Abstract:
In a method and a bar blade for milling spiral bevel gears and hypoid gears, a shaft of the bar blade has a blade profile that is formed by the cross-section of at least one cutting area, at least two free areas, and at least one top area. The blade profile is provided with a first cutting edge for a first flank, a second cutting edge for a second flank that faces the first flank, and a top cutting edge for the bottom of a tooth space. The first and the second cutting edge are embodied as principal cutting edges for completely cutting the first or second flank. The top cutting edge is configured for completely cutting the bottom of the tooth space such that the tooth space comprising the complete final geometry is created in one milling process by means of one and the same bar blade.

Description:
FIELD OF THE INVENTION 
   This invention relates to a method for milling spiral bevel gears and hypoid gears using profile-sharpened or profile-sharpened and additionally form-ground bar cutting blades each having a shank and at one end of the shank a cutting edge profile which, for producing a tooth slot, enables a first tooth flank, at least a portion of the bottom of the tooth slot, and least a portion of a second tooth flank lying opposite said first tooth flank to be cut. 
   Furthermore, the present invention relates to a profile-sharpened or profile-sharpened and additionally form-ground bar cutting blade for milling spiral bevel gears and hypoid gears, with a shank and with a cutting edge profile formed at one end of the shank by the intersection of at least one rake surface, at least two clearance surfaces and at least one top surface, said cutting edge profile including, for producing a tooth slot, a first cutting edge for a first tooth flank, a second cutting edge for at least a portion of a second tooth flank opposite said first tooth flank, and a top cutting edge for at least a portion of the bottom of the tooth slot. 
   Finally, the present invention relates to a use of at least one profile-sharpened or profile-sharpened and additionally form-ground bar cutting blade of the aforementioned type. 
   BACKGROUND OF THE INVENTION 
   Such a method and such a bar cutting blade are known from DE 694 05 978 T2 which will be discussed in greater detail further below. To quote from the aforementioned document already at this point, in a bar cutting blade referred to as “profile-sharpened”, the top surface and the two clearance surfaces are ground to restore and resharpen the cutting blade. In this type of bar cutting blade the rake surface is not ground. Such profile-sharpened bar cutting blades may be used for the removal of stock from the outside or concave flank of a tooth slot (outside cutting blade), from the inside or convex flank of a tooth slot (inside cutting blade) and/or from the bottom of the tooth slot (rough cutting blade). By contrast, in a bar cutting blade referred to as “profile-sharpened and form-ground’, sharpening involves grinding the top surface, the two clearance surfaces and the rake surface. 
   From U.S. Pat. No. 1,667,299 from the year 1928, a form cutting blade is known which is reground only on the rake surface for sharpening. According to the aforementioned definition, a form cuffing blade is a cutting blade known as a “form-ground” cutting blade. Such a form cutting blade or form-ground cutting blade is no bar cutting blade but has a short shank dimensioned as wide as possible in the direction normal to the cutting blade longitudinal axis, because when such a cutting blade is reground stock is ground off in a direction normal to the cutting blade longitudinal axis. The usable profile length for regrinding is determined by the thickness of the form cutting blade normal to the cutting blade longitudinal axis. The profile of the form cutting blade is fixed and cannot be altered by resharpening. The form cutting blade has its two clearance surfaces relieved in an arc-shaped configuration. A variety of pressure angles are obtained by suitably coarsely stepped cutting blades. The shape of the clearance surfaces results necessarily from a selected clearance angle on the top cutting edge and the necessary relief grind. The design constraints with regard to a technologically advantageous clearance and rake angle are accordingly high. 
   From the article “Spiral- und Hypoidkegelrãder nach dem Spiroflex-Verfahren” (spiral and hypoid bevel gears according to the Spiroflex technique) by Erich Kotthaus, German journal “Werkstatt und Betrieb”, 1967, pages 602-606, the following additional aspects result in this context. In a form cutting blade the tangent of the flank clearance angle must be equal to the tangent of the normal pressure angle times the tangent of the top clearance angle. For grinding, the cutter head with the form cutting blades held therein is mounted on a special sharpening machine, and each of the form cutting blades has its rake surfaces reground individually in the indexing head until the wear marks on the cutting edges are abraded. In order to be able to cut as many teeth as possible per cutting blade, a long usable profile length is required. The space requirements of a cutting blade on the circumference of the cutter head are hence dependent on the profile length on the cutting blade and the space between two adjacent form cutting blades, which is necessary to ensure passage of the grinding wheel necessary for sharpening. The higher the space requirements, the lower the performance of the cutter head because fewer form cutting blades can then be accommodated on the same circumference and, hence, fewer cuts can be taken per unit of time. 
