METHOD FOR MANUFACTURING A RACK COMPRISING VARIABLE-PITCH TOOTHING IMPLEMENTING A ROUGHING STEP FOLLOWED BY A FINISHING STEP AND A HELICAL BROACHING TOOL

Abstract

A method for manufacturing a rack including a toothing with variable pitch includes a roughing step, in which a blank of the rack is produced, the blank including at least one extra thickness on at least one zone of the toothing compared to a desired dimensional feature of the rack. The method also includes a finishing step, in which the at least one extra thickness of the blank is removed.

Claims

1. A method for manufacturing a rack comprising a variable-pitch toothing, comprising a blank-forming step in which a blank of the rack is produced, the blank comprising at least one additional thickness on at least one area of the toothing with respect to a desired dimensional characteristic of the rack, wherein the method also comprises a finishing step carried out by means of a helical broach method using a broach tool in which the at least one additional thickness of the blank is removed.

2. The manufacturing method according to claim 1, wherein the at least one additional thickness is comprised between 0.02 mm and 1 mm.

3. The manufacturing method according to claim 1, wherein the finishing step comprises a positioning phase in which the broach tool is positioned with respect to the blank of the rack so as to form a predetermined angle.

4. The manufacturing method according to claim 1, wherein the finishing step comprises a broach phase in which the broach tool performs a rotational movement along an axis of rotation extending along a length of the broach tool.

5. The manufacturing method according to claim 4, wherein, during the broach phase, the broach tool performs a translational movement along a translation axis extending along the length of the broach tool.

6. The manufacturing method according to claim 4, wherein, during the broach phase, the blank of the rack performs a translational movement along an axis extending along a length of the blank of the rack.

7. The manufacturing method according to claim 1, wherein, during the broach phase, a translation speed of the blank of the rack or a translation speed of the broach tool or a rotation speed of the broach tool is variable.

8. The manufacturing method according to claim 7, wherein the speed of translation of the blank of the rack or the speed of translation of the broach tool is correlated to the speed of rotation of the broach tool according to a broach curve corresponding to a reduction ratio between the rack and a pinion with which it is intended to cooperate.

9. A broach tool allowing the implementation of the manufacturing method according to claim 1 comprising at least one helical flute extending over a length of the broach tool, the flute corresponding to a groove of a pinion with which the rack is intended to cooperate.

Description

[0055] Thus, the flute extends helically over the entire length of the broach tool. The broach tool is suitable for producing helical broach. The invention will be better understood, thanks to the description below, which relates to an embodiment according to the present invention, given by way of non-limiting example and explained with reference to the appended diagrammatic drawings, in which:

[0056] FIG. 1 is a representation at a first instant of a finishing step of a method according to the invention;

[0057] FIG. 2 is a representation at a second instant of the finishing step of the method according to the invention;

[0058] FIG. 3 illustrates, in a schematic perspective view, a portion of a power steering mechanism for a vehicle comprising a pinion which meshes with a rack with variable-pitch toothing manufactured according to the method which is the object of the invention;

[0059] FIG. 4 is a representation of a broach curve used by a method according to the invention.

[0060] The invention concerns a method for manufacturing a rack 2 comprising a toothing with variable pitch P1 implementing two distinct steps comprising a blank-forming step then a finishing step F.

[0061] During the blank-forming step, the blank 1 of the rack 2 is made using, for example, a machining method. The term «machining method» means a method for removing material by cutting chips by means of a moving cutting tool, preferably a rotating cutting tool such as a milling cutter, which is driven in rotation around its own central axis to achieve a cutting effect.

[0062] The blank-forming step produces a blank 1 of the rack 2, that is to say a rack 2 which comprises additional thicknesses on an area of the toothing. Thus, the blank 1, due to the presence of excess material, is not conform to the dimensional characteristics desired for the rack 2. The blank 1 cannot therefore be used directly in a vehicle power steering system.

[0063] The production of the blank 1 requiring less precision than the direct production of the rack 2, an adjustment of the machining tool used by the machining method is facilitated. Thus, the method according to the invention makes it possible to reduce a time necessary for adjusting the machining tool.

[0064] The blank 1 is made by cutting a toothing in a rectilinear bar, preferably metal for problems of mechanical strength during the use of the rack 2. The toothing extends substantially transversely to a length L2 of the bar.

[0065] The toothing has a variable pitch P1, that is to say that the interval P1 which axially separates two successive teeth 4 varies according to the position and the curvature of said teeth 4 along the length L2 of the bar.

[0066] This makes it possible in particular to vary the reduction ratio R of the rack 2 as a function of the considered meshing area 8, 9, 10.

