Patent Publication Number: US-2022226832-A1

Title: An apparatus for pulverizing material including a stationary housing

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for pulverizing a material, the apparatus comprising at least one sawblade and a plurality of collecting notches for collecting cut material, and a stationary housing having an interior space housing the at least one sawblade. 
     The present invention also relates to a method for producing the stationary housing of the apparatus. 
     BACKGROUND 
     There exists different apparatus for pulverizing material, such as wood, pellets, and plastic. 
     US2014339344A1 discloses apparatus for pulverizing a super absorbent polymer that comprises: an inlet portion for feeding the super absorbent polymer, a pulverizing portion for pulverizing the super absorbent polymer fed through the inlet portion, and an outlet portion for discharging the pulverized super absorbent polymer from the pulverizing portion. The pulverizing portion comprises a rotary drum to which a plurality of rotary blades is attached; at least one fixed blade for pulverizing the super absorbent polymer in cooperation with the plural of rotary blades, and a housing for holding the rotary drum and the at least one fixed blade. 
     WO2007054981A1 discloses an apparatus for cutting raw material, that consists of fixed knives with the shape of sickles and knives on an axis that are attached to teeth-units on a teeth-holder. The teeth-holders enter in between the sickle-shaped knives except uppermost and thus a space for intake is formed where material is drawn in by the knives and is cut between the knives on the axis and the fixed knives. 
     In many applications, the size of the particles in the pulverized material is important. For example, finely pulverized wood powder burns better and dries faster than wood powder including larger particles. Thus, it is desired to control the size of the cut material. 
     U.S. Pat. No. 3,241,776 discloses a comminuting apparatus including a stationary housing defining an interior space and having an interior wall facing the interior space, a plurality of spaced apart sawblades is disposed in the interior space and arranged rotatable with respect to the stationary housing. The sawblades have a periphery provided with cutting teeth for cutting the material and a plurality of collecting notches for collecting the cut material. The interior wall of the housing is provided with a plurality of recesses facing the periphery of the sawblades and arranged to deflect cut material in a radial direction back into contact with the cutting teeth of the sawblades. The apparatus further comprises a sieve including a screen provided with apertures of predetermined size disposed below the sawblades for receiving the cut material and to determine the maximum particle size of the cut material that is permitted to exit from the interior space. A disadvantage with using a screen provided with apertures to determine the size of the cut material is that the apertures can be clogged with the cut material. This is particularly a problem if the cut material is moist. For example, it is difficult to pulverize wood with this apparatus, since wood is often moist. This comminuting apparatus is intended for coarsely grind the material, and is not suitable for pulverization of material into powder of fine grinded material. 
     Further, if wood is to be cut, it is a desire to be able to pulverize a whole log without the need of firstly dividing the log into smaller pieces. 
     SUMMARY 
     An aim of the present invention is to provide an improved apparatus for pulverizing material. 
     This aim is achieved by an apparatus as defined in claim  1 . 
     The apparatus comprises a stationary housing having an interior space and at least one interior wall facing the interior space, at least one sawblade disposed in the interior space and arranged rotatable with respect to the stationary housing in a rotational direction, an inlet opening for feeding material to be pulverized to the interior space, and an outlet opening for dispatching the pulverized material from the interior space. The sawblade is disc-shaped and has a periphery provided with a plurality of cutting teeth for cutting the material and a plurality of collecting notches for collecting the cut material. The interior wall is provided with a plurality of recesses facing the periphery of the sawblade, arranged to receive the cut material from the collecting notches of the sawblade, and to revert the cut material to the periphery of the sawblade so that the material will be cut multiple times before it is dispatched from the apparatus. The interior wall provided with the recesses surrounds a main part of periphery of the at least one sawblade, and the inlet opening and the outlet opening are arranged in the interior wall and in direct communication with the interior space so that the material enters and leaves the interior space during one revolution of the at least one sawblade. 
     With the term that the outlet opening is arranged in direct communication with the interior space of the stationary housing is meant that there is no obstacle, such as sieve or screen provided with apertures, disposed between the outlet opening and the interior space. 
     The sawblade is rotating with a high speed. The cutting teeth of the sawblade will cut the material fed to the interior space through the inlet opening. The collecting notches will collect and house the cut material. In the beginning of a revolution, the cut material is rough. Since the sawblade is rotating with respect to the stationary housing, each of the collecting notches will face each of the recesses of the stationary housing once per revolution of the sawblade. 