   While form cutting blades such as the one from the year 1928 cut the complete tooth slot in a single milling pass, machining techniques and quality demands brought about their replacement already a few years after their introduction, substituting a group of form cutting blades having inside and outside cutting blades of the type then used for decades in the manufacture of spiral bevel gears and hypoid gears (see U.S. Pat. No. 2,024,494 from the year 1935 and Gear Handbook by D. W. Dudley, McGraw-Hill, 1962, pages 20-24 and 20-25). U.S. Pat. No. 2,024,494 describes a cutter head with alternating inside and outside cutting blades, with which both bevel gear flanks of a tooth slot can be finished in a single cut using the same machine settings. It is only in a period after 1960 that these form cutting blade groups, which have been in use since 1935 and are comprised of at least one form cutting blade for cutting the concave flank and one form cutting blade for cutting the convex flank, have been replaced in each case by a group of at least two bar cutting blades. The reasons therefor and the advantages achievable with groups of bar cutting blades are clearly described in the aforementioned article “Spiral- und Hypoidkegelrader nach dem Spiroflex-Verfahren”. In this technique, each group of bar cutting blades includes two finishing cutters (one for the concave and one for the convex tooth flank), each having an associated roughing cutter for performing the roughing cut. The combination roughing and finishing cutters are received in a common slot. The mounting of two cutting blades in one slot and the small shank cross-section of these bar cutting blades allow a substantially denser population of cutting blades than would be possible with form cutting blades. 
   According to the current state of the art, bevel gear milling cutters continue to be used in the form of bar cutting blades. The bar cutting blades used are of high speed steel or carbide. When machining bevel gears in one milling pass, two different cutting edge profile designs of bar cutting blade are used in a cutter head. The bar cutting blades embodying the one design of cutting edge profile machine with the cutting edge arranged on the outside diameter the concave tooth flank (outside cutting blade). Bar cutting blades of this profile design have a special cutting edge geometry leading generally to a positive rake angle. The term positive or negative rake angle is defined, for example, in DIN 6581 of May 1966, page 8, FIG. 13. The bar cutting blades embodying the second design of cutting edge profile machine with the cutting edge arranged on the inside diameter the convex tooth flank (inside cutting blade). Bar cutting blades of this profile design have likewise a special, yet different, cutting edge geometry leading generally also to a positive rake angle. The possibility exists to use one or two roughing cutters in addition to the previously described bar cutting blades. 
   The number of bar cutting blades adapted to be positioned on a cutter head is limited. Due to the bar cutting blade geometry employed in the prior art, at least two different geometry designs have to be inserted into the cutter head in alternation. In this arrangement only half of the cutting blades can be involved at a time in the generation of the respective tooth flank final geometry. 
   In the method known as the Oerlikon method for manufacturing bevel gears, the cutter head is equipped with several cutting blade groups each comprised of three bar cutting blades. Each group includes an outside cutter, an inside cutter and a roughing cutter. On each Oerlikon bar cutting blade at least one rake surface and two lateral clearance surfaces at the cutting end are reground. Such cutting blades are designated as triplex flank ground cutting blades or—according to the above definition—as profile-sharpened and additionally form-ground bar cutting blades. Further details relating to the Oerlikon method are contained, for example, in the introductory part of the description of DE 19624685 C1. 
   In a method according to EP 0 203 085 B1, bar cutting blades of a profile design are used that enable the roughing cutter to be eliminated. Therefore, a group of bar cutting blades includes only two bar cutting blades, which is the reason why more bar cutting blade groups can be accommodated on a cutter head than with the aforementioned Oerlikon method. These bar cutting blades are reground on only two surfaces in the direction of the shank, so that a coating can be applied to the rake surface of these cutting blades which does not necessarily have to be renewed after sharpening, hence prolonging the life between regrinds. Such cutting blades are referred to as duplex flank ground cutting blades or—in accordance with the above definition—as profile-sharpened bar cutting blades. 
