Patent Description:
One of the main factors limiting accuracy in large machines tools are thermal deformations produced by the internally generated heat and the changing environmental conditions.

The thermal errors caused by thermal deformations can be of an order of magnitude equivalent to the geometrical errors of the machine, severely affecting the accuracy of the machine.

Over the years, the different sources of error have been studied and it is estimated that approximately <NUM>% of the total error of a machine tool is made up of geometric and thermal sources.

In the <NUM>, thermal errors were formally recognised as the major source of error on manufactured parts. The three main mechanisms that produce a structural temperature change are:.

Thermal effects within a machining environment have a great influence on the structure of a machine tool and therefore affect the accuracy of the machined part. Any kind of expansion and contraction will create distortion in the machine tool and affect its positioning accuracy. Over the years, researchers have shown that this thermal distortion of the machine tool can generate <NUM>-<NUM>% of all dimensional errors in precision machined parts. Therefore, any small change in temperature can make a significant difference to the machined quality of a component.

The main reasons why machines are sensitive to these thermal effects are, on the one hand, the lack of symmetry of machine structure and, on the other hand, the uneven distribution of masses. That means that the dimensional evolution of the machine's structural assembly upon temperature changes is not uniform.

This unequal lengthening or shortening of the different parts of the structural bodies of the machine, as well as the different speeds of these variations due to the unequal distribution of masses, leads to twisting and bending of the parts.

<FIG> shows a schematic representation at an increased temperature of the cross-rail and two columns of a vertical lathe. The expansion of the metal can be seen to cause the cross-rail in particular to arch in the XZ and XY planes.

<FIG> shows the quantitative results of a simulation of the displacement of the tip of a tool mounted to a milling head of a vertical lathe in the X, Y and Z directions when exposed to a temperature change of <NUM> over a period of <NUM> hours. <FIG> graphically shows the results of the same simulation. The colour map shows the distribution of the deformation across the structure of the vertical lathe such that it can be observed that the largest deformation occurs at the top of the lathe, especially in the cross-rail.

To overcome these drawbacks different solutions have been disclosed.

<CIT> teaches a method for compensating for the misalignment between the centre of the tool and the axis of the turning table which comprises moving the milling head along guides in the milling head support, wherein the guides move the milling head simultaneously in the z and y-directions and have an angle of <NUM> - <NUM>° with respect to the ZX plane. Such a solution is limited in the range of displacement it can compensate for and is dependent on the milling head being moveable in the z-direction.

<CIT> teaches a method for compensating for thermal error in a mineral goods lathe bed which comprises the use of cooling pipes embedded in the lathe bed and changing the internal material of the lathe-bed to one of low thermal conductivity and low thermal shrinkage. However, as this document explains in paragraph [<NUM>], it is directed towards compensating for local temperature rises as a result of the operation of the machine. Furthermore, the cooling is not localised to a critical area which causes misalignment of the machine.

<CIT> discloses a machine tool comprising a bed defining a base and a column mounted in the rearward portion of the bed. A spindle head is mounted on the front face of the column. The column comprises an interior space which allows a liquid to be circulated to maintain the temperature of the column regardless any change in the room temperature.

<CIT> discloses a machine tool and a method according to the preamble of claims <NUM> and <NUM>.

A first aspect of the invention relates to a machine tool comprising.

According to the invention the machine comprises a conduit configured to allow the flow of air in a closed circuit, the conduit comprising a surface part and an underground part, the surface part being an inner cavity of the support structure and the underground part comprising a piping configured to be buried in the soil beneath the structure.

The air can be recirculated in the closed circuit by means of a fan or any other suitable driving means.

The inner cavity of the surface part can comprise sealing means to prevent the escape of the air inside de conduct. The inner cavity can also comprise ducts or pipes. The piping of the underground part can also be sealed.

By means of convection, the air brings the two parts of the conduit into thermal contact, the surface part which is influenced by the environment temperature and the underground part buried in the soil at a much more stable temperature and thus reducing the temperature gradient of the surface part which is influenced by the low temperature gradient of the subsoil through which the air flows in the underground part.

The structural bodies of the machines are traditionally made of cast iron or welded steel. In both cases they are usually structures consisting mainly of an outer surface as a shell and a "hollow" interior where stiffeners or ribs are included to improve the structural capabilities of the part while incorporating as little weight as possible.

According to the invention the surface part of the conduit is the cavity of the support structure (hollow interior of the structural bodies forming the support structure) such that the temperature stabilizing effect of the air directly affects the structure of the machine (the metallic outer shell of the structural bodies forming the support structure) and thus a decrease in the thermal variation of the machine is obtained which leads to greater dimensional stability in the machine tool structure and therefore improves the precision of the workpiece during machining.

In some embodiments the underground part comprises a serpentine tube.

In some embodiments the underground part comprises a duct embedded in the foundation of the machine.

In some embodiments the underground part is buried more than <NUM>, and preferably between <NUM>-<NUM>.

In some embodiments the length of the underground part is more than <NUM> and preferably more than <NUM>.

