Abstract:
Centering device acting upon an already present first ring ( 14   a ) of a straight or double helical toothing of gears, sprocket wheels or pulleys to obtain the second ring ( 14   b ) of the toothing generated by a pinion shaped cutter ( 16 ), a central groove ( 19 ) separating said first ring and said second ring. The centering device comprising a retractable pin ( 20, 120 ), cooperating during the positioning step of the gear ( 13 ), with at least a tooth ( 15 ) of the first ring ( 14   a ), having a first waiting position ( 20   a   , 120   a ) and a second operating position ( 20   b   , 120   b ) defining the angular position of the gear ( 13 ) clamped on the rotary supporting base ( 18 ), the angular position being functionally correlated to the plane which contains the axis ( 25 ) of the gear ( 11 ) and the axis ( 26 ) of the cutter ( 16 ).

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This Application is a continuation of International Application PCT/GB98/03301, filed Nov. 4, 1998, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention concerns a supporting and centering device for use in the production of double-toothing sprocket wheels or pulleys and method of using same. 
     The sprocket wheels or pulleys to which the invention refers include a central groove which separates the two toothed rings and whose teeth are usually inclined from 2° to 60°, but can be straight. The sprocket wheels or pulleys to which the invention refers are advantageously of the type used together with the so-called positive drive belts such as those produced by the Goodyear Company and known by the trade name of “EAGLE”. 
     The invention seeks to enable sprocket wheels to be produced using generating blades or pinion instruments which move along an axis parallel to the axis of the sprocket wheel and rotate in coordination therewith, both in the forward and in the return movement, the axes being perpendicular to the plane of the workpiece-bearing table. The sprocket wheels to which the invention relates may include teeth which coincide with respect to the center line or are offset, the offset distance being advantageously 50% of the pitch. 
     The invention is used to correctly position a workpiece in order to obtain the desired reciprocal positioning of the two mating rings of teeth of the sprocket wheel. The state of the art covers sprocket wheels with a straight or bi-helical double-toothing. Reference is made, for example, to DE-A1-2717515 where one positioning method for carrying out toothing is described. The wheels comprise two coaxial rings of oblique and converging teeth, arranged substantially symmetrically with respect to an intermediate plane of separation, in a substantially herringbone design. Double-toothing wheels are normally worked by positioning and clamping a workpiece and subsequently activating a generating blade or cutting instrument to act on the periphery of the workpiece. Depending on the shape of the teeth, the sprocket wheels may be used to transmit movement by means of positive drive belts. 
     On the one hand it is not normally possible to work small double-toothing wheels on a large scale with machines having a single positioning, and on the other hand such a machine involves prohibitive production costs and low productivity. There have also been proposals to work the double-toothing sprocket wheels using machines with a supporting base or workpiece-bearing table and with a generating blade or pinion instrument cutting tool which works with a to-and-fro movement parallel to the axis of the piece and rotates at the same time in a coordinated manner with the piece to be worked. 
     In this case the unworked workpiece is placed with its axis vertical and associated with a shaft, cooperating with the supporting base or piece-bearing table, which determines the starting position with respect to the position of the generating blade or pinion instrument. In this procedure, two rings of teeth are made in two successive steps; the first step leads to formation of teeth on a first ring and the second step is for formation of the teeth on the other ring. 
     However, with this procedure there is the problem of correctly positioning the workpiece on the machine for the second ring of teeth to be made so as to achieve the precise respective positions of the first and second ring of teeth. It should be noted that in order to obtain a satisfactory result the axes of the teeth must necessarily meet along the center line of the sprocket wheel within extremely limited tolerances. 
     The play in the connection between the workpiece to be worked and the shaft which clamps the workpiece to the supporting base makes it even more difficult to achieve the correct positioning. Therefore it becomes necessary to carry out long and complex operations to adapt the position of the generating blade or pinion instrument with respect to the workpiece and vice versa, according to data and measurements monitored by specific equipment. 
     Also with this method, the cost of production of the sprocket wheels is very high and one of the reasons for this is simply because of the additional operations of removing the shavings. 
     The smaller the sprocket wheel to be made, the greater the problem; even though it is possible to achieve larger sprocket wheels with a sufficient degree of precision, and with overall production times which are reasonable with respect to the total cutting time, until now it has been practically impossible to make small sprocket wheels of this type with any guarantee of reliability and at reasonable cost. 
