EP2367645B1 - Method and rolling die for producing a screw having a variable thread pitch - Google Patents

Description

Background of the invention

The present invention relates to a method and means for making a screw with a continuous thread of variable pitch. In this case, the term of a "continuous" thread indicates that it is a single, continuous thread, and serves to delimit against a screw with two separate threads.

Related prior art

A screw with a continuous thread with variable pitch is, for example, in the WO 2009/015754 described. By a suitable variation of the thread pitch can be generated when screwing the screw into a component an internal stress in the bond between the screw and the component. According to the teaching of the cited patent, the variation of the thread pitch is to be selected such that the residual stress of a composite stress, which occurs under load of the component, is opposite, so that at least the voltage peaks of the resulting composite stress are reduced under load of the component. Such a screw with variable pitch can, for example, for reinforcing components, for. As laminated supports, or used to introduce forces into a component.

To make a screw with a desired variable thread pitch, it is advisable to mill the thread from a blank. Modern cutting machines can be relatively easily programmed according to the desired thread progression. The disadvantage here, however, the relatively large loss of material in the machining and the comparatively long processing time, which limits the throughput.

A method according to the preamble of claim 1 or 2 is known from DE 60 2004 004 057 T2 known.

Summary of the invention

The invention has for its object to provide a method for producing a screw with a continuous thread with variable pitch, which can be performed quickly and inexpensively, as well as means for performing this method.

This object is achieved in a first embodiment by the method according to claim 1 and in a second embodiment by a method according to claim 2. Further, it is achieved by a rolling die according to claim 7 or a rolling die according to claim 8. Advantageous developments are specified in the dependent claims.

In the method according to the invention, a blank is rolled between two dies, wherein a rolling profile is formed in each die, comprising a family of curved, non-parallel depressions. This is an essential difference to known rolling processes for the formation of threads with constant pitches, in which the rolled section is formed by a family of straight, parallel and equidistantly arranged recesses.

According to the first embodiment, the recesses are formed and arranged so that the center lines of adjacent recesses can be brought into coincidence by a displacement in the rolling direction by a constant distance T. Further, the slopes of the center lines, which are defined as the quotient of the changes of the position of the center line in the direction transverse and in the direction parallel to the rolling direction, at the respective intersections of the center lines are identical with a line parallel to the rolling direction. Incidentally, these slopes are proportional to the thread pitch in the line-corresponding portion of the finish-rolled screw, i. the portion of the screw which is formed by a portion of the rolling die which extends along said lines parallel to the rolling direction.

In this respect, the course of each indentation or its center line reflects the course of the variable pitch of the finished screw.

The inventor has found that a variable pitch screw with a so-formed rolling die is in practice uncomplicated and surprisingly surprising to the inventor - Can train low rolling pressure. The above-defined geometry of the recesses according to the first embodiment has the consequence that there is virtually no material transfer in the axial direction of the blank apart from the rolling of material into the depressions to form the thread, whereby the rolling forces can be kept surprisingly low.

The uncomplicated behavior during rolling with this geometry of the rolling die is surprising to the person skilled in the art. For example, the inventor is aware of attempts to form two separate threads with different but respectively constant thread pitch on a blank in one and the same rolling process with a two-part rolling die. This has proven to be difficult in practice, since the blank tends to tilt transversely to the rolling direction. It is a surprising result of the rolling process according to the first embodiment that no such tilting occurs during rolling, but that variable threads of excellent quality can be rolled easily and simply.

The above-described geometry of the recesses of the rolled section is thus chosen so that the volume transport of the material in the axial direction is minimal, which is a reason for the relatively low rolling pressure and the uncomplicated rolling behavior. However, the inventor has found that a scheduled volume transport in the axial direction may well be desirable. Assuming that the blank is cylindrical and thus has a constant volume per unit length, this means that after a rolling process without volume transport in the axial direction and the finished rolled thread over its entire length has a constant volume per unit length. In fact, however, in a low pitch range, ie lower pitch, the screw requires more material per unit length to form the thread than in a high pitch area. If this additionally required material is missing during rolling, it may happen that the thread diameter decreases in the region of low thread pitch, or that the thread is not completely "filled" in the rolling process. The local lack of material is referred to below as "volume defect". For this reason, it would be advantageous in certain applications if during the rolling process material from such axial sections of the blank where a higher pitch threaded section is to be formed is transferred to an axial region in which a lower pitch threaded section is to be formed.

