Abstract:
A device for machining a part by metal cutting comprising a massive block in a bore of which is located a sliding column guided in a front bearing and a rear bearing, at least the front bearing and the section of the column sliding therein being continuously convex in cross-section with three lobes distributed at 120° about a central axis. The front and rear bearings are hydrostatic journal bearings. The column supports a sliding block on which is mounted a slide in which is incorporated a turret head for dispensing cutting tools.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for machining a workpiece by removing chips, forming part of a machine tool and comprising a positioning and longitudinal-displacement means. This may involve machining a workpiece, for example made of metal, be it cylindrical (by turning, drilling, shaving, cylindrical grinding) or not (by milling, drilling, grinding) with a view to obtaining a workpiece of predetermined size and shape. 
     2. Description of the Prior Art 
     The machine tools or machines for machining known to date consist of a support base on the ground, supporting or incorporating a bed for supporting the guide elements. These guide elements are made in the form of slideways of parallelepipedal—generally rectangular—cross section, which are not very stiff and therefore have to be supported along their entire length by a bed which thus gives the machine its precision and rigidity. This bed thus generally covers the entire surface of the machine and constitutes an obstacle to the efficient removal of chips. 
     To overcome this drawback, certain turning machines have been designed with a bed which is inclined to the operator or which is vertical. In this configuration, the latter constitutes an impediment to loading or unloading the machine. In particular, when installing automated production lines: 
     it makes it impossible to obtain a natural flow of workpieces when the exit point differs from the entry point, which means that factory layouts are not as optimized as they might be; 
     it means that the feed robot has to be installed on the same side of the machine as the operator, which results in difficulties with access for settings or maintenance. 
     As they lie in the flow of chips and cutting fluids, these beds and their guide systems require protection. This protection, which has to be telescopic to allow for the movements of the machine, and therefore very expensive, is not readily compatible with high speeds and accelerations. Used in a mass-production environment, for example for machining parts for the automotive industry, these protection elements quickly deform and thus offer passages through which the chips can pass, which has an adverse effect on the reliability and life of the machine. Once they have been in use for a certain amount of time, they act more as an obstacle to removing and cleaning away the chips than as a true barrier against the mechanical aggression of these chips. 
     Certain turning machines have been designed with a bed facing back towards the rear of the headstock, while at the same time maintaining guidance on slideways. Protection demarcating the chip zone from the mechanical-precision zone is still, however, very tricky to achieve and cleaning out chips which have managed to get in is almost impossible. 
     Furthermore, because of their presence, these beds fix the configuration of the axes, of the spindles and of the movements of the tools or workpieces with respect to the floor, and their relative positions. 
     In multi-spindle milling or punching-drilling machines, it is very important, for maintaining precision under the force exerted during cutting, that the resultant of the forces exerted on the guides by the cutting force be centred with respect to the guides so that they do not cause any deflection of the position of the axis of the spindle. This is very tricky to achieve with straightedge guides, or leads to U-shaped straightedge support structures which are therefore open and not very rigid. 
     A guide profile must have mechanical properties that are adequate for obtaining correct operation and good precision of the machines, namely: 
     1—It must have maximum bending stiffness to withstand the cutting forces and form, with the mass of the turret system for changing tools and controlling additional axes (for example the X, Y, Z axes) with the tool fixed at the end of the spindle and the spindle of a machining centre or with the mass of any other machining or workpiece holder system fixed at its end, an assembly whose natural frequency of vibration is high enough that it does not disrupt the cutting process. 
     2—Its torsional stiffness must be high enough that, while withstanding the overturning moment that is due to the cutting force, does not affect the precision of the machine. 
     3—Its mass must be low so that speeds and accelerations compatible with the current design of so-called “agile” machines, namely a speed of 1 m/s and an acceleration of 10 m.s −2  can be obtained economically. 
     4—Its shape must be simple, cylindrical, and its profile must be as constant as possible so that it can be machined by surface grinding and so that hydrostatic guide bearings can be developed (eliminating wear), and so that effective protection against the ingress of chips into the bearings can be achieved simply by scraping over a continuous profile. 
     5—Its shape must be able, without the addition of an additional guidance system, to angularly position the system fixed to the end of this column. 
     Points 1, 2, 3 and 4 are advantageously optimized by the use of a cylindrical column of circular profile, this having maximum moment of inertia for minimum mass. This solution has already been used to produce guides fixed to a bed. By contrast, as it requires an additional angular-positioning system, it does not satisfy point 5, a condition which is essential for simplifying chip protection. 
