WO2017138881A1

WO2017138881A1 - Cutting tool assembly with controlled resilience using hyperelastic materials

Info

Publication number: WO2017138881A1
Application number: PCT/SE2017/050131
Authority: WO
Inventors: Qilin FU; Cornel Mihai NICOLESCU
Original assignee: Plasmatrix Materials Ab
Priority date: 2016-02-12
Filing date: 2017-02-10
Publication date: 2017-08-17
Also published as: EP3414037A4; EP3414037A1; JP2019508276A; SE540533C2; CN108778581A; SE1650185A1; CN108778581B

Abstract

The invention relates to a cutting tool assembly (10) comprising a tool head (11) with a seat (12) for receiving a cutting insert (13). The cutting tool assembly comprises a hyperelastic material (15) provided to control the resilience of the cutting tool assembly and to dampen shocking force and localized vibrations. The hyperelastic material may be provided as layer or coating, or in the form of an entire tool part made of a hyperelastic material.

Description

CUTTING TOOL ASSEMBLY WITH CONTROLLED RESILIENCE USING HYPERELASTIC

MATERIALS

TECHNICAL FIELD

[0001 ] The invention relates to a cutting tool assembly provided with a hyperelastic dampening material provided to gradually build up the physical contact between the cutting tool and a workpiece, and to absorb shocking forces and vibration energy during a metal cutting operation, and to thereby provide a controlled resilience of the cutting tool.

BACKGROUND

[0002] Cutting force variations during machining incurs disturbance to the edge of the cutting insert that is in contact with the workpiece. When contact is made between the cutting insert and the workpiece a sudden increase of cutting force induces localized stress distribution and fracture in the area close to the edge of the cutting insert due to the shocking force effect. During machining when the edge of the cutting insert has a constant contact with the workpiece, a wide frequency range of dynamic cutting force excites the edge of the cutting insert and induces localized flaking and spalling in the region close to the edge of the cutting inserts. Both of the shocking force at the entrance of a cutting process and the wide frequency dynamic cutting force, accelerate the tool wear of cutting inserts with unpredictable tool failure and increase the production cost.

[0003] From US 5 738 468 it is known to ensure toughness of cutting inserts and a high fracture strength of cutting inserts. This is inter alia achieved by supporting the cutting insert with shims having a contacting surface that conforms to the surface profile of the cutting insert, which is to be arranged at a distance from the lower surface of the cutting edge of the cutting insert.

[0004] In US 201 1/0008576 A1 a method of providing a cutting tool with a coating comprising carbon pillars is disclosed. The coating may e.g. be applied to increase the dynamic stiffness of a cutting tool or the like.

[0005] During machining, the cutting process and the vibrational properties of the tooling structures interacts and form a closed loop system. The closed loop system has stable operation regions and unstable regions defined by the depth of cut, where the tooling structure vibration amplitude grows with time. Under conditions where the cutting tool overhang length to diameter ratio (L/D) exceeds five, the cutting tool stiffness decreases and the depth of cut for the stable region decreases and approaches nearly zero, such that the cutting process cannot be performed under stable conditions. To perform stable machining at long overhang length, it is known to embed vibration damping solutions in the tooling structure and these solutions can be divided into three major groups:

• Tuned mass dampers that transmit the vibrational kinetic energy to the tuned mass;

• Constrained layer damping in critical joint interfaces, which damping transforms the vibrational potential strain energy to heat; and

• Piezo actuators that apply a counteracting force to the tool shank to attenuate the vibration tendency.

[0006] These solutions usually resolve the problems associated with structure vibrations over the frequency range typically below 3000 Hz either in bending or torsional modes.

[0007] Tooling market investigations reveal that only less than 5% of the machining cases are performed by a cutting tool with an overhang length longer than 5 times its diameter. At shorter overhang length where the L/D ratio is less than 5, damping solutions were usually not necessary as the machining process is usually deemed as stable in terms of machine tool regenerative chatter. However, the cutting force variation and dynamic excitations at the edge of the cutting insert is naturally occurring, independent of the tooling structure configuration and overhang length to diameter ratio.

