Patent Application
20210039172
The present disclosure generally relates to a mass damper. In particular, a mass damper for a cutting tool and a cutting tool comprising the mass damper, are provided.
Cutting tools are usually implemented as cantilever structures and their stiffness substantially decreases with an increased overhang length to diameter ratio. When machining a metal workpiece with a cutting tool, the cutting generates periodic forces on the cutting tool and vibrations arise. Vibrations of the cutting tool during a machining process are undesirable for several reasons. The surface of the workpiece may be destroyed, the cutting tool may break, the machining process may fail etc.
One known solution to suppress vibrations of cutting tools is to provide a tuned mass damper comprising a damping mass such that the vibration energy of the cutting tool is transmitted to the added damping mass. Thereby, the added damping mass vibrates instead of the cutting tool and the cutting tool can be held steady during operation.
In order for mass dampers to function efficiently, it is important to match the resonance frequency of the mass damper to the vibration frequency of the vibrating cutting tool. For this purpose, some cutting tool mass dampers are provided with a tuning mechanism to adjust or tune the resonance frequency of the mass damper. Furthermore, some prior art cutting tools are provided with complex mechatronic parts for measuring the vibration frequency of the cutting tool and for adjusting the stiffness of a spring element in response to measured vibration frequency.
U.S. Pat. No. 3,447,402 A discloses a machine tool boring bar body assembly. The assembly comprises a circular boring bar body having a central bore, ring-shaped viscoelastic absorber elements, a cylindrical damper mass and a tuning mechanism.
US 2016067787 A1 discloses a boring bar for machining operations. The boring bar has an internal chamber within which a vibration dampening mass is supported at each axial end by resilient buffer members. A vibration adjusting piston is linearly moveable with the tool holder and has dampening adjustment engagement with the mass.
The tuning process required for prior art cutting tools is intricate for end users. In addition, an erroneous tuning may lead to costly damages of the tool and/or the workpiece during machining. Calling a professional service technician to make the correct tuning is expensive for most end users.
One object of the present disclosure is to provide a mass damper for a cutting tool that provides an automatic self-tuning.
A further object of the present disclosure is to provide a mass damper for a cutting tool that provides a reliable and efficient vibration damping over time.
A still further object of the present disclosure is to provide a mass damper that enables the use of a damping mass having a relatively large volume (relative to the cutting tool) and/or the use of a relatively compact and stiff cutting tool.
A still further object of the present disclosure is to provide a mass damper for a cutting tool that has a simple, cheap and/or reliable construction.
A still further object of the present disclosure is to provide a cutting tool comprising a mass damper which cutting tool solves one, several or all of the foregoing objects.
According to one aspect, there is provided a mass damper for a cutting tool, the mass damper comprising at least one damping mass; and at least one spring element arranged to support the damping mass, wherein the spring element comprises a nanostructure material with a structural size of 100 nm or less in at least one dimension.
Throughout the present disclosure, the structural size of the nanostructure may alternatively be referred to as a grain size. The nanostructure of the spring element having a structural size of 100 nm or less in at least one dimension provides for a stiffness that is dependent on the vibrational frequency of the spring element. Thereby, a self-tuning function can be realized in the mass damper. The mass damper thereby constitutes a self-tuned mass damper.
A wide range of materials may have a nanostructure material with a structural size of 100 nm or less in at least one dimension. Examples include polymeric materials, resin, such as thermoset resin (resin prior to a curing process), nanocellulose, metal and graphene. Further examples of nanostructure materials with a structural size of 100 nm or less in at least one dimension include various materials doped with nanoparticles (e.g. carbon nanoparticles and the like), nanofibers (e.g. carbon nanotubes) and nanoflakes (e.g. graphene nanoflakes). For example, polymers may be doped with these dopants. Nanostructure materials according to the present disclosure may or may not be cross-linked.
