Abstract:
Cold thread forming tap or screw tap with a shaft, said shaft having a chuck portion at the one end and a forming or cutting profile portion on the other end, the forming or cutting profile portion being realised as a separate profile element and being fixable centrically and secured against rotation on the shaft by means of a fixing device, characterised in that the profile element has a small axial length and on the outside a lead-in or starting tap cone, respectively, that juts and/or indentations are formed at the end of the shaft and on that side of the profile element facing the shaft, which interlock in a positively fitting manner, that an automatic centring of the profile element takes place on the shaft when the profile element is put onto the shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is the National Stage filing under 35 U.S.C. 371 from PCT/EP02/01913 now PCT Publication No. WO03/011508 having an international filing date of Jul. 17, 2002, claiming priority from German Application No. DE2001010136293.5 filed Jul. 25, 2001, now German Patent No. DE3934621 published on Sep. 4, 1997, on behalf of Fette GmbH, the entire contents of which are hereby incorporated herein in their entireties. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   It is known to create interior (or female) threads with the aid of a screw tap or a thread forming tap. Screw taps evolve from screw bolts by generating cutting edges through the introduction of axis parallel or slant flutes. In the lead-in region, a so-called starting taper is provided, which is mostly realised in conical form. The hole in the workpiece is pilot-drilled with the core diameter. 
   Thread forming taps are also geometrically derived from a screw bolt. However, they do not remove any material, but only displace it. The workpiece is therefore pilot-drilled with the average pitch diameter. The thread forming tap has an ascending profile on the cone point, which progressively displaces the material. 
   It is known to increase the wear resistance of thread forming taps through selection of the material they are made from and by heat treatment. On this occasion, particular advantages are brought about by coating with titanium nitride (Maschinenmarkt, Würzburg 92 (1986) 40, pages 88 to 92). But it is also known to form thread forming taps from hard metal (Maschine+Werkzeug 12/95, pages 26 to 28). 
   From DE 39 34 621 C2, a thread forming tap has become known, with a cylindrical chuck shaft and a form-giving region with polygonal cross-section differing from the annular shape, as well as with thread corrugations on the outer perimeter which correspond to the thread which is to be produced. In the region of the cross section corners of the form-giving region, cylindrical ridges from particularly hard and wear resistant material are soldered into corresponding, axially running grooves, which ridges have a contour on the outer perimeter matching the thread corrugations. Thread forming taps have the same thread profile as screw taps, but have a polygonal cross section in their profile portion, with three or six “corners”, e.g. 
   The profile portion of thread forming taps and screw taps has a first forming region by which the thread is essentially produced, as well as a guiding portion following thereafter. This serves for the guiding of the tool when it is turned back from the bore, after completion of the interior thread. 
   Thread forming taps and screw taps are relatively expensive to manufacture and consist of a high-quality material, as the case may be. After wear or damage, the tool is no more useful. 
   The invention has the objective to create a cold thread forming tap or screw tap which can be produced with little expenditure of material and manufacture. 
   BRIEF SUMMARY OF THE INVENTION 
   In the tool according to the invention, the forming or cutting profile portion is realised as a separate profile element, which has at least two complete profile convolutions besides to a lead-in or starting taper cone, respectively, and which is fixable centrically and secured against rotation on the shaft by means of a fixing device. 
   The profile element can be plate-shaped or rather have a short axial length; besides to the lead-in and starting taper cone, respectively, it has relatively few complete profile convolutions, two to four for instance. 
   The shaft for the tool according to the invention can be manufactured in conventional manner and conventional dimensions from a material highly capable of bearing, particularly a suited steel. It has not to be discarded upon wearing of the profile element, but can be reused for many times. The invention makes also possible the provision of a plurality of different profile elements with different diameters, different pitches and so on, which can each be connected with one common unity shaft. Only the profile element is needed to consist of a suitable hard material. Through this, the material expense is significantly less than with conventional tools. 
   A relatively high torque has to be transmitted from the shaft to the profile element. According to the invention, juts and/or indentations are formed at the end of the shaft and on that side of the profile element facing the shaft, which interlock in a positively fitting manner. At the same time, these juts and indentations are formed such that they effectuate an automatic centring when the profile element is put onto the shaft. 
   The juts and indentations have complementary lateral areas inclined to the axis of the shaft and the profile element, respectively, which are in gearing for the purpose of centring and transmission of torque. These areas can be formed plane or crowned. Preferably, only the mentioned areas are in gearing, which means that the end areas of the profile element and the shaft have a distance from each other, when the profile element is attached on the shaft. 
   Preferably, the areas are formed on ribs and grooves, respectively, which are disposed radially crosswise, the centre point of the cross being on the axis of the shaft or the profile element, respectively. In order to facilitate the putting on of the profile element, the edges of the ribs and grooves can have a chamfer. 
   In the invention, it was found that with modern machine tools, more or less long guiding portions on cold thread forming taps or screw taps are no more necessary in order to turn out the tool without damaging the thread, when screwing it out of the produced thread bore. 