[0008] The current disclosed invention focuses on these machining cases where the tooling structure has a high stiffness. Conventionally, the rigidity in such structures may be too high. As a consequence of the high rigidity of the cutting tools, the shocking forces during metal cutting operations frequently fractures the cutting edge due to the fragile property of most carbide materials and ceramics. The currently disclosed invention applies means of controlled resilience with hyperelastic property in the cutting tool assembly to reduce the tool fracture damages while maintaining the sufficient rigidity to ensure accuracy of machined parts.

[0009] For example in milling metal cutting processes, the cutting edge is excited by a shocking force at the entrance of a workpiece during each rotation of the cutting tool. To enhance the cutting tool's performance against the fracture failure due to the shocking forces, it is common to add a higher amount of binder (ex. Cobalt) in the carbide insert to improve its strength and to enhance its toughness (fracture resistance), at a price of reduced wear resistance and reduced tool life by abrasive wear. The currently disclosed invention separates the design of resilience (toughness or fracture resistance) and hardness of cutting tool assembly. Thus, the toughness (fracture resistance) of carbide inserts can be obtained without sacrificing the abrasive wear resistance property.

[0010] In a prior art study, a tool shank with a hydraulic oil chamber was provided beneath the cutting inserts to adjust the resilience of the cutting inserts. (Fleischer, J. , Becke, C, Pabst, R., 2008. Improving tool life by varying resilience and damping properties in close proximity of the cutting edge. Production Engineering 2, 357-364). The resilience was controlled by the voltage applied on a piezo-actuator which exerts a compression pressure on the hydraulic oil. The study was, however, only empirical and the underlying mechanism was not described. Tooling structure design usually aims for the highest rigidity and their stiffness is usually linear due to the construction (mostly steel or cast iron) material's linear elastic modulus. The rigid construction of tooling structural is detrimental to cutting insert tool life as it increases the peak amplitude of impact shocking force at the entrance of a cutting process. The high peak amplitude directly increases the maximum stress in cutting inserts which might lead to fracture of cutting inserts. Certain resilience in the construction facilitates to reduce peak amplitude of the shocking force, such as by changing the geared

transmission to belt transmission in the spindle. It was found that it is not just to reduce the stiffness of tooling structure, as a reduction of stiffness in tooling structure might further reduce the tool life of cutting inserts instead of prolong.

[001 1 ] The invention is based on the notion that the hyperelastic property of a tooling structure vouches for reduced shocking force amplitude and prolonged tool life of cutting inserts.

[0012] The hyperelastic property means that the stiffness of the tooling structure is not linear. At the entrance of a cutting process, the stiffness of tooling structure at the cutting point is substantially lower than that during cutting process for a short period of time, typically less than 1 ms. After that, the stiffness of tooling structure retains its maximum stiffness when the contact between the edge of the cutting insert and the workpiece is constant.

Generally, the hyperelastic property in tooling structures decreases the amplitude of the derivative of cutting force to time (dF/dt), which reduces the shock wave energy generated at the entrance of a cutting process. The available time frame for the edge of the cutting insert to build up a full contact with the workpiece is also prolonged by the hyperelastic property a tooling structure. This is illustrated in figure 1 .

[0013] The most familiar hyperelastic materials are natural rubbers, which behave rather different from ceramics or metals. Thermally heating of the rubber will lead it to shrinkage instead of expansion as in most conventional structure materials. Stretching of the rubber bands increases its temperature as the internal inter-molecular bonding restores only a small fraction of the mechanical energy and dissipates the larger fraction energy as heat. Stretching the rubber gradually increases the stiffness of the rubber, thus rubber possesses hyperelastic property. The hyperelastic property of rubbers is recoverable due to the elastic viscous sliding between the large sizes molecular. The typical behavior of rubbers compared to metals under indentation, reveals its hyperelastic property and recoverability. This is illustrated in figure 2.