In some embodiments the support structure comprises two vertical columns joined by a transversal crossbeam such that at least one transversal carriage is displaceable along the transversal crossbeam in a horizontal direction X, whilst the tool carrier support is mounted in the transversal carriage and displaceable in a vertical direction Z. In these embodiments the surface part of the conduit is formed by an inner cavity in the vertical columns and an inner cavity in the transversal superior crossbeam.

A second aspect of the invention relates to a method for improving thermal stability in a machine tool comprising.

The method according to invention comprises the steps of:.

In some embodiments the support structure (<NUM>) comprises two vertical columns (<NUM>) joined by a transversal crossbeam (<NUM>) such that the tool carrier support (<NUM>) is mounted on a carriage (<NUM>) displaceable along the transversal crossbeam (<NUM>) in a horizontal direction X, whilst the tool carrier (<NUM>) is mounted in the tool carrier support (<NUM>) and displaceable in a vertical direction Z. In these embodiments the closed conduit is formed with a surface part comprising an inner cavity (<NUM>) in the vertical columns (<NUM>) and an inner cavity (<NUM>) in the transversal superior crossbeam (<NUM>) and the underground part comprises the piping (<NUM>) buried in the soil (<NUM>) beneath the structure.

The machine tool can be a horizontal machining center, a vertical machining center, a milling machine, a grinding machine or a vertical lathe.

To complete the description and in order to provide a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate embodiments of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out.

<FIG> illustrates two vertical lathes as known in the state of art, comprising a workpiece support <NUM> for supporting at least one workpiece for allowing machining of the workpiece and a support structure <NUM>. A tool carrier support <NUM> is mounted on a carriage <NUM> displaceable along the transversal crossbeam <NUM> in a horizontal direction X. The tool carrier support <NUM> supports a tool carrier <NUM> movable in a vertical direction Z and configured for carrying and driving a tool for machining a workpiece supported by the workpiece support <NUM>.

In the machine shown in <FIG> the structure comprises two vertical columns <NUM> and the cross beam <NUM>.

<FIG> shows a cross section of a vertical lathe according to <FIG> and a conduit <NUM> configured to allow the flow of air in a closed circuit. The conduit <NUM> comprises a surface part and an underground part, the surface part being an inner cavity <NUM> of the support structure <NUM> and the underground part comprising comprises a piping <NUM>. The piping <NUM> comprises a duct <NUM> embedded in the foundation <NUM> of the machine and a serpentine tube <NUM> buried in the soil <NUM> beneath the support structure <NUM>. <FIG> shows a perspective view of the conduit <NUM> according to the invention.

The machine comprises a fan <NUM> to recirculate the air in the conduit <NUM>.

<FIG> illustrates a cross section of the transversal cross beam <NUM> showing the inner cavity <NUM> which forms part of the conduit <NUM>, where the air flows. In <FIG> the cross beam <NUM> has been represented in its position over the vertical column <NUM>. The figure also shows the carriage <NUM>, tool carrier support <NUM> and the tool carrier <NUM>.

<FIG> illustrates a cross section of a vertical column <NUM> showing the inner cavity <NUM> which forms part of the conduit <NUM>, where the air flows.

<FIG> illustrates two typical structural parts of the structural body of a machine tool to show that the parts are hollow inside and that the hollow spaces can be used to conform the cavity <NUM>.

<FIG> illustrates a foundation <NUM> for the machine tool according to an embodiment of the invention. The foundation <NUM> includes ducts <NUM> which form part of the conduit <NUM>.

<FIG> shows a schematic illustration of a front and plan view of the vertical lathe of <FIG> at a high temperature causing deformation of the columns <NUM> and cross beam <NUM> in the ZX and XY planes.

<FIG> shows the quantitative results of a simulation of the effect of temperature changes over time on the displacement of the centre of a tool on a milling head with respect to the axis of a turning table.

<FIG> shows the qualitative results of the simulation of <FIG> showing the distribution of deformation across the vertical lathe of <FIG>.

<FIG> and <FIG> show the variation over time of the tool tip position in X, Y and Z axes in a vertical lathe as illustrated in <FIG>. The doted lines illustrate the position of the tool in a vertical lathe with a refrigeration system according to the invention. The continuous lines illustrate the position of the tool in a vertical lathe without a refrigeration system. The conditions of this measures were the following:.

In this text, the term "comprises" and its derivations (such as "comprising", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements.

Claim 1:
Machine tool with improved thermal stability comprising:
- a workpiece support (<NUM>) for supporting at least one workpiece for allowing machining of the workpiece
- a support structure (<NUM>) comprising a tool carrier support (<NUM>), wherein the tool carrier support (<NUM>) supports a tool carrier (<NUM>) configured for carrying and driving a tool for machining a workpiece supported by the workpiece support (<NUM>) through a relative movement between the workpiece and the tool, - a conduit (<NUM>) configured to allow the flow of air in a closed circuit, the conduit (<NUM>) comprising a surface part, said surface part being an inner cavity (<NUM>) of the support structure (<NUM>), the machine tool being characterised in that it further comprises an underground part, said underground part comprising a piping (<NUM>) configured to be buried in the soil (<NUM>) beneath the support structure (<NUM>).