Improved equipment for lapping internal surfaces

Abstract: In improved equipment for lapping internal surfaces, use is made of a composite lap (12), the rod (2) of which is held by the tool spindle (11) and inserted with a marginal degree of clearance into a cone (3) that carries the abrasive shell (4), and a chucking mechanism (1) located beneath and coaxial with the spindle and lap; errors are avoided, particularly with small-bore work, by embodying the lap rod (2) in three rigidly associated sections (7, 8 and 9), namely, a polygonal tip (7), insertable and fastened with a transverse pin (10) in a matching polygonal socket (13) offered by the cone (3), a flexible coupling section (8), and a shank (9) uppermost that is screwed into or otherwise clamped to the spindle. Work to be lapped is held in a chuck (1) suspended from at least three cables (5, 6) threaded through relative peripheral mountings (14) afforded by the chuck itself.

Description: The invention relates to improved equipment for lapping internal surfaces. Conventionally, use is made of lapping machines, or lapping equipment, to effect polishing and truing operations that call for a high quality precision finish, for example, such as presented by the bores of parts manufactured to prescribed fit tolerances; a typical case in point is that of the barrels of hydraulic and pneumatic cylinders. The operation effected with lapping equipment is a finishing process; work to be lapped may be either heat-treated or untreated, heat treatment being adopted where the work to be guaranteed a given degree of surface hardness. An initial machining or grinding may be made, whereupon the lapping operation will constitute the final step. Considerable accuracy is obtainable with lapping machines, down even to fraction of 1 µm, and it is therefore essential to avoid any contingency that may lead to dimensional inaccuracies, such as errors in shape. One such contingency, when operating with a machine designed for lapping internal surfaces, is created at the moment when the lap is introduced into the bore to be finished. The main difference between this type of machine and those for external lapping, which can also finish plane surfaces, is that it is structured principally for finishing cylindrical surfaces of circular cross section, and will normally operate in the vertical axis with the lap entering the work from above. In order to avoid the occurence of errors, that is, of any unintentional widening of the bore occasioned on introduction of the lap, as mentioned above, the work is chucked in a floating fixture, and the lap held in such a way as to allow a barely perceptible transverse oscillation. This carefully calculated freedom of movement is invisible to the naked eye (though verifiable by measuring the work, given the tolerances that are generally prescribed), but sufficient to enable the bore and the lap to adapt gently to one another as the lap enters the work and the pass commences. Notwithstanding the close tolerances characteristic of lapping machine construction, it will rarely occur that one has the necessary faultless coaxial alignment when the lap is inserted into the bore of the work; moreover, the lack of any self-alignement facility results in a conical or 'banana' defect that becomes more pronounced as the departure from coaxial alignement becomes greater. Given that internal lapping generates a profile referred to the lapped bore itself, rather than to any external datum (e.g. perpendicular or concentric alignement with other surfaces etc.), care must be taken to achieve coaxial alignement between the lap and the bore of the work without in any way forcing either component. Currently, self alignement is achieved by supporting the floating chuck with a system of slides at right angles to one another, in such a way that the work can move freely in the appropriate direction, and using a lapping tool of composite type design. In one such composite embodiment, the lap comprises : a support rod, the top end of which is associated with a floating spindle; a cone, ensheating the rod and allowed a certain freedom of transverse movement relative thereto; and a abrasive diamond shell that is fitted rigidly over the cone. The freedom of the cone to shift in relation to the rod is obtained in this instance by leaving a slight clearance between the two parts and supporting the cone on two moderate loose pins; these are fitted one at either side, at dissimilar distance from the end of the rod, and thus interconnect the two parts. Accordingly, the cone and the rod are prevented from separating, and a marginal float is permitted, the extent of which being dependent upon the clearance allowed. The lap and the chucking fixture thus described are suitable for bores of a given diameter, but tend to create problems when lapping notably small bores. More exactly, with a small diameter lapping tool of the composite type in question, the walls of the cone and the diamond shell are necessarily thin, so that if an obstacle is encountered, or at all events, should the tool's speed of rotation drop for any reason, the rod becomes subject to increased torque. This higher torque is transmitted through the rod to the cone and shell by way of the two pins, with stresses concentrated particularly on the opposite ends of the pins. However, the cone and the abrasive shell will often not be thick enough to sustain the thrust with which they are invested by the ends of the pins, and