Coated abrasive product incorporating selective mineral substitution
CROSS-REFERENCE TO RELATED CASE 
     This is a continuation of application Ser. No. 721,869 filed Apr. 10, 1985 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to coated abrasive products and is especially concerned with coated abrasive products using two or more different abrasive minerals. 
     The mineral used in coated abrasive products made in the United States of America conventionally meets American National Standards Institute, Inc. (ANSI) standards, which specify that the particle size distribution for each nominal grade falls within numerically defined limits. According to the ANSI standards, any nominal grade is made up of three particle size fractions, viz., a &#34;control&#34; fraction, an &#34;overgrade&#34; fraction containing large particles nominally one fraction coarser than the control fraction, and a &#34;fine&#34; fraction containing small particles finer than the control fraction. Additionally ANSI standards permit the inclusion of up to 0.5% particles coarser than the overgrade fraction. The percentage of particles falling within each fraction varies from grade to grade; in general, however, about 50-60% are in the control fraction, about 10% in the overgrade fraction and about 30-40% in the fine fraction. When considered as a total, the sum of the three fractions is referred to as &#34;full grade.&#34; 
     As used in the preceding paragraph, the term &#34;grade&#34; refers to a specified combination of abrasive particles as related to the standard mesh screens through which the particles will or will not pass. To illustrate, ANSI publication B74.18-1977 provides that a coated abrasive product having a nominal Grade 50 mineral coat will contain a control fraction which will pass through a 48.5-mesh (1 Std.) screen but not through a 58.5-mesh (3 Std.) screen, an overgrade fraction that will pass through a 37-mesh (38 GG) screen but not a 48.5-mesh (1 Std.) screen, and a fine fraction that will pass through a 58.5-mesh (3 Std.) screen. Additionally, Grade 50 may include up to 0.5% of extra-coarse particles that pass through a 32-mesh (32 GG) but not through a 38-mesh (38 GG) screen. The term &#34;mesh&#34; refers to the number of openings per lineal inch in the screen. Grading systems employed in foreign countries also utilize screens but vary somewhat as to the exact particle size, the number of screens and the percentage of particles falling in the several fractions that collectively make up a &#34;full grade&#34;. Like the ANSI system, the Japanese grading system employs three fractions; the European grading system effectively includes four fractions, the coarsest three of which correspond roughly to the ANSI overgrade and control fractions. As a point of interest, the various grading systems are all intended to provide complete utilization of all the particles obtained during the process of crushing the originally supplied lumps of raw abrasive mineral. 
     For any given abrading operation, some types of abrasive mineral are more effective than others. For most metal abrading operations, however, the most widely used mineral has long been fused aluminum oxide, or alumina. In recent years, superior minerals have been developed by the co-fusion of alumina and zirconia; see, e.g., U.S. Pat. Nos. 3,181,939, 3,891,408, and 3,893,826. Another recently developed superior mineral, described in U.S. Pat. No. 4,314,827, is a non-fused synthetic alumina-based mineral containing certain metal oxide and/or spinel additives. Both the co-fused alumina:zirconia and the non-fused ceramic products are significantly more expensive than the conventional fused alumina, as, of course, are the coated abrasive products made with such minerals. Other slightly superior--and comparatively expensive--alumina-based minerals may be obtained by specially heat treating or coating conventional fused alumina. 
     It has been suggested that various types of minerals can be blended in making coated abrasive products; see, e.g., U.S. Pat. No. 3,205,054. One commercial product embodying this concept incorporates a full-grade blend of conventional fused alumina and the significantly more expensive co-fused alumina:zirconia. See also U.S. Pat. Nos. 2,410,506 and 3,266,878, showing the use of inexpensive &#34;diluent&#34; grain blended with diamond particles of the same grade. U.S. Pat. No. 3,996,702 describes the blending of co-fused alumina:zirconia with flint, garnet, or fused alumina of the same grade, and U.S. Pat. No. 4,314,827 suggests blending non-fused alumina-based abrasive grain with conventional fused alumina of the same grade. 
     In the manufacture of molded fabric-reinforced abrasive grinding wheels, several combinations of abrasive grain have been suggested for use in different layers of the construction. For example, U.S. Pat. No. 1,616,531 describes the use of different particle size mineral in the various abrasive layers. U.S. Pat. No. 3,867,795 describes the blending of expensive co-fused alumina:zirconia with flint, emery, silicon carbide, fused alumina, etc. in the various layers of relatively thin snagging wheels for use on portable grinders. One suggested construction in the latter patent utilizes conventional fused alumina in one layer with a blend of co-fused alumina:zirconia and a coarser garnet in the work-contacting surface. 
