Abstract:
A hob having a helically curved tooth profile. A plurality of segments covering only a limited angle range are attached to a base body for forming the tooth profile. The segments abut each other in the circumferential direction of the base body, and the segments form rows in the direction of the longitudinal axis of the base body. The segments each comprise a segment foot and a segment profile disposed diagonally thereon. The diagonal arrangement of the segment profile forms a coiled slope of the helically curved tooth profile in each segment and the tooth profile can be built up simply by arranging the segments in rows in the circumferential and the longitudinal directions.

Description:
CLAIM TO PRIORITY 
     This application is a National Phase of International Application PCT/EP2009/007346, filed on Oct. 13, 2009, which claims priority to German Application No. 10 2008 062 147.1, filed on Dec. 16, 2008 and German Application No. 10 2009 016 257.7, filed on Apr. 3, 2009, the entire contents of all of these applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a hob having a helical tooth profile. 
     2. Description of Related Art 
     A hob of this kind is disclosed in EP 0 050 270 A2. Hobs of this kind having helical tooth profiles are usually used for producing tooth profiles on gearwheels, for example on spur gears for transmissions. In order to form the helical tooth profile of the hob, cutting bodies are usually arranged in a distributed manner along a helix. These cutting bodies are often formed as cutting inserts, in particular indexable cutting inserts, which are secured in an exchangeable manner to a cross-sectionally roof-shaped carrier. 
     DE-A 27 00 525 discloses a multipart hob, in which the tooth profile is formed by a plurality of encircling disks which are arranged in a row and each carry a disk-like gear rim. The individual disks are formed as annular disks which are arranged on a basic body in a row in the longitudinal direction. 
     In operation, not only the cutting bodies but also the insert seats for the cutting inserts can become worn, and so the cutting insert support, into which the insert seats are incorporated, also has to be replaced from time to time. 
     In order to avoid disassembling the hob known from DE-A 27 00 525 into its individual components, it is provided according to EP 0 050 270 A2 for the tooth profile of the hob to be formed by tooth segments that adjoin one another in the circumferential direction, with the individual tooth segments being secured in a thread-shaped guide of the basic body. 
     In this configuration, it is therefore necessary to produce the basic body with an attached helical guide in a comparatively complicated manner. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to specify a hob which is easy to produce. 
     The object is achieved according to the invention by a hob having the features of claim  1 . Preferred configurations of the hob can be gathered from the dependent claims. 
     The hob is characterized in that a multiplicity of individual segments are provided in order to form the helical tooth profile, said segments being secured to a usually cylindrical basic body. Each individual segment covers in this case a limited angular range and has preferably only precisely one helical turn. The segments themselves comprise in each case a segment root and a segment profile. They are secured to the basic body by way of the segment root. The segment profile is therefore a portion of the helical tooth profile. The segment profile is arranged in an oblique manner on the segment root. The segment root itself extends preferably perpendicularly to the longitudinal direction, so that lateral surfaces that are adjacent to the segment root run perpendicularly to the longitudinal direction over their entire length. 
     The particular advantage of this configuration can be seen in that fact that, on account of the oblique orientation of the segment profile with regard to the segment root, the helix pitch of the tooth profile within each individual segment is reproduced by the oblique position of the segment profile. On account of this measure, it is therefore possible to dispense with an additional helical guide on the basic body. The basic body itself is therefore formed without a helical guide. The tooth profile is preferably formed solely by arranging the individual segments in a row on the one hand in the circumferential direction and on the other hand in the longitudinal direction of the basic body. 
     The expression helix pitch is generally understood to mean the distance between two portions of the helix of the tooth profile following a revolution through 360° in the longitudinal direction. 
     On account of this configuration, the hob is easy to assemble and produce. All that is needed is to secure the individual segments to the hob. The hob is formed in an easy manner as a cylindrical body. The individual segments themselves are likewise comparatively easy to produce. 