   Cutter heads in which all the bar cutting blades are arranged on a circle such that alternately one bar cutting blade works the concave flank and the next bar cutting blade works the convex flank of one and the same tooth slot, are used in the method referred to as the single indexing manufacturing method. In this method, a tooth slot continues to be machined in one milling pass until the final geometry is obtained. Then an indexing movement to the next tooth slot takes place whereupon this next tooth slot is machined in the next milling pass. By contrast, cutter heads in which the bar cutting blades are arranged in groups find application in the method known as the continuous manufacturing method, in which one cutting blade group machines the convex and the concave flanks in a tooth slot while subsequently the next group of cutting blades enters the next tooth slot where it machines the two tooth flanks. Pertinent details are contained, for example, in the Handbook of Bevel and Hypoid Gears by Hermann J. Stadtfeld, Rochester Institute of Technology, 1993, page 35. 
   A feature shared by the known methods described in the foregoing is that the machining of a tooth slot invariably requires at least two bar cutting blades whose cutting edge profile is designed to enable the cutting blades jointly to generate a complete final geometry in one milling pass. Furthermore, proper positioning of the individual bar cutting blades of a group of cutting blades in a cutter head is critical and involves an elaborate technique. 
   A method and a bar cutting blade of the type initially referred to are known from DE 694 05 978 T2 referred to initially. The bar cutting blade is of the profile-sharpened type having its primary or first cutting edge used as outside or inside cutting blade while yet including a second cutting edge on the rake surface in the region of its secondary cutting edge. To obtain the second cutting edge, a slot is produced in the rake surface which forms said second cutting edge whose rake angle differs from the rake angle of the first cutting edge. The second cutting edge cuts a portion of the bottom of the tooth slot as well as a portion of the flank opposite the flank cut by the first cutting edge. The reason for such an elaborate second cutting edge does not become readily apparent from DE 694 05 978 T2. In this document however express reference is made to U.S. Pat. No. 4,575,285. This U.S. patent is based on a prior art in which a cutting blade group is comprised of three cutting blades, i.e., an inside cutting blade, an outside cutting blade and an additional cutting blade for roughing the bottom of the tooth slot. The second cutting edge, which is produced by means of the slot, enables each inside and outside cutting blade to cut not only the associated tooth flank, but also a portion of the opposite flank and a portion of the bottom of the tooth slot. The purpose of this is to omit the need for the roughing cutter and to reduce the cutting blade group to two cutting blades. Hence, two cutting blades per group continue to be necessary in order to be able to generate a tooth slot to its complete final geometry. 
   Accompanying  FIG. 6  illustrates the engagement of a group of cutting blades including an outside cutting blade  60  and an inside cutting blade  66  in a tooth slot  51 . The outside cutting blade  60  has a primary cutting edge  61   v  and a secondary cutting edge  61   x  opposite the primary cutting edge  61   v . The inside cutting blade  66  has a primary cutting edge  67   x  and a secondary cutting edge  67   v  opposite the primary cutting edge  67   x . With its primary cutting edge  61   v , the outside cutting blade  60  machines a first flank  53  of the later tooth slot  51  to provide it with its final geometry. With its secondary cutting edge  61   x , this cutting blade machines simultaneously another flank, not shown in  FIG. 6 , which lies opposite the first flank  53 . However, said other flank is not part of the tooth slot  51  with its final geometry but is an intermediate flank serving to facilitate the work of the primary cutting edge  67   x  of the adjoining inside cutting blade  66  of the group, which cutting blade machines a second flank  54  of the tooth slot  51  opposite the first flank  53  to its final geometry. Accordingly, the second cutting edge provided on the profile-sharpened bar cutting blade according to DE 694 05 978 T2 can at best slightly improve the cutting operation of the secondary cutting edge because it has a rake angle differing from the negative rake angle of the secondary cutting edge and amounting to zero degrees in the embodiment illustrated in DE 694 05 978 T2. 
   It is an object of the present invention to provide a method and a bar cutting blade of the type initially referred to, which enable the positioning of the cutting blades in a cutter head to be simplified and the machining of bevel gears to be performed with substantially enhanced effectiveness. Furthermore it is an object to indicate a special use of the bar cutting blade. 
