Abstract:
In a multi-axis machine tool comprising a slide way aligned on a longitudinal axis, a traveling column coupled via one end with the slide way, an arm mounted slidably to the column and traversable at least in a first direction normal to the longitudinal axis of the slide way, and a machining head carried by the arm, the material used for the arm is one typified by low thermal expansion.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a multi-axis machine tool comprising a slide way extending along a longitudinal axis, a column associated slidably by way of one end with the slide way, an arm coupled slidably with the column, and a machining head mounted to the arm.  
         [0002]     In particular, the invention is applicable to the art field of numerically controlled multi-axis machine tools used typically to perform high-speed milling and other such machining operations.  
         [0003]     Conventionally, such machine tools are utilized particularly in the aerospace industries for contouring and drilling parts made of aluminum and composite materials, and in the automobile sector for producing master models, and models for bodywork, internal parts and dies.  
         [0004]     Multi-axis machines of the type in question present a structure made entirely of steel which, in the course of the various movements, will absorb heat produced by the electric motors needed to induce motion and by the friction generated between moving parts during operation.  
         [0005]     The exposure of these parts to heat can result in their deformation.  
         [0006]     In particular, deformation of the arm caused by heat is especially critical.  
         [0007]     Indeed thermal expansion changes the dimensions of the arm, especially the predominating dimension, and consequently the position of the machining head, relative to a reference rigidly associated with the work, with the result that the accuracy of machining operations is jeopardized.  
         [0008]     In the light of the foregoing, the main object of the present invention is to provide a multi-axis machine tool unaffected by the aforementioned drawbacks.  
         [0009]     In particular, the object of the invention is to set forth a multi-axis machine tool of which the machining accuracy will not be rendered unreliable as the result of heat generated by the machine during operation.  
       SUMMARY OF THE INVENTION  
       [0010]     The stated objects are realized according to the present invention in a multi-axis machine tool comprising a slide way aligned on a longitudinal axis, a travelling column coupled via one end with the slide way, an arm mounted slidably to the column and traversable at least in a first direction normal to the longitudinal axis of the slide way, and a machining head carried by the arm. Of the various machine components, at least the arm is fashioned from a material, preferably composite or ceramic, typified by low thermal expansion. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The invention will now be described in detail, by way of example, with the aid of the accompanying drawings, in which:  
         [0012]      FIG. 1  shows a multi-axis machine tool according to the present invention, viewed in perspective;  
         [0013]      FIG. 2  shows a first detail of the machine in  FIG. 1 , viewed in section;  
         [0014]      FIG. 3  shows a second detail of the machine in  FIG. 1 , viewed in section;  
         [0015]      FIG. 4  shows a third detail of the machine in  FIG. 1 , viewed in section.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     With reference to the drawings, numeral  1  denotes a multi-axis machine tool, in its entirety.  
         [0017]     The multi-axis machine tool  1  comprises a slide way  2  extending along a longitudinal axis X of the machine  1  that defines a first direction of motion, and a column  3  coupled by way of one end  3   a  to the slide way  2 . The column  3  is also rotatable relative to the slide way  2  about a first axis B perpendicular to the aforementioned longitudinal axis X. The machine  1  further comprises an arm  4  coupled slidably with the column  3 , capable of movement along at least a first direction Z perpendicular to the axis X of the slide way  2  and coinciding with the first axis B about which the column  2  is rotatable relative to the slide way  2 . Also forming part of the machine  1  is a machining head  5  mounted to the arm  4 , such as can be fitted with a tool (not illustrated).  
         [0018]     The machining head  5  is pivotable relative to the arm  4  about two mutually perpendicular axes C and A, identifiable in particular as a second axis A parallel to the axis X of the slide way  2 , and a third axis C perpendicular both to the axis X of the slide way  2  and to the first direction Z along which the arm  4  is traversable relative to the column  3 .  
         [0019]     The arm  4  is coupled to the column  3  by way of a sleeve  6  and able to traverse along the selfsame column  3  in the first direction Z. The sleeve  6  presents an outer surface  6   a  of substantially prismatic geometry, and an internal bore  6   b  shaped substantially to match a cross sectional profile of the arm  4 . More exactly, the arm  4  appears as a right prism and is thus slidable internally of the sleeve  6  so as to allow the arm  4  a further degree of freedom relative to the column  3 , along a second direction Y perpendicular both to the first direction Z and to the longitudinal axis X of the slide way  2 .  
         [0020]     In particular, the second direction Y along which the arm  4  and column  3  are slidably coupled coincides with the second rotational axis C of the machining head  5 . Thus, the machining head  5  is capable of movement along and/or about six distinct axes X, Y, Z, A, B and C and can be positioned freely at any given point on a three-dimensional workpiece.  
         [0021]     Preferably, at least the arm  4  will be made of a low thermal expansion material (LTEM). In the context of the present specification and the appended claims, a low thermal expansion material would be one having a coefficient of linear thermal expansion (CLTE) much lower than that of steel, which is in the region of 11*10 −6 ° C. −1 . Advantageously, the material employed will have a CLTE of less than 3*10 −6 ° C. −1 , possibly between −1*10 −6 ° C. −1  and 1*10 −6 ° C. −1 , and preferably between −0.5*10 −6 ° C. −1  and 0.5*10 −6 ° C. −1 .  
         [0022]     Likewise advantageously, the low thermal expansion material in question will be a composite containing carbon fiber. In particular, carbon fiber typically has negative coefficients of linear thermal expansion ranging between −1*10 −6 ° C. −1  and −0.3*10 −6 ° C. −1 . The structure of carbon fiber composite is such that coefficients of linear thermal expansion CLTE, whether positive or negative, will in any event be close to zero.  