     Although products of the type described in the preceding paragraphs have managed to reduce the overall cost of the mineral applied in the coated abrasive construction, there has remained a strong desire to obtain the benefits of the superior mineral products while further minimizing the amount of the superior mineral present. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides coated abrasive products having excellent abrading effectiveness, utilizing the advantages inherent in superior abrasive grains while minimizing the quantity of such grains actually employed. Indeed, in some instances synergistic effects are obtained, the construction actually performing better than coated abrasive products in which only the superior mineral is present. 
     The present invention combines a minor portion of superior abrasive grains and the balance, correspondingly constituting a major portion, of inferior abrasive grains in such a way that most of the superior grain is concentrated in the coarsest portion. The unexpectedly good performance contributed by the superior grain can sometimes be detected in quantities as low as 1% by weight, but 3% of the superior grain contributes more consistently significant improvement. For most purposes, the superior abrasive grain will constitute 5% to 30% (preferably 10% to 20%) of the total mineral weight. It is technically feasible to add up to 50% of the superior grain, but the additional cost generally will not justify doing so. Thus, the invention can be broadly characterized as a coated abrasive product having a specified nominal grade of abrasive granules firmly adherently bonded to a sheet backing, the particle size of the granules ranging from large, or coarse to small, or fine. The granules consist essentially of two types of mineral, one type being present as a minor portion and demonstrably superior to an equivalent grade of the other type in the abrading operation for which the coated abrasive product is intended to be used, most of the superior mineral being concentrated in the coarser portion of the particles. 
     As will be shown, products corresponding to the invention can be made utilizing either a single application of blended abrasive grains or a multiple coating operation in which the first mineral coat does not conform to conventional mineral grading specifications because it exceeds the limits for fine particles, and the second mineral coat does not conform to conventional mineral grading specifications because it exceeds the limits for coarse particles. In this construction, the coarse fraction, which consists essentially of the superior mineral, is present in the second coat. The overall composition of the two mineral layers is, however, in full compliance with mineral grading specifications. 
     DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS 
     Although the terms &#34;superior&#34; and &#34;inferior&#34; might seem to involve a considerable degree of subjectivity, those skilled in the coated abrasive art are quite capable of making such judgments. It is, of course, true that superiority or inferiority depends to some degree on the type of workpiece and the abrading conditions employed. Thus, for an ultimate determination of relative &#34;superiority&#34; and &#34;inferiority&#34; for two types of abrasive grain, coated abrasive products made with each of the two types should be tested under the specific grinding conditions of interest, using workpieces of the type to be abraded. For the present most commercially significant abrading operations, however, it has been found that a test involving the abrasion of cold rolled steel with coated abrasive products having only one specific type of abrasive grain bonded to the backing will, when compared to an identical construction involving a different abrasive grain, yield test results that are highly reliable in categorizing abrasive grain as to relative superiority or inferiority. This test will now be described in more detail. 
     A pre-weighed cold rolled steel workpiece (SAE 1018) 1 inch×2 inches×71/4 inches (approximately 2.5×5×18 cm), mounted in a holder, is positioned vertically, with the 1-inch×71/4 inch (2.5×18-cm) face confronting a 14-inch (approximately 36-cm) diameter 65  Shore A durometer serrated rubber contact wheel over which is entrained a Grade 50 belt to be tested. The workpiece is then reciprocated vertically through a 71/4-inch (18-cm) path at the rate of 20 cycles per minute, while a spring-loaded plunger urges the workpiece against the belt with a force of 25 lbs (11.3 kg) as the belt is driven at 5500 surface feet (about 1675 meters) per minute. After one minute elapsed grinding time, the workpiece is pulled away from the moving belt, the first workpiece-holder assembly removed and reweighed, the amount of stock removed calculated by subtracting the abraded weight from the original weight, and a new pre-weighed workpiece and holder mounted on the equipment. Using four workpieces, this procedure is repeated for a total of 88 minutes or until the cut per minute is 25 grams or less, whichever occurs sooner. With coarser or finer grades of mineral, abrading force may be respectively increased or decreased and final cut figures likewise adjusted. 
     Because there is inevitably some variation among presumably identical belts and presumably identical workpieces, the total cut values are considered accurate to ±5%; thus, if a belt from one lot cuts over 10% more than a belt from another lot, the first belt is deemed &#34;superior&#34; and the second &#34;inferior&#34;. As might be expected, a higher degree of reliability is achieved if duplicate belts are tested. 
     Using the test procedure just described, the total cut values tabulated below were obtained for a series of belts made to ANSI standards using solely the type of coated abrasive mineral indicated. In each case, the cut figure is the average of at least two belts. 
     
