Abstract:
A tool for lapping internal surfaces of holes. Lapping tools conventionally have an expandable working portion for variable diameter operation. Known tools tend to expand in a barrel shape leading to inaccurate lapping. The invention seeks to avoid this by providing a working portion of a tubular tool with slots in a plurality of rows each having several slots. Slots in each row are staggered relative to those in adjacent rows.

Description:
BACKGROUND OF THE INVENTION 
     For lapping a cylindrical hole of small diameter it is a common practice to employ a lapping tool constituted by a tube having an expandable working portion which carries diamond cutters externally. Within the tube is provided a cone movable with a sliding motion and integral with a controlling rod. By movement of the cone, the working portion of the lapping tool is expanded. 
     A lapping tool of this kind normally incorporates, in order to allow the expansion of its working portion, either one longitudinal slot extending from one end of the tube to the other, or a series of alternating longitudinal slots beginning from one end of the tube and terminated at a short distance from the other end. Unfortunately, however, such a lapping tool does not retain a cylindrical form when expanded. It tends to assume a swollen form similar to that of a barrel. This shape prevents lapping with a hole of accurate dimensions. To obtain accuracy within half a micron for instance, it is necessary both that each lapping action remove very little material (otherwise the lapping tool is overworked at the end and is worn to a conical shape) and that the lapping tool retain a perfectly cylindrical shape when expanded. 
     OBJECT OF THE INVENTION 
     An object of the present invention is to provide a lapping tool capable of expansion at its working portion in a relatively uniform manner. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided a lapping tool comprising a tubular member having a working portion at one end thereof, which working portion has an abrasive external surface, a frusto-conical internal surface, and a plurality of slots disposed in parallel rows, the slots of one row being staggered relative to those of an adjacent row, and the tool further comprising a control rod having a frusto-conical portion axially slidable within the working portion whereby the diameter of said working portion is adjustable. 
     Preferably, the tubular member is elongate and the rows are parallel to the longitudinal axis of the tubular member i.e. lie along generatrices thereof. 
     Preferably, said working portion has a diamond-coated external surface. 
     Preferably, the slots of each row are staggered in relation to the slots in each adjacent row. 
     Preferably, the tubular member is securable to the control rod for rotation therewith at least primarily or principally by means of frictional contact between the frusto-conical portion and said frusto-conical inner surface. 
     In one embodiment, the tubular member is securable to the control rod for rotation therewith solely by means of said frictional contact. 
     In said one embodiment, said tubular member may be constituted by an abrasive-coated sleeve which has a force fit on said frusto-conical portion. 
     In a further embodiment, the control rod is slidably keyed to a part of the tubular member, which part is linked to the working portion by a zone of smaller wall thickness. 
     According to a further aspect of the invention, there is provided a lapping machine incorporating a said lapping tool, wherein the control rod of the lapping tool is connected to a spindle of the machine in such a way that the frusto-conical portion is fixed against axial movement and the tubular member is connected to an expansion rod of the machine. 
     Preferably, the tubular member is connected to the expansion rod of the machine by a thread. 
     Preferably, said tubular member is screwed into a support tube connected to said expansion rod and includes a notched part in engagement with a notched ring slidably mounted on the support tube and immobilized angularly relative to this by resilient means arranged to bias the notches of the ring to a position between the notches of the tubular member. 
     Preferably, the angular immobilized of the ring relative to the support tube is produced by a peg extending from the support tube and slidable with its free end in a slot of the ring. 
     Expediently, the slot is L-shaped and has a portion extending perpendicular to the longitudinal axis of the tubular member which allows the locking of the ring in a position for adjustment of the lapping tool position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an axial sectional view of a first embodiment of lapping tool; 
     FIG. 2 is a section along line II--II of FIG. 1; 
     FIG. 3 shows schematically the working portion of the lapping tool in opened-out form; 
     FIG. 4 is a schematic side view of the lapping tool; 
     FIG. 5 is a schematic side view, with a partial section of a second embodiment of lapping tool; and 
     FIG. 6 shows a sectional view of a lapping machine for the simultaneous lapping of holes whose axes are close together. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 to 4 show a lapping tool constituted by a tube 1 and an expansion control rod 2 movable within tube 1. At its free end, rod 2 incorporates an externally frusto-conical part 2a which bears against an internally frusto-conical part 1a of the tube 1. Part 1a constitutes the working portion of the lapping tool and is provided externally with a coating of abrasive, e.g. diamond abrasive. 
     At its end 1b remote from part 1a, tube 1 is secured for common rotation to rod 2 by a key 3 movable in an elongate slot 4 of the rod. Tube 1 is made with thinner walls in its central part 1c positioned between part 1a and its end 1b, to which a rotational drive is applied. 
     Part 1a incorporates slots 5, formed for instance, by electro-erosion (spark erosion). These are disposed in rows parallel to the longitudinal central axis of the tube in such a way that several separate slots 5 form each row. The slots are alternated, that is to say, each slot is located adjacent an unslotted part interposed between two slots of the adjacent rows. In the example shown, the rows of slots are not evenly spaced but are grouped in pairs, the distance separating the two rows of each pair being less than that separating two adjacent rows belonging to respective neighboring pairs. 
