Abstract:
The invention concerns a spindle ( 1 ) for machining inner grooves and comprising a cylindrical input guide ( 15 ) of diameter D 1  and a cutting portion ( 20 ). Said spindle is characterised in that the cutting portion ( 20 ) is subdivided into a plurality of cutting sections ( 25 ) separated by realigning guides ( 26 ) having a diameter equal to diameter D 1  of the input guide ( 15 ), so as to accurately centre the grooves relative to the hole previously machined in the workpiece ( 30 ).

Description:
DESCRIPTION  
         [0001]    The invention relates to machine tools which remove material and more particularly to broaches for machining internal splines.  
           [0002]    Broaches for machining internal splines are tools which are well known. Made of cutting tool material, for example of tungsten carbide, they comprise, in succession, from front to rear, a cylindrical inlet guide of diameter D 1  followed by a cutting section consisting of a number of cutting teeth the diameter of which increases progressively from the diameter D 1  of the inlet guide. The teeth themselves comprise slots so as to reproduce the profile of the internal splines that are to be machined. The workpiece is pierced beforehand, along the axis of the splines that are to be machined, with a reference bore of diameter D 1 , increased by a functional clearance E allowing the inlet guide to slide in the reference bore. This reference bore constitutes the reference with respect to which the splines that are to be machined need to be centered. The splines are machined by the passage of the broach through the reference bore in a back and forth movement. The broach is, to begin with, centered in the reference bore by the inlet guide. However, when the inlet guide has left the reference bore, the broach is likely to deflect radially as the splines are machined, which means that the machined splines are no longer exactly centered on the reference bore.  
           [0003]    To overcome that, it is known practice for two or three stages of unslotted teeth to be arranged behind the teeth for machining the splines, so as to allow the reference bore to be touched up and recentered with respect to the finished splines. The disadvantage of such a solution is that it does not allow the machined splines to be centered rigorously with respect to the reference bore already machined in the workpiece.  
           [0004]    U.S. Pat. No. 5,224,804 discloses a broach for boring crankshaft bearing mounts on a combustion engine, this broach comprising a succession of cutting portions of diameters which increase from front to rear, these cutting portions being separated by “pilot segments” at the diameter of the cutting portions preceding them. A broach such as this makes it possible to improve the mutual concentricity of the bearing mounts but does not allow these bearing mounts to be positioned rigorously in the space of these bearing mounts, because random drift, arising out of the necessary clearance left between the pilot segments and the bores made by the preceding cutting portions, builds up. Thus, the greater the amount of material removed by the broaching operation, the more cutting portions are needed and the greater the resulting drift becomes. There are two reasons why this necessary clearance is not insignificant:  
           [0005]    1. It has always to exist so that the pilot segments can slide without forcing in the bores,  
           [0006]    2. it has to be large so that this clearance still actually exists when the cutting edges of the broach begin to wear.  
           [0007]    The invention proposes a broach for internal splines that allows the machined splines to be centered rigorously with respect to the reference bore. Such a broach is notable in that the cutting section is subdivided from front to rear by a number of cutting portions separated from one another by cylindrical recentering guides of the same diameter D 1  as the inlet guide. In other words, two contiguous cutting portions are separated from one another by at least one recentering guide, and preferably by just one. The recentering guides allow the broach to be recentered on the reference bore while the splines are being machined and thus make it possible to ensure rigorous centering of the machined splines with respect to the reference bore.  
           [0008]    The broach of the invention cannot be likened to the one disclosed in U.S. Pat. No. 5,224,804 because according to the invention, the recentering guides slide along the reference bore during the broaching operation. As a result, the drift is at most equal to the necessary clearance left between the reference bore and the recentering guides. There is therefore no build-up of drift as there was in the aforementioned patent, and the positioning of the broached surfaces, in this instance the splines, in space, is far better. 
       
    
    
       [0009]    The invention will be better understood, and the advantages it affords will become more clearly apparent from a detailed exemplary embodiment of the broach and from the attached figures.  
         [0010]    [0010]FIG. 1 illustrates a broach according to the invention.  
         [0011]    [0011]FIG. 2 illustrates the broach of FIG. 1 in cross section. 
     