         ______________________________________                                    
Mineral                   Time,    Total Cut,                             
Designation                                                               
        Type of Grade 50 Mineral                                          
                          Minutes  Grams                                  
______________________________________                                    
AO      Conventional fused alumina                                        
                          56       2779                                   
AZ      Co--fused alumina-zirconia                                        
                          56       4580                                   
CUB     Non-fused alpha alumina                                           
                          88       8094                                   
        containing certain metal                                          
        oxides and/or spinels                                             
HT      Heat-treated fused alumina                                        
                          --       --                                     
______________________________________                                    
 
    
     The mineral designations listed above will be used in the following description and examples. 
    
    
     EXAMPLES 1-3 
     Each of the following examples was prepared using a conventional cloth backing, viz., rayon drills saturated with a blend of synthetic rubber latex and phenolic resin. A conventional calcium carbonate-filled phenol-formaldehyde make coat was applied, the mineral electrostatically coated in conventional manner, the make coat precured, a conventional calcium carbonate-filled size coat applied, and both make and size coats then final cured. The only difference between conventional ANSI Grade 50 coated abrasive belt stock and the products of these examples, then, resided in the specific abrasive grain, or combination of grains, employed. In each of the examples made according to the invention, the abrasive grain was a blend of (1) the fine and control fractions of conventional Grade 50 fused alumina mineral, and (2) as a replacement for the coarse (overgrade) fraction, an equivalent weight of a full grade of Grade 40 superior mineral. (While it might be supposed that the overgrade fraction present in the full grade of the Grade 40 mineral would be excessively coarse for use in Grade 50, such is not the case in actual practice. There is considerable overlap in these two grades, but, as in normal manufacturing procedures, pre-coating screening removes any particles--perhaps 1%-- that are larger than ANSI standards permit for Grade 50 products.) 
     Endless belts 3 inches (7.6 cm) wide×132 inches (335 cm) long were prepared from both conventional coated abrasive material and coated abrasive material made in accordance with the experimental examples. These belts were then entrained over a 20-inch (51-cm) diameter 65 Shore D durometer rubber contact wheel, serrated at a 45° angle to the lateral surfaces of the wheel, lands being 3/4 inch (approximately 19 mm) wide and grooves one-third that dimension. The belts were then driven at 7380 surface feet (2250 meters) per minute while sets of pre-weighed metal test bars having either a rectangular or a circular cross section (approximate area 0.5-1 in 2 , or about 3.2-6.4 cm 2 ) were urged against the belt under a pressure of either 100 or 150 psi (690 or 1035 kPa). Sets of 15 pre-weighed bars of SAE 1095 steel, 1018 steel, and 304 stainless steel were employed, while sets of  10 pre-weighed bars of Waspalloy and Inconel 600 were employed. Each bar was run for 5 seconds. Total cut figures are tabulated below: 
     
                                           TABLE I                                 
__________________________________________________________________________
              Total Cut, Grams, for Grade 50 Coated Abrasive Product      
              Indicated                                                   
                              304                                         
              1095 Steel                                                  
                      1018 Steel                                          
                              Stainless Steel                             
                                      Waspalloy                           
                                              Inconel 600                 
Example                                                                   
      Mineral 100 psi                                                     
                  150 psi                                                 
                      100 psi                                             
                          150 psi                                         
                              100 psi                                     
                                  150 psi                                 
                                      100 psi                             
                                          150 psi                         
                                              100 psi                     
                                                  150 psi                 
__________________________________________________________________________
Control A                                                                 
      100% AO 195 266 180 221 253 317 176 134 537 415                     
Control B                                                                 
      100% CUB                                                            
              342 468 355 397 358 570 389 325 767 671                     
Control C                                                                 
      100% AZ 280 409 281 280 301 495 456 348 699 566                     
Control D                                                                 
      100% HT 226 307 241 275 290 389 --  --  --  --                      
1     90:10 AO:CUB*                                                       
              276 357 241 289 320 444 263 174 725 567                     
2     90:10 AO:AZ*                                                        
              248 335 335 267 307 426 --  --  --  --                      
3     90:10 AO:HT*                                                        
              191 307 174 --  231 --  --  --  --  --                      
__________________________________________________________________________
 *All ratios are by weight. The densities of AO, CUB and HT are           
 substantially the same, so the weight ratios and volume ratios are       
 essentially the same. Because AZ has a considerably higher density, it   
 would theoretically be necessary to employ a highe r weight to arrive at 
 10% volume concentration; practically, however, the comparatively small  
 amount of AZ present does not justify such an adjustment.   If a straight
 line is drawn between the 100% AO and 100% CUB cut figures, it will be
 observed that the total amount of metal cut by Example 1 lies considerably
 above the interpolated value that would be predicted. The same is true for
 Examples 2 and 3, where the blends of &#34;superior&#34; AZ and HT minerals with
 the &#34;inferior&#34; AO perform better than would be expected.
 
