Abstract:
A manufacturing method for a worm wheel, the worm wheel meshed with an operational worm at usage, has steps of preparing a worm wheel, in which at least a surface layer of a tooth portion is made of synthetic resin, preparing a machining worm, in which pitch of teeth of the machining worm in an axial direction thereof is the same as that of an operational worm, diameters of tip and root portions of the tooth of the machining worm are equal to or more than those of the operational worm and abrasive grains are provided on a surface of the machining worm by means of electro-deposition, disposing the machining worm in a twisting position relative to the worm wheel and rotating the machining worm and the worm wheel with meshing each other so as to machining the tooth portion of the worm wheel.

Description:
[0001]     The present invention claims foreign priority to Japanese patent application No. 2004-332322, filed on Nov. 16, 2004, the contents of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a worm gear reducer which is used being incorporated into an electric power steering device, a nursing bed and so forth. The present invention also relates to a manufacturing method for a worm wheel composing the worm gear reducer.  
         [0004]     2. Description of the Background Art  
         [0005]     In order to reduce a force needed for operating a steering wheel when a driver gives a steering angle to vehicle wheels to be steered (Vehicle wheels to be steered are usually front wheels except for a special vehicle such as a forklift), a power steering device is generally used. As for this power steering device, an electric power steering device, in which an electric motor is used as an assisting power source, has recently spread. As compared with a hydraulic type power steering device, the electric type power steering device is advantageous as follows. The electric type power steering device can be made compact and further the weight can be reduced. Furthermore, an intensity of assisting torque can be easily controlled. Therefore, a loss of power caused in an engine can be decreased. In this case, a reduction gear is incorporated into this electric power steering device. Concerning this reduction gear, it is conventional to use a reversible type worm gear reducer having a large lead angle, the power transmitting direction of which can be reversed.  
         [0006]      FIGS. 17 and 18  are views showing an example of the worm gear reducer of the background art into which the above electric type power steering device is incorporated. This worm gear reducer is provided in the reduction gear housing  1  which is fixed to the electric motor  1 . The worm gear reducer includes: a worm shaft  4 , in the intermediate portion in the axial direction of which the worm  3  is provided; and a worm wheel  5  meshed with this worm  3 . The worm shaft  4  is pivotally supported inside the housing  2  by a pair of ball bearings  6 ,  6  which are outwardly engaged with both end portions in the axial direction of the worm shaft  4 . When one end portion of the worm shaft  4 , that is, the left end portion of the worm shaft  4  in  FIG. 17 , is connected with the output shaft  7  of the electric motor  1 , this worm shaft  4  can be freely rotated being driven.  
         [0007]     The worm wheel  5  is pivotally arranged inside the housing  2 , and the rotary central axis of the worm wheel  5  itself is arranged at a twisting position with respect to the worm shaft  4 . When the tooth portion  8  formed in the outer circumferential edge portion of the worm wheel  5  is meshed with the worm  3 , torque can be freely transmitted between the worm wheel  5  and the worm shaft  4 . In this connection, in the example shown in the drawing, the inner end portion and the intermediate portion in the radial direction of the worm wheel  5  are made of metal, and the outer end portion (the hatched portion shown in  FIGS. 17 and 18 ) in the radial direction including the tooth portion  8  is made of synthetic resin  9 . Due to the above structure, it is possible to reduce an intensity of the knocking and sliding noise generated in the meshing portion at the time of operation. This worm wheel  5  is outwardly fixed to an intermediate portion of the steering shaft  10 . Due to this structure, it is possible to transmit a rotating drive force generated by the electric motor  1  to the steering shaft  10  via the worm gear reducer.  
         [0008]     When the lead angle θ defined between the worm  3  and the worm wheel  5  is sufficiently large, a reverse operation of the worm gear reducer can be conducted. That is, a rotation can be transmitted from the worm wheel  5  to the worm  3 . In this connection, in order to increase the lead angle θ between the worm  3  and the worm wheel  5  while the reduction ratio of the worm gear reducer and the outer diameter of the worm wheel  5  are being maintained constant, as shown in  FIGS. 19A and 19B , it is necessary to decrease the outer diameter of the worm  3 . Accordingly, in the worm gear reducer shown in  FIGS. 17 and 18 , as compared with the worm gear reducer in which the reversible operation not needed and the lead angle θ thereof is small, the outer diameter of the operational worm  3  is greatly decreased.  