     When a collecting notch faces a recess, the cut material in the collecting notch is transferred to the recess by means of the centrifugal force. The cut material travels along the length of the recess. When the particles in the cut material travels in the recess, the speed of the particles is increased until the material reaches the end of the recess, where the speed of the cut material is reduced, and the cut material is redirected by the rear wall of the recess towards the periphery of the sawblade. When the cutting tooth reaches the rear end of the recess, the cut material will be cut a second time and the cut material is collected in the collecting notch. Thus, the size of the material will be reduced. Each time a cutting tooth and a collecting notch pass by one of the recesses during the revolution, the material is cut one more time. The pulverized material leaves the interior space of the housing through the outlet opening after one revolution of the sawblade. 
     The number of times the material is cut depends on the number of cutting teeth and the number of recesses. For example, if the number of recesses is 5 and the number of cutting teeth is 10, the material is cut 5*10=50 times. Thus, the size of the material can be controlled by the number of recesses in the stationary housing, and the number of cutting teeth. 
     The apparatus according to the invention makes it possible to achieve a fine grinded powder having particles of a certain size. The invention makes it possible to omit the sieve in order to control the size of the powder. There is no need of a classifier in order to control the size of the powder, which will reduce the costs for manufacturing the powder. The invention makes it possible to pulverize a wet material, such as wood, which would clog the holes in the sieve. 
     In one aspect, the interior wall including the recesses surrounds at least 60% and preferably at least 70% of the periphery of the at least one sawblade. This makes it possible to increase the number of recesses in the wall, and by that reduce the size of the pulverized material. 
     In one aspect, the at least one sawblade is disposed so that the distance between the cutting teeth and portions of the interior wall located at the rear ends of the recesses is less than 8 mm, preferably less than 4 mm, and most preferably less than 2 mm. The redirected material is cut between the cutting tooth and the interior wall at the rear end of the recess. The smaller distance between the cutting teeth and the interior wall at the rear ends of the recesses the smaller is the size of the cut material. Further, a small distance between the cutting teeth and the interior wall ensures that most of the redirected material is cut another time. 
     In one aspect, the apparatus comprises at least three sawblades put together to form a cutting unity and each of the sawblades abuts against the neighboring sawblade in the cutting unity. Thus, it is prevented that a gap is formed between the sawblades and it is ensured that all material coming into contact with the cutting unity is pulverized. 
     In one aspect, the shape of the recesses, in a cross-section perpendicular to the rotational axis, is tapering in a direction reverse the rotational direction of the at least one sawblade. Thus, the height of the recess in a radial direction is increasing towards the rear end of the recess. This makes it easier to empty the collecting recesses, to reduce the speed of the flow of cut material, and to redirect the flow of cut material. 
     In one aspect, the length of the recesses is larger than the length of the collecting notches in a cross-section perpendicular to the rotational axis. In one aspect, the length of the recesses is at least three times larger than the length of the collecting notches in a cross-section perpendicular to the rotational axis. This gives the cut material time to be transferred to the recess in one end of the recess, and to redirect the cut material towards the tooth in the other end of the recess. 
     In one aspect, each recess has a narrow first end and a wide second end, and the wide second end of the recess has a guiding surface designed to guide the flow of cut material towards the periphery of the at least one sawblade. The second end of the recess has a wall designed to form a guiding surface for directing the flow towards the sawblade. The wall can be designed in different ways, for example, the wall can be bent so that the guiding surface is concave, or the wall can be straight and arranged so that the guiding surface is about perpendicular to the periphery of the sawblade. 
     In one aspect, the guiding surface is concave and the length of the recesses, in a cross-section perpendicular to the rotational axis, is at least twice the bending radius of the guiding surface. Preferably, the guiding surface is concave and the length of the recesses, in a cross-section perpendicular to the rotational axis, is at least 2.5 times larger than the bending radius of the guiding surface. Most preferably, the length of the recesses is at least three times larger than the bending radius of the guiding surface. Due to the fact that the length of the recesses is significantly larger than the bending radius of the second end of the recesses, the particles in the cut material is allowed to accelerate to a high speed before they reach the concave guiding surface at the second end and is redirected towards the cutting tooth of the sawblade. Thus, it is ensured that the cut material reaches the cutting teeth and is cut multiple times. 
     In one aspect, the number of recesses is more than three, and preferably more than five. The number of times the material is cut depends on the number of recesses. If the number of recesses is increased, the number of times the material is cut is increased and accordingly the size of the cut particles is reduced. 
     In one aspect, the at least one sawblade comprises a disc-shaped body and the cutting teeth protrude from an outer edge of the body in a radial direction. The distance the cutting teeth is protruding from the outer edge of the body determines cutting depth of the material. 
     In one aspect, the collecting notches have a front end and rear end with respect to the rotational direction, and the cutting teeth are disposed at the rear ends of the collecting notches. 
     In one aspect, the width of the cutting teeth is larger than the width of the body in an axial direction. This enables a cutting of a wider part of the material. With the term “in an axial direction” is meant along an axis parallel with the rotational axis. 
     In one aspect, the collecting notches are evenly distributed along the periphery of at least one sawblade, the periphery of the at least one sawblade is provided with a plurality of empty notches without any cutting teeth arranged between the collecting notches, the at least one sawblade comprises a first sawblade and a second sawblade rotated in relation to the first sawblade so that the empty notches of one of the sawblades are aligned with the collecting notches of the other sawblade in an axial direction. This makes it possible to cut a whole log without the need of dividing the log into smaller pieces. 
     In one aspect, the interior space of the stationary housing extends along the rotational axis of shaft, and the housing comprises a plurality of plate-shaped pieces having two parallel main sides, wherein the-plate shaped pieces are attached to each other with the main sides facing each other so that that the pieces together form the housing with the interior space. This makes it easier to manufacture the stationary housing. 
     In one aspect, the apparatus comprises at least one rotatable shaft defining a rotational axis, and the at least one cutting plate is attached to the rotatable shaft and arranged rotatable in a rotational direction about the rotational axis. 
     The stationary housing can, for example, be manufactured by moulding, and subsequently machining, for example, milling of the interior surface to achieve the recesses in the interior space. However, it can be tricky and time-consuming to achieve the recesses on the interior surface of the interior space by machining since the interior space is long and narrow, and it is difficult to reach the interior surface with the machining tool. It is important that the stationary housing has a high mechanical strength so that it can withstand great mechanical stress. Suitably, the housing is made of a hard material, such as cemented carbide. Such hard materials are difficult to machine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures. 
         FIG. 1  shows an example of an apparatus for pulverizing a material including a stationary housing and sawblades. 
         FIG. 1 a    illustrates how the cut material is moved from a collecting notch of the sawblade to a recess of the housing. 
         FIG. 1 b    illustrates how the cut material is directed from the end of the recess of the housing towards a periphery of the sawblade so that the material is cut a second time. 
         FIG. 2  shows the apparatus of  FIG. 1  in a perspective view. 
         FIG. 3  shows an example of sawblade in a side view. 
         FIG. 4  shows an example of a plurality of sawblades  FIG. 5  shows the sawblades of  FIG. 4  attached to a rotary shaft. 
         FIG. 6  shows an example of an apparatus for pulverizing a material in a perspective view. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of an apparatus  1  for pulverizing a material according to the invention in a cross-section perpendicular to a rotational axis  11 .  FIG. 2  shows the apparatus  1  in a perspective view. The apparatus  1  can be used for pulverizing various types of material, for example, wood or plastic. The apparatus  1  comprises a stationary housing  3  having an interior space  5  and at least one interior wall  7  facing the interior space  5 , as shown in  FIG. 2 . The shape of the interior space  5  is substantially cylindrical and has a central axis that coincides with the rotational axis  11 . The apparatus  1  further comprises one or more sawblades  9  disposed in the interior space  5  and arranged rotatable with respect to the stationary housing  3  about the rotational axis  11 . The one or more sawblades  9  are disc-shaped and have a substantially circular periphery. Preferably, the apparatus comprises two or more sawblades  9 . The number of sawblades  9  needed depends on the width of the material to be pulverized. The rotational axis  11  of the one or more sawblades  9  coincides with the central axis of the interior space  5 . The rotational direction R of the one or more sawblades  9  is illustrated by an arrow in the  FIG. 1 . The periphery  12  of sawblade  9  is provided with a plurality of cutting teeth  15  for cutting the material and a plurality of collecting notches  17  for collecting the cut material. The cutting teeth  15  have cutting edges for cutting the material. 
     The shape of the collecting notches  17  may vary. In this example, the collecting notches  17  are U-shaped. Alternatively, the collecting notches  17  can be V-shaped. The collecting notches  17  are disposed in front of the cutting teeth  15  with respect to the rotational direction R so that the material cut by the teeth can be collected. The cutting teeth  15  are disposed at rear ends  33  of the collecting notches  17 , as shown in  FIG. 3 . When the tooth  15  has cut the material, the cut material is collected in the collecting notch  17 . 
     The interior space  5  is designed for housing the one or more sawblades  9 . The at least one sawblade  9  is disposed in the interior space  5  at a distance from the interior wall  7  of the stationary housing  3  so that a gap  18  is formed between the periphery  12  of the sawblade  9  and the interior wall  7  of the housing  3  for housing a flow of cut material, as shown in  FIG. 2 . The width of the gap  18  varies depending on the position of the sawblade  9  with respect to the interior wall  7  of the housing  3 . Preferably, the gap is larger than 0.2 mm. 
     The apparatus further comprises an inlet having an opening  19  for feeding material to be pulverized to the interior space  5 , and an outlet having an opening  21  for dispatching the pulverized material from the interior space  5 . The material to be pulverized enters the interior space  5  of the housing  3  through the inlet opening  19 . The pulverized material leaves the interior space  5  of the housing  3  through the outlet opening  21  after one revolution of the sawblade  9 . The inlet opening  19  and the outlet opening  21  are arranged in the interior wall  7 . Both the inlet opening  19  and the outlet opening  21  are arranged in direct communication with the interior space  5 . The cut material is provided direct to the outlet and do not pass through any sieve or other device for determining the size of the output material. Thus, the apparatus does not contain any sieve or other device for determine the maximum particle size of the cut material that is permitted to exit from the interior space. 
     The interior wall  7  is provided with a plurality of recesses  23  facing the periphery  12  of the sawblade  9 , and accordingly facing the cutting teeth  15  and the collecting notches  17 . The recesses  23  are designed so that they encourage a flow of cut material in the rotational direction of the cutting plates. The recesses  23  are arranged to receive cut material from the collecting notches  17  of the sawblade  9 , and to revert the cut material to the periphery  12  of the sawblade  9  so that the material will be cut multiple times before it is dispatched from the apparatus. The number of recesses  23  may vary. The number of times the material is cut depends on the number of cutting teeth  15  and the number of recesses  23 . Thus, number of recesses  23  is selected in dependence on the desired particle size. The recesses  23  are elongated in a tangential direction with respect to the periphery  12  of the sawblade  9 , as shown in  FIG. 1 . The larger number of recesses, the more time is the material cut during a revolution of the sawblade  9 . Accordingly, the particle size of the pulverized material depends on the number of recesses. Preferably, the number of recesses  23  is more than three, and most preferably more than 10. 
     The interior wall  7  provided with the recesses  23  surrounds a main part of the periphery  12  of the one or more sawblades  9 . Preferably, at least 60% of the periphery of the sawblade  9  is surrounded with the interior wall  7  provided with the recesses  23  to provide large number of recesses. More preferably, at least 70% of the periphery of the sawblade  9  is surrounded with the interior wall  7  provided with the recesses  23 . Large number of recesses makes it possible to pulverize the material to a desired particle size during one single revolution. 
     The recesses  23  have a first end  24  and a second end  25 , as shown in  FIG. 1 . The recesses are arranged in succession so that the first end  24  of a recess  23  abuts the second end  25  of the next recess  23  in the succession. 
     In one aspect, the length L of the recesses  23 , in a cross-section perpendicular to the rotational axis  11 , is larger than the length L 2  of the collecting notches  17 . Preferably, the length L of the recesses  23  is at least two times larger than the length L 2  of the collecting notches  17  in a cross-section perpendicular to the rotational axis  11 , and more preferably, the length L of the recesses  23  is at least three times larger than the length L 2  of the collecting notches  17 . The shape of the recesses  23  may vary. In one aspect, the shape of the recesses  23 , in a cross-section perpendicular to the rotational axis, is tapering in a direction reverse the rotational direction R of the at least one sawblade  9 . Thus, the first end  24  is a narrow and the second end  25  is wide. 
     The second end  25  of the recess has a guiding surface  27  designed to guide the flow of cut material in the recess towards the periphery  12  of the at least one sawblade  9 . The design of the guiding surface  27  may vary. Preferably, the guiding surface  27  is concave. However, the guiding surface  27  can also be straight and extend in a radial direction of the sawblade  9 . The second end  25  of the recess has a wall designed to form the guiding surface  27  for directing the material flow towards the cutting teeth  15 . The wall of the second end  25  is bent so that the guiding surface  27  is concave. The concave guiding surface  27  has a bending radius. In one aspect, the length L of the recesses, in a cross-section perpendicular to the rotational axis  11 , is at least twice the bending radius of the guiding surface  27 . Preferably, the length L of the recesses, in a cross-section perpendicular to the rotational axis  11 , is at least 2.5 larger than the bending radius of the guiding surface  27 . Most preferably, the length L of the recesses is at least three times larger than the bending radius of the guiding surface  27 . Due to the fact that the length L of the recesses  23  is significantly larger than the bending radius of the second end  25  of the recesses, the particles in the cut material is allowed to accelerate to a high speed before they reach the concave guiding surface at the second end  25  and is redirected towards the cutting tooth  15  of the sawblade. Thus, it is ensured that the cut material reaches the cutting teeth  15  and is cut multiple times. 
     The size of the cut material depends on the distance between the cutting teeth  15  and the portions  26  of the interior wall  26  located at the second ends  25  of the recesses in the radial direction. In this example, the location of the interior wall  26  at the second end  25  of the recess is the same as the location of interior wall  26  at the first end  24  of the next recess. To ensure that the material is pulverized and not cut, the distance between the cutting teeth  15  and the portions  26  of the interior wall at the second end of the recess is less than 8 mm, preferably less than 4 mm, and most preferably less than 2 mm. For example, the distance between the cutting teeth  15  and the portion  26  of the wall is less than 1 mm. Large number of recesses and a short distance between the between the cutting teeth  15  and the portions  26  ensures that the material can be pulverized to a desired particle size during one single revolution. 
     The sawblade  9  is disposed so that the distance between the cutting teeth  15  and the interior wall  26  at the second ends  25  of the recesses is less than 8 mm, preferably less than 4 mm, and most preferably less than 2 mm. 
     In one aspect, the recesses  23  are elongated in a direction parallel to the rotational axis  11 , as shown in  FIG. 2 . In this example, the recesses  23  extend in parallel with the rotational axis  11 . Alternatively, the recesses  23  may extend at an angle with respect to the rotational axis  11 . For example, the recesses  23  may form a herringbone pattern on the interior wall  7 . The 
     In one aspect, the housing  3  comprises a plurality of plate-shaped pieces  28  having two parallel main sides  29 , as shown in  FIG. 2 . The plate-shaped pieces  28  are attached to each other with the main sides  29  facing each other so that that the pieces  28  together form the housing  3  with the interior space  5 . This facilitates the manufacturing of the housing  3 . 
       FIG. 1 a    illustrates how the cut material is moved from the collecting notch  17  of the sawblade  9  to the recess  23  of the housing  3  when the collecting notch  17  is facing the first end  24  of the recess.  FIG. 1 b    illustrates how the cut material is directed from the second end  25  of the recess  23  of the housing  3  towards the periphery  12  of the sawblade  9 . 
     The at least one sawblade  9  is rotating with a high speed. The cutting teeth  15  of the sawblade  9  will cut the material fed through the inlet opening  19  to the interior space  5 . The collecting notches  17  will collect and house the cut material. In the beginning of a revolution, the cut material is rough. Since the sawblade  9  is rotating with respect to the stationary housing  3 , each of the collecting notches  17  will face each of the recesses  23  of the stationary housing  3  once per revolution of the sawblade  9 . The material is cut each time the portions  26  of the interior wall  26  faces one of the cutting teeth  15 . When a collecting notch  17  is facing a recess  23  of the stationary housing  3 , the cut material in the collecting notch is transferred to the first end  24  of the recess by means of the centrifugal force, as shown in  FIG. 1 a   . When the cut material travels in the recess, the speed of the material is increased until the material reaches the second end  25  of the recess, where the speed of the cut material is reduced and the cut material is redirected by the guiding surface  27  of the recess towards the periphery  12  of the sawblade  9 , as shown in  FIG. 1 b   . When the cutting tooth  15  reaches the second end  25  of the next recess due to the rotation of the sawblade  9 , the cut material will be cut a second time and collected in the collecting notch  17 . Thus, the size of the material will be reduced. Each time a cutting tooth and a collecting notch pass one of the recesses  23 , the material is cut one more time. The cut material is moved in the gap  18  until it reaches the outlet opening  21  where it is dispatched. The number of times the material is cut depends on the number of cutting teeth  15  and the number of recesses  23 . For example, if the number of recesses  23  is 5 and the number of cutting teeth is 10, the material is cut 5*10=50 times. Thus, the size of the material can be controlled by the number of collecting notches in the stationary housing  3 , and the number of cutting teeth  15 . The apparatus makes it possible to pulverize material into very small particles. 
       FIG. 3  shows an example of sawblade  9  in a side view. The sawblade  9  comprises a disc-shaped body  30  having an outer edge  13 . The periphery  12  of the sawblade is provided with a plurality of notches  17 ,  35  evenly distributed along the periphery. Some of the notches are collecting notches  17 . The sawblade  9  further comprises a plurality of cutting teeth  15  attached to the body  30 . The cutting teeth  15  are made of a harder material than the body  30 . The collecting notch  17  comprises a front end  32  and a rear end  33  with respect to the rotational direction of the sawblade  9 . The cutting teeth  15  are attached to the rear end  33  of the collecting notches. The cutting teeth  15  protrude from the outer edge  13  of the body  30  in a radial direction. The distance between the rotational axis  11  and the front end  32  is less than the distance between the rotational axis  11  and the upper part of the cutting teeth  15 . This is, for example, achieved by having an inclined portion  34  of the outer edge  13  of the body  30 , as shown in  FIG. 3 . The inclined portion  34  slopes towards the cutting teeth  15 . 
     Further, the distance between the rotational axis  11  and the rear end  33  of the collecting notches is less than the distance between the rotational axis  11  and an upper end of the cutting teeth  15 . The distance the cutting teeth  15  protrude from the outer edge  13  of the body  30  determines the cutting depth of the material. The periphery  12  of the sawblade  9  is provided with a plurality of empty notches  35  without any cutting teeth  15 . The empty notches  35  are arranged between the collecting notches  17 . Thus, a collecting notch  17  is always followed by an empty notch  35 . The empty notches  35  are larger than the collecting notches  17 . The distance between the empty notches  35  and the collecting notches  17  is the same. In one aspect, the width of the cutting teeth  15  is also larger than the width of the body  30  in an axial direction, i.e. in a direction parallel to the rotational axis  11  of the sawblades. 
     In many applications, it is a desire to be able to feed an object with a width, such as a plank of wood or a log, directly to the pulverizing apparatus without cutting it into smaller pieces. In such case, the apparatus comprises a plurality of sawblades  9  put together to a cutting unity. 
       FIG. 4  shows an example of a plurality of sawblades  9  put together to form a cutting unity  40 . Preferably, the cutting unity comprises at least three sawblades, and preferably five or more sawblades. The sawblades  9  are arranged as closed to each other as possible to enable pulverization of the material. If there is a gap between the sawblades, a part of the material may enter the gap and will not be pulverized. Preferably, the each of the sawblades  9  is arranged in mechanical contact with the neighboring sawblades in the cutting unity. This means that each sawblade abuts against the neighboring sawblade in the cutting unity  40 . Thus, it is prevented that a gap is formed between the sawblades and it is ensured that all material coming into contact with the cutting unity is pulverized. 
     The sawblades  9  are rotated in relation to each other so that a front part of the empty notches  35  of the sawblades are aligned with the collecting notches  17  of the sawblade  9  in a direction parallel to the rotational axis  11  of the sawblades. The empty notches  35  and the collecting notches  17  together form a slot  36  in parallel with the rotational axis for collection of cut material. As seen from  FIG. 4 , the width of the cutting teeth  15  is larger than the width of the body  30  in an axial direction as well as in a radial direction. Due to the fact that empty notches  35  are longer than the collecting notches  17  along the periphery of the sawblade  9 , grooves  37   a - b  are formed between the sawblades  9  and between the cutting teeth  15 . Those grooves allow the cutting teeth  15  to be wider than the body  30  of the sawblade  9  in the axial direction. As seen from the figure, the sawblades are arranged so that the cutting teeth  15  and the grooves  37   a - b  form a plurality of groups along the periphery of the cutting unit. The cutting teeth  15  within the same group are aligned in an axial direction. Every second group has the same configuration of the cutting teeth  15  and the grooves  37   a - b . However, two neighbouring groups have the cutting teeth  15  and grooves  37   a - b  arranged so that each groove of a first group is followed by a tooth in the next group along the periphery of the sawblade  9 . By that a piece of material can be cut along its whole width. The cutting teeth  15  in two neighbouring groups cooperate to cut the material along the width of the material. 
       FIG. 5  shows the sawblades  9  of  FIG. 4  attached to a rotary shaft  38 . The apparatus  1  comprises at least one rotatable shaft  38  defining a rotational axis. The one or more cutting plates  9  are attached to the rotatable shaft  38  and by that arranged rotatable in a rotational direction about the rotational axis R. A part of the shaft  38  and the sawblades  9  are disposed in the interior space  5  of the housing  3 . 
       FIG. 6  shows an example of an apparatus  1  for pulverizing a material shown in a perspective view. The apparatus comprises the stationary housing  3  as described with reference to  FIGS. 1 and 2 , a shaft  38  provided with one or more sawblades  9 , and a motor  42  for driving the rotation of the shaft  38 . The apparatus further comprises a feeding part  44  for feeding the apparatus with material to be pulverized. 
     In the following, a method for producing the stationary housing  3  is described. The stationary housing  3  is manufactured using one or more sheets, such as metal sheets. For example, the one or more sheets are made of cemented carbide. Suitably, the width of the one or more sheets is between 0.5 and 100 mm, preferably between 1 and 50 mm, and most preferably between 5 and 30 mm. 
     The method comprises:
         1. Defining a plurality of plate-shaped parts by dividing the housing  3  along the central axis into a series of successive plate-shaped parts having the same width as the metal sheets. This step is, for example, carried out by using a CAD program. The design of the housing including the interior space can be determined in the CAD program. A CAD model of the housing with the desired shape can be defined in the CAD program. The CAD model of the housing is divided along its central axis into a series of successive plate-shaped parts having the defined width using the CAD program. The width of the plate-shaped parts is determined by the width of the sheets used for manufacturing the housing.   2. Determining the outer and interior contours  4   a ,  4   b  for each of the plate-shaped parts in a cross-section perpendicular to a central axis  11  of the housing, wherein the interior contour  4   b  is the contour of the interior space  5 . This can, for example, be done in the CAD program.   3. Cutting one or more sheets having a width corresponding to the width of the plate-shaped parts into a plurality of pieces  28 , each piece  28  having a shape corresponding to the outer and inner contours  4   a ,  4   b  of one of said plate-shaped parts, and comprising two parallel flat main sides  29 , as shown in  FIGS. 1 and 2 . The cutting is, for example, made by waterjet cutting, laser cutting or flame cutting. These methods are suitable for cutting hard materials.   4. Attaching the plate-shaped pieces  28  to each other with the main sides  29  facing each other in an order corresponding to the defined series of successive plate-shaped parts, so that the pieces together form the stationary housing  3  with the interior space  5 . The pieces  28  are, for example, attached to each other gluing, welding, bolting, or screwing.       

     The method uses cutting of thin sheets to achieve the recesses  23 , instead of machining a moulded object. Thus, it is possible to use a hard material, such as cemented carbide, in the housing. Further, no machine tool is needed. The method makes it possible to manufacture a housing with a long and narrow internal space provided with recesses  23  on the interior wall. This method is simple, cost-effective and fast. 
     The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims. For example, the shapes of the recesses and the collecting notches may vary. 
     REFERENCE LIST 
     
         
           1  Apparatus for pulverizing a material 
           3  Stationary housing 
           4   a  Outer contour 
           4   b  Inner contour 
           5  Interior space 
           7  Interior wall 
           9  Sawblade 
           11  Rotational axis 
           12  Periphery of the sawblade 
           13  Outer edge of the sawblade 
           15  Teeth 
           17  Collecting notches 
           18  Gap 
           19  Inlet opening 
           21  Outlet opening 
           23  Recesses 
           24  First end of the recess/narrow end 
           25  Second end of the recess/wide end 
           26  portion of the interior wall 
           27  Guiding surface 
           28  Plate-shaped pieces 
           29  Main sides 
           30  Body 
           32  Front end of collecting notch 
           33  Rear end of the collecting notch 
           34  Inclined portion 
           35  Empty notches 
           36  Slot 
           37   a - b  Grooves 
           38  Rotary shaft 
           40  Cutting unit 
           42  Motor 
           44  Feeding part