Abstract:
Method for dressing or profiling an essentially cylindrical grinding worm on a machine suitable for continuous generating grinding in the diagonal method. The method comprises providing an essentially cylindrical gear used as a dressing tool with the dressing tool being provided with an abrasive coating on that surface which is active in the dressing process and bringing the dressing tool into contact with the grinding worm and profiling at least one flank of the grinding worm in a tangential or diagonal method.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a method for dressing or profiling of an essentially cylindrical grinding worm.  
         BACKGROUND OF THE INVENTION  
         [0002]    Cylindrical grinding worms are used for continuous generating grinding operations. The economic efficiency of the process and the precision of the machined gears are significantly influenced by the dressing or profiling process.  
           [0003]    Today, dressing (profiling) is mostly performed on the machine. This requires the employment of dressing devices. Those machines are expensive and they require highly precise slide guides and movements in the dressing process that are very accurately synchronized with the movement of the grinding worm. Moreover, the dressing devices require considerable space and isolation from contamination, e.g. by minute grinding chips or dressing tool particles, which may in part become quite costly.  
           [0004]    Another disadvantage of the well-known dressing devices is that for control-technical reasons, the dressing process must be performed at a slow revolution of the grinding worm. This problem increases with an increase in the number of starts of the grinding worm. These circumstances do not only result in a long dressing time but deformations and possibly displacements of the grinding worm, which have a negative effect on the geometry of the ground gears, occur at working speed. Moreover, dressing with commonly used dressing devices is not performed at the very spot where the workpiece is subsequently machined. However, it is desirable for precision purposes that the dressing process takes place at the very spot at which the grinding worm will subsequently machine the workpiece.  
           [0005]    One method is known according to which a diamond coated dressing gear is used for profiling a grinding worm. With this method, grinding worm and dressing gear are arranged at crossed axes. The two objects will rotate according to the ratio of the tooth number of the dressing gear to the number of starts of the grinding worm. Feed occurs either radially up to the nominal center distance followed by a process of cutting free, or both objects are adjusted to the nominal center distance and a feed motion is performed in the direction of the workpiece or of the tool circumference. A first grinding worm flank is profiled during this process and the direction of the feed motion is subsequently reversed and the second flank of each grinding worm tooth space is profiled.  
           [0006]    With this method, the grinding worm receives a globoid like shape. The process is performed very quickly. However, the precision of the subsequently ground gears is in most cases insufficient for mounting the gears into low-noise gear boxes without finishing the tooth flanks, e.g. by shave-grinding. However, such an additional operation is highly undesirable.  
           [0007]    One reason for the insufficient precision of the ground wheels lies in the insufficient precision of the dressing gears. Roughly simplified, the following holds true: at dressing, the enveloping geometry of the dressing gear is transferred to the grinding worm and during the subsequent grinding operation it is transferred from the grinding worm to the workpiece. There is line contact on transferring the enveloping geometry of the dressing gear to the grinding worm. Under ideal conditions, the entire surface of each flank of the dressing gear takes part in the profiling of the grinding worm. Each flank modification which the toothed gears to be ground shall receive must therefore be provided for in the dressing gear. As a result, each different tooth design requires a corresponding differently designed dressing gear.  
           [0008]    It is an object of the invention to develop a method that can be applied at working speed of the grinding worm to be dressed and wherein the grinding worm is dressed or profiled at the very spot where machining is subsequently performed. It is desirable that the method is inexpensive, operates with precision, requires little dressing time, minimal space, and shows a low risk of contamination of critical machine components.  
         SUMMARY OF THE INVENTION  
         [0009]    In the inventive method the dressing tool is essentially in the form cylindrical toothed gear. It is provided, at least in the area of its active surface, with a corresponding abrasive coating. The dressing tool is chucked on the machine instead of the workpiece, for example. It can, of course, also be chucked on the workpiece spindle axially displaced relative to the workpiece. It is furthermore possible to chuck the dressing gear at a different location relative to the grinding worm. This, however, requires a more complicated design.  
           [0010]    The cylindrical grinding worm makes it possible to manufacture essentially cylindrical toothed gears with high precision in an axial or diagonal method. In this process dressing gears can be employed which are unmodified in their flank line. Such dressing gears can be manufactured with more precision than gears that are modified in their flank line. The flank line modifications required for the gears to be machined can be created very precisely with the cylindrical grinding worm by corresponding machine movements in the axial or diagonal method.  
           [0011]    On dressing of an essentially cylindrical grinding worm in the tangential method only a small strip of the dressing gear flank, practically only one line per flank, is active. In contrast thereto, as has already been mentioned, the entire surface of each and every flank of the dressing gear is used for dressing of a globoid-like grinding worm. A line can, of course, be produced with more precision than a surface. Therefore, it can be taken as a fact that gears which are ground by an essentially cylindrical grinding worm are more precise than gears ground by a globoid-like grinding worm, even if both grinding worms are being shaped by a gear type dressing tool. Moreover, as has also been mentioned already, for dressing of a globoid-like grinding worm, a different dressing gear design is required for each workpiece gear design.  
           [0012]    It follows from the foregoing that dressing of a globoid-like grinding worm using gear type dressing tools leaves much to be desired, particularly with respect to the precision and flexibility that can be accomplished.  