     Specifically, additional prismatic guidance amounts to: 
     either designing a bed which has chip retainers and requires the use of telescopic protectors which are incompatible with good operational reliability in a mass-production environment; 
     or to using a key system, or any other currently known system for preventing rotation which, depending on its position: 
     at the front: constitutes a difficulty with scraping and sealing which is not compliant with point 4, 
     at the rear: constitutes an asymmetry in the way that torque is taken up, thus creating eccentricity and bending in the column, which is detrimental to the high precision desired. 
     Document U.S. Pat. No. 4,604,008 A discloses a machining device comprising, in a conventional chassis, a longitudinal-displacement means with rotation being prevented, using a four-lobed cross section, the assembly itself being capable of being rotated. Document U.S. Pat. No. 3,263,530 A discloses a head of a precision boring device of four-lobed cross section. Document EP 0 172 299 A discloses a compact machine tool. None of these is able to dispense with a bed. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a machining device that overcomes the drawbacks that result from the use of a bed supporting straight edges for guiding moving parts, and which overcomes the shortcomings of the guide profiles hitherto used. 
     The device in accordance with the invention comprises a massive block in a first bore of which there is housed a sliding column guided in a front bearing and a rear bearing, at least the front bearing and that section of the column that slides therein being of continuously convex cross section with three lobes 120° apart about a central axis, a spindle, preferably with an incorporated motor, being positioned in a second bore in the said massive block. 
     Thus, according to the invention, the bed is dispensed with and replaced by a massive block of dense and inexpensive material, for example a block of natural or reconstituted granite (a composite material made of granite chips held together by a binder of the chemical resin type), the block supporting the special guide of the invention allowing work at an unsupported overhang. 
     Advantageously, for reasons concerned with the simplicity of achieving the precision of the machined workpieces without resorting to sophisticated expansion-compensation software, the coefficient of expansion of the block is similar to that of steel, namely of the order of 10×10 −6  meters per meter per degree Celsius. 
     Advantageously, the block has two perfectly planar and parallel opposite faces, for the attachment and relative-position adjustment of the bearings that guide the mobile column or columns. 
     The block may have other bores, which may house various sliding columns of the same type as the main column or of a different type, or alternatively a spindle or any other device. 
     The bores, with no dimensional accuracy, allow the guide columns to pass through from one bearing to the other. 
     The block may be positioned either horizontally (with the axes of the bores horizontal, and the flat faces vertical), which makes it possible to assemble a frontal machine, or a turning machine with a tailstock for a lathe, or a horizontal-spindle milling unit for a machining centre; or vertically with a view to machining workpieces whose natural axis of transportation and handling is vertical (flat workpieces). 
     According to the invention, the same block may support, in particular: 
     either a guide column alone, supporting a spindle with built-in motor, for example for turning, milling, grinding, which spindle may have a number of degrees of freedom with respect to the movement of the column; 
     or a guide column alone supporting a tool-holder turret which may have a number of degrees of freedom with respect to the movement of the column; 
     or a built-in fixed spindle or a guide column supporting a spindle with a built-in motor, together with one or more guide columns supporting a tool-holder turret which may have a number of degrees of freedom. 
     Joining two or more blocks together using rigid and adequately sized spacer pieces makes it possible to obtain multi-spindle machines in which the relative position of the spindles is due merely to the shape and size of inexpensive static parts. 
     It will be understood that a block in accordance with the invention constitutes a standardized element, acting as a central unit. A number of central units make it possible to construct assemblies having from 1 to 4 simultaneous cutting units, the assemblies being incorporated into a singlespindle machine, the axis of which can be positioned either horizontally or vertically, or into a machine with two horizontal spindles face to face or parallel, which may be synchronized or independent. This concept may, by the juxtaposition of modules, be extended to an entire machining line. It means that the central unit alone can be marketed, it being possible for the complete machine to be designed and assembled around this central unit by an industrial partner, while at the same time being able to guarantee the level of precision and of performance. 
     The particular guide profile used according to the invention is a cylinder with a profile that is optimized between a triangular cross section which is well able to transmit overturning moments and gives good angular positioning, and a circular cross section which has the best rigidity and best machinability. In a different context, such a shape (sometimes known as a “polygon” or “epitrochoid”) has been used to guide boring heads, according to document WO-A-93/15299. The use of a continuously non-concave multi-lobe cross section for precisionless mechanical guidance with the transmission of significant torque can be extended by the present invention to hydrostatic guidance giving precision compatible with the standards of exceptional precision that relate to guide structures for machine tools. 
