"TABLE GROUP FOR A MACHINE TOOL"

[0001]The present invention relates to the field of machine tools, and in particular to the field of large machine tools, such as a vertical lathe. In particular, it is an object of the present invention to provide a table group of a machine tool, intended to support the semi-finished product to be machined.

[0002]Such machine tools are used for performing machining operations on large semi-finished products, such as shafts for marine engines, tanks for the oil industry or parts of power plants.

[0003]Due to the criticality of such components, extremely tight machining tolerances are generally required. Therefore, machine tool manufacturers are constantly researching and developing solutions that can guarantee these tolerances.

[0004]It is the purpose of the present invention to provide a table group for a large machine tool that meets the needs of the sector.

[0005]Such an object is achieved by a table group according to claim 1 . The claims dependent thereon describe further advantageous variant preferred embodiments . [0006]The features and advantages of the table group according to the present invention will be apparent from the following description, given by way of non-limiting example, according to the accompanying figures, in which:

- figure 1 shows a machine tool according to an embodiment of the present invention;

- figure 2 shows a table group of the machine tool in figure 1 according to an embodiment of the invention;

- figure 3 shows the support assembly of the table group in figure 2, according to an embodiment of the invention; - figure 4 is a cross-section view of the table group, which shows a main skid of the table group;

- figure 5 is a cross-section view of the table group, which shows a dog of the table group;

- figure 6 shows a dog of the table group, according to an embodiment of the invention;

- figure 7 is a cross-section view of the dog in figure 6, which shows a secondary skid and a piston.

[0007]With reference to the accompanying figures, a large machine tool is indicated by reference numeral 1 as a whole.

[0008]For example, said machine tool is a vertical lathe. [0009]For simplicity of exposition, reference will hereinafter be made to a vertical lathe, but the invention is likewise referred to a milling machine or a generic machining center. [0010]In the illustrated embodiment (figure 1), the machine tool 1 comprises a pair of benches 2, each extending prevalently along a longitudinal gantry translation axis Y, parallel to each other, resting on a reference plane, e.g., the ground plane. The machine tool 1 further comprises a pair of uprights 4, each extending prevalently vertically, parallel to each other and adapted to be moved upon command along the gantry translation axis Y. [0011]The machine tool 1 further comprises a crossbar 6, having extending prevalently transversally, translatable supported by the two uprights 4; in particular, the crossbar 6 can translate upon command along a vertical crossbar translation axis W, perpendicular to the reference plane.

[0012]The machine tool 1 further comprises a head 8 translatably supported by the crossbar 6; in particular, the head 8 can translate upon command along a transverse head translation axis X, orthogonal to the crossbar translation axis W or parallel to the reference plane.

[0013]The machine tool 1 further comprises a spindle (not shown), carrying a tool for mechanical machining, translatably supported by the head 8; in particular, the spindle can translate upon command along a vertical spindle translation axis Z, perpendicular to the reference plane.

[0014]For supporting the semi-finished product to be subjected to the turning (or milling) process and rotating it about a rotation axis C, the machine tool 1 further comprises a table group 10, generally arranged between the benches 2.

[0015]In the illustrated embodiment (figures 2 and 3), the table group 10 comprised a fixed, tub-shaped support assembly 12 and a typically circular table 14, rotatable about the rotation axis C, supported by the support assembly 12.

[0016]The table group 10 further comprises at least one electric motor 16 rotationally actuating the table 14, operatively connected to said table 14 through a kinematic chain.

[0017]For example, each motor 16 is located on the reference plane, with a horizontal axis, adjacent to the rotary table 14, and the kinematic chain comprises a bevel gear redirecting device 18 to achieve rotation with a vertical axis.

[0018]The table 14 is further provided with a toothed ring gear 20 (figure 4), operatively connected to the motor 16, e.g., through said redirecting device 18, to be driven in rotation. [0019]For example, the table 14 comprises an upper portion 14a, bearing the circular upper face 14' for supporting the semi-finished product, and a lower portion 14b, projecting axially downwardly from the upper portion 14a. The upper portion 14a has a first maximum diameter Dl, while the lower portion 14b has a second maximum diameter D2; the first maximum diameter Dl is greater than the second maximum diameter D2 so that the table has a stepped configuration.

[0020]Preferably, the ring gear 20 is fixed coaxially to the lower portion 14b of the table 14.

[0021]According to an embodiment, the support assembly 12 comprises an annular wall 21, a bottom 23, which inferiorly closes the annular wall 21, and a band 22 in the form of a circular ring gear, coaxial to the rotation axis C, placed at the upper end of the annular wall 21, to circumferentially delimit the opening towards an inner compartment 25 of the support assembly. A peripheral region of the upper portion 14a of the table 14 is superimposed on the band 22. [0022]A labyrinth device 30 is operative between the band

22 and the peripheral region of the upper portion 14a to achieve a seal between the rotating table 14 and the fixed support assembly 12.

[0023]The table group 10 further comprises a plurality of hydrostatic skids 40 for supporting the table 14 by means of pressurized fluid, capable of providing a lifting action FI, directed from the bottom upwards, which tends to lift the table 14 relative to the support assembly 12, thereby detaching it from contact with the latter and allowing the rotation with low friction.

[0024]Preferably, the hydrostatic skids 40 are located below the ring gear 20 to cooperate with a lower face 20' of said ring gear 20 to form a pressure chamber 40' or meatus. [0025]According to a preferred embodiment, each hydrostatic skid 40 has an inner pocket 42, for forming an inner meatus, and an outer pocket 44, located radially external to the inner pocket 42, for forming an outer meatus. [0026]Preferably, the table group 10 further comprises a first hydraulic pump and a first circuit, connected to the first hydraulic pump, for supplying pressurized fluid to the inner pockets 42 of all the hydrostatic skids 40, and a second hydraulic pump and a second circuit, connected to the second hydraulic pump, for supplying pressurized fluid to the outer pockets 44 of all the hydrostatic skids 40.

