US2003131A - Method and apparatus for manufacturing articles of bonded granules - Google Patents

Description

R. C. BENNER ET AL May 28, 1935.

METHOD AND APPARATUS FOR MANUFACTURING ARTICLES OF BONDED GRANULES Original Filed Feb. 13, 1933 3 SheetsSheet 1 INVENTORS May 28, 1935. R. c. BENNER ET AL 2,003,131

METHOD AND APPARATUS FOR MANUFACTURING ARTICLES 0F BONDED GRANULES Original Filed Feb. 13, 1933 5 Sheets-Sheet 2 Mm. Pans Mm. 61mm MAX P0175 MIN. 67mm! MA/r Pan E Mm. 61mm MAX. GRAIN MIN. PORL' f 5 MAX/MUM P005 (1 MIN/MUM 6mm MAX. P0175 Mm. PURE INVENTORS May 28, 1935. R. c. BENNER El AL 2,003,131

METHOD AND APPARATUS FOR MANUFACTURING ARTICLES OF BONDED GRANULES Original Filed Feb. 15, 1935 5 Sheets-Sheet 3 xv: ii! ill}. 1

INVEITTORS C. 7'- rvwzrn TON Patented May 28, 1935 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR MANUFAC- TURING ARTICLES OF BONDED GRAN- ULES Original application February 13, 1933, Serial No. 656,500. Divided and this application December 14, 1933, Serial No. 702,332

21 Claims.

This invention relates to the manufacture of articles formed of bonded particles, such as abrasive and refractory wheels and shapes. More particularly, our invention is concerned with the manufacture of articles of an abrasive or ceramic nature, according to which a different relation of the particles and the bond therefor is obtained in the resulting articles than has heretofore been obtainable, and according to which articles of this nature having a considerably superior character are produced.

This application is a continuation in-part of our earlier filed application Serial No. 245,659, filed January 10, 1928, and is a division of .our copending application Serial No. 656,500, filed February 13, 1933. p

The invention will hereinafter be described with reference to the manufacture of abrasive articles, particularly abrasive wheels, but this is by way of illustration, and it will be understood that the invention applies equally to the manufacture of articles of a ceramic nature, whether intended for abrasive purposes or not, and where the particular disposition ofthe bond with reference to the individual particles is of the peculiar character sought to be obtained with the present invention.

Heretofore there have been two pricipal methods in use for the forming of abrasive and refractory articles. One of these (called the puddle process) employs a very wet mixture or slip comprizing approximately 20% of water. The other process is known by various names according to the method used in applying pressure, and consists in the molding under pressure of a damp mix containing only about 2.5% of water.

The present invention pertains to that method in which a damp mix is used and molded under pressure, as distinguished from the puddle process. Heretofore the puddle process has been considered to produce articles having the most desirable distribution of bond, grains and pores, but from a commercial standpoint it is less satisfactory because several days time is required for the drying of the article before it can be burned, and the finished articles has to be dressed and cut to shape. In the use of a damp mix, the product has had less desirable characteristics, but the drying time is shortened to a period of a few hours, and the article can be formed in the mold directly to the shape and size desired, so that very little triming or dressing of the molded article is necessary.

By the use of the present invention articles are produced by the damp method, which are superior to those which have heretofore been pro-- duced, and without the loss of time incident to the puddle process and with all of the advantages of the damp mix method.

' If an abrasive or ceramic article be closely ob served three things in the structure of the article can be noticed. If, for instance, the article be an abrasive wheel, the abrasive grainscan be seen, the bond-which cements the abrasive grains together can be seen, and the pore space can be noticed. These three things can be very much more clearly noticed if the article be impregnated with a synthetic resin so that the resin fills the pore spaces, the wheel then baked to set the resin, and then polished.

When the article has been treated in this way, the surface shows- (1) The grains in cross-section,

(2) The bond, which can be readily distin guished from the resin which fills the pore spaces by the contrast in color.

(3) The voids or pores can be discerned readily by reason of the resin which fills them and which contrasts in color from the grain and the bond.

