Advanced automation enables pieces which are to be machined be measured and their optimal machining surfaces to be determined at a high speed. Conventionally the pieces are positioned on the feed line so that they will be cut at predetermined points while they pass an axially stationary revolving cutter. Adequate accuracy required for optimal cutting can not be reached when the position of a piece to b machined has to be changed at a high speed. Therefore it is sometimes more advantageous to let the piece travel to the machining station in the same position in relation to the fe line as in which it was, in the measuring phase and to set th cutting device in stead in such an axial movement for the machining that the desired cutting point is reached. This means moving a revolving cutter head in its axial direction during the machining.

The above described arrangement has been employed e.g. in machines for edging of boards as disclosed e.g. in the US patent 4,239,072. The arrangement presupposes fast and long axial movements of the cutter head in order to achieve new edging widths as fast as possible. The axial movement of the cutter head must during the edging be as even as possible to produce a smooth edge in the board. This is easy to realize by servo control when the motor rotating the cutter head move in axial direction with the cutter head. When the edging speed increases the required acceleration becomes so high tha an electric motor can not tolerate it. Also the base of the machine must be very rigid and sturdy. For this reason the electric motor must be detached from the axial movement and torsion be transmitted from the motor to the ^* cutter head through a coupling device which during loading allows the cutter head to move axially approx. 200 mm.

Extention of a shaft is usually realized by using a groove shaft which is also commonly used as a part in articulated shafts. The axial movement of the cutter head during machini should, however, be carried out very smoothly. In a groove shaft the torque aria of the contact surfaces transmitting the torque is very small, for which reason the contact forces and consequently the axial friction forces are great. A servo- controlled positioning cylinder can exert even great adjustin forces but it can not master the sharp jerky resistance of a great friction force.

It is an object of the present invention to provide an axial coupling the friction resisting the axial movement of which i very small.

The invention is described in more detail in the following with reference to the accompanying drawing in which:

Fig. 1 is an end view of an embodiment of the present invention with portions broken away for clarity; and

Fig. 2 is a sectional view taken along line A-A of Fig. 1.

The coupling comprises two hub plates 1 and 2 one of which is the shaft of a stationary electric motor or other driving device and the other one to the shaft of an axially movable cutter head. The hub plates are connected to each other by three levers 3 one end of each lever being secured through a ball joint 4 to the hub plate of the motor and the other end through a ball joint to the hub plate of the cutter head.

The ball joint comprises an outer ring 5 having a spherical inner surface and an inner ring 6 having an outer spherical surface. The outer ring is disposed in the lever 3 and the inner ring is mounted on a screw bolt 7 which is fastened in the hub plate. Three screw bolts or pins are disposed on each

hub plate symmetrically with regard to the centre of the hub plate parallell to the shafts. A distance piece 8 is mounted on the screw bolt between the hub plate and the inner ring. slot 9 in the lever having the same width as the diameter of the collar 10 of the distance piece prevents the lever from tilting.

The distance of the hub plates from each other is chosen to position the coupling in the middle of its axial movement range when all the articulation points are on the same plane in the middle of the distance between the hub plates.

The chosen number of the levers, i.e. three, determines exactly the axial position of the hub plates in relation to the rotation axis. A larger amount of levers could also be used but it would require extreme accuracy in positioning th articulation points in order to avoid internal forces. Only two levers could also be used.

Example

A coupling constructed in accordance to Fig. 1 and 2 having the following dimensions:

distance between the centres of the ball joints in one lever

385 mm; distance of the centres of the ball joints from the centre o the shafts 480 mm; distance between the hub plates 90 - 230 mm, i.e. axial movement 230 mm.