   SUMMARY OF THE INVENTION 
   This object is accomplished in accordance with the invention in that for milling a bevel gear at least one bar cutting blade is used with which each tooth slot is generated to a complete final geometry in one complete milling pass. 
   Furthermore, proceeding from a bar cutting blade of the type initially referred to, this object is accomplished in accordance with the present invention in that the first and the second cutting edge are designed as cutting edges for completely cutting the first and the second tooth flank, respectively, and that the top cutting edge is designed for completely cutting the bottom of the tooth slot, thus enabling the tooth slot to be generated to its complete final geometry using one and the same bar cutting blade in one milling pass. 
   With regard to the use, the object of the invention is accomplished in that at least one profile-sharpened or profile-sharpened and additionally form-ground bar cutting blade of the invention is employed in a method for milling spiral bevel gears and hypoid gears with a cutter head, wherein the or each bar cutting blade is arranged in an end face of the cutter head on a circle and in use has the cutting end of the shank protrude from the end face of the cutter head. 
   The method of the invention can be carried out substantially more easily than the known methods because a single completely cutting bar cutting blade is provided with a cutting edge profile that enables the cutting blade to generate each tooth slot to a complete final geometry in a single milling pass. The entire cutting edge profile of the bar cutting blade entering the tooth slot (outside and inside cutting edges and one top cutting edge) produces the gear final geometry. As a result, the number of cutting edges actively involved in the generation of the gear surface can be doubled while using the same cutter head. The volume of chips being cut per bar cutting blade of the invention can be equally increased a significant amount because the entire profile entering the tooth slot is involved in the machining operation. Wear of the individual bar cutting blade is diminished, because on each cutting blade the complete profile entering the tooth slot is subjected to wear phenomena. The volume of chips to be cut per cutting edge is reduced per gear, resulting in an increased parts output per cutter head population. The positioning of the individual bar cutting blade of the invention in a cutter head is accomplishable with greater ease than in the case of a pair of cutting blades or a group of cutting blades. 
   In the prior art the bar cutting blade works only one flank. A clearance space exists on the opposite side between the bar cutting blade and the adjacent tooth flank. Chip flow is invariably directed to this clearance space, with the tendency for the chips to enter the clearance space and being trapped between the gear and the bar cutting blade, resulting in damage to the tooth flank. 
   The cutting edge geometry of the bar cutting blade of the invention results in a changed chip flow which has a beneficial effect on the tooth flank surfaces. Because the cutting edge profile of the bar cutting blade of the invention encompasses the tooth slot completely on entering the tooth slot, the chip is prevented from being trapped in a clearance space between the tooth flank and the bar cutting blade. 
   The bar cutting blade of the invention is suitable for use with both the generating method and the non-generating method. 
   Moreover, the bar cutting blade of the invention may be used for performing both roughing and finishing cuts. 
   The bar cutting blade of the invention may have a cutting edge profile in which the rake surface and/or the clearance surfaces may be curved in any desired shape. 
   The cutting edge profile design of the bar cutting blade of the invention results in very small rake angles of the cutting edges and consequently in high cutting forces. Conveniently therefore, the bar cutting blade of the invention is used on modern NC hobbing machines. 
   The field of application of the completely cutting bar cutting blade of the invention is preferably the single indexing method. 
   In the use of the bar cutting blade of the invention, all the bar cutting blades inserted in the cutter head can be involved in generating the tooth slots to their complete final geometry. This affords the advantage of enabling machining time to be significantly reduced (by up to 50%). Still further, the individual bar cutting blade of the invention is exposed to materially reduced wear. 
   Novel cutter heads with odd slot numbers may hence find application. By contrast, the prior art conventionally uses cutter heads with even slot numbers, because cutting blade groups with bar cutting blades of two different profile designs are employed. 
   Cutting blade logistics is dramatically simplified because in use of the bar cutting blade of the invention all the cutting blades may absolutely have the same cutting edge geometry on a cutter head. Equally, the cutting blade turnaround volume and the related costs can be reduced significantly. 
   Advantageous embodiments of the method and of the bar cutting blade of the invention are the subject-matters of the subclaims. 
   When in an embodiment of the method of the invention the final geometry is generated by the hobbing method, the manufacture of pinions is possible in particularly simple manner. 
   When in another embodiment of the method of the invention the final geometry is generated by the plunge milling method, the manufacture of ring gears is possible in particularly simple manner. 