     An object of the invention is to achieve a device which will allow the precise positioning and centering of the workpieces required for producing a second ring of teeth in the production of double-toothing sprocket wheels. 
     SUMMARY OF THE INVENTION 
     Another object of the invention is to obtain the precise positioning of the teeth of the second ring with respect to the teeth of the first ring cheaply and in a short time. 
     A further object is to provide a simple, functional device which will allow the workpiece to form the sprocket wheel to be centered precisely with respect to a cutting blade or pinion instrument, and which will allow the correlated centered position to be maintained throughout the working step. 
     The invention is set forth and characterised in the respective independent claims, while the dependent claims describe other characteristics of the idea of the main embodiment. 
     The invention is applied to production of double-toothing sprocket wheels generated typically by generating blades or pinion instruments. 
     This invention also provides a method of producing double-toothing sprocket wheels which will allow a drastic reduction in the time required to center and correctly position the workpiece and therefore, more generally, reduce the times and costs of production, while at the same time guaranteeing extreme precision and reliability even for smaller sprocket wheels. 
     The centering device according to the invention operates in association with the shaft of the supporting base of the workpiece to be worked and is placed, for any size of sprocket wheel, in a pre-set position with respect to the start-of-work position of the blade device or pinion instrument. The device cooperates with any tooth, or with any pair of teeth, of the first ring of teeth. 
     The pre-set position is correlated to the extent of the desired staggering, or alignment, of the teeth of the first and second ring of the sprocket wheel. 
     The centering device comprises at least one retractable pin arranged with its axis substantially parallel to the axis of rotation of the sprocket wheel and cooperating with the separating grooves between the teeth of the first ring of the sprocket wheel which is to be made. 
     According to one variant, the centering device has a fork element which cooperates with a tooth of the first ring. 
     According to another variant, the pin or fork element cooperates with the groove, or with two teeth, substantially along the pitch circle of the teeth. 
     Hereinafter, for simplicity of explanation, we shall describe only the situation with the retractable pin, but the situation with the fork element cooperating with a tooth shall be considered equivalent thereto. 
     According to the invention, the retractable pin can assume at least a first position, or waiting position, wherein it is arranged in a high position above the support surface of the supporting base and a second position, or operating position, wherein it cooperates with two teeth of the first ring, then protruding to a small extent from the said surface. 
     According to a variant, the retractable pin cooperates with spring means which is able, when compressed or released, to define respectively the waiting position or the operating position. 
     The centering device is kinematically connected to the supporting base so that it rotates together with the base, with the sprocket wheel and possibly with the axial positioning and clamping shaft; the sprocket wheel is mounted coaxially with respect to the base and the shaft. 
     According to another variant, the position of the centering device with respect to the axis of rotation of the workpiece can be adjusted according to the diameter of the sprocket wheel which is to be made. 
     According to the invention, after the first ring of teeth has been made, the workpiece is positioned at a work station so that the second ring may be made, the first ring being arranged so as to cooperate with the supporting base. 
     In this condition, the retractable pin of the centering device is in the waiting position and is then inserted into a groove separating two teeth of the first ring; then the axial positioning and clamping shaft is applied, which thereby centering the sprocket wheel with respect to the supporting base and at the same time clamping it with respect thereto. When the sprocket wheel is clamped onto the supporting base, the centering device is in the operating position. 
     The contact between the retractable pin and the walls of the teeth between which it is inserted during the clamping step causes a possible adjustment rotation of the sprocket wheel with respect to the axis of rotation. This causes the pair of teeth to be correctly positioned with respect to the generating device or pinion instrument. 
     As work progresses in making the second ring, the centering device, since it is fixed to the base, moves together with the sprocket wheel, the supporting base and possibly also the axial positioning and clamping shaft, thus maintaining the workpiece in the correct position for the duration of the whole operation. 
     According to a variant, the retractable pin of the centering device is placed orthogonal to the axis of rotation of the sprocket wheel. 
