Abstract:
The invention relates to a method for milling fishbone-type notches ( 3 ) which are arranged on the perimeter of a turbine shaft ( 2 ) and which are used to receive blade footings. A step-tapered preform ( 15 ) of the cross-section ( 3 ) of the notch is pre-milled in preferably three machining steps. On the basis of this preform ( 15 ), at least the substantial part of the fishbone-type cross-section form which is characterised by undercuts in the step flanks is milled in one tool passage by means of a profile milling cutter ( 21 ).

Description:
DESCRIPTION  
         [0001]    Method for Milling Fir-Tree Grooves  
           [0002]    The invention relates to a method for milling fir-tree grooves, which are used to accommodate blade roots, on the circumference of a turbine shaft. It also relates to a profile milling cutter for carrying out the method.  
           [0003]    An axial fir-tree connection is used predominantly in gas and steam turbine engineering to fix turbine blades which are pushed in substantially in the axial direction, in order to optimally absorb long-term loads acting on the blades during operation as a result of centrifugal forces. These centrifugal forces act as static and dynamic bending forces. Securing the blades by means of what are known as fir-tree grooves transmits the forces in a staggered form via a plurality of pairs of teeth. If such fir-tree grooves which are used to secure blades are not curved, but rather are straight, they can most easily be produced by reaming.  
           [0004]    However, a drawback of straight fir-tree grooves is the unfavorable introduction of the forces and moments from the projecting exit edge of the profile into the blade root and the relative difficulty of matching the centers of gravity of the blade section and of the blade root on one radius. Securing the blade root in fir-tree grooves in the shape of an arc of a circle provides a suitable remedy to this problem, in particular since the arc shape is close to the shape of the blade. Usually, however, producing fir-tree grooves which are curved in this way without any problems requires special milling machines (“Turbomaschinen im Kraftwerk” [Turbomachines in power plants] by Prof. Dipl.-Ing. Gerhard Schilg, VEB Verlag Technik Berlin, 1st edition, 1978; pp. 247 ff.).  
           [0005]    The invention is based on the object of providing a method for producing fir-tree grooves of this type which does not require any special milling machines for this purpose, but rather can be carried out by means of a milling unit which has been placed onto a conventional turbine rotor turning machine. Furthermore, it is intended to provide a milling tool which is particularly suitable for carrying out the method.  
           [0006]    With regard to the method, according to the invention this object is achieved by the features of claim 1. For this purpose, first of all a preform of the groove cross section, which tapers in stepped form, is premilled in preferably three machining stages. The main part of the fir-tree cross-sectional shape, which is characterized by undercuts in the step flanks, is milled out of this preform in a single tool pass by means of a profile milling cutter. In this way it is possible to effectively produce in particular curved fir-tree grooves.  
           [0007]    The method particularly advantageously makes it possible on the one hand for the load required for the milling operation to be within the capacity of a milling unit of this type. On the other hand, the high machining rate during milling of the undercuts, which are shaped as root hooks, into the step flanks of a preform of the fir-tree cross section, is also managed. This places high demands on the performance of the milling unit if the shaping is to take place substantially in one operation. Hitherto, to manage the high machining rate different tools have been used in a plurality of operations, but the cutting edges of these tools have to be reground undesirably frequently on account of the wear.  
           [0008]    With regard to the milling tool for carrying out the method, the invention provides a profile milling cutter having the features of claim 3 which is particularly suitable for producing that part of the fir-tree cross section which is provided with root hooks. The profile milling cutter is preferably provided with carbide disposal tips.  
           [0009]    The milling of the cross section of the preform which tapers in stepped form is expediently carried out by means of end mill cutters, which are likewise advantageously provided with carbide disposal tips. A finish-milling operation which may still be required can be carried out using shape finish-milling cutters as hitherto. The tool geometry of these cutters produces the final contour. This is because any allowance with respect to the final contour which may remain after the milling of the root hooks by means of the profile milling cutter is sufficiently small—with respect to the machining rate to be achieved—to protect the machine and tool from damage or destruction.  
