Tool for applying surface coated abrasives for use on a machine for abrasion machining of cylindrical surfaces on workpieces
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a tool for applying surface coated abrasives for use on a machine for abrasion machining of cylindrical surfaces on workpieces, especially crankshaft journals and crankpins. 
     2. Description of the Prior Art 
     Machines of this type described, for example, in patent application EP-A-0 366 506 comprise, for each bearing surface to be machined on a workpiece, a pivoting arm mobile vertically and carrying three surface coated abrasive application pads disposed substantially at the three corners of an equilateral triangle. A first of the three pads is mounted at a median top position on the arm and the other two in lateral bottom positions on two jaws articulated to the arm and coupled so that they can be clamped together by synchronous pivoting in opposite directions due to the action of common maneuvering means. The arm is moved vertically by a balancing cylinder with regulated feed pressure. When the jaws are clamped the three pads apply surface coated abrasives to the bearing surface to be machined with a uniform pressure. 
     In the prior art machines the pads which apply the surface coated abrasives have an application surface in the shape of a circular arc subtending a relatively small angle, usually less than 30°. These tools give good results in terms of the bearing surface finish. 
     For honing cylindrical bearing surfaces on workpieces &#34;hones&#34; are used whose surface in contact with the bearing surface to be machined is in the shape of a circular arc subtending an angle of up to 60°. The hones wear asymmetrically during machining so that in practise they exert pressure and therefore their action of honing the cylindrical bearing surface on the workpiece over an angle significantly less than 60°. 
     In other prior art machines using surface coated abrasives the tools in contact with each cylindrical bearing surface to be machined comprise two opposed surface coated abrasive application pads disposed on two jaws of a clamp, each pad having a circular arc shape application surface subtending an angle of almost 180°. The drawback of these prior art pads used in pairs is poor distribution of the application pressure. The pressure is inevitably concentrated in the median part of the circumferential length of the circular arc shaped surface of the two opposed pads. 
     None of the prior art tools constitutes an entirely satisfactory response to the severe demands in terms of precision that apply nowadays, for example with respect to the absence of shape defects and to the straightness of crankshaft journals and crankpins. 
     The present invention is directed to a tool for applying surface coated abrasives enabling more accurate machining by abrasion of cylindrical bearing surfaces on workpieces such as crankshaft journals and crankpins. 
     SUMMARY OF THE INVENTION 
     The surface coated abrasive application tool of the present invention is for a machine for abrasion machining of cylindrical bearing surfaces on workpieces such as crankshaft journals and crankpins of the type comprising for each bearing surface to be machined a pivoted arm mobile vertically and carrying three surface coated abrasive application pads disposed substantially at the three corners of an equilateral triangle. According to the invention, each pad has a circular arc shape concave application surface subtending an angle of greater than 60° and less than 120° so that the three pads enclose substantially all of the bearing surface to be machined. 
     By virtue of their enveloping shape over slightly less than 120° and the specific way they are mounted on the arm, the three pads apply the surface coated abrasive against the bearing surface to be machined not only all with the same pressure but also with substantially uniform pressure over all their circumferential length. This ensures optimal compensation of any shape defects (ovalization, out-of-round, etc) of the bearing surface. 
     To enable optimal location of the top pad of a machine according to application EP-A-0 366 506 relative to the bearing surface to be machined the pad is advantageously mounted with limited mobility in horizontal translation perpendicular to the axis of the bearing surface to be machined. 
     On this same type of machine each lateral pad is advantageously mounted with limited freedom to oscillate perpendicularly to the axis of the bearing surface to be machined. 
     Each pad preferably comprises a rigid base faced with a more flexible material having a low coefficient of friction. 
     This facing may comprise a synthetic rubber vulcanized onto the base, for example. 
     The facing may advantageously have along its axis of curvature a hardness which increases from the middle towards both ends. 
     In one preferred embodiment of the invention the facing may comprise five sections, a median section of low hardness followed towards each end by an intermediate section of medium hardness followed by an end section of high hardness. 
     These three hardnesses may be Shore hardnesses of 90, 95 and 98, for example. 