This can be achieved according to the second embodiment in that the slope of the center lines of the recesses at a first end of the rolling die, on which the rolling process of the blank begins, in relation to the slope at - viewed in the rolling direction - opposite portion of a second end of the rolling die at which the rolling process is finished is varied. Namely, if one increases the pitch of the recesses, or in other words, the distance of the recesses in a region of the first end compared to the opposite region of the second end viewed in the rolling direction, this leads to a compression of the corresponding portion of the blank during rolling, so that material is transported into the corresponding axial area of the finished screw. The reverse effect occurs when the pitch of adjacent recesses in the region of the first end of the rolling die is reduced in proportion to the slope in the corresponding region at the second end. This creates a transport of material volume out of the corresponding axial area during rolling.

wobei P₂₁ die mittlere Steigung der (Mittellinie der) Vertiefungen in einem ersten Bereich am zweiten Ende des Walzbackens ist, die geringer ist als die mittlere Steigung P₂₂ der Vertiefungen in einem zweiten Bereich am zweiten Ende des Walzbackens, und wobei P₁₁ und P₁₂ die mittleren Steigungen in denjenigen Bereichen am ersten Ende des Walzbackens sind, die dem ersten bzw. dem zweiten Bereich - in Walzrichtung betrachtet - gegenüberliegen. Hierbei bedeutet der Begriff "in Walzrichtung betrachtet gegenüberliegend", dass die einander entsprechenden Bereiche durch zwei zur Walzrichtung parallele Linien begrenzt werden.This principle can be used to compensate for the volumetric defect described above in low pitch threads. According to the second embodiment, the rolling profile is therefore chosen such that the following inequality holds: $$\frac{P_{21}}{P_{11}}<\frac{P_{22}}{{\mathrm{<figure-callout\; id="12"\; label="P"\; filenames="imgf0001.png"\; state="\{\{state\}\}">P</figure-callout>}}_{12}}.$$

wherein P _{21 is} the mean slope of the (center line of) depressions in a first region at the second end of the rolling die that is less than the mean slope P _{22 of} the depressions in a second region at the second end of the rolling die, and P ₁₁ and P _{12 are} the mean slopes in those areas at the first end of the rolling die, which are opposite to the first and the second area, as viewed in the rolling direction. Here, the term "viewed in the rolling direction opposite" means that the corresponding areas are bounded by two lines parallel to the rolling direction.

Note that, unlike the geometry of the first embodiment, P ₂₁ = P ₁₁ and P ₂₂ = P ₁₂ , so that both fractions result in Equation 1 above, indicating a lack of volume transport in the axial direction.

Additionally or alternatively, a volume defect can also be compensated for by selecting a smaller cross-section of the thread tooth by varying the flank angle and / or the thread depth for the finish-rolled thread in a region of lesser thread pitch. So can be made with less available material, the same thread diameter.

In the case of the rolling die, those depressions whose center line in the region of the first end of the rolling jaw have a greater pitch are preferably formed deeper in the region of the first end of the rolling jaw than those whose center line has a smaller pitch in the region of the first end of the rolling jaw. Since recesses with a larger pitch are spaced further apart in the area of the first end, it is advantageous for the rolling process if these recesses are formed deeper. Preferably, the recesses in the region of the first end of the rolling die are V-shaped in cross-section and at least in depth at least to ± 10% proportional to the slope of the center line at the first end of the rolling die.

