This invention relates generally to a orbital grinding. More specifically, this invention relates to a new and improved orbital grinding process, and to the abrading tools used therein, for use the orbital grinding of comparatively softer materials, to produce elongated two-dimensional forms. The process and tools of this invention find particular utility in the formation and restoration of two-dimensional or near two-dimensional workpieces such as graphite electrodes used in electrical discharge machining.

BACKGROUND OF THE INVENTION

Electrical discharge machining, usually referred to as EDM, is a well known process for the machining or forming of complex and intricate shapes into the surface of conductive metals. In this process, a graphite electrode is first formed wherein one face thereof has a configuration or image formed thereon which is the negative image of that to be machined onto the workpiece. Thereafter, the graphite electrode and the workpiece are properly mounted into an EDM machine where the imaged face of the electrode and the workpiece are brought into close proximity, while an electrical potential is maintained across the two pieces. As the gap between the two pieces narrows, the electric potential will discharge across the gap causing arcing at the point or points of nearest proximity. Such arcing will cause minute volatilization of the metal workpiece at these locations with the metal

dispersed from the site. By careful control of the operating parameters, very detailed and intricate machining can be performed by this process, so that the finished workpiece will have a surface which, though slightly oversized or undersized, will be the exact negative image of the graphite electrode. If the electrode face consists of a detailed convex form, the workpiece will be formed to have a slightly oversized mating concave depression. On the other hand, if the electrode provides a concave form, the workpiece will result ^' with a slightly undersized mating convex form. Because the machining process is electrical rather than physical, EDM finds particular utility in its ability to machine rather hard metals such as tool steels, titanium and the like.

In addition to the machining or formation of three- dimensional surfaces, as described above, EDM is also utilized in two-dimensional machining applications. For example, holes of any desired cross sectional configuration can be machined into or through a metal workpiece utilizing a graphite electrode having that same cross sectional form, but slightly smaller than that of the machined hole. In this application, the face of the electrode brought into proximity of the workpiece is flat, or at least the same as that of the workpiece. The arcing action will then remove metal from the workpiece directly opposed from the electrode, thus forming a hole directly under the electrode having the same configuration of the

electrode. As the electrode is advanced into the workpiece, more and more metal is removed until the operation is ceased, or until the electrode passes all the way through the workpiece, leaving a recess or hole having the same cross sectional configuration as the electrode, but having a periphery slightly larger by an amount equal to the arc distance.

While the graphite electrodes are comparatively far more stable than the metal workpiece in this application, the electrodes do wear to some degree and must be periodically replaced or restored before further use. In the case of three-dimensional electrodes, the entire three-dimensional form must be restored, which may require rather sophisticated machining in and of itself. Indeed, orbital abrading techniques have been utilized to machine or remachine the three-dimensional form into the face of such electrodes. The restoration of two-dimensional electrodes need not, however, address the formation or reformation of a complex or even simple three-dimensional contact face. Since this face is merely a flat surface, all that need be done is to flatten or reflatten that face by a simple grinding or sawing operation. Even with two- dimensional electrodes, however, rather intricate machining may be required to extend the sides thereof if the cross sectional form is more complex than a true circle. To be more specific, it should be noted that two dimensional electrodes consist of an elongated electrode body having a uniform cross section throughout its length

with a flat surface at the end, parallel to the electrode section. The electrode body is usually a machined extension of a rectangular graphite base block. The rectangular base block portion is an essential part thereof for mounting and securing purposes into the EDM machine. When such an electrode is advanced into a workpiece, an arc is maintained only at the narrow gap between the flat end surface of the electrode and the flat surface of the workpiece immediately adjacent to the electrode. Accordingly, the only place where the electrode is worn is at the end thereof which is eroded away, while the corner edges at the end are somewhat rounded. This erosion will, of course, progressively shorten the electrode body. While the original form of the electrode body can readily be reformed by merely reflattening the end face, it is usually necessary to remachine a length of the base block portion to relengthen the electrode body portion. Such machining or forming operation may be rather complex if the cross section of the electrode body is complex.