Throughout the present disclosure, a nanostructure may be referred to as a material microstructure observed at the nm scale (e.g. one billionth of a meter), such as on the atomic or molecular level. For the purpose of this application, the term “nanostructure” typically refers to structures having a minor dimension that is greater than about 1 nanometer but typically substantially less than about 100 nm. The nanostructure of the spring element may have a structural size of 100 nm or less in at least one dimension at room temperature (20° C.) or at typical machining environment temperatures (e.g. 0° C. to 60° C.). The nanostructure material of the spring element according to the present disclosure may be prepared by a synthetic approach, such as mixing (e.g. adding nanoparticles into thermoset resin) and blending (e.g. mixing thermoset resin and polymeric materials, and then heat up to obtain a well-distributed mixture of substances that cannot be separated from each other), top-down patterned approaches including chemical vapor deposition (CVD) or molecular beam epitaxy (MBE).
The nanostructure material of the spring element may have a structural size of 100 nm or less in at least two dimensions. Alternatively, or in addition, the nanostructure material of the spring element may have a structural size of 40 nm or less, such as 20 nm or less, in at least one dimension, such as in two dimensions or in three dimensions. The spring element may be constituted by a solid piece of nanostructure material.
Identification of the nanostructure size can be made by means of a scanning electron microscope method, a transmission electron microscope method and an X-ray diffraction method. The X-ray diffraction method measures the dispersion of the X-ray diffraction pattern to decide the particle size.
The damping mass may be supported by the spring element on only one end or side of the damping mass and the damping mass may be unsupported on an opposite end or side. This may be suitable when the damping mass has a relatively short length. The damping mass may be supported by only one spring element on one end of the damping mass, e.g. by only one spring element constituted by a solid piece of nanostructure material. Alternatively, the damping mass may be supported by two or more spring elements on only one end of the damping mass. In this case, the spring elements may be arranged in a stack.
As a further alternative, the spring element may be placed on the external surface of the damping mass (i.e. between the ends of the damping mass) along the axis of the cutting tool, on one or multiple locations. For example, grooves may be formed on an external cylindrical surface of the damping mass and the spring element is placed in the grooves, in contact with a cavity surface of a tool body.
Throughout the present disclosure, the nanostructure material may be amorphous. The nanostructure material may for example comprise multiple types of weak bonding, e.g. Van der Waals bonding. Alternatively, or in addition, the activation energy of the nanostructure material may be on the temperature band between 0° C. and 60 ° C. The temperature band or range between 0° C. and 60 ° C. may constitute a temperature range of a typical machining environment.
The spring element may be connected to the damping mass by an adhesive connection, an interference fit, or a force fit, such as a press fit. The nanostructure material of the spring element may be self-adhesive (e.g. comprising, or being constituted by, a sticky material). In this case, the nanostructure material can adhere to the damping mass without using any additional adhesive. In some applications, the nanostructure material needs not to be heated to provide the adhesive connection by means of the nanostructure material itself. Alternatively, an additional adhesive may be applied to provide the adhesive connection. According to one variant, the spring element according to the present disclosure does not comprise 3 M® 112 viscoelastic polymers.
According to a further aspect, there is provided a cutting tool comprising a tool body and a mass damper according to the present disclosure arranged to damp vibrational movements of the damping mass relative to the tool body. The cutting tool may be constituted by a metal cutting tool. The cutting tool may alternatively be referred to as a machine tool. Vibrational movements of the damping mass according to the present disclosure are dominated by radial and torsional vibrations.
The cutting tool may be stationary or rotatable. In either case, a rotation relative to a workpiece may be generated to machine the workpiece.
The tool body may comprise a cavity and the damping mass may be arranged within the cavity. Alternatively, the damping mass may be arranged external on a cutting tool.
In a further example, the cutting tool may comprise multiple cavities and one or several damping masses arranged in each of the cavities. For example, a cutting tool with multiple cutting inserts (e.g. a milling tool), the cutting tool may comprise multiple cavities located under each of the cutting inserts, where one or multiple damping masses are displaced.
Each of the damping mass and the cavity may have a conical shape substantially conforming to each other. The conical shapes of the damping mass and the cavity may each taper towards a base of the cutting tool and taper away from a cutting head of the cutting tool. The conical shape of the damping mass increases the stiffness of the tool body since the bending moments of inertia on the tool body are increased at locations having a predefined distance to a cutting force loading point.
The cross section of the cavity and the damping mass are not limited to a circular shape. For example, the cross section may be in the shape of a square.
The cutting tool may further comprise a viscous fluid having a frequency dependent stiffness and the viscous fluid may be arranged within the cavity.
