Abstract:
A method for producing a cellular wheel, wherein a tool having a cylindrical inner shell, the diameter of which corresponds to the inner diameter of the subsequent sleeve, is placed on the free end edges of the fins for fixing the angular orientation of the fins. A molding material mixture is applied in the cells bounded by the inner shell of the tool and the fins and cured. After removing the tool, the subsequent sleeve is placed on the free end edges of the fins fixed in place by the cured molding material mixture. The free end edges of the fins are joined to the subsequent sleeve to the cells. The cured molding material mixture is thermally treated and is removed from the cells.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/EP2011/068713 filed Oct. 26, 2011, claiming priority based on European Patent Application No. 10 189 873.2, filed Nov. 3, 2010, the contents of all of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a method for producing a cellular wheel from metal, having sleeves which lie concentrically with respect to a rotational axis and fins which are arranged between sleeves which follow one another, are oriented radially with respect to the rotational axis, are joined with an end edge to a sleeve, and are fixed in their angular position before the joining of the free end edges to a following sleeve. 
     PRIOR ART 
     The method of downsizing has belonged for some years to the main topics in the construction of new, supercharged engines. The fuel consumption and therefore the exhaust gas emissions of a vehicle can be reduced by way of downsizing. These aims are becoming more and more important nowadays, since the high energy consumption by way of fossil fuels contributes greatly to air pollution and more and more strict legal measures are forcing the automotive producers to act. Downsizing is understood to be the substitution of a large-volume engine by an engine with a reduced engine displacement. Here, the engine power output is to be kept constant by supercharging of the engine. The aim is to achieve the same power output values with small-volume engines as with naturally aspirated engines of the same power output. New findings in the field of downsizing have shown that, in particular in the case of very small gasoline engines with an engine displacement of 1 liter or less, the best results can be achieved with pressure wave supercharging. 
     In the case of a pressure wave supercharger, the rotor is configured as a cellular wheel and is enclosed by an air and exhaust gas housing with a common shell. The development of modern pressure wave superchargers in order to supercharge small engines leads to cellular wheels with a diameter in the order of magnitude of 100 mm or less. In order to achieve a maximum cell volume and also for the purpose of weight reduction, cell wall thicknesses of 0.5 mm or less are aimed for. At the high exhaust gas entry temperatures of approximately 1000° C., in practice only highly heat-resistant steels and alloys are suitable as materials for the cellular wheel. The production of dimensionally stable and highly precise cellular wheels with a low cell wall thickness is scarcely possible today or else is associated with considerable additional costs. 
     It has already been proposed to form the chambers of a cellular wheel from Z-shaped profiles which are in a row next to one another and overlap partially. However, the production of a cellular wheel of this type is associated with high time outlay. In addition, placing Z-profiles next to one another in a row and fixing them in an accurate position can scarcely be carried out with a precision which is sufficient for maintaining the required tolerances. 
     It has also already been proposed to produce a cellular wheel from a solid body by eroding the individual cells. However, it is not possible by way of this method to achieve cellular wall thicknesses of less than 0.5 mm. A further substantial disadvantage of the eroding method is the associated, high material and machining costs. 
     A method of the type which is mentioned at the outset is disclosed in WO 2010/057319 A1. The fixing of the fins which project to the outside from a sleeve in a predefined angular position takes place via a tool with fingers which can be pushed on the end side in the axial direction between the fins, fill the intermediate space between adjacent fins, and in the process move the fins into the predefined angular position and fix them in the latter. After a next sleeve has been pushed on, it is welded by means of a laser beam to the free end edges of the fins which lie beneath it, by means of a bead on plate weld. Subsequently, the sleeve is fitted with further fins which are positioned one after another and are joined by means of a laser beam to the sleeve by means of a hollow seam, and, as described above, is fixed in a predefined angular position and is fitted with further fins. This operation is repeated in accordance with the construction of the cellular wheel to be produced until the final setting and joining of the outermost sleeve. 