are forced outwards and distorted as result. In such situation, bulges are produced in the outer surface of the diamond shell which, though not large, are sufficient to enlarge the cross section of the abrasive surface and its action at a given point, with the result that the lapped surface will be inaccurate; it can also happen that the lap will bind against the bore of the work and break. When lapping a bore of small diameter and/or where especially a great precision is required, problems can also arise with movement of the chucking fixture; more exactly, the chuck unit is not replaced with a lighter one to suit the smaller work, and it happens that the weight of the smaller lap required for the small bore is often insufficient to shift the chuck and the work, in order to bring them into coaxial alignement. Furthermore, it is not always effectively possible to replace the chuck with another of different size to suit heavier or lighter work. Accordingly, the object of the invention is one of overcoming the drawbacks mentioned above, from the standpoint of the lapping tool. The stated object is achieved with lapping equipment as characterised in the appended claims. Improved equipment according to the invention is of the type utilizing a chuck to hold the work, and a composite lapping tool that consists in a vertical rod ensheathed by a marginally loose fitting cone supporting a diamond shell, and features a lap rod embodied in three rigidly associated sections: a bottom section of polygonal transverse profile engaging to an exact fit in a corresponding axial socket offered by the cone and locked in position axially by means of a diametral pin; a middle section, possessing a degree of flexibility commensurate with the tolerances prescribed for the work, whilst ensuring sufficient torsional rigidity to function as a flexible coupling; and a top section connected to the spindle. One of the advantages of the invention is that of its adaptability to different sizes of bore to be lapped, and to varying sizes of workpiece, inasmuch as torque is transmitted from the rod to the cone through polygonal mating surfaces; accordingly, the area through which these two parts make contact is generous; and distributed uniformly about the axis of the abrasive shell, however small the overall diameter. Moreover, the three-stage embodiment of the lap rod is instrumental in obtaining a more accurate seating of the lap internally of the bore to be finished, thanks to the combined elasticity and resilience of the middle section and its ability to function as flexible coupling. Another advantage of the invention is that it is simple in construction, and therefore advantageous from the cost standpoint; in effect, the only parts requiring any significant degree of precision are the polygonal bottom section of the lap rod and the matching socket of the cone. Still another advantage of the invention is that the stresses transmitted through the lap rod to the cone and shell can be reduced practically at will, since mating contact occurs across the entire expanse of the polygonal profile offered by the bottom section of the rod, and accordingly, this same polygonal profile can be lengthened or shortened, so as to reduce or increase mechanical pressure on the cone during the lapping process. The invention, which is defined in claim 1, will now be described in detail, by way of example, with the aid of the accompanying drawings, in which: fig. 1 is the axial section through a lapping tool forming part of improved equipment according to the invention; figs. 2 and 3 are views of the lap shown in fig. 1, in side elevation, and half-elevation/half-section, respectively, which illustrate two of its component parts, namely the rod and the cone. The improved equipment disclosed, of the type used for lapping internal surfaces, consists essentially in a vertically disposed composite lapping tool 12, fastened directly or indirectly in conventional fashion to an overhead spindle 11, and a chucking fixture 1 mounted to the bed of the lapping machine (not illustrated). The composite lap 12, illustrated in fig. 1, consists in a rod 2, located uppermost and connected by its top end to the spindle 11, a cone 3, located below and carried by the rod 2, and an abrasive diamond shell 4 that ensheaths the cone 3 to an exact fit. The cone 3 exhibits an axial hole 13 with an open top end in which the bottom end of the rod 2 is freely inserted, the degree of clearance allowed being compatible with the tolerances prescribed for the lapping operation. The diamond shell 4, which constitutes the abrasive part proper of the composite lap 12, is fitted to and removable from the cone 3 by conventional means such as tock nuts 23 and 24, for example, that screw onto corresponding threads offered by the ends of the cone 3 and clamp the shell 4 from either end, as illustrated in fig. 1. According to the invention, the rod 2 is embodied in three sections 7, 8 and 9, which are associated rigidly and coaxially one with the next. The bottom section or tip, denoted 7, exhibits a polygonal, preferably hexagonal transverse profile (discernable in fig. 2), and a diametrically disposed hole 25 serving to accomodate a pin. The polygonal tip 7 is insertable to an exact fit in a matching polygonal socket 13a afforded by the axial hole 13 of the cone 3, which also presents a diametrically disposed hole 26 positioned to coincide coaxially with the hole 25 of the tip 7. 