chip breaking body in the chip breaker

shown in FIG.2. In this embodiment the chip breaking body is formed only by two planes,

the pivoting plane 12 and the up-curl guiding plane 14.

To ensure the chip is broken by the leverage chip breaking method, the curl-up guiding plane 14 shown in FIG.10 cannot be removed for further simplification based on the

structural design as shown in FIG.10, because plane 14 is necessary to ensure that the chip

is formed into a lever for chip breaking. However, the pivoting plane 12 and up-curl guiding plane 14 in the chip breaker design as shown in FIG. l 1 can be combined into an

up-curl guiding and pivoting surface which is designed such that it provides both the up- curl guiding function for forming the chip into a level of curled form and the pivoting

function for the leverage chip breaking process. FIG.12 shows a tenth embodiment of the chip breaker of the present invention, showing another design of the chip breaking body in

the chip breaker as shown in FIG.2. In this design, the chip breaker 22 has a chip breaking body formed by a side-flow restricting plane 13 and an arcuate up-curl guiding and pivoting surface 96. The parameter b is the same as the one shown on FIG.2. The radius

__, height H and the centre co-ordinates x and y for the arcuate surface 96 is determined

such that surface 96 will work together with the side-flow restricting plane 13 to form the chip into a lever of curled form, and as soon as the head of the chip has received a reaction force, it will work, through the contact of the plane 96 with the back of the chip, as the

fulcrum for pivoting in the leverage chip breaking process. As an example for designing the arcuate up-curl guiding and pivoting surface 96, in turning of low carbon steels at

cutting speeds from l OOm/min to 300m/min, feed rates from 0.03 mm/rev to 0.3 mm/rev, the parameters R, H, x and y can be from 1.5 mm to 5 mm, from 1 mm to 6 mm, from 0

mm to 0.5 mm and from 1 mm to 6 mm, respectively.

In the production of the chip breaker of the present invention, if the chip breaker is produced not by compression and sintering but by machining, the production of the chip

breaking body as shown in FIG.2 may be difficult and costly. An assembled design for the chip breaking body can avoid such production problems. This is shown in FIG.12 which

shows an eleventh embodiment of the chip breaker of the present invention, being another

modification of the chip breaking body in the chip breaker as shown in FIG.2. In this

embodiment the chip breaking body is in an assembled chip breaker comprises the main

part 22 and a separate part 94. The chip breaking body is formed by the pivoting plane 12,

up-curl guiding plane 14 and side-flow restricting plane 93. The planes 12 and 14 are on the main part 22. Plane 93 is on the separate part 94. There is a gap g between the main part 22 and the separate part 94. By varying the value of g the position and effectiveness

of plane 93 can be adjusted. The assembled chip breaker is pressed on the top of the tool insert 18 by the clamp (not shown) of the tool holder (not shown). The clamp engages on both the top 17 of the main part and the top 95 of the separate part of the assembled chip

breaker.

The manufacturing of the chip breaker involves only simple processes of machining and

surface hardening in the case of tool steel manufacturing materials, or compression and sintering in the case of tungsten carbide or ceramic manufacturing materials.

In the present drawings only embodiments of the chip breaker constructed according to the present invention have been illustrated for some lathing processes. This is because

problems caused by continuous chips in those processes are more serious compared to other

metal cutting processes, such as milling. However chip breakers of the present invention are applicable for other metal cutting processes.

In the above described drawings, only embodiments of the chip breaker for cutting with flat rake face insert tools using tool holders with a clamp on top of the chip breaker have been

illustrated. This is not to be construed as limitative and chip breakers with other ways of being clamped, such as having a hole in the body of the chip breaker for locating the clamp or for fixing the chip breaker together with the tool insert on the tool holder, can be

constructed according to the present invention. It is also possible to construct the chip breaker together with a tool insert to form a chip breaker insert of the present invention,

which combines at least one chip breaking body of the present invention and the tool insert.

The chip breaker of this invention works under machining conditions with and without a coolant. In dry cutting with the chip breaker, the chip will be broken into small pieces of less than one circle; when a coolant is applied to cutting with the chip breaker, the chip will still be broken into small pieces in less than one circle, which is similar to dry cutting,

and the radii of the broken chip circles will be smaller and more consistent compared to those in dry cutting, because the ductility of the chip material is reduced by cooling.

In general, although the surfaces of the chip breaker have been shown planar (except for

surface 96 in FIG. l 1), they may be of other surface shapes provided they can still provide their functions in the chip leverage process.

Claims (19)

1 . Apparatus for breaking chips in a metal cutting process in which a chip is parted from a workpiece at a shear plane of chip formation, comprising lever forming means for formin" a chip into a lever for exerting a force to increase shear strain at the shear plane until fracture occurs at the shear plane.

2. Apparatus as claimed in claim 1 wherein the lever forming means comprises a pivoting surface for bending a said chip towards a reaction surface and forming a fulcrum point in contact with the said chip.

3. Apparatus as claimed in claim 2 further comprising a guiding surface for guiding said chip to flow towards the said reaction surface.

4. Apparatus as claimed in claim 3 further comprising a side-flow restricting surface for restricting sideways flow of the said chip.

5. Apparatus as claimed in Claim 4 further comprising a tool main cutting edge engaging surface and wherein the angle Ψ between the side-flow restricting surface and the tool

main cutting edge engaging surface is represented by the equation :

ψ = 90° - η where η represents the chip side-flow angle.

6. Apparatus as claimed in any one of claims 3 to 5 wherein the guiding surface and the pivoting surface are formed together as a single arcuate surface.

7. Apparatus as claimed in any one of claims 3 to 6 further comprising a supplementary

reaction surface for providing a supplementary reaction when in contact with the body of

the said chip.

8. Apparatus as claimed in claim 1 comprising a plurality of surfaces for modifying the direction of movement of the chip.

9. Apparatus as claimed in any one of claims 2 to 8 wherein the position of at least one said surface is adjustable.

10. Apparatus for breaking chips in a metal cutting process comprising means for

increasing shear strain at the shear plane of chip formation until fracture of a said chip occurs at the shear plane irrespective of machining conditions.

1 1. Apparatus as claimed in Claim 10 comprising a plurality of surfaces for modifying the direction of movement of the said chip.

12. Apparatus as claimed in Claim 1 1 wherein the position of at least one said surface is adjustable.

13. In combination, apparatus as claimed in any one of the preceding claims and a tool insert, the tool insert providing a reaction surface for the lever.

14. ln combination, apparatus as claimed in any one of claims 1 to 12 and a workpiece, the workpiece providing a reaction surface for the lever.

15. Apparatus as claimed in any one of claims 1 to 12 constructed together with a tool

insert.

16. Apparatus as claimed in any one of the claims 1 to 12 fabricated in more than one

part.

17. A method of breaking chips in a metal cutting process compπsing the step of increasing shear strain at the shear plane of chip formation until fracture of a said chip occurs at the shear plane irrespective of machining conditions.

18. A method of breaking chips in a metal cutting process in which a chip is parted from a

workpiece at a shear plane of chip formation, the method comprising the step of forming a

chip into a lever for exerting a force to increase shear strain at the shear plane until fracture

occurs at the shear plane.

19. A method as claimed in claim 18 further comprising the steps of (1 ) applying a force

to a head of a said chip, said head forming a load-lifting point of said lever, and (2) providing a fulcrum between the head and the root of the chip.