US3334041A

US3334041A - Coated abrasives

Info

Publication number: US3334041A
Application number: US392741A
Authority: US
Inventors: Hugh N Dyer; James W Sprague; Bruce W Duke; Charles W Marshall; Loran A Henderson
Original assignee: Norton Co
Current assignee: Saint Gobain Abrasives Inc
Priority date: 1964-08-28
Filing date: 1964-08-28
Publication date: 1967-08-01
Anticipated expiration: 1984-08-01
Also published as: GB1073666A

Description

' Aug. 1, 1967 H. N. DYER ETAL COATED ABRASIVES Filed Aug. 28, 1964 A G-UE SHALL. A. ENDEESON BY INVENTORS HUGH N. DYER JAMES w SPE BEL/CE M DUKE Z'HAELES W. MAE ORAN ATTORNEY United States Patent 3,334,041 COATED ABRASIVES Hugh N. Dyer, Loudonville, James W. Sprague, Clay, Bruce W. Duke, Amsterdam, Charles W. Marshall, Troy, and Loran A. Henderson, Elnora, N.Y., asslgnors to Norton Company, Troy, N.Y., a corporation of Massachusetts Filed Aug. 28, 1964, Ser. No. 392,741 20 Claims. (Cl. 204-284) The present invention relates generally to coated abrasives and more specifically to coated abrasives 1n the form of discs or "belts especially adapted for use in electrolytic grinding operations.

Electrolytic grinding, or the electrolytic removal of material from an electrically conductive work-piece augmented by contact between the workpiece and an abrasive grinding tool has long been a recognized area of use for metal-bonded abrasive wheels. Only very recently have efforts been made to supplant the bonded wheel by a coated abrasive disc or belt in order to gain the advantages of the coated products ability to conform to irregular surfaces and the increased abrasive area of a belt as contrasted to a wheel. Metal-bonded wheels have been quite limited as to both shape and size, keeping the electrolytic grinding process in consequence limited to a relatively small area of the metal grinding field.

Accordingly, it is an object of the present invention to provide a coated abrasive construction suitable for use in electrolytic grinding.

Another object of the invention is the provision of coated abrasive articles for electrolytic grinding having improved conductivity over those known to the art.

Additional objects, if not specifically set forth herein will be readily apparent to one skilled in the art from the following detailed description of the invention.

In the drawings:

FIGURE 1 shows a cross-section of a coated abrasive belt formed in accordance with the present invention.

FIGURE 2 illustrates a cross-section of another form of coated abrasive belt made in accordance with this invention.

FIGURE 3 represents a still further modification of the present invention illustrating again a coated abrasive belt in cross-section.

' FIGURE 3A represents an enlarged portion of FIG- URE 3 showing the relationship of the conductive coating to the channels or passages through the abrasive belt.

Generally, the present invention contemplates the use in an electrolytic grinding abrasive of a water-resistant, non conductive backing, preferably in the form of woven textile fibres, to which backing, in any manner well known inthe coated abrasive art is firmly anchored a plurality of non-conductive abrasive grains. The adhesive or adhesives used in anchoring the grains to the backing are non-conducting electrically and are designed to give maximum anchorage between the abrasive grains and the backing during wet grinding operations. Laminated or bonded to the abrasive coated face of the non-conductive backing is a layer of conductive material through which the tips of the abrasive grains protrude. Likewise, laminated or bonded to the back of the backing is another conductive layer of material and extending between the two conductive layers and through the non-conductive backing are a plurality of conductive connections. This construction provides a product having the pliability and life of a conventional coated abrasive product and yet having excellent front-to-back conductivity for use in electrolytic grinding. The prior art has suggested the use of relatively in-- flexible woven or perforated metal backings, i.e. completely conductive backings which have very poor life when subjected to the flexing inherent in their use, for

example, as an abrasive belt. The use of conductive adhesives has also been suggested. In this type of adhesive the holding power of the adhesive is weakened in order to obtain conductivity and the resultant abrasive product is a poor compromise which will not hold up well in use.