           [0010]    In a particularly advantageous configuration, the profile milling cutter has a number of cutting tool tips which corresponds to the number of individual grooves which are to be milled for the fir-tree contour, these cutting tool tips being arranged in a uniform distribution and offset over the circumference of the cutter. In this way, it is possible to deal with a particularly high machining rate, since generally only one carbide cutting edge or carbide cutting tool tip per undercut or root hook is acting on the workpiece. This takes into account the limited available spindle power of the milling unit in a particularly advantageous way and, on account of the relatively small number of working steps, in particular also in a time-saving manner, given the relatively high machining rate which is to be achieved. 
       
    
    
       [0011]    Exemplary embodiments of the invention are explained in more detail below with reference to drawings, in which:  
         [0012]    [0012]FIG. 1 diagrammatically depicts the sequence of milling firstly a preform of the groove cross section, from which the fir-tree cross-sectional shape is then milled in a single tool pass by means of a specially configured profile milling cutter,  
         [0013]    [0013]FIG. 2 diagrammatically depicts the result of the method stages which have been passed through in FIG. 1,  
         [0014]    [0014]FIG. 3 shows a side view of the profile milling cutter used for what is substantially the last milling stage,  
         [0015]    [0015]FIG. 4 shows a front view of the profile milling cutter shown in FIG. 3 viewed from the direction indicated by arrow IV in that figure,  
         [0016]    [0016]FIG. 5 shows a cross-sectional view of the profile milling cutter in section plane V-V from FIG. 3,  
         [0017]    [0017]FIG. 6 shows a cross-sectional illustration similar to that shown in FIG. 5 corresponding to section plane VI-VI in FIG. 3, and  
         [0018]    [0018]FIG. 7 shows a cross-sectional illustration similar to those shown in FIGS. 5 and 6, corresponding to section line VII-VII in FIG. 3. 
     
    
       [0019]    Parts which correspond to one another are provided with identical reference symbols in all the figures.  
         [0020]    A fir-tree groove  3 , the definitive cross-sectional shape of which is hatched with a greater density of lines in FIG. 1, is to be milled into the circumferential region  1  of a turbine shaft  2 , the longitudinal axis of which runs approximately perpendicular to the plane of the illustration shown in FIG. 1 and FIG. 2. This milling takes place in stepwise form, initially so as to produce a preform of the groove cross section, which tapers in stepped form in the radial penetration direction  9 , in three method stages ( 1 )-( 3 ) indicated by numbers in circles. In method stage ( 1 ), an end mill cutter  4  of relatively large operative diameter  5 , starting from a prefabricated support region  6  for the blade root, mills inward—based on the axis of the turbine shaft—over a penetration depth  7 . In this way, first of all a preform for the outer root hooks  8 , which are subsequently to be milled out on both sides, of the fir-tree groove  3  is created in the outer circumferential region  1 .  
         [0021]    The further milling operations which follow in the radial penetration direction  9  and are carried out by means of the end mill cutters  10  and  11  for production of the complete preform  15  of the fir-tree cross section follow in the method stages ( 2 ) and ( 3 ) indicated by numbers in circles. In method stage ( 2 ) of FIG. 1, the end mill cutter  10  shapes the penetration region  12 , and the end mill cutter  11  shapes a penetration region  13  of smaller internal diameter  14 . The intermediate result created by the three milling operations ( 1 ) to ( 3 ) is a preform  15  of the fir-tree groove  3  which tapers in stepped form in three stages in the radial penetration direction  9 . The cross section of this preform  15  of the fir-tree groove  3 , which tapers in stepped form in the radial penetration direction  9  starting from the support region  6  for the blade root, is illustrated in FIG. 2 on the left-hand side of the axis of symmetry  14 . This cross section is illustrated in the left-hand part of FIG. 2 by hatching  16  which runs from the top left to the bottom right.  