     One illustrative embodiment of a tool according to the invention is described in more detail hereinafter by way of non-limiting example and with reference to the appended diagrammatic drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general view of the lower part of an arm carrying a tool in accordance with the present invention. 
     FIG. 2 is a partial cross-section through a carrier on the line II--II in FIG. 1 and to a larger scale. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, the tool in accordance with the invention is used on a machine as described in patent application EP-A-0 366 506 for abrasion machining of cylindrical bearing surfaces on workpieces, especially machining of crankshaft journals and crankpins using surface coated abrasives. FIG. 1 shows an arm 1 mounted on a vertically mobile slider (not shown) to pivot freely about a horizontal axis. The arm 1 carries two jaws 2a and 2b articulated to it by horizontal pivot pins 3a and 3b and coupled by toothed segments 4a, 4b so that they can be clamped together by synchronous pivoting in opposite directions relative to the arm 1 due to the action of a common maneuvering cylinder 5. 
     To machine by abrading the cylindrical bearing surface 6 a first pad 7 is mounted in a median position on the arm 1 and two other pads 8a and 8b are mounted in lateral positions on the two jaws 2a and 2b so that when the three pads 7, 8a and 8b are clamped against the cylindrical bearing surface 6 to be machined by the cylinder 5, in order to apply surface coated abrasives 9 against the bearing surface 6, the three pads 7, 8a and 8b are substantially at the three corners of an equilateral triangle by virtue of the balancing of the mass of the arm 1 by balancing means (not shown) such as a regulated feed pressure cylinder, the three pads are applied against the bearing surface 6 to be machined with a uniform pressure. 
     According to the invention each pad 7, 8a and 8b subtends an angle of slightly less than 120° so that the three pads envelop the bearing surface 6 to be machined almost completely. 
     As shown by the double-headed arrow 10, the top median pad 7 is mounted on the arm 1 with limited mobility in horizontal translation perpendicular to the axis of the bearing surface 6. As shown by the double-headed arrows 11a and 11b the bottom lateral pads 8a, 8b are mounted on the jaws 2a and 2b by horizontal pivot pins 12a and 12b with limited freedom to oscillate perpendicular to the axis of the bearing surface 6 to be machined. The mobility of the three pads 7, 8a and 8b ensures an optimal match of the pads to the bearing surface 6 to be machined, independently of tolerances of the arm 1 and especially any play that there may be at the toothed segments 4a, 4b coupling the two jaws 2a and 2b. 
     As shown in FIG. 2 in particular, the top pad 7 comprises a rigid, in this example metal base 13 whose circular arc shape concave interior surface has a facing 14 of a more flexible material with a low coefficient of friction, for example a vulcanized synthetic rubber. A dovetail profile 15 improves the fixing of the facing 14 to the base 13. 
     Given that abrasion machining of the cylindrical bearing surface 6 involves not only rotational movement of the bearing surface 6 but also, in the known manner, an oscillatory movement of the bearing surface 6 along its axis (in order to obtain a &#34;cross-hatched&#34; machining pattern), and given that the width of each pad 7, 8 is less than the width of the bearing surface 6 to be machined, the two end portions of the width of the bearing surface 6 are machined only alternately whereas the median part is machined continuously, which can lead to defects in terms of the straightness of the bearing surface. To compensate such defects of straightness the facing 14 may advantageously comprise along its axis of curvature a plurality of sections of different hardness, increasing from the middle towards both ends. In FIG. 2 these different hardnesses are symbolically represented by the subdivision of the facing 14 into five sections, namely a median section 14a, two intermediate sections 14b and two end sections 14c, these sections having respective Shore hardnesses of 90, 95 and 98, for example. 
     It goes without saying that the above embodiment has been described by way of non-limiting illustrative example only and that many modifications and variations are feasible within the scope of the invention. 
     For example, the tool in accordance with the invention could be used on machines other than that described in patent application EP-A-0 366 506 even though the latter machines give particularly favorable results. 
     Note also that in the context of the invention the pads can have an angular length greater than 60° and less than 120°, preferably between about 90° and 110°.