Orbital grinding, on the other hand, is a completely different machining operation which involves the physical abrasion of a workpiece by a tool having a roughened abrading surface. Unlike conventional grinding, orbital grinding does not utilize a rotating grinding wheel, but rather brings the tool and workpiece together, whereby at least one of which is in orbital motion without rotation. In this process, the working tool is usually formed of a

rather hard material and typically has a three dimensional configuration in its working face. By orbiting either the tool or the workpiece, or both, while the two pieces are in contact and biased against each other, using a rather small radius of orbit, the complimentary negative configuration of the tool is worked into the workpiece. In this process, the orbital grinding machine is typically provided with a stationary mounting means and a nonrotating, orbital mounting means opposed thereto. ith the abrading tool mounted on one of the mounting means and the workpiece mounted on the other, the two are brought into physical contact where the nonrotating orbital motion between the two pieces causes the relatively harder tool to physically abrade the workpiece to produce a negative compliment of the tool's working surface. Which piece is stationary and which is orbiting will vary depending on the particular application, while in more advanced applications, both can be made to orbit to effect rather complex configurations. Unlike conventional grinding techniques, orbital grinding utilizes a rather small relative movement in having a radius of orbit, typically from 0.0020 to 0.0030 inch, (51 to 76 micrometers) at relatively high oscillation rates. Because of the orbital motion between the tool and the workpiece, the resulting machined configuration in the workpiece cannot be of identical size to that of the tool. However, because of the very small displacement of the two pieces during working, orbital grinding is capable of producing rather

detailed and intricate ground configurations with a high degree of resolution, in three-dimensional form. The degree of resolution achieved in the workpiece surface can, of course, be no better than the degree of resolution in the working tool. In order to effect a high degree of resolution in the finished product, therefore, it is somewhat essential that the tool be made of a relatively hard material, such as a hard steel alloy, so that the abrading tool is not significantly eroded during the grinding operation which would thereby reduce resolution.

While orbital grinding is usually utilized to produce a three-dimensional form on the workpiece surface, as described above, some limited degree of two-dimensional machining may naturally result, depending upon the design of the image to be machined into the surface of the workpiece.

SUMMARY AND OBJECTS OF THE INVENTION This invention is predicated on the development of a process and grinding tools for orbital grinding machines which can be used to readily machine or form elongated bodies having a uniform or nonuniform cross sections, which in essence shapes the sides of such elongated forms without shaping any three-dimensional face image. Accordingly, the process and abrading tools of this invention find particular utility in the forming of two- dimensional graphite electrodes for use in the EDM process, as described above, and in the restoration of such electrodes after they have been shortened due to

excessive use, as well as the formation of other elongated forms having any desired uniform cross-sectional configuration. In addition, certain embodiments of this invention fine particular utility in the making of very narrow elongated workpieces having either straight or tapered sides without any significant degree of breakage of the elongated product. Since orbital grinding machines typically have rather large working areas which are measured in feet rather than inches, the formation of EDM graphite electrodes with such conventional orbital grinding machines will greatly speed up production thereof because a significant number of electrodes can be produced or restored simultaneously in a single operation.

It is therefore an object of this invention to provide a new, simple and inexpensive process and abrading tools for orbital grinding machines which can be utilized to machine elongated bodies having straight or tapered sides.

Another object of this invention is to provide a simple and inexpensive abrading tool for use in conjunction with orbital grinding machines for the machining of two dimensional graphite electrodes used in

EDM.

Still another object of this invention is to provided simple and inexpensive abrading tools for the machining of two-dimensional EDM electrodes by utilizing orbital grinding machines which will permit the simultaneous

processing of a significant number of electrodes to thereby speed production.

A further object of this invention is to provide a process for the production or restoration of two- dimensional graphite electrodes utilized in the EDM process which utilizes a simple and inexpensive abrading tool in an orbital grinding process thus permitting the processing of a significant number electrodes simultaneously to thereby speed production. An even further object of this invention is to provide a process for producing narrow, elongated forms in relative soft workpiece materials, having tapered sides which could not be produced previously without considerable breakage. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is an isometric view of a typical abrading tool in accordance with one embodiment of this invention.

Figure 2 is a sectional view of the tool shown in Figure 1 taken at line II-II. Figure 3 illustrates an alternative sectional view to that shown in Figure 2 wherein the side walls of the hole through the tool are tapered.

Figure 4 is an isometric view of a two dimensional graphite electrode as could be produced or reconditioned by the abrading tools depicted in Figure 1, 2 and 3.

Figure 5 is an isometric view of an alternative workpiece form as could be produced by the abrading tools depicted in Figures 1, 2 and 3.