Brief description of the figures

Fig. 1A

eine Draufsicht auf einen Walzbacken nach dem Stand der Technik zum Walzen eines Gewindes mit konstanter Gewindesteigung, sowie eines Rohlings und eines fertig gewalzten Gewindes;

Fig. 1B

eine Draufsicht auf eine Stirnfläche des Walzbackens von Fig. 1A an dessen erstem Ende;

Fig. 1C

eine Draufsicht auf eine Stirnfläche des Walzbackens von Fig. 1A an dessen zweitem Ende;

Fig. 2A

eine Draufsicht auf einen Walzbacken nach einer ersten Ausführungsform der Erfindung, sowie eines Rohlings und eines fertig gewalzten Gewindes;

Fig. 2B

eine Draufsicht auf eine Stirnfläche des Walzbackens von Fig. 2A an dessen erstem Ende;

Fig. 2C

eine Draufsicht auf eine Stirnfläche des Walzbackens von Fig. 2A an dessen zweitem Ende;

Fig. 2D

eine vergrößerte und vereinfachte Draufsicht auf den Walzbacken von Fig. 2A;

Fig. 2E

eine perspektivische Ansicht des Walzbackens von Fig. 2A;

Fig. 3A

eine Draufsicht auf einen Walzbacken nach einer zweiten Ausführungsform der Erfindung, sowie eines Rohlings und eines fertig gewalzten Gewindes;

Fig. 3B

eine Draufsicht auf eine Stirnfläche des Walzbackens von Fig. 3A an dessen erstem Ende;

Fig. 3C

eine Draufsicht auf eine Stirnfläche des Walzbackens von Fig. 3A an dessen zweitem Ende.

Further advantages and features of the invention will become apparent from the following description in which the invention with reference to two embodiments with reference to the accompanying drawings will be described. Show:

Fig. 1A

a plan view of a rolling die according to the prior art for rolling a thread with a constant pitch, and a blank and a finished rolled thread;

Fig. 1B

a plan view of an end face of the rolling die of Fig. 1A at its first end;

Fig. 1C

a plan view of an end face of the rolling die of Fig. 1A at its second end;

Fig. 2A

a plan view of a rolling die according to a first embodiment of the invention, as well as a blank and a finished rolled thread;

Fig. 2B

a plan view of an end face of the rolling die of Fig. 2A at its first end;

Fig. 2C

a plan view of an end face of the rolling die of Fig. 2A at its second end;

Fig. 2D

an enlarged and simplified plan view of the dies of Fig. 2A ;

Fig. 2E

a perspective view of the rolling of Fig. 2A ;

Fig. 3A

a plan view of a rolling die according to a second embodiment of the invention, as well as a blank and a finished rolled thread;

Fig. 3B

a plan view of an end face of the rolling die of Fig. 3A at its first end;

Fig. 3C

a plan view of an end face of the rolling die of Fig. 3A at its second end.

Description of the Preferred Embodiments

Fig. 1A FIG. 12 is a plan view of a prior art rolling die 10 that can be used to roll a constant pitch lead screw.

The rolling die 10 has a first end 12 and a second end 14. During rolling, a blank 16 is rolled from the first end 12 of the rolling die 10 toward the second end 14. On the surface of the rolling die 10, a rolled profile is formed, which is formed from a plurality of rectilinear, parallel and equidistant depressions 18. The recesses 18 in the region of the first and second end 12, 14 are in Figs. 1B and 1C can be seen, each showing a plan view of one of the end faces 20, 22 of the rolling die 10. A screw 19 with finished rolled thread is shown in the region of the second end 14 of the rolling die 10.

As in Figs. 1A, 1B and 1C As can be seen, the cross-section of the recesses 18 changes between the first and second ends 12, 14 of the rolling jaw 10. However, the cross-sections of all recesses 18 at the first end 12 are identical (see Fig. 1B ), and the same applies to the cross sections 18 at the second end of the rolling die 10 (see Fig. 1C ). Furthermore, the center lines of the recesses 18 are arranged parallel to each other and equidistant.

Fig. 2A shows a plan view of a rolling die 24, which is for a method for producing a screw 26 with a continuous thread 28 variable pitch which is also suitable in Fig. 2A is shown. The screw 26 can be made of the same blank 16, which in the embodiment of Fig. 1A has been shown and which is rolled from a first end 30 of the rolling jaw 24 toward a second end 32. Fig. 2E shows a perspective view of the rolling die 24th Fig. 2B and Fig. 2C show plan views of end faces 36 and 38 in the region of the first and second end 30, 32 of the rolling die 24th

As in Fig. 2A it can be seen, consists of the rolling profile of the rolling die 24 of a plurality of elongated recesses 34, unlike the dies 10 of Fig. 1A however not straightforward, not parallel and not equidistant. The geometry of the recesses 34 is determined by Fig. 2D described in more detail, in which the top view of the rolling jaws 24 is shown enlarged, and in the sake of clarity, only the center lines 34 'of the respective elongated recesses 34 are located.