[0014] The key factor that enables the hyperelastic property of rubber is the size of its large and long chain molecules with a dimension typically below 100 nm. With the reduced size of molecules in rubberlike materials, crystal size or grains in composites below 100 nm, also enables the material's hyperelastic property. It has been found that composites, with nano-structures of which the dimension is below 100 nm, also exhibits hyperelastic behavior and recoverability (Fu et al., 2016. High dynamic stiffness mechanical structures with nanostructured composite coatings deposited by high power impulse magnetron sputtering. Carbon 98, 24-33.).

[0015] Hyperelastic property refers to materials' behavior that maintain elastic property at high strains (typically above 5%). Particularly in indentation tests, hyperelastic materials mostly recover its surface indent, and make it difficult to measure the surface hardness with surface indent area measurements.

[0016] Most materials undergo 'elastic deformation', 'plastic deformation' and then 'fracture', under steadily increased load condition. Steel materials for example, enters the plastic deformation state at a strain of less than 0.05%. Natural rubber on the other hand, can be deformed to a greater extent without entering the plastic deformation region, and can easily exceed 50% strain while maintaining an elastic behavior. Therefore, we usually call rubber a 'Hyperelastic' material as we rarely observe any plastic deformation behavior in rubbers.

[0017] The stiffness of 'elastic' materials is nearly constant while the 'hyperelastic' materials exhibits a stiffness property that is non-linear, due to the large geometric deformation. With the same external load, components with hyperelastic property reacts and deforms slower. The longer impact time (within a fraction of a second) avoids the excitation of vibration energy at high frequencies which are detrimental to fragile materials.

[0018] The hyperelastic property of rubbers is due to its long chain molecules and grains, that are entangling and sliding over each other while subjected to an external load. The entangling and sliding behavior is 'reversible', i.e. the object restores its original geometry while the external load is removed.

[0019] Rubbers also demonstrates viscoelastic property due to the nanometer size grains that create a high internal grain boundary surface area to volume ratio, which favors the vibration energy dissipation capacity. However, the requirement for a material to demonstrate 'hyperelastic' property and 'viscoelastic' property is different, particularly on the grain size.

[0020] Viscoelastic property requests the grain size to be strictly below 20 nm whereas the hyperelastic property can be obtained even with a larger grain size above 100 nm.

[0021 ] Rubber are not suitable for cutting insert applications. The major limitation for rubber in the shim application, is the temperature variations during metal cutting process. Ser e.g. KUS, Abdil, et al. Thermocouple and infrared sensor-based measurement of

temperature distribution in metal cutting. Sensors, 2015, 15.1 : 1274-1291 .

[0022] Shims are placed underneath the cutting insert and is 2-3 mm to the cutting zone, where intense heat is generated. Particularly in the region where the shim is seated, the temperature can rise to 90 Celsius degrees, or even higher. For natural rubbers, the elastic modulus will decrease by an order of magnitude when the temperature is increased from 20 Celsius degrees to 90 Celsius degrees. The damping property will also decrease with the increased temperature from 20 Celsius degrees to 90 Celsius degrees.

[0023] Substantially reduced elastic modulus with increased temperature will lose the stiffness of the cutting insert holding structure, and the insert will lose clamping and become destroyed in machining.

[0024] Another reason natural rubbers and polymers are not suitable is due to their low elastic modulus. Stiffness in the normal direction of a cutting insert can be calculated as:

[0025] ^k = ^T

[0026] With a predefined stiffness k, the higher is the bulk modulus K, the higher can be the thickness of the part, t. The elastic modulus of polymers and rubbers are normally 100 kPa, versus an elastic modulus of 100 GPa for metals. Thus, the thickness of a shim should be divided by 1 000 000, to have the same stiffness if the shim is made of rubbers. The shim has a thickness of about 3 mm, and that means the thickness of the polymers can only be 3 nm, which is not possible. Further, the resilience functionality also depends on the thickness and volume of the layer, to allow the recoverable elastic flow. Without a certain volume, the object will not demonstrate the resilience property at all.