     The fixing of the fins which project to the outside from a sleeve in a predefined angular position by way of a tool, by way of which, according to WO 2010/057319 A1, molding pieces are inserted between the fins, and which is associated with the disadvantage that said molding pieces can scarcely be removed any more from the fin intermediate spaces without damage of the joined parts. The reason lies in the fact that the locally high temperatures which occur during the welding lead to mechanical stresses and therefore to jamming of the molding parts and the fins. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of providing a simple and inexpensive method of the type stated at the outset with the avoidance of the disadvantages of the prior art, which method makes it possible to produce a cellular wheel with the required precision. A further aim of the invention is the provision of a method for producing a cellular wheel which is suitable for use in a pressure wave supercharger for supercharging internal combustion engines, in particular for supercharging small gasoline engines with an engine displacement in the order of magnitude of 1 liter or less. In particular, the method is to make the inexpensive production possible of dimensionally stable and highly precise cellular wheels with a cellular wall thickness of 0.5 mm or less. 
     It leads to the achievement according to the invention of the object that 
     (a) a tool is placed onto the free end edges of the fins with a cylindrical inner circumferential face, the diameter of which corresponds to the internal diameter of the following sleeve, in order to fix the fins in their angular position, 
     (c) a molding material mixture comprising a fire-resistant basic molding material and a binding agent which can be hardened and can be released from the molding basic material by thermal treatment is introduced into the cells which are delimited by the inner circumferential face of the tool and the fins, and is hardened, 
     (d) after the removal of the tool, the following sleeve is placed onto the free end edges of the fins which are fixed in their angular position by the hardened molding material mixture, 
     (e) the free end edges of the fins are joined to the following sleeve by welding or brazing to the cells, and 
     (f) the hardened molding material mixture is treated thermally and the basic molding material is removed from the cells. 
     The sleeves and fins of the cellular wheel are preferably composed of a highly heat-resistant metallic material made from metal sheet, preferably with a thickness of less than 0.5 mm. 
     The cellular wheel which is produced by way of the method according to the invention is preferably used in a pressure wave supercharger for supercharging internal combustion engines, in particular gasoline engines with an engine displacement of 1 liter or less. 
     The production of sleeves with fins which project radially from them and are joined with an end edge to a sleeve can take place in different ways. The methods which are preferred in conjunction with the present invention are described in WO 2010/057319 A1. 
     The tool preferably has longitudinal grooves which emanate radially from the inner circumferential face and into which the free end faces of the fins which have an oversize are inserted before the introduction of the molding material mixture, and the oversize of the fins which projects out of the hardened molding material mixture after the removal of the tool is removed before the following sleeve is placed onto the free end edges of the fins. In one particularly preferred embodiment, the inner circumferential face of the tool can have bead-like elevations on both sides of the longitudinal grooves. 
     The fins are preferably joined to the sleeves by welding or brazing by means of a laser or electron beam. 
     The thermal treatment is preferably carried out only on the finished cellular wheel and the basic molding material is removed from all cells at the same time. 
     The finished cellular wheel is expediently subjected to stress-relief annealing, the hardened molding material mixture preferably being removed from the cells of the cellular wheel only after the stress-relief annealing. 
     For example, a core sand, in particular a quartz sand, can be used as fire-resistant basic molding material. 
     The binding agent is preferably an organic binding agent, in which the hardening reaction is accelerated by a gaseous catalyst or which is hardened by reaction with a gaseous hardener. In one particularly preferred embodiment of the method according to the invention, a two-component system with a solution of a polyol, in particular of a phenolic resin, as first component and with a solution of a polyisocyanate as second component is used as binding agent, and a gaseous tertiary amine, in particular triethylamine, is guided through the molding material mixture for hardening. 
     The essential core concept of the method according to the invention is the use of the core sand mixtures which are used during the casting of metals for providing a molding wall for the liquid metal to fix adjacent fins in their predefined angular position before the joining by the formation of a molding piece which can be introduced in a simple way between the fins and can be removed again. 