More specifically it has now been found that if a conventional coated abrasive belt is made on a woven textile backing using a good water-resistant maker adhesive, e.g. phenolic resin and a good size adhesive, e.g. phenolic resin, as has been heretofore known in the art the resultant belt has the requisite life and grain adhesion required for good performance. Such belt may be converted to an electrolytic grinding belt by forming a conductive layer on the front and back thereof (with the non-conductive abrasive grains protruding through the front conductive layer) and providing a plurality of conductive channels running through the belt to joint the front and back conductive layers together. The resultant belt will compare favorably for flex life and operating life with a conventional abrasive belt and will give excellent results in electrolytic grinding operations.

The conductive layers may be the same or different on opposlte sides of the belt. Generally on the face or grain side of the belt the conductive layer is either a laminated illustrated in FIGURE foil or a coating of metal applied directly to the belt as by metal spray, vacuum metallizing, electroplating or the like. In either instance, care must be taken before actual electrolytic grinding commences that the abrasive grain tips (which are non-conductive) must protrude from the film or metal coating layer. This may be accomplished by mechanical abrasion or by deplating, i.e. mounting the belt on the electrolytic grinding machine and reversing the current flow to plate the metal from the tips of the grain onto a dummy workpiece. Since the abrasive tips are the highest point in the plane of the belt, this step of clearing the tips is relatively uncomplicated.

The back side or the side opposite the abrasive coated surface may carry a foil laminate,-a metal layer (sprayed, vacuum-deposited, electroplated, etc.) or a Wire screen laminate. Generally, the only requirement is that the coating or laminate be conductive and not sufficiently stiif or brittle to impair the operation of the belt.

The invention requires that there must be electrically conductive means connecting the front and back electrically conductive layers together. Preferably, extending through the belt are a plurality of channels of conductivity connecting the opposing conductive surfaces of the belt to each other. The actual form or physical structure of such channels may vary widely. One very suitable approach is the use of metal grommets and rivets as is 1 of the drawings. A second type of conductive channel may be a series of conductive wires running tlfrough the woven belt and connecting the two conductive layers. Alternatively, the channels may be formed by conductive lacquer so applied as to form a continuous path through the belt. Where a sprayed or otherwise directly applied metal layer exists, it is usually preferable to use a belt wherein a plurality of perforations extend from one face to the other. These perforations generally are desirable in any belt for use in electrolytic grinding in order to insure proper flow of the electrolyte in the actual grinding area. Variations of belts with perforations are described in connection with FIG- URES 2 and 3 of the drawings. Where the perforations are provided in the belt, the metal coating sprayed on the abrasive side is so applied as to extend through such perforations and thereby provide the requisite channels of conductivity from one side of the belt to the other.

The number of conductive channels is not critical, but the total must be sufficient and the current density per channel sufficiently high to provide current carrying capacity for the whole system. Currents in the neighborhood of 1000 amps. per square inch are common during the grinding operation, and currents as high as 3000 amps. per

square inch have been conducted through the belts during deplating operations (removing metal from the tips of the abrasive grains). Generally, it may be stated that the conductive channels may either be few in number but of very high capacity or may be numerous and of lower capacity per channel.

Referring now to the drawings, FIGURE 1 illustrates a cross-section of an abrasive belt composed of a nonconductive Woven backing member 11 having a layer of abrasive grains 12 bonded thereto by a water-resistant adhesive 13. Over the abrasive grains 12 is applied a conductive foil 14. This foil 14 is adhered to the abrasivecoated surface by a suitable adhesive as is more fully described below. The opposite side of woven backing 11 is laminated by a suitable adhesive to a wire screen 15. Both the foil 14 and the wire screen 15 are electrically conductive. Rivets or grommets 16 of conductive material are provided along both edges of the belt 10 running entirely through the belt and in contact with both the foil 14 and the wire screen 15 and providing channels of conductivity through the belt 10. It will be noted that the abrasive grain 12 is shown protruding through the foil 14. This may be accomplished in the laminating procedure but if not can be done by deplating or mechanical abrading as described above. The foil may be any conductive metal foil such as aluminum, copper, tin, lead or the like and has been used in thicknesses ranging from .00065" to .005 in thickness Heavier foils can be used if desired although the range given above appears to be adequate. The foil is applied to the abrasive side of the woven non-conductive backing after lightly sizing the abrasive with a suitable water-resistant adhesive. The specific adhesive is not critical and one which has been found effective is a blend of epoxy ester and polyamide resin in methyl ethyl ketone solvent. These materials are commercially available as Epon 1001-X-75 and Versamid 140. A 1:1 blend of these components with 15% methyl ethyl ketone solvent added produces a good laminating adhesive. After placing the foil on the adhesive-coated surface the laminate is run through a set of pressure rolls, one steel and the other rubber (Shore A-2 hardness45) with the abrasive surface towards the rubber roll. Air operated pistons supply a pressure of 15 pounds per inch of roll width to force the foil into intimate contact with the adhesive-coated abrasive surface. Following laminating the material is wound on a core of not less than 16" diameter with the abrasive-foil surface convex to insure no separation of foil will occur. The material is then cured for 16 hours at 135 F. followed by /2 hour at 175 F., /2 hour at 200 F., /2 hour at 225 F. and a final /2 hour at 250 F. Where foil is applied to the opposite surface of the non-conductive woven backing, the same procedure is followed except that in such instance the material is wound with the abrasive surface concave. Suitable grommets for use in this type of construction are aluminum grommets having a wall thickness of from 0.006" to 0.01". The average grommet has a total cross-sectional area of about 0.090 in. and for a 4 x 107" belt it has been found that the preferred number of grommets is 18 where the maximum amperage through the belt is to be 1200 amps. and the workpiece is to be one square inch.