[0027] Cutting tools with short tool overhang length to diameter ratio, usually does not have the problem of regenerative tool chatter. The dynamic force variation, however, still excites the tooling structure and induces forced vibration to the tooling structure components. Especially at the regions close to the edge of the cutting insert, high frequency excitation accelerates the wearing of edge of the cutting insert in the form of localized flaking and spalling effect.

SUMMARY OF THE INVENTION

[0028] It is an object of the disclosed invention to prolong the tool life of cutting inserts by means of an improved shocking force resistance. It is a further object of the disclosed invention to prolong the tool life of cutting inserts by means of a high damping performance to eliminate the localized flaking and spalling phenomenon. The invention relates to a wide range of cutting tools, including milling, turning, drilling, broaching, gear hobbing etc. Other mechanical products that endures shocking forces and has a minimum rigidity requirement, may also benefit from the type of design that has a hyperelastic high dynamic stiffness damper in accordance with the invention.

[0029] The invention relates to a cutting tool assembly comprising a tool head with a seat for receiving a cutting insert. The cutting tool assembly further comprises a hyperelastic material provided to dampen vibrations produced during operation of the cutting tool assembly. The dampening is achieved in that the resilience of the cutting tool is controlled, and specifically the hyperelastic material is provided to dampen shocking force and localized vibrations.

[0030] As explained in this specification a hyperelastic material has proven to be particularly efficient when it comes to the transient process of building up a full physical contact between a cutting tool and a workpiece, and to thereby reduce wear and prolong the effective operational time of a cutting insert.

[0031 ] In a specific embodiment the hyperelastic material is provided as a resilient layer between the seat and the cutting insert. The hyperelastic material has a controlled stiffness and shows a high vibration damping property, especially if one dimension of the internal grains, atom cluster or long chain molecules is substantially smaller than 200 nm. [0032] In another specific embodiment a shim is arranged between the seat and the cutting insert, the layer of hyperelastic material being provided between the shim and the cutting insert.

[0033] In yet another specific embodiment the layer of hyperelastic material is provided as a surface coating to the shim and /or the cutting insert.

[0034] In another specific embodiment the cutting tool assembly further comprises a tool shank and a tool clamp, the tool clamp being arranged to hold the tool shank, wherein the tool shank supports the tool head, and wherein a layer of hyperelastic material is provided between the tool shank, and the tool clamp.

[0035] The layer of hyperelastic material may be provided as a separate part.

[0036] The hyperelastic material should preferably have an elastic modulus between 0.1 GPa and 550 GPa, so as to ensure the stiffness of the cutting tool structure.

[0037] The hyperelastic material is preferably comprised in the group comprising polymers, composites, metals and metal alloys to a major part comprised of nano-sized grains, atom clusters or long chain molecules with at least one internal structure dimension which is less than 200 nm, preferably less than 100 nm, or even less than 10 nm.

[0038] In a specific embodiment of the invention the cutting insert is made of a hyperelastic material, and in another specific embodiment the shim is made of a hyperelastic material.

[0039] The invention further relates to a shim for use in a cutting tool assembly that comprises a tool head with a seat for receiving a cutting insert, the shim being provided to be arranged between the seat and the cutting insert, and having a flat shape that corresponds to the shape of the seat and the shape of the cutting insert wherein the shim is provided with a layer of hyperelastic material on at least one of its flat sides, facing either the seat or the cutting insert.

[0040] For cutting inserts application, the hyperelastic property provided by a surface coating on the shim, will ensure that the cutting edge experiences a gradual increase of cutting force, due to the resilience behavior of the hyperelastic materials. For those without hyperelastic property, the cutting edge will experience a sudden increase of cutting force, which excites high frequency vibration energy that is detrimental to the fragile tungsten carbide materials. [0041 ]

[0042] The invention further relates to a cutting insert for use in a cutting tool assembly that comprises a tool head with a seat for receiving said cutting insert, the cutting insert having a shape that corresponds to the shape of the seat or a shim arranged between the seat and the cutting insert, wherein the cutting insert is provided with a layer of hyperelastic material on at least one of its sides, which side is arranged to face the seat or shim, such that the layer of hyperelastic material provides a dampening effect between the cutting insert and a supporting surface.

