Patent Publication Number: US-6220536-B1

Title: Milling machine, method of crushing ore by use of the milling machine, and method of manufacturing the milling machine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a milling machine which produces crushed stones by crushing an arbitrary ore through use of predetermined crushing members stored in a shell main unit, to a method of crushing an ore through use of the milling machine, and to a method of manufacturing the milling machine. 
     2. Description of the Related Art 
     In principle, a shell main unit of a conventional milling machine is formed into the shape of a hollow cylinder. A milling machine has a shell main unit of one of the below-described types intended for improving the efficiency of crushing ore by dropping and rotating grinding members provided in the shell main unit. For example, when the shell main unit is viewed from the side, an ore-feeder portion of the shell main unit is formed into the shape of a hollow cylinder, whereas an outlet portion of the shell main unit is formed into the shape of a hollow truncated cone tapered toward the direction of discharge. Another type of shell main unit has a reverse structure; namely, the ore-feeder portion of the shell main unit is formed into the shape of hollow truncated cone, whilst the outlet portion of the same is formed into the shape of a hollow cylinder. The grinding member is formed into the shape of a sphere or a deformed rectangular polyhedron. 
     The efficiency of crushing ore has been known to be improved by uniform and balanced distribution of grinding members within the shell main unit. The reason for this is that uniform distribution of grinding members results in crushing action uniformly acting on ore, thereby enabling excessive crushing of ore and energy loss. Since the amount of crushing energy to be dissipated is proportional to the size of a substance to be crushed, crushing action is uniformly exerted on ore by uniform distribution of crushing members, thereby improving the quality of crushed stones. 
     However, in the milling machine which has conventionally been utilized, the shape of the milling machine makes attainment of uniform distribution of crushing members within the shell main unit difficult. For this reason, the conventional milling machine experiences difficulty in greatly improving the efficiency of crushing ore and is apt to cause excessive crushing of ore. 
     SUMMARY OF THE INVENTION 
     Accordingly, the object of the present invention is to provide a milling machine which enables an improvement in an efficiency of crushing ore through uniform distribution of grinding members within a shell main unit and an improvement in the quality of crushed stones by prevention of excessive crushing of ore. 
     More specifically, according to a first aspect of the present invention, there is provided a milling machine for crushing ore into crushed stones comprising: 
     a hollow shell main unit which rotates about a rotation axis, wherein the shell main unit further includes 
     a first hollow cylindrical section which is disposed on an ore supply side of the shell main unit and is tapered such that the inner diameter thereof becomes greater toward the ore supply side, and 
     a second hollow cylindrical section which is disposed on the ore outlet-port side, is tapered such that the inner diameter thereof becomes greater toward the ore supply side, and has an internal space continuing from the first cylindrical section. 
     In this milling machine, the shell main unit assumes the foregoing shape, grinding members can be uniformly distributed within the entire shell main unit. Accordingly, the efficiency of crushing the ore can be improved to a much greater extent, thereby enabling a reduction in energy loss. 
     Preferably, the cone angle of the second cylindrical section is greater than the cone angle of the first cylindrical section. Since the first cylindrical section is tapered, the grinding members can be prevented from accumulating in a specific location of the first cylindrical section. Since the cone angle of the second cylindrical section is greater than that of the first cylindrical section, crushed stone and the grinding members are conveyed to the crushed-stone-outlet side through utilization of a difference in peripheral speed arising from rotation of the shell main unit, thereby enabling effective output of the ore to the outside. Accordingly, ore to be conveyed is prevented from accumulating in a connection section between the first cylindrical section and the second cylindrical section, thereby enabling uniform arrangement of the grinding members. 
     Preferably, the cone angle of the first cylindrical section is very small, and the cone angle of the second cylindrical section is significantly larger than that of the first cylindrical section. As a result, the ore and the milling members can be distributed more uniformly within the entire shell main unit. In other words, since the first cylindrical section assumes a very small cone angle, the grinding members can be uniformly distributed without accumulating in a specific location. Since the cone angle of the second cylindrical section is made so as to become significantly larger than that of the first cylindrical section, the crushed ore and the grinding members are conveyed to the outlet side through utilization of a difference in peripheral speed arising from rotation of the shell main unit, thereby effectively outputting the ore to the outside. Accordingly, the ore to be conveyed is prevented from accumulating in a connection section between the first cylindrical section and the second cylindrical section, thereby rendering the grinding members more uniform. 