[0027]The table group 10 further comprises detection means adapted to detect the lifting action FI generated by the main skids 40; e.g., the detection means comprise at least one pressure sensor (not shown) for detecting the fluid pressure operating in the main pressure chamber 40'; preferably, the table group comprises a first pressure sensor operating along the first circuit and a second pressure sensor operating along the second circuit.

[0028]In a preferred embodiment, in which a plurality of pressure sensors, which detect the pressure in only a few main skids, is provided, the detected pressure values are interpolated to estimate the value of the pressure also on the other skids and thus calculate the lifting action FI.

[0029]The fluid escaping from the main chambers 40' of the main skids 40 is collected on the bottom 23 of the support assembly 12, preferably by gravity, and returned by a pumping device 50 to the hydrodynamic unit, which pressurizes the hydrostatic skid supply circuit.

[0030]For example, the table group 10 comprises twelve main skids, placed angularly equispaced. [0031]Furthermore, according to the invention, the table group 10 comprises a plurality of dogs 60, adapted to operate by means of pressurized fluid on the table 14, providing a contrasting action F2, from the top downwards, contrary to the lifting action FI. [0032]Preferably, said dogs 60 are placed above said ring gear 20 to cooperate with an upper face 20'' of said ring gear 20 to make a secondary pressure chamber 60' or meatus.

[0033]According to further embodiments of the invention, the main skids 40 operate on a lower face 120' of a main biasing portion 120 integral with the table 14 and the dogs 60, and secondary skids 70 thereof, on an upper face 220' of a secondary biasing portion 220 integral with the table 14. [0034]According to a preferred embodiment, each dog 60 comprises a body 62 intended to be fixed to the support assembly 12, within which at least one piston chamber 64 is obtained which can be fed with pressurized fluid through a control circuit 66. [0035] The dog 60 further comprises a sealed sliding piston 68 in the piston chamber 64, preferably dual acting, along an actuation direction K.

[0036]The dog 60 further comprises a hydrostatic secondary skid 70, which can be biased by the piston 68 to be pushed towards the upper surface 20'' of the ring gear

20, with which it forms the secondary pressure chamber

60'.

[0037]The secondary skid 70 is preferably movable along the actuation direction K relative to the piston 68. In other words, in a condition in which the piston 68 is fixed, the skid 70 is movable along the drive direction K to vary the height of the corresponding secondary pressure chamber 60'.

[0038]For this purpose, the secondary skid 70 is engaged with the piston 68 by at least one grub screw (not shown) accommodated with play in a piston housing 72 and a skid housing 74, aligned along said direction of actuation K. [0039]The fluid escaping from the secondary pressure chambers 60' of the secondary skids 70 is collected, preferably by gravity, and returned by the pumping device 50 to the hydrodynamic unit.

[0040]According to an embodiment, the piston can be actuated by generic piston actuating means, e.g., electrical . [0041]We will assume that the main skids 40 of the table group 10 can support a maximum load Pmax, that the table 14 has a weight Ptav, and that the semi-finished product to be machined has a weight Psem.

[0042]In a step of setting of the table group 10, the table 14 is resting on the support assembly 12 and the main skids 40 are actuated until the table 14 is raised. According to the pressure value detected by the pressure sensors, it is thus possible to determine the weight of the table Ptav, which constitutes the tare weight of the system. [0043]The pressure of the fluid operating on the main skids 40 increases when the semi-finished product is placed on table 12; the weight Psem of the semi-finished product can be determined based on this value, measured by the pressure sensors.

[0044]The total load on the table 14 is thus Ptot = Ptav + Psem.

[0045]Having defined a nominal load P* (with Ptot < P* < Pmax, i.e. greater than the total load, but either lower than or equal to the maximum load that can be borne by the main skids), preferably equal to the maximum load (P* = Pmax), the dogs 70 are supplied with pressurized fluid which operates in thrust on the respective pistons 68 to obtain an additional load equal to the contrast action F2 = P* - Ptot, i.e. equal to the difference between the nominal load and the total load weighing on the table. [0046]Consequently, the main skids 40 operate in a condition so that they feel a load equal to the nominal load P* and are supplied by pressurized fluid to generate a lifting action FI = P*, i.e., equal to the nominal load.

[0047]With the same load acting on the skids, the height of the main pressure chamber 40' is constant. This means that the upper face 14' of the table 14 is at a constant height relative to the tool carried by the machine tool head 6, even when the weight of the semi-finished product varies.

[0048]The variation in the weight of the semi-finished product occurs because different semi-finished products having different weights are machined at different times, or because the weight changes during the machining of a predetermined semi-finished product due to chip separation .

[0049]In other words, the table group 10 comprises electronic management means operatively connected to the main skids 40, the dogs 60 and the detection means, configured to control the dogs 60 to generate a contrasting action F2 such that the lifting action F2 is equal to a predetermined constant nominal load P*. [0050]Furthermore, the load being equal on the main skids, the stiffness of the main skids is constant. Indeed, it is well known that the stiffness of hydrostatic skids is not linear, but varies as a function of the load on the skids themselves. On the other hand, the invention makes it possible to keep the stiffness constant and, given the proportionality between the weight load and the stiffness, to keep it practically equal to the maximum stiffness.

[0051]All this makes it possible to reduce the machining tolerances ensured by the table group, according to the needs of the sector, as mentioned above.

[0052]It is apparent that a person skilled in the art may made changes to the machine tool described above, all of which are contained within the scope of protection as defined in the following claims in order to satisfy contingent needs.