    
       [0012]    A conventional broach will first of all be described, and reference will firstly be made to FIG. 1. The broach  1  consists of a straight rod  2  of geometric axis  3 . The broach  1  is made of cutting tool material, for example tungsten carbide. The front  4  and the rear  5  of the broach  1  will be defined. At the front  4 , the broach  1  comprises attachment means  10  allowing the broach  1  to be grasped and pulled from rear  5  to front  4 . These attachment means  10  comprise, from front  4  to rear  5 , a bulb  11  and a groove  12  allowing the broach  1  to be grasped via the bulb  11  by a gripper, not depicted. The broach  1  then comprises, toward the rear  4 , an inlet guide  15  preceded by a chamfer  16 , the inlet guide  15  being cylindrical of diameter D 1  centered on the geometric axis  3 . The inlet guide  15  is followed immediately toward the rear  5  by a cutting section  20  consisting of a number of teeth  21  which are cutting forward  4 , the diameter D of which increases progressively from front  4  to rear  5  from a value equal to the diameter D 1  of the inlet guide ( 15 ) to a diameter D 2 , not depicted, corresponding to the outside diameter of these splines that are to be machined.  
         [0013]    Reference will be made provisionally to FIG. 2. The teeth  21  are laterally separated by slots  22  penetrating the broach  1  radially toward the geometric axis  3  down to a diameter D 3  at most equal to the diameter D 1  of the inlet guide  15  and preferably smaller than it. The term “laterally” is to be understood as meaning along the circumference of the broach  1  passing through the teeth  21 . It will be appreciated that the flanks of the slots  22  have a shape that complements the flanks of the splines, not depicted, that are to be machined. The teeth  21  may be arranged along circumferences of the broach  1 . Most often, they are arranged in a single helix so as to give a constant cutting force.  
         [0014]    Reference is made once again to FIG. 1. According to the invention, the cutting section  20  is divided from front  4  to rear  5  into a number of cutting portions  25  separated one from the next by a recentering guide  26 . The cutting portions  25  obviously consist of the previously defined teeth  21 . The recentering guides  25  are cylindrical of diameter equal to the diameter D 1  of the inlet guide, these recentering guides also being centered on the geometric axis  3 . The recentering guides  26  each have a chamfer  27  at the front  4  each.  
         [0015]    The way in which the broach  1  works is as follows: the workpiece  30  is already pierced with a bore, unreferenced, of diameter D 1  increased by a functional clearance E allowing the inlet guide  15  and the recentering guides  26  to slide. The broach is introduced into the reference bore in the workpiece  30  and pulled from rear  5  to front  4 . The inlet guide  15  enters the workpiece  30  first and centers the broach  1 . While the teeth  21  of the first cutting portion  25   a  enter the workpiece  30  and begin to machine the splines, the inlet guide  15  leaves the workpiece  30  and is no longer able to center the broach  1 . As a result, the teeth  21  are therefore likely to deviate radially, but this deviation is very soon halted by the arrival of the first recentering guide  26   a  which then recenters the broach  1  on the reference bore already pierced in the workpiece  30 . The first recentering guide  26   a  successively centers the teeth  21  of the first cutting portion  25  then of the second cutting portion  25   b  and so on and so forth as the broach  1  advances through the workpiece  30 . It will be appreciated that the chamfers  27  return the recentering guides  26  to face the reference bore in the event of deviation and thus make it easier for the recentering guides  26  to enter the reference bore. The length of the recentering guides  26  is not critical. It will preferably be at most equal to one quarter of the diameter D 1  so as to have a good distribution of the pressures exerted by the recentering guides  26  on the reference bore.  
         [0016]    In order not to increase the length of the broach  1  excessively, each cutting portion  25  will comprise axially, that is to say from front  4  to rear  5 , at least three teeth  21 . In general, however, it is preferable not to have more than six teeth  21  per cutting portion  25 , so as not to allow an excessive amount of deviation of the broach  1  to develop, as it might not be possible for this to be compensated for by the next recentering guide  26  without developing an excessive force that would run the risk of breaking the broach  1  or the workpiece  30 .  
         [0017]    The invention is not restricted to the exemplary embodiment thereof which has just been given but also covers all alternative forms supported by the claims.  
         [0018]    The broach will usually have a circular profile centered on the geometric axis  3 , but the invention also applies to non-circular profiles. In any event, the cross section of the recentering guides  26  will be identical to that of the inlet guide  15 .  
         [0019]    The attachment means  6  are usually at the front  4  of the broach  1 , but the invention may just as easily be applied to a broach comprising attachment means at the rear  5  or combined ones at the front  4  and the rear  5 , these last two configurations making it possible to eliminate or reduce the tensile load imposed on the broach  1 .  
         [0020]    The cutting section  20  may also be supplemented toward the rear  5  by several stages of teeth, not depicted, these teeth having no slots and allowing the reference bore to be finished after the recentering guides have passed through and rubbed against it.