FIGURE 2 of the drawings illustrates a modification of the present invention wherein the belt 20 consists of the non-conductive woven textile backing member 21 with non-conductive abrasive grain 22 bonded thereto by waterresistant adhesive 23. Laminated to the grain-coated surface of member 21 is again a foil layer 24 as in the construction of FIGURE 1. Laminated to the back surface of member 21 is another layer of conductive foil 25. Extending through the belt 20 from one layer of conductive foil 24 to the other layer 25 are a plurality of perforations 26 to provide adequate electrolyte flow through the belt when in use. These may vary in size and number as desired, but preferably are of /s diameter and are arranged in a staggered pattern offset from the web direction in order to prevent marking the workpiece with a pattern when the belt is used in electrolytic grinding. With this construction the conductive channels may be provided by the rivets used in FIGURE 1 or by conductive wires or as illustrated in FIGURE 2 by filling some of the perforations 26 with a conductive lacquer 27. A suitable, commercially-available lacquer is (dag) Dispersion No. 235. An alternate method of providing the conductive channels is to extend the conductive foil layers 24 and 25 beyond the edge of the backing 21 and join them along the edge of belt 20 as shown at 28 in FIG- URE 2.

FIGURE 3 shows another modification wherein a belt 30, again formed from a non-conductive woven textile backing 31 with a layer of non-conductive abrasive grain 32 bonded thereto by a non-conductive, water-resistant adhesive 33, is provided with perforations 34 as was the belt of FIGURE 2. The conductive layers on the front and back surface of the belt 30 are provided in this modification by the application to the belt of a metal spray to form two layers of

metal

35 and 36 on opposite faces of the belt 30. As is more clearly shown in the enlarged portion of belt 30 illustrated in FIGURE 3A, the metal which is sprayed on the belt surface to form

conductive layers

35 and 36 extends through perforations 34 as shown at 37 to form conductive channels connecting the two

conductive layers

35 and 36 together. In applying the metal spray, various techniques known to the art of metal application may be used including fiame spraying, vacuum metallizing, plasma arc or the like. Generally it is desirable to sandblast or otherwise roughen and clean the surfaces of the belt before applying the metal to insure proper adhesion. It has been found that the conductive layers of sprayed metal have sufficient adhesion to prevent metal shedding when the coated belts are run wet against a hardened steel platen on a conventional grinding machine under 50 psi. workpiece pressure. Copper has been found to be the preferred metal since the adhesion is much higher than that of sprayed zinc or aluminum. The thickness of the sprayed metal layer should range from about 0.003" to 0.01" on the back or non-abrasive side of the belt to from about .005" to 0.02" on the front. Good results have been achieved using a coating of 0.005" on the back and 0.008" on the front surface of electrolytic grinding belts. Obviously, additional conductive channels may be formed in the metal sprayed belts using conductive rivets, wires, etc. to supplement the metal coated perforations if desired. The spray-applied metal must be removed from the tips of the non-conductive abrasive particles before use in an electrolytic grinding process and, as described above, may be done by deplating or mechanical abrasion.

