DOUBLE-ROTATABLE SPINDLE HEAD FOR MACHINE TOOLS.
This invention relates to a double-rotatabfe 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, as disclosed, for instance, in EP-A-0513716.
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.
Accordingly, the present invention provides a double-rotatable spindle head of
non-perpendicular axis type for machine tools, comprising a first half-head connected
by a connecting element to the machine tool structure so as to pivot about a first axis
and, for supporting the tool spindle, a second half- head coupled to the first half-head
on a flat surface and pivoted to it about a second axis perpendicular to said flat
surface, characterised by a first direct motor having a stator and a rotor for rotating said
first half-head with respect to said machine tool structure and a second direct motor
having a stator and a rotor for rotating said second half-head with respect to said first
half-head.
A preferred embodiment of the invention is described in detail hereinafter with
reference to the accompanying drawings, in which:

Figure 1 is a side view of a double-rotatable head of the invention in a
configuration with the tool axis parallel to the first axis;
Figure 2 is a vertical section therethrough in the same configuration as Figure 1 ;
Figure 3 is a side view thereof in a configuration with the tool axis at an angle of
110° to the first axis; and
Figure 4 is a vertical section therethrough in the same configuration as Figure 3.
As can be seen from the figures, the double-rotatable head of the invention
comprises, in traditional manner, a first half-head 2 applied to the machine tool structure
(not shown) via a connection element 4, and a second half-head 6 supporting the tool
spindle 8. The first half-head 2 can rotate with respect to the machine structure, and
hence to the connection element 4 about a first axis 10 by virtue of the presence of
guide bearings 12 applied to said connection element 4 and engaging a circumferential
groove 14 provided in the casing of the first half-head 2.
The first halfrhead 2 is rotated with respect to the connection element 4 by a
direct motor, the stator 16 of which is rigid with said connection element 4 and the rotor
18 of which is rigid with said first half-head 2.

The second half-head 6 and the first half-head 2 are coupled together on a flat
surface, the representation of 'which in the plane of the' drawing is indicated by the line
20. This surface forms an angle of 35° with the first axis 10.
The second half-head 6 can rotate with respect to the first half-head 2 , about a
second axis 22, by virtue of the presence of guide bearings 24 applied to said first half-
head 2 and engaging a circumferential groove 26 provided in the casing of the second
half-head 6.
The second half-head 6 is rotated with respect to the first half-head 2 by a direct
motor, the stator 28 of which is rigid with said first half-head 2 and the rotor 30 of which
is rigid with said second half-head 6.
Both the direct motors, i.e. that interposed between the connection element 4 and
the first half-head- 2 and that interposed between the first half-head 2 and the second
half-head 6, can be either permanent magnet synchronous motors or induction motors.
The angular position of the first half-head 2 to the connection element 4 and the angular
position of the second half-head 6 to the first half-head 2 are controlled by respective
position transducers 32, 34.
In operation, while the rotation of the direct motor interposed between the
connection element 4 and the first half-head 2 causes both the half-heads

6 and 2 to rotate about the first axis 10 and 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-head 2
and the second half-head 6 causes this latter to rotate about the second axis
22 to hence vary the angle between the first axis 10 and 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 aforegoing 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.

WE CLAIM :
-1. A double-rotatable spindle head of non-perpendicular axis type for machine tools,
comprising a first half-head (2) connected by a connecting element (4) to the machine
tool structure so as to pivot about a first axis (10) and, for supporting the tool spindle
(8), a second half- head (6) coupled to the first half-head (2) on a flat surface (20) and
pivoted to it about a second axis (22) perpendicular to said flat surface (20),
characterised by a first direct motor having a stator (16) and a rotor (18) for rotating said
first half-head (2) with respect to said machine tool structure and a second direct motor
having a stator (28) and a rotor (30) for rotating said second half-head (6) with respect
to said first half-head (2).
-2. A double-rotatable head as claimed in claim 1, comprising a first transducer (32)
for controlling the angular position of the first half-head (2) with respect to machine tool
structure and a second transducer (34) for controlling the angular position of the second
half-head (6) with respect to the first half-head (2).
-3. A double-rotatable head as claimed in claim 1, wherein said flat surface (20)
forms with said first axis (10) an angle less than 45° and preferably between 35° and
40°.

A double-rotatable
spindle head of non-perpendicular
axis type for machine tools, with
a first half-head (2) pivoted to
the machine structure (4) about a
first axis (10) and, for supporting
the tool spindle (8), a second
half-head (6)-coupled to the first
half-head (2) on a flat surface (20)
and pivoted to it about a second
axis (22) perpendicular to said
flat surface (20), characterised by
comprising a first direct motor (16,
18) for rotating said first half-head
(2) with respect to said machine
structure (4) and a second direct
motor (28, 30) for rotating said
second half-head (6) with respect
to said first half-head (2).