Figure 6 is a side view of the tool and a graphite block as mounted within a orbital grinding machine prior to the formation an electrode as shown in Figure 2.

Figure 7 is a photograph of a more complicated tool as utilized in accordance with another embodiment of this invention.

Figure 8 is a photograph of the workpiece produced by the tool shown in Figure 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT Reference to Figure 1 will illustrate one embodiment of this invention which comprises nothing more than a hard metallic plate _1 , made of a material such as hardened steel. A hole 12 is provided perpendicularly through the face of plate 3^ at the approximate center thereof. Said hole 12 has a cross sectional configuration as will mate with that of the electrode to be produced or reconditioned, or other such form to be machined. The actual parameters of hole 12 must be slightly larger than that of the intended or reconditioned electrode by an amount equal to the radius of orbit utilized in the orbital grinding operation. That is to say, hole 12 will extend beyond the configuration formed thereby by a distance equal to the radius of orbit, which is typically from 0.0020 to 0.0030 inch, (51.0 to 76.0 micrometers), in most applications. A surface portion 14 on the face of plate _1_0 surrounding hole 12 is roughened to provide a rough surface finish, preferable about 250-300 RMS. The outer parameters of the roughened area 14 should be

determined by the block of graphite to be processed such that when the tool JL is brought into contact with the graphite block in an orbital abrading machine, the roughened area is sufficient in size to cover the entire graphite block throughout the relative orbital motion. The inner parameters of roughened area 14 should, of course, extend to the very edge of hole 12. Lastly, a plurality of holes 16 are usually provided through plate 10 for the purpose of locating and securing plate JL0_ within the orbital grinding machine.

While a number of suitable procedures and materials could be utilized to produce plate 3^, good results have been accomplished by fabricating the grinding tool from D- 2 or AISI 4140 alloy plate, 3/8 to 1/2 inch (0.95 to 1.27 cm) thick, and heat treating the plate to 60-62 Rockwell C. Hole 12 can be easily cut to form using wire electrical discharge machining.

In operation, the abrading tool as described above and a graphite block workpiece must be mounted into an orbital grinding machine in a suitable manner, one of which is illustrated in Figure 6. In this particular application, the graphite block 20 is secured within a V- block 22 attached to the lower platen or orbital table 24. A holding fixture 30 is attached to the upper platen 32 such that abrading tool 10 can be attached to a tubular holding fixture 32 and thus spaced away from platen 32 so as to provide adequate open space above hole 12 in tool 10. The orbital abrading action is commenced after platen

32 is lowered so that roughened area 14 on the face of tool 10 comes into contact with peripheral portion of graphite block 20 and is biased thereagainst. As the graphite block 20 is orbited against abrading tool 10 utilizing a small radius of orbit, the outer periphery of the top surface of graphite block will be eroded away by abrading tool 10 so that the only portion thereof not so abraded will be that portion which is directly under hole 12. As the abrading action continues, the outer portion of graphite block 20 is progressively abraded away, with the result that the center portion not abraded by tool 10 will get progressively longer extending upwardly through hole 12, thereby forming a "post" extending progressively further from the graphite base block from which it is ground. That portion of graphite block 20, i.e. the post, that extends upwardly through hole 12 will have a cross sectional area mating with the cross sectional area of hole 12 as described above, and will become the electrode body portion of the structure in those situations where EDM electrodes are being formed.

During the abrading operation, a flushing fluid- coolant, such as mineral spirits, is admitted through inlet 34 to flush away the abrasion debris and prevent the tool and workpiece from becoming overheated. When utilizing an abrading tool as illustrated in Figure 1, the finished graphite electrode will be as illustrated in Figure 2.

The process for restoring a worn electrode is substantially the same as described for producing an electrode except that the worn electrode is inserted instead of a fresh graphite block. The abrading action then is merely to abrade away a predetermined length of the remaining graphite block portion making that a shorter portion while lengthening the electrode portion.

While the above described process is limited to the production of a single electrodes or workpieces, it should be kept in mind that the working area in most conventional orbital grinders is far greater than needed for producing a typical EDM electrode. Accordingly, the use of such orbital grinding machines lends itself to the production or restoration of a large number of electrodes or workpieces in a single operation which would speed up production significantly. This can be done by providing a plurality of abrading tools as described above for a single orbital grinding operation, or one abrading tool having a plurality of holes 12, either of which can abrade a plurality of graphite blocks in a single operation.

