Abstract:
A robotic machining tool employing an endless machining belt is disclosed. The tool includes a front pulley and a rear pulley which guide the machining belt, a drive unit which turns the rear pulley, a spindle about which the front pulley is free to rotate, and two wheels which flank the front pulley and are mounted idly on the spindle of the front pulley. The two wheels have an outside diameter greater than that of the front pulley in order to roll over a surface to be machined and in order to define a machining distance between the machining belt guided around the front pulley and the surface to be machined.

Description:
This invention relates to a machine tool employing an endless machining belt designed to be mounted on a robot arm for the purpose of carrying out operations of polishing, grinding, deburring, shaving, brushing, etc., on any workpiece such as for example an exhaust casing of a turbine engine. 
     BACKGROUND OF THE INVENTION 
     The exhaust casing of a turbine engine is made up of several elements assembled together by weld beads which must be machined to produce a smooth profile along the weld bead and between the assembled elements. 
     DESCRIPTION OF THE PRIOR ART 
     The prior art includes a machine tool employing an endless abrasive belt guided around a driven pulley and a drive pulley whose axes of rotation are parallel, the driven pulley being mounted on the piston rod of a ram whose function is to separate the pulleys from each other and thus tension the abrasive belt between the pulleys. 
     Where the workpieces are relatively complex, this tool must be manipulated and guided manually by an operator, which makes the machining operations slow and expensive and relatively dangerous for the operator. 
     There are many other drawbacks to manual use of the machine tool by an operator. There is no way of defining a precise machining distance, that is to say a thickness of material remaining after machining, and therefore the smoothness of weld beads after machining depends entirely on the skill of the operator. Also, while the tool is being manipulated the abrasive belt may escape from the pulleys, whereupon the tool has to be stopped and the operator must intervene to put the abrasive belt back in position. 
     It is a particular object of the invention to provide a simple, effective and inexpensive solution to the problems of the prior art. 
     SUMMARY OF THE INVENTION 
     To this end, the invention provides a robotic machining tool, employing an endless machining belt, comprising two pulleys, one at the front and the other at the rear, to guide the machining belt, drive means for turning the rear pulley, while the front pulley rotates idly on a spindle carried by a support guided translationally on the body of the tool, and ram means for tensioning the belt between the two pulleys, in which tool the front pulley is flanked by two wheels rotating idly on the spindle of the front pulley, which two wheels have an outside diameter greater than that of the front pulley in order to roll over a surface to be machined and in order to define a machine distance between the machining belt guided around the front pulley and the surface to be machined, and are made of an electrically conducting material and are each connected by a conducting element to a terminal of an electrical energy source whose other terminal is intended to be connected to the workpiece, the tool further comprising, connected to means for controlling the position and path of the tool, means for detecting the passage of an electric current between each wheel and the workpiece. 
     The machining distance is defined as the distance between the working outer surface of the belt and the outer peripheral surfaces of the wheels. 
     When machining a weld bead on a surface of a workpiece, the machine tool is moved along the weld bead with the wheels situated on either side of the bead and in permanent contact with the surface of the work so that the thickness of the weld bead projecting from the work, after machining, is defined and constant all the way along the weld bead. 
     If the weld bead connects to non-aligned surfaces of the workpiece, each wheel is placed in contact with one surface of the workpiece, and the machining belt can machine the weld bead between the two misaligned surfaces. 
     In addition, the machining belt is prevented from escaping from the front pulley by the wheels mounted on either side of the pulley. This avoids sudden stopping of the machine operation and the intervention of an operator to put the belt back in position on the pulleys. 
     The wheels are preferably removably attached to the spindle of the front pulley. The machine distance can thus be changed by simply replacing the wheels mounted on the tool with other wheels having a different outside diameter. 
     In accordance with another feature of the invention, the wheels are made of an electrically conducting material and are each connected by a conducting element to one terminal of an electrical energy source whose other terminal is intended to be connected to the workpiece. 
     When a wheel is in contact with a surface to be machined of an electrically conducting part, and is rolling for example along a weld bead, an electric current passes between the wheel and this surface and is detected by appropriate means provided on the tool which transmit corresponding signals to control means of the tool. As soon as one of the wheels loses contact with the surface to be machined, the control means modify the position and path of the tool to ensure that both wheels are brought back into contact with the surface to be machined. 
     The wheels can be made of a wear-resistant metallic material and are electrically insulated from each other and from the rest of the tool. 