     The large grinding members and the large pieces of ore concentrate in the first cylindrical section. In the second cylindrical section, the grinding members and the pieces of ore are uniformly arranged so as to become smaller in diameter toward the crushed-stone-outlet side. Therefore, the first cylindrical section provides the grinding members and the ore with the maximum drop and peripheral speed. Since the ore is crushed by the grinding members of large diameters, the ore is crushed with maximum physical impact. In the second cylindrical section, the grinding members and the pieces of ore become gradually smaller in drop and peripheral speed toward the crushed-stone-outlet side. The ore is crushed by the grinding members having smaller diameters, thereby preventing excessive crushing of the ore and resulting in an improvement in quality of crushed stones. 
     Preferably, the longitudinal center of the shell main unit resides in the first cylindrical section. As a result of the cylindrical section tapered with a very small cone angle being made longer than the cylindrical section tapered with a significantly large cone angle, the ore and the grinding members are prevented from accumulating in the vicinity of the exit of the shell main unit, which would otherwise be caused by an excessive increase in the efficiency of discharge. 
     Preferably, the ratio between the cone angle of the first cylindrical section and the cone angle of the second cylindrical section and the ratio between the axial length of the first cylindrical section and the axial length of the second cylindrical section are set such that the grinding members achieve a substantially uniform distribution within the shell main unit. Consequently, uniform distribution of the grinding members within the shell main unit enables improvement in the efficiency of crushing ore, as well as improvement in the quality of crushed stone. 
     Preferably, a liner is provided on the internal wall surface of the first and second cylindrical sections in the shell main unit. As a result, the efficiency of crushing ore can be improved in association with uniform distribution of the grinding members within the shell main unit. 
     Preferably, the milling machine further comprising: an ore storage section for storing ore which is to be fed; an ore conveying section for conveying the ore stored in the ore storage section to the shell main unit; at least a pair of outer ring members provided around the outer periphery of the shell main unit; and a drive apparatus for rotating the shell main unit. 
     In this milling machine, the ore fed from the ore storage section is supplied into the shell main unit by way of the ore conveying section, and the shell main unit is rotated by the drive apparatus by way of the outer ring members. 
     Preferably, a crushed-stone outlet port is provided on the end of the crushed stone outlet side portion of the second cylindrical section, and a partition plate is provided so as to become spaced away from the crushed-stone outlet port by a given gap. Consequently, uncrushed stone or the grinding members of large diameters can be prevented from outputting from the shell main unit. 
     According to a second aspect of the present invention, there is provided a method of crushing ore through use of a milling machine that comprises a hollow shell main unit made by continuously joining together a first cylindrical section—which is provided on an ore supply side and is tapered with a very small cone angle so as to have a larger inner diameter toward the ore supply side—and a second cylindrical section which is provided on a crushed stone outlet side and is tapered with a cone angle significantly greater than that of the first cylindrical section such that the inner diameter of the second cylindrical section becomes smaller toward the crushed stone outlet side, the method comprising the steps of: 
     feeding ore and grinding members into the shell main unit and rotating the shell main unit; 
     uniformly distributing the grinding members within the shell main unit through rotation of the shell main unit; and 
     crushing the ore to crushed stone of predetermined size through rotation and drop of the grinding members. 
     Under the crushing method by use of the milling machine, the grinding members are actively and uniformly distributed by means of the shape of the first cylindrical section tapered with a very small cone angle and the shape of the second cylindrical section tapered with a significantly large cone angle, thereby enabling an improvement in the efficiency of crushing ore with the grinding members. 
     According to a third aspect of the present invention, there is provided a method of manufacturing a milling machine having a shell main unit, wherein the hollow shell main unit is manufactured by continuously joining together a first cylindrical section—which is provided on an ore supply side and is tapered with a very small cone angle so as to have a larger inner diameter toward the ore supply side—and a second cylindrical section—which is provided on a crushed stone outlet side and is tapered with a cone angle significantly greater than that of the first cylindrical section such that the inner diameter of the second cylindrical section becomes smaller toward the crushed stone outlet side—through setting of a ratio of cone angle between the first cylindrical section and the second cylindrical section and a ratio of axial length between the first cylindrical section and the second cylindrical section such that the grinding members are uniformly distributed within the shell main unit. 