         [0023]     Alternatively, the low thermal expansion material could be a ceramic material or a composite of silicon carbide and its derivatives, albeit no limitation is implied.  
         [0024]     To enable the arm  4  to traverse on the column  3  along the first direction Z, the sleeve  6  presents at least one shoe  7  coupled with a first rail  8  mounted to the column  3  and extending along the selfsame first direction Z. In particular, the first rail  8  can be accommodated in an opening  9  presented by the column  3  and extending parallel to the first direction Z. In this instance, as discernible from  FIG. 1 , the column  3  is equipped with a rolling shutter  10  composed of two parts  10   a  and  10   b  that accompany the translational movement of the sleeve  6 , hence also of the arm  4 , along the selfsame column  3 ; this ensures that chips from machining will not interfere accidentally with the operation of the first rail  8  and the first shoe  7 . Whilst in the preferred embodiment of  FIG. 2 , the column  3  is equipped with a single rail  8  and the sleeve  6  with a single shoe  7 , the column  3  might equally well be furnished with two parallel and mutually opposed rails  8 , and the sleeve  6  with two corresponding shoes  7  engaging the two rails  8 .  
         [0025]     The machine tool  1  further comprises a first linear electric motor  11 , wired to a master control unit (not illustrated), by which motion is induced in the sleeve  6 . The first linear motor  11  presents a stator  12  mounted to the column  3 , extending parallel to the first rail  8 , and a magnet  13  associated with the sleeve  6 .  
         [0026]     To advantage, the sleeve  6 , like the arm  4 , is fashioned from a low thermal expansion material. In this instance, given that the magnet  13  is metallic and thus liable to expand thermally, a layer  14  of resilient material will be interposed between the magnet  13  and the sleeve  6 , as illustrated in  FIG. 2 , so as to absorb the thermal expansion of the metal. The resilient material could be a film of adhesive or a layer of resin, both familiar to a person skilled in the art.  
         [0027]     Preferably, moreover, the first shoe  7  and the first rail  8  will also be metallic. Accordingly, a layer  14  of resilient material is interposed likewise between the shoe  7  and the sleeve  6  in order to prevent internal stresses from being generated at the interface between these same components.  
         [0028]     Similarly, to enable the movement of the arm  4  transversely to the column  3 , that is to say along the second direction Y, the arm  4  is furnished with at least one second rail  15  extending parallel with the second direction Y and coupled with a second shoe  16  mounted to the sleeve  6 , and more exactly in the bore  6   b  of the sleeve  6 . Likewise in this instance, the machine tool  1  is equipped with a second linear electric motor  17 , wired to the aforementioned master control unit, by which motion is induced in the arm  4 . The second linear motor  17  presents a stator  18  mounted to the arm  4 , extending parallel to the second rail  15 , and a magnet  19  associated with the sleeve  6 ; more exactly, the magnet  19  is installed in the bore  6   b  of the sleeve  6  as illustrated in  FIG. 3 .  
         [0029]     Advantageously, a layer  14  of the aforementioned resilient material will be interposed between the stator  18  and the arm  4 , serving to absorb thermal expansion.  
         [0030]     Moreover, the second shoe  16  is made of metal. The second rail  15 , on the other hand, is fashioned from a low thermal expansion material. In practice, the second rail  15  could be machined directly from the material of the arm, or embodied separately and applied to the arm  4 . A layer  14  of resilient material is interposed between the sleeve  6  and the second shoe  16 , as indicated in  FIG. 3 , so as to prevent internal stresses from being generated at the interface between the two components. Likewise in  FIG. 3 , only a single second shoe  16  and a single second rail  15  are illustrated, whereas the arm  4  might equally well be furnished with two mutually opposed rails  15 , and the sleeve  6  with two corresponding second shoes  16  engaging the two second rails  15 .  
         [0031]     The column  3  engages with the slide way  2  by way of a third shoe  20 , presented by the end  3   a  of the column  3  and coupled with a third rail  21  mounted to the slide way  2 .  
         [0032]     A third linear motor  22  comprises a stator  23  mounted to the slide way  2 , extending parallel to the third rail  21 , and a magnet  24  associated with the end  3   a  of the column  3  (see  FIG. 4 ). Like the first two linear motors, the third linear electric motor  22  is wired to the master control unit, by which its movements are coordinated with those of the other two motors.  
         [0033]     In a preferred embodiment, the column  3  will be embodied in the same low thermal expansion material as the arm  4  and the sleeve  6 . Here too, a layer  14  of the aforementioned resilient material is applied to the magnet  24  of the third linear motor  22 , and preferably to the third shoe  20 , as a means of absorbing thermal expansion. A single third shoe  20  and a single third rail  21  are shown in  FIG. 4 , whereas use might be made of two third shoes  20  and two third rails  21 .  
         [0034]     The drawbacks mentioned at the outset are overcome by the present invention, and the stated objects duly realized.  
         [0035]     First and foremost, the adoption of a low thermal expansion material for the construction of the arm is instrumental in limiting elongation and thus maintaining the position of the machining head steady in relation to the workpiece. Accordingly, a machine tool according to the present invention guarantees greater precision in machining than is possible with machines of the prior art, and this same precision is unaffected by the temperatures registering in its component parts.  
         [0036]     Adopting composite materials, moreover, the overall weight of the machine can be reduced in comparison with machines of the prior art, and the power of electric motors thus trimmed, without any loss of performance. The reduction of weights and rated power also means that less heat is generated directly by the linear electric motors, and less also by friction, so that elongation of the arm is reduced further.