An article made by previously known methods, when treated in this fashion and examined, discloses numerous patches of small area over its entire surface where the grains are very densely clustered and where there is a very sparse amount of bond and a very little area of visible pore space. Other patches will be seen wherein the grains are sparsely distributed, as compared with the first described areas, and there is a predominance of bond or of pore space.

If this article be an abrasive article, this characteristic conglomeration' of cubical sections having a wide variation in the grain, pore, and bond distribution, has a very noticeable effect, which efiects have heretofore been recognized, but could not be avoided. The abrasive grains, of course, perform the grinding operations. The bond. is ordinarily softer, and the pore space determines, with the size of the abrasive grains, the cutting characteristics of the bonded abrasive mass. In those portions of the abrasive article where there are dense clusters of grains, the wheel will be very hard. Where the grains are relatively sparse, and there isa predominance of bond, the action is one where there is little cutting, but in which heat is generated by reason of the friction between the bond and the material being abraded. Where the wheel structure has a predominance of pore space, the wheel is cent the arbor hole.

soft and the cutting action is quite different from where there is a dense clustering of the grains.

If the abrasive article be a wheel, this variation in the character of very small portions of the entire mass shows itself in the character of the work done. If the work is rough work, such as finishing castings, such local irregularities show up as pits and irregular open sections and cause the wheel to vibrate objectionably. If the wheel is for precision work, the work will show chatter marks and slight irregularities when closely examined, and which cannot be avoided. If the article, instead of being an abrasive article, is a refractory article, the non-uniformity of structure also has its effect.

While refractory articles are generally of a low porosity as contrasted with a grinding wheel wherein a high porosity is desirable, non-uniformity of structure results in uneven strains under varying temperature conditions, which strains manifest themselves in a tendency to rupture and crack.

Not only have articles made by the usual methods possessed this characteristic variation between difierent small portions in the same area of the article, but the average distribution of grain and bond and pore space in one area of the article may differ widely from the average in another relatively large section of the article. For instance, if the article he an abrasive wheel, and the wheel is put on a spindle which is mounted to turn very freely, one side of the wheel will invariably be found to be at least somewhat heavier than the other side. This has .been recognized by the industry, and relatively tute a large portion of the grinding art, in which extremely small tolerances for balance have been established, and some of these operations require wheels that are very closely balanced without being corrected by the application of metal adja- The methods used before our invention produced a large portion of wheels in which the condition of balance was such that they had to be rejected for .use in connection with those operations requiring wheels of small tolerances.

In the usual practice of forming articles of this nature, the abrasive grains and the bonding material are very carefully mixed for a long time in agitating mixers so as to secure as complete uniformity in the distribution of the bond as is and to avoid the condition referred to which exists where pressure is applied to a thick mass of a mixture of this nature. We have found, however, that such a method of filling the mold does not result in uniform distribution of the mix in-the mold. The operator cannot, by throwing the material into the mold with a scoop, absolutely distribute evenly the number of scoopfuls which go into any one area; he-cannot rake the material around inthe mold to assure an absolutely even distribution; and finally, he cannot tamp all areas to the same extent and with the same degree of pressure. Moreover, the tamping of one area may tend to disturb the grains in the adjacent areas. It is for these reasons that the best recognized practice prior to the present invention results in a wheel or other article in which there is not only a great probability of an uneven distribution of the material in defierent large sections of the article, but also the very wide distribution of grains, bond and pore space-in adjacent small areas or patches of the article.

According to the present invention, a method and apparatus are provided according to which articles are produced wherein there is consistently a much more uniform distribution of grains, bond and pore space in different large areas of the article and the uniform absence of widely differing distribution of bond, grains and pore space in adjacent small patches or areas of the article, with the result that the waste due to unbalance or non-uniformity is very materially reduced, the operation of balancing, in the case of wheels, is made much less difficult, and the grinding characteristics are comparable to those obtained with an absolute uniformity of pore space, bond and grain.