         [0009]     The electric type power steering device including the aforementioned worm gear reducer is operated as follows. When a driver operates a steering wheel so as to give a steering angle to the wheels to be steered, the steering shaft  10  is rotated. Then, a torque sensor (not shown) detects a rotary direction and torque of this steering shaft  10  and sends a signal expressing the thus detected value to a controller (not shown). This controller energizes the electric motor  1  and rotates the steering shaft  10  via the worm gear reducer in the same rotating direction as that of the operation of the steering wheel. As a result, the steering shaft  10  is rotated being given an assisting torque, which is generated by the electric motor  1  when it is energized, in addition to the torque generated by the operation of the steering wheel. Therefore, it is possible to decrease an intensity of torque, which is required for the driver to operate the steering wheel, by an intensity of torque generated by the assisting power. On the other hand, for example, when the electric motor  1  connected with the worm shaft  4  is out of order, the worm gear reducer can be reversed by operating the steering wheel. Due to the foregoing, it is possible to prevent the occurrence of a case in which the steering wheel can not be operated because of the problem of the electric motor  1 .  
         [0010]     In this connection, in the case where the worm wheel  5  having the above-described worm gear reducer is manufactured, it is common that the tooth portion  8  of this worm wheel  5  is machined with a hob cutter as described in Japanese Patent Unexamined Publication Nos. JP-A-2002-254478 and JP-A-2003-334724.  FIG. 20  is a view showing circumstances in which the tooth portion  8  is machined with the hob cutter  11 . In the case where this tooth portion  8  is machined, as shown in the drawing, the central axis of the intermediate material  12  (the worm wheel  5  in the state before the tooth portion is formed) of the worm wheel  5  and the central axis of the hob cutter  11  are arranged at positions so that the two central axes can be twisted from each other. In this state, the intermediate material  12  and the hob cutter  11  are respectively rotated at speeds according to the gear ratio of the worm gear reducer. Under this condition, the outer circumferential edge portion of the intermediate material  12  is cut with the hob cutter  11  to form the tooth portion  8  in the outer circumferential edge portion.  
         [0011]     In the case where the tooth portion  8  is formed as described above, when the hob cutter  11  is used, the diameter of which is the same as that of the worm  3 , it is possible to extend the contact area of the tooth portion  8 , which has already been machined, with the worm  3 , that is, the contact pressure can be decreased. Therefore, the durability of the worm gear reducer can be sufficiently ensured. However, as described before, in the case of the worm gear reducer incorporated into the electric type power steering device, it is necessary to make the electric type power steering device operate reversibly, that is, it is necessary to make the lead angle θ larger. Therefore, as compared with the worm gear reducer which is not reversibly operated, the lead angle θ of which is small, in the case of the worm gear reducer incorporated into the electric type power steering device, the outer diameter of the worm  3  is greatly decreased. Therefore, since the hob cutter  11  has pluralities of slits along with an axial direction thereof, when using the hob cutter  11  the diameter of which is the same as that of the worm  3 , it becomes impossible to sufficiently ensure the mechanical strength of the hob cutter  11 .  
         [0012]     For the above reasons, the hob cutter  11 , the diameter of which is larger than that of the worm  3 , is actually used. However, only when the hob cutter  11  of large diameter is used as described above, the contact area of the tooth portion  8 , which has already been machined, with the worm  3  (shown in FIGS.  17  to  19 ) is so small (it leads increase of contact pressure) that it becomes difficult to ensure the durability of the worm gear reducer. Therefore, in the actual case, after the tooth portion  8  has been machined with the hob cutter  11 , the tooth portion  8  and the worm  3  are meshed with each other. Then, while a load is being given to this meshing portion, the worm gear reducer is driven being subjected to the running-in process. Due to the foregoing, the tooth face of the tooth portion  8  is machined, so that the contact area of the tooth portion  8  with the worm  3  can be increased. However, when the worm gear reducer is driven being subjected to running-in process as described above, the manufacturing cost of the worm gear reducer is raised.  
         [0013]     Furthermore, recently, in order to enhance the efficiency of the worm gear reducer, the tooth face of the worm  3  is subjected to the mirror-finish process in many cases. In the case of the worm  3 , the tooth face thereof is subjected to the mirror-finish process, even when this worm  3  is driven being subjected to the running-in process, it is impossible to cut the tooth face of the tooth portion  8 . Accordingly, there is a possibility that an object of conducting the running-in process on the worm  3  can not be accomplished.  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention has been accomplished in view of the above circumstances. It is an object of the present invention to realize a manufacturing method for a worm wheel in which a contact area of the contact portion of the worm wheel with the worm can be sufficiently extended. It is another object of the present invention to realize a worm gear reducer in which a contact area of the contact portion of the worm wheel with the worm can be sufficiently extended.  
         [0015]     According to a first aspect of the present invention, there is provided a manufacturing method for a worm wheel, the worm wheel meshed with an operational worm at usage, comprising the steps of:  
         [0016]     preparing a worm wheel, in which at least a surface layer of a tooth portion is made of synthetic resin;  
         [0017]     preparing a machining worm, in which pitch of teeth of the machining worm in an axial direction thereof is the same as that of an operational worm, diameters of tip and root portions of the tooth of the machining worm are equal to or more than those of the operational worm and abrasive grains are provided on a surface of the machining worm by means of electro-deposition;  
         [0018]     disposing the machining worm in a twisting position relative to the worm wheel; and  
         [0019]     rotating the machining worm and the worm wheel with meshing each other so as to machining the tooth portion of the worm wheel.  
