Eddy current vibration absorber assembly for cutting tool

A cutting tool includes a cutting insert mounted to a head attached to a collar at a first end of the cutting tool. A shank is located at a second, opposite end of the cutting tool. A central cavity extends inwardly from the first end toward the shank. An eddy current vibration absorber assembly is disposed within the central cavity. The eddy current vibration absorber assembly includes an absorber mass made of an electrically conductive material, a magnetic material proximate the absorber mass, and a support member for supporting the absorber mass within the central cavity. The eddy current vibration absorber assembly is tuned by selectively adjusting a distance between the absorber mass and the magnetic material.

FIELD OF THE INVENTION

In general, the invention relates to a cutting tool and, more particularly, to a cutting tool, such as a boring bar, with a tunable dynamic vibration absorber assembly that utilizes eddy currents for vibration suppression.

BACKGROUND OF THE INVENTION

During a metal cutting operation, any vibratory motion between a cutting tool and workpiece may lead to undesirable cutting performances such as poor workpiece surface finish and out-of-tolerance finished workpieces. Furthermore, such vibration may cause the cutting tool or the machine tool to become damaged.

To reduce these vibrations, the metal removal rate can be decreased. However, this approach interferes with production and only minimally reduces the amount of vibration.

Instead of decreasing metal removal rates, tuned boring bars have been manufactured using a heavy mass supported by rubber elements. The rubber elements are responsible for providing the stiffness and damping for the dynamic absorber system. Stiffness and damping are material specific properties. Therefore, it is impossible to design the dynamic absorber package to specified stiffness and damping parameters for optimum performance using rubber elements alone. Another method to introduce damping that has been employed is the addition of a viscous fluid in the dynamic absorber cavity within the boring bar shank. While this method does increase damping, the rubber elements and viscous fluid must be chosen carefully to ensure compatibility. Otherwise, the viscous fluid can deteriorate the rubber elements and alter the performance of the boring bar. Therefore, there is a need to provide a dynamic absorber that solves the above-mentioned problems.

SUMMARY OF THE INVENTION

The problem of using rubber elements to suppress vibration in a cutting tool is solved by providing an eddy current dynamic absorber. The eddy current effect arises from the interaction of a magnet that produces a strong magnetic field and a mass made of a highly electrically conductive material. The relative motion between the magnet and the conductive material causes the conductive material to experience a change in the magnetic flux (i.e. varying magnetic field strength) that is described by Faraday's law of induction, thereby creating a flow of electrons within the conductive material. As the current flows, it creates small loops, which are referred to as eddy currents. The direction of the eddy current obeys Lenz's law, which states that an induced current always flows in the direction opposite to the source that produced the induced current. Due to the direction of rotation of the eddy currents, a magnetic field is produced that is opposite in direction to the relative motion that created them. This opposite magnetic force resists the relative motion and provides damping of vibrations during a cutting operation. The amount of damping that can be produced is determined by the conductivity of the conductive material, the magnetic strength of the magnet, the thickness of the conductive material, the surface area of the magnet, and the distance between the conductive material and the magnet. These factors can be used to mathematically predict the amount of damping obtainable which will make it possible to build a dynamic absorber to the specified damping parameters needed to optimize the performance of the tuned eddy current vibration absorber assembly.

In one aspect of the invention, a cutting tool comprises a cutting insert mounted to a head attached to a collar at a first end of the cutting tool. A shank is located at a second, opposite end of the cutting tool. A central cavity extends inwardly from the first end toward the shank. An eddy current vibration absorber assembly is disposed within the central cavity. The eddy current vibration absorber assembly comprises a magnetic material proximate an absorber mass made of electrically conductive material, and a support member for supporting the absorber mass within the cavity, wherein the eddy current vibration absorber assembly is tuned by selectively adjusting a distance between the absorber mass and the magnetic material.

In another aspect of the invention, an eddy current vibration absorber assembly comprises an absorber mass made of an electrically conductive material; a magnetic material proximate the absorber; and a support member for supporting the absorber mass within a cavity, wherein the eddy current vibration absorber assembly is tuned by selectively adjusting a distance between the absorber mass and the magnetic material.

In yet another aspect of the invention, a method of tuning an eddy current vibration absorber assembly, the assembly comprising an absorber mass made of an electrically conductive material; a magnetic material proximate the absorber; and a support member for supporting the absorber mass within a cavity, the method comprising selectively adjusting a distance between the absorber mass and the magnetic material.