[0043] Other features of the invention will be apparent from the detailed description below.

BRIEF DESCRIPTION OF DRAWINGS

[0044] Various embodiments and examples related to the invention will now be described with reference to the appended drawings, of which;

Fig. 1 is a diagram of the cutting force variation during machining;

Fig. 2 is a diagram illustrating the hyperelastic material under compression and

indentation load;

Fig. 3 shows a cutting tool assembly according to a specific embodiment of the

invention.

Fig. 4a shows the cutting tool assembly of fig. 3 in a detailed view;

Fig. 4b shows a detailed and exploded view of the cutting tool assembly of fig. 3;

Fig. 5 is a sectional view of a cutting tool assembly according to an alternative

embodiment of the invention; and

Fig. 6 is an exploded view of the cutting tool assembly in fig. 5.

DETAILED DESCRIPTION OF EMBODIMENTS

[0045] Below, specific embodiments of the invention are explained in detail. The invention relates to a cutting tool assembly 10, used for turning, drilling, machining or milling etc. The cutting tool assembly 10 comprises a tool shank 16 which is arranged to be fixed in a tool cassette or the like. A hyperelastic damper 18 may be arranged around the tool shank 16. The tool shank 16 is connected to tool head 1 1 . The tool head 1 1 and the tool shank 16 may either be fixedly or releasably connected to each other. The tool head 1 1 may also be a part of a cutting tool assembly not including a tool shank, and such arrangements are also part of the scope of the invention although not shown in the figures.

[0046] The tool head 1 1 comprises at least one seat 12 where a cutting insert 13 is to be arranged. The cutting insert 13 may have symmetric, e.g. polygonal shape with multiple cutting edges 21 . Thus, the cutting insert 13 may be rotated and re-used a number of times before it as to be replaced for a new cutting insert. The cutting insert 13 may also have similar top and bottom surface, such that it may be turned up-side down to double its numbers of use. The cutting insert 13 may be arranged directly in the seat 12, but most often the cutting insert is supported by a shim 14 arranged between the seat 12 and the cutting insert 13. The shim 14 supports the cutting insert 13 and protects the seat 12 from wear. Both the shim 14 and the cutting insert 13 are expendables.

[0047] In the shown embodiment the cutting insert 13 and the shim 14 are attached to the tool head 1 1 by means of a by fastening screw 20. Other fastening or locking

mechanisms are also applicable and are known to the skilled person. Further, in the shown embodiment, the cutting insert 13 includes protrusions 24, which are arranged to be received in corresponding notches 27 in the shim 14 in order to correctly position the cutting insert 13 with respect to the shim 14 and ultimately with respect to the tool head 1 1 .

[0048] In accordance with a specific embodiment of the invention a hyperelastic damper 15 is arranged between the cutting insert 13 and the shim 14. However, the hyperelastic damper 15 can be provided either between the cutting insert 13 and the shim 14 or between the shim 14 and the seat 12. The hyperelastic damper 15 can be either a separate component in the cutting tool assembly in the disclosed invention, or integrated into the components as a surface coating layer. The hyperelastic damper 15 can also be applied on both the top and bottom surfaces of a cutting insert 13. This is useful for cutting inserts 13 that are provided with a similar top and bottom surface, as it assures that there will always be a dampening layer close to the cutting insert 13 regardless of which side of the cutting insert that is up or down.