   When in a still further embodiment of the method of the invention the method involves a rough milling process and/or a finish milling process, the wide variety of uses of the bar cutting blade of the invention becomes apparent. 
   When in an embodiment of the bar cutting blade of the invention the cutting edge profile is formed by the intersection of one and the same rake surface with at least the two clearance surfaces and the top surface, the cutting edge profile of the bar cutting blade of the invention can be manufactured most easily regardless of whether it is a profile-sharpened or a profile-sharpened and additionally form-ground bar cutting blade. 
   When in another embodiment of the bar cutting blade of the invention the cutting edge profile is formed by the intersection of two relatively angled rake surfaces with at least the two clearance surfaces and the top surface, the rake angles on the two primary cutting edges may be selected optimally independently of each other. 
   When in another embodiment of the bar cutting blade of the invention the first and second cutting edges have rake angles equaling zero degrees in either case, this is accomplishable with a plane surface as the rake surface in simple manner by arranging the rake surface parallel to the reference plane against which the rake angle is measured. 
   When in another embodiment of the bar cutting blade of the invention the first and second cutting edges have rake angles greater than zero degrees in either case, good milling results are obtained also when the bar cutting blade is made of tool steel instead of carbide. 
   When in another embodiment of the bar cutting blade of the invention the first and the second cutting edges have rake angles smaller than zero degrees in either case, this is accomplishable in simple manner by using two relatively angled rake surfaces which protrude like a rake surface curved in convex fashion relative to the reference plane against which the rake angle is measured. Preferably, such a bar cutting blade should be made of carbide. 
   When in another embodiment of the bar cutting blade of the invention the first and second cutting edges have rake angles one of which is greater than zero degrees and the other of which is smaller than zero degrees, this is accomplishable in simple manner with a plane surface as the rake surface, which is angled in the one or the other direction relative to the reference plane against which the rake angle is measured. 
   When in another embodiment of the bar cutting blade of the invention the rake surface is worked into the shank unalterably, the rake surface is not ground during sharpening. Hence a duplex flank ground cutting blade or a profile-sharpened cutting blade is involved. 
   When in another embodiment of the bar cutting blade of the invention the rake surface between the first and second cutting edges is curved in a concave configuration, positive rake angles result on these two cutting edges. 
   When in another embodiment of the bar cutting blade of the invention for form-grinding the bar cutting blade, the rake surface is a rake surface to be reground, a triplex flank ground or profile-sharpened and form-ground bar cutting blade is involved, which affords special advantages. In such a cutting blade the rake surface and the two lateral clearance surfaces are freshly coated upon each regrinding operation. This improves the useful life expectancy of the cutting blades considerably. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will be described in more detail in the following with reference to the accompanying drawings. In the drawings, 
       FIG. 1  is a front view of a duplex flank ground or profile-sharpened bar cutting blade illustrating a first embodiment of the bar cutting blade of the present invention; 
       FIG. 2  is a side view of the bar cutting blade of  FIG. 1 . 
       FIG. 3  is a top plan view of the bar cutting blade of  FIG. 1 . 
       FIG. 4  is a front view of a triplex flank ground or profile-sharpened and form-ground bar cutting blade with a rake surface to be reground, illustrating a second embodiment of the bar cutting blade of the present invention; 
       FIG. 5  is a side view of the bar cutting blade of  FIG. 4 . 
       FIG. 6  is a top plan view of the bar cutting blade of  FIG. 4 . 
       FIG. 7  is a front view of a triplex flank ground or profile-sharpened and form-ground bar cutting blade, in which the rake surface to be reground is arranged different from the embodiment of  FIG. 4 , illustrating a third embodiment of the bar cutting blade of the present invention; 
       FIG. 8  is a top plan view of the bar cutting blade of  FIG. 7 . 
       FIG. 9  is a front view of a triplex flank ground or profile-sharpened and form-ground bar cutting blade with a rake surface to be reground, which, in contrast to the other embodiments, is curved in a concave configuration, illustrating a fourth embodiment of the bar cutting blade of the present invention; 
       FIG. 10  is a top plan view of the bar cutting blade of  FIG. 9 . 