     According to the invention, the use of a cylinder of continuously convex three-lobed shape gives a distribution of guide pressures the result of which is perfectly centred. 
     Furthermore, the localization of the pressure zones around the three lobes makes it possible to optimize the position of the points at which fluid is injected into the hydrostatic bearings and thus makes it possible to limit the cost of the pressure unit. 
     Such a profile can therefore guide the main movement of an independent cutting unit ready to be incorporated into any equipment asset dedicated to machining or, by incorporating a spindle into the basic block, may constitute the guidance for the Z movement (parallel to the axis of the spindle) of the spindle (machine with moving spindles or tailstocks) or of the tool holder (machine with headstocks), but also advantageously support the tailstock of a turning machine for machining long workpieces that need support at the opposite end to the end where the workpiece is held or any other system for holding or handling workpieces. Such a configuration leaves the installer of a machining line numerous possibilities for laying out a robot-controlled automatic loading system, given the fact that the spindle is directly accessible from 4 sides. 
     Other specific features and advantages of the invention will emerge from the description which follows, given by way of indication and with no implied limitation, relating to the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows, in perspective, a central unit comprising a fixed spindle and a tool-holder column according to the invention; 
     FIG. 2 shows, in perspective, the design of a central unit comprising a fixed spindle and two tool-holder columns according to the invention; 
     FIG. 3 illustrates the hydrostatic linear guidance according to the invention; 
     FIG. 4 shows the distribution of pressure over the lobes of the hydrostatic linear guidance according to the invention; 
     FIG. 5 shows a section along the line  5-5  of the central unit of FIG. 1, with an alternative form of spindle; 
     FIG. 6 shows the breakdown of the movements of the axes to obtain a Y movement according to the invention; 
     FIG. 7 shows, in perspective, a central unit comprising a fixed spindle and a tool-holder column according to the invention, the unit being equipped with a Y axis and a C axis. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts one possible design of a central machining unit to which the invention applies. 
     The block  1 , made of natural granite or of granite and polymer resin composite, supports a spindle  2  with integrated motor  2  and a slide/saddle assembly consisting of a column  3  equipped at its end with a slide block  4  on which there moves along a transverse axis X a slide  5  in which is incorporated the turret  6  distributing cutting tools  7 . The column  3  is guided longitudinally along the longitudinal axis Z 1  by two bearings  8  and  9  (cf. also FIGS. 3 to  5 ). These two bearings are fixed to two perfectly planar and parallel surfaces  10  and  11  of the block  1 . The precision of the parallelism of these two faces is the only precision element of this basic block. 
     A third support face  12  allows the tailstock or loading system options to be attached. 
     The polygonal external profile of the body of the spindle  2  according to the invention makes it possible, in certain configurations of the machines, either for automatically feeding the lathe with raw material in the form of bar stock or, for allowing two-spindle lathes to be loaded or alternatively for replacing the axis Z 1 , to make the spindle  2  able to move along an axis Z 2 . The body of the spindle  2  is then guided by a set of two hydrostatic bearings, a front one  13  and a rear one, respectively (the rear one bearing against the face  11  and not being depicted), in exactly the same way as the column  3  is guided. 
     According to the embodiment depicted, in order to obtain an autonomous assembly, the motors have been incorporated into the mechanics and their variable-speed drives have been inserted in a chest  15  fixed directly onto the back of the block  1 . Adjustments in the motor speed control loops on which the precision and performance in terms of speed and acceleration of the machine depend, are therefore completely incorporated into the central unit, which makes it possible to envisage the sale of this unit to partners who are manufacturers of equipment assets so that it can be adapted, by integration into a machine of its design, to the specific needs of its local market, particularly as far as safety standards, man-machine interface, and type of numerical control are concerned. FIG. 2 shows, in perspective, a central unit comprising a fixed spindle  2  and two tool-holder columns according to the invention. Positioned on a massive block  21  which is deeper than the block  1  but which can also come from a common mould partitioned for casting the block  1 , is a second column  17  which is perfectly identical to the column  3 . It too is guided in two hydrostatic bearings, a front bearing  18  and a rear bearing (not depicted) which are respectively identical to the bearings  8  and  9 . It is fitted at its end with a slide block  19  (identical to 4) on which there moves, along a transverse axis X 2 , a slide  22  (identical to 5) into which is incorporated the turret  23  (identical to 6) for distributing cutting tools  24 . Likewise, the motors are incorporated into the mechanics and their variable-speed drives inserted in a chest  25  fixed directly to the back of the block  21 . It is therefore possible to use perfectly identical elements in the two sets of tools, hence achieving an appreciable reduction in production costs and stocks. 