     As a result, in a case where ore is crushed by use of the milling machine manufactured by the foregoing manufacturing method, the efficiency of crushing ore can be improved by uniform distribution of the grinding members within the shell main unit, thereby preventing excessive crushing of ore and enabling improvement in the quality of crushed stone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and forming a part of the specification, illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings: 
     FIG. 1 is an external view showing a milling machine according to one embodiment of the present invention; 
     FIG. 2 is a view for illustrating the principle elements of an ore feeder portion; 
     FIG. 3 is a cross-sectional view showing the structure of a shell main unit, outer ring members and a classifier; 
     FIG. 4 is an end view showing the cross section of the shell main unit taken along line X—X shown in FIG. 3; 
     FIG. 5 is a side view showing the structure of a partition plate; 
     FIG. 6 is a cross-sectional view showing the principle elements of the partition plate and a classifier; 
     FIG. 7 is a view for illustrating a drop of grinding members during the operation of the milling machine; 
     FIG. 8 is a view for illustrating the distribution of the grinding members during the operation of the milling machine; and 
     FIG. 9 is a view for illustrating the operation of the classifier. 
    
    
     Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will now be described by reference to the accompanying drawings. 
     As shown in FIG. 1, a milling machine A 1  according to a first embodiment comprises a hopper B 10  serving as an ore storage section; a shell main unit C 1 ; outer ring members D 1 ; a partition plate E 1 ; a classifier F 1 ; and a drive unit G 1 . 
     As shown in FIG. 2, the hopper B 10  assumes the shape of a vessel, and a chute B 20  is formed at the lower end of the side surface of the hopper B 10 . The chute B 20  has an outlet port for ore. When ore Q 1  is fed into the hopper B 10  from the outside, the ore Q 1  is fed into an ore supply section C 10  of the shell body C 1 , which will be described later, by way of the chute  20  in metered amounts. The chute B 20  acts as an ore delivery section. 
     As shown in FIG. 3, the shell main unit C 1  comprises the ore supply section C 10 , a first cylindrical section C 20 , and a second cylindrical section C 30 . 
     The ore supply section C 10  is substantially formed into the shape of a disk, and an ore inlet port C 12  is formed in the center of the ore supply section C 10  for the purpose of feeding of the ore Q 1 . The portion of the ore supply section C 10  where the ore inlet port C 12  is formed protrudes to the outside in a tapered manner. The ore supply section C 10  serves as a side wall of an ore-supply side portion of the shell main unit C 1 . The chute B 20  is fitted to the ore inlet port C 12 , and the ore Q 1  is fed into the shell main unit C 1 . 
     Specifically, the ore supply section C 10  has an outer wall section C 11  and a liner C 14 , and the outer wall section C 11  substantially assumes the shape of a disk. An opening where the ore inlet port C 12  will be formed is formed in the center of the outer wall section C 11 , and the area of the ore supply section C 10  where the opening is formed protrudes toward the outside in a tapered manner. 
     A collector section B 30  is disposed below the ore feed portion C 12  for the purpose of collecting overflowing ore or water. 
     The liner C 14  is formed from metal or rubber over the internal wall surface of the shell main unit, i.e., the inner surface of the outer wall section C 11 . The liner C 14  is uniformly formed so as to extend from the inner periphery of the outer wall section C 11  up to the internal wall surface of the ore inlet port C 12 . 
     As shown in FIGS. 3 and 4, the first cylindrical section C 20  substantially assumes a cylindrical shape and is positioned in the vicinity of the ore supply section in the longitudinal direction of the shell main unit C 1 . The first cylindrical section C 20  is actually formed into the shape of a truncated cone which is tapered with a very small cone angle (not shown) such that the inner diameter of the first cylindrical section C 20  becomes slightly greater toward the ore supply section. More specifically, the first cylindrical section C 20  is formed such that an inner diameter S 2  of the outlet-side portion thereof becomes smaller than an inner diameter S 1  of the supply-side portion of the same. Here, the cone angle of the tapered first cylindrical section C 20  corresponds to the angle which the outer surface of the first cylindrical section C 20  forms with a plane surface passing through the rotation axis of the first cylindrical section C 20 . 
     In short, the first cylindrical section C 20  comprises an outer wall section C 21  and a liner C 22 , and the outer wall section C 21  substantially assumes the shape of a cylinder and is formed into a truncated cone which is tapered by a very small cone angle such that the inner diameter thereof becomes greater toward the supply side portion of the outer wall section C 21 . 
     As shown in FIG. 4, the liner C 14  is provided on an internal wall surface serving as the interior of the shell main unit, i.e., the inner surface of the outer wall section C 21 . More specifically, a plurality of substantially rectangular plane liners C 22  formed from metal or rubber are uniformly laid on the internal surface of the first cylindrical section C 20 . As shown in FIG. 4, a one-half or more of each liner C 22  is formed into a substantially oval-shaped protuberance, and the first cylindrical section C 20  is formed so as to assume a longitudinal dimension T 1 , as shown in FIG.  3 . 