       FIGS. 11   a - f  are views of the size and position of the rake angles for different embodiments of the bar cutting blade of the invention, i.e., in  FIG. 11   a  for the embodiment of  FIG. 1 , in  FIG. 11   b  for the embodiment of  FIG. 4 , in  FIG. 11   c  for the embodiment of  FIG. 7 , in  FIG. 11   d  for an embodiment which, while corresponding to  FIG. 4 , has its rake surface angled in inverted position relative to the reference plane, in  FIG. 11   e  for the embodiment of  FIG. 9 , and in  FIG. 11   f  for an embodiment otherwise not shown having two relatively angled rake surfaces; 
       FIG. 12  is a view of a known bar cutting blade group comprised of outside cutting blade and inside cutting blade as engaged with a tooth slot; 
       FIG. 13  is a view of the bar cutting blade of the invention as engaged with a tooth slot; and 
       FIG. 14   a  is a top plan view of a cutter head equipped with known bar cutting blades and, by comparison, of a cutter head equipped with bar cutting blades of the invention ( FIG. 14   b ), each shown during the machining of a bevel gear using the single indexing method. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-3  shows a bar cutting blade generally designated as  10  and preferably made of carbide, having a shank  12  of rectangular cross-section. The bar cutting blade  10  has at a cutting end  14  with which it protrudes, when in use, from an end face  42  of a cutter head  40  (illustrated in  FIGS. 14   a  and  14   b ) a cutting edge profile including a first cutting edge  16  for a concave tooth flank  53 , a second cutting edge  18  for a convex tooth flank  54 , and a top cutting edge  20  for the bottom  52  of a tooth slot  51  of a bevel gear  50  (shown in  FIG. 13 ). 
   Extending between the first cutting edge  16  and the second cutting edge  18  is a rake surface  22  which in this embodiment is plane and not reground when sharpening the bar cutting blade  10 . The cutting edge profile is formed by the intersection of the rake surface  22  with two clearance surfaces  17 ,  19  and one top surface  21  ( FIG. 3 ). It will be understood that more than two clearance surfaces and more than one top surface may be provided, for example, when each of these clearance surfaces has a secondary surface. The first and the second cutting edge  16 ,  18  are designed as primary or forming cutting edges for completely cutting the concave and, respectively, convex tooth flanks, and the top cutting edge  20  is designed to completely cut the bottom  52  of the tooth slot, so that in one milling pass using one and the same bar cutting blade  10  the tooth slot  51  can be produced to its complete final geometry. With reference to  FIGS. 13 and 14   a  and  14   b  a method for milling spiral bevel gears and hypoid gears is described in more detail further below. 
   The bar cutting blade  10  illustrated in  FIG. 1  is a bar cutting blade referred to as a duplex flank ground or profile-sharpened bar cutting blade in which the rake surface  22  is worked into the shank  12  unalterably as becomes apparent from  FIG. 2 . 
     FIGS. 4-6  show in the same views as in  FIGS. 1-3  a bar cutting blade referred to as a triplex flank ground or profile-sharpened and form-ground bar cutting blade  11  which has a rake surface  24  to be reground which, in contrast to the rake surface  22 , does not extend into the shank  12 . Those parts of the bar cutting blade  11  of  FIGS. 4-6  that coincide with corresponding parts of the bar cutting blade  10  of  FIG. 1  are assigned like reference numerals and therefore do not need to be described again in  FIGS. 1-3 . 
     FIGS. 7 AND 8  show as a third embodiment of the bar cutting blade of the invention the triplex flank ground or profile-sharpened and form-ground bar cutting blade  11  with the rake surface  24  to be reground in a front view ( FIG. 7 ) and in a top plan view ( FIG. 8 ). In contrast to the embodiment of  FIGS. 4-6  which shows the general case regarding the orientation of the rake surface  24 , the embodiment of  FIGS. 7 AND 8  relates to the special case in which the rake surface  24  is oriented in such manner that the two cutting edges  16  and  18  are in a symmetrical relationship to each other resulting in like rake angles Y S  (illustrated in  FIG. 11   c ). 
   In the bar cutting blade  11  of  FIGS. 7 AND 8  the rake angle Y S  differs from the rake angle of the bar cutting blade  11  of  FIGS. 4-6 , being measured according to the representation of  FIGS. 11   a - f  to be discussed in greater detail further below between the rake surface and a reference plane B of the cutting edge (cf., for example, the representation of  FIG. 11   a ), said reference plane of the cutting edge being the plane of projection in  FIGS. 3 ,  6 ,  8 ,  9  and  11   a - f . In  FIGS. 4-6  the rake angle Y S  on the first and the second cutting edges  16 ,  18  is unequal to zero degrees, while being equal to zero degrees in  FIGS. 7 and 8 . 