The invention may be understood by reference to the accompanying drawings, in which:

Figure 1 is a more or less schematic drawin showing one formof apparatus for carrying out the invention;

Figure 2 shows more or less diagrammatically the method used in calculating the grain and pore distribution as applied to an abrasive wheel embodying the present invention;

Figure 3 is a view similar to Fig. 2 showing only part of a wheel, but in which the diagrammatic representation of the variation between grain and bond in different small areas is comparable to that obtaining in good quality wheels made in accordance with the present general practice;

Figure 4 is merely a legend for the explanation of Figs. 2 and 3;

Figure 5 is a typical graph actually plotted against an abrasive wheel formed by a damp mix process in accordance with the best known. methodsprior to the present invention, showing the maxima and minima percentages in grain distribution in diiferent small areas of a wheel;

Figure 6 is a similar view of another wheel showing a characteristic curve;

Figure 7 is a corresponding typical curve for the grain distribution in an abrasive wheel made in accordance with the present invention; and 7 Figures 8, 9 and 10 correspond to Figs. 5, 6, and '7, respectively, except that the curves are plotted on the pore distribution in different small areas instead of the grain distribution.

Referring first to Fig. 1 of the drawings, 2 designates a turntable having a mold thereon. The moldillustrated is for forming a wheel, and has a central post 4 for forming the opening at the center of the wheel, but it will be understood that the mold may be of any suitable or preferred shape. At the top of the mold is a strikeoff ring 5.

Suspended above the mold is a riddle 6 carried by a frame I which is vibrated by any suitable driving means (not shown) at a uniform rate. The area of the riddle is preferably at least 25% larger than the area of the mold. The riddle is provided with a lower screen 8, and there are also preferably provided one or more screens 9 above the lower screen, although these may be dispensed with. The mesh size of the lower screen 8 is preferably less than the mesh size of the upper screen. In the riddle which we prefer to use the meshes of the lowermost screen do not exceed five times the average diameter of the grains and preferably are approximately four 'meshes coarser than the mix to be distributed.

, The height of the riddle above the mold is such that the distance between the lowermost screen and the bottom of the mold is about 18 inches, but this distance can be varied. It is preferable, however, that the riddle shall not be so high that the falling grains will produce any considerable impact when they fall into the mold.

In carrying out the invention, the grain and the bond are mixed in the usual manner in the proportions commonly used, and for a length of time corresponding to the length of time now considered advisable, and until there has been obtained as good distribution of the grain and the bond as canbe obtained by the stirring and agitating of the mix. This mix is then placed in the riddle, being distributed around in the riddle as evenly as possible, and while the riddle is vibrating. The operation of the riddle is of course to sift the mix down into the mold, and the mix falls into the mold in clusters of only av few grains. While the mold-is being filled in this manner, it is preferably rotated on the turntable at a uniform speed. By reason of the fact that the riddle is preferably 25% larger in diameter than the mold, the uniform filling of the mold is insured. When the mold is filled to the top of the strike-off ring, the excess mix is carefully removed, by removing the ring and leveling with a straight edge, the operation being done care-- fully so that the mix remaining in the mold is not disturbed or is not compressed at any point and is of uniform thickness.

After the mold has been leveled off, the mix in the mold is consolidated into the finished article. This is done by pressure in a press having a plunger arranged for cooperation with the mold, and the pressing operation may be done in the con-. ventional or any preferred manner.

By carrying out the process in this manner, the article has a uniformity of distribution of grain, bond and spore space which distinguishes it in appearance, particularly when impregnated with a resin and polished, as hereinbefore described, from articles'of a like nature made by other processes. This quality is due to a number of things. The mix is so deposited in the mold that it is uniformly distributed and this uniform'distribution is not disturbed by subsequent operations. .The maximum increments or clusters which fall into the mold are not in excess of one cubic centimeter, and, if the mesh of the riddle bears the relation to the mesh of the grain above described, these increments or clusters will be smaller. than one cubic centimeter and contain only a relatively few grains as contrasted with a large mass oi many cubic centimeters and containing many thousands of grains which is thrown into the mold when even a very small scoop is used. The riddling action also serves to distribute the bond. over the grain, because in passing through the riddle excess bond is transferred from some of the. grains onto other grains which may be deficient in the amount of bond clinging to them. This is by virtue of the rubbing action of the grain upon the wire produced by the screen of the riddle, causing the excess bond material to be rubbed off one particle and deposited on others.

