US2410506A - Coated abrasive - Google Patents

Description

Novt s, 194e. H. F. KIRCHNER mL 2,410,506

COATED ABRASIVE 3 Sheets-Sheet lv1 Filed July 15, 1942 NOV. 5, 1946. H p, K|RC||NER ETAL 2,410,506

COATED ABRAS IVE Filed July 15, 1942 3 sheets-sheet 2' Nov. 5, 1.946,' v H. P. KIRCHNER COATED ABRAS I VE Filed July 15, 1942 3 sheetsL-sheet s -ETP INVENTOR@ HENRY 1. KIRCHNER Mm ALM-Rr L. BALL ATTORNEY Patente 1946 COATED ABRASIVE Henry P. Kirchner, Niagara Falls, and Albert L. Ball, Lewiston, N. Y., assig'nors to The (Jarborundum Company, Niagara Falls, N. Y., a corl poration of Delaware Application July 15, i942, Serial No. 451,024

(C1. sl-issl 8 Claims.

This invention relates to abrasive coated belts or sheets, and has for one of its objects the provision of such coated abrasives in which the abrasive consists wholly or in part of diamond grit. Other objects will become apparent upon consideration of the following disclosure.

'Ihe belt or sheet of this invention is useful for grinding to shape, finishing, and polishing many different kinds of objects and materials, but nds particular utility in working on hard alloys, such as cemented tungsten carbide, boron carbide, and similar substances, or on hard ferrous alloys such as special iron-tungsten alloys employed as dies and cutting tools. When employed as abelt, coated abrasive of this invention may be used for the purpose, among others, of sizing, finishing, and polishing the interior working surface of v Wire, bar and tube drawing dies made of cemented hard carbide. These dies require nishing and polishing after they are made by being molded and sintered. After a certain amount of use, such dies require a refinishing and repolishing to remove surface wear defects and to restore the mirror finish necessary to their proper functioning. In addition, after such dies have been used for such time that their amount of wear exceeds a certain tolerance, their bore must be expanded to the next size and the die again finished and polished. Coated abrasives as provided by the present invention are suited for all these above-mentioned die finishing and polishing operations, and their use greatly shortens the time and expense required by the use of hitherto known abrasive materials in carrying out such working and finishing operations on dies, tools, etc., made of hard alloy materials.

vThe present invention will be more readily understood by reference to the accompanying'drawings, which are illustrative only and are not to be construed by way of limiting the present invention. In the drawings:

Figure 1 is an enlarged cross-section of a typical diamond coated leather abrasive product, as taught -by the present invention.

Figure 2 is an enlarged cross-section of a diamond coated leather abrasive product, the leather being reinforced by a layer of flexible material united. to the back thereof.

Figure 3 is an enlarged cross-section of a diamond coated leather abrasive belt having a relatively stii reinforcing layer joined thereto, said reinforcing layer having a quick-detachable splicen l Figure i is a Times-section of a diamond coated Figure 6 is a cross-sectional view of a draw-'- ing die as molded and sintered, and before finishing.

Figure 7 is a cross-sectional view of. a used drawingA die, and illustrates typical defects in the bore which must be removed. l

Figure 8 is a diagrammatic view of a belt grinding and polishing machine for nishing the bores of dies, cylinders and the like. i

Figure 9 is an enlarged cross-section of a diamond coated leather abrasivebelt having a narrow reinforcing member in the center of the back thereof.

Figure 10 is a cross-section through a die being finished by the belt shown in Figure 9.

Figure 11 is an enlarged cross-section of a diamond coated leather belt having grain coated on the side portions of its face only, leaving the e. center portion devoid of grain. n

Figure 12 is a, cross-section through a die be ing finished by the belt shown in Figure 11.

Figure 13 is an enlarged cross-section of a diamond coated leather abrasive belt having a longitudinal groove in the back face thereof.

Figure 14 is a crossmtection through a die being finished by the belt ,fi'lown in Figure 13.

Figures 15-20, inclusive, are reproductions of charts produced by a brush surface analyzer, Figures 15, 1'?, and 19 being analyses of surfaces of cemented tungsten carbide abraded by diamond leather belts of the present invention, and Figures 16, 18, and 20 being analyses of surfaces of cemented tungsten carbide abraded by belts composed of' backings of plied cotton tape with coatings of. diamond grit.

