Tool holder

A tool holder has a cutter arbor. The cutter arbor has a shank portion detachably mounted on a spindle, and a mechanism for holding a cutting tool. In the cutter arbor portion, a cavity portion is formed. In the cavity portion, a vibration damping material is filled at a filling rate so as to be capable of obtaining predetermined vibration damping characteristics. Thus, it is possible to provide high vibration damping characteristics, effectively damp vibrations during cutting and improve working precision, without using any special materials, such as damping materials, although the tool holder has a usual shape.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese application No. 2001-377059, filed Dec. 11, 2001, the content of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a tool holder for attaching a cutting tool to a spindle of a machine tool.

2. Description of Related Art

In machine tools, a cutting tool is exposed continuously to cutting resistance forces and so forth during a cutting process for a work piece, so that vibratory forces are unavoidably generated. Since vibrations generated in the cutting tool have a harmful influence on working precision in machine tools, so that various measures are conventionally taken.

In general, the cutting tools and tool holders are made of materials having high vibration damping characteristics to inhibit vibrations during cutting processes, and/or tool holders are integrally formed with damping materials by welding or shrinkage fit to inhibit vibrations.

In addition to the enhancement of the vibration damping characteristics of the tool holder itself, it is a well-known art to incorporate a damping mechanism in a spindle unit. For example, Japanese Patent Laid-Open No. 1994-277806 discloses a damping mechanism in which elastic body and a viscous fluid are filled between a spindle unit and a spindle head to damp vibrations by the elastic deformation of the elastic body and the viscous resistance of the fluid.

However, it is not possible to expect effective damping capacity by depending on the vibration damping characteristics of the materials themselves of the tools and tool holders like conventional measures. In particular, vibrations are transferred to a machined surface in finishing machining, so that the working precision on the machined surface deteriorates. In addition, the damping materials have a problem in production costs since the damping materials are generally expensive. Moreover, there is a problem in that it is not obtain a damping action effective in the prevention of fretting which is caused on the inner peripheral surface of a tapered hole of a spindle contacting a taper shank of the tool holder.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a tool holder which has high vibration damping characteristics, which can effectively damp vibrations during cutting processes and which is effective in the improvement of working precision, without using any special materials, such as damping materials, although the tool holder has a usual shape.

In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, a tool holder for a spindle of a machine tool, said tool holder comprises an arbor having a shank portion which is detachably attached on the front end of the spindle, and a mechanism for holding a cutting tool, a cavity portion defined in the arbor; and a vibration damping material charged in the cavity portion at a predetermined charging rate in a manner that the damping material shows vibration damping characteristics.

According to the present invention, by vibrations produced during a cutting work, sand particles filled in the cavity portion collide with each other and with the walls of the cavity portion to convert vibrational energy into thermal energy, so that vibrations can gradually attenuate.

Referring now to the accompanying drawings, a preferred embodiment of a tool holder according to the present invention will be described below.

FIG. 1shows a tool holder in this preferred embodiment. Reference number2denotes an arbor constituting a body of a tool holder. On the front end portion of the arbor2, a tool mounting portion4is provided. A cutting tool6is attached into the tool mounting portion4to be fixed thereto by means of a bolt (not shown).

The arbor2has a shank portion7and a flange portion8. On the end portion of the shank portion7, a pull stud9is fixed. By inserting the shank portion7into a tapered hole open on a spindle and by gripping and pulling the pull stud9by means of a collet connected to a draw bar built in a spindle, the arbor2can be fixed to the spindle.

As shown inFIG. 2, a ring-shaped groove is formed in the end face of the flange portion8of the arbor2. By sealing the ring-shaped groove with a cover member10, an annular cavity portion12filled with a vibration damping material therein is defined coaxially with the arbor2. This cavity portion12is preferably arranged nearer to the cutting tool6.

In this preferred embodiment, the vibration damping material is a viscous fluid, such as an oil, in which sand is densely mixed so that sand particles contact each other.

As the vibration damping material, only fine sand densely charged may be applicable. In addition to sand, fine glass particles, ceramic fine particles and metal particles may be used. The particle size and shape of the particles are not uniform but preferably uneven.

With this construction, the operation of the tool holder in this preferred embodiment will be described below.

When a cutting work is carried out, if the spindle is moved while rotating, the cutting tool6mounted on the tool holder cuts a work piece. In the cutting process, vibratory forces are generated by cutting resistance forces and so forth, and transferred from the cutting tool6to the arbor2to vibrate the arbor2. By the vibrations, the sand particles charged in the cavity portion12start to move to collide with each other and with the walls of the cavity portion12. Thus, vibratory energy is converted into thermal energy, so that the vibrations gradually attenuate. Therefore, in the finish machining, it is possible to prevent vibrations from being transferred to the machined surface. Since the movement of the particles is converted into thermal energy, it is possible to obtain a larger damping action than conventional tool holders which use special damping materials. In addition, by adjusting the charging rate and grain size of the particles, it is possible to obtain damping characteristics which are effective in the prevention of fretting wear caused on the outer peripheral surface of the shank portion7.

By thus charging particles, such as particles of sand, in the cavity portion12in the arbor2, it is possible to provide damping characteristics, which are not possessed by usual tool holders, to the tool holder to damp vibrations during the cutting work.

Furthermore, a viscous fluid may be charged at a flowable charging rate without using particles. In this case, vibratory energy is changed to thermal energy by the shear of the fluid according to the movement of the fluid, so that vibrations can be damped. Although the whole tool holder is unbalanced so that the center of gravity is slightly shifted from the shaft center, the fluid in the cavity portion12automatically compensates the unbalance of the tool holder in accordance with the rotation of the tool holder during working.

As described above, according to the present invention, a tool holder comprises a cutter arbor having a shank portion which is detachably mounted on a spindle, and a mechanism for holding a cutting tool, wherein a cavity portion is formed in the arbor, and a vibration damping material is filled in the cavity portion at a filling rate so as to be capable of obtaining predetermined vibration damping characteristics. Therefore, it is possible to provide high vibration damping characteristics, effectively damp vibrations during cutting and improve working precision, without using any special materials, such as damping materials, although the tool holder has a usual shape.