     Molding material mixtures which are suitable for use with the method according to the invention are essentially all core sand mixtures. 
     The fire-resistant basic molding material, for example quartz sand, is preferably present in a pourable form, with the result that it can be filled between the fins and can be compressed there. A firm cohesion between the particles of the basic molding material is produced by the binding agent, with the result that the molding piece which is formed is given the required mechanical stability. 
     Both organic and inorganic binding agents can be used. Binding agents are preferred here, the hardening of which can be carried out substantially at room temperature. Here, the hardening takes place usually by way of a chemical reaction which is triggered, for example, by virtue of the fact that a gas is guided as catalyst through the molding material mixture which is to be hardened and is filled and compressed between the fins. 
     Particularly preferred molding material mixtures contain organic binding agents, in which the hardening reaction is accelerated by a gaseous catalyst or which are hardened by reaction with a gaseous hardener. In the field of casting, these methods are called “cold box” methods. 
     A polyurethane binder is suitable, in particular, as binding agent for the use according to the invention, that is to say a two-component system with a first component consisting of the solution of a polyol, for example of a phenolic resin, and with a solution of a polyisocyanate as second component. A system of this type is known, for example, from U.S. Pat. No. 3,409,579 A. The two components of the polyurethane binder are brought to reaction by a gaseous tertiary amine being guided through the mixture of basic molding material and binding agent after the filling and compressing of the molding material mixture. The hardening reaction of polyurethane binders is a polyaddition, that is to say a reaction without separation of by-products, such as water. 
     During the heating of the hardened molding material mixture, the organic constituent parts of the binding agent break down. The basic molding material becomes pourable again, and can be removed from the cells by simple tilting of the finished cellular wheel. 
     The finished cellular wheel is preferably subjected to stress-relief annealing. It has proved particularly advantageous here if the hardened molding material mixture is removed from the cells of the cellular wheel only after the stress-relief annealing. 
     The customary materials in the field of casting for the production of cores can substantially be used as fire-resistant basic molding material. For example, quartz or zircon sand is suitable. Further suitable sands are, for example, olivine, chrome ore sand and vermiculite. 
     Furthermore, synthetic basic molding materials can also be used, such as hollow aluminosilicate spheres or spherical ceramic basic molding materials. Said spherical ceramic basic molding materials contain, for example, mullite, corundum, cristobalite in different proportions as minerals. They contain aluminum oxide and silicon dioxide as essential fractions. Typical compositions contain, for example, Al 2 O 3  and SiO 2  in approximately identical proportions. In addition, further constituent parts can be contained, such as TiO 2 , Fe 2 O 3 . The diameter of the spherical basic molding materials is preferably less than 1000 μm, in particular less than 600 μm. Synthetically produced fire-resistant basic molding materials, such as mullite, are also suitable. Said synthetic basic molding materials do not have a natural origin and can also have been subjected to a special shaping process, such as in the production of hollow aluminosilicate microspheres or spherical ceramic basic molding materials. 
     Glass materials can also be used as fire-resistant synthetic basic molding materials. They are used, in particular, either as glass spheres or as glass granulate. Customary glasses can be used as glass, glasses which have a high melting point being preferred. Glass beads and/or glass granulate which are/is produced from cullet are/is suitable. Borate glasses are likewise suitable. 