     FIG. 3 is a detailed perspective view of one embodiment of a longitudinal axis produced according to the invention. The guide column  3  is a cylinder which is made up of three different coaxial sections: 
     The first section  27 , at the front, is of continuously convex shape with three lobes, according to the invention; it allows the torque to be taken up as close as possible to the cutting force and is guided by a front hydrostatic bearing  8  determining a first point  35  on the axis  36  of the column and its orientation  37 . Ingress of chips into the bearing is halted by a scraping seal with continuous profile  28 . The uniformity of the exterior shape of the surface of the column  3  gives its precision and makes sure that the scraper  28  is effective. The second section  29 , which is the intermediate section, is radially widened to form two ears. The first is for securing a nut  30  which collaborates with a ball screw  31 . The second ear is for securing a mobile point  32  of a system  33  for measuring the position of the axis. 
     The third section  34 , at the rear, is circular; it has only to locate a second point  38  of reference on the axis  36  of the column. It is guided hydrostatically by the bearing  9 . In this example, the motor  39  for the numerical control of the axis is incorporated into the bearing  9 , its rotor being shrink-fitted directly onto the ball screw  31  to avoid any additional coupling system, this making it possible to control the inertia of the axis and reduce the number of parts involved in the bill of materials for the machine. It is advantageous to produce the bearing  40 , the axial reference bearing for the axis, in the front bearing  8 , so that heating of the ball screw by the motor does not affect the precision of the axis. 
     FIG. 4 shows the distribution of pressure on the lobes of the hydrostatic linear guide according to the invention. As the pressure zones  41 ,  42  and  43  are axisymmetric irrespective of the position of the force F that is to be withstood, there is no shifting of the axis  36  under the cutting force. 
     FIG. 5 shows a section along the line  5-5  of FIG. 1 of a central unit in an alternative two-axis form designed according to the invention. This central unit is formed of a spindle  2 , fixed longitudinally in this alternative form, the spindle being fixed to the massive block  1  by means of a flange  43  and centered by the cylinder  44 . The rotor  46  of the drive motor is shrink-fitted into the rear of the spindle, on its shaft  45 . The stator  49  of the motor is shrink-fitted into a sleeve  47  which provides cooling through the circulation of water containing glycol through the spiral groove  48 . At the front, a chuck  50  allows the workpiece to be held during the machining process. 
     The column  3  guided on the bearings  8  and  9  forms excellent protection for the passage and movement of the cables and pipes  51  and for the motor  52  of the turret  6 . 
     FIG. 6 shows the geometrical breakdown of the movements of the axes to obtain a Y movement according to the invention. The end of the column supporting the tool-holder turret may advantageously be fitted with a ring  53  for orientating the slide block, of the axis X, which, by numerical calculation, makes it possible to command an additional axis labelled Y perpendicular to the X-axis and to the Z-axis (cf. also FIG.  7 ). If A is the distance between the axis of rotation of the workpiece, which axis is determined by the axis of the spindle  2 , and the axis of orientation of the ring, then: 
     Y=A sin α α being the angle of inclination of the slide block  4 . 
     A correction on the X-axis needs to be incorporated: 
     
       
           X=A .cos α−( A−Xp )= Xp−A (1−cos α) 
       
     
     Xp being the value read by the X-axis positioning system and Xp the actual value obtained on the workpiece [sic] 
     and the value of the angular orientation of the workpiece needs to be compensated for: 
     
       
           C=Cp+α=Cp +arcsin( Y/A ) 
       
     
     Cp being the value read by the system for orientating the workpiece (C-axis) and Cp the actual value obtained on the workpiece. 
     FIG. 7 shows, in perspective, the design of a central unit comprising a fixed spindle and a tool-holder column according to the invention, the unit having a Y-axis and a C-axis. The spindle is angularly positioned by the built-in motor, the position being read by a sensor built into the motor using a principle that is known in the art. Inserted between the column  3  and the slide block  4  is an orientation ring  53  comprising a motor and an angular encoder which is accurate to {fraction (1/000)} th  of a degree. 
     FIG. 8 shows, in perspective, a machine tool comprising two devices formed in accordance with the present invention with spindles facing each other, constituting a horizontal-axis two-spindle turning machine. The two blocks  1  and  21  are joined together using rigid and adequately sized spacer pieces  56 .