     Expediently, the segment profile is arranged obliquely on the segment root at an angle which correlates with the helix pitch of the entire tooth profile. As a result, in a simple configuration, the segment root can have a cuboidal shape in plan view. During assembly, the individual segment roots are arranged in a simple manner in an orientation perpendicular to the longitudinal direction in a row adjacent to one another starting from a stop at the edge. 
     Furthermore, it is preferably provided that the segment root has a width which corresponds to the helix pitch. Thus, no further spacer elements are necessary between two adjacent segment roots which are adjacent to one another in the longitudinal direction within a row. 
     With regard to easy assembly, the segments of one row are arranged without an offset with respect to one another as seen in the circumferential direction. The individual segments are thus oriented in the longitudinal direction of the basic body. 
     Expediently, it is therefore provided that an even number of segments cover exactly 360° and preferably the individual segments which are adjacent to one another have the same segment angle, i.e. they cover in each case the same angle. Preferably, three segments are provided, each at 120°, per circumference. Alternatively, these segments can also cover different angles. 
     In order to ensure that the segment profiles that are adjacent to one another in the circumferential direction merge into one another in a flush manner and without an offset, the segment roots of two segments that are adjacent to one another in the circumferential direction are arranged in a manner offset with respect to one another in the longitudinal direction. The offset in the longitudinal direction corresponds here preferably to the proportion of the helix pitch that is reproduced by the respective segment profile. This proportion depends here on the segment angle which is covered by the respective segment. In this case, the offset x is determined in particular generally by the formula: 
               x   =       α     360   °       ⁢   xs       ,         
wherein α is the segment angle which is covered by the segment and s is the helix pitch. In the case of a 120° segment, the offset is therefore ⅓ of the helix pitch.
 
     This offset is preferably formed in a simple manner in that a compensating segment is arranged at the edge of each row and has a compensating width which corresponds to the offset required. Generally, the compensating width is selected in such a way and is correlated with the helix pitch, that two segment roots that are adjacent to one another in the circumferential direction are arranged in an offset manner with respect to one another in the longitudinal direction so that their segment profiles adjoin one another in a flush manner. 
     With regard to positionally accurate and easy assembly, the basic body has guides that run preferably in the longitudinal direction and along which the segments are positioned on the basic body. Expediently, it is provided here that adjacent segments butt against one another in each case at the guides, so that one guide is used for in each case two adjacent segments. 
     The guides are in this case preferably formed by grooves, in which there is accommodated an insert element which protrudes out of the groove incorporated into the basic body and thus forms a kind of stop as a guide for the segments. Expediently, the insert element is formed here as what is known as a feather key, which in the simplest case is an elongate profile element that is made in particular of metal and preferably formed in length, width and cross-sectional form in such a way that it is accommodated in the respective groove up to a tolerance margin. This configuration with the grooves in the basic body and the feather key arranged therein makes it possible to produce the basic body easily, since it is only necessary to incorporate in a simple manner, for example by milling, grooves that run in the longitudinal direction. 
     Expediently, it is furthermore provided that the segments have receptacles, into which the feather key engages, at their segment roots. The receptacles are formed in this case expediently at the edge. Two mutually adjacent receptacles of adjacent segment roots therefore together cover the width of the feather key. 
     In order to secure the elements to the basic body, the latter expediently has screw holes. In this case, at least one, preferably several, for example two or three, screw holes are provided for each of the segments. The screw holes of one row of segments form rows of screw holes, wherein the screw holes of segments which are adjacent to one another in the circumferential direction are arranged in a manner offset with respect to one another in the longitudinal direction. The offset corresponds here to the offset of the segment roots. 
     Preferably, there are provided different types of segments which are arranged alternately in the circumferential direction of the basic body. Preferably, there are provided here in particular two different types, which differ only with regard to the arrangement of cutting bodies. The cutting bodies are generally formed preferably as indexable cutting inserts. The indexable cutting inserts are furthermore preferably arranged generally in the manner of staggered toothing alternately on an inside and an outside of a roof-shaped cross-sectional profile of the segment profiles. The arrangement of the cutting bodies on the segment profiles of different types is now chosen such that the desired alternating arrangement of the cutting bodies is ensured in the case of staggered toothing, even in the case of successive segment profiles. 
     For a simple construction of the hob, the basic body expediently has at one end an annular stop, on which the individual segments or compensating segments are positioned. 
     The basic body is formed preferably as a hollow cylinder to be secured preferably in a form-fitting manner on a driveshaft of a machine tool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While various embodiments of the invention are illustrated, the particular embodiments shown should not be construed to limit the claims. It is anticipated that various changes and modifications may be made without departing from the scope of this invention. 
         FIG. 1  shows a perspective view of a hob; 
         FIG. 2A-2C  respectively show a side view and an end view of a basic body having in each case a row of segments extending in the longitudinal direction; 
         FIG. 3A-3C  show different views of a first segment type; 
         FIG. 4A-4C  show different views of a second segment type; and 
         FIG. 5  shows a perspective illustration of a basic body. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the exemplary embodiment, the hob comprises a hollow cylindrical basic body  2  which is formed in the manner of a drum and to which a tooth profile  3  has been attached. The tooth profile  3  is wound helically on the lateral cylinder surface of the basic body  2  with a helix pitch s (cf.  FIG. 2B ). The basic body  2  (cf. in this regard in particular  FIG. 5 ) has a central shaft receptacle  4 , with which it can be secured to a driveshaft of a machine tool. At its rear end, the basic body  2  has an annular stop  6 . A plurality of rows of screw holes  8  are provided in the lateral surface of the basic body  2 . Introduced alongside these are grooves  12  that run in the longitudinal direction  10 . 
     Fitted to this basic body  2  are a multiplicity of individual segments  14 A,B, the detailed structure of which can be gathered in particular from  FIGS. 3A to 3C  and  4 A to  4 C. 
     As can be seen from  FIGS. 2A to 2C , the individual segments  14 A,B are secured to the lateral surface of the basic body  2  in a manner arranged in rows in the longitudinal direction  10 , so that in each case individual rows  16  that form a lateral surface sector are formed. Each row  16  has preferably at least five segments. In the exemplary embodiment, each row  16  covers a segment angle α of exactly 120°. Preferably, the segments  14 A,B of each row  16  are arranged without an offset with respect to one another in the circumferential direction. 
       FIG. 2A  shows in this case the row  16  that forms a first sector,  FIG. 2B  the row  16  that forms a second sector and  FIG. 2C  the row  16  that forms a third sector. 
     The individual segments  14 A,B are in each case formed from a segment root  18  and from a segment profile  20  formed on the segment root  18 . The segments  14 A,B are preferably integrally formed, and are thus worked from one workpiece. The segment profiles  20  of the individual segments  14 A,B form as a whole the helical tooth profile  3 . 
     As can be seen from the side illustrations in  FIGS. 2A to 2C , in each case compensating segments  26  are arranged at the edge—as seen in the longitudinal direction  10 —of the individual rows  16 . These compensating segments  26  cause two adjacent segments  14 A,B to be offset with respect to one another in the longitudinal direction  10 . This ensures that the segment profiles  20  of two segments  14 A,B that are adjacent to one another in the circumferential direction are flush with one another. The width of these compensating segments  26  is selected accordingly. As can be seen from  FIG. 2A , the first segment  14 A of the first sector is secured directly adjacent to the annular stop  6 . In the case of the second sector, first of all a thin compensating segment  26  is arranged at the annular stop  6 , the width of said compensating segment  26  corresponding to a third of the width of a normal segment  14 A,B. A compensating segment  26  of this kind having a ⅓ width is in turn provided at the end opposite the annular stop  6 . 
     Finally, in the case of the third sector according to  FIG. 3C , a compensating segment  26  having a ⅔ width is arranged directly at the annular stop  6 . The compensating segments  26  are formed substantially by modified segment roots  18 . 
     The width B of each segment  14 A,B is determined by the segment root  18 . The width B corresponds here to the helix pitch, that is the distance (in the longitudinal direction  10 ) between two portions of the helix of the tooth profile  3  following a revolution through 360°. The pitch and thus the width B lie for example in the region of 22 mm. In the exemplary embodiment, each row  16  comprises a total of eleven segments  14 A,B. 
     The individual segments  14 A,B, or the individual rows  16  butt against one another centrally over the grooves  12 . Insert elements, specifically feather keys  27 , are inserted into these grooves  12 , said insert elements on the one hand being accommodated in the grooves  12  but on the other hand engaging in corresponding receptacles  28  (cf. in particular  FIGS. 3A ,  4 A) at the edge of the segment root  18 . Together with the feather keys  27  the grooves  12  form guides for the segments  14 A,B. 
     In the exemplary embodiment, the segments  14 A,B are preferably designed to cover in each case the same angular range (segment angle α). As an alternative, the division of the individual segments  14 A,B can also be unequal. 
     In the exemplary embodiment, preferably precisely two different segment types, specifically the segments  14 A ( FIG. 3A to 3C ) and the segments  14 B ( FIG. 4A to 4C ) are provided. 
     When the hob is put together, the two segment types  14 A,B are secured in each case in a manner alternating with one another in the circumferential direction. In the circumferential direction, in each case one type B segment is therefore arranged between two type A segments. In the geometry selected in the exemplary embodiment, this means that the different segment types are arranged in each case in an alternating manner in the longitudinal direction  10 , too. 
     As is apparent from  FIGS. 3A and 4A , the segments  14 A,B are formed in the manner of annular segments. In the exemplary embodiment, the inside diameter d i  of each segment  14 A,B (and thus the outer radius of the lateral cylinder surface of the basic body  2 ) is approximately 165 mm. In the exemplary embodiment, the outside diameter d a  of the annular segment is for example 270 mm. 
     The segment profile  20  is approximately roof-shaped in cross section. Arranged on each of the roof sides are cutting bodies  30 , which are formed preferably as indexable cutting inserts. The cutting bodies  30  are in this case arranged in each case in a manner alternating in pairs on one or the other roof side. 
     As can be seen in particular from  FIGS. 3B ,  4 B, the roof-shaped segment profile  20  runs obliquely with respect to the longitudinal direction  10 . The helix pitch of the helical tooth profile  3  is thus reproduced by the oblique position of the segment profile  20  on the segment root  18 . the segment root  18  itself is—as seen in a radial projection—formed in a rectangular manner so that its longitudinal sides run parallel to the annular stop  6  or perpendicular to the longitudinal direction  10 . 
     The difference in the configuration of the two segment types  14 A,B can be seen from a comparison of  FIGS. 3B and 4B . The segment profile  20  is arranged specifically in a manner rotated merely through approximately 180° with respect to a segment center M. As a result, in the segment type A according to  FIG. 3A , at the edge of the segment  14 A in each case a pair of cutting bodies  30  is provided on the side which is remote from the annular stop  6 . Conversely, the pairs of cutting bodies  30  at the edge in type B are arranged on the side which is remote in  FIG. 4A  and oriented towards the annular stop  6 . Apart from this arrangement of the segment profile  20  rotated through 180° with regard to the center point M, the segments  14 A,B are formed in an identical manner. 
     In the exemplary embodiment, each segment  14 A,B is secured to the basic body  2  by in each case three screws  24 . The screws  24  are locked following assembly, for example using grub screws, and adhesively bonded. This prevents the ingress of dirt. 
     The patents and publications referred to herein are hereby incorporated by reference. 
     Having described presently preferred embodiments the invention may be otherwise embodied within the scope of the appended claims.