     The filling of the fin intermediate spaces with the molding material mixture preferably takes place by way of the methods which are used in the field of casting for core production. For example, the molding material mixture can be shot into the fin intermediate spaces by means of a core shooter with the aid of compressed air and can be compressed by means of a jogging device. The shot and compressed molding material mixture is subsequently hardened, for example by way of gassing. Said gassing is preferably carried out at room temperature. Here, the gassing duration lies in the order of magnitude of seconds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Further advantages, features and details of the invention result from the following description of preferred exemplary embodiments and using the drawing which serves merely for explanation and is not to be interpreted as being restrictive. In the drawing, in diagrammatic form, 
         FIG. 1  shows a longitudinal section in the rotational axis through a cellular wheel for a pressure wave supercharger, 
         FIG. 2  shows a section perpendicularly with respect to the rotational axis of the cellular wheel from  FIG. 1 , according to the line I-I, 
         FIG. 3  shows a longitudinal section through a drive shaft having a flanged sleeve for the construction of the cellular wheel from  FIG. 1 , 
         FIG. 4  shows a longitudinal section through the drive shaft, which is inserted into a receiving plate, with a flanged sleeve from  FIG. 3 , 
         FIG. 5  shows the longitudinal section from  FIG. 4  with the inner sleeve, which is placed onto the flanged sleeve and is fitted with fins, of the cellular wheel from  FIG. 1  and with a first tool which is placed onto the fins, 
         FIG. 6  shows a section through the arrangement from  FIG. 5  according to the line II-II, 
         FIG. 7  shows an enlarged detail from  FIG. 6  of the arrangement from  FIG. 5  which is filled with core sand, 
         FIG. 8  shows the section from  FIG. 6  through the arrangement from  FIG. 5  with the first intermediate sleeve, which is placed onto the fins of the inner sleeve and is fitted with fins, of the cellular wheel from  FIG. 1  and with a second tool which is placed onto the fins of the first intermediate sleeve, 
         FIG. 9  shows an enlarged detail from  FIG. 8 , which corresponds to  FIG. 7 , of the arrangement which is filled further with core sand, 
         FIG. 10  shows the section from  FIG. 8  through the arrangement from  FIG. 5  with the second intermediate sleeve, which is placed onto the fins of the first intermediate sleeve and is fitted with fins, of the cellular wheel from  FIG. 1  and with a second tool which is placed onto the fins of the second intermediate sleeve, 
         FIG. 11  shows an enlarged detail from  FIG. 10 , which corresponds to  FIG. 9 , of the arrangement which is filled further with core sand, 
         FIG. 12  shows a further enlarged detail from  FIG. 11  with a tool variant, and 
         FIG. 13  shows an enlarged detail of the surroundings of a welding point in the case of the use of the tool variant from  FIG. 12 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A cellular wheel  10  (shown in  FIGS. 1 and 2 ) of a pressure wave supercharger (not shown in the drawing) comprises a cylindrical inner sleeve  12  which lies concentrically with respect to a rotational axis y of the cellular wheel  10 , a cylindrical outer sleeve  14  which lies concentrically with respect to the inner sleeve  12 , and two intermediate sleeves  18 ,  20  which are arranged between the inner sleeve  12  and the outer sleeve  14  concentrically with respect to them. The inner annular space between the inner sleeve  12  and the first intermediate sleeve  18 , the middle annular space between the first intermediate sleeve  18  and the second intermediate sleeve  20 , and the outer annular space between the second intermediate sleeve  20  and the outer sleeve  14  are divided by strip-shaped fins  16  which lie radially with respect to the rotational axis y into a multiplicity of inner cells  22  and middle cells  24  which are arranged offset rotationally with respect to the former, and outer cells  26  which are once again arranged offset rotationally with respect to the former. The cellular wheel  10  which is shown by way of example with a diameter D and a length L of, for example, 100 mm has 36 inner cells  22 , 54 middle cells  24  and 54 outer cells  26 . The inner sleeve  12 , the intermediate sleeves  18 ,  20 , the outer sleeve  14  and the fins  16  have a uniform wall thickness of, for example, 0.4 mm and are composed of a highly heat-resistant metallic material, for example Inconel 2.4856. In the direction of the rotational axis y, said parts have an identical length L in accordance with the length of the cellular wheel  10  and extend between two end sides of the cellular wheel  10  which lie perpendicularly with respect to the rotational axis y. 
     A flanged sleeve  32  which is arranged concentrically with respect to the drive shaft  30  is seated on a drive shaft  30  (shown in  FIG. 3 ) for a cellular wheel  10  which is shown in  FIGS. 1 and 2 . The rotational axis of the drive shaft  30  corresponds to the rotational axis y of a cellular wheel  10  which is placed on the drive shaft  30 . In the following text, using  FIGS. 4 to 13 , the step-by-step construction of the cellular wheel  10  (shown in  FIGS. 1 and 2 ) on the flanged sleeve  32  which is arranged on the drive shaft  30  will be explained in greater detail. 
