The invention relates to an axial thread rolling head, comprising a head part, in which at least three thread rolls are rotatably mounted, wherein the thread rolls amongst themselves border an insertion section for a workpiece and wherein the thread rolls each have a profile for forming an outer thread on the workpiece. The invention further relates to a method for forming an external thread on a workpiece with an axial thread rolling head.
With such axial thread rolling heads, external threads are created on generally cylindrical workpieces through cold forming. The thread rolls have a profile on their circumference for the forming of the external thread. The workpiece is inserted into the insertion section of the axial thread rolling head during the course of a relative movement between the axial thread rolling head and the workpiece, wherein the external thread is created by the profile of the thread rolls. Axial thread rolling heads are known, in which after complete formation of the external thread on the workpiece, in particular a complete insertion of the workpiece into the insertion section of the axial thread rolling head, the axial thread rolling head is opened by an mechanical actuation mechanism. The thread rolls are thereby moved outward in the radial direction and the axial thread rolling head can be pulled off the workpiece without the thread rolls colliding, i.e. engaging, with the workpiece. This results in a simple processing with precisely produced external threads.
However, disadvantageous thereby is a comparatively large space requirement for the radial movement of the thread roller bearing including the thread rolls arranged on it in the working space of the processing machine. This working space must naturally also include the workpiece contour. Moreover, a not insignificant installation space is required for the actuation mechanism for radial adjustment of the thread rolls outwards in the axial thread rolling head itself. Due to the increasingly desired component size reduction, for example in the automotive industry, the space available for processing is becoming increasingly smaller. One example is the production of external threads on cylindrical appendages of fuel injection rails of an automobile. Pressure lines can be connected to the formed external threads. Among other things due to the enormous operating pressures, there are very high requirements for the accuracy and thus impermeability of the produced threads. A rolling die for the pressing of threads is known from EP 0 552 713 A1, which is non-opening. The advantage is that this rolling die is built comparatively small and requires no space for opening. However, in the case of the known rolling die, a considerable deflection of the thread rolls results during the course of the processing, among other things because the rolling die does not have a head plate and the thread rolls are thus little reinforced against deflection due to the small size of the rolling die. This deflection leads to process errors on the workpiece. In particular, the external thread produced on the workpiece does not always have the provided profile.
The deflection can be reduced to a limited scope through the provision of a head plate. A deflection of the thread rolls can also be compensated to a certain degree through a skewing of the axles of the thread rolls with respect to the longitudinal axis of the insertion section. However, support measures for preventing a deflection of the thread rolls can naturally only be taken in a limited and constructive manner in the case of the specified diameter of the thread to be produced on the workpiece on one hand and the maximum cross-section of the axial thread rolling head on the other hand. In particular in the case of the processing of high-strength materials with tensile strengths e.g. of more than 1,000 N/mm2, a deflection of the thread rolls cannot be completely prevented through constructive measures.
Based on the named state of the art, the object of the invention is thus to provide an axial thread rolling head and a method of the initially named type, with which comparatively large diameters in the case of small outer dimensions of the axial thread rolling head can be produced reliably and precisely in the specified manner.
The invention solves this object through the subjects of claims 1 and 16. Advantageous embodiments are found in the dependent claims, the description and the figures.
The invention solves the object for one through an axial thread rolling head, comprising a head part, in which at least three thread rolls are rotatably mounted, wherein the thread rolls amongst themselves border an insertion section for a workpiece and wherein the thread rolls each have a profile for forming an outer thread on the workpiece, wherein the thread rolls are held non-adjustably in the radial direction in the head part and wherein the profile of the thread rolls is respectively designed asymmetrically such that the thread rolls with their asymmetrical profile form a symmetrical external thread on the workpiece. The axial thread rolling head can also comprise a shaft, which holds the head part.
The basic function of the axial thread rolling head is similar to that described above for the state of the art. The thread rolls, also called profile rolls, of the axial thread rolling thread according to the invention thus border in a generally known manner amongst themselves an insertion section for example for a cylindrical workpiece to be processed. Through a suitable relative movement between the axial thread rolling head and the workpiece, the workpiece is inserted into the insertion section, wherein the cross-section of the workpiece is large enough that the profile of the thread rolls engages in the outer surface of the workpiece and an external thread is created in the workpiece through cold deformation. For this, the thread rolls have in the generally known manner a profile progressing over its circumference. In the case of axial thread rolling heads, as applicable according to the invention, the thread rolls have a profile formed from incline-free profile ribs, that is a rib profile. The distance between the ribs thereby corresponds with the incline of the thread to be formed on the workpiece. This is known per se. For example, three thread rolls can be provided, which are provided over the circumference of the insertion section respectively distributed spaced with respect to each other at 120°.