         [0020]     Note that it is sufficient that at least the tooth face of the tooth portion which contacts with the operation worm is made by synthetic resin.  
         [0021]     In this connection, the machining worm, in which the tooth pitch in the axial direction of which is the same and the diameters of the tip and root portions of which are the same as those of the operational worm, means that the shape and size of the machining worm are the same as those of the operational worm.  
         [0022]     According to a second aspect of the present invention, as set forth in the first aspect of the present invention, it is preferable that the diameter of the tip and root portions of the machining worm are a little larger than the diameters of the tip and root portions of the operational worm.  
         [0023]     According to a third aspect of the present invention, as set forth in the first aspect of the present invention, it is preferable that the manufacturing method for the worm wheel, further comprising the step of:  
         [0024]     prior to the step of machining of the tooth portion of the worm wheel, preparing a cylindrical intermediate material for the worm wheel; and  
         [0025]     forming a shape of an outer peripheral face of the intermediate material into a shape close to a shape of the tooth portion of the worm wheel relative to the original cylindrical shape of the intermediate material, wherein the forming of the shape of the outer peripheral face of the intermediate material is conducted by other than the machining worm.  
         [0026]     According to a fourth aspect of the present invention, as set forth in the first aspect of the present invention, it is preferable that the synthetic resin of the worm wheel includes glass fiber.  
         [0027]     According to a fifth aspect of the present invention, as set forth in the first aspect of the present invention, it is preferable that a shape of the operational worm is a cylindrical or a middle-recessed drum shape which has a large diameter portions at both ends thereof and a small diameter portion at a middle thereof.  
         [0028]     According to a sixth aspect of the present invention, as set forth in the first aspect of the present invention, it is preferable that the tooth portion of the worm wheel is machined with the machining worm while a machining portion is cooled by air without using machining oil.  
         [0029]     According to a seventh aspect of the present invention, there is provided a worm gear reducer, comprising:  
         [0030]     an operational worm; and  
         [0031]     a worm wheel meshing with the operational worm, at least a surface layer of a tooth portion thereof being made of synthetic resin,  
         [0032]     wherein the worm wheel is formed by disposing a machining worm in a twisting position relative to the worm wheel and rotating the machining worm and the worm wheel with meshing each other so as to machining the tooth portion of the worm wheel, and  
         [0033]     wherein pitch of teeth of the machining worm in an axial direction thereof is the same as that of an operational worm, diameters of tip and root portions of the tooth of the machining worm are equal to or more than those of the operational worm and abrasive grains are provided on a surface of the machining worm by means of electro-deposition.  
         [0034]     Especially, in the worm gear reducer according to the seventh aspect of the present invention, the worm wheel is manufactured by the manufacturing method according to any one of the first through sixth aspects of the present invention.  
         [0035]     According to an eighth aspect of the present invention, as set forth in the seventh aspect of the present invention, it is preferable that surface roughness Ra on a worm tooth face of the tooth portion is 0.1 μm or less. Note that the surface roughness Ra (arithmetical mean roughness) is defined in Japanese industrial Standard (JIS) as JIS B 0601-1994. When the above constitution is adopted, it is possible to reduce a sliding resistance of the meshing portion of the worm wheel with the worm, and the worm gear reducer can be highly effectively operated.  
         [0036]     According to a ninth aspect of the present invention, there is provided a machining worm for machining a worm wheel meshed with an operational worm, comprising:  
         [0037]     a tooth portion in which pitch of teeth in an axial direction thereof is the same as that of an operational worm, diameters of tip and root portion of the tooth are equal to or more than those of the operational worm,  
         [0038]     wherein abrasive grains are provided on a surface of the machining worm by means of electro-deposition.  
         [0039]     As described above, in the case of the manufacturing method for a worm wheel of the present invention, concerning the tool used for machining the tooth portion of the worm wheel, a machining worm is used, the shape and size of which are substantially the same as those of the operational worm. Therefore, even when the running-in process described before is not conducted after machining has been conducted by the worm used for machining, it is possible to extend a contact area of the worm wheel after machining with the worm at the time of use can be extended, that is, the contact pressure can be decreased. Different from the case in which the hob cutter is used, according to the present invention, the tooth portion formed on the outer circumferential face of the machining worm is spirally continued and no slits are formed in the middle. Therefore, even when the machining worm is used, the shape and size of which are substantially the same as those of the operational worm, in other words, even when the machining worm, of which outer diameter is small, is used, the mechanical strength of the machining worm can be sufficiently ensured.  