     In a case where the first cylindrical section C 20  assumes an internal diameter of about 2000 mm and cone angle α is defined as α=(S 1 −S 2 )/T 1 , the first cylindrical section C 20  assumes a very small cone angle of 0.01≦α≦0.03 or thereabouts, which through experimentation has been found to be preferable. 
     As shown in FIG. 3, the second cylindrical section C 30  is substantially formed into the shape of truncated cone and is provided in the vicinity of a crushed stone outlet side of the shell main unit C 1  in its longitudinal direction. The second cylindrical section C 30  is formed in such a way as to assume a smaller internal diameter toward its outlet side and is sharply tapered with a cone angle greater than that of the first cylindrical section C 20 . In short, the second cylindrical section C 30  is formed such that the inner diameter S 3  of the outlet side thereof becomes sharply smaller than the inner diameter S 2  of the supply-side portion of the same, as will be described later. 
     Specifically, the second cylindrical section C 30  comprises an outer wall section C 31  and a liner C 32 . The outer wall section C 31  is formed such that the inner diameter thereof becomes smaller toward the outlet side and is formed into the shape of a truncated cone whose cone angle is greater than that of the first cylindrical section C 20 . 
     Similar to the case of the first cylindrical section C 20  as shown in FIG. 4, in the second cylindrical section C 30  the liner C 32  is provided on the internal wall surface serving as the interior of the shell main unit, i.e., the internal surface of the outer wall section C 31 . More specifically, a plurality of substantially rectangular plane liners C 32  formed of metal or rubber are uniformly laid on the internal surface of the second cylindrical section C 30 . The liner C 32  assumes the same cross section as that of the liner C 22 , as shown in FIG. 7, 
     As shown in FIG. 3, the second cylindrical section C 30  is formed so as to assume a longitudinal length of T 2 , and a crushed-stone outlet port C 40  is formed in the crushed-stone-outlet-side portion of the second cylindrical section C 30 . 
     In a case where crushed stones assume a size of several millimeters or thereabout, the second cylindrical section C 30  assumes a sharp cone angle θ, shown in FIG. 3, of 30°≦θ≦50° or thereabouts, which through experimentation has been found to be preferable. The angle θ corresponds to the cone angle of the second cylindrical section C 30 . Here, the cone angle of the tapered second cylindrical section C 30  corresponds to the angle which the outer surface of the second cylindrical section C 30  forms with a plane surface passing through the rotation axis of the second cylindrical section C 30 . 
     The first cylindrical section C 20  and the second cylindrical section C 30  are connected together seamlessly, thereby constituting the hollow shell main unit C 1 . The axial length T 1  of the first cylindrical section C 20  is set as to be longer than the axial length T 2  of the second cylindrical section C 30 . 
     Given that γ=T 2 /(T 1 +T 2 ), a preferred ratio between the length T 1  and the length T 2  is 0.32≦γ≦0.39 or thereabouts, which through experimentation has been found to be preferable. 
     As shown in FIG. 3, the longitudinal center W of the shell main unit C 1  resides in the first cylindrical section C 20 . 
     As shown in FIGS. 1 and 3, each of the pair of outer ring members D 1  assumes the shape of a substantially circular strip and is provided along and integrally with the outer peripheral surface of the shell main unit C 1 ; more particularly, one of the outer members D 1  is provided in the vicinity of the ore supply section of the shell main unit C 1  and the other is provided in the vicinity of the outlet side of the same. The pair of outer ring members D 1  is provided on a group of tire sets G 5 , and each tire set G 5  comprises two tires G 10 . Although FIG. 1 shows only two tires sets G 5  provided on the front side of the milling machine A 1 , another group of tires GI is provided on the back side of the milling machine A 1  behind the outer ring members D 1 . Since the outer ring members D 1  remain in pressed contact with the group of tires G 5 , the outer ring members D 1  are also rotated in conjunction with the tires G 5  when the group of tires G 5  is rotated by a drive unit G 1 . 
     As shown in FIG. 5, the partition plate E 1  is formed substantially into the shape of a disk and is disposed so as to become spaced from the crushed-stone outlet port C 40  of the shell main unit C 1  by only a gap U, as shown in FIG.  6 . Here, the partition plate E 1  in FIG. 6 is drawn in accordance with a cross-sectional view taken along line Y—Y shown in FIG.  5 . 
     As shown in FIGS. 5 and 6, an outer diametrical portion of the partition plate E 1  is formed by joining together five fan-shaped slit members E 10  which are segmented with respect to the center of the partition plate E 1 . The five slit members E 10  are fixed together through use of joint members E 12  through welding. An inner diametrical portion of the partition plate E 1  is formed from a single disk-shaped gravel-stop member E 20 , and joint members E 22  are fixed to the gravel-stop member E 20  by welding. 