Figure 1 shows in enlarged cross-section an abrasive article typical of the present-invention. The article there shown is composed of a leather backing i, a coating 2 of a substance such as methyl cellulose, with or without a finely divided crevice filler such as clay, Whiting, or non-diamond abrasive which prevents penetration of the making adhesive coating 3, which may be a synthetic resin, into the leather, a layer of abrasive granules 5, composed wholly or partly of diamond grit and a sizing coat t of adhesive, which may likewise be a synthetic resin, applied `after the abrasive. grit, for the purpose of strengthening the bond between the grit and the base. Preferably the abrasive is applied to the flesh side of "the leather, although it may be applied to the reverse side if desired, or to any cut surface of the leather. A coating of a waterproofing substance is applied to the sides and back of the leather, thereby 'rendering the abrasivearticle impervious to moisture and able to be used with a liquid coolant.

A typical diamond-coated leather abrasive such as shown in Figure 1 is made in the manner now set forth. It will, of course, be understood that such method is set out merely for the purpose of illustration and that numerous-variations are possible, some of which will be given below.

A diamond coated leather belt is made by cutting'a piece of chrome tanned leather 7 feet long, 1/4 inch wide and :54,4 inch thick. Obviously such leather should be as uniform as possible as far as dimensions, strength, resilience and grain 'are concerned. The flesh side of the cut piece is buffed on a 40 grit, bonded silicon carbide wheel to smooth it, after which the same side of the leather is size coated with a solution of methyl cellulose (5% by weight in- Water) and'hot dried for 30 minutes at 18o-200 F. The purpose of this coating is to seal and stop the pores in the leather,

material, which may be, among others, that sold under the trade name "Viscol Triple Action, which is a vulcanized vegetable oil in solution. It is, of course, understood that other suitable waterproofing agents may be employed.

When the leather is waterproofed and when waterproof adhesives are employed as the making or grain bonding coat and also the sizing coat, the coated abrasive oi the present invention may be used with a liquid coolant. Such coolant allows the abrasive to take heavier cuts at higher speeds than it can without its use, since the coolant keeps the temperature of the coated abrasive below that at which it would be harmed, and also prevents the work piece from becoming locally ovcrheated and heat checked. In addition the coolant cleans detritus from the surface of the work piece, and prevents the scatthereby preventing undue penetration of the leather backing by the making adhesive with the resulting undesirable embrittling of the backing. In some instances a second coating .of size may be necessary to render this face of the leather sufciently impervious to the making adhesive.

After the sizing coat or coats have dried, a making coat of liquid synthetic resin is applied to the sized leather surface. A suitable resin, among others, is one sold under the trade name "Bakelite XR 10190, which is an unmodified phenol-aldehyde resin which, when fully cured, sets to a firm, strong, and waterproof condition. This resin may be mixed in the proportions of '75% by weight of the liquid resin to 25% by weight denatured ethyl alcohol. While the making coat is still sticky, suitable grit is applied to it to form a uniform layer thereon. This grit may be wholly composed of diamonds, or maybe diamond grit uniformly mixed with any desired proportion vof other abrasive. The abrasive granule size may also be varied. Both factors depend on-the work to be done with the abrasive product. In one instance, a mixture of 20% by weight of 220 grit diamonds and 80% by weight of 220 grit silicon carbide may be used. The diamond 'and silicon carbide grit may also be used in ratios of 40/60, 80/20, etc., respectively.

The belt is now heatedvto dry the resin making coat. For the Bakelite resin XR. 10190 such treatment consists of heating it at 160 F. for. 30

minutes, then `at 180 F. for 30 minutes, and

finally at 200 F. for 30 minutes. A sizing coat is then applied to the grit coated side of the belt to strengthen the bond between grit and tering of dust cut from the work piece by the abrasive.

The making andsizing coats c'ure to a somewhat stiff condition. To enable thebelt to assume thecontour of the guiding and driving pulleys as well'as to enable it to conform to the surfaces of curved work-pieces, it must be treated to cause uniform localized cracking of these coats without injuring the bond in locations where it is not cracked. 'One vmethod of accomplishing this is to gently flex the belt in the hands, and then to pull it over a large diameter pulley, such as a 6inch pulley, followed by flexing over'successively smaller pulleys such as 3-inch and then l-inch. The belt is then flexed over a 90 Wooden corner in directions along its length and then at 45 to its length in each direction. The belt is also flexed cross-wise of its length by supporting it at its longitudinal axis and bending down the edges.