         [0040]     In the worm gear reducer of the present invention described above, the worm wheel manufactured by the manufacturing method of the present invention is used. Therefore, it is possible to extend a contact area of the meshing portion of the worm wheel with the worm. Accordingly, the durability of the worm gear reducer can be sufficiently ensured.  
         [0041]     In a case where executing the manufacturing method for a worm wheel of the present invention, as described in the second aspect of the present invention, it is preferable that the tip diameter and the root diameter of the tooth portion of the machining worm are respectively a little larger than the diameters of the tip and the root of the worm meshed with the operational worm wheel, for example, the tip diameter and the root diameter of the machining worm are respectively larger than those of the operational worm by 1 to 10%, more preferably by 1 to 3%.  
         [0042]     Due to the foregoing, a clearance of the worm gear reducer can be made a positive value (&gt;0). Wherein the clearance of the worm gear reducer is defined as a distance along with a direction which is commonly perpendicular to central axes of the worm wheel and the operational worm, and the clearance is the distance between a circle of a tooth root of the worm wheel and a circle of a tooth tip of the operational worm, or defined between a circle of the tooth tip of the worm wheel and a circle of the tooth root of the operational worm, when meshing the worm wheel after the completion of machining worm wheel with the operation worm.  
         [0043]     In general, when the worm gear reducer is used, it is possible to prevent the occurrence of interference of the tip (root) of the operational worm with the root (tip) of the worm wheel in the case where the clearance of the worm gear reducer is positive (&gt;0) relative to a case where the clearance of the worm gear reducer is zero. As a result, it is possible to prevent the meshing portion of the worm with the worm wheel from being locked.  
         [0044]     According to the second aspect of the present invention, it is possible to prevent the tip (root) of the worm wheel after the completion of machining from interfering with the root (tip) of the operational worm. As a result, it is possible to prevent the meshing portion of the worm with the worm wheel from being locked. Further, by the entire meshing portion of the worm wheel with the worm, it is possible to generate a gap between the tip (root) of the worm wheel and the root (tip) of the worm at all times. Therefore, lubricant can easily proceed to a position between the tip (root) of the worm wheel and the root (tip) of the worm. Accordingly, the meshing portion can be lubricated in an excellent condition, and the durability of the worm gear reducer can be sufficiently ensured.  
         [0045]     When executing the manufacturing method for a worm wheel of the present invention, as described in the third aspect of the present invention, prior to machining of the tooth portion of the worm wheel, it is preferable to prepare a cylindrical intermediate material for the worm wheel, and forming a shape of an outer peripheral face of the intermediate material into a shape close to a shape of the tooth portion of the worm wheel relative to the original cylindrical shape of the intermediate material. Wherein the forming of the shape of the outer peripheral face of the intermediate material is conducted by other than the machining worm.  
         [0046]     In this case, for example, the forming of the shape of the outer peripheral face of the intermediate material is conducted is injection molding of the synthetic resin or machining conducted by a hob cutter of which diameter is larger than that of the machining worm.  
         [0047]     Due to the foregoing, a machining removal of the machining worm can be decreased. Therefore, life of the machining worm can be prolonged, and further the entire machining time can be reduced. Accordingly, the manufacturing cost can be decreased. Especially, when injection molding of synthetic resin is adopted for the means except for the machining conducted by the machining worm, it is possible to enhance the yield of material (synthetic resin). Accordingly, the manufacturing cost can be further reduced.  
         [0048]     According to the fourth aspect of the present invention, even in the case in which synthetic resin constituting the objective worm wheel includes glass fiber, the manufacturing method for a worm wheel of the present invention can be preferably executed.  
         [0049]     That is, when synthetic resin composing the worm wheel contains glass fiber, the mechanical strength and the rigidity can be enhanced. Accordingly, the durability of the tooth portion made of this synthetic resin can be enhanced. However, since the cutting resistance (and the grinding resistance) of the synthetic resin containing glass fiber is higher than that of the synthetic resin not containing glass fiber, when machining is conducted with a hob cutter having small diameter, it is difficult to sufficiently ensure the durability of this hob cutter. On the other hand, since the mechanical strength of the machining worm of the present invention is sufficiently higher than that of the hob cutter of the same size, even when synthetic resin containing glass fiber is ground (or cut), it is possible to ensure a sufficiently high durability. Therefore, according to the present invention, it is possible to manufacture a highly durable worm wheel made of synthetic resin containing glass fiber at low manufacturing cost.  
         [0050]     According to the fifth aspect of the present invention, the manufacturing method for the worm wheel of the present invention can be executed even when the operational worm is a cylindrical worm or a middle-recessed drum shaped worm. Wherein the meddle-recessed drum shape has a large diameter portions at both ends thereof and a small diameter portion at middle thereof. In this case, the middle-recessed drum worm is defined as a worm in which the shape of bus line of the root (broken line β) is a middle-recessed drum shape irrespective of the shape of a bus line of the tip (solid line α) as a model is shown in  FIGS. 21A and 21B .  