     As shown in FIGS. 5 and 6, brackets C 34  protruding from the edge of the second cylindrical section C 30  are fastened to the joint members E 12  by means of bolts E 40 , and the joint members E 12  are fastened to the joint members E 22  by means of bolts E 50 . 
     As shown in FIG. 6, each of the brackets C 34  is fastened to the corresponding joint member E 12  by way of an elongated hole C 34   a  formed in the bracket C 34 . Therefore, the position of the partition plate E 1  can be adjusted in the axial direction of the shell main unit C 1 . As shown in FIG. 6, the elongated hole C 34   a  is formed so as to be longer in the axial direction of the shell main unit C 1 . As a result, the gap U between the crushed-stone out let port C 40  can be changed in accordance with the volume of ore Q 1  to be fed into the milling machine A 1  as well as with the size of crushed stones R 1  to be outputted. 
     As shown in FIG. 5, a plurality of grinding member stop slits E 14  are formed in each of the slit members E 10 , and a plurality of gravel-stop holes E 24  are formed in the gravel-stop member E 20 . Furthermore, an opening E 30  of substantially circular shape is formed in the gravel-stop member E 20 . Preferably, the grinding member stop slits E 14  and the gravel-stop holes E 24  are formed so as to become tapered such that the holes and slits have a greater diameter at their outlet-side portions than at their diameter in their shell-main-unit-side portions. Even if crushed stones or debris enter the grinding member stop slits E 14  or the gravel-stop holes E 24 , they will be readily released. 
     As shown in FIG. 6, the classifier F 1  is substantially formed into the overall shape of a cylinder, and the outer periphery of the classifier F 1  is formed into a cylindrical member F 10 . The cylindrical member F 10  is fastened to the shell main unit C 1  by means of bolts and rotates simultaneous with rotation of the shell main unit C 1 . A screen member F 20  is provided along the outer periphery of the cylindrical member F 10  while being divided in three segments in the axial direction thereof and permits selective passage of only crushed stone of a certain particle size. As shown in FIG. 6, an opening not having a screen is formed in a forward end section F 30  of the cylindrical body F 10 . The opening allows discharge of crushed stones which is of greater than a certain particle size and cannot pass through the screen member F 20 . The classifier F 1  has a capability of classifying crushed stone which is of certain particle size and can pass through the screen member F 20  and crushed stone which is of greater than a certain particle size and cannot pass through the screen member F 20 . 
     The drive unit G 1  comprises a motor and gears and is arranged so as to transmit torque to the group of tires G 5 . 
     The operation and advantageous results of the present invention will now be described. 
     As shown in FIG. 2, grinding members P 1 —which comprise a mixture of different sized grinding members and are each formed into the shape of a sphere—are housed in the shell main unit C 1 . The ore Q 1  fed from the hopper B 10  is supplied in metered amounts into the shell main unit C 1  along a slope of the chute B 20  by way of the ore inlet port C 12 . In a case where the ore is fed with water, a predetermined amount of water is also supplied to the shell main unit C 1  together with the ore Q 1 . 
     By force of gravity, the ore Q 1  is supplied from the hopper B 10  and the chute B 20  to the shell main unit C 1 . Therefore, the milling machine A 1  does not require an ore supply apparatus which forcefully supplies ore into the shell main unit C 1  by means of a commonly-employed drive unit. 
     As shown in FIG. 3, since the crushed stone outlet port C 40  is sufficiently larger in diameter than the ore inlet port C 12 , the ore Q 1  is smoothly conveyed and discharged and does not accumulate in the vicinity of the ore supply section C 10 . Therefore, there is no need to squeeze the ore Q 1  into the ore supply section C 10 . Since the milling machine A 1  does not need the foregoing forceful ore supply apparatus, the milling machine A 1  can accordingly be formed into a simple and inexpensive structure. Alternatively, the milling machine may be formed by use of a forceful ore supply apparatus such as that mentioned previously. In this case, even when a large volume of ore is fed into the milling machine A 1 , the ore can be smoothly fed and discharged. 
     As shown in FIG. 7, when the shell main unit C 1  is rotated by the drive unit G 1  by way of the tires G 10  and the outer ring members D 1 , the grinding members P 1  are raised by action of the plurality of liners C 22 . When the liner C 22  is raised to such an angle in relation to the horizontal plane thereof so as to be unable to hold the grinding members P 1 , the grinding members P 1  are dropped as shown in the drawing. As a result, the ore Q 1  positioned directly below the grinding member P 1  is crushed by means of the grinding members P 1 . 
     As mentioned previously, the first cylindrical section C 20  is tapered with a very small cone angle, and the second cylindrical section C 30  is tapered with a cone angle greater than that of the first cylindrical section C 20 . Further, the axial center of the shell main unit C 1  resides in the first cylindrical section C 20 . As shown in FIG. 8, the grinding members P 1  are distributed uniformly (in a horizontal direction) within the shell main unit C 1 . The reason for this is will now be described. 
     The movement of the grinding members P 1  is pursuant to the basic principle of a milling machine; namely, larger grinding members move toward an opening having a large diameter, and smaller grinding members move toward an opening having a small diameter. As mentioned above, the first cylindrical section C 20  is tapered with a very small cone angle such that the inner diameter of the first cylindrical section C 20  becomes greater toward the ore supply section. Therefore, the grinding members P 1  do not move toward any direction because of variations in the diameter of the grinding members P 1 . If the first cylindrical section C 20  is tapered with a large cone angle, the grinding members P 1  having large diameters concentrate in an area designated by P 10  shown in FIG. 8 (i.e., the side of the shell main unit Q 1  into which the ore Q 1  is fed), whereas the grinding members P 1  having smaller diameters concentrate in an area designated by P 20  shown in FIG. 8 (i.e., the center of the shell main unit C 1 ). Such concentration of grinding members in any location does not occur in the present embodiment. 
     Conversely, if the first cylindrical section C 20  is not tapered at all, there is a reduction in the effect of discharging the ore Q 1  toward the outlet side by means of a difference in circumferential speed. As a result, the ore Q 1  accumulates in the position designated by P 10  shown in FIG.  8 . 
     Accordingly, the first cylindrical section C 20  is set so as to be tapered with a very small cone angle, thereby enabling uniform distribution of the grinding members P 1 . For this reason, the efficiency of crushing the ore Q 1  through rotation and drop of the grinding members P 1  can be improved. 
     The very small cone angle of the first cylindrical section C 20  is determined according to the material, size, shape, and volume of the grinding members P 1  fed into the milling machine A 1 , as required. 
     The second cylindrical section C 30  is tapered toward the outlet side with a cone angle larger than that of the first cylindrical section C 20 . With this configuration, the tapered section having a large cone angle causes a great difference in circumferential speed, and therefore the ore Q 1  conveyed from the first cylindrical section C 20  can be sufficiently discharged to the crushed-stone outlet port C 40 . Therefore, the ore Q 1  and the grinding members P 1  are prevented from accumulating in the area designated by P 20  shown in FIG. 8, which would otherwise be caused by insufficient conveyance. As a result of the second cylindrical section C 30  being tapered with a cone angle greater than that of the first cylindrical section C 20 , the distribution of the grinding members P 1  can be made uniform, thereby enabling an improvement in the efficiency of crushing the ore Q 1  through rotation and drop of the grinding members P 1 . 
     The cone angle of the second cylindrical section C 30  is determined according to the material, size, shape, and volume of the grinding members P 1  fed into the milling machine A 1 , as required. 
     The axial center W of the shell main unit C 1  resides in the first cylindrical section C 20 , and the first cylindrical section C 20  is longer than the second cylindrical section C 30 . Accordingly, the portion of the first cylindrical section C 20  tapered with a very small cone angle is longer than the portion of the second cylindrical section C 30  tapered with a large cone angle. For this reason, neither the ore Q 1  nor the grinding members P 1  accumulate in the vicinity of the crushed-stone outlet port C 40 . In other words, the ore Q 1  and the grinding members P 1  are prevented from accumulating in an area designated by P 30  shown in FIG. 8 (i.e., in the vicinity of the crushed-stone outlet port C 40 ), which would otherwise be caused by an excessive increase in the efficiency of discharge. 
     As mentioned above, by means of the ratio (or balance) among the very small cone angle of the first cylindrical section C 20 , the large cone angle of the second cylindrical section C 30 , the length of the first cylindrical section C 20 , and the length of the second cylindrical section C 30 , the grinding members P 1  can be uniformly distributed within the shell main unit C 1 . As a result, the efficiency of crushing the ore Q 1  through rotation and drop of the grinding members P 1  can be improved, thereby enabling a reduction in energy loss. 
     The grinding members P 1  and the pieces of ore Q 1  having the largest diameters concentrate in the first cylindrical section C 20 . Further, in the tapered portion of the second cylindrical section C 30 , the grinding members P 1  and the pieces of ore Q 1  are uniformly arranged so as to become smaller in diameter toward the crushed-stone outlet port C 40 . Therefore, the first cylindrical section C 20  provides the grinding members P 1  and the ore Q 1  with the maximum drop and peripheral speed. Since the ore Q 1  is crushed by the grinding members P 1  of large diameters, the ore Q 1  is crushed with maximum physical impact. In the second cylindrical section C 30 , the grinding members P 1  and the pieces of ore Q 1  become gradually smaller in drop and peripheral speed toward the crushed-stone outlet port C 40 . The ore Q 1  is crushed by the grinding members P 1  having smaller diameters, thereby preventing excessive crushing of the ore and resulting in an improvement in quality of crushed stones. 
     The ore Q 1  is efficiently crushed into crushed stones R 1  of predetermined size by the uniformly-distributed grinding members P 1 . The thus-crushed stone R 1  is discharged to the classifier F 1  in direction of arrow Ua shown in FIG. 9 from the gap U between the crushed-stone outlet port C 40  and the partition plate E 1 . 
     In a case where a large volume of ore Q 1  is fed into the milling machine A 1  and where the crushed stone R 1  is discharged in large amounts, the crushed stone R 1  is discharged to the classifier F 1 , as designated by arrow E 14   a  shown in FIG. 9, even from the grinding member stop slits E 14  formed in each of the slit member E 10 . If the size of the grinding member stop slits E 14  is set to a predetermined value or smaller, the grinding members P 1  are prevented from being discharged from the grinding member stop slits E 14  and are retained. 
     When the volume of ore Q 1  fed into the milling machine A 1  becomes greater than the foregoing volume, the crushed stone is discharged to the classifier F 1 , as designated by arrow E 24   a  shown in FIG. 9, even from the gravel-stop holes  24  formed in the gravel-stop member  20 . If the size of the gravel-stop member E 24  is set to or smaller than a predetermined value, the uncrushed pieces of ore Q 1  and the grinding members P 1  are prevented from being discharged from the gravel-stop holes E 24  and are retained. 
     The opening E 30  formed in the inner diametrical portion of the gravel-stop hole E 24  is used as a drain, as indicated by arrow of E 30   a  shown in FIG. 9, in the event of the gap U, the grinding member stop slits E 14 , or the gravel-stop holes E 24  becoming clogged or in the event of an excessive amount of water being fed into the milling machine A 1 . Further, the opening E 30  is also used as an observation window for observing the shell main unit C 1  from the outside. In normal operations, the opening E 30  is not used for discharging the crushed stones R 1 . 
     The gap U is in principle formed so as to be greater than the width of the grinding member stop slit E 14  or the width of the gravel-stop slit E 20 . Consequently, the pieces of crushed stone R 1  larger than the gap U are prevented from being discharged from the shell main unit C 1 . 
     The classifier F 1  classifies the crushed stones R 1  conveyed from the second cylindrical section c 30  into crushed stone R 10  which can pass through the screen member f 20  and crushed stone R 20  which is greater in particle size than the crushed stone R 10 . 
     In the previously-described milling machine A 1  according to the present embodiment, the first cylindrical section C 20  is tapered with a very small cone angle, and the second cylindrical section C 30  is tapered with a cone angle far greater than that of the first cylindrical section C 20 . With this structure, the very small cone angle of the first cylindrical section C 20  enables uniform distribution of the grinding members P 1  and the ore Q 1  without involvement of accumulation of the grinding members P 1  and the ore Q 1 . Further, since the second cylindrical section C 30  is tapered with a great cone angle, the ore Q 1  crushed in the shell main unit C 1  and the grinding members P 1  are conveyed to the outlet-port side in the shell main unit C 1  by utilization of a difference in peripheral speed arising from rotation of the shell main unit C 1 , thereby effectively discharging the ore Q 1  to the outside. More specifically, the ore Q 1  that is conveyed from the first cylindrical section C 20  to the second cylindrical section C 30  is prevented from accumulating in a connection path between the first cylindrical section C 20  and the second cylindrical section C 30 , thereby rendering the grinding members P 1  more uniform. As a result, the efficiency of crushing the ore Q 1  through rotation and drop of the grinding members P 1  can be improved, and therefore energy loss can be diminished. 
     The grinding members P 1  and the pieces of ore Q 1  having the largest diameter concentrate in the first cylindrical section C 20 . Further, in the tapered portion of the second cylindrical section C 30 , the grinding members P 1  and the pieces of ore Q 1  are uniformly arranged so as to become smaller in diameter toward the crushed-stone outlet port C 40 . Therefore, the first cylindrical section C 20  provides the grinding members P 1  and the ore Q 1  with maximum drop and peripheral speed. Since the ore Q 1  is crushed by the grinding members P 1  having large diameters, the pieces of ore Q 1  are crushed with the maximum physical impact. In the second cylindrical section C 30 , the grinding members P 1  and the ore Q 1  become gradually smaller in drop and peripheral speed toward the crushed-stone outlet port C 40 . The ore Q 1  is crushed by the grinding members P 1  having smaller diameters, thereby preventing excessive crushing of the ore and resulting in an improvement in quality of crushed stones. 