     According to a further variant, the centering device comprises an auxiliary centering element which is arranged, during the initial equipping and preparation step, above the supporting base and in cooperation with the retractable pin. The auxiliary centering element has the function of centering the generating tool with respect to the retractable pin before the cycle is started, and therefore it allows for positioning of the generating tool in the correct angular position with respect to the sprocket wheel to be worked. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
     FIG. 1 shows a diagram of an assembly for use in making double-toothing sprocket wheels which embodies the invention; 
     FIG. 2 shows a detail of FIG. 1 from a direction “A”; 
     FIG. 3 shows a lengthwise section of the cantering device of the assembly of FIG. 1; 
     FIG. 4 shows a detail of the operating position of a retractable pin with respect to the sprocket wheel; 
     FIG. 5 shows a variant of the arrangement of the preceding figures; and 
     FIG. 6 shows a further variant of the arrangement. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the attached figures the reference number  10  denotes generally a centering device comprised by apparatus embodying the invention for producing double-toothing sprocket wheels  11 . 
     In the embodiment shown here we shall describe the situation of sprocket wheels  11  with a bi-helical toothing and cooperating with a supporting surface of a workpiece-bearing table which is substantially horizontal and with a clamping shaft which passes through the hollow sprocket wheel. 
     The production of sprocket wheels with a straight double-toothing, or of double-toothing pulleys for drive belts, or workpiece-bearing tables which are vertical or in any case inclined to the horizontal are all situations which are to be considered equivalent. 
     The bi-helical sprocket wheels  11  comprise two different rings, the first ring  14   a  and the second ring  14   b , of teeth  15  being inclined at a desired angle, but opposite, with respect to the median plane  29 . 
     The rings  14   a ,  14   b  are separated from each other by a central groove  19 . 
     The centering device  10  is used in association with an assembly to position and center the workpieces  13 , in order to determine the correct positioning of the workpieces  13  to obtain the desired angular positions between the rings  14   a ,  14   b  of the teeth  15 . 
     To be more exact, the centering device  10  cooperates with a pinion instrument  16  in order to place the second ring  14   b  in the desired position with respect to the first ring  14   a.    
     The pinion instrument  16 , which has teeth which reproduce the teeth  15  to be made on the workpieces  13 , works by generation and moves to-and-fro (from  16   a  to  16   b  and vice versa) and along a travel path  12 , which passes through the central groove  19  parallel to the axis of rotation  25  of the workpiece  13 . 
     The two axes  26 - 25  (of the pinion instrument  16  and the workpiece  13 ) progressively approach one another (each of them being in motion) until they define the desired depth of the teeth  15 . 
     The centering device  10  is put in a pre-set position with respect to the plane where the axis  25  joins the axis  26  of the pinion instrument  16  in its start-of-work position. 
     In the embodiment shown here, the positioning and centering assembly comprises a supporting base  18  or workpiece-bearing table; the workpiece  13  is placed on the supporting surface  18   b  thereof. 
     The supporting base  18  has a coupling cavity  18   a  entered from above by a shaft  17 , which positions and clamps the workpiece  13  to the base  18 , while centering the shaft  17 . The base  18  enables the workpiece  13 , and also possibly the shaft  17 , to rotate. 
     The centering device  10  is radially displaceable with respect to the base  18  and is mounted, in this case, on a slider which slides on a guide  24  which is formed radially in and with respect to the base  18 . 
     The centering device  10  comprises a main body  21  inside which a retractable pin  20  is resiliently constrained; the upper part of the retractable pin  20  is spherical in order to make the reciprocal positioning between the centering device  10  and the workpiece  13  more precise. 
     The retractable pin  20  may assume at least a first position, or waiting position,  20   a  wherein it is arranged in a high position above the plane lab, and a second position, or operating position,  20   b  when it has entered a groove  22  between two teeth  15 , protruding, for this purpose as much as necessary from the surface  18   b.    
     In this case, the retractable pin  20  is associated with spring means  27  of the cup type, housed inside the main body  21 . 
     The retractable pin  20  is of a replaceable type and its spherical part mates with the transverse sides of the grooves  22  between two successive teeth  15 . 
     Moreover, the retractable pin  20  is made of a material of great hardness and strength, at least in its upper part. 