         [0051]     In this case, as the machining worm, a cylindrical worm or a middle-recessed drum worm is adopted in accordance with the cylindrical or middle-recessed drum shaped operational worm.  
         [0052]     Further, when the manufacturing method for a worm wheel of the present invention is executed, according to the sixth aspect of the present invention, it is preferable that machining by the machining worm is conducted without using machining lubricant, that is, by dry-machining while the machined portion is being cooled by air (air-cooling).  
         [0053]     Due to the foregoing, the tooth portion can be effectively machined by the machining worm. Concerning abrasive grains which are electro-deposited on the surface of the machining worm, about half of the outer diameter of the abrasive grain is embedded in the surface of the machining worm. Therefore, at the time of machining the tooth portion, the feeding speed of the machining worm toward the worm wheel (the intermediate material of the work wheel) must be set lower than about a half of the outer diameter of the abrasive grain. Accordingly, in order to prevent the deterioration of the machining efficiency, it is necessary to increase the rotating speed of the machining worm to a value 5 to 10 times as high as the rotating speed of the conventional hob cutter. Of course, the rotating speed of the intermediate material is increased by the same ratio as that of the rotating speed of the machining worm. However, the following problems may be encountered. When machining is conducted at high speed as described above, a quantity of heat generated from the machined portion is increased. Accordingly, synthetic resin to be machined is melted. As a result, clogging is caused in the machining worm, and it becomes difficult to effectively conduct machining after occurring of the clogging. In order to solve the above problems, machining conducted by the machining worm is executed by means of dry-machining while air-cleaning is being conducted on the portion to be machined. Then, the synthetic resin, which has been melted in the above process, can be made into a film-shape by quenching and blown away. Therefore, it is possible to prevent the occurrence of clogging of the machining worm, and the tooth portion can be effectively machined.  
         [0054]     In a case where the worm gear reducer of the present invention is executed, according to the eighth aspect of the present invention, it is preferable that the surface roughness Ra of the tooth face of the worm is 0.1 μm or less. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0055]      FIG. 1A  is a front view of an operational worm according to Embodiment 1 of the present invention;  
         [0056]      FIG. 1B  is a front view of a machining worm according to Embodiment 1 of the present invention;  
         [0057]      FIG. 2  is an enlarged sectional view of a surface layer portion of the machining worm;  
         [0058]      FIG. 3  is a front view showing an intermediate material of a worm wheel;  
         [0059]      FIG. 4A  is a front view showing a method of machining a tooth portion of the worm wheel with the machining worm;  
         [0060]      FIG. 4B  is a sectional view taken on line X-X in  FIG. 4A .  
         [0061]      FIG. 5  is a front view of the worm wheel.  
         [0062]      FIG. 6  is an enlarged view corresponding to portion A in  FIG. 17  which shows a state in which the worm wheel after machining and the operational worm are meshed with each other.  
         [0063]      FIG. 7A  is a front view showing the operational worm according to Embodiment 2 of the present invention;  
         [0064]      FIG. 7B  is a front view of the machining worm according to Embodiment 2 of the present invention;  
         [0065]      FIG. 8  is a partial sectional view showing a relation of the size between the machining worm and the operational worm in detail.  
         [0066]      FIG. 9  is a front view showing an intermediate material of the worm wheel.  
         [0067]      FIG. 10  is a front view showing the worm wheel.  
         [0068]      FIG. 11  is an enlarged view corresponding to portion A in  FIG. 17  which shows a state in which the worm wheel after machining and the operational worm are meshed with each other.  
         [0069]      FIG. 12A  is a front view showing the operational worm according to Embodiment 3 of the present invention  FIG. 12B  is a front view showing the machining worm according to Embodiment 3 of the present invention  FIG. 13  is an enlarged view corresponding to portion A in  FIG. 17  which shows a state in which the worm wheel after machining and the operational worm are meshed with each other.  
         [0070]      FIG. 14A  is a front view showing the operational worm according to Embodiment 4 of the present invention;  
         [0071]      FIG. 14B  is a front view showing the machining worm according to Embodiment 4 of the present invention;  
         [0072]      FIG. 15  is an enlarged view corresponding to portion A in  FIG. 17  which shows a state in which the worm wheel after machining and the operational worm are meshed with each other.  
         [0073]      FIG. 16  is a front view of an intermediate material of the worn wheel of Embodiment 5 of the present invention.  
         [0074]      FIG. 17  is a sectional view showing a portion of a power steering device into which a worm gear reducer is incorporated.  
         [0075]      FIG. 18  is an enlarged view of portion A in  FIG. 17 .  