    
     EXAMPLE 4 
     A coated abrasive product was made by the same procedure as in Example 1, ANSI Grade 80 mineral being substituted for the ANSI Grade 50 and all coating weights adjusted appropriately. In other words, in this Example 4, the coarse fraction was made up of the full grade of Grade 60. Belts were prepared in the same manner as for Examples 1-3 and tested on a comparable piece of equipment, the differences being that the belt speed was 5500 surface feet (about 1675 meters) per minute and the pressure applied to the workpiece was either 30 or 75 psi (respectively about 207 or 517 kPa). For convenience in comparing results, cut figures have been converted to percentages, conventional fused alumina at 30 psi (207 kPa) being assigned the value of 100%. 
     
                                           TABLE II                                
__________________________________________________________________________
              Total Cut, Grams, for Grade 80 Coated Abrasive Product      
              Indicated                                                   
                          304 Stain-        Cast aluminum,                
              1095 Steel                                                  
                    1018 Steel                                            
                          less Steel                                      
                                Waspalloy                                 
                                      Inconel 600                         
                                            300 Series                    
Example                                                                   
      Mineral 30 psi                                                      
                 75 psi                                                   
                    30 psi                                                
                       75 psi                                             
                          30 psi                                          
                             75 psi                                       
                                30 psi                                    
                                   75 psi                                 
                                      30 psi                              
                                         75 psi                           
                                            30 psi                        
                                                75 psi                    
__________________________________________________________________________
Control E                                                                 
      AO      100                                                         
                 111                                                      
                    100                                                   
                        89                                                
                          100                                             
                             178                                          
                                100                                       
                                   -- 100                                 
                                          99                              
                                            100 218                       
Control F                                                                 
      CUB     143                                                         
                 310                                                      
                    115                                                   
                       244                                                
                          127                                             
                             308                                          
                                129                                       
                                   -- 121                                 
                                         210                              
                                            112 332                       
4     90:10 AO:CUB                                                        
              121                                                         
                 177                                                      
                    135                                                   
                       135                                                
                          232                                             
                             622                                          
                                195                                       
                                   -- 210                                 
                                         422                              
                                            109 348                       
__________________________________________________________________________
 
    
     It will be observed from the foregoing table that in almost every instance products containing only 10% of the CUB mineral performed more effectively than products made with either 100% of the &#34;inferior&#34; conventional fused alumina or 100% of the &#34;superior&#34; CUB mineral. This result is considered surprising and synergistic. Even in those instances where belts made with the blended mineral did not actually cut more stock than those made with either of the two component minerals, total cut was more than would be predicted from a linear interpolation based on the amount of the superior mineral present. 
     EXAMPLES 5-8 
     Coated abrasive belts were made as in Examples 1 and 4, (i.e., each containing 10% CUB) in Grades 36, 50, 60, and 80. These belts were then tested according to the method described earlier in connection with evaluating &#34;superior&#34; and &#34;inferior&#34; minerals; the tests were, however, run for a predetermined period of time, rather than to a predetermined cutting rate. This time was 40 minutes for the Grade 50 belts and 30 minutes for Grades 36, 60, and 80. The control belts for each grade were conventional products made with fused alumina. Results are tabulated below: 
     