The axial thread rolling head according to the invention is designed in a non-opening manner. In particular, no actuation mechanism is provided, which moves the thread rolls outward in the radial direction after the forming of the external thread on the workpiece. Rather, the thread rolls are non-adjustable in the radial direction; that is, they are arranged fixed in the axial thread rolling head. Thus, in the case of the axial thread rolling head, the cross-section of the insertion section of the axial thread rolling head is unchangeable except for an unavoidable deflection of the thread rolls in the course of processing.
According to the invention, an unavoidable deflection of the thread rolls in the course of the processing is taken into account and compensated for in that the thread rolls have a suitable asymmetrical profile or respectively rib profile. In the case of the asymmetrical profile according to the invention, opposite lying profile flanks are thus respectively not mirror-symmetrical, in particular with respect to a line running in the radial direction through the bottom or respectively the deepest point between the profile flanks, at least via a processing section of the thread rolls engaging in a forming manner with the workpiece in the course of the processing. The asymmetrical profile according to the invention is thereby adjusted so that it forms a symmetrical thread profile on the workpiece taking into consideration the deflection of the thread rolls to be expected for the respective material and cross-section of the workpiece to be processed on one hand and the constructive conditions of the axial thread rolling head, in particular its dimensions, on the other hand.
Through the design of the axial thread rolling head according to the invention, it is possible to also process in a transformative and precise manner workpieces with a proportionately large cross-section using an axial thread rolling head with a proportionally small outer contour. Compared to the state of the art, the required space is reduced since the axial thread rolling head is non-opening. No complex and space-consuming actuating mechanism is needed to move the thread rolls outward in the radial direction. It is simultaneously ensured through the asymmetrical design of the profile of the thread rolls that the thread designed on the workpiece is precisely symmetrical. The desired external threads are thus produced reliably and precisely.
It is ensured through suitable selection of the design boundary conditions of the axial thread rolling head, in particular for example materials and material strengths, that the requirements with respect to function, stability and durability are safely met. In particular, the durability of the axial thread rolling head and its thread rolls can be ensured through the selection of a suitable material and a corresponding heat treatment.
According to the invention, a forming processing of the external thread of the workpiece can take place through the asymmetrical profile of the thread rolls both during insertion of the workpiece into the insertion section as well as during removal of the workpiece from the insertion section. The profile of the external thread to be produced is thus first designed completely through an insertion into and a removal of the workpiece from the insertion section. In particular, another deformation takes place through the asymmetrical profile of the profile rolls also during removal of the workpiece. Please note that, in this context, an insertion or a removal of the workpiece in or out of the insertion section does not necessarily mean that the workpiece must be moved in the axial direction. The insertion or respectively removal refers rather to a relative movement between the workpiece and the axial thread rolling head in the course of which the workpiece is inserted into the insertion section or respectively removed from the insertion section of the axial thread rolling head. In order to bring about this relative movement, a movement of the axial thread rolling head or also of the workpiece is possible. A movement of both the axial thread rolling head and the workpiece is also possible.
According to a further embodiment, the profile of the thread rolls can form respectively a pointed profile with straight flanks. The profile flanks lie respectively in a straight plane, wherein the planes of the respectively opposite-lying profile flanks lie at an angle to each other. In the case of a pointed profile, the angle formed between two opposite-lying profile flanks is less than 90°. On their summit, neighboring profile flanks can merge in a rounded head area. Accordingly, opposite-lying profile flanks can merge in an also rounded bottom area.
According to a further embodiment, it can be provided that a leading flank angle of the profile of the thread rolls lies respectively in an area between 28.0° and 30.0°, and is preferably 29.5°. A trailing flank angle of the profile of the thread rolls can then lie respectively in an area between 30.0° and 32.0° and can be preferably 31.0°. In this context, a leading flank angle is the angle between the profile flanks leading during insertion of the workpiece into the insertion section and a radial running through the bottom or respectively deepest point between opposite-lying profile flanks. Accordingly, a trailing flank angle is the angle between the trailing profile flanks lying opposite the leading profile flanks during insertion of the workpiece into the insertion section and a radial running through the bottom or respectively deepest point between opposite-lying profile flanks. According to the invention, the leading flank angle and the trailing flank angle are different so that the profile has the asymmetry explained above. If the opposite-lying profile flanks, that is leading and trailing profile flanks, do not lie in a respectively straight plane, but rather have e.g. a curvature, the angles can be designed respectively e.g. between a tangent placed at half the height on the curved profile flanks and the respective radial. Through the named angle areas, a deflection of the thread rolls is taken into consideration and a particularly high dimensional accuracy of the produced asymmetrical external thread is achieved. The described angle ratios can be available for the entire profile e.g. except for a start and an end area of the profile.