As in Fig. 2D it can be seen, the center lines 34 'of each two adjacent recesses are formed and arranged so that they can be brought by a shift in the rolling direction by a constant distance T in line. The center lines 34 'have a pitch which is defined as the quotient of the changes .DELTA.y or .DELTA.x of the position of the center line in the direction transverse (y-direction) and parallel (x-direction) to the rolling direction. Due to translation symmetry in the rolling direction, the slopes of each centerline at each intersection are identical to a line 40 parallel to the rolling direction, and this pitch is proportional to the pitch in section 42 of the finished screw 26 (see also FIG Fig. 2A ).

In Figs. 2B and 2C It can be seen that the distances between adjacent recesses 34 in the y-direction, ie a direction transverse to the rolling direction at both the first and at the second end 30, 32 of the rolling die 24 change. This change in the distances reflects the variable thread pitch, since the distances represent a "local" pitch of the screw, so the local thread pitch of the screw. Note that the local thread pitch P = dy / dφ is proportional to the in Fig. 2D slope Δy / Δx shown, since a certain distance .DELTA.x during unrolling of the blank corresponds to a certain rolling angle .DELTA..phi.

It should be noted, however, that the mean pitches of the recesses 34 in-viewed in the rolling direction-are identical to opposing areas at the first and second ends 30, 32 of the rolling die 24. For explanation, see Fig. 2B a first region 44 of the first end and in Fig. 2C a first portion 46 of the second end of the rolling die 24 is shown. Each of these areas has six recesses 34, which means that the average pitch of the recesses 34 in the opposing areas 44, 46 is identical.

Further shows Fig. 2B a second region 48 of the first end of the rolling die 24, the width of which corresponds to that of the first region 44, but in which the mean slope of the recesses 34 is greater, because only four recesses fit into the region 48. The second region 48 of the first end A second region 50 of the second end is opposite, in which the average pitch is larger than in the first portion 46 of the second end, but equal to that in the opposite portion 48 of the first end.

The fact that the average pitches in - viewed in the rolling direction - opposite sections 44/46 and 48/50 at the first and at the second end 30, 32 of the rolling die 24 are identical, has the consequence that there is virtually no material volume transport in the axial direction of the blank (or γ-direction of the rolling die 24) (except for the transport when filling the recesses 34). As a result, the rolling process with relatively low rolling forces is feasible and can be performed easily and quickly.

Furthermore, it has been shown in experiments by the inventor that the blank 16 during the rolling of the rolled section of Fig. 2A or 2D shows no tendency to cross, so that the screw 26 with continuous thread variable pitch - amazingly for the inventor - easy and uncomplicated rolling.

Another difference between the rolling die 24 according to the first embodiment and the rolling die 10 of Fig. 1A to 1C From the prior art is that such wells 34, whose center line in the region of the first end 30 of the rolling jaw 24 have a greater pitch, are formed deeper in the region of the first end 30 than those whose center line in the region of the first end 30 a has smaller slope, like Fig. 2B can be seen directly. When rolling 10 of Fig. 1B however, the depths of all depressions 18 at the first end 12 of the rolling die 10 are identical. By adjusting the cutting depth of the recesses 34 in the region of the first end 30 of the rolling die 24 to the Slope or the spacing of adjacent recesses can be ensured that between two adjacent recesses peaks are formed, which are all at least approximately at the same level and thereby simultaneously come into contact with the blank 16. As Fig. 2B can be seen, in the first embodiment, the recesses 34 in the region of the first end 30 of the rolling die 24 in cross-section V-shaped, and their depth is proportional to the slope of the center line 34 'in the region of the first end 30 of the rolling die 24, or to the distance of adjacent recesses 34th

Since the blank 16 used is cylindrical and therefore has a constant volume per unit length, the screw 26 produced with the rolling jaw 24 also has a constant volume per unit length, because the geometry of the rolled section is chosen such that a volume transport in the axial direction when rolling the blank 16 is avoided. However, the finished screw 26 requires more material in an area of lesser thread pitch, where the turns are closer together. If the thread pitch varies greatly over the length of the thread of the screw, it can happen that the thread is not completely "filled" in places when rolling, because there is not enough material, or that the diameter of the thread decreases in this area.

