Improved equipment for lapping internal surfaces

Abstract: In improved equipment for lapping internal surfaces, 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 (see for example DE-A-1 677 168). 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 follow a barely perceptible transverse oscillation. This carefully calculated freedom of movement is invisibe 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 alignment 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 alignment 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 work chucking fixture. The stated object is achieved with an improved chucking fixture as defined in claim 1. Advantageous embodiments of the invention are described in the dependent claims. The improved work chucking fixture according to the invention is suspended from at least three cables which pass through respective peripheral mountings integral with the chuck; thus, chuck and work can shift bodily in any given plane, and are provided with a cushioning system at one and the same time. The mobility of the chuck can be adjusted by tautening or slackening the cables to suit the weight of the work, and offset mechanical resistance, if any, generated by the lapping action. The invention will now be described in detail, by way of example, with the aid of the accompanying drawings in which: figs. 1 and 2 illustrate the work chucking fixture according to the invention, in plan and axial section, respectively; fig. 3 illustrates a lapping tool utilizable with the work chucking fixture, as in figures 1 and 2, not described in the present specification but subject matter of parent EP-A-0 314 634 of the same Applicant. The chucking fixture 1 which, for example, might be a conventional type self-alignining chucking mechanism, this is suspended from at least three longitudinal elements, embodied as cables 5 and 6 anchored to fixed supports provided by the side walls of a vertically disposed hollow frame 16 (see fig. 1) that is made fast to the machine bed in conventional manner. In the embodiment illustrated, the cables 5 and 6 are arranged in two pairs, set at different heights in order to avoid mutual contact; it will be seen, in fact, that the two cables each pair are parallel and disposed at right anglesto those of the remaining pair. The cables 5 and 6 associate with chuck 1 by way of four peripheral hollow mountings 14, fitted in pairs and occupying position is that are diametrically opposed in relation to the vertical axis of chuck 1. It will be observed from fig. 1 that the diameter of the passage 15 through each mounting 14 decreases from the ends toward the middle, creating what is a substantially biconical, hourglass type profile the waisted portion of which is rounded so, as to avoid any eccessive build-up of tension at that point. The minimum diameter of the passage 15 substantially matches the diameter of the respective cable 5 or 6, in such a way as to prevent any unwanted sideways movement of the chuck in relation to the cables 5 and 6. The two ends of each cable 5 and 6 pass through the side walls of the frame 16, and are prevented from working free by relative fixed and removable locking elements 17 located externally of the frame. In the embodiment illustrated in figs. 1 and 2, each cable 5 and 6 is secured by one fixed element 17, and one removable element, or clamp. 18 denotes a device encircling each cable 5 and 6 at one end, located between the relative clamp 17 the frame 16, which serves to adjust the tension of the cable with which is associated. Each such device 18 consist in an externally threaded hollow member 19 associated rigidly with the cable 5 or 6, an adjuster nut 20 screwed onto the threaded member 19, and a set of belleville disc springs 21 through which the threaded member 19 is inserted. Where the device is assembled such that the springs 21 impinge on the clamp 17, a distance collar 22 may also be added, as illustrated in figs. 1 and 2. By moving the nut 20 one way or other along the threaded member 19, the relative cable 5 and 6 can be tensioned or slackened to suit the size of the work, and more important, to suit its weight. The cables 5 and 6 will need to be well tightened for large and bulky items so as to avoid too loose a suspension, whereas a slacker setting is needed for lighter work in order to permit a reasonable freedom of movement as the lap enters the bore. At all events the cables 5 and 6 are always sure of a certain degree of elasticity thanks to the action of the springs 21, which will rarely be compressed to, the limit. It will be clear then, from figs. 1 and 2, that the chuck 1 is capable of movement in practically any direction; the arrangement of the cables 5 and 6 permits a marginal degree of rotation not only about the vertical axis, but about any given horizontal or inclined axis too.

Claims: Equipment for lapping internal surfaces, comprising: a composite lapping tool (12) 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 lapping tool (12), characterized in that the chucking mechanism (1) is suspended from at least three longitudinal elements, embodied as cables (5, 6) and made to pass through respective peripheral hollow mountings (14) afforded by the chucking mechanism itself. Equipment as in claim 1, characterized in that the work chucking mechanism (1) is freely suspended from two pairs of parallel cables (5, 6) disposed at right angles one to the other. Equipment as in claim 1, characterized in that the two pairs of cables (5, 6) are located at dissimilar height one from the other. Equipment as in claim 1, characterized in that the diameter of the passage (15) through each peripheral hollow mounting (14) decreases from the ends towards the middle. Equipment as in claim 1 or 2, characterized in that the tension of the cables (5, 6) is adjustable by flexible means, commensurately with and inversely proportionate to the dimensions and weight of the work to be lapped. Equipment as in claim 1 or 2, characterized in that the two ends of each cable (5, 6) are inserted through respective permanent supports (16) and prevented from working loose by fixed or removable locking elements (17) applied externally of the supports, and characterized in that at least one end of each cable (5, 6) associates with a flexible tension-adjustment device (18) located between the locking element (17) and the permanent support (16). Equipment as in claim 6, characterized in that the supports (16) are afforded by the side walls of a vertically disposed hollow frame (16). Equipment as in claim 6, characterized in that the single flexible tension-adjustment device (18) consists in an externally threaded hollow member (19) ensheathing and rigidly associated with the cable (5 or 6), in adjuster nut (20) screwed onto the threaded member (19), and a spring (21) through which at least the threaded member (19) is inserted, in such a way that the nut (2) can be screwed back or forward along the threaded member to increase or slacken the tension of the relative cable (5, 6).

Inventors: CASSANELLI GIANFRANCO, CASSANELLI, GIANFRANCO
Applicants: TRIMATE SRL, TRIMATE S.R.L.
Application year: 1988
Main classification: B24B41
IPC codes: B24B33, B24B41, B23Q1