DETAILED DESCRIPTION OF THE INVENTION

Referring now toFIGS. 1-3, a cutting tool10, such as a boring bar, is shown according to an embodiment of the invention. Although the present invention is directed to a boring bar10for boring deep holes in work pieces, the principles of the invention can be applied to any cutting tool that produces vibrations when cutting a work piece.

A cutting tool, such as a cutting tool12, such as a cutting insert, may be mounted in a conventional manner to a head14attached to a collar16at one end18of the boring bar10. A shank20is located at the opposite end22of the boring bar10. The boring bar10has a central cavity24extending inwardly from the end18toward the shank20. In the illustrated embodiment, the boring bar10includes a coolant tube26extending along a central, longitudinal axis28of the boring bar10for providing coolant proximate the cutting insert12. It will be appreciated that the coolant tube26is optional and can be omitted. For example, the coolant tube can be replaced with a threaded stud or a solid stud with threaded ends. In another example, the shank20can be manufactured without the central cavity20as a solid shank.

Use of the boring bar10in a metalworking operation will produce vibrations that travel through the boring bar10, thereby affecting the stability of the cutting process. For this reason, the boring bar10is provided with an eddy current vibration absorber assembly, shown generally at30, according to an embodiment of the invention that will dampen the vibrations traveling through the boring bar10.

In general, Faraday's law of induction is a basic law of electromagnetism predicting how a magnetic field will interact with an electric circuit to produce an electromotive force (EMF), which is a phenomenon called electromagnetic induction. It is a fundamental operating principle of transformers, inductors, and many types of electrical motors, generators and solenoids.

Faraday's law states that the EMF is also given by the rate of change of the magnetic flux:

where ϵ is the electromotive force (EMF) and Φ is the magnetic flux. The direction of the electromotive force is given by Lenz's law, which states that the direction of current induced in a conductor by a changing magnetic field due to Faraday's law of induction will be such that it will create a field that opposes the change that produced it. Lenz's law is shown by the negative sign in Faraday's law of induction:

which indicates that the induced voltage (ϵ) and the change in magnetic flux (δΦ) have opposite signs. It is a qualitative law that specifies the direction of induced current, but says nothing about its magnitude. Lenz's law explains the direction of many effects in electromagnetism, such as the direction of voltage induced in an inductor or wire loop by a changing current, or why eddy currents exert a drag force on moving objects in a magnetic field.

Eddy currents (also called Foucault currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material.

According to Lenz's law, an eddy current creates a magnetic field that opposes the magnetic field that created it, and thus eddy currents react back on the source of the magnetic field. For example, a nearby conductive surface will exert a drag force on a moving magnet that opposes its motion, due to eddy currents induced in the surface by the moving magnetic field. This effect is employed in eddy current brakes, which are used to stop rotating power tools quickly when they are turned off. The current flowing through the resistance of the conductor also dissipates energy as heat in the material.

Referring now toFIG. 3, the eddy current vibration absorber assembly30comprises a generally cylindrical absorber mass32adjacent a magnetic material34. In the illustrated embodiment, the magnetic material34comprises one or more permanent magnets34disposed within the cavity24proximate the shank20of the cutting tool10. However, it will be appreciated that the invention is not limited by the type of magnetic material, and that the invention can be practiced with any desirable magnetic element that produces a magnetic field, such as an electromagnetic field, and the like.

At least a portion of the absorber mass32is made of highly electrically conductive material, such as copper, aluminum, and the like. In the illustrated embodiment, the absorber mass32has two portions: a first portion32amade of a highly electrically conductive material, such as copper, aluminum, and the like; and a second portion32bmade of a different material with a relatively higher density, such as tungsten, and the like. The second portion32bis provided in the illustrated embodiment so that the absorber mass32has an adequate amount of total mass to adequately dampen vibrations in the cutting tool10. However, it will be appreciated that the absorber mass32can be made of a single piece of electrically conductive material, so long as the mass of the absorber mass32is sufficient to provide adequate dampening of vibrations in the cutting tool10. The first portion32acan be secured to the second portion32bby press fitting, brazing, welding, and the like. In the illustrated embodiment, for example, the first portion32ais press fit to the second portion32b.