     The support of the front pulley is advantageously rotatable about an axis approximately parallel to the longitudinal axis of the ram. When the abovementioned means do not detect the passage of a current between one of the wheels and the surface to be machined, the support of the front pulley can be rotated about the longitudinal axis of the ram, until this wheel is in contact with the surface and the means once again detect the passage of a current between the wheel and the surface to be machined. 
     The tool comprises sensors for sensing the position of the piston of the ram, such as for example two sensors of the end-of-travel position of the ram piston (the fully retracted and fully extended positions) and a sensor for sensing an intermediate position in which a machining belt is stretched between the pulleys of the tool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be understood more clearly, and other details, features and advantages of the present invention will become apparent from the following description, given by way of non-restrictive example with reference to the appended drawings, in which: 
         FIG. 1  is a schematic front view of the machine tool according to the invention; 
         FIG. 2  is a partial schematic front view of the machine tool of  FIG. 1 , on a large scale; 
         FIG. 3  is a partial schematic side view of the machine tool of  FIG. 1 , on a larger scale; 
         FIGS. 4-6  are highly schematic partial perspective views of the front part of the machine tool according to the invention, and show steps in a process of machining off a weld bead connecting two aligned walls of a workpiece; and 
         FIG. 7  is a partial schematic view of the machine tool according to the invention, seen from the front, and shows a step in a process of machining off a weld bead connecting two misaligned walls of a workpiece. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1 , this is a schematic view of a machine tool  10  according to the invention comprising at its front end a driven pulley  12  and at its rear end a driving pulley  14 , these pulleys  12 ,  14  having parallel axes of rotation and being capable of driving and guiding an endless machining belt  15  such as an abrasive belt. The tool  10  is designed to be carried by a robot arm  16  to carry out operations of polishing, grinding, deburring, shaving, brushing, etc., on any workpiece such as for example an exhaust casing of a turbine engine. 
     As will be described in greater detail later, the tool  10  is moved by the robot arm  16  backwards or forwards in such a way that the machining belt  15 , driven and guided by the pulleys  12 ,  14 , is applied by the front pulley  12  to a surface of a workpiece to machine this surface by abrasion. 
     Here, the tool  10  is elongated in shape and comprises at the rear a base  18  mounted on one end of the robot arm  16 , and at the front a body  20  which is guided translationally on the base  18  along the longitudinal axis of the tool (double arrow  21 ). 
     The base  18  has drive means  22  for turning a spindle  24  on which the drive pulley  14  is mounted. The driven pulley  12  is mounted idly on a spindle  26  that is parallel to the spindle  24  of the driving pulley  14  and that is mounted at the front end of the body  20  of the tool. 
     The tool  10  also comprises a ram  30  whose cylinder  32  is mounted on the base  18  of the tool with a piston rod  34  connected to the rear end of the body  20  of the tool for the translational movement of the body  20 . Carried at the rear end of the body  20  is a slider  36  engaged with a rail  38  mounted on the base  18  of the tool for the translational guidance of the tool body. 
     When the pulleys  12  and  14  are engaged in the ends of a machining belt  15 , the piston rod  34  of the ram is extended until the belt  15  is stretched between the pulleys  12 ,  14 . 
     The tool includes three sensors  40 ,  42  and  44  for detecting the position of the piston rod  34  of the ram. These are connected to a control unit of the tool  10 . The sensors  40  and  42  transmit signals to the control unit when the piston rod  34  of the ram is in a fully extended position and in a fully retracted position, respectively. The sensor  44  emits a signal when the piston rod of the ram is partially extended and the pulleys  12 ,  14  of the tool are far enough apart to stretch a belt between the pulleys of the tool, as is the case in  FIG. 1 . 
     The body  20  of the tool is connected at its front end to a U-shaped fork  46  which has two arms  48  parallel and a certain distance apart and containing bearings for the ends of the spindle  26  on which the front pulley  12  is to rotate. This fork  46  is mounted so that it can rotate on the front end of the body  20  about an axis approximately parallel to the longitudinal axis of the tool. In the example shown, the fork  46  is supported by a spindle  50  located centrally in and guided rotationally by a corresponding bore in the body  20  of the tool and turned by drive means carried by the tool. 
     The tool  10  also comprises two identical wheels  60  mounted so as to turn freely on the spindle  26  of the front pulley  12 , on either side of this pulley  12 . The wheels  60  are located between the pulley  12  and the arms  48  of the fork  46 , parallel to the arms  48  of the fork, and are separated from these arms and from the front pulley. 
