Abstract:
A lapping tool has an elongate shank mechanically coupled to a platform, having a generally planar working surface, by a universal joint comprising a ball and socket. A cavity is provided for entrapping grinding paste. Normally, the parts are made of electrically conductive materials so that current can be passed from the shank via the working surface to surface of a workpiece that is to be polished.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to lapping tools. 
     2. Description of Prior Art 
     Lapping tools are well-known and used for “polishing” working surfaces by applying an abrasive action using random manual manipulation or machinery constrained (automatic) movements so that the abrasive material smooths and polishes a workpiece surface. The abrasive material may be in the form of a diamond paste and/or the lapping tool itself may be impregnated with a suitable abrasive material. The abrasive material is usually diamond grit but other abrasive materials may be used. Traditionally, the working surface is polished in a number of stages in sequence, using at first relatively course granules or grit and using much finer abrasive particles for the final stages, for example. 
     It is already known to simultaneously apply ultrasonic signals to cause high frequency vibration to the lapping tool during lapping procedures. Additionally, in some cases, electrical power can also be applied to provide an electrical discharge between the lapping tool and the workpiece surface or, in the presence of a suitable electrolyte, an electrolytic reaction. Such additional techniques, broadly stated, improve the lapping procedures and considerably reduce the time required to carry out the polishing procedures. 
     A lapping tool is known that is formed of more than one component, where the components are capable of relative pivotable movement. Such a tool is however incapable of making use of the additional techniques. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to overcome this problem. 
     According to the invention there is provided a lapping tool having an elongate shank and a platform with a generally planar lower operative surface, in which the platform and an end of the shank are connected together by a pivotable joint, and including an electrically conductive path extending from the shank to the lower working surface. 
     The pivotable joint is preferably a universal joint, and may comprise a ball and socket. 
     The ball is preferably provided on the end of the shank. 
     The shank and the platform may be formed of electrically conductive material. The platform may be formed at least in part of plastics material that is impregnated with electrically conductive material to form the said conductive path. 
     The planar working surface may be impregnated with abrasive material. 
     The planar working surface may be provided with a cavity exposed at its undersurface, in which abrasive material can be constrained in use. 
     The ball and socket may be held together by a removable spring clip, or a coiled spring or a screw cap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Lapping tools according to the invention will now be described by way of example with reference to the accompanying drawings in which: 
     FIG. 1 is a sectional side view of one tool; 
     FIG. 2 is a sectional side view of another tool; and 
     FIG. 3 is a sectional side view of a further tool. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, in FIG. 1 a lapping tool comprises an elongate shank  10  (only part of which is shown and comprises a cylindrical metallic rod of any suitable length) having an end  11  shaped as a spherical ball. A metallic platform  12  has a lower generally planar working surface  13  formed with a cavity  14  for receiving and holding abrasive/grinding paste  15  in use. As explained above, abrasive paste for polishing is conventionally a “diamond paste” and can be provided in various possible grades and used for coarse and fine polishing as applicable. 
     The ball  11  is held in and against a spherical socket  16  formed in an upper part of the platform  12  by a circlip  17 . An electrically conductive path is therefore formed between the shank  10  and the working surface  13  of the platform  12 , to enable electrical current to pass from the shank via the surface  13  to a surface of a workpiece (not shown) immediately below and adjacent the surface  13  in use. Thus, the described lapping tool can be used in conjunction with electrical discharge and electrolytic techniques, known per se. It will be appreciated that the shank may also be exposed to high frequency vibrations caused by ultrasonic or other high frequency signals. 
     In FIG. 2, the lapping tool is generally similar to the lapping tool shown in FIG. 1 except that it is provided with a coiled spring  18  to hold the shank  10  and platform  12  together. 
     Similarly, in FIG. 3 the shank  10  and the platform are held together by a screwed cap  19 . The ball  11  fits into a cylindrical cavity  20  formed in an upper part of the platform  12  and a bias spring  21  is provided in the cavity  19  under the ball  11 . Generally, the periphery of the ball  11  will remain in good electrical contact with the inner surface of the cavity  20 , but if desired the spring  21  may be formed of electrical conductive material to provide or to ensure a good electrical connection between the ball  11  and the platform  12 . 
     Lapping tools of the invention may comprise several other different arrangements. For example, the universal joint, the ball  11  and socket  16 , may be provided in other forms and may also be replaced by a pivotable joint that pivots about a single axis. The pivotable joint is provided to allow the platform  12  to pivot relative to the end of the shank so that its working surface  13  can “float” across the surface of a workpiece in use. This enables the polishing to take place more effectively and efficiently especially if the workpiece surface is not flat or has imperfections that may otherwise disrupt the relatively free polishing movement of the platform over the surface of the workpiece. Where the lapping tool is held in a vice or tool holder, as is the case in an automatic machine, it may sometimes be an advantage or be sufficient to allow the platform to pivot only about a single axis. 
     It will appreciated that the described universal joint could be formed by a socket formed on the end of the shank  10  for cooperating with a ball provided or formed on an upper part of the platform. The socket or ball provided on the platform may also be positioned, if preferred, on a side of the platform. 
     Normally, the shank  10  is formed of conductive metallic material. However, it is possible where the shank material is non-conductive, or is a relatively poor conductor of electricity, to have a conductive strip or similar extending along its length to provide an electrically conductive path. The same can apply to the platform  12  where a strip or ring of conductive material can be used as a suitable electrical conductor. 
     For convenience, easy of manufacture and cost, it may be preferable to form the platform at least partially of plastics material. In that case an electrically conductive plastics material will be required, such as for example as a normal plastic material that is sufficiently impregnated with conductive material so as to provide the necessary electrical path for current to pass from the shank  10  to the working surface  13 . 
     In the described lapping tools grinding paste  15  is used. It is possible alternatively or additionally to form the surface  13 , normally with an inherently more wearable material than otherwise, using at least a layer of material impregnated with grinding material, such as industrial diamond grit. This means that the platform  12  will have to be replaced from time to time, or perhaps more often than in the described arrangements. However, as the platform  12  is a separate detachable component of the lapping tool, replacement is not a significant practical difficulty. In such arrangements, the platform  12  is then replaced as required with a new platform or with an unserviceable (i.e. used) platform that has been “re-layered” with a new impregnated working surface layer. 
     It is noted that the surface  13  has been described as “generally planar”. This expression is intended to include a surface that in one plane may however be curved to some extent, as may be required for use in polishing an arcuate surface of a workpiece, such as an outer surface of a shaft to form a polished bearing surface.