                       TABLE III                                                   
______________________________________                                    
Lab Tests                                                                 
Example Grade    Abrading Force kPa                                       
                               Total Cut, grams                           
______________________________________                                    
Control G                                                                 
        36       206           1356                                       
5       &#34;        &#34;             2316                                       
Control A                                                                 
        50       172           1672                                       
6       &#34;        &#34;             2588                                       
Control H                                                                 
        60       139           1236                                       
7       &#34;        &#34;             2026                                       
Control E                                                                 
        80       103            962                                       
8       &#34;        &#34;             1661                                       
______________________________________                                    
 The Grade 50 and Grade 80 belts were then field tested against the same
 controls, where results in grinding various cold rolled or tool steel
 workpieces were as follows:
 
    
     
                       TABLE IV                                                    
______________________________________                                    
Field Tests                                                               
          No. of Pieces Finished                                          
Example Grade   Wrench Handles                                            
                              Breaker Bars                                
                                       Chisels                            
______________________________________                                    
Control A                                                                 
        50       600                                                      
6       &#34;       1000                                                      
Control E                                                                 
        80                    140      65                                 
8       &#34;                     285      95                                 
______________________________________                                    
 
    
     The preceding examples have all described coated abrasive products in which the abrasive grain was applied in a single coating. As has been pointed out above, coated abrasive products have sometimes been made by applying the abrasive grain in two separate stages, typically drop coating the bottom portion and subsequently electrostatically coating the top portion. This two-step procedure offers certain advantages in the practice of the present invention, where it is possible to divide the abrasive grains so that the first layer contains substantially no coarse particles, the second layer containing a disproportionately large percentage of coarse particles. Since, in practicing the present invention, the coarse particles are predominantly made up of a comparatively expensive &#34;superior&#34; mineral, the effect of the two-coat system is to provide a higher concentration of these particles in the abrading surface that initially contacts the material to be abraded. The following examples illustrate this type of contruction. 
     EXAMPLES 9-13 
     ln each of these examples, one half the total weight of Grade 50 abrasive grain was applied in a first trip containing substantially only the fine and control fractions of conventional fused alumina, while the second half of the Grade 50 mineral was applied in the form of a blend of minerals containing, in an amount sufficient to constitute the ANSI standard coarse fraction for the two mineral layers combined, a specified percentage of a mineral superior to fused alumina. To help put the results into perspective, several controls were also provided. The nature of the examples and controls, together with the results of abrading tests similar to those described in Table I, is tabulated below: 
     
                                           TABLE V                                 
__________________________________________________________________________
                            Total Cut, Grams, for Grade 50                
                            Coated Abrasive Product Indicated             
                                        304 Stain-  Inconel               
                            1095 Steel                                    
                                  1018 Steel                              
                                        less Steel                        
                                              Waspalloy                   
                                                    600                   
      First    Second       100                                           
                               150                                        
                                  100                                     
                                     150                                  
                                        100                               
                                           150                            
                                              100                         
                                                 150                      
                                                    100                   
                                                       150                
Example                                                                   
      Mineral Coat                                                        
               Mineral Coat psi                                           
                               psi                                        
                                  psi                                     
                                     psi                                  
                                        psi                               
                                           psi                            
                                              psi                         
                                                 psi                      
                                                    psi                   
                                                       psi                
__________________________________________________________________________
Control A                                                                 
      Single coat full grade 50 AO                                        
                            195                                           
                               266                                        
                                  180                                     
                                     221                                  
                                        253                               
                                           317                            
                                              176                         
                                                 134                      
                                                    537                   
                                                       415                
Control B                                                                 
      .sup. Single coat full grade 50 CUB                                 
                            342                                           
                               468                                        
                                  355                                     
                                     397                                  
                                        358                               
                                           570                            
                                              389                         
                                                 325                      
                                                    767                   
                                                       671                
Control C                                                                 
      Single coat full grade 50 AZ                                        
                            280                                           
                               409                                        
                                  281                                     
                                     280                                  
                                        301                               
                                           495                            
                                              456                         
                                                 348                      
                                                    699                   
                                                       566                
Control D                                                                 
      Single coat full grade 50 HT                                        
                            226                                           
                               307                                        
                                  241                                     
                                     275                                  
                                        290                               
                                           389                            
                                              -- -- -- --                 
Control I                                                                 
      Full grade 50 AO                                                    
               Full grade 50 CUB                                          
                            325                                           
                               432                                        
                                  279                                     
                                     394                                  
                                        453                               
                                           603                            
                                              -- -- -- --                 
Control J                                                                 
      &#34;        Full grade 50 AZ                                           
                            285                                           
                               414                                        
                                  277                                     
                                     344                                  
                                        407                               
                                           523                            
                                              -- -- -- --                 
9     Fine &amp; control                                                      
               90:10 fine &amp; control                                       
                            221                                           
                               341                                        
                                  231                                     
                                     276                                  
                                        266                               
                                           369                            
                                              242                         
                                                 -- 650                   
                                                       --                 
      grade 50 AO                                                         
               grade 50 AO:full grade                                     
               40 CUB                                                     
10    Fine &amp; control                                                      
               80:20 fine &amp; control                                       
                            292                                           
                               388                                        
                                  324                                     
                                     345                                  
                                        318                               
                                           433                            
                                              266                         
                                                 -- 696                   
                                                       --                 
      grade 50 AO                                                         
               grade 50 AO:full grade                                     
               40 CUB                                                     
11    Fine &amp; control                                                      
               80:20 fine &amp; control                                       
                            253                                           
                               368                                        
                                  254                                     
                                     258                                  
                                        374                               
                                           501                            
                                              440                         
                                                 -- 510                   
                                                       --                 
      grade 50 AO                                                         
               grade 50 AO:full grade                                     
               40 AZ                                                      
12    Fine &amp; control                                                      
               80:20 fine &amp; control                                       
                            348                                           
                               501                                        
                                  360                                     
                                     451                                  
                                        422                               
                                           609                            
                                              454                         
                                                 -- 727                   
                                                       --                 
      grade 50 AO                                                         
               grade 50 CUB:full grade                                    
               40 CUB                                                     
13    Fine &amp; control                                                      
               80:20 fine &amp; control                                       
                            337                                           
                               440                                        
                                  296                                     
                                     347                                  
                                        374                               
                                           501                            
                                              -- -- -- --                 
      grade 50 AO                                                         
               grade 50 AZ:full grade                                     
               40 AZ                                                      
__________________________________________________________________________
 