• Erstens könnte anstatt eines zylindrischen Rohlings ein Rohling mit veränderlichem Querschnitt verwendet werden. Dieser Rohling hätte in Bereichen, in denen ein Gewindeabschnitt geringer Gewindesteigung auszubilden ist, einen etwas größeren Durchmesser als in Bereichen, in denen einen Abschnitt mit vergleichsweise großer Gewindesteigung auszubilden ist.
• Diese Lösung ist jedoch insofern nachteilig, als sie eine aufwendige Fertigung des Rohlings erforderlich macht.

The lack of material in the area of lower thread pitch is referred to below as "volume defect". To compensate for the volume defect, three approaches are proposed herein:

• First, instead of a cylindrical blank, a blank of variable section could be used. This blank would have a slightly larger diameter in areas where a threaded portion of low pitch is to be formed than in areas where a portion of comparatively large pitch is to be formed.
• However, this solution is disadvantageous in that it requires a complicated production of the blank.

A second solution is to vary the cross section of the thread tooth of the thread 28 by varying the flank angle and / or the thread depth so that the finish rolled thread tooth has a smaller cross-sectional area in a region of lesser thread pitch and thus the volume defect is compensated. Thus, the thread can have a sharper flank angle, so that the thread in the longitudinal section of the screw considered narrower and with a sharper flank and therefore less material is needed. This can be very easily implemented in the method according to the first embodiment by making the widths of the recesses 34 at the second end of the rolling die 24 narrower and / or less deep in areas of lesser thread pitch.

The third and preferred solution is to design the rolled profile so that a targeted volume transport from areas of greater thread pitch in areas of lower thread pitch is caused, which compensates for the volume defect just. This third variant is described in the second embodiment, which is described below with reference to FIG Figs. 3A to 3C is described.

Fig. 3A FIG. 12 shows a plan view of a rolling die 52 according to a second embodiment of the present invention having a first end 54 and a second end 56. On the rolling jaw 52 is similar to in Fig. 2A a rolling profile consisting of a plurality of elongated, curved, non-parallel depressions 58 is formed. The course of the recesses 58 is based on that of Fig. 2A which, however, has additionally been modified for a particular intended volume transport.

Figs. 3B and 3C again show the top view of the end faces 60 and 62 of the first and second end 54, 56 of the rolling die 52. The as can be seen by comparing Fig. 2C and 3C 1, the rolling profile in the second embodiment at the second end 56 of the rolling jaw 52 is identical to that at the second end 32 of the rolling jaw 24 of the first embodiment. This is because the rolling operation at the second end is finished, and apart from the volume defect correction, the same type of screw is to be manufactured with both embodiments. The difference between the first and second embodiments is in the shape of the rolled profile at the first end of the rolling die 52, as by comparison of FIG Fig. 3B and 2 B can be seen.

According to the second embodiment of Figs. 3B and 3C the thread pitches in - viewed in the rolling direction - opposite portions of the first and second end 54, 56 of the rolling die 52 are no longer identical. In Fig. 3B For example, a first portion 64 of the first end 54 of the rolling jaw 52, which includes five recesses 58, is shown. This area is - viewed in the rolling direction - at the second end 56 of the rolling jaw 52, a region 66 opposite, in the six recesses 58 fall. In other words, the mean slope P ₁₁ in the first region 64 of the first end 54 greater than the average pitch P ₂₁ in the first region 66 of the second end 58. This has the consequence that takes place during rolling of the blank 16, an axial material transport in the region 66 corresponding portion of the thread , Since the thread section corresponding to the area 66 is a section with a low thread pitch, the volume defect in this area described above can be compensated in this way.