The following specific examples are illustrative of the performance in electrolytic grinding operations of belts made in accordance with the present invention:

Example 1 Belts according to the construction illustrated in FIG- URE 1, except that these were perforated as illustrated in FIGURES 2 and 3, were prepared by taking conventional, resin-bonded, water-resistant coated abrasive materials (Grits 24-50, Speed-wet Metalite Cloth), which were made on X weight cotton, desized drills cloth using a phenolic resin maker and size adhesive, and laminating to the abrasive side thereof as described in connection with FIGURE 1 a 0.003 1235-O-Dry aluminum foil. A 30 mesh, 0.01" wire diameter, type 34 stainless steel wire screen was then placed against the back of the coated abrasive material under tension and an adhesive solution of butadiene-acrylonitrile latex and a phenol-formaldehyde resin in a 1:1 ratio (31.8% total solids) was doctored over the screen. The latex used was Hycar 1041 while the phenolic resin was Durez 12687. The adhesive was cured for 1 hour at 150 F. followed by a further cure of 1 hour at 275 F. The completed foil/abrasive backing/wire screen combination was then perforated with a pattern of A3" holes on centers, in staggered array, off-set 4 from the web direction to prevent ridging or patterning of the workpiece. The total number of perforations was 2975 :25 for each 4" x 107" section. Belts measuring 4"x 10 were cut from the perforated material and joined by cutting the ends at a 45 angle and joining with a single skive adhesive point in conventional'fashion. Front-to-back conductivity was obtained by using as the conductive channels aluminum grommets and washers, set with the center line of the grommets 75 from the belt edge. An average of 9 evenly spaced grommets and washers were used on each side of the belt.

These belts were used in electrolytic grinding of 1" square stock Firth-Sterling M-2 Star-MO High Speed Steel workpiece, hardened to Rockwell C-65, using an electrolytic belt grinding machine. This equipment generally is of the type disclosed in US. Letters Patent No. 2,997,437 to Richard A. Whitaker. The belt tension in each case was .45#, the belt speed was 1150 surface feet per minute and the voltage was 7.0 volts D.C. The platen was perforated tungsten carbide while the electrolyte was Anocut #SO-90, 2#/gal. solution delivered at the rate of 5 gallons per minute filtered. The following results were achieved:

Average Stock Re- Belt Grit Current moval Rate (amps/in!) (infi/min.)

24 200 38Xl0- 36 400 53 l0- 40 450 50 10' 50 400 33 l0' Example 2 Belts according to the construction illustrated in FIG- URE 2, utiliZingfoil/abrasive backing/foil lamination were made and run and the data recorded was the applied voltage and resultant current conducted. A 150-X Speedwet Metalite Cloth Belt with 0.00017 aluminum foil laminated to both the abrasive and non-abrasive side of the belt using an epoxy adhesive and perforated as described in connection with Example 1 was prepared with conductive lacquer filling some of the holes to provide front-to-back conductivity. The voltage applied was 17.5 volts and the average current conducted was 500 amps. per square inch. A similar belt with the edges of the foil extending beyond the sides of the belt and interconnected to each other when placed under 17.5 volts conducted 320 amps, per square inch of current.

Example 3 FIGURES 3 and 3 A were made Jbyfirstperforating-grit 60 Speed-wet Metalite Cloth abrasive material withfa;

staggered pattern of A;"holes on F/s" centers oifsetA from the web direction. 4" x 107" belts were made from this material using a 55 joint angle. Both sides of'the belt were sandblasted using a Vacu-Blast Jr. sandblasting unit with'aluminum oxide grain. Sandblasting was continued only long enough to de-lustre the material. Both the front and back of the abrasive material was flamesprayed with copper using a Metco type -4-E metallizing gun. The gun was directed at the surfaces of the belt at an angle .to insure penetration of fthe metal into'the perforations in order to provide front-to-back conductive paths. These belts were run on the Hammond unit under the same conditions as the belts described in Example 1 except that here the workpiece pressure varied from 16 to 50 p. s.i. whereas in Example 1 the pressure was applied by hand and' not measured (although it could be described as moderate).

Average I Stock Re- Belt Grit Current moval Rate (ampsJinJ) v (ins/min.)