    
     Example 9 contains 5% CUB based on the total weight of mineral present. Similarly, Examples 10-13 contain 10% &#34;superior&#34; mineral based on the total weight of mineral present. 
     It will be observed that the performance of Examples 9-13 is significantly better than would be predicted from a linear interpolation between Control A and Controls B, C, and D (as appropriate) based on the percentage of &#34;superior&#34; mineral present. 
     EXAMPLES 14-17 
     The following examples were all prepared according to ANSI standards for Grade 40 product made on phenolic resin-bonded drills cloth backings, using conventional backing, make, size, and coating techniques except for the type of abrasive mineral and, for two of the examples, the method of applying such mineral. Endless belts were prepared from each lot of material and tested on SAE 1018 steel according to the method described earlier in connection with evaluating &#34;superior&#34; and &#34;inferior&#34; mineral; all tests were, however, run for a predetermined length of time (221/2 minutes) instead of to a predetermined cutting rate, using a force of 43 lbs (19.5 kg). Results are tabulated below: 
     
                       TABLE VI                                                    
______________________________________                                    
                                Total                                     
                                Cut,                                      
Example Mineral                 Grams                                     
______________________________________                                    
Control K                                                                 
        Full grade 40 AO        2051                                      
Control L                                                                 
        Full grade 40 CUB       4308                                      
14      95:5 full grade 40 AO:full grade 40 CUB                           
                                2236                                      
15      95:5 fine &amp; control fractions Grade 40                            
                                2501                                      
        AO:full grade 36 CUB                                              
16      70:30 full grade 40 AO:full grade 40 CUB                          
                                3085                                      
17      70:30 fine &amp; control fractions grade 40                           
                                3999                                      
        AO:full grade 36 CUB                                              
______________________________________                                    
 
    
     The preceding examples have all been related to the manufacture of coated abrasive belts. The same principles and general types of construction are also applicable to the manufacture of coated abrasive discs made on 30-mil (about 0.76-mm) vulcanized fiber backing. The following examples are all Grade 50 products, made to conventional coating standards, with all components being conventional except for the mineral or mineral blend employed. 
     EXAMPLES 18-20 
     Cured 7-inch (17.8-cm) diameter discs were first conventionally flexed to controllably crack the hard bonding resins, mounted on a beveled aluminum back-up pad, and used to grind the face of a 1-inch (2.5-cm)×71/4-inch (18.4-cm) 1.25-cm×30-cm 1018 cold rolled steel workpiece. Each disc was driven at 5000 rpm while the portion of the disc overlying the beveled edge of the back-up pad contacted the workpiece with a force of 10 lbs (4.5 kg) or 15 lbs (6.8 kg), generating a disc wear path of 18.9 in 2  (about 120 cm 2 ). Each disc was used to grind 10 separate workpieces for 1 minute each, the cumulative cut figures being shown in Table VII below: 
     