The uniform vibration of the screen severs at regular intervals the streams of material passing through the screen and thus makes the masses or groups of grains falling into the mold of uniform length as well as of uniform area. These small portions or masses, as explained above, contain relatively fewgrains, and generally do not exceed 15 times the volume of the coarsest grain in the mix. As stated above, the volume would rarely exceed one cubic centimeter, and would.

ordinarily be considerably less than this.

The addition of particles to the mold'at uniform velocity is also of importance, and this is accomplished by suspending the vibrating screen at a definite height above the mold and vibrating it at a uniform speed. The small groups of grains thus .fall into the mold with substantially equal impact, and because of the relatively short distance the impact is relatively slight.

Uniform distribution of'the mix over the mold is secured, and this is produced by having the screen larger than the mold, by keeping the screen covered with mix, and by rotating the mold while it is being filled. r I

By securing uniform distribution of the mix over the surface of the mold, terracing of the mix, which results when it is piled up, is prevented. The step 01 filling the mold to excess and then striking off the excess removes any possibility of there being irregularities in the depth or thickness of the material in the mold. I

The advantages of the present method and apparatus are made more fully apparent by the detailed inspection of the articles made bythe use of the present invention.

In articles made according to the present invention the difference between the average grain content of those areas in which the grain concentration is a minimum, and the average grain content of those areas in which the grain concentration is a maximum is less than 60% of the grain content of an area having a minimum grain concentration, whereas the diiferences in articles made by processes heretofore known have been 100% or greater. Likewise the pore distribution in articles made in accordance with our invention is characteristically more uniform than has been obtainable by methods employed heretofore. The difference between the average pore space of those areas in which pore space is a minimum and the average pore space of those areas in which pore space is a maximum is less than 60% of the pore space of an area having a minimum pore space. On the other hand, the usual methods of manufacture have resulted in articles in which the difference was 100% or Q cordance with the best known prior art practice and of the same grain and'bond proportions and of the same grain size is likewise treated. Then the impregnated surface of each wheel is ground and polished, as hereinbefore described. When the wheels have been thus treated .it is easily possible for one familiar with abrasive wheels to scan the surface and pick out small areas over the surface where the grains are most densely arranged, and to'pick out other small areas where the grains are least densely arranged.

The article under examination is ordinarily divided into a number of areas of equal size. This is indicated in Figs. 2 and 3 where the radial lines divide the surface of the wheel into six segments. The operator, in examining a small wheel, picks one spot having the maximum grain concentration in each segment. He also picks a small area having the maximum pore area in the same segment. This is done in each of the six segments, the areas marked off being the same size in each case. In the usual practice of making such comparisons, the average area of each spot chosen is that covered by not less than 50 nor more than 100 maximum sized grains of the grit of which the article is composed, and

they are ordinarily between one-quarter and onehalf inch square.

Since the number of areas required to give a fair and reasonable average of the maxima and of the minima percentages of each constituent must vary with the area of the wheel and also with the grit size of the abrasive grain used, we have found it desirable to determine the number of observations to make from the formula N-f/ZE in which N is the number of areas to be examined, A is the area in inches of the broadest surfaceof the article, and G is the average mesh size of the abrasive grain.

To illustrate this method let us assume that we wish to know how many areas to measure in a wheel 30 inches in diameter composed of grain ranging in size from those which will pass through a 58 and not through a 60 mesh to the linear inch screen to those which will pass through a 28 mesh screen but which will not pass through a screen with 30 meshes to the linear inch. The area of a 30 inch diameter wheel is 707 square inches and the average mesh size of the particles ranging from 30 to 60 grit is 45. The number of observations to make, therefore, is

for each constituent viz. grain, bond and pore space.