     As shown in  FIG. 4 , the drive shaft  30  with the flanged sleeve  32  arranged on it is inserted and fixed into a central hole  38  of a circular receiving plate  34  in such a way that the flanged sleeve  32  rests with its drive shaft-side end of a base face  36  over the entire end-side circumference, and the rotational axis y, corresponding to the rotational axis y of the cellular wheel  10 , of the drive shaft  30  lies perpendicularly on the base face  36 . 
     According to  FIGS. 5 and 6 , the inner sleeve  12 , the internal diameter and length of which are adapted to the external diameter and the length of the flanged sleeve  32 , is placed concentrically onto the flanged sleeve  32  with the fins  16  which were joined beforehand in a positionally accurate manner with an end edge to the inner sleeve  12 , and projects with the free end edge radially to the outside, in the axial direction y until contact with the base face  36  of the receiving plate  34 , and is welded to said flanged sleeve  32  by means of an NC-controlled laser beam. 
     A first tool  40  in the form of a hollow cylinder with a cylindrical inner circumferential face  41 , the diameter of which corresponds to the internal diameter of the first intermediate sleeve  18 , has longitudinal grooves  42  which are arranged in a positionally accurate manner, project radially from the inner circumferential face  41  into the first tool  40  and run parallel to the axial direction y. The first tool  40  is placed and fixed in a positionally accurate manner onto the base face  36  by means of a centering annular disk  39 , the outer circumferential face of which bears against an annular edge  37  which projects coaxially with respect to the axial direction y or hole  38  from the base face  36  of the receiving plate  34  and the internal diameter of which corresponds to the external diameter of the first tool  40 . Subsequently, the inner sleeve  12  which is fitted with the radially outwardly protruding fins  16  and has already been welded to the flanged sleeve  32  is introduced concentrically into the first tool  40  with the drive shaft  30  in the axial direction y until contact with the base face  36  of the receiving plate  34 , by means of an introduction aid for the fins. Here, the fins  16  of the inner sleeve  12  which have an oversize e are guided in the longitudinal grooves  42  of the first tool  40 . 
     As shown in  FIG. 7 , the inner cells  22  which are delimited in a preliminary manner by the inner circumferential face  41  of the first tool  40  instead of the first intermediate sleeve  18  which is to be placed in a later step are filled with a molding material mixture F. After the compression, the molding material mixture F which fills the inner cells  22  completely is hardened. 
     A suitable molding material mixture is composed, for example, of a natural or synthetic basic molding material having a mean grain size of from 0.2 to 0.36 mm and an AFS number of from 52 to 45, for example quartz sand H32, with a binding agent on a polyurethane basis with trimethylamine as catalyst for the hardening process. The molding material mixture can be filled into the cells by way of a commercially available core shooter and can be compressed by way of a jogging device. 
     After the hardening of the molding material mixture F, the partially produced cellular wheel which is constructed on the flanged sleeve  32  is ejected together with the drive shaft  30  in the axial direction y out of the first tool  40 , and the fins  16  of the inner sleeve  12  which project with the oversize e out of the hardened molding material mixture F are removed, for example by grinding, down to a dimension which corresponds to the internal diameter of the first intermediate sleeve  18 . Since the fins are anchored firmly in the hardened molding material mixture, the removal of the oversize e can take place without problems by grinding. 
     The first intermediate sleeve  18  with the fins  16  which are joined to it on one side and project radially to the outside is placed in the axial direction y in a positionally accurate manner onto the free end edges of the fins  16  which are enclosed by the hardened molding material mixture F. Subsequently, the first intermediate sleeve  18  is welded by means of a laser beam to the free end edges of the fins  16  of the inner sleeve  12  which lie underneath, by means of a bead on plate weld, with the formation of the inner cells  22 . 