           [0013]    The essentially cylindrical grinding worm can be dressed in a position that it will occupy on the machine when the workpiece is subsequently machined. In contrast to the prior art of dressing cylindrical grinding worms, optimal conditions for a high precision of the grinding worm and the gear to be subsequently ground are thereby achieved. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0014]    The drawing shows a perspective view of a gear grinding machine of the prior art for toothed wheels.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    The machine of the FIGURE is comprised of a bed  1  on which a radial slide  2  displaceable in direction X and a tangential slide  3  displaceable in direction Y are arranged.  
         [0016]    An axial slide  4  is displaceable in direction Z. The axial slide  4  is connected by its linear guides with a swivel slide  5 . The axial slide  4  can be swiveled in direction A with the help of this swivel slide and can, because the swivel slide  5  is connected with the tangential slide  3 , be displaced in direction Y.  
         [0017]    The workpiece spindle  6  and its drive are housed inside of the axial slide  4 . Workpiece spindle  6  holds a workpiece (not illustrated).  
         [0018]    The radial slide  2  carries the tool spindle (not illustrated), the tool  7  embodied as a cylindrical grinding worm, and the tool drive  8 ; it further carries the base for receiving a conventional dressing device. Such a dressing device is comprised, for example, of one or more diamond-coated dressings discs (not illustrated) with bearings and drives. Advantageously, the axis of the dressing disc(s) runs parallel to the rotational axis of the grinding worm  7 .  
         [0019]    The dressing device is displaceable in direction U (parallel to X) by a U slide  9  and in direction V (parallel to Y) by a V slide  10 . When working with one dressing disc, for example, the dressing process is performed as follows:  
         [0020]    The dressing disc is adjusted to depth (relative to the grinding worm  7 ) in direction U and is set in rotation in order to create the dressing-cutting speed. It is furthermore displaced in direction V synchronously to the rotation of the grinding worm such that the required lead of the grinding worm is created.  
         [0021]    The drawing shows distinctly that the working location and the dressing location are located opposite of each other, at least, approximately diametrically relative to the grinding worm diameter. For precision purposes, however, they should coincide.  
         [0022]    Although the dressing gears recommended herein show significant advantages over the dressing gears required for dressing/profiling of globoid-like grinding worms, dressing gears designed for dressing/profiling of globoid-like grinding worms can also be employed for dressing/profiling of essentially cylindrical grinding worms. However, in that event, shifting the dressing gear or working in the diagonal method are practically impossible if the gears to be ground require a considerable amount of helix modification.  
         [0023]    In the inventive method the dressing wheel is, for example, chucked on the spindle  6  instead of the workpiece. The U slide  9 , the V slide  10 , the guides associated therewith, drives and controls for the slide movements, the dressing wheels, the bearings thereof, drives and controls become obsolete thereby. The space required for the dressing gear for the subsequent grinding operation of the workpiece already exists.  
         [0024]    The inventive method does not require a differently designed dressing gear for each different workpiece tooth design, either. Rather, the profiling or dressing of cylindrical grinding worms requires only a single dressing gear design for practically all gears having the same basic rack tooth profile. It should be noted, however, that strictly speaking, each change of a profile modification will lead to a change in the basic rack tooth profile. However, calculations show that in many cases a certain range of workpiece parts can be machined with a basic rack tooth profile that takes into account a certain profile modification. This applies in particular to cases in which the profile modification shows only a profile crowning. The dressing gear does not require any flank line modification. It can be manufactured more easily and more precisely than dressing gears required for dressing of globoid-like worms. The dressing gear can practically have any helix angle, even, for example, a helix angle of 0°. The dressing process can be performed at the number of revolutions of the worm, at which the worm will subsequently machine the workpieces. It poses no difficulty whatsoever to very precisely dress or profile even multi start worms at a very high number of revolutions of the worm. Each and every movement of the tangential or diagonal method, required for dressing the worms can be performed via the machine axes which already exist.  
         [0025]    At least the flanks of the grinding worm chucked in the tool spindle are profiled in the tangential or diagonal methods. In this dressing process, the machine is used like a hobbing machine for hobbing worm wheels using the tangential method or as for diagonal hobbing. In this process, only the technological parameters of feed, advance, and number of revolutions of the grinding worm  7  have to be coordinated with the actual dressing task to be performed.  
         [0026]    Because the grinding worm can be rotated during the dressing process at a high number of revolutions, the dressing time is very short. In the dressing process described herein, the right and the left flanks of the grinding worm  7  can be dressed in a single pass or successively in separate passes. The method described herein makes it also possible in certain cases to dress the area of the root of the teeth of the grinding worm  7 , the tip area or the transitional area between the tip area and the flanks, in a single pass.  
         [0027]    In the dressing process described herein the grinding worms  7  are dressed with high precision and with high economic efficiency. As a result the workpieces/gears machined by the grinding worms can also be machined economically and, in particular, with high precision.  
         [0028]    Because only a small part of the width of the dressing tool is used in many cases when operating using the tangential method or the diagonal method with a small axial component of feed motion, the tool can, e.g., after having become worn out in a certain area of its width, be shifted and portions of the tool width, that have not been used yet can be utilized. This measure can significantly reduce the costs for the dressing tool and thereby increase the economic efficiency of the dressing process. In the case of a spur toothed dressing tool the shifting movement consists of merely displacing the dressing tool in the direction of its axis. In the case of a helical dressing tool, in addition to the displacement in the direction of the tool axis, a rotation corresponding to the helix of the enveloping screw of the dressing tool is required.  
         [0029]    While the invention has been described with reference to preferred embodiments it is to be understood that the invention is not limited to the particulars thereof. The present invention is intended to include modifications which would be apparent to those skilled in the art to which the subject matter pertains without deviating from the spirit and scope of the appended claims.