In Figure 2 the small areas outlined and marked a are areas of maximum grain concentration. Those areas marked b are areas in which there is a predominance of pore space. These areas are then examined microscopically under a cross ruled ocular, and in each dense spot examined the relative area occupied by grains is determined, and in each porous spot the relative area occupied by grains is also determined. Likewise, in each dense spot the relative area of pore space is calculated and in each porous spot the relative pore area is also observed. In a similar manner, the relative areas occupied by bond in each of the spots is determined. Average values for the grain concentration in the dense spots can thus be calculated and average values of the areas of the grains in the spots can be calculated.

In Figure 3 the same procedure is followed and the areas marked a are areas of maximum grain concentration, and the areas marked b are the areas of maximum pore space.

The results may be plotted. graphically. Figs.

5 and 6 are for the results on areas of maximum grain concentration in abrasive wheels made in accordance with the best known prior methods and considered to be of good quality,

whereas Fig. '7 is plotted against a typical wheel corresponding graph for a typical wheel made ,1

in accordance with the present invention.

Taking Fig. 5, for example, the six.points plotted at the top represent the grain concentrations observed in the respective six areas examined. The dotted line marked Average" represents the average grain area for the six points plotted. The lower graphin Fig. 5 shows the grain concentration in the areas of minimum grain concentration, and the dotted line indicates the average. It will be seen that the two lines marked Average for these two curves are separated by 22 Fig. 6 likewise shows a similar curved plotted against another typical wheel of the best prior art construction, the separation between the two lines marked Average being even greater than in Fig. 5. It will be seen that these curves are very abrupt ones, jumping widely from one side of the average line to the other. In Fig. 7 it will be noted that the maximum and minimum average lines are separated by less than 15% and that there is not the wide fluctuation to each side of the average line that obtains in Figs. 5 and 6. Because the average lines of Fig. 7 are much closer together than the average lines of Figs. 5 and 6, Fig. 7 shows that a wheel embodying the present invention has a much more uniform distribution of grains than did the wheels on which the curves for Figs. 5 and 6 were plotted.

The fact thatthe curves in Fig. 7 fluctuate less widely to opposite sides of the respective average lines, and that the average lines are closer together, whereas in Figs. 5 and 6 there is a wide fluctuation of each curve to opposite sides of its average line and the average lines are widely separated, indicates that there is a much morein the difierent small areas examined and also.

a separation'bf approximately 30% between the average line for the maximum and the average line for the minimum. In Fig. 10 the average lines are very much closer together, being separated by about 15%.

As previously noted, an examination of Figs. 5 and 6 shows a very wide fluctuation in the areas, examined from one side of the average line to the other. In other words, the average lines are not only further apart than they are in Fig. 7, but the graphs indicate much wider fluctuations. The same is true in Figs. 8 and 9 with respect to the pore distribution, while I0 shows very little variation to opposite sides of the average lines. An inspection of Figs. 7 and 10 shows that in most cases the variation from one side of the average line to the other is less than 5 and the separation between the average lines is less than 20.

In the illustrations given for normal objects in Figs. 5 and 6 and 8 and 9, the average lines are spaced considerably further apart and indicate that the variation in the small areas for both grain and pore spaces fluctuates more than to each side of the average line.

An article made in accordance with the present invention may be further characterized in this way: That when the pore spaces of the arti- .cle are filled with a substance to make the pore spaces stand out in contrast to the grains and the surface of the article is then polished, the article has a uniformly stippled appearance subour invention and one made by the prior art prac-' tice cannot be illustrated readily by the drawings I but it can be detected readily in the wheels themselves by observation.

The differences which have been pointed out above in connection with an article produced by practicing the method herein described and with the use of the apparatus as herein disclosed, clearly show that by the use of the present invention, bonded abrasive articles may be produced which are physically difierent from similar objects as produced 'at the present time by the generally accepted methods and with the generally used apparatus. This physical and structural difference which is effected in the articles through the use of the present invention, manitests itself in abrasive wheels, for instance, for precision grinding, for an abrasive wheel made in accordance with the present invention will produce work'which is noticeably more free of chatter marks than wheels which have heretofore been made.