         [0022]    The end mill cutters  4 ,  10  and  11 , which are provided with different operative diameters  5 , bear, in their active milling circumferential region, carbide cutting edges  17 ,  18 ,  19  which are each part of a carbide cutting tool tip, in particular of a disposal cutting tool tip.  
         [0023]    In a subsequent method stage ( 4 ) (FIGS. 1, 2), the main part of the fir-tree cross-sectional shape  3  is milled from the preform  15  over the entire penetration depth  20  of the profile milling cutter  21  in a single tool pass. The flank region  43  of the definitive cross-sectional shape of the fir-tree groove  3  is illustrated in FIG. 2 to the right of the axis of symmetry  14  and to the left of the hatching lines  22 .  
         [0024]    The following part of the description of the figures deals with the structural design of the profile milling cutter  21  for the majority of the final shaping of the fir-tree groove  3  from the preform  15  in a method stage ( 4 ). On the circumferential side, the milling cutter base body  23  of the profile milling cutter  21  is provided with a number of cutting points  25 ,  26 ,  27  which are arranged offset in the axial direction  24  of the milling cutter base body  23 , radially project with respect to the milling cutter axis of rotation  37  and are stepped in terms of their operative diameters  28 ,  29 ,  30  (FIGS.  5  to  7 ). Each cutting point  25  to  27  is part of a carbide cutting tool tip  31 ,  32  or  33  which is exchangeably fixed to the milling cutter base body  23 . The cutting tool tips  31  to  33  are disposal cutting tool tips. Each operative diameter  28  to  30  is assigned just one cutting tool tip  31  to  33  on the circumference of the milling cutter base body  23 . The cutting point  25  to  27  of each disposal cutting tool tip  31  to  33  projects circumferentially beyond the shank region of the milling cutter base body  23  which supports them.  
         [0025]    An important feature is considered to reside in the fact that cutting tool tips  31  to  33  which are provided with different operative diameters  28  to  30  are arranged offset in the circumferential direction  34  with respect to one another on the milling cutter base body  23 . They are in each case arranged offset by 120° with respect to one another (FIGS.  5  to  7 ). This offset in the positioning of the cutting tool tips  31  to  33 , which is present in the circumferential direction with respect to the milling cutter axis of rotation  36 , makes the profile milling cutter  21  work more uniformly. The cutting tool tips  31 ,  32 ,  33 , which are each positioned individually on its circumference, do not simultaneously come into contact in each case with the flanks of the preform  15  of the fir-tree groove  3 . Rather, they do so in succession at temporal intervals. This avoids load peaks and reduces the maximum drive power  21  which the milling drive is required to produce.  
         [0026]    The cutting edges  35  which between them enclose the cutting points  25 - 27  form approximately a right angle with one another, as can be seen particularly clearly from the shape of the cutting tool tip  33  in FIG. 3. The angle bisector  36  (FIG. 3) between the two cutting edges  35  of a cutting point  25 - 27  forms approximately a right angle with the milling cutter axis of rotation  37 .  
         [0027]    The disposable tips  31  to  33  have a square contour. The sides of the square formed by the cutting edges  35  are the tip covering surfaces, and in each case the outer tip covering surface forms or includes the tool face of a cutting edge  35 .  
         [0028]    Each cutting tool tip  31  to  33  is a perforated disposable tip with a central securing hole, the hole axis of which is oriented approximately perpendicular to the tip covering surfaces  38  of the disposable tip  31  to  33 .  
         [0029]    Beads  40 ,  41 ,  42 , which are in each case shaped approximately in the form of an arc segment, are formed on the circumference of the milling cutter base body  23  in order to receive and support the cutting tool tips  31  to  33 .  
         [0030]    The final shape of the fir-tree cross section (right-hand part of FIG. 2) is milled in substantially one tool pass from the preform  15  in an advantageously time-saving way by means of the profile milling cutter  21 . If this cannot be achieved completely, the final shape is produced using separate milling tools in further passes. However, this only requires a small volume of material to be removed, with a low demand on power from the milling unit and with a correspondingly low level of tool wear.