A 60 600 x10- B so 575 234x10- The abrasive grain, water-resistant abrasive grain binders and flexible, water-resistant, backings used in the present invention may be selected from any of the many known types used in the coated abrasive art. As indicated above, these components of the abrasive articles of the present invention must, however, be electrically non-conductive. The backing is preferably a woven structure but non-conductive films such as Mylar of the like, non-woven backings, etc., may be used. While described primarily in connection with an abrasive belt, the present invention may be utilized in other forms if desired. For example, this laminated material may be made up in the form of circular discs which can be used in equipment designed for electrolytic grinding using metalbonded wheels.

While the various constructions specifically described herein are preferred, obviously the different electrically conductive coatings may be combined as desired as can the various electrically conductive connecting means described to give variations of laminates all of which are within the scope and purview of the present invention. Obviously, many variations and modifications can be made without departing from the spirit and scope of the invention described herein, so that only such limitations should be imposed as are contained in the appended claims. We claim: 1. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(A) an electrically non-conductive flexible backing material having a front surface and a back surface; (B) a plurality of electrically non-conductive abrasive grains adhesively bonded to the front surface of said backing material; (C) an electrically conductive layer of material over lying said front surface and bonded thereto, said abrasive grains protruding through and above said electrically conductive layer when electrolytic grinding is taking place; (D) an electrically conductive layer of material overlying said back surface and bonded thereto; and

' (E) electrically conductive means connecting said layer of electrically conductive material on said front surface with said layer of electrically conductive material on said back surface.

2. A coated abrasive material as in claim 1 wherein said material is in the form of an endless belt.

- 3. A coated abrasive material as in claim 1 wherein said material is in the form of a circular disc.

4. A watenresistant coated abrasive material especially adapted for electrolytic grinding which comprises:'

(A) an electrically non-conductive flexible backing material having a front surface and a back surface;

(B) a plurality of electrically non-conductive abrasive grains adhesively bonded to the front surface of said backing material;

(C) an electrically conductive layer of metal overlying said front surface and bonded thereto, said abrasive grains protruding through and above said electrically conductive layer when electrolytic grinding is taking place;

(D) an electrically conductive layer of metal overlying said back surface and bonded thereto; and

(E) electrically conductive means connecting said layer of electrically conductive material on said front surface with said layer of electrically conductive material on said back surface.

5. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(A) an electrically non-conductive flexible backing material having a front surface and a back surface; (B) a plurality of electrically non-conductive abrasive grains adhesively bonded to the front surface of said backing material;

(C) an electrically conductive layer of foil overlying said front surface and bonded thereto, said abrasive grains protruding through and above said electrically conductive foil when electrolytic grinding is taking place;

(D) an electrically conductive layer of material overlying said back surface and bonded thereto; and (E) electrically conductive means connecting said layer of electrically conductive material on said front surface with said layer of electrically conductive material on said back surface.

6. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(D) an electrically conductive layer of foil overlying said back surface and bonded thereto; and

7. A Water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(C) An electrically conductive layer of foil overlying said front surface and bonded thereto, said abrasive grains protruding through and above said electrically conductive foil when electrolytic grinding is taking p ace;

(D) An electrically conductive layer of wire screen overlying said back surface and bonded thereto; and

8. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(C) An electrically conductive layer of material overlying said front surface and bonded thereto, said abrasive grains protruding through and above said electrically conductive layer when electrolytic grinding is taking place;

(D) An electrically conductive layer of material overlying said back surface and bonded thereto; and (E) Electrically conductive rivets connecting said layer of electrically conductive material on said front surface with said layer of electrically conductive material on said back surface.

9. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(D) An electrically conductive layer of material overlying said back surface and bonded thereto; and (E) Electrically conductive grommets connecting said layer of electrically conductive material on said front surface with said layer of electrically conductive material on said back surface.

10. A water-resistant coated abrasive material especially adapted for electrolytic-grinding which comprises:

(D) An electrically conductive layer of material overlying said back surface and bonded thereto; and (E) Electrically conductive wires connecting said layer of electrically conductive material on said front surface with said layer of electrically conductive material on said back surface.

11. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(B) A plurality of electrically non-conductive abrasive grains adhesively bonded to the front surface of said backing;

(D) An electrically conductive layer of material overlying said back surface and bonded thereto;

(E) a plurality of perforations extending through said electrically non-conductive backing and said electrically conductive layers bonded thereto; and

(F) electrically conductive means connecting said layer of electrically conductive material on said front surface with said layer of electrically conductive material on said back surface.