     In operation, driving torque is furnished by a spindle of a lapping machine to the expansion control rod 2. This torque is transmitted to the working part 1a of the lapping tool, partly via the tube 1 (coupled to rod 2 by key 3) and partly by virtue of the friction between working part 1a and the conical part 2a of rod 2. This frictional drive is predominant through the effect of the thinning of the part 1c which imparts to tube 1 a certain angular flexibility. 
     When the tube 1 is moved axially upwards relative to the rod 2, working part 1a is spread and its diameter increased in a uniform manner throughout its length. It remains perfectly cylindrical externally. 
     In the embodiment of FIG. 5, the lapping tool is constituted by a simple ring 6 forming a sleeve on a frusto-conical expansion rod 7. Ring 6 incorporates slots 5a disposed as the slots 5. The expansion may be obtained by manually forcing the ring 6 onto the rod, and during that operation the ring 6 remains externally perfectly cylindrical. This lapping tool is particularly suitable for use in manual lapping of open holes. 
     The geometric quality of the lapping tool is enhanced by limiting the stresses on the tube of the lapping tool. This is achieved by causing the tube and control rod to rotate as one body at least predominantly through the friction of the expansion cone within the tube. The lapping tool then has great inertia under the varying stresses of machining, under the stresses of progressive expansion or under variations of contact stresses. 
     Where the tube of the lapping tool can be connected to the control rod solely through the intermediary of the expansion cone, the tube can be constituted by a simple diamond-encrusted sleeve (FIG. 5) made a forced fit on the expansion cone 8a. Expansion is then effected manually by forcing the sleeve onto the cone. 
     Where the control rod is slidably keyed relative to a part of the tube that is connected to the working part of this same by a zone of reduced wall thickness, this zone imparts an angular suppleness to the tube in such a way that the part of the torque transmitted to this by the cone is greater than that which is transmitted to it through the intermediary of the key. 
     FIG. 6 shows a plurality of lapping tools fitted to a mass-production automatic machine intended for simultaneous lapping of several closely adjacent holes. 
     Each lapping tool has an expansion control rod 8 which incorporates in its lower part an expansion cone 8a and is fixed directly to a spindle 9 of the machine by a nut 10. 
     Each lapping tool has a tube 11 which is mounted on the respective expansion cone 8a and is screwed into a support tube 12. A linking ring 13 is fitted in the upper part of the support tube 12 and held there by a nut 14 and a washer 15 of plastic material. A draw bar 16 which is screwed onto the expansion rod 17 of the machine is engaged in an axial bore of the rod 8 and secured to the linking ring 13 by a key 18 which passes through draw bar 16, through longitudinal slots 19 of the rod 8 and engages in recesses of linking ring 13. 
     When the expansion rod 17 rises, it thus takes with it support tube 12 and therefore the tube 11 of the lapping tool which causes radial expansion of the working part 11a of the lapping tool by sliding over expansion cone 8a which is fixed axially. 
     Below its threaded part, the tube 11 is provided with a notched collar 11b, with which is engaged the notched lower edge of a ring 20 which is fitted over support tube 12 and is secured thereto by a peg 21 secured to support tube 12 and engaged in an L-shaped aperture 22 of ring 20. A compression spring 23 is located between the ring 20 and a shoulder of support tube 12, and tends to hold the notches of ring 20 engaged with those of the collar 11b. 
     To effect an adjustment of the lapping tool, it is totally collapsed by moving the rod 17 downwards, so that it exerts no stress on the tool through the expansion cone 8a. Then the ring 20 is moved upwards normally, compressing the spring 23, and the ring is rotated slightly so that the peg 21 is engaged in the horizontal part of the aperture 22, which maintains the ring in its retracted position. Adjustment is then effected by screwing the tube 11 of the lapping tool into or out of the support tube 12. The notched ring 20 is then replaced in its initial position, which fixes the tube 11 in its new position (i.e. prevents rotation of tube 11). The reference numeral 24 identifies a split washer which is enclosed in a channel provided in the upper part of the control rod 8, near the nut 10. When it is desired to retract the control rod 8 from the spindle 9, nut 10 is unscrewed, and bears against the washer 24 thus exerting a force on the rod 8 which extracts it from the spindle 9. 
     The lapping machine which has just been described makes it possible to obtain continuous expansion of working part 11 during lapping, while driving the tube 11 solely through friction with the expansion cone 8a. The lapping tool can be exchanged and adjusted with an excellent degree of precision. This can even be achieved in a cramped space, which makes it possible to produce multiple lapping heads that can operate with very small spacing between the individual lapping tools. It is possible to adjust the lapping tool at any time manually to attain a dimension with very high precision, of the order of 0.5. micron. 
     It will be seen that the machine just described has the controlling rod of each lapping tool connected to a respective spindle of the machine in such a way that the expansion cone is axially fixed, while the tube of each lapping tool is connected to a respective expansion rod of the machine. Because the tube does not transmit rotation torque, it can be linked to the expansion rod of the machine by a simple thread. This allows the diameter of the working part of the lapping tool to be regulated easily. 
     It will be understood that the claims are intended to cover all changes and modifications of the preferred embodiments of the invention, herein chosen for the purpose of illustration, which do not constitute departures from the spirit and scope of the invention.