That the method used is materially different from present methods is evidenced by the fact that in production work the number of rejections 'of wheels because of their being out of balance overa long extendedrun of production has been less than 2%,-whereas it has been the experience that as high as 30%, and always considerably greater than 2%, of the wheels produced by the previously known methods for use in connection with grinding operations requiring close tolerances as to balance, have had to bediscarded for out of balance exceeding the limits set.

scribed in pages 51 and 52 thereof) is disclosed by penetration tests. According to such a test six abrasive wheels made in accordance with the present invention and six made according to Searles method were burned in the same kiln at the same time and under the same conditions of temperature and atmosphere. The same bond was used and the same size of abrasive was used and the abrasives in .both cases were of like quality. The kiln glaze was removed from the surfaces of the wheels after they were fired, and the new surfaces were tested for hardness under the same conditions and by the same methods. This test comprised subjecting the wheels at four places on each face to the impact action of a This difference between the number of times, and then measuring the depth to which the chisel penetrated the wheel. According to this method the difference in the structure of two abrasive wheels, or the difference in structure of two areas in one abrasive wheel, are indicated by the difference in the depth of penetration of the tool; the penetration into an area where the granular material is sparse is greater than the degree of penetration into an area where the grain concentration is the maximum. The results of these tests are given in the following table:

Ratio of maximum penetration to minimum penetration in each. wheel By our method By Sear-1e 's method 110 to 100 135 to 100 116 to 100 138 to 100 117 to 100 140 to 100 118 to 100 146 to 100 120 to 100 148 to 100 This impact penetration test is a standard test used to determine the relative hardness of bonded abrasive articles. The above figures show that wheels made by the Searle method are not as uniform as those made by the present process, since the maximum penetration in a given wheel is from 35% to 48% greater than the minimum penetration in the same wheel; whereas the maximum penetration is only to great--' er than the penetration in a wheel made in accordance with our invention. This illustrates that by using the same materials and subjecting them to the same conditions, a wheel embodying the present invention has a physical resistance to penetration that is substantially uniform over the surface of the wheel.

The methods of testing as herein described are the best available ones at present known to the abrasive art for determining the character of an abrasive structure. The method of dividing the object into a number of sections and selecting in each section a space of maximum'grain and a space of maximum pore distribution, and then calculating the areas by the use of a cross ruled ocular is similar to the method well-known to geologists in the study and analysis of rock formferred apparatus, it will be understood that changes and modifications may be made therein and variations may be made in the procedure within the contemplation of our invention and under the scope of the following claims.

We claim:

1. The method of making an article of uniform density from a mixof bonded grain which comprises adding the mix to a mold in small uniformly distributed portions not exceeding substantially one cubic centimeter in volume and then applying pressure to the material in the mold simultaneously over the entire surface of the material and before any small areas have been individually tamped orcompacted.

2. The method of making an article of uniform density from a mix or grains and bond which comprises adding the mix to the mold incentimeter in volume until the said mold is filled to excess, striking off from the mold the excess mix above a predetermined plane, and then pressing the mix in the mold.

3. The method of making an article of uniform density from a mix of grain and bonding material which comprises adding the mix to the mold in small uniformly distributed portions not exceeding substantially one cubic centimeter in volume, removing the excess mix above a predetermined plane, and then pressing.

4. The method of making an article of uniform density from a mix of grains and a bond which comprises adding the mix to a mold in uniformly distributed portions not substantially exceeding fifteen times the volume of the coarsest grain, striking off the uniformly distributed mix to a predetermined upper limiting plane, and then pressing.

5. The method of making an article of uniform density from a damp mix of grains and bonding agent which comprises depositing the mix in the mold from a distributing device of the sifter or gyratory riddle type operated above the mold, and subsequently applying pressure to the contents of the mold simultaneously over the entire surface of the material and before any small areas have been individually tamped or compacted.

6. The method of making an article of uniform density from a damp mix of grains and bonding agent which comprises adding the mix to the mold in small uniformly distributed portions not exceeding substantially one cubic centimeter in volume, simultaneously rotating the mold, and subsequently pressing the contents of the mold.