12. A coated abrasive material as in claim 10 wherein said material is in the form of an endless belt.

13. A coated abrasive material as in claim 10 wherein said material is in the form of a circular disc.

14. A water-resistance coated abrasive material especially adapted for electrolytic grinding which comprises:

(A) an electrically non-conductive flexible woven backing having a front surface and a back surface;

15. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(D) an electrically conductive layer of foil overlying said back surface and bonded thereto; v

(F) electrically conductive means connecting said layer of electrically conductive foil on said front surface with said layer of electrically conductive foil on said back surface.

16. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(D) an electrically conductive layer of wire screen overlying said back surface and bonded thereto;

(F) electrically conductive means connecting said layer of electrically conductive foil on said front surface With said layer of electrically conductive wire screen on said back surface.

17. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(C) an electrically conductive layer of sprayed metal overlying said front surface and bonded thereto, said abrasive .grains protruding through and above said electrically conductive layer when electrolytic grinding is taking place;

(D) an electrically conductive layer of sprayed metal overlying said back surface and bonded thereto;

(F) electrically conductive means connecting said layer of electrically conductive sprayed metal on said front surface with said layer of electrically conductive sprayed metal on said back surface.

18. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(C) an electrically conductive layer of sprayed metal overlying said front surface and bonded thereto, said abrasive grains protruding through and above said electrically conductive layer when electrolytic grinding is taking place;

(D) an electrically conductive layer of sprayed metal overlying said back surface and bonded thereto; (E) a plurality of perforations extending through said electrically non-conductive backing and said electrically conductive layers bonded thereto; and

(F) sprayed metal extending through said perforations and bonded to the walls thereof forming electrically conductive connections between said layers of electrically conductive sprayed metal bonded to the front and back surfaces of said electrically non-conductive backing.

19. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(B) a plurality of electrically non-conductive abrasive 25 grains adhesively bonded to the front surface of said backing;

(F) electrically conductive lacquer extending through said perforations and bonded to the walls thereof forming electrically conductive connections between said layers of electrically conductive sprayed metal bonded to the front and back surfaces of said electrically non-conductive backing.

20. A water-resistant coated abrasive material especially adapted for electrolytic grinding which comprises:

(C) an electrically conductive layer of vacuum deposited metal overlying said front surface and bonded thereto, said abrasive grains protruding through and above said electrically conductive layer when electrolytic grinding is taking place;

(D) an electrically conductive layer of vacuum deposited metal overlying said back surface and bonded thereto;

(F) electrically conductive means connecting said layer of electrically conductive material on said front surface with said layer of electrically conductive vacuum deposited metal on said back surface.

References Cited UNITED STATES PATENTS 3,162,588 12/1964 Bell 204224 X JOHN H. MACK, Primary Examiner. D. R. VALENTINE, Assistant Examiner.

Claims

1. A WATER-RESISTANT COATED ABRASIVE MATERIAL ESPECIALLY ADAPTED FOR ELECTROLYTIC GRINDING WHICH COMPRISES: (A) AN ELECTRICALLY NON-CONDUCTIVE FLEXIBLE BACKING MATERIAL HAVING A FRONT SURFACE AND A BACK SURFACE; (B) A PLURALITY OF ELECTRICALLY NON-CONDUCTIVE ABRASIVE GRAINS ADHESIVELY BONDED TO THE FRONT SURFACE OF SAID BACKING MATERIAL; (C) AN ELECTRICALLY CONDUCTIVE LAYER OF MATERIAL OVERLYING SAID FRONT SURFACE AND BONDED THERETO, SAID ABRASIVE GRAINS PROTRUDING THROUGH AND ABOVE SAID ELECTRICALLY CONDUCTIVE LAYER WHEN ELECTROLYTIC GRINDING IS TAKING PLACE; (D) AN ELECTRICALLY CONDUCTIVE LAYER OF MATERIAL OVERLYING SAID BACK SURFACE AND BONDED THERETO; AND (E) ELECTRICALLY CONDUCTIVE MEANS CONNECTING SAID LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL ON SAID FRONT SURFACE WITH SAID LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL ON SAID BACK SURFACE.