     In manufacturing the milling machine A 1 , the milling machine A 1  is set so as to attain an optimum ratio between the cone angle of the first cylindrical section C 20  and the second cylindrical section C 30  and an optimum ratio between the length of the first cylindrical section C 20  and the length of the second cylindrical section C 30 , in order to uniformly distribute the grinding members P 1  within the shell main unit C 1 . Therefore, uniform distribution of the grinding members P 1  can be realized more actively, and the efficiency of crushing the ore Q 1  by means of the grinding members P 1  can be improved. 
     As mentioned previously, the very small cone angle of the first cylindrical section C 20  and the large cone angle of the second cylindrical section C 30  depend on various elements such as (1) the inner diameter of the first cylindrical section C 20  and that of the second cylindrical section C 30 ; (2) a length ratio between the first cylindrical section C 20  and the second cylindrical section C 30 ; (3) the material, size, shape, and volume of the grinding members P 1  to be used; (4) the material, size, shape, and volume of ore to be crushed; (5) the performance of the liner to be used; and (6) the rotation speed of the shell main unit C 1 . Therefore, the cone angles of the cylindrical sections C 20  and C 30  cannot be calculated directly. 
     For these reasons, according to (1) the preset inner diameters of the first and second cylindrical sections C 20  and C 30 ; (2) the material, size, shape, and volume of the grinding members P 1  to be used; (3) the material, size, shape, and volume of ore to be crushed; (4) the performance of the liner to be used; and (5) the rotation speed of the shell main unit C 1 , the ratio of cone angle between the first cylindrical section C 20  and the second cylindrical section C 30  is set such that the grinding members P 1  are uniformly distributed within the shell main unit C 1 , in the manner as mentioned previously. Further, the length ratio between the first cylindrical section C 20  and the second cylindrical section C 30  is set such that the grinding members P 1  are uniformly distributed within the shell main unit C 1 . Thus, the cone angles and the lengths of the first and second cylindrical sections C 20  and C 30  are selected through balancing (or tuning), as required. 
     Although each of the grinding members P 1  assumes the shape of a sphere in the foregoing description, the present invention is not limited solely to that shape. A grinding member of arbitrary shape, size, and material, such as a deformed rectangular polyhedron or a regular polyhedron, may also be used as the grinding member, as required, according to the volume, shape, material, and size of ore to be crushed and according to a desired shape, size, and volume of crushed stone. Metal, ceramics, or rubber may preferably be used as the material of the grinding member P 1 . However, the material is not limited solely to these materials, and arbitrary selection and use of another material may also be feasible. 
     The expression “uniform distribution of grinding members” used herein does not signify completely uniform distribution of grinding members but uniform spreading of grinding members without tending to move toward any place, which would otherwise adversely affect the efficiency of crushing ore. Accordingly, even in the present embodiment, it is assumed that grinding members slowly accumulate in the vicinity of a connection section between the first cylindrical section C 20  and the second cylindrical section C 30 , thereby resulting in a slight increase in the thickness of a layer of grinding members. The tendency of the grinding members to move toward any position, which would not affect the efficiency of crushing ore, falls within the scope of the present invention. 
     The term “uniform” used in the description “The grinding members P 1  and the pieces of ore Q 1  having the largest diameter concentrate in the first cylindrical section C 20 . Further, in the tapered portion of the second cylindrical section C 30 , the grinding members P 1  and the pieces of ore Q 1  are arranged so as to become smaller in diameter toward the crushed-stone outlet port C 40 ,” signifies that the large pieces of ore Q 1  are crushed by the large grinding members P 1 , and the small pieces of ore Q 1  are crushed by the small grinding members P 1 . In short, the grinding members and the pieces of ore are uniformly arranged in decreasing order of magnitude, thus preventing a state of irregular imbalance, such as the small pieces of ore Q 1  being crushed by the large grinding members P 1  and the large pieces of ore Q 1  being crushed by the small grinding members P 1 . 
     The shape, size, material, and operating method of individual components according to the present invention may be arbitrarily determined within the extent to which the foregoing object, the foregoing operation, and advantageous result to be described later of the present invention are accomplished. As a matter of course, modifications of these elements shall fall within the scope of the present invention. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principle of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended Claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.