As seen inFIG. 3, the absorber mass32has a first end36proximate the magnetic material34, and a second, opposite end38proximate the collar16. In the illustrated embodiment, the absorber mass32has a total length of about 5.0 inches (127 mm), wherein the exterior surface of the first portion32ahas a length of about 4.0 inches (101.6 mm) and the exterior surface of the second portion32bhas a length of about 1.0 inches (25.4 mm). Thus, there is about a 4:1 ratio in the relative length between the first portion32aand the second portion32b.

The absorber mass32is supported within the cavity24by a support member40. In the illustrated embodiment, the support member40is made of a suitable material to provide some stiffness or rigidity, but allow the absorber mass32to move within the cavity24. For example, the support member40can be made of a relatively strong, lightweight material, such as titanium, and the like. In the illustrated embodiment that includes the coolant tube26, the support member40is annular in shape to allow the coolant tube26to pass through the support member40. In another embodiment in which the coolant tube26is omitted, the support member40can be a solid member. One end40aof the support member40is secured within a cavity42of the collar16and the opposite end40bof the support member40is secured within a cavity44of the absorber mass32. The support member40can be secured to the collar16and the absorber mass32by press fitting, brazing, welding, and the like. In the illustrated embodiment, the support member40is press fit into the collar16and the absorber mass32.

As noted above, the absorber mass32is suspended within the cavity24only by the support member40and the absorber mass32is allowed to move within the cavity24. It is noted that the coolant tube26(if included) does not provide any additional support for the absorber mass32. In addition, the absorber mass32has an outer diameter that is smaller than the inner diameter of the cavity24that enables the absorber mass32to freely move in two directions perpendicular to the longitudinal axis28of the cutting tool10(i.e. in the y- and z-directions). Further, the second end38of the absorber mass32is separated from the collar16by a small distance46to allow clearance between the collar16and the absorber mass32. Thus, the support member40acts as a cantilever beam and the absorber mass32acts as a point mass on a tip of the support member40(i.e. cantilever beam). It should be noted that the absorber mass32is fixed in the x-direction, and that the absorber assembly30can be tuned by adjusting the distance48between the absorber mass32and the magnetic material34.

Referring now toFIG. 4, a schematic representation of the cantilever of the eddy current vibration absorber assembly30is described. The eddy current vibration absorber assembly30has a natural frequency, Wn, that can be defined by the following equation:

The stiffness, K, is given by the following equation:

The mass, M, is given by the following equation:
M=(33/140)Mb+M(Eq. 5)
where,
Mb=mass of beam (kg),
Mt=mass at tip (kg),

The amount of the highly electrically conductive material32aof the absorber mass32is determined by how much damping is required. If the amount of damping is met by the amount of material32aand the mass requirement is not met, then the material32bhaving the relatively higher density is added until the mass requirement is met. The length, L, outer diameter (OD) and inner diameter (ID) of the support member40is selected to meet a specific stiffness requirement.

Referring back toFIG. 3, the electrically conductive material32aof the absorber mass32is separated from the magnetic material34by a very small distance48when the eddy current vibration absorber assembly30is mounted within the central cavity24of the cutting tool10. The distance48can be in a range between about 0.001 inches (0.025 mm) to about 0.100 inches (2.54 mm). In the illustrated embodiment, the distance48is about 0.01 inches (0.25 mm). The small distance48allows the eddy current vibration absorber30to be tuned by selectively adjusting the distance48between the absorber mass32and the magnetic material34.

FIGS. 5 and 6shows test results for the eddy current vibration absorber assembly30of the invention. Specifically,FIG. 5shows a graph of real data (m/N) as a function of the distance48between the absorber mass32and the magnetic material34, andFIG. 6shows a graph of imaginary data (m/N) as a function of the distance48between the absorber mass32and the magnetic material34. InFIGS. 5 and 6, the distance48was selectively adjusted between about 0.50 inches (12.7 mm) and 0.010 inches (0.25 mm). As seen inFIGS. 5 and 6, the distance48of about 0.50 inches (12.7 mm) provided the least desirable damping, while the distance48of about 0.010 inches (0.25 mm) provided the most desirable damping.

As described above, the cutting tool10, such as a boring bar, includes an eddy current vibration absorber assembly30that utilizes eddy currents to dampen vibrations of the cutting tool10. The method of using eddy currents for suppression of vibrations generated by the cutting tool provides for superior damping capability, as compared to conventional damping methods using an absorber mass with rubber elements.

The patents and publications referred to herein are hereby incorporated by reference.

Having described presently preferred embodiments the invention may be otherwise embodied within the scope of the appended claims.