The viscous fluid may thereby constitute a part of the mass damper. The viscous fluid may comprise a nanostructure material having a structural size of 100 nm or less in at least one dimension, such as between 1 nm and 100 nm, such as between 5 nm and 100 nm, such as between 5 nm and 20 nm. Alternatively, or in addition, the viscous fluid may comprise nanosized fillers.
The nanostructure material of the spring element may have a frequency dependent stiffness such that a resonance frequency of the damping mass substantially matches, or matches, a vibration frequency of the tool body over a vibration frequency range up to 4000 Hz, such as from 100 Hz to 1000 Hz, such as from 200 Hz to 600 Hz. A substantial match in this regard covers differences between resonance frequencies and vibration frequencies of up to 20%, such as up to 10%, such as up to 5%, such as up to 1%, as well as perfect matches. The stiffness of the spring element may be proportional to a vibration frequency of the cutting tool with a power of two.
Further details, advantages and aspects of the present disclosure will become apparent from the following embodiments taken in conjunction with the drawings, wherein:
a: schematically represents a perspective view of a cutting tool;
b: schematically represents an exploded perspective view of the cutting tool in
a: schematically represents a nanostructure material having a nanostructure in three dimensions;
In the following, a mass damper for a cutting tool and a cutting tool comprising the mass damper, will be described. The same reference numerals will be used to denote the same or similar structural features.
The cutting tool 10 further comprises a mass damper 22. The mass damper 22 of this example comprises one damping mass 24 and one spring element 26 for supporting the damping mass 24. The damping mass 24 may for example be made of tungsten material. In the example of
In this example, the spring element 26 is annular and substantially flat, e.g. the diameter of the spring element 26 is at least 30 times the thickness of the spring element 26. However, alternative shapes of the spring element 26 are possible, including symmetrical shapes, e.g. O-ring shape, asymmetrical shapes, e.g. having varying extensions in radial directions, and spring elements 26 comprising an amorphous nanostructure material. The thickness of the spring element 26 may for example be approximately 1 mm.
The spring element 26 constitutes the only support of the damping mass 24 relative to the tool body 12. Thus, the damping mass 24 is unsupported on the right end in
In prior art cutting tools, the mass is usually supported at both ends by a spring element, such as in the boring bar of US 2016067787 A1. Due to the necessity of the tuning mechanism, the mass needs at least two spring elements to become isolated from the tool body. In contrast, with the mass damper 22 according to the present disclosure, the damping mass 24 may be supported by a spring element 26 on only one of its ends since the tuning mechanism is eliminated.
When designing a damped cutting tool 10, the cutting tool 10 should have an optimal stiffness and an optimal damping. In a typical prior art cantilever cutting tool, such as the boring bar of US 2016067787 A1, the strives for a stiffer cutting tool and for a larger damping mass are conflicting. A larger damping mass requires a longer, and less stiff, cutting tool.
However, an increased stiffness in the mass damper 22 according to the present disclosure will reduce the movement amplitude of the damping mass 24 and avoid collision between the damping mass 24 and the tool body 12. For example, if the tool body 12 has an external diameter of 20 mm and a length of 200 mm, the length of a prior art damping mass may be 40 mm.
By supporting the damping mass 24 from only one end, a space of 4 mm can be gained for the damping mass 24 in the axial direction. Thus, with the mass damper 22 according to the present disclosure, the damping mass 24 can be made 4 mm longer, e.g. 10% heavier, and the spring elements 26 can increase the stiffness by 10% while maintaining the same Eigen frequency.
When the mass damper 22 is vibrating with the same amount of energy, an increase in stiffness by 10% on the spring elements 26 will reduce the vibration amplitude of the damping mass 24 by 5%. The 5% reduction of vibration amplitude the damping mass 24 will reduce the requirements on the surface finish of the cavity 14 under the same design, and thereby lower production costs.
Moreover, by supporting the damping mass 24 by one or more spring elements 26 on only one end of the damping mass 24, an axial space within the cavity 14 is now “free”. This axial space may for example be 4 mm. The volume of the whole cavity 14 can thereby be reduced, e.g. by reducing the axial depth of the cavity 14 with 4 mm. Thereby, the stiffness of the tool body 12 can be increased. With the same damping efficiency but a higher stiffness in the tool body 12, the vibration amplitude of the cutting tool 10 will be further reduced to benefit the machining process.
A damping mass 24 according to
The cutting tool 10 in
Furthermore, the mass damper 22 in
The cutting tool 10 in
The cutting tool 10 in
Most engineering materials have a nearly constant stiffness over the frequency range between 20 Hz and 4000 Hz, such as rubbers, elastomers, steels and ceramics. However, the stiffness of some materials becomes sensitive to temperature when the material is experiencing a phase change (e.g. re-polymerization and re-crystallization), such as memory alloys. The sensitive behavior of material stiffness over temperature is caused by the phase transformation where the structural size inside the materials are reduced to 100 nm or less, such as to 40 nm or less, such as to 20 nm or less.
The theory of time-temperature superposition explains that materials having a mechanical property sensitive to temperature are also sensitive to frequency changes. Such materials undergo a decrease in stiffness when the temperature is increased and an increase in stiffness when the temperature is decreased. This property makes such materials a good choice for the one or more spring elements 26 in order to provide a self-tuning effect of the mass damper 22.
Viscoelastic materials do not necessarily have the described frequency dependent stiffness property. Typical viscoelastic materials, such as rubbers and elastomers, have a nominal structural size between 100 nm and 1000 nm at room temperature near 20° C. and do not show any significant frequency dependent mechanical properties. Materials having a nanostructure where the structural size is 100 nm or less, preferably 40 nm or less, more preferably 20 nm or less, in at least one dimension, will provide a frequency dependent mechanical stiffness. For cutting tool applications, the nanostructure material shall have the nanostructure with a structural size of 100 nm or less at temperatures of industrial machining operation environments (e.g. o ° C. to 60° C.) and/or at room temperature (e.g. approximately 20° C.).
When the nanostructure of the material has one dimension that is 100 nm or less, such as 20 nm or less, the nanosized grains form massive grain boundaries with weak bonding between molecules. These weak bonding are sensitive to temperature. A slight temperature rise will provide the thermal energy to overcome the activation energy of the bonding in the grain boundaries. The time-temperature superposition theory tells that the decrease of excitation frequency (increase of time) has the same effect as increase of temperature.
A nanostructure material according to the present disclosure may be constituted by an amorphous material. In such amorphous material, the stiffness of the material changes gradually over a wide temperature band, such as over a temperature range in a typical machining environment. Furthermore, a nanostructure material according to the present disclosure may have a nanostructure with long chain molecules (nanozise in cross section of the molecule) entangled with each other. Examples of such materials include thermoset resin and nanocellulose. Nanostructure materials according to the present disclosure, such as thermoset resin, can lose 90% of stiffness over a temperature range between 0° C. and 60° C., because there are multiple types of weak bonding (inter-molecular and interatomic Van der Waal bonding) overcoming the activation energy at different temperatures, such as H—H Van der Waal bond, H—C Van der Waal bond, O—H Van der Waal bond etc. The gradual change of stiffness over temperature reflects, i.e. corresponds to, the gradual change of stiffness over frequency, i.e. the preferred frequency dependent stiffness for self-tuning mass damper applications.
Rubbers may also have a temperature dependent stiffness, and the stiffness of rubbers typically varies drastically over the temperature band between −50° C. and 0° C., which is outside the targeted machining operation environment. This means that the activation energy of the weak bonding is in the temperature band between −50° C. and 0° C. Moreover, the extent of stiffness change over a frequency range between 100 Hz and 1000 Hz for rubbers is too low to achieve the self-tuning function.
Furthermore, in order to stabilize the stiffness property of a material over a targeted frequency range, nanostructured particles, fibers and/or flakes can be used as fillers and mixed to the material (e.g. to a thermoset resin material or a nanocellulose material). Examples of such fillers include nanostructured polymer particles (e.g. ground down thermoplastic polymers or elastomers), nanostructured fibers (e.g. carbon nanotubes) and nanostructured flakes (e.g. graphene).
After being heated up within a typical machining environment, the nanostructure material does not necessarily change into a fluid phase. The nanostructure material can undergo a re-polymerization process and form another chemical compound that is stable on the elevated temperature with a different stiffness property. After being heated up to a higher temperature, for example to a temperature of 120° C. or higher, the re-polymerization process of the nanostructure material can become irreversible, and the nanostructure material will lose the frequency dependent stiffness property.
Materials with a nanostructure, where at least one dimension is 100 nm or less, such as 20 nm or less, will have a frequency dependent stiffness controlled by the bonding strength in the grain boundaries over a typical machining temperature. The mass damper 22 comprising one or more spring element 26 having such nanostructure material will have a self-tuning function since the stiffness of the spring element 26 increases when the vibration frequency of the cutting tool 10 increases. The nanostructure material of the spring element 26 according to the present disclosure may have a structural size of 100 nm or less, such as 40 nm or less, such as 20 nm or less, in one direction, two directions, or three directions. The mass damper 22 according to the present disclosure thereby obviates complex mechatronics parts on a cutting tool holder that measure vibration frequency and then adjust the stiffness of the spring elements based on the measured vibration frequency.
It is important to match the Eigen frequency fn of the mass damper 22 with the vibration frequency ft of the cutting tool 10 in order to obtain the highest damping efficiency. The Eigen frequency fn of the mass damper 22 should match with the vibration frequency ft of the tool 10, and can be expressed as:
where k is the stiffness of the spring element 26 and m is the mass of the damping mass 24. The stiffness k of the spring element 26 depends for example on the geometry and material used.
When the vibration frequency of the cutting tool 10 decreases, the stiffness of the spring element 26 will be reduced. As a consequence, the Eigen frequency of the mass damper 22 will be reduced to match the frequency change in the cutting tool 10. Conversely, when the vibration frequency of the cutting tool 10 increases, the stiffness of the spring element 26 will be increased and the Eigen frequency of the mass damper 22 will be increased to match the frequency change in the cutting tool 10. This self-tuning effect of the mass damper 22 is highly valuable for the machine tool industry, for example in cases where the vibration frequency is drifting over time and is different in different machines. The self-tuning property of the mass damper 22 eliminates the intricate tuning process necessary for prior art cutting tools.
By reformulating equation (1), the stiffness k of the spring element 26 can be expressed as:
k=m×(2πfn)2=m×(2πft)2 (2)
The mass m of the damping mass 24 is usually a fixed term. The stiffness of the spring element 26 is thus dependent on the vibration frequency ft of the cutting tool 10. The stiffness of the spring element 26 may change proportionally to the vibration frequency of the cutting tool 10 with a power of two. The frequency dependent stiffness may take place over various frequency bands. For turning and milling tools, the vibration frequency typically changes between 100 Hz and 1000 Hz. The stiffness of the material of the spring element 26 therefore needs to change 100 times to cover the full range of frequency change. For metal cutting applications, the vibration frequency typically changes less. For example, if the vibration frequency changes between 200 Hz and 600 Hz, the stiffness of the material of the spring element 26 needs to change nine times over the targeted frequency band for making perfect tuning.
The stiffness kmd1 of the mass damper 22 in
where X is the displacement amplitude of the damping mass 24, F is the force on the damping mass 24, G is the shear modulus of each spring element 26, t1, is the thickness of each spring element 26, and A is the cross sectional area (perpendicular to the thickness) of each spring element 26.
The stiffness kmd2 of the mass damper 22 in
where X is the displacement amplitude of the damping mass 24, F is the force on the damping mass 24, G is the shear modulus of the spring element 26, t2 is the thickness of the spring element 26, and A is the cross sectional area (perpendicular to the thickness) of the spring element 26.
If the stiffnesses of the mass dampers 22 in
Thus, by supporting the damping mass 24 on only one end of the damping mass 24, the thickness of the spring element 26 can be further reduced to enable a larger damping mass 24 and/or a shorter (stiffer) tool body 12.
While the present disclosure has been described with reference to exemplary embodiments, it will be appreciated that the present invention is not limited to what has been described above. For example, it will be appreciated that the dimensions of the parts may be varied as needed. Accordingly, it is intended that the present invention may be limited only by the scope of the claims appended hereto.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SE2018/050192 | 2/28/2018 | WO | 00 |