     A second tool  50  in the form of a hollow cylinder with a cylindrical inner circumferential face  51 , the diameter of which corresponds to the internal diameter of the second intermediate sleeve  20 , has longitudinal grooves  52  which are arranged in a positionally accurate manner, project radially from the inner circumferential face  51  into the second tool  50  and run parallel to the axial direction y. The second tool  50  is placed and fixed onto the base face  36  in a positionally accurate manner by means of a centering annular disk, the outer circumferential face of which bears against an annular edge  37  which projects coaxially with respect to the axial direction y from the base face  36  of the receiving plate  34  and the internal diameter of which corresponds to the external diameter of the second tool  50 . Subsequently, the first intermediate sleeve  18  which is fitted with the radially outwardly projecting fins  16  and has already been welded to the fins  16  of the inner sleeve  12  is introduced concentrically into the second tool  50  with the drive shaft  30  in the axial direction y until contact with the base face  36  of the receiving plate  34 , by means of an introduction aid for the fins. Here, the fins  16  of the first intermediate sleeve  18  which have an oversize e are guided in the longitudinal grooves  52  of the second tool  50  ( FIG. 8 ). 
     As shown in  FIG. 9 , the middle cells  24  which are delimited in a preliminary manner by the inner circumferential face  51  of the second tool  50  instead of the second intermediate sleeve  20  which is to be placed in a later step are filled with the molding material mixture F. After the compression, the molding material mixture F which fills the middle cells  24  completely is hardened. 
     After the hardening of the molding material mixture F, the partially produced cellular wheel which is constructed on the flanged sleeve  32  is ejected together with the drive shaft  30  in the axial direction y out of the second tool  50 , and the fins  16  of the first intermediate sleeve  18  which project with the oversize e out of the hardened molding material mixture F are removed, for example by grinding, down to a dimension which corresponds to the internal diameter of the second intermediate sleeve  20 . 
     The second intermediate sleeve  20  with the fins  16  which are joined to it on one side and project radially to the outside is placed in a positionally accurate manner in the axial direction y onto the free end edges of the fins  16 , enclosed by the hardened molding material mixture F, of the first intermediate sleeve  18  middle cells  24 . Subsequently, the second intermediate sleeve  20  is welded by means of a laser beam to the free end edges of the fins  16  of the first intermediate sleeve which lie underneath, by means of a bead on plate weld, with the formation of the middle cells  24 . 
     A third tool  60  in the form of a hollow cylinder with a cylindrical inner circumferential face  61 , the diameter of which corresponds to the internal diameter of the outer sleeve  14 , has longitudinal grooves  62  which are arranged in a positionally accurate manner, project radially from the inner circumferential face  61  into the third tool  60  and run parallel to the axial direction y. The third tool  60  is placed and fixed in a positionally accurate manner onto the base face  36 . Here, the third tool  60  bears against the annular edge  37  which projects coaxially with respect to the axial direction y from the base face  36  of the receiving plate  34 . Subsequently, the second intermediate sleeve  20  which is fitted with the radially outwardly projecting fins  16  and has already been welded to the fins  16  of the first intermediate sleeve  18  is introduced concentrically into the third tool  60  with the drive shaft  30  in the axial direction y until contact with the base face  36  of the receiving plate  34 , by means of an introduction aid for the fins. Here, the fins  16  of the second intermediate sleeve  20  which have an oversize e are guided in the longitudinal grooves  62  of the third tool  60  ( FIG. 10 ). The third tool  60  which is in contact with the base face  36  of the receiving plate  34  is centered in a positionally accurate manner by the peripheral annular edge  37  which is arranged concentrically with respect to the hole  38  and projects from the base face  36  of the receiving plate  34 . 
     As shown in  FIG. 11 , the outer cells  26  which are delimited in a preliminary manner by the inner circumferential face  61  of the third tool  60  instead of the outer sleeve  14  which is to be placed in a later step are filled with the molding material mixture F. After the compression, the molding material mixture F which fills the middle cells  26  completely is hardened. 