[0067] Thus, in the example of a steering mechanism for a vehicle, such as that illustrated in FIG. 3, in which the rack 2 meshes with a pinion 6, itself driven for example by an assistance motor and/or or by a steering column 7 connected to a steering wheel. It is possible to provide a short pitch P1 in the middle area 8 of the rack 2, so as to obtain greater precision of the steering manoeuvres in the vicinity of the straight line, then to increase the pitch P1 when moving away from the middle area to the end areas 9, 10 of the rack 2, so as to accelerate large-scale movements, in particular during parking manoeuvres. The difference in behaviour of the steering movements in the middle area 8 and the extreme areas 9, 10 is represented by a curve of the reduction ratio as illustrated in FIG. 4.

[0068] The curve of the reduction ratio R illustrates a reduction coefficient of the rack 2 with variable pitch as a function of a rotation D of the pinion 6. For a rotation angle D of the pinion 6 comprised between −10° and 10°, that is to say in the middle area 8, the reduction ratio R is substantially constant in order to promote driving precision and the steering wheel feeling in a straight line. Whereas for rotation angles D of the pinion 6 comprised substantially between −10° and −130° and 10° and 130°, that is to say in the extreme areas 8, 9, the reduction ratio D increases greatly thus making it possible to favour the trajectory of the vehicle.

[0069] After the completion of the blank-forming step, the method according to the invention implements a finishing step F comprising a positioning phase then a broach phase.

[0070] The positioning phase consists of positioning the blank 1 previously produced facing a broach tool 11 with a view to carrying out the broach phase. For this, the blank 1 is fixed on a first sliding carriage 12 so that the blank 1 can perform a translational movement along an axis X extending along the length of the blank 1 of the rack 2.

[0071] Furthermore, the broach tool 11 is mounted on a second carriage 13 so as to allow a rotational movement along an axis of rotation Y extending along a length of the broach tool and a translational movement along an axis of translation Z also extending along the length of the broach tool 11.

[0072] The broach tool 11 comprises a plurality of helical flutes 14 extending over the length of the broach tool 11. More precisely, the flutes 14 correspond to a trace of the grooves 15 of the pinion 6 with which the rack 2 is intended to cooperate, when the pinion 6 travels over the rack 2. In other words, the flutes 14 of the broach tool 11 are identical to the grooves 15 of the pinion 6 when the latter are extended over the entire length of the broach tool 11. The flutes 14 have cutting edges capable of notching and removing the material in which the blank 1 is made.

[0073] The broach tool 11 also comprises grooves 16 extending substantially transversely to the flutes 14. The purpose of the grooves 16 is to evacuate the elements or shavings of material cut by the flutes 14.

[0074] During the positioning phase, and as represented in FIG. 1, the broach tool 11 is positioned at a first end of the blank 1.

[0075] Furthermore, the broach tool 11 and the blank 1 of the rack 2 form a predetermined angle A corresponding to the angle formed between the rack 2 and the pinion 6 with which it is intended to cooperate.

[0076] Following the positioning phase, the finishing step carries out the broach phase as illustrated in FIG. 2, in which the additional thicknesses of the blank 2 are removed by the broach tool.

[0077] During the broach phase, the broach tool 11 is brought into contact with the blank 1 so as to engage the broach tool 11 on the blank 1 in the manner of the pinion 6 on the rack 2.

[0078] The broach tool 11 then performs a movement of translation and rotation while the blank 1 performs a movement of translation. Thus, the broach tool 11 travels over the toothing of the blank 1 in the manner of the pinion 6 on the rack 2. When the broach tool 11 arrives at the level of the second end of the blank 1, the entire length of the broach tool 11 has passed through the blank 1 so that the whole of the blank 1 has been travelled by the broach tool 11.

[0079] The translation speed of the blank 1 of the rack 2 or the translation speed of the broach tool 11 is correlated to the rotation speed of the broach tool 11 according to a broach curve similar to the curve of the reduction ratio illustrated in FIG. 4.

[0080] Thus, the broach tool 11 travels over the blank 1 with a movement similar to the movement of the pinion 6 on the rack 2. In this way, the broach tool 11 removes the material exactly in the areas which will hinder subsequent operation of the pinion 6/rack 2 couple.

[0081] After the completion of the broach phase, the blank 1 no longer comprises any additional thickness. The blank 1 has become a rack 2 which can be installed in a power steering system of a vehicle.

[0082] According to one characteristic of the invention, the broach phase is carried out after a heat treatment phase. Indeed, during the latter, deformations can occur such as swelling and twisting of the rack, in particular when the rack has significant helix angles. Thus, carrying out the broach phase after the heat treatment phase advantageously confers the geometric quality of the flanks.

[0083] Of course, the invention is not limited to the embodiments described and represented in the appended figures. Modifications remain possible, in particular from the point of view of the constitution of the various elements or by substitution of technical equivalents, without thereby departing from the scope of protection of the invention.