[0049] The cutting tool assembly 10 usually has a minimum requirement of stiffness. From this follows that the applied hyperelastic damper 15 should have a stiffness that is at least comparable to said minimum requirement of stiffness, or higher than the minimum requirement. In order to meet this stiffness requirement for hyperelastic damper 15 made of materials with low elastic modulus, such as rubber or polymer, the thickness of the hyperelastic damper needs to be substantially reduced to ensure its stiffness property. [0050] Such hyperelastic materials, e.g. rubbers and polymers, usually have low tribology performance. Therefore, for such materials, a surface shield 19 is preferably applied to protect the surface integrity and improve the tribology performance of hyperelastic damper 15. The surface shield 19 can either be a thin sheet metal that has been cut into the same geometry with adaptable surface profiles as the cutting insert 13 and the shim 14. In the embodiment shown in figures 3 and 4 a-b both the hyperelastic damper 15 and the surface shield 19 are provided as thin sheet layers with a common geometry that matches the profiles of the cutting insert 13 and the shim 14. Hence, the hyperelastic damper 15 and the surface shield 19 have cut-outs 26 and 27, respectively, adapted to allow the protrusions 27 of the cutting insert 13 access to the notches 27 in the shim 14. Further, all parts include a central through hole 22 for allowing passage of the fastening screw 20. The through hole 22 of the cutting insert 13 includes a chamfer 23 for allowing sinking of the head of the fastening screw 20, to thereby correctly position the cutting insert 13.

[0051 ] Both the hyperelastic material 15 and the surface shield 19 may be comprised as a coating layer by other means such as plasma spraying and plasma coating technology. The present invention is not related to the method of applying such coating, but to articles comprising such a coating. The coating method in itself is described in US 201 1/0008576 A1 and EP 2 434 525 A1 .

[0052] The hyperelastic damper 15 can be a surface coating by plasma coating technology which forms a composite layer of a hyperelastic material of a type comprised in the group comprising polymers, composites, metals and metal alloys that to a major part is comprised of nano-sized grains, atom clusters or long chain molecules with at least one internal structure dimension which is less than 200 nm. In a preferred embodiment said at least one internal structure dimension is less than 100 nm, and in a further preferred embodiment said at least one internal structure dimension is less than 10 nm.

[0053] The stiffness of the material expressed as elastic modulus is preferably from 0.1 GPa, up to 550 GPa, so as to ensure the rigidity of cutting tool assembly. The component made by the hyperelastic material should ensure a stiffness that is comparable to the minimum requirement of cutting tool stiffness, typically in the range of a few Newtons per μηι. The lower the elastic modulus of the hyperelastic material is, the thinner the thickness of the component must be to ensure a stiffness of the component. The outer surface area of the component is limited by the geometric dimension of the cutting tools, where the hyperelastic material is to be applied. The inner grain boundary surface area of the component made by the hyperelastic material is proportional to the dimension ratio between the component and the diameter of the nano-sized grains or long chain molecules. For example, the component may have a dimension of 10mm x 10mm X 1 mm, and the nano-sized grain dimension may be 5 nm. In such a case the external surface area is only 240 mm², but the internal surface area of the component is 6χ 10⁷ mm², which leads to a ratio of 2.5x 10⁵.

[0054] The higher the elastic modulus of the hyperelastic material is, the higher the thickness of the component may be without exceeding the stiffness requirement of the cutting tools, and the internal grain boundary surface area is directly proportional to the thickness and inversely proportional to the dimension of the nano-sized grain.

[0055] The nano-sized grain, atom clusters or molecular dimensions are typically below 200 nm, and preferably below 10 nm and the material is preferably made by a composite structure.

[0056] The hyperelastic material can be either added to the surface as a layer of coating, or as an independent part that can be assembled. For example such parts may be provided by high speed compaction, by which compact nano-structured composites may be achieved. The shim or the cutting insert itself can be made of such composites compacted by high speed compaction.

[0057] In general, the production process of similar material is requested to have a low process temperature and short process time, as the nano-sized inclusions tends to migrate and diffuse to agglomerate and form larger crystals. The plasma coating process has low process temperature, but long process time. The high speed compaction, has low process temperature and short process time simultaneously and has more potential.