                       TABLE VII                                                   
______________________________________                                    
                    Total Cut, Grams,                                     
                    for Coated Abrasive                                   
                    Product Indicated                                     
Example Grade 50 Mineral  10 lbs.  15 lbs.                                
______________________________________                                    
Control M                                                                 
        Full grade 50 AO  114      176                                    
Control N                                                                 
        Full grade 50 CUB 394      535                                    
18      95:5 fine &amp; control grade                                         
                          260      378                                    
        50 AO:full grade 40 CUB                                           
19      90:10 fine &amp; control grade                                        
                          316      456                                    
        50 AO:full grade 40 CUB                                           
20      2-trip - 1/2 full grade                                           
                          262      360                                    
        50 AO followed by 1/2                                             
        (90:10 fine &amp; control grade                                       
        50 AO:full grade 40 CUB)                                          
______________________________________                                    
 
    
     Once again it is noted that the abrading effectiveness of the examples is significantly greater than could have been predicted from a linear interpolation between Controls M and N. 
     EXAMPLES 21-28 
     Cured 7-inch (17.8-cm) diameter Grade 24 discs were prepared using different combinations of abrasive grains and tested under a 15-lb (33-kg) load in substantially the same manner as in Examples 18-20, but using an 8-inch (20-cm) long work piece. Results are tabulated below: 
     
                       TABLE VIII                                                  
______________________________________                                    
                                 Total                                    
                                 Cut,                                     
Example Mineral                  Grams                                    
______________________________________                                    
Control O                                                                 
        Full grade AO             50                                      
Control P                                                                 
        Full grade CUB           673                                      
Control Q                                                                 
        Full grade Si.sub.3 N.sub.4 --coated SiC                          
                                 604                                      
        (&#34;SNAG&#34;), as in U.S. Pat. No.                                     
        4,505,720                                                         
Control R                                                                 
        70:3O full grade AO:full grade CUB                                
                                 468                                      
21      70:30 (fine &amp; control fractions AO):                              
                                 574                                      
        (control &amp; coarse fractions CUB)                                  
Control S                                                                 
        90:10 full grade AO:full grade CUB                                
                                 247                                      
22      90:10 (fine &amp; control fractions AO):                              
                                 321                                      
        coarse fraction CUB                                               
23      90:9:1 (fine &amp; control fractions AO):                             
                                 287                                      
        coarse fraction CUB:coarse fraction AO                            
Control T                                                                 
        95:5 full grade AO:full grade CUB                                 
                                 196                                      
24      95:5 full grade AO:coarse fraction CUB                            
                                 200                                      
Control U                                                                 
        97:3 full grade AO:full grade CUB                                 
                                  96                                      
25      97:3 full grade AO:coarse fraction CUB                            
                                 121                                      
Control V                                                                 
        99:1 full grade AO:full grade CUB                                 
                                  50                                      
26      99:1 full grade AO:coarse fraction CUB                            
                                  58                                      
Control W                                                                 
        70:30 full grade AO:full grade SNAG                               
                                 361                                      
27      70:30 (fine and control fractions AO):                            
                                 434                                      
        control and coarse fractions SNAG                                 
Control X                                                                 
        90:10 full grade AO:full grade SNAG                               
                                 173                                      
28      90:10 (fine &amp; control fractions AO):                              
                                 250                                      
        coarse fraction SNAG                                              
______________________________________                                    
 
    
     It will be noted that the performance of the coated abrasive products made in accordance with the invention is not only consistently superior to that of coated abrasive products made with full grade blends but also superior to the performance that would be predicted by interpolating between the individual cut figures for the minerals blended. 
     It will be appreciated that the foregoing examples are only illustrative and that numerous changes can be made without departing from the invention. For example, more than one type of &#34;superior&#34; mineral, &#34;inferior&#34; mineral, or both may be employed. Similarly, the weight of abrasive grain applied in each layer of a multiple-coated product can be varied; further, more than two mineral layers may be applied.