It is essential that the belt joint be as strong and flexible as possible. One manner of making a, satisfactory 'joint is to skive the belt ends an'd slightly roughen them with abrasive paper. A cement such as that sold under the trade name Sea Lion Waterproof Belt Cement, which is a "cel1uloid type cement containing nitrocellulose and camphor in a solvent of methyl acetone, and which is made by the Chicago Belting Company, Chicago, Illinois, is applied to both ends in two coats, the first being rubbed in thoroughly and let dry for 20 .to 30 minutes. The second coat is then applied, and the splice clooed at' once. The splice is placed between plates and clamped under moderate pressure. The cement ris allowed to set 10 minutes, the clamps are removed, and the'c'ement is allowed to dry in.

the air, after which the belt is ready for use.

Another manner of making a satisfactory joint is to skive the ends of the belt for a suitable distance. The beveled surfaces are then painted with a suitable adhesive bonding composition such as a solution of 10% by weight of a polyvinyl backing. Such size may be composed of a mixv F. held for 16 hours.

To render the belt waterproof itis coated on its back and sides with a suitable waterproofing acetal'resin sold under the trade name Alvar #l1-90 dissolved in 90% acetone. The skived ends are hot air dried for 5 to 10 minutes, are then confronted, and are hot pressed at 300 F. for 2 minutes on each side at a pressure of 500 to 600 pounds per square inch.

It will, of course, be understood that the above coating, sizing and curing procedure is applicable to various leather-backed diamond coated abrasives besides belts, and that numerous modifications of this procedure are possible. Thus the leather thickness may be varied, from a minlmum in cases where light cuts are taken and it is desired to have the coated abrasive ilexible a layer 'i of suitable adhesive.

enough to conform to curves of short radius, to a maximum of leather thickness available where straight surfaces or curves of long radius are to be cut, where the driving and guiding wheels are of large diameter and where coarse grit is employed for heavy cuts. Fairly thick' resilient belts are also useful for polishing operations as will be shown below. The width of theleather may likewise vary as desired in making belts and in the case of sheets may be any desired or available value.

In general, the' stronger leather is the more satisfactory for such diamond abrasive belts, al-

though satisfactory belts are made with leather which has a tensile strength of as vlow as about 2,000 pounds per square inch. Leathers ranging up to a tensile strength of 11,670 pounds per square inch have been used satisfactorily. Resilience of the leather is another factor taken into consideration when belts are made. In general, the more resilient leathers are chosen for the finer grit belts or sheets, since, as will be later explained, the excellence of leather as a backing for abrasive products of this kind appears to be due to its pronounced resilience.

Experience has shown that for removal of line defects from cemented carbide dies an abrasive of 220 grit size, in the ratio of 20 parts by weight of diamonds, 80 parts by weight of silicon carbide is satisfactory. To succeed the 220 grit belt, a 400 grit belt is generally used with diamonds and silicon carbide in the ratio 40/60. This removes scratches left by the 220 grit belt. A 500 grit belt, with diamonds and silicon carbide in the ratio 5/95 may be used to follow the 400 grit belt, although usually it is necessary only to fol- 'low with the nal belt 8'00 grit abrasive, dia-` mond/silicon carbide in the ratio 5/95. The 800 grit leather belt is eminently satisfactory for putting a iinal polish and lustre on a die if the belt is a fairly thick soft backing, for example .045 inch or .050 inch thick. The ratios of diamond to silicon carbide grit are obviously capable of being widely varied. The function of the silicon carbide is primarily to -dilute the diamond grit and make possible different cutting rates at the same grit size and overall grit concentration on the belt. uniform distribution of diamonds on the belt than would ordinarily be possible without it.

Figure 2 illustrates a diamond-leather abrasive product similar to that shown in Figure 1, with the exception that it is provided, on the face 4opposite the abrasive, with a layer of flexible reinforcing material 8 cemented to leather I by be composed, among other things, of metal foil. Its purpose is to strengthen the leather longitudinally and laterally, and also to prevent failure of the leather due to repeated exing. Ad-

It also allows the securing of a moreother at interface It. To prevent lateral separation of the ends II and I2 of the belt during operation, one of the interfitting portions of hooks I0 and I3 is provided with a groove and the other with a rib, the rib and groove being such that they snap by each other when the belt ends are disengaged. .The belt shown in Figure 3 .is run past the work piece in the direction shown by the arrow, so that end II of the belt is not sc uied up during working. The leather is waterproofed on at least its exposed surfaces inthe Material t may hesive 'I may be a hide glue, a silicate adhesive,

or if the product is to be waterproof, a heat hardenable resin such as a phenol-aldehyde condensation product. The other elements in Fig ure 2 are the same as those in Figure 1.

Figure 3 illustrates a diamond-leather abrasive belt having a relatively stiff reinforcing member 8 secured to the back surface of the leather as by a layer of a suitable adhesive 9, which may be a heat hardenable synthetic resin such as phenol-aldehyde condensation product. Leather I has coated on its upper or front face a layer of a sizing material such as methyl cellulose, a layer of a resin making coat, a layer of diamonds 'of suitable grit size, and a sand size coating over manner described in connection with Figure 1.

Figure 4 shows a diamond-leather abrasive product having a reinforcing backing I5 which may 'be metallicor non-metallic with a layer of e leather i6-secured thereto as by a layer of adhesive. Leather. I6, which may be in one piece or made up of several pieces cemented together, is shaped to a desired cross-section in order properly to t the contour of the work piece being abraded. A layer of diamonds Il is secured to The leather facing may also be cupped, slotted,

or embossed to give the same effect.

Figure 6 shows a rough cored hard carbide drawing die or nib 22 as received from the sintering furnace. It willbe seen that the bore is divided roughly into 5 portions, the first portion, 23, being known as the'bell angle or radius; the second portion. 2t, being designated the entering angle; the third portion, 25, being known asthe approach angle; the fourth portion, 26, a right cylindrical bore being designated as the bearing;

and the last portion, 2l, being known as the back relief angle. The die bore must be finished to size and to such contour that the upper portions 23, 2t, and 25 become curved and blend smoothly into the lower cylindrical or bearing portion, and that the portion 2l also blends into the bearing portion. The final finished bore of the die is indicated by the dotted line 23 in Figure 2.

InFigure '7 there is shown ahard carbide.

drawing die 2e which, after extended use,` has developed grooves in the bore. Such grooves usually occur at the approach angle as shown in the figure, although they may also develop in the bearing. Such grooves, which are shown greatly exaggerated as to depth in Figure 'I at 30, must be removed by refinishing and repolishing the bore of the die. With shallow grooves it is possible to remove them and still have the bore of a size within allowable tolerances. If such grooves' are deep, however, the bore must be expanded to the next usable size and reiinished and repolished. The dotted line 3| indicates the contour of the bore after being repolished.

refinished and Use of the apparatus diagrammatically shown in Figure 8 is a convenient way in which the abrasiveproduct of the present invention may be utilized, when in the form of belts, to accomplish the above-mentioned types of work on hard carbide dies or'dies of other hard material. The apparatusconsists of a top guide and driving wheel 32 for abrasive belt 33, a floating bottom guide wheel 34,2. rotatable table 35 driven by a means such as pinion 436, two top intermediate idler guide rolls 31, and two bottom intermediate idler guide rolls 38. Guide wheels 34 may have its axle weighted to maintain and apply greater tension in the belt and thus pressure of the belt against the work piece. All these elements are supported on a suitable frame, not shown. Wheel 34 is adjustable in a vertical direction in order to accommodate belts of different lengths. Idler guide rolls 31 and 38 are adjustable vertically and also to and from the line between the centers of wheels 32 and 34, Whereas wheels 31 are shown inside the belt and wheels 38 outside the belt, either set may be employed inside or outside the belt, depending on the contour of the bore being finished in die 22. Die 22 is held on table 35 by a suitable chuck, not shown. Table 35 is rotated by means 36 in synchronism with the movement of belt 33 through the die, and thus the die is uniformly abraded in each peripheral-zone of its bore. Adjustment of table 35 vertically, suitable adjustment of guide rolls 31-and 38, and a proper choice of weighting on wheel 34 make possible the attainment of desired belt pressure in any vertical zone of the bore.-

By `suitable cutting and gauging operations the rough cored die shown in -Figure 6 may be iinished entirely by use of diamond abrasive leather belts on the machine shown in Figure 8. The removal f grooves or scratches and repolishing of the die, as shown in Figure '7, aswell as the openingup of badly worn dies to the next size, and refinishing and repolishing the same, may likewise be carried out by use oi' diamond abrasive leather belts on the device in Figure 8.

As an example of the procedure followed in bringing the bore ,of the rough cored nib shown in Figure 6 to the finished size and contour, the

-following steps are carried out. A leather belt having a mixture of 220 grit abrasive thereon, said abrasive consisting of a mixture of diamond grit and silicon carbide grit in the ratio of 20/80 is employed to remove a large amount of material to cause the bore to approach finished shape. A leather belt with a mixture of 400 grit diamonds and 400 grit silicon carbide in the ratio 40/60 is then employed to remove the scratches resulting from the 220 grit belt and to bring the `bore to a. partial mirror finish. As a final step a leather belt is used which is provided with 800 grit abrasive, said abrasive consisting of 800 grit diamonds and 1000 and finer grit. silicon carbide in the ratio /95, for the purpose of vremoving the scratches left by the 400 grit and to develop the final lustre, which is such that the finished surface is a veritable mirror.

A similar procedure is followed in the opening of a die to a larger size or in removing vgrooves or scratches. .The choice of the grit size of the first belt employed depends upon the amount of stock to be removed. If it is small, a grit size of 'around 400 can first be used, followed by the 800 grit for finishing and polishing.

The following illustrates 'the -saving in time,

in the resizing 0f a warn die of cemented tungsten carbide 1 inch long, from 3A inch diameter to its next useful size of H inch diameter. Ordinary procedure for accomplishing this is as follows:

' A. The die is rebored with a diamond tipped tool at not-over .001 inch per cut with very slow feed; time, 8 to 12 hours.

B. The bore of the die is now lapped with 120 boron -carbide held by olive oil in the meshes of a wire braid clad linen core, the wire being securely twisted on to a hard wood stick which is held by hand in the rotating' die.

C. The bore of the die is lapped with 320 boron carbide as in B.

D. The die bore is lapped with #5 diamond dust in olive oil lin meshes of a wire clad linen core twisted on a steel mandrel.

E. The bore of the die is lapped with #6 diamond dust on a hardwood stick mandrel.

F. Some dies are occasionally given a further finishing polish with #6 diamond dust and olive oil held on a cloth.

The lapping steps (B-F incl.) require from 3 to 4 hours. The total time is thus from 11 to'16 hours to enlarge a 1%, inch die to H inch, and obviously the time involved increases with the size of the die.

When diamond-coated leather rough cutting and polishing belts made in. accordance with the present invention are employed in the manner set out below for enlarging die of the same dimensions, the increase in diameter can be accomoverconventional practice, possible by using a plished in about 12 minutes actual cutting time, while the polishing can be done in about 4 or 5 minutes. belts or diamond-coated leather sheets" is meant a coated abrasive having a backing composed entirely or predominantly of leather, abrasive grit being coated on` the leather and retained by an adhesive, the abrasive being composed wholly of diamond grit or partly of diamond grit, and partly of another kind or kinds of abrasive grit. The term leather as here used means the general class of animal, fish and bird hides, regardless of kind or nature of treatment in the processing of the hide. The belts shown in Figures 9, 11, and 13, are modifications of that shown in Figure 1. Such modifications allow the belt faithfully to conform to curvedA work piece surfaces and enable the proper grinding pressures to be applied where desired. Except as noted the waterproofing, adhesive, and abrasive coatings of these belts are the same as those in Figure 1.

The belt illustrated in Figure 9 consists of a layer 40 of leather such as has been before described as useful in such belts, with a layer of diamonds or a' mixture of diamonds with other abrasive adhered to its working face in the same manner as that described in connection with Figure 1. A reinforcing member 39, which may -be among other suitable materials, a thin springy strip of steel tape, made into a belt by butt or lap silver soldering its ends, is shown placed in a groove in the back of leather 40. In one example, which is illustrative only, the strip 39 was 11g inch wide, 0.005 inch thick, and was placed at the center of a belt A inch wide, Another layer 4I of leather or suitable material is secured tothe leather 40 and strip 39 by a suitable adhesive. One such 4adhesive is that previously described which is sold under the trade-name Sea Lion Waterproof Belt Cement, which is a cellulold" type cement containing nitrocellulose and cam- By the-term diamond-coated leather f ber 60, or if it is thin enough, neither leather member need be grooved, since the leather will deform sufficiently under the combining presssure to conform closely to and enclose strip 39.