The reverse effect occurs in a second region 70 at the second end 56 of the rolling jaw 52, which is opposite to a second region 68 at the first end 54 of the rolling jaw 52 - viewed in the rolling direction. As Figs. 3B and 3C 2, the average pitch P _{22 of} the second area 70 at the second end of the rolling jaw 52 is greater than the average pitch P ₁₂ at the opposite area 68, as viewed in the rolling direction, which means that a material transport from that corresponding to the area 70 Section of the thread takes place. This is useful because the corresponding area of the thread is a high pitch area where therefore less material per unit length is needed to form the thread.

wobei P₂₁ die mittlere Steigung der Vertiefungen in einem ersten Bereich am zweiten Ende des Walzbackens ist, P₂₂ die mittlere Steigung der Vertiefungen in einem zweiten Bereich am zweiten Ende des Walzbackens ist und P₁₁ und P₁₂ die mittleren Steigungen in den Bereichen am ersten Ende des Walzbackens sind, die dem ersten und dem zweiten Bereich - in Walzrichtung betrachtet - gegenüberliegen, und wobei ferner gilt: P₂₁<P₂₂. Die obige Ungleichung definiert somit eine lokale Umverteilung von Material in axialer Richtung, die über eine globale Streckung oder Stauchung hinausgeht.It should be noted that by varying the thread pitch in - viewed in the rolling direction - opposite portions at the first and second ends of the rolling die both a global stretching or compression of the thread and a redistribution of materials in the axial direction can be achieved. For the correction of the volume defect described above, however, a global stretching or compression is not sufficient, but material must be transferred from a region of higher thread pitch in a region of lesser thread pitch. A criterion for such a redistribution is given by the following inequality: $${\mathrm{P}}_{\mathrm{21}}\mathrm{/}{\mathrm{P}}_{\mathrm{11}}\mathrm{<}{\mathrm{<figure-callout\; id="22"\; label="P"\; filenames="imgf0001.png"\; state="\{\{state\}\}">P</figure-callout>}}_{\mathrm{22}}\mathrm{/}{\mathrm{<figure-callout\; id="12"\; label="P"\; filenames="imgf0001.png"\; state="\{\{state\}\}">P</figure-callout>}}_{\mathrm{12}}\mathrm{.}$$

where P _{21 is} the mean slope of the pits in a first region at the second end of the rolling die, P _{22 is} the mean slope of the pits in a second region at the second end of the rolling die, and P ₁₁ and P _{12 are} the mean slopes in the regions at the first End of the rolling die, which are the first and the second region - viewed in the rolling direction - opposite, and further where: P ₂₁ <P ₂₂ . The above inequality thus defines a local redistribution of material in the axial direction, which goes beyond a global stretching or compression.

wobei ΔV der Volumendefekt der i-ten Windung und d_G0 ein "zylindrischer Ersatzdurchmesser" des fertigen Gewindes ist, d.h. der Durchmesser eines Ersatzzylinders, der die gleiche Länge und das gleiche Volumen hat, wie das fertige Gewinde. Hierbei ist dp(i) die Steigungsänderung pro Winkeländerung Δϕ, die proportional zu einer Änderung ΔX der Vertiefungen in Walzrichtung ist.The rolling dies of Figs. 3A to 3C can for example be constructed as follows: Starting point of the rolling dies without volume transport, as in Fig. 2A is shown to be. The geometry of the wells of the rolling die without volume transport can then be based on a desired shape of the finished screw and using the in conjunction with Fig. 2A to 2E construct named criteria. As explained above, the average pitches in - compared to opposite areas in the rolling direction at the first and second ends of the rolling die are initially identical. In a second step, the slopes at the first end can then be varied to produce the desired volume transport. For this purpose, a correction value dp (i) is preferably added to the slope of the i-th depression at the first end, which is calculated as follows: $$\mathit{dp}\left(i\right)=\frac{\mathrm{\Delta }V\left(i\right)}{{d_{G0}}^2\pi /4}.$$

where ΔV is the volume defect of the ith turn and d _{G0 is} a "cylindrical replacement diameter" of the finished thread, ie the diameter of a replacement cylinder having the same length and volume as the finished thread. Here dp (i) is the slope change per angular change Δφ, which is proportional to a change ΔX of the depressions in the rolling direction.

In this way, the slope corrections at the first end can be calculated for each turn. The correction leads to a displacement of the recesses at the first end of the rolling die, as determined by the comparison of Fig. 3B With Fig. 2B is apparent. The individual recesses may then be modified by smooth functions to result in the desired variation at the first end of the rolling die and the desired thread form at the second end of the rolling die.

