The invention also relates to a tool holder adapted to be employed in a cutting tool as mentioned above, comprising an elongated body having a centre axis, and in one end of the elongated body supporting a circular insert, which is rotatably mounted in bearings about a second axis, being substantially perpendicular to the centre axis.

BACKGROUND OF THE INVENTION The development within the car and aeroplane industry towards lighter and more heat resisting constructions has resulted in that conventional materials, such as cast iron and steel are replaced with new materials, e. g. metal matrix composites and high strength or thermal resistant superalloys.

To machine these materials in a milling operation using conventional cutting tools is considered difficult and time consuming. The reason is that high temperatures and pressures appear during the milling operation, possibly leading to deformation hardening or phase transformation of some materials. Furthermore, to simultaneously use high cutting data is considered even more complicated. Cutting data is comprised from the variables, i. e. cutting speed, depth of cut and feed rate, which determine performance and quickness of the milling operation, at the same time as required

surface finish is maintained. In the pursuit of good metal cutting economics maximisation of cutting data is an evident goal.

Instead of employing conventional cutting tools, cutting tools having rotary inserts provide for a better solution. Rotary inserts imply that circular inserts start to self rotate during engagement with a work piece, enabled by the friction appearing between the insert and the work piece, as well as the fact that the insert is rotary mounted in the cutting tool. Due to the rotating movement wear and heat is uniformly distributed around the insert. This heat and wear distribution imply that the temperature at the cutting zone between insert and work piece is kept at an acceptable level, and thus the areas where deformation hardening or phase transformation occur is avoided. Obviously, the heat and wear distribution also result in longer insert life time and longer interval between necessary tool changes.

Circular inserts have a large nose radius causing difficulties to follow surface contour in finishing operations, but permits high feed rate at rough machining.

In order to optimise cutting data, possibilities to affect and adjust the variables determining cutting data need to be provided for.

One of the variables which is determining cutting data is, as mentioned above, the cutting speed, i. e. the speed at which the insert removes material from the work piece.

The cutting speed, in turn, is determined by the rotary speed of the milling cutter, and the rotary speed of the insert about its axis is determined by the inclination angle of the insert in relation to the work piece together with the cutting speed. Different materials require different rotary speeds and different inclination angels in order to gain an optimal cutting speed and thus an effective milling operation. Accordingly, it is important that a correct angular adjustment in relation to work piece is carried out in order to maximise performance during milling operation.

US 4 477 211 show how tool holders 58,60 comprising rotary, circular inserts are connected to a tool adapter 56. The tool holders are fixed to the tool adapter by means of dovetail joints 12. The inserts of each tool holder exhibit different inclination angles, which may be adjusted by altering the appearance of the dove tail joint at the end 12. However, this require a new tool holder for every desired inclination angle adjustment and is a complicated procedure when optimising cutting data.

US 3 329 065 shows a cutting tool 1 comprising a milling cutter 5. A plurality of suspension attachments 6 are integrated with the milling cutter 5, which attachments each support a circular insert 7. According to this document the inclination angle of the insert is approximately 35° and not changeable. Thus, an optimisation of cutting data can not be achieved.

Other disadvantages are the bearing arrangement of the rotary, circular insert. The shaft on which the insert is connected is mounted in radial bearings and in a thrust bearing. With this arrangement radial and axial forces may be absorbed, but not bending momentum. Furthermore, there is no separate raceway for the radial bearings, which causes stresses on the shaft. This results in a more expensive manufacturing process, since the shaft has to be treated in hardening and grinding operations, but also make the shaft more sensitive to shock loads and vibrations.

SUMMARY OF THE INVENTION An object of the present invention is to optimise cutting data and tool life time of rotary, circular inserts during milling operations.

This object is achieved by means of a cutting tool as initially defined and which is characterised in that each second part may be variably pivoted and clamped in the first part about a third axis being parallel with the first axis. Hereby, each insert can be angular adjusted in relation to the work piece to be machined, whereby optimal cutting data can be achieved.

Furthermore, this object is also achieved by means of a tool holder as initially defined and which is characterised in that the elongated body has a circular cross-section.

Hereby is achieved that the tool holder may be variably angular adjusted about the centre axis when it is arranged in the milling cutter.

In a preferred embodiment the first part comprises a tool adapter which is rotatable about the first axis, and the second part comprises tool holders, whose centre axes coincide with the third axes, and which each support one of the circular inserts, wherein the tool holders are pivotal and clampable in the tool adapter. Hereby, each insert may be separately angular adjusted and may also be changed independently of the other inserts.

Preferably, the tool holders have ends with circular cross-sections, which by means of torque proof clamping means are pivotal and clampable in circular holes in the tool adapter. Hereby, a variable adjustment of the tool holders is possible.

Suitably, the torque proof clamping means is a hydraulically actuated bushing. Hereby is achieved a viscous damping of the tool holder during the machining, which improves machining result as well as tool economy. Furthermore, due to the large contact surface of the hydraulically actuated bushing, a more stable clamping is achieved compared to a screw holder exhibiting a non-uniform clamping.

Advantageously, the tool holders are rotation symmetrically arranged in a circle about the first axis. Hereby, the milling cutter exhibits a stable rotation behaviour around the first axis.

Every circular insert is preferably rotatably mounted in bearings on a pivot having a rotation axis coinciding with the second axis. Hereby, inserts having a large spectrum of cutting angles, compared to a case where the pivot is not perpendicular to the third axis, can be employed.

Preferably, the pivot is mounted in tapered roller bearings. Hereby, a combined loading capacity in both radial and axial directions and ability to absorb bending momentum is achieved. Furthermore, tapered roller bearings have separate raceways whereby the shaft is not subjected to direct stresses from the roller bearings.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be more thoroughly described by means of an exemplifying embodiment and with references to the drawings, on which: Fig. 1 shows a schematic presentation of plane milling according to related art using rotary, circular inserts; Fig. 2 shows a perspective view of a milling cutter according to the present invention; Fig. 3 shows an exploded view of a tool holder according to the present invention; Fig. 4 shows an exploded view of the milling cutter; Fig. 5 shows a hydraulically actuated bushing, and Fig. 6a-b shows the tool holders with different inclination angles.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Fig. 1 shows a schematic presentation of plane milling according to related art using rotary, circular inserts, wherein the figure is only intended to demonstrate the principle of milling using rotary, circular inserts. In general, the invention is not limited to plane milling, but other milling operations, such as axial-and contour milling are also conceivable.

In this figure a cylindrical milling cutter is shown, which is rotary fixed about a rotation axis in a not shown milling machine. Inserts are arranged along the periphery of the cylindrical milling cutter, and are rotary mounted in bearings in a plane being essentially perpendicular to the rotation axis of the milling cutter. During machining, as the milling cutter rotates, the inserts are in engagement with a work piece and material will be removed from the work piece. Due to the friction between the work piece and the inserts as well as the fact that the inserts are rotary mounted in the milling cutter the inserts will start to rotate. This rotating movement imply that a new cutting edge continuously will engage the work piece, whereby wear and heat will be distributed around the respective insert. This increases the performance of the inserts and leads to longer interval between necessary tool changes.

Fig. 2 shows a milling cutter 10 according to a preferred embodiment of the present invention. The milling cutter 10 is rotatable about a first axis I and comprises a tool adapter 12, two tool holders 14 and torque proof clamping means in this case in form of two hydraulically actuated bushings 16. The tool adapter has a substantially cylindrical design with an end portion 18, adapted to be fixed in a not shown milling machine. The tool holders 14 are protruding perpendicularly from an under side of the tool adapter 12, in which they are removably arranged by means of the hydraulically actuated bushings 16, but also pivotal and clampable about respective third axes III, which are parallel to the first axis I. Furthermore, the tool holders 14 are circumferentially divided around the tool adapter 12 so that they are rotation symmetrically arranged about the first axis I, whereby stability is obtained during milling.

On a protruding end of each tool holder 14 a circular insert 20 is supported, which insert is rotary mounted in bearings about a second axis II. This second axis II is situated in a plane being substantially perpendicular to the first axis I. Preferably, the plane is completely perpendicular to the first axis I, which results in larger freedom to

choose inserts having different cutting edge angles compared to axes situated in non- perpendicular planes.