The belt shown in Figure 9, when employed to finish a die bore, as shown in Figure l0, by reason of the reinforcing strip 39 exerts considerable pressure on the work piece at the center in a location opposite strip 39. The diiculty, sometimes experienced with belts not having such strip 33, of securing the desired grinding pressure at the center of the belt when grinding curved surfaces, such as the bore of die d2 shown in Figure 10, is thus overcome. At the same time the belt flexes laterally and longitudinally quite readily, and thus conforms faithfully to the surface being ground.

`In Figure l1 there is shown a belt having a leather backing d3 with an abrasive layer of diamonds or a mixture of diamonds with other abrasive coated thereon as before described, with the exception that the center portion d@ of the belt is coated by a waterproofing. coat only. 'I'hus the center portion of the belt consists of exible tanned waterproofed leather only, and the belt readily flexes laterally at the center, as shown in Figure 12, when grinding on an interior curved surface, allowing substantially all the abrasive coated' portions of the belt to come into contact with the work piece and to exert substantially uniform grinding pressure. As an alternative the center portion of the belt may be coated in the same manner as the other portions of the Working face except that the abrasive is omitted.

Figure 13 shows a belt employing a leather base 615 having an abrasive layer thereon of diamonds or a mixture of diamonds with other abrasive coated thereon as before described. The back of the leather is provided with a slot or groove 436, whereby the belt may ex readily laterally. Obviously more than one groove may be employed, if desirable, to produce the same result. As shown in Figure 14, such belt conforms readily to the surface of the bore oi a die i2 or the like. It has been found that by proper choice of depth and width `of the groove i6 correlated with. the width and thickness of the leather belt backing and the diameter of the bore to be ground, the belt can be made to exert substantially uniform pressure throughout the extent of its width on the die bore and thus each abrasive grain cuts eiciently,

It hasbeen found that leather-backed waterproof sheets or belts possess great and unexpected advantages over backings such as cloth or other cellulosic material. These advantages, which may result from the greater resilience of leather as compared with cloth and the like, reside in the ability to secure much smoother surfaces on Work pieces ground with such leather sheets or belts. In every grit size, belts with leather backings yield surfaces which are smoother and more lustrous than those ground with belts having plied tape backings of the same thickness as the leather used in the comparable leather belt.

The superior surfaces obtainable with leather belts as compared with those produced by use of plied tape backed belts are strikingly shown in the charts reproduced in Figures 15-20, inclusive. These charts are produced by the brush y surface analyzer in which a tracer point connected to a crystal pick-'up isA traversed over the surface to be analyzed. Current developed by vibrations of the tracer, suitably amplified, op crates a recording pen, Which'marks a path on a graph moving in a known chosen speed relationship with the traversing speed of the tracer. The recorded line on the chart thus is a reproductionof the surface analyzed, with vertical and horizontal dimensions multiplied by chosen known factors. A

In the charts reproduced in Figs. 15-20, incl., horizontal distances on the work piece are multiplied 80 times. Thus 10 inches of chart length represent the tracing of a 1/8 inch length of work piece surface. In the charts each small verticalY division represents 10 micro-inches (a micro-inch being l millionth of an inch) as measured on the work piece surface.

The charts reproduced in Figures 15-20, inclusive, were made from six different surfaces ground on the same piecebf cemented tungsten carbide.` The leather and the tape belts employed the same size grit, that is, the producing the surfaces analyzed in Figures 15 and '16, Figures 17 and 18, and Figures 19 and 20,were identical as to backing thickness, width, adhesives in both making and sizing coats, and grit size and amount. The only difference was that in Figures 15, 17 and 19 the backing was of leather and in Figures 16, 18 and 20 the backing was of plied cotton tape. In the belts employed to grind the surfaces reproduced in Figures 15 and 16 the grit was employed in aratio of 20 parts by weight of diamonds to 80 parts by weight of SiC, designated 20/80. In the belts employed to grind the surfaces reproduced in Figures 16 and 17 the grit was employed in a ratio of 40 parts by weight of diamonds to 60 parts by weight .of SiC, Ai0/6() Whereas, the belts producing the surfaces analyzed in Figures 19 and 20 employed 5 parts by Weight of diamonds and 95 parts by weight of SiC, 5/95. The use of silicon carbide or other grit less hard than diamondsisfor the purpose, among others, as above set out, of diluting the diamond grit, thus allowing its uniform distribution on the belt.

As is readily apparent by comparison of Figures 15, 17 and 19 With Figures 16, '18 and 20, respectively, the surfaces obtained by use of leatherbacked belts are much smoother and more uniform than those obtained with plied tape backed belts. In every instance the maxima and minima of the traced curves in Figures 15, 1'? and 19 are less than those in 16. 18 and 20 respectively, and the curves in Figures 15, 17 and 19 are more regular and uniform. \The comparison between leather and tape backed belts becomes more striking as the grit size is decreased, as can be seen by comparing Figures 19 and 20, where 800 grit was employed.

. The results given by the brush analyzer are borne out visually. backed belts `give surfaces which are more -lustrous and of higher specular quality than do 800 grit diamond leatheubacked belt is very 5 In all cases the leathern polishing of drawing dies and the like, where-extreme accuracy and as near perfection of polish as possible are required for satisfactory operation, and the better polished the die surface the better will be the product and the more economical the use of the die.

One reason for the superiority of leather over cloth and the like as a backing for waterproof diamond sheets and belts is its greater resilience. This greater resilience allows the leather belt to conform faithfully to the surface being ground and thus the grit to contact it evenly and unlformly. Repeated contacts of this sort between the work and the grit, especially if it is ne, smooths and polishes the work to a mirrorlike finish. The canvas, being less resilient, does not conform to the work surface so faithfully. Thus certain grits take deeper'cuts than others and continue to do so on repeated contacts, scratching the surface even though the grit may be ne and preventing the attainment of the required mirror finish on the hard carbide and alloy dies, tools,

and the like.

An added advantage in the use of leather is that its resilient nature is favorable toward a belt maintaining its cutting rate. This is a valuable attribute where skilled high wage labor is used in die finishing operations. A diamond belt made on a cotton base was tested for dropping off in cutting rate per unit interval of 12 ing comprised of a, layer of leather cemented to a reinforcing layer of metal, a layer of abrasive grain on the layer of leather, said grain comprising diamonds, and a grain bonding coat of an adhesive on the leather.

3. A coated abrasive belt comprising a backing consisting of a layer of leather cemented to a time when grinding hard cemented carbide, and

compared to another diamond belt substantially the same except that the backing was leather. Such advantage is clearly shown in the following table:

Per cent of initial cutting rate C L th ecentage otton ea er erence, Tx-me interval base belt base belt degree of superiority reinforcing layer of metal by a layer of a cured phenol-aldehyde resin., a layer of abrasive grain on thelayer of leather, said grain comprising diamonds, and a grain bonding coat of a cured phenol-aldehyde resin on the leather, the `leather being waterproofed at least on its exposed surfaces.

4. A vflexible coated abrasive comprising a backing comprised of a layer of leather, the working face of the leather having uniformly distributed portions of greater height than the remainder of such face, a layer of abrasive grain on such raised portions of the Working face of the leather, said grain comprising diamonds, and a grain bonding coat of a cured resin on the raised portions of the working face of the leather.

5. A flexible coated abrasive belt comprising a backing of leather, a layer of abrasive grain on the leather, said grain comprising diamonds, and a grain bonding coat of an adhesive on the leather, said leather backing having incorporated therein a reinforcing strip of relatively stiff springy material, said. reinforcing strip being located a substantial distance from the abrasive coated face of the leather backing.

6. A flexible coated abrasive beltl comprising a backing of leather, a layer of abrasive grain on the leather, said grain comprising diamonds, and

a grain bonding coat of an adhesive on the leather, said leather backing having at least one longitudinal groove in the back thereof at approximately its center.

7. A flexible coated abrasive belt comprising a backing of leather, a layer of abrasive grain on the side portions of the working face of the belt, the center of the belt being devoid of Vabrasive grain,.said grain comprising diamonds, and a grain bonding layer of an adhesive on the leather.

8. A coated abrasive waterproof belt comprising a backing consisting of a layer of flexible resilient leather, said leather having a pore stopping sizing coat on one surface and being waterprcofed on its other surfaces, a coating of abrasive grain on the sized surface of the leather, said abrasive grain comprising at least a substantial amount of diamonds, and a grain bonding coat of a cured phenoll-aldehyde Waterproof resin on the sized surface of the leather.

HENRY P. KIRCHNER. ALBERT L. BALL.

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