Note that in the dies 24 of Fig. 2 or 52 from Fig. 3 continuously change the slopes of the centerlines 34 'of the recesses 34. To put it clearly, this means that the recesses are not bent at any point, which would correspond to a sudden change in the thread pitch of the rolled screw. such abrupt changes would be obtained if, in the finished screw threaded sections with different but within the section constant thread pitches would abut each other. Such a die might be simpler in construction, but more expensive to manufacture than the dies disclosed herein. The rolling jaws shown here with the smooth, kink-free depressions can be produced by milling. This is not readily possible with rolling jaws with kinked depressions. Although the rolling jaw could be composed of several separately manufactured parts at the kinks, the inventor has found that such a composite rolling jaw tends to be susceptible to wear. Alternatively, it would be possible to produce a rolling jaw with kinked recesses in an erosion process, which, however, is much more expensive than a milling process. Therefore, the rolling jaws proves to be particularly advantageous with a smooth, kink-free course of the wells.

LIST OF REFERENCE NUMBERS

10

dies

12

first end of the rolling 10

14

second end of the rolling 10

16

blank

18

deepening

19

screw

20

End face at the first end of the rolling 10th

22

End face on the second end of the rolling 10th

24

dies

26

screw

28

Thread of the screw 26

30

first end of the rolling die 24

32

second end of the rolling die 24th

34

deepening

34 '

Center lines of the recesses 34

36

Front side at the first end of the rolling die 24th

38

Front side at the second end of the rolling die 24th

40

Line parallel to the rolling direction

42

Section of the thread 28

44

first area at the first end of the rolling die

46

first area at the second end of the rolling die 24th

48

second area at the first end of the rolling die 24th

50

second area at the second end of the rolling die 24th

52

dies

54

first end of the rolling jaw 52

56

second end of the rolling die 52

58

deepening

60

End face at the first end of the rolling jaw 52nd

62

Front side at the second end of the rolling jaw 52nd

64

first area at the first end of the rolling jaw 52

66

first area at the second end of the rolling jaw 52

68

second area at the first end of the rolling jaw 52nd

70

second area at the second end of the rolling jaw 52nd

Claims (14)

1. A method for manufacturing a screw (26) comprising a continuous thread (28) with a variable thread pitch,
   wherein a blank (16) is rolled between two rolling dies (24),
   wherein in each rolling die a rolling profile is formed that comprises a host of depressions (34),
   wherein as a result of a virtual displacement in the direction of rolling by a constant distance (T) the centre lines (34') of adjacent depressions (34) can be aligned, and wherein the slopes of the centre lines (34'), being defined as the quotient of the changes in position of the centre line (34') in directions transverse and parallel to the direction of rolling, respectively, are identical at the intersections of the centre lines (34') with a line (40) that is parallel to the direction of rolling,
   wherein the rolling die (24, 52) has a first and a second end (30, 32; 54, 56) spaced apart from each other in the direction of rolling, wherein the direction of rolling points from the first end towards the second end of the rolling die (24, 52),
   characterized in that the rolling profile of each rolling die comprises a number of curved non-parallel depressions (34, 58),
   wherein the depressions (34, 58) in the region of the second end (32, 56) are designed in such a manner that the finish-rolled thread (28) in a region with a smaller thread pitch has a more acute flank angle than in a region with a larger thread pitch, and/or wherein a blank with a variable cross section is used, which blank has a larger diameter in a region in which a thread section with a smaller thread pitch is to be formed, than in a region in which a thread section with a larger thread pitch is to be formed, and/or
   wherein those depressions (34, 58) whose centre lines (34') in the region of the first end (30, 54) have a larger slope, are deeper in the region of the first end (30, 54) than those whose centre lines (34') in the region of the first end (30, 54) comprise a smaller slope.
2. A method for manufacturing a screw comprising a continuous thread with a variable thread pitch,
   wherein a blank (16) is rolled between two rolling dies (52),
   wherein in each rolling die (52) a rolling profile is formed that comprises a host of depressions (58),
   wherein the rolling die (52) has a first end and a second end spaced apