Robotic machining tool employing an endless machining belt

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.

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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially toFIG. 1, this is a schematic view of a machine tool10according to the invention comprising at its front end a driven pulley12and at its rear end a driving pulley14, these pulleys12,14having parallel axes of rotation and being capable of driving and guiding an endless machining belt15such as an abrasive belt. The tool10is designed to be carried by a robot arm16to 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 tool10is moved by the robot arm16backwards or forwards in such a way that the machining belt15, driven and guided by the pulleys12,14, is applied by the front pulley12to a surface of a workpiece to machine this surface by abrasion.

Here, the tool10is elongated in shape and comprises at the rear a base18mounted on one end of the robot arm16, and at the front a body20which is guided translationally on the base18along the longitudinal axis of the tool (double arrow21).

The base18has drive means22for turning a spindle24on which the drive pulley14is mounted. The driven pulley12is mounted idly on a spindle26that is parallel to the spindle24of the driving pulley14and that is mounted at the front end of the body20of the tool.

The tool10also comprises a ram30whose cylinder32is mounted on the base18of the tool with a piston rod34connected to the rear end of the body20of the tool for the translational movement of the body20. Carried at the rear end of the body20is a slider36engaged with a rail38mounted on the base18of the tool for the translational guidance of the tool body.

When the pulleys12and14are engaged in the ends of a machining belt15, the piston rod34of the ram is extended until the belt15is stretched between the pulleys12,14.

The tool includes three sensors40,42and44for detecting the position of the piston rod34of the ram. These are connected to a control unit of the tool10. The sensors40and42transmit signals to the control unit when the piston rod34of the ram is in a fully extended position and in a fully retracted position, respectively. The sensor44emits a signal when the piston rod of the ram is partially extended and the pulleys12,14of the tool are far enough apart to stretch a belt between the pulleys of the tool, as is the case inFIG. 1.

The body20of the tool is connected at its front end to a U-shaped fork46which has two arms48parallel and a certain distance apart and containing bearings for the ends of the spindle26on which the front pulley12is to rotate. This fork46is mounted so that it can rotate on the front end of the body20about an axis approximately parallel to the longitudinal axis of the tool. In the example shown, the fork46is supported by a spindle50located centrally in and guided rotationally by a corresponding bore in the body20of the tool and turned by drive means carried by the tool.

The tool10also comprises two identical wheels60mounted so as to turn freely on the spindle26of the front pulley12, on either side of this pulley12. The wheels60are located between the pulley12and the arms48of the fork46, parallel to the arms48of the fork, and are separated from these arms and from the front pulley.

The wheels60have an outside diameter greater than that of the front pulley12and are designed to roll over a surface to be machined as the tool travels over this surface. Also, when a machining belt15is mounted on the tool, it is prevented from slipping off the pulley12by the wheels60mounted on each side of the pulley12.

The distance, measured on a radius from the axis26of rotation of the pulley12, between the outer working surface of the belt15and the outer peripheral surfaces of the wheels60, 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 wheels60are held permanently in contact with a workpiece comprising a weld bead62that must be machined off, this weld bead will have, after machining, a thickness e theoretically equal to this machining distance. The weld bead62to be machined is narrower than the belt15and than the pulley12so that the wheels60can roll along on either side of the bead without coming into contact with it.

The machining distance C can be modified by replacing the wheels60mounted on the tool with other wheels having a different outside diameter. The wheels are therefore mounted removably on the tool10.

To ensure that the wheels60are always in contact with the workpiece, the tool includes means70for generating and detecting an electric current between the wheels60and the workpiece, and these means are connected to the control unit74controlling the tool10and the robot arm.

The wheels60are made of an electrically conducting material and are connected by conducting elements64to 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 elements64and to the workpiece by appropriate means such as electric wires66.

The conducting elements64are fixed to the fork46and are each pressed against the outer peripheral surface of a wheel60, preferably under spring pressure. The wheels60are in rubbing contact with the conducting elements64and are preferably made of a wear-resistant metallic material such as a tungsten-based composite material for example. The wheels60are insulated electrically from each other and from the other components of the tool10. The conducting elements64are also insulated electrically from each other and from the rest of the tool10. An electrical insulator68is also mounted between the fork56and the body20of the tool.

The means70detect the passage of an electric current between each of the wheels60and the workpiece and transmit corresponding signals72to the control unit74controlling the tool10and the robot arm16to modify the position and path of the tool as a consequence.

FIGS. 4-6show steps in a process of machining a weld bead80between two aligned walls82,84of a workpiece.

The front pulley12of the tool is advanced toward the weld bead80(FIG. 4) until at least one of the wheels60of the tool makes contact with one wall84of the workpiece. This contact sends a current between the wheel60and the workpiece which is detected by the means70.

If only one wheel60is in contact with a wall84of the workpiece, as inFIG. 5, the unit74tells the fork46to pivot until the other wheel60also makes contact with the other wall82of the workpiece, this contact causing a current to pass between the wheel and the wall82which is detected by the means70(FIG. 6). The tool is now in position to machine the weld bead80, with the wheels one on either side of the weld bead80and the axis26of rotation of the front pulley12approximately parallel to the walls82,84of the workpiece. The tool10is now moved backwards or forwards along the weld bead so that the belt15is applied to the weld bead80by the pulley12and grinds off the excess bead material, that is it grinds off the thickness greater than the predetermined machining difference. The tool10may also possess means for aligning the path of the tool10with the weld bead80.

In the example shown inFIG. 7, the tool10is used to grind down a weld bead80′ between two walls82′,84′ that are misaligned relative to each other, in the sense that they form a step or shoulder. The unit74tells the fork46to pivot so that each wheel60is in contact with a wall82′,84′ of the work. In this case the axis26of rotation of the front pulley12is inclined relative to each of the walls82′,84′.

In the abovementioned examples, the fork46may be free to pivot about the longitudinal axis of the tool through a small angle, without the control unit74having 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.