     The wheels  60  have an outside diameter greater than that of the front pulley  12  and are designed to roll over a surface to be machined as the tool travels over this surface. Also, when a machining belt  15  is mounted on the tool, it is prevented from slipping off the pulley  12  by the wheels  60  mounted on each side of the pulley  12 . 
     The distance, measured on a radius from the axis  26  of rotation of the pulley  12 , between the outer working surface of the belt  15  and the outer peripheral surfaces of the wheels  60 , defines a machining distance C, corresponding to a thickness of material projecting from a surface after this surface has been machined ( FIGS. 2 and 3 ). Thus, when the wheels  60  are held permanently in contact with a workpiece comprising a weld bead  62  that must be machined off, this weld bead will have, after machining, a thickness e theoretically equal to this machining distance. The weld bead  62  to be machined is narrower than the belt  15  and than the pulley  12  so that the wheels  60  can roll along on either side of the bead without coming into contact with it. 
     The machining distance C can be modified by replacing the wheels  60  mounted on the tool with other wheels having a different outside diameter. The wheels are therefore mounted removably on the tool  10 . 
     To ensure that the wheels  60  are always in contact with the workpiece, the tool includes means  70  for generating and detecting an electric current between the wheels  60  and the workpiece, and these means are connected to the control unit  74  controlling the tool  10  and the robot arm. 
     The wheels  60  are made of an electrically conducting material and are connected by conducting elements  64  to a terminal of a source of electrical energy whose other terminal is connected to the workpiece, which is itself made of electrically conducting material ( FIG. 2 ). The source of electrical energy is connected to the conducting elements  64  and to the workpiece by appropriate means such as electric wires  66 . 
     The conducting elements  64  are fixed to the fork  46  and are each pressed against the outer peripheral surface of a wheel  60 , preferably under spring pressure. The wheels  60  are in rubbing contact with the conducting elements  64  and are preferably made of a wear-resistant metallic material such as a tungsten-based composite material for example. The wheels  60  are insulated electrically from each other and from the other components of the tool  10 . The conducting elements  64  are also insulated electrically from each other and from the rest of the tool  10 . An electrical insulator  68  is also mounted between the fork  56  and the body  20  of the tool. 
     The means  70  detect the passage of an electric current between each of the wheels  60  and the workpiece and transmit corresponding signals  72  to the control unit  74  controlling the tool  10  and the robot arm  16  to modify the position and path of the tool as a consequence. 
       FIGS. 4-6  show steps in a process of machining a weld bead  80  between two aligned walls  82 ,  84  of a workpiece. 
     The front pulley  12  of the tool is advanced toward the weld bead  80  ( FIG. 4 ) until at least one of the wheels  60  of the tool makes contact with one wall  84  of the workpiece. This contact sends a current between the wheel  60  and the workpiece which is detected by the means  70 . 
     If only one wheel  60  is in contact with a wall  84  of the workpiece, as in  FIG. 5 , the unit  74  tells the fork  46  to pivot until the other wheel  60  also makes contact with the other wall  82  of the workpiece, this contact causing a current to pass between the wheel and the wall  82  which is detected by the means  70  ( FIG. 6 ). The tool is now in position to machine the weld bead  80 , with the wheels one on either side of the weld bead  80  and the axis  26  of rotation of the front pulley  12  approximately parallel to the walls  82 ,  84  of the workpiece. The tool  10  is now moved backwards or forwards along the weld bead so that the belt  15  is applied to the weld bead  80  by the pulley  12  and grinds off the excess bead material, that is it grinds off the thickness greater than the predetermined machining difference. The tool  10  may also possess means for aligning the path of the tool  10  with the weld bead  80 . 
     In the example shown in  FIG. 7 , the tool  10  is used to grind down a weld bead  80 ′ between two walls  82 ′,  84 ′ that are misaligned relative to each other, in the sense that they form a step or shoulder. The unit  74  tells the fork  46  to pivot so that each wheel  60  is in contact with a wall  82 ′,  84 ′ of the work. In this case the axis  26  of rotation of the front pulley  12  is inclined relative to each of the walls  82 ′,  84 ′. 
     In the abovementioned examples, the fork  46  may be free to pivot about the longitudinal axis of the tool through a small angle, without the control unit  74  having to intervene or even requiring a change in the position and path of the tool. This allows the tool to adjust to any height differences between the walls of a workpiece and/or any imperfections in these walls.