   As a material for the profile element, a hard metal, but also rapid machining steel, or an other known material can be used. It shall be understood that the outer diameter of the shaft of the thread forming tap has to be at least somewhat smaller than the core diameter of the thread which is to be created. By all means, one shaft can be used for profile elements for the production of different thread dimensions. 
   Different possibilities are conceivable to undertake a fixation of the separated profile element on the shaft. According to one form of the invention, one of them consists in that the profile element is fixed on the shaft by means of a screw in an axial thread bore. According to this, the profile element is provided with a centric bore, the screw having a conical portion e.g., which co-operates with a conical sink of the bore of the profile element. The screw does not contribute to the torque transmission from the shaft to the workpiece. 
   Alternatively, a threaded stem can be formed on the shaft, which either co-operates with a thread bore of the profile element, or a nut is provided, which is screwed onto the stem in order to fix the profile element on the shaft. 
   Finally, the profile element can be soldered on the shaft. However, the interchangeability is aggravated in this case, because the profile element has first to be heated when it is to be removed from the shaft. 
   In the invention described so far, one single profile element is centrically attached on the shaft. Another solution according to the invention provides that two or more identical plate-shaped separate form profiles are fixed on the front side of the shaft, in a manner secured against rotation. As well as the profiles described above, the forming profiles have thread profiles in the portion projecting over the shaft, for the creation of the interior thread. Such a tool is comparable to a thread rolling die for the creation of interior threads, with the exception that in the invention the “rolls” are fixed on the shaft in a manner secured against rotation. Furthermore, it is not necessary to use annular or rather polygonal bodies, but only portions thereof, because only the portions projecting over the shaft contribute to the creation of the thread. The tool described at last is particularly suited as a forming tool. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in more detail below, by means of realisation examples represented in drawings. 
       FIG. 1  shows the side view of a shaft for a cold thread forming tap according to the invention. 
       FIG. 2  shows the front view of the shaft after  FIG. 1   
       FIG. 3   a  shows the side view of the shaft after  FIG. 2 , whereas  FIGS. 3   b  and  3   c  represent details. 
       FIG. 4  shows the top view on a profile element according to the invention. 
       FIG. 5  shows a magnified view of a profile element similar to  FIG. 4 . 
       FIG. 6  shows the side view of the profile element after  FIG. 5   
       FIG. 7  shows the rear view of a profile element according to the invention for a thread cutting tool. 
       FIG. 8  shows the side view of the profile element after  FIG. 7   
       FIG. 9  shows a screw for the fixation of one of the shown profile elements on a tool shaft, the shaft after  FIG. 1 , e.g. 
       FIG. 10  shows the front view of another embodiment of a profile element according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   While this invention may be embodied in many different forms, there are described in detail herein a specific preferred embodiment of the invention. This description is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiment illustrated. 
   In  FIG. 1 , a tool shaft  10  is represented which has a chuck portion  12  on the one end and on the other end a portion  14  for connection with a profile element after  FIG. 4 . From the end view of shaft  10  after  FIG. 2  one can recognise that it has a centric thread bore  16  and crosswise arranged ribs  24 , the cross of the ribs  24  coinciding in its centre point with the axis of the shaft  10  or the bore  16 , respectively. 
   In  FIG. 4 , a plate-shaped forming profile element  20  is represented, which has a forming initiation cone as well as a thread profile (not shown) for the making of an interior thread according to the cold form grooving or thread forming method. The perimeter of the forming profile element  20  is polygonal, which is not represented in  FIG. 4 . Into further details of the profile element  20  will still be dwelled on in connection with  FIGS. 5 and 6 . 
   The profile element  20  has a centric bore  22  and crosswise radially arranged grooves  18  on one side. The grooves  18  are arranged and formed such that they fittingly accommodate the ribs  24 . For the sake of an easier insertion, the grooves  18  and/or the ribs  24  have a chamfer on the longside edges. Through the positively fitting gearing of ribs  24  and grooves  18 , a positive fitting is obtained, whilst the remaining area of the profile element  20  either sits plain on the remaining area of the end area of the shaft  10 , or preferably has a certain separation from the latter. Through the bore  22 , the shaft of a screw, like that represented in  FIG. 9  at  26 , is inserted and screwed into the thread bore  16 , in order to connect the profile element  20  with the shaft  10  fixedly and secured against rotation. Thus, a two-part cold thread forming tool is made, and it is easily possible to attach another profile element on the shaft  10  by unscrewing the screw  26 . 