'7. The method of making an article of uniform density from a mix of grains and bonding agent which comprises distributing the mix over an area. greater than and including the mold from a device of the sifter or gyratory riddle type operated above the mold while simultaneouslyv rotating the mold, subsequently striking off the excess material from the mold, and pressing the material.

8. The method of producing an article of uniform density from a mix of grains and a bonding agent which comprises shaking the mix into a mold from a device of the sifter or vibrating riddle type operated above the mold and having a screen whose meshes do not exceed five times the average diameter of the particles of grain in the mix, striking off the excess mix from the mold, and pressing.

9. The method of producing an article of uniform density from a mix of grains and a bond which comprises shaking the mix from a device of the sifter or vibrating riddle type into a rotating mold, striking off the excess material from the mold, and pressing.

10. The method of producing an article of uniform density from a mix of grains and a bonding agent which comprises shaking the mix 'through a screen whose mesh is less than five times the average diameter of the particles of grain in the mix into a mold, striking ofi any excess mix from the mold, and pressing the mix in the mold.

11. The method of making an article of substantially uniform density from a mix of grains and a bond which comprises the steps of adding the mix to the mold in small uniformly distributed portions and at a substantially uniform velocity, striking off excess mix from the mold, and pressing.

12. Apparatus for the manufacture of bonded articles comprising a riddle having a screen, the mesh of the screen not exceeding five times the diameter of the average particles of grain to be passed therethrough, and a mold spaced below the screen of the riddle to receive screened material directly, the area of the riddle exceeding the area of the mold. I

13. Apparatus for producing an article of uniform density comprising a rotatable mold, and a riddle spaced above the mold whose area exceeds the area of the mold, said riddle having a screen therein, the mesh of which does not exceed five times the average diameter of the particles which pass through the riddle into the mold.

14. The method of making an article of uniform density from a mix of grains and bond which comprises shaking the mix uniformly from a uniformly vibrated riddle into a mold while maintaining a substantially even distribution of material in the riddle, striking off excess materialfrom the mold, and pressing.

15. The method of making an article of uniform density which consists in continuously adding to the mold small particles of mix, said particles being added uniformly across the mold until it is filled to provide in the mold a substantially uniform layer, and consolidating the mix.

16. The method of making an article of uniform density which consists in continuously adding to the mold small particles of mix, said particles being added uniformly across the mold until it is filled to a predetermined upper limiting plane, and consolidating the mix, the particles being added to the mold at a substantially uniform velocity.

17. The method of forming an article from a damp mix comprising grains and a bond, which comprises gradually filling an empty mold form to a desireddepth of fullness with the mixture by adding the mixture in small increments of a relatively few grains and not exceeding a cubic centimeter in volume, and maintaining the depth to which the mold is filled uniform throughout its area, and allowing such small increments to settle normally, and then applying pressure to the material in the mold simultaneously over the entire surface of the material and before any small areas have been individually tamped or compacted.

18. The method of making an article of uniform density from a mix of bonded grain which comprises adding the mix to a mold in small uniformly distributed portions not exceeding one cubic centimeter. in volume, striking off excess material to leave a fiat uniformly distributed layer defined by upper and lower substantially parallel surfaces and then applying pressure to the contents of the mold.

19. The method of making an article of uniform density from a mix of bonded grain which comprises adding the mix to a fiat bottom mold in small uniformly distributed portions of comparatively uniform size and not exceeding one cubic centimeter in volume, striking off excess on a fiat bottom from a device of the sifter or vibratory riddle type operated above the mold, striking off the deposited miX to a substantially uniform depth and then pressing.

21. The method of making an article of uniform density from a mix of bonded grain which comprises depositing the mix in a mold cavity from a device ofthe sifter or vibrating riddle type operated above the mold cavity, striking of! excess mix, and pressing the contents of the mold between substantially parallel flat surfaces.

RAYMOND C. BENNER. PRESCOTT H. WALKER. WILLIAM G. SOLEY.

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