A ratio between the diameter of the cylindrical section and the thread spacing of the profiles of the thread rolls is also important for the stability of the system. The thread spacing is the distance between the middle of two neighboring thread pitch sections in the direction of the thread roll longitudinal axis. In the case of single-start threads, as are in particular affected according to the invention, the thread spacing corresponds with the thread pitch, i.e. the section in the direction of the thread roll longitudinal axis, around which e.g. a screw nut would be moved by 360° on the thread in the case of a full revolution. The larger the thread spacing or respectively the thread pitch, the greater are the radial forces occurring in the case of an axial thread rolling head during operation. Thus, the thread spacing or respectively the thread pitch must not be too large specifically in the case of small axial thread rolling heads, which have in particular a small diameter and thus also generally a small wall thickness. A ratio between the diameter of the thread rolls and the diameter of the workpiece to be processed can be respectively less than 1.4. A ratio between the diameter of the at least partially cylindrical section of the head part and the diameter of the workpiece to be processed can also be less than 3.8. The head part can be made of several sections. The head part can also have several cylindrical sections. The at least partially cylindrical section referenced in the aforementioned embodiments is the section mounting the thread rolls, which has the largest cross-section or respectively diameter.
According to a particularly practical embodiment, which also enables a particularly simple production, the head part can comprise a base section and a head plate fastened on the base section. The head plate ensures additional stability of the axial thread rolling head and reduces a deflection of the thread rolls. The thread rolls can be rotatably mounted in the base section and/or the head plate and can be held non-adjustably in the radial direction. The base section can be designed as one piece. It can comprise in particular one or more cylindrical sections of the head part. In the case of the non-opening axial thread rolling head according to the invention, the one-piece design of the head part leads to a particularly high stability and loading capacity during operation. According to a further embodiment, the thread rolls can be fixed respectively between a stop surface of the base section and a stop surface of the head plate in the axial direction.
The thread rolls can also be rotatably mounted on axial pins held in the base section. These are hereby also particularly practical embodiments. For the thread rolls, corresponding receivers can be provided in the base section and in the head plate. In a further embodiment, it can be provided that the longitudinal axes of the axial pins are tilted into the insertion section with respect to the insertion direction of the workpiece such that the insertion section tapers in the insertion direction of the workpiece. The processing is hereby simplified and a deflection of the thread rolls is already compensated for to a certain extent. It can also be provided that the head plate is screwed together with the base section by means of several fitting screws.
According to a further embodiment, it can be provided that the thread rolls in the longitudinal direction of the thread rolls arranged behind each other have respectively a first asymmetrical profile section and a second asymmetrical profile section for forming respectively an external thread on a workpiece, wherein the thread rolls can be respectively selectively aligned with the first profile section or the second profile section to the entrance of the insertion section in order to form an external thread selectively with the first profile section or the second profile section on the workpiece. In the case of this embodiment, the profiles are designed in duplicate on each thread roll, and namely in the opposite direction starting from the two front sides of the thread rolls or respectively starting from the middle of the thread rolls. Depending on the direction in which the thread rolls are inserted into the head part, the first or the second profile section can then be used for the thread formation on the workpiece. The length of the thread rolls is thereby designed such that a complete molding of the thread to be produced on the workpiece is possible without the rear one of the two profile sections seen in the insertion direction of the workpiece engaging with the workpiece. Furthermore, the first profile section and the second profile section can be respectively mirror-symmetrical to an axis extending perpendicular to the longitudinal axis of the respective thread roll. However, it is also conceivable that the first profile section and the second profile section are respectively not mirror-symmetrical to an axis extending perpendicular to the longitudinal axis of the respective thread roll, i. e. are designed differently. A greater flexibility is hereby achieved during the processing since the two profile sections can be adjusted for the processing of different workpieces.
The invention also relates to a method for forming an external thread on a workpiece with an axial thread rolling head according to the invention, in which the workpiece is inserted into the insertion section of the axial thread rolling head through a relative movement between the workpiece and the axial thread rolling head in the axial direction and is removed from it again, wherein a forming processing of the external thread of the workpiece takes place through the asymmetrical profile of the thread rolls both during insertion of the workpiece into the insertion section as well as during removal of the workpiece out of the insertion section. As already initially explained, it is possible to also form external threads on workpieces made of high-strength materials with the axial thread rolling head according to the invention. For example, the workpieces processed according to the invention can have tensile strengths of more than 600 N/mm2, preferably more than 900 N/mm2, even more preferably more than 1000 N/mm2. As also initially explained, in particular the production of external threads on cylindrical appendages of fuel injection rails of an automobile is possible according to the invention.