10 denotes the aforementioned pin, which inserts to an exact fit in the holes 25 and 26 offered by the hex tip 7 and socket 13a, thereby furnishing the means by which the rod and cone parts 2 and 3 of the composite tool 12 are connected, and constituting the sole means by which the cone 3 is supported. The length of the hex tip 7 will depend first and foremost on the external diameter of the abrasive shell 4: the smaller the diameter of the shell 4, the greater the length of the hex tip 7. Given, in fact, that torque generated by the spindle motor is transmitted from the rod 2 to the cone 3 by way of the hex tip 7, the pressure with which the cone 3 is invested by the tip 7 will be proportionally less (transverse dimensions remaining par) the greater the lateral surface area of the tip 7 that enters into contact with the cone 3, i.e. the longer the axial dimensions of the tip 7. More exactly, the hex tip 7 coincides with the centre of balance of the composite tool, so as to avoid overloading the bottom end and ensure better balance overall. The middle section of the rod 2 denoted 8, referred to hereinafter as a coupling (the reasons for which will shortly become clear) is embodied in material possessing a given degree of flexibility that will be commensurate with the tolerances prescribed for the lapped work, though sufficient at all events to ensure good torsional rigidity. In the example illustrated, the coupling 8 consists in a longitudinal element embodied as a tightly coiled helicoid, i.e. with no gaps between the turns of the helix, generated in the opposite direction to that in which the lap 12 turns with the spindle 11; the coupling 8 is accomodated freely by the axial hole 13 of the cone 3, the degree of clearance being compatible with the prescribed lapping tolerances. The top section, or shank 9 of the rod 2 exhibits a stretch of diameter substantially identical to that of the coupling 8, an intermediate boss 27 of wider transverse dimensions, and a screw thread 28 at the very top end. The boss 27 affords two flat parallel faces that can be engaged by a key or wrench to the end of tightening the screw thread 28 in the female thread of a holder, integral with or fitted to the spindle 11. The method of assembling an improved lapping tool 12 according to the invention is substantially the same as for a conventional composite tool, and therefore will not be described. As regards the operation of the composite lap 12, it will be seen that the abrasive shell 4 is capable of movement in relation to the rod 2 by virtue both of the clearance existing between rod and cone 3, and of the elastic properties of the coupling 8, which are such as to make the rod advantageously flexible while ensuring that it retains a sufficient degree of torsional rigidity. The inclusion of the coupling 8 provides a further marked advantage, namely, the capacity to function as a safety device in the event of some unforeseen obstacle slowing down or jamming the shell 4. More exactly, the coupling 8 can be proportioned in the manner of a shear pin, i.e. capable of transmitting torque only up to a given maximum value, in excess of which it will break, preventing damage either to the work or to other components of the equipment.

Claims: Equipment for lapping internal surfaces, comprising : a composite lapping tool (12) that consists in a vertical rod (2) uppermost, carried by a spindle (11), a cone (3) at bottom which ensheaths the rod in a lose fit so as to enable a marginal freedom of transverse movement between rod and cone, this transverse movement being compatible with the tolerances prescribed for the lapping operation, and a diamond shell (4) ensheathing and rigidly associated with the cone (3); and a work chucking mechanism (1), positioned beneath the spindle and capable of oscillating about an axis disposed substantially coaxial with, or at least parallel to, that of the tool (12), characterised in that the rod (2) is embodied in three rigidly connected section (7, 8, 9): a bottom section (7) of poligonal transverse profile enganging to an exact fit in a corresponding axial socket (13a) afforded by the cone (3) and accomodating a diametral pin (10), a middle section (8), consisting in a longitudinal helicoid element, the helical generation of which occurs about the axis of the rod (2) in the opposite direction to that of the rotation of the tool (12), possessing a moderate degree of transverse elasticity to permit a sufficient freedom of movement for a good fit of the diamond shell (4) in the internal surfaces and ensuring sufficient transverse rigidity to avoid that the diamond shell (4) binds against the internal surfaces and breaks, that the bore has an incorrect profile, and ensuring sufficient torsional rigidity to function as a flexible coupling; and a top section (9) connected to the spindle (11). Equipment as in claim 1, characterised in that the middle section (8) of the lap rod (2) presents a safety device, realized by the breaking of this middle section, that avoids the reaching of a given maximum torque transmissible from the rod of the cone (3), so as to forestall damage attributable to an obstruction encountered during the lapping operation, or at all events, to a drop in the speed of rotation of the abrasive shell (4).

Inventors: CASSANELLI GIANFRANCO, CASSANELLI, GIANFRANCO
Applicants: TRIMATE S A S DI LONGO SILVANA, TRIMATE S.A.S. DI LONGO SILVANA E C.
Application year: 1988
Main classification: B24B33
IPC codes: B23Q1, B24B33