[0058] In case the hyperelastic material is provided by plasma coating the substrate is typically but not limited to tungsten carbide, silicon carbide, any carbide materials, or steel.

[0059] For example, either the shim 14 or the cutting insert 13 may be made of a hyperelastic material. One example is to build the shim 14 or the cutting insert 13 of tungsten carbide powder mixed with nano-sized impregnates that has been jointly compacted with high speed impaction to be condensed and transformed to a solid that has hyperelastic properties. The tungsten carbide powder can also be milled down to nano-size by a ball milling process prior the compaction or fast sintering processes.

[0060] In figures 5 and 6 an alternative embodiment is shown in which a hyperelastic damper 18 is provided between the tool shank 16 and the tool adapter 17. In similarity to the embodiment shown in figures 3 and 4a-b, the hyperelastic damper 18 may be arranged as a separate layer arranged between the tool shank 16 and the tool adapter 17 or as a coating on either the tool shank 16 or the tool adapter 17. In another embodiment the tool shank 16 is made of a hyperelastic material.

[0061 ] Now, with reference to figure 1 , the characteristics of the hyperelastic material will be described. The hyperelastic damper has a stiffness that is non-linear under the impact of cutting forces typically below 1000 N. As illustrated in figure 1 there is a sudden increase of cutting force at the entrance of a cutting process until the cutting insert build a full and stable contact with the workpiece. The hyperelastic damper has a non-linear stiffness property, and its stiffness is relatively low at small amplitudes of indentation force. With reduced stiffness at small amplitudes of cutting force during the entrance stage of a cutting process, the hyperelastic damper functions as a cushion for the cutting insert to move in the same direction as the cutting force and elongate the time frame of the entrance of a cutting process. The slope of cutting force increment will be reduced due to the hyperelastic damper and the shock wave exerted by the sudden increase of cutting force becomes less harmful. As a consequence, the localized fracture along the cutting edge of the cutting insert is suppressed, and the tool life is improved.

[0062] The hyperelastic material can be made of rubber, polymers, composites, metals, ceramics or alloys. The criterion of becoming a hyperplastic material is that the internal grain size is smaller than 200 nm in at least one dimension. Typically, this includes materials comprising grains, atom clusters or long chain molecules with at least one internal structure dimension which is less than 200 nm. In a preferred embodiment said internal structure dimension is less than 100 nm, or even less than 10 nm.

[0063] The small size grains inside the material exhibit recoverable viscous-elastic flow along the grain boundaries under compression or tension stress. The recoverable viscous-elastic flow enables the hyperelastic property as it changes the elastic modulus of the material. A substantially amount of mechanical energy of the loading is instantly transformed to thermal energy due to the recoverable viscous-elastic flow.

[0064] After a full contact is established between the cutting edge of a cutting insert and a workpiece, the stiffness of the hyperelastic material used in the hyperelastic damper increases due to the increased cutting force and remains the same until the exit phase of a cutting process, shown in figure 1 . The metal cutting process exerts vibrational energy that excites the cutting tool and machine structure over a wide frequency band. The hyperelastic material has large internal grain boundary surface due to their small grain size, typically less than 10 nm, and the grain boundary surface functions as impedance for the vibrational energy transmission. The grain boundaries efficiently transform the vibrational strain energy to thermal energy by the diffusion of atoms or dislocations in the grain boundaries. With reduced grain size below 20 nm, the grain boundary facilitates the diffusion of dislocations instead of blocking them, due to the reason that the maximum allowed number of dislocations inside each grain or long chain molecule decreases with reduced grain size below 20 nm. The hyperelastic material thus performs vibration damping and eliminates the detrimental effect of the excited vibration energy in the form of reduced flaking and spalling, and improved surface integrity of cutting edges. The direct benefit to the end user is a reliable and predictable machining process, well controlled energy consumption of metal cutting processes and reduced production cost due to the prolonged tool life.