     According to the invention, the upper part of the workpiece  13  is always worked on by the pinion instrument  16 ; therefore, after the first ring  14   a  has been formed, the same workpiece  13  is transferred to another machine arranging the first ring  14   a  on the surface of  18   b  of the supporting base  18 . The workpiece  13  is moreover positioned in such a way as to align one of the grooves  22  with the retractable pin  20 . 
     When the axial positioning and clamping shaft  17  is lowered to clamp the workpiece  13  onto the surface  18   b , the pin  20  assumes its operating position  20   b , and is inserted inside the groove  22  in a substantially median position. 
     Although not essential in the case of FIG. 4, the spherical end of the retractable pin  20  is shaped with respect to the groove  22  in such a way that the points of contact  20   c  with the walls of the teeth  15  lie on the pitch circle  115 . 
     The retractable pin  20  comes into contact with the side walls  15   a  of the teeth  15  between which it is inserted, and causes a possible adjustment rotation and a consequent centering of the workpiece  13  with respect to the axis  25 , and therefore the correct positioning with respect to the start-of-work position  16   a  of the pinion instrument  16 . 
     The pre-set positioning of the centering device  10 , and therefore of the retractable pin  20  according to the desired angular positioning of the first ring  14   a  and the second ring  14   b , guarantee the precise positioning of the workpiece  13  with respect to the pinion instrument  16  which thus is found to be, in the start-of-work position  16   a , already correctly arranged to begin working on the second ring  14   b.    
     When a workpiece  13  of a greater or lesser diameter is being worked, the centering device  10  is radially displaced with respect to the shaft  17  by moving the slider  23 , on which the shaft is mounted, along the guide  24 . 
     The supporting base  18  includes grooves  28 , which can extend in any direction or orientation whatsoever, which facilitate the removal of the sprocket wheel  11  once its forming is complete, by reducing the adhesion of the workpiece to the workpiece-bearing table resulting from the surface tension of the cutting liquid. 
     In the case of FIG. 5, instead of the retractable pin  20  entering a groove  22  between two teeth  15 , there is a fork-type pin  120 , resiliently mounted using spring means  127 , which cooperates with a main body  121  and includes two spherical elements  30  which cooperate with the sides of a tooth  15  in order to center the sprocket wheel  11  itself. The fork-type pin  120  has a waiting position  120  (not shown) and an operating position  120   b.    
     In the variant shown in FIG. 6, the centering device  10  comprises an auxiliary centering element  31  which is used in the initial equipping and setting-up of the tool  16 . 
     The auxiliary centering element  31  consists of a plate  32 , suitable for having the base  18  rest thereon, to which a block  33  is attached; the block  33  contains inside a retractable pin  34  equipped with a protruding head, round in shape, advantageously spherical. 
     The retractable pin  34  is housed inside a housing  35 , set within the block  33 , inside which it is able to slide vertically, for example resiliently supported and having a short defined section. 
     With the auxiliary centering element  31  in the working position, the retractable pin  34 , is arranged in an axial extension of the retractable pin  20  inserted inside the base  18 . 
     Under the containing body  35 , at a substantially central position of the plate  32 , there is a hollow bush  36  with an axial hole  37  which is able to cooperate with the head of the retractable pin  20 . 
     The height of the base  18  of the plate  32  plus the block  33  corresponds substantially to the height “1” of the tool  16  taken from the base  18  itself. 
     During the setting-up step, before the workpiece  13  is arranged on the base  18  and working begins, the auxiliary centering element  31  is positioned on and clamped to the base  18  making the head of the retractable pin  20  cooperate with the axial hole  37  of a bush  36  inserted in the plate  32 . 
     Then, the tool  16  is displaced radially into proximity with the auxiliary centering element  31  until one of the grooves defined between its teeth comes into contact with the protruding head of the retractable pin  34 , with a consequent rotation of the tool  16  being carried out to adjust the angle thereof. 
     Then, when its correct angular position has been fixed, the tool  16  is retracted, the auxiliary centering element  31  is discharged from the base  18  and in its place the workpiece  13  on which the second ring of teeth has to be made is positioned, with a consequent centering of the workpiece  13  with respect to the retractable pin  20 . 
     Due to the axial alignment of the retractable pin  20  and the retractable pin  34 , the tool  16  is already in the precise angular position without needing any further adjustments to its position with respect to the workpiece  13 .