         [0076]      FIGS. 19A and 19B  are partial sectional views for explaining a relation between the outer diameter of the worm and the lead angle θ.  
         [0077]      FIG. 20  is a partial front view showing a method of machining a tooth portion of the worm wheel with a hob cutter, which is an example of the conventional method.  
         [0078]      FIGS. 21A and 21B  are schematic illustrations showing an outline of the form of a middle-recessed drum worm. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Embodiment 1  
       [0079]     FIGS.  1  to  6  are views showing Embodiment 1 of the present invention which corresponds to the first and fourth through eighth aspects of the present invention. In this connection, the characteristics of this embodiment are the manufacturing method for a worm wheel  5   a  and the structure of a worm gear reducer including the worm wheel  5   a . The structure and operation of the other portions are the same as those of the conventional structure shown in  FIG. 17 . Therefore, the duplicated drawings and explanations are omitted or simplified here, and the characteristic portions of this embodiment will be mainly explained as follows.  
         [0080]     In this embodiment, as the tool used for machining the tooth portion  8   a  of the worm wheel  5   a , the machining worm  13  shown in  FIG. 1B  is used. This machining worm  13  is composed in such a manner that a large number of abrasive grains  14 ,  14  (shown only in  FIG. 2 ) are electro-deposited on the surface thereof, of which shape and size are the same as those of the operational worm  3  meshed with the tooth portion  8   a  of the worm wheel at the time of use as shown in  FIGS. 1A and 6 . In this connection, the fine hatched portions shown in  FIGS. 1B and 4 , the portions shown in  FIGS. 1B and 4 , which can be seen as if they were painted out black, are portions in which a large number of abrasive grains  14 ,  14  are electro-deposited. In this embodiment, the intermediate material shown in  FIG. 3 , the outer circumferential face of which is formed cylindrical, is used for the intermediate material  12   a  for making the above worm wheel  5   a . Note that, in this embodiment, the synthetic resin  9   a  composing the outer end portion in the radial direction of this intermediate material  12   a  contains glass fiber.  
         [0081]     In this embodiment, in the case where the tooth portion  8   a  of the worm wheel  5   a  is machined with the machining worm  13 , as shown in  FIG. 4 , the central axis of the intermediate material  12   a  and the central axis of the machining worm  13  are arranged at positions where the two axes are twisted to each other, and the intermediate material  12   a  and the machining worm  13  are respectively rotated at speeds corresponding to the reduction gear ratio of the worm gear reducer. When the above machining worm  13  is pushed to the outer circumferential face of the above intermediate material  12   a  under this condition, the outer circumferential face of the above intermediate material  12   a  is ground (or cut). This grinding (or cutting) work is conducted until a distance between the central axis of the machining worm  13  and the central axis of the intermediate material  12   a  becomes a predetermined value. For example, a distance between the central axis of the machining worm  13  and the central axis of the intermediate material  12   a  after the completion of machining becomes a distance between the central axis of the worm wheel after the completion and the central axis of the worm. When machining is conducted as described above, the tooth portion  8   a  (shown in  FIGS. 5 and 6 ) is formed on the outer circumferential face of the intermediate material  12   a.    
         [0082]     The outer diameter of the intermediate material  12   a  before conducting the above machining can be arbitrarily determined. For example, it is possible to determine this outer diameter of the intermediate material  12   a  so that a sufficiently large clearance can be generated between the tip of the worm wheel, which has been completed, and the root of the worm.  
         [0083]     In this connection, as described before, when machining is conducted as described above, the feeding speed of the machining worm  13  toward the intermediate material  12   a  must be a value approximately not more than a half of the outer diameter of each abrasive grain  14 . Therefore, in order to prevent the deterioration of the machining efficiency, the rotating speed of the machining worm  13  is made to be 5 to 10 times as high as that of the conventional hob cutter. Of course, the rotating speed of the intermediate material  12   a  is increased by the same ratio as that of the rotating speed of the machining worm  13 . In this case, machining by the machining worm  13  is conducted by means of dry-machining while the machined portion is being cleaned by air.  
         [0084]     In this embodiment, the surface roughness Ra of the tooth face of the worm  3 , which is meshed with the worm wheel  5   a  after the completion as shown in  FIG. 6 , is made to be not more than 0.1 μm.  
         [0085]     In the case of the manufacturing method for a worm wheel of the present embodiment and also in the case of the worm gear reducer of the present embodiment described above, concerning the tool used for machining the tooth portion  8   a  of the worm wheel  5   a , the machining worm  13 , the shape and size of which are substantially the same as those of the operational worm  3 , is used. Therefore, even when the running-in process described before is not conducted after machining has been completed by the machining worm  13 , as shown in  FIG. 6 , it is possible to extend a contact area of the tooth portion  8   a  of the worm wheel  5   a  with the operational worm  3  can be extended, that is, the contact pressure can be decreased. Further, in the present embodiment, the synthetic resin  12   a  composing the worm wheel  5   a  contains glass fiber. Therefore, it is possible to ensure a sufficiently high durability of the worm gear reducer.  