The advantage with this construction is that the inclination angle of the inserts, in relation to the work piece to be machined, can be adjusted, which is not possible using related art solutions having rotary, circular inserts. This is made possible since the tool holders can be pivoted about the third axis III and be clamped in a desired position.

Together with the rotational velocity of the milling cutter, the inclination angles determine the rotary speed of the inserts, but also the effective cutting velocity, i. e. the speed by which material is removed from the work piece. The effective cutting velocity is an important variable, which together with depth of cut and feed rate of the work piece, determine the performance, efficiency and result of the cutting operation.

Different materials have different metallurgical properties and therefore require different combinations of the above mentioned variables in order to make the milling operation as effective as possible. Thus, a careful choice of the inclination angle is important. This will be more thoroughly explained in the end of description.

Fig. 3 shows the tool holder 14. It has an elongated portion 22 with a circular cross- section, and a longitudinal centre axis which coincide with the third axis III. The elongated portion 22 with the circular cross-section is adapted to, via the earlier mentioned hydraulically actuated bushing, be pivotally fixed in the tool adapter 12, so as to adjust the inclination angle of the insert. In an other end of the tool holder 14 the circular insert 20 is arranged on a pivot 24 by means of a screw 26. The screw 26 is designed to have a somewhat countersunk screw head in order not to influence the chip flow during milling. The pivot 24 itself is fixed in a housing 30 by means of a shaft nut 34, and has a rotation axis coinciding with the second axis II. The pivot 24 is able to rotate freely in the housing 30, while it is supported by a bearing arrangement means comprising two tapered roller bearings 32 allowing the above mentioned rotating movement. By means of these bearings 32 the pivot 24 can effectively absorb necessary axial and radial forces but also generated bending moments.

Fig. 4 shows an exploded view of the milling cutter and how the various parts are fitted together. Evidently, at an under side 36 of the tool adapter circular bore holes 38 are disposed. These holes are arranged in a circle, which is concentric with the contour of the tool adapter and thus have the first centre axis I as centre point. The holes 38 are adapted for the tool holders 14, which may be introduced into the holes via the hydraulically actuated bushings 16 (also shown in figure 5). The main object of the hydraulically actuated bushing 16 is to clamp and fix the tool holders 14 in the holes 38. The bushing has an outer diameter D and an inner diameter d. The bushing may with a small clearance be pressed onto the circular end 22 of the tool holder 14, and the bushing 16 may then, with the outer diameter D, be fitted into one of the holes 38 at the under side of the tool adapter 12. Between the inner diameter d and the outer diameter D are formed a not shown, closed, inner space, which is in contact with the surroundings through a nipple 42. This inner space may under pressure be filled with, or emptied of, oil or other suitable, viscous, compressible medium. When enough oil is pressed, via nipple 42, into the closed space, the inner diameter d will diminish while the outer diameter D increases. In this way the tool holder having such a bushing, can be variably angular adjusted about the third axis III in order to be fixed in a desired position in the tool adapter 12. Another important advantage with this bushing is that viscose damping of the tool holder is gained, which restrains vibrations.

Fig. 6a-b shows from below the milling cutter 10 with two tool holders 14. The purpose with the figure is to show inserts with different inclination angles As. During milling of a work piece lying in the same plane as the paper, engagement with the work piece will take place at point A. This point A will during rotation of the milling cutter 10 about axis I, travel in a circular path C having I as centre. The inclination angle B5 is defined as an angle between a tangent T of the circle C in point A and a normal N to the front side of the insert in point A. Thus, in figure 6a the inclination angle k, l is approximately 30°, while the inclination angle XS2 in figure 6b is 0°, since the tangent T2 and the normal N2 coincide. The latter angle imply no rotation of the insert during milling.

Appropriate inclination angle during milling is dependent on many factors, such as chosen material, milling method and desired machining finish, but normally inclination angles As between-60° and +60° are used.

Furthermore, it is of course possible to use larger number of tool holders than those two being shown in the figures. The main thing is that the tool holders are symmetrically distributed about the tool adapter, so that the milling cutter is balanced during its rotation. This is clearly shown in figures 6a-b.