Machine-tool with machining head mounted for moving on a horizontal rail
FIELD OF THE INVENTION 
     The present invention is related to the field of machine-tools. More precisely, the invention relates to a machine-tool equipped with a machining head mounted on the front face of a rail and being movable longitudinally along that rail. 
     BACKGROUND OF THE INVENTION 
     In such a machine, the rail is fitted at its ends, either by fastening, or by wedging, or by guiding without play, on vertical columns. 
     The movable machining head, which can be a sliding drill head, or a boring slide, or a milling carriage, or a lathe slide, is generally guided by sliding ways placed on the front face of the rail. 
     During the displacement of the machining head, the rail becomes deformed, on the one hand, by bending since it has to support the weight of the machining head, and on the other hand, by twisting since it is subjected to a torsional couple which is equal to the product of the weight of the machining head and the distance separating the axis of the rail from the line of application of said weight. Such deformation affects the accuracy of the displacement of the machining head on the rail. 
     It is known that the deformation due to the torsional couple can be eliminated by fitting s balance weight which is fixed to the machining heed and placed on the other side of the rail, but the disadvantage of this solution is that it increases the weight of the machining head, increasing as s result the deformation due to bending. 
     FIELD OF THE INVENTION 
     It is the object of the present invention to improve the accuracy of movement of the machining head on the rail by virtually eliminating the deformation of the rail caused by the torsional couple without increasing the deformation due to bending. 
     This object is reached by the fact that a couple of forces is applied to the rail, which couple has the same value as the torsional couple exerted by the machining head, but is directionally opposite thereto and applied by static means provided in the rail. 
     The invention therefore relates to a machine-tool of the type comprising: 
     two vertical spaced apart columns, 
     a horizontal rail with two ends, both being fixed respectively on said two columns, and 
     a machining head mounted on said rail via guide means allowing its displacement along said rail, 
     said machining head being placed in facing relationship to said rail and exerting thereon a torsional couple under the action of its weight, 
     machine-tool wherein static means are further provided for exerting on said rail an antagonistic torsional couple of substantially the same value as the torsional couple exerted by the weight of said machining head on said rail, but directionally opposite thereto, said static means being disposed inside said rail. 
     Advantageously, the static means for exerting the antagonistic torsional couple comprise at least one tube extending along the axis of said rail and being subjected to a torsional force, said tube having a first portion which is fast with the median part of said rail and at least a second portion embedded in one of the ends of said rail. 
     Preferably, one of said tube portions is mounted on said rail so as to be pivotable; means being provided between said rail and said tube portion for adjusting the torsional force exerted on said tube. 
     Preferably, said rail comprises in its median part, a transversal plate, and the static means for exerting the antagonistic torsional couple comprise at least one tube which extends along the axis of said rail and is subjected to a torsional force, said tube having a first end fixed on said transversal plate and a second end fitted in a pivotable manner in one of the ends of said rail with interposition of means for controlling the torsional force exerted on said tube. 
     Advantageously, the means for controlling the torsional force exerted on said tube comprise a control screw cooperating with a lever arm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood on reading the following description given with reference to the accompanying drawings, in which: 
     FIG. 1 is a front view of the machine-tool according to the invention, 
     FIG. 2 is across-section along line II--II of FIG. 1, 
     FIG. 3 is a longitudinal section of the rail equipped with static means for exerting an antagonistic torsional couple, and 
     FIG. 4 is a side view of the rail. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     The drawing shows a machine-tool 1 comprising a machining head 2 disposed on the front face 3 of a rail 4 of which the ends 5a, 5b are respectively fitted in vertical columns 6a, 6b, either by fastening, or by wedging, or by guiding without play. The machining head 2 is guided by horizontal sliding ways 7a, 7b provided on the front face 3 of the rail 4. The machining head 2 of weight P exerts on the rail 4 a torsional couple which is equal to the product of the weight P and the distance d separating the axis 8 of the rail 4 from the line of application of the weight P. 
     As clearly illustrated in FIGS. 3 and 4, the rail 4 is equipped with two tubes 11a, 11b which exert thereon a couple of forces which is directionally opposite to the torsional couple exerted by the weight P of the machining head 2. The two tubes 11a, 11b extend along the axis 8 of the rail 4 and they are disposed on either side of at least one transversal plate 12 provided in the median part 13 of the rail 4. The first ends 14a, 14b of the two tubes 11a, 11b are fixed on the transversal plate or plates 12 by welding or by any other method. The tube portions 15a, 15b adjacent the other ends of the tubes 11a, 11b are carried by support bearings 16a, 16b mounted on the ends 5a, 5b of the rail 4 so as to allow the pivoting movement of tubes 11a, 11b with respect to the rail 4. The torsional force is exerted on tube portions 15a, 15b via control screws 17 which are in resting contact on radial lever arms 18 fixed respectively on the tube portions 15a, 15b. The control screws 17 are carried by the rail 4 and are placed in such a way as to be accessible in order to allow the control of the antagonistic couple exerted on each one of the tubes 11a, 11b. 
     The antagonistic couple exerted by the two tubes 11a, 11b has a value which is substantially equal to the couple exerted by the weight P of the machining head 2, but it is directionally opposite thereto. In practice, the correction couple will be controlled by placing the machining head 2 in the median part 13 of the rail 4, and by controlling that said head returns to the vertical position by using a precision level. 
     In the abovedescribed embodiment, two tubes 11a, 11b are provided. But it is obviously possible to obtain the same results with only one tube, for example 11a, which would, on its own, exert the antagonistic couple. Also, the two tubes 11a, 11b can be replaced by only one tube extending throughout the length of the rail 4, one opening being then provided to allow the passage of the tranversal plates 12. The tubes 11a, 11b can also be fixed to the ends 5a, 5b of the rail 4, the means for controlling the twisting force exerted on the tubes 11a, 11b being then provided close to the median part 13 of the rail 4.