         [0086]     In the case of the present embodiment, being different from the case in which the hob cutter  11  (shown in  FIG. 20 ) is used, the tooth portion formed on the outer circumferential face of the machining worm  13  is spirally continued and no slits are formed in the middle portion. Therefore, even when the machining worm  13  of which shape and size are substantially the same as those of the operational worm  3 , that is, when the machining worm  13  of which outer diameter is small is used, the mechanical strength of the machining worm  13  can be sufficiently ensured. Accordingly, even in the case where the synthetic resin  12   a  composing the worm wheel  5   a  contains glass fiber as described in this embodiment and the cutting resistance (grinding resistance) of this synthetic resin  12   a  is high, the durability of the machining worm  13  can be sufficiently ensured.  
         [0087]     In this embodiment, machining conducted by the machining worm  13  is executed by means of dry-machining while air-cleaning is being conducted on the portion to be machined. Therefore, when machining is conducted at high rotating speed, the synthetic resin  12   a , which has been melted in the above process, can be made into a film-shape by rapid quench and blown away. Therefore, it is possible to prevent the occurrence of clogging of the machining worm  13 , and the tooth portion  8   b  can be effectively machined.  
         [0088]     Further, in this embodiment, the surface roughness Ra of the tooth face of the operational worm  3  meshed with the tooth portion  8   a  of the worm wheel  5   a  after the completion is made to be not more than 0.1 μm. Therefore, it is possible to reduce a sliding resistance of the meshing portion of the worm wheel  5   a  with the worm  3 , and the worm gear reducer can be highly effectively operated.  
       Embodiment 2  
       [0089]     FIGS.  7  to  11  are views showing Embodiment 2 of the present invention which corresponds to the first and fourth through eighth aspects of the present invention. In this embodiment, concerning the tool used for machining the tooth portion  8   b  of the worm wheel  5   b , the machining worm  13   a  shown in  FIG. 7B  is used. This machining worm  13   a  is composed in such a manner that a large number of abrasive grains  14 ,  14  (shown in  FIG. 2 ) are electro-deposited on the surface of the worm, the shape and size of which are the same as those of the operational worm  3  meshed with the tooth portion  8   b  of the worm wheel at the time of use as shown in  FIGS. 7A and 11 . In this connection, the fine hatched portions (which can be seen as if they were painted out black) shown in  FIG. 7B , is a portion in which a large number of abrasive grains  14 ,  14  are electro-deposited. Due to the foregoing, the portion, on which a large number of abrasive grains are electro-deposited in the machining worm  13   a , is made a little larger than the corresponding portion of the operational worm  3 . Specifically, although pitch P of the teeth in the axial direction is the same, the diameter D 13a  of the tip circle and the diameter d 13a  of the root circle are made a little larger than the diameter D 3  of the tip circle of the operational worm  3  and the diameter d 3  of the root circle, for example, by 1 to 10% with respect to the operational worm  3 , preferably 1 to 3%, and by 0.1 to 1.0 mm in the actual size, preferably 0.1 to 0.3 mm in the actual size. (D 13a &gt;D 3 , d 13a &gt;d 3 ) Accordingly, as the detail is shown in  FIG. 8 , the diameter D 13a  of the tip circle of the machining worm  13   a  shown by a solid line and the diameter d 13a  of the root circle are a little larger than the diameter D 3  of the tip circle of the worm  3  shown by a two-dotted chain line and the diameter d 3  of the root circle, that is, D 13a &gt;D 3 , d 13a &gt;d 3 .  
         [0090]     In the same manner as that of Embodiment 1 described before, in this embodiment, concerning the intermediate material  12   a  for making the above worm wheel  5   b , as shown in  FIG. 9 , the outer circumferential face of the intermediate material  12   a  is formed into a cylindrical face, and the outer edge portion in the radial direction of the intermediate material  12   a  is made of the synthetic resin  9   a  containing glass fiber.  
         [0091]     In this embodiment, in the case where the tooth portion  8   b  of the above worm wheel  5   b  is machined, the above machining worm  13   a  is used, and by the same machining method (shown in  FIG. 4 ) as that of Embodiment 1 described before, the outer circumferential face of the intermediate material  12   a  is ground (or cut).  
         [0092]     In the case of the manufacturing method for a worm wheel of the present embodiment and also in the case of the worm gear reducer of the present embodiment described above, in the same manner as that of Embodiment 1, concerning the tool used for machining the tooth portion  8   b  of the worm wheel  5   b , the machining worm  13   a , the shape and size of which are substantially the same as those of the operational worm  3 , is used. Therefore, even when the running-in process described before is not conducted after machining has been completed by the machining worm  13   a , it is possible to extend a contact area of the worm wheel  5   b  with the operational worm  3 , that is, the contact pressure can be decreased.  
