Double-rotatable spindle head for machine tools

A double rotatable spindle head of non-perpendicular axis type has a first half head and a second half head. The first half head is connected to the machine structure and rotates about a first axis. The second half head is coupled to the first half head on a flat surface and rotates about a second axis perpendicular to the flat surface. The rotation of the first and second half heads is achieved by a first and second direct motor, respectively.

This invention relates to a double-rotatable spindle head for machine tools.

Double-rotatable spindle heads for machine tools are known. They are divided substantially into double-rotatable heads with perpendicular axes and double-rotatable heads with non-perpendicular axes. Both comprise a first half-head pivoted to the machine structure about a first axis and a second half-head pivoted to the first half-head about a second axis. However while in double-rotatable heads of the first type the two axes of rotation are mutually perpendicular, in machines of the second type the two axes of rotation are not mutually perpendicular.

Double-rotatable heads of the first type, which often present serious limitations in terms of their ability to make the tool approach the surface of the workpiece, and in particular penetrate into the cavity in the workpiece, do not generally present difficulties in orientating the tool axis in the desired spatial direction, in that they enable this orientation to be achieved by rotating said axis about one or both the relative axes of the machine.

Double-rotatable heads of the second type are more advantageous than those of the first type with regard to approaching the surface of the workpiece with their tool, but at the same time are more complicated in terms of their programming and their operational precision. In particular, in this respect even though variations in the tool axis orientation can be thought of as rotations thereof about a physical axis, in reality they are achieved by rotation about a virtual axis as result of several combined machine movements, which can totally or partially involve the three linear X, Y and Z axes and the two rotation axes.

For these reasons, double-rotatable heads with non-perpendicular axes have intrinsically to be more precise than those with perpendicular axes, this precision being generally obtained by constructing these heads mainly for indexed positioning, achieved generally by Hirth toothed precision rings.

However Hirth rings, which are precise and practically without slack, enable only discrete positions to be obtained. This means that the angular positions obtainable for the tool depend on the combination of discrete angles of rotation and are therefore not infinitely variable.

According to the present invention the problem is solved by a double-rotatable spindle head of non-perpendicular axis type for machine tools having direct drive motors.

As can be seen from the figures, the double-rotatable head of the invention comprises, in traditional manner, a first half-head2applied to the machine tool structure (not shown) via a connection element4, and a second half-head6supporting the tool spindle8. The first half-head2can rotate with respect to the machine structure, and hence to the connection element4about a first axis10by virtue of the presence of guide bearings12applied to said connection element4and engaging a circumferential groove14provided in the casing of the first half-head2.

The first half-head2is rotated with respect to the connection element4by a direct motor, the stator16of which is rigid will said connection element4and the rotor18of which is rigid with said first half-head2.

The second half-head6and the first half-head2are coupled together on a flat surface, the representation of which in the plane of the drawing is indicated by the line20. This surface forms an angle of 35° with the first axis10.

The second half-head6can rotate with respect to the first half-head2about a second axis22, by virtue of the presence of guide bearings24applied to said first half-head2and engaging a circumferential groove26provided in the casing of the second half-head6.

The second half-head6is rotated with respect to the first half-head2by a direct motor, the stator28of which is rigid with said first half-head2and the rotor30of which is rigid with said second half-head6.

Both the direct motors, i.e. that interposed between the connection element4and the first half-head2and that interposed between the first half-head2and the second half-head6, can be either permanent magnet synchronous motors or induction motors. The angular position of the first half-head2to the connection element4and the angular position of the second half-head6to the first half-head2are controlled by respective position transducers32,34.

In operation, while the rotation of the direct motor interposed between the connection element4and the first half-head2causes both the half-heads6and2to rotate about the first axis10and hence cause the tool axis to describe a conical surface, which becomes cylindrical when the two axes are parallel, rotation of the direct motor interposed between the first half-head2and the second half-head6causes this latter to rotate about the second axis22to hence vary the angle between the first axis10and the spindle axis.

More specifically, this angle can vary from 0° to 110° and assume all intermediate values between these two end values.

Consequently, by suitably controlling the two direct motors the spindle axis can be made to assume any spatial direction within a cone having a vertex angle of 220°.

From the a foregoing it is apparent that the double-rotatable head of the invention is particularly advantageous compared with traditional double-rotatable heads, and in particular:it enables the tool axis to be orientated with continuity in any spatial direction with high positioning precision and with elimination of slack and friction between the relatively-moving parts,it enables the tool to be positioned in strict proximity to the surface of the workpiece,it ensures good penetrability of the tool and its spindle into any cavities in the workpiece,it enables undercuts to be machined.