     FIGS. 9 AND 10  show as a fourth embodiment of the bar cutting blade of the invention the triplex flank ground or profile-sharpened and form-ground bar cutting blade  11  with a rake surface  26  to be reground in a front view ( FIG. 9 ) and in a top plan view ( FIG. 10 ). In contrast to the other embodiments, the rake surface  26  between the first and second cutting edges  16 ,  18  is curved in a concave configuration. In this case both cutting edges  16 ,  18  result in a positive rake angle. 
     FIGS. 11   a - f  show the size and the position of the rake angles Y S  for different embodiments of the bar cutting blade of the invention, i.e., in  FIG. 11   a  for the bar cutting blade  10  of  FIGS. 1-3 , in  FIG. 11   b  for the bar cutting blade  11  of  FIGS. 4-6 , in  FIG. 11   c  for the bar cutting blade  11  of  FIGS. 7 and 8 , in  FIG. 11   d  for the bar cutting blade  11  which, while corresponding to  FIGS. 4-6 , has its rake surface  24  angled in inverted position relative to the reference plane B, in  FIG. 11   e  for the bar cutting blade  11  of  FIGS. 9 and 10 , and in  FIG. 11   f  for a bar cutting blade  11 ′ otherwise not shown, which has its two rake surfaces  24   v  and  24   s  angled relative to each other. 
     FIGS. 11   a ,  11   b  and  11   d  show bar cutting blades  10  and  11  corresponding to the representations of  FIGS. 1-6 , respectively, in which the first and second cutting edges  16 ,  18  have rake angles Y S  of which one is greater than zero degrees and one smaller than zero degrees. 
   The first cutting edge  16  cuts the concave tooth flank  53 , and the second cutting edge  18  cuts the convex tooth flank  54 , which is why the rake angles on these two cutting edges are designated as Y SV  and Y SX , respectively, in accordance with the representations of  FIGS. 11   a - f . In the bar cutting blade of  FIG. 11   a  the rake angle Y SV  is smaller than zero degrees, and the rake angle Y SX  is greater than zero degrees. The same applies to the rake angles of the bar cutting blade  11  of  FIG. 11   b . In the bar cutting blade  11  of  FIG. 11   d  the rake angle Y SV  is greater than zero degrees, and the rake angle Y SX  is smaller than zero degrees. 
   In the bar cutting blade  11  of  FIG. 11   c  the first cutting edge  16  and the second cutting edge  18  have rake angles Y SV  and Y SX  respectively, both equaling zero degrees. 
   In the bar cutting blade  11  of  FIG. 11   e  in which the rake surface  24  is curved, the first cutting edge  16  and the second cutting edge  18  have rake angles Y SV  and Y SX , respectively, both being greater than zero degrees. 
   In the bar cutting blade  11 ′ of  FIG. 1   f  in which the rake surface  24  is composed of two relatively angled rake surfaces  24   v ,  24   x , the first cutting edge  16 , which is formed by the intersection of the clearance surface  17  and the rake surface  24   v , and the second cutting edge  18 , which is formed by the intersection of the clearance surface  19  and the rake surface  24   x , have each a rake angle Y SV  and Y SX , respectively, which is smaller than zero degrees. 
   A feature common to all the embodiments shown in  FIGS. 11   a - f  is that very small rake angles result which lie in a range of between 0 and ±10°. At positive rake angles the bar cutting blade of the invention may be made of tool steel. In cases where the rake angles equal zero degrees and at negative rake angles the tool should be made of carbide, because at such rake angles tool steel has less favorable wear characteristics. 
     FIG. 13  shows the bar cutting blade  10  of the invention in engagement with the tooth slot  51 . When cutting the tooth slot  51 , the cutting edge profile of one and the same bar cutting blade  10  generates the tooth slot  51  to its complete final geometry in a single milling pass. From a comparison between the  FIGS. 12 and 13  it becomes readily apparent that the bar cutting blade  10  is substituted for the known cutting blade group comprised of the outside cutting blade  60  and the inside cutting blade  66 . The primary cutting edges  61   v  and  67   x  of the known cutting blade group have been replaced by corresponding forming cutting edges  16  and  18 , respectively, of one and the same cutting blade  10 . 