         [0093]     Especially, in the case of the present embodiment, since the above machining worm  13   a  is used, as shown in  FIG. 11 , the clearance of the worm gear reducer, in which the worm wheel after the completion of machining and the worm are meshed with each other, can be made to be a positive value. Accordingly, in the case of the worm gear reducer including the worm wheel  5   b  manufactured by the manufacturing method of the present embodiment, it is possible to prevent the occurrence of interference of the tip (root) of the worm wheel  5   b  with the root (tip) of the operational worm  3 . As a result, it is possible to prevent the meshing portion of the worm with the worm wheel from being locked. Further, in the entire meshing portion of the worm wheel  5   b  with the operational worm  3 , the clearance can be generated at all times between the tip (root) of the worm wheel  5   b  and the root (tip) of the worm  3 . Therefore, lubricant can easily proceed to between the tip (root) of the worm wheel and the root (tip) of the worm. Accordingly, the meshing portion can be lubricated in an excellent condition. Accordingly, the durability of the worm gear reducer can be sufficiently ensured. Other points of the structure and operation of this embodiment are the same as those of Embodiment 1 described above.  
       Embodiment 3  
       [0094]      FIGS. 12 and 13  are views showing Embodiment 3 of the present invention which corresponds to the first and fourth through eighth aspects of the present invention. In the case of Embodiment 1 described before, an object to be manufactured is the worm wheel  5   a  which is meshed with the cylindrical operational worm  3 . On the other hand, in this embodiment, as shown in  FIG. 13 , an object to be manufactured is the worm wheel  5   c  which is meshed with the middle-recessed drum worm  3   a . In this embodiment, in the same manner as that of Embodiment 1 described before, the machining worm  13   b  shown in  FIG. 12B  is composed in such a manner that a large number of abrasive grains  14 ,  14  (shown in  FIG. 2 ) are electro-deposited on a surface of the middle-recessed drum worm, the shape and size of which are the same as those of the operational worm  3   a  at the time of use shown in  FIGS. 12A and 13 . In this connection, the fine hatched portion (which can be seen as if they were painted out black) shown in  FIG. 12B  is a portion in which a large number of abrasive grains  14 ,  14  are electro-deposited. The other points of the structure and operation are the same as those of Embodiment 1 described before.  
       Embodiment 4  
       [0095]      FIGS. 14 and 15  are views showing Embodiment 4 of the present invention which corresponds to the first, second and fourth through eighth aspects of the present invention. In the case of Embodiment 2 described before, an object to be manufactured is the worm wheel  5   b  which is meshed with the cylindrical operational worm  3 . On the other hand, in this embodiment, as shown in  FIG. 15 , an object to be manufactured is the worm wheel  5   d  which is meshed with the middle-recessed drum operational worm  3   a . In this embodiment, in the same manner as that of Embodiment 2 described before, a machining worm  13   c  shown in  FIG. 14B  is composed in such a manner that, a large number of abrasive grains  14 ,  14  (shown in  FIG. 2 ) are electro-deposited on a surface of the middle-recessed drum worm in which pitch P in the axial direction is the same and the diameters of the tip circle and the root circle are respectively a little large as compared with the worm  3   a  at the time of use shown in  FIGS. 14A and 15 . In this connection, the fine hatched portion (which can be seen as if they were painted out black) shown in  FIG. 14B  is a portion in which a large number of abrasive grains  14 ,  14  are electro-deposited. The other points of the structure and operation are the same as those of Embodiment 2 described before.  
       Embodiment 5  
       [0096]      FIG. 16  is a view showing Embodiment 5 of the present invention which corresponds to the first through eighth aspects of the present invention. In this embodiment, as shown in  FIG. 16 , before the tooth portion of the worm wheel is machined with the machining worm, by a means (for example, by means of injection molding of synthetic resin or by means of cutting with a hob cutter of which diameter is larger than that of the machining worm), the shape of the outer circumferential face of the intermediate material  12   b  for making the worm wheel is formed into a shape which is closer to the shape of the tooth portion after the completion than the cylindrical face.  
         [0097]     In the case of this embodiment, a machining removal of the machining worm can be decreased. Therefore, life of the machining worm can be prolonged, and further the entire machining time can be reduced. Accordingly, the manufacturing cost can be decreased. Especially, when injection molding of synthetic resin is adopted as the means except for the machining conducted by the machining worm, it is possible to enhance the yield of material (synthetic resin). Accordingly, the manufacturing cost can be further reduced. The other points of the structure and operation of this embodiment are the same as those of Embodiments 1 to 4 described before.  
         [0098]     While there has been described in connection with the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modification may be made therein without departing from the present invention, and it is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the present invention.