   From the detail representations after  FIGS. 3   b  and  3   c  it can be recognised that the juts or ribs  24  are trapezoid in cross section, i.e. that they have slant side faces, as one of them is represented at  25 . The grooves  18  are provided with complementary side faces (not shown), so that only the areas  25  of the ribs  24  and the grooves are brought into gearing with each other upon putting the profile element  20  onto the shaft end  24 , but not the end areas of shaft  10  and profile element  20 . Because the radial ribs  24  and the radial grooves  18  stand perpendicular with respect to each other, a centring of the forming element  20  on the shaft  10  takes place automatically, for the areas  25  and the complementary areas of the grooves run parallel to diameters which stand perpendicular with respect to each other. The crossing point of the diameters is naturally situated on the axis of the profile element  20  and shaft  10 . 
   In the case shown the areas  25  are planar. However, they can also be crowned and have the desired effect of centring and torque transmission in the same manner. 
   The profile element  20  is represented in some more detail in  FIGS. 5 and 6 . One recognises from the front view after  FIG. 5  that the profile element  20  is polygonal, i.e. that is has four “corners”, the stroke between the corners being denoted with h. It is to be understood that a thread creation by non-cutting forming takes place only in the corner regions. In the  FIGS. 5 and 6 , grooves  18  are provided, which co-operate with corresponding ribs of a shaft like that represented in  FIG. 1 , in the same manner as has been elucidated in connection with  FIGS. 1 to 4 . The grooves  18 , again, have slant wall faces on their sides, which co-operate with slant wall faces of ribs. As can be recognised in  FIG. 6 , the profile element  20  has a forming initiation cone  28  in conventional manner and a proper profile portion  30 , with two to four convolutions of a forming thread. As indicated at  32 , the bore  22  has a sink  32  for the accommodation of the cone  34  of the head of the screw  26  after  FIG. 9 . 
   In the  FIGS. 7 and 8 , a profile element  40  for a thread-cutting tool (screw tap) is represented. As can be recognised from  FIG. 8 , the profile element  40  has a conical starting taper  42  as well as a thread cutting portion  44 . The thread cutting portion  44  consists of four bars  46  disposed at 90° separation, which are provided with corresponding thread portions on the perimeter, as this is known from screw taps. Between the bars  46 , flutes  48  are disposed, respectively. The profile element  40  has a bore  50 , which has a sink  52  on the end which is on the left side in  FIG. 8 , for the accommodation of a screw head of a screw after  FIG. 9 , for instance. On the side facing the not shown shaft, the profile element  40  has crosswise arranged ribs  54 , comparable with the ribs  24  after  FIGS. 3   a  and  3   b . The ribs  54  have slant faces  56 , which co-operate with the slant faces of the grooves of the not shown shaft for the profile element  40 . 
   The attachment of the profile element after  FIGS. 7 and 8  on the tool shaft is the same as that of the profile elements as described in  FIG. 4 . Only the disposition of the ribs and grooves is reversed with respect to  FIGS. 2 and 4 . Consequently, it can be offhand replaced by an other one after wearing, or even be replaced by a profile element which is shaped otherwise. 
   In  FIG. 10 , a shaft of a cold thread forming or thread cutting tool, e.g. a shaft after  FIG. 1 , is represented with its front side  62  at  60 . On the front side  62 , two profile elements  64  and  66 , respectively, are fixed in a perimeter spacing of 180°, with the aid of screws  68  and  70 , respectively, which are comparable to the screw  26  after  FIG. 9 . The profile elements  64 ,  66  resemble e.g. on the perimeter the profile element  20  after  FIG. 4 , and are therefore suited for cold form tapping. Preferably, even four such profile elements will have to be preferred, so that a polygon similar to that after  FIG. 5  evolves again. As the generating thread is disposed around the perimeter in the profile elements  64 ,  66 , it is also imaginable to turn the profile elements  64 ,  66  at a certain angle after previous disassociation, in order to bring another profile region into action, which has not be worn yet. Through this, the profile elements  64 ,  66  resemble so-called indexable inserts with respect to their application. 
   It is to be understood that profile elements like those after  FIG. 10  can be used for thread cutting also. 
   The above disclosure is intended to be illustrative and not exhaustive. This description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the claims where the term “comprising” means “including, but not limited to”. Those familiar with the art may recognize other equivalents to the specific embodiments described herein which equivalents are also intended to be encompassed by the claims. 
   Further, the particular features presented in the dependent claims can be combined with each other in other manners within the scope of the invention such that the invention should be recognized as also specifically directed to other embodiments having any other possible combination of the features of the dependent claims. For instance, for purposes of claim publication, any dependent claim which follows should be taken as alternatively written in a multiple dependent form from all prior claims which possess all antecedents referenced in such dependent claim if such multiple dependent format is an accepted format within the jurisdiction (e.g. each claim depending directly from claim  1  should be alternatively taken as depending from all previous claims). In jurisdictions where multiple dependent claim formats are restricted, the following dependent claims should each be also taken as alternatively written in each singly dependent claim format which creates a dependency from a prior antecedent-possessing claim other than the specific claim listed in such dependent claim below. 
   This completes the description of the preferred and alternate embodiments of the invention. Those skilled in the art may recognize other equivalents to the specific embodiment described herein which equivalents are intended to be encompassed by the claims attached hereto.