     FIG. 14   a  shows in a top plan view a cutter head  40  from the end face  42  thereof protrude known bar cutting blades, that is, outside cutting blades  60  and inside cutting blades  66 . The cutting blades  60  and  66  are arranged on a circle  49 . The direction of rotation of the cutter head  40  is indicated by an arrow  44 . The cutter head  42  rotates about an axis  46 . The bevel gear  50  is a ring gear rotating about an axis  56 . 
     FIG. 14   b  shows for comparison the cutter head  40  equipped with bar cutting blades  10  of the invention. It would also be possible for the bar cutting blades  10  to be provided in a cutting blade ring according to  FIG. 14   a  in addition to the bar cutting blades  60 ,  66 . In  FIG. 14   a  the outside cutting blade  60  machines the concave flank  53 , and the inside cutting blade  66  machines the convex flank  54  of the tooth slot.  FIG. 12  is a detail of the representation of  FIG. 14   a , showing that the cuffing blades  60 ,  66  machine also the bottom  52  of the tooth slot  51 . In  FIGS. 14   a  and  14   b  the bevel gear  50  is machined using the single indexing method. This means that the tooth slot  51  is machined in one milling pass until its desired final geometry is obtained. Then an indexing movement takes place, that is, the bevel gear  50  is indexed to the next tooth slot. This next tooth slot  51  is then machined in another milling pass until it is finished, etc. In  FIG. 14   b  one and the same bar cutting blade  10  machines the tooth slot  51  using the first and the second cutting edge  16  and  18 , respectively, of the bar cutting blade  10 , working the concave and convex flank  53  and  54 , respectively, and using the top cutting edge  20  for working the bottom  52  of the tooth slot according to the representation of  FIG. 13 . The first and the second cutting edges  16 ,  18  are primary or forming cutting edges for completely cutting the concave and convex flanks  53  and  54 , respectively, so that the tooth slot  51  is worked with one and the same bar cutting blade  10  in one milling pass until its complete final geometry is generated. Then follows an indexing movement, the next tooth slot  51  is produced to its complete final geometry in the next milling pass, etc. The bar cutting blades  10  are also arranged on a circle  49  and protrude from the end face  42  of the cutter head  40 . 
   The method of the invention which is carried out using the cutter head  40  and the arrangement of the bar cutting blades  10  of  FIG. 14   b  differs from the known method carried out according to  FIG. 14   a  in that during cutting of the tooth slot  51  with the cutting edge profile of one and the same bar cutting blade  10  in one milling pass the tooth slot  51  is produced to its complete final geometry. In the method of the invention the number of cutting edges actively involved in the production of the teeth of the bevel gear  50  is double the number of the known method, using the same cutter head. The volume of chips being cut per bar cutting blade  10  is significantly higher than the volume removed per known cutting blade  60  or  66 , because in the bar cutting blade  10  of the invention the entire profile entering the tooth slot  51  is involved in the cutting action. The profile design of the bar cutting blade of the invention results in very small rake angles of the forming cutting edges  16 ,  18  and, hence, in extremely high cutting forces which, however, do not pose any problems on the currently available NC hobbing machines. 
     FIGS. 14   a  and  14   b  show each a ring gear as bevel gear. While in the arrangement chosen in  FIGS. 11   a - f  the left-hand cutting edge  16  is assigned to the concave flank, and the right-hand cuffing edge  18  to the convex flank of a tooth, this assignment could also be reversed depending on the workpiece to be milled, as, for example, in the representation of  FIGS. 14   a  and  14   b.    
   The field of application of the completely cutting bar cutting blades of the invention is predominantly the single indexing or circular arc manufacturing method, but it will be understood that the bar cutting blades of the invention may also find application in the continuous manufacturing method. 
   Considering that all the bar cutting blades  10  of the invention inserted in the cutter head  40  are involved in the generation of the final geometry of the bevel gear teeth, the method of the invention affords the advantage over the known method of  FIG. 8   a  of enabling the machining time to be reduced significantly (by up to 50%), with the individual bar cutting blade being subjected to substantially reduced wear.