It should be obvious that the air foil configuration as illustrated for the electrode and the abrading tool is merely an example of the cross-sectional configurations that can be produced according to this invention, as a post of practically any cross section can be produced. For example, multiple holes could be provided in a single tool to produce multiple, parallel elongated body portions or posts. The one limiting factor, however, is that

because the abrading tool is moving in an orbital path, inside corners cannot be produce to have a sharp angle of intersection. Instead, all inside corners will have a rounded intersection having a radius equal to the radius of orbit of the orbital grinding motion. Outside corners, however, can be produced having rather sharp angles of intersection. Even tapered posts can be formed by providing a progressively decreasing radius of orbit. The degree of taper on the sides of the post will be a direct measure of the degree of the orbit radius decrease.

In the orbital grinding of relatively thin and elongated electrodes or workpieces according to the above described embodiment of this invention, it has been found that as the length of the unmachined workpiece, i.e. post, increases, there is an increasing risk of breaking the post. While the tool as shown in Figures 1 and 2 abrades only the horizontal surface of the base block transverse to the post, and does not itself abrade or even contact the side surfaces of the post, there is, nevertheless, some lateral force applied on the sides of the post due to the fact that flushing fluid and debris will normally be present between the tool and post. Therefore, as the tool and post are in relative orbital motion, and the fluid and debris are squeezed from the gap by the relative motion, some lateral force will be applied against the side surfaces of the post. When relatively thin post sections are being formed, tools as shown in Figures 1 and 2 will tend to cause considerable breakage of the posts being

formed in proportion to the magnitude of post length over pose section. One technique for reducing this breakage is to utilize a tool having a hole with tapered sides as illustrated in Figure 3, or otherwise having an increasing section away from the abrading surface of the tool, so that the area of very close proximity between the tool and side surfaces of the post is significantly reduced. Such a relationship will significantly lessen the lateral forces applied to the sides of the post, as the pressure of the flushing fluid and debris thereagainst is progressively reduced as the gap distance increases. It should be readily apparent that since the roughened surface 14 of the tool is the only surface that abrades the workpiece, that the cross-sectional configuration of the post will be determined only by the dimensions of the hole 12 encircling roughened surface area 12, provided, of course, that the sides of hole 12 or the passage therethrough is no less confining than the hole 12 configuration at surface 12, at least when a uniform orbital motion is being utilized.

As noted above, the inventive process can also be utilized with a variable orbital motion in the event tapered post sections are desired. The use of such a variable orbital radius can be utilized, not only to produce a post with tapered sides, but can be further utilized to minimize the lateral forces on the sides of the post to further minimize breakage. In this embodiment, a tool with a hole having either straight or

tapered sides can be utilized, while the radius of orbit is programmed or manually adjusted to be progressively reduced during the grinding operation. The radius of orbit can be either continually decreasing or periodically decreased.

EXAMPLE The photographs shown in Figures 7 and 8 illustrate a practice as described above utilizing a decreasing radius. In this application, a graphite electrode for making a die for a motor end bell was desired. The electrode was to be provided with a number of slightly tapered, radial fins encircling a disk form, as shown in Figure 9. The tool utilized to produce the electrode shown in Figure 9, was fabricated from a high carbon tool steel, and is shown in the photograph in Figure 8. The sides of each hole in the tool shown in Figure 8 was straight. Initial orbital grinding was started on the cylindrical workpiece blank at an orbital radius of 0.132 inch (0.335 cm). The orbital radius was decreased 0.0011 inch, (28.0 micrometers), for every 0.010 inch (0.025 micrometers) of grinding depth, until the final orbital radius of 0.03 inches (0.08 cm) was reached, to produce the desired fins having a depth of 0.97 inch (2.46 cm). The bottom surfaces of the fin holes in the tool were rounded so the outermost edge of each fin would be rounded, as shown in Figure 7. The grinding time was two hours.

In addition to the above, it should be obvious that other modifications and embodiments could be utilized

without departing from the spirit of this invention. For example the plate 1^ itself could be fabricated from any material hard enough to readily abrade the graphite or whatever the workpiece material is. While a hard steel is preferred because of its hardness and ease of fabrication, other materials could be utilized if desired. In addition, holes 16 could be eliminated if other means are utilized for securing and positioning the abrading tool in place within the orbital grinding machine.