     After the hardening of the molding material mixture F, the partially produced cellular wheel which is constructed on the flanged sleeve  32  is ejected together with the drive shaft  30  in the axial direction y out of the third tool  60 , and the fins  16  of the second intermediate sleeve  20  which project with the oversize e out of the molding material are removed, for example by means of grinding, down to a dimension which corresponds to the internal diameter of the outer sleeve  14 . 
     The outer sleeve  14  is placed in the axial direction y onto the free end edges of the fins  16  of the second intermediate sleeve  20  which are enclosed by the hardened molding material mixture F. Subsequently, the outer sleeve  14  is welded by means of a laser beam to the free end edges of the fins  16  of the second intermediate sleeve  20  which lie underneath, by means of a bead on plate weld, with the formation of the outer cells  26 . 
     The drive shaft  30  with the then finished cellular wheel  10 , the cells  22 ,  24 ,  26  of which are still filled with the hardened molding material mixture F, is then subjected to stress-relief annealing at a temperature of, for example, 550° C. for three hours. During this thermal treatment, a thermal regeneration of the basic molding material takes place at the same time. Here, the binding agent decomposes and the basic molding material which is freed from the binding agent pours readily out of the cells  22 ,  24 ,  26 . 
     During the stress-relief annealing, the inhomogeneous stress field which is produced during the joining of the end edges of the fins to the following sleeve merges into a state of equilibrium with a homogeneous stress field, with the result that, after the subsequent removal of the basic molding material from the cells, the cellular wheel is free of distortion and accordingly balancing can be dispensed with. 
     In the variant (shown in  FIG. 12 ) of a first tool  40 , bead-like elevations  43  which project from the inner circumferential face  41  and lead to correspondingly shaped depressions in the molding material mixture F which is filled and hardened between the fins  16  are arranged on both sides of the longitudinal grooves  42 . As shown in  FIG. 13 , said depressions  43 , after the first tool  40  is pulled off and the first intermediate sleeve  18  is pushed  40  onto the free end edges of the fins  16  which are shortened by the oversize e and project radially from the inner sleeve  12 , lead to channels  47  which are arranged on both sides of the fins  16  and are open with respect to the intermediate sleeve  18  which rests thereon. If the first intermediate sleeve  18  is then welded by means of a laser beam  45  to the free end edges of the fins  16  of the inner sleeve  12  which lie underneath, by means of a bead on plate weld  46 , the channels  47  make satisfactory pressing of the intermediate sleeve  18  via hold-downs  44  onto the free end edges of the fins  16  possible, and therefore optimum welding. A further advantage of the channels  47  is seen in the fact that an inert gas which is used to prevent the oxidation can be guided on a direct path to the contact faces of the intermediate sleeve  18  which are susceptible to oxidation with the free end edges of the fins  16 . 
     It goes without saying that the variant (shown in  FIGS. 12 and 13 ) of the first tool  40  can also be applied to the second and third tools  50 ,  60 . 
     LIST OF DESIGNATIONS 
     
         
           10  Cellular wheel 
           12  Inner sleeve 
           14  Outer sleeve 
           16  Fins 
           18  First intermediate sleeve 
           20  Second intermediate sleeve 
           22  Inner cells 
           24  Middle cells 
           26  Outer cells 
           30  Drive shaft 
           32  Flanged sleeve 
           34  Receiving plate 
           36  Base face of  34   
           37  Annular edge on  34   
           38  Central hole in  34   
           39  Centering annular disk 
           40  First tool 
           41  Inner circumferential face of  40   
           42  Longitudinal grooves in  40   
           43  Bead-like elevations 
           44  Hold-down 
           45  Laser beam 
           46  Bead on plate weld 
           47  Channels 
           50  First tool 
           51  Inner circumferential face of  50   
           52  Longitudinal grooves in  40   
           60  First tool 
           61  Inner circumferential face of  60   
           62  Longitudinal grooves in  40   
         e Oversize of  16