[0065] At the exit phase of a cutting process, the hyperelastic damper functions again to reduce the slope of cutting force decrement (dF/dt) due to the recovering of the elastic deformation and the reduced stiffness at reduced cutting force. The reduced slope of cutting force decrement reduces the harmless of the shocking wave energy exerted by the cutting process.

[0066] Above the invention has been described in reference to specific embodiments. The invention is however not limited to any of these embodiments, and is only limited by the scope of the following claims.

Claims

1 . A cutting tool assembly (10) comprising a tool head (1 1 ) with a seat (12) for receiving a cutting insert (13), characterized in that the cutting tool assembly further comprises a hyperelastic material (15, 18) provided to dampen vibrations, the hyperelastic material is comprised in the group comprising polymers, composites, metals and metal alloys to a major part comprised of nano-sized grains, atom clusters or long chain molecules with at least one internal structure dimension which is less than 200 nm, wherein the hyperelastic material has an elastic modulus between 0.1 GPa and 550 GPa, and wherein the layer of hyperelastic material (15) is provided as a surface coating to the cutting insert (13) and /or to a shim (14) arranged between the seat

(12) and the cutting insert (13).

2. The cutting tool assembly (10) according to claim 1 , wherein the hyperelastic material (15) is provided as a dampening layer between the seat (12) and the cutting insert

(13) .

3. The cutting tool assembly (10) according to claim 1 , wherein a shim (14) is arranged between the seat (12) and the cutting insert (13), the layer of hyperelastic material

(15) being provided between the shim (14) and the cutting insert (13).

4. The cutting tool assembly (10) according to claim 1 or 3, wherein a shim (14) is

arranged between the seat (12) and the cutting insert (13), the layer of hyperelastic material (15) being provided between the seat (12) and the shim (14).

5. The cutting tool assembly (10) according to anyone of the preceding claims, which cutting tool assembly (10) further comprises a tool shank (16) and a tool clamp (17), the tool clamp (17) being arranged to hold the tool shank (16), wherein the tool shank

(16) supports the tool head (1 1 ), and wherein a layer of hyperelastic material (18) is provided between the tool shank (16), and the tool clamp (17).

6. The cutting tool assembly (10) according to anyone of the preceding claims, wherein the hyperelastic material is comprised in the group comprising polymers, composites, metals and metal alloys to a major part comprised of nano-sized grains, atom clusters or long chain molecules with at least one internal structure dimension which is less than 10 nm.

7. A shim (14) for use in a cutting tool assembly (10) that comprises a tool head (1 1 ) with a seat (12) for receiving a cutting insert (13), the shim (14) being provided to be arranged between the seat (12) and the cutting insert (13), and having a flat shape that corresponds to the shape of the seat (12) and the shape of the cutting insert (13) wherein the shim is provided with a layer of hyperelastic material on at least one of its flat sides, facing either the seat (12) or the cutting insert (13), wherein the

hyperelastic material is comprised in the group comprising polymers, composites, metals and metal alloys to a major part comprised of nano-sized grains, atom clusters or long chain molecules with at least one internal structure dimension which is less than 200 nm, and wherein the hyperelastic material has an elastic modulus between 0.1 GPa and 550 GPa.

8. A cutting insert (13) for use in a cutting tool assembly (10) that comprises a tool head (1 1 ) with a seat (12) for receiving said cutting insert (13), the cutting insert (13) having a shape that corresponds to the shape of the seat (12) or a shim (14) arranged between the seat (12) and the cutting insert (13), wherein the cutting insert (13) is provided with a layer of hyperelastic material on at least one of its sides, which side is arranged to face the seat (12) or shim (14), such that the layer of hyperelastic material provides a dampening effect between the cutting insert (13) and a supporting surface, wherein the hyperelastic material is comprised in the group comprising polymers, composites, metals and metal alloys to a major part comprised of nano- sized grains, atom clusters or long chain molecules with at least one internal structure dimension which is less than 200 nm, and wherein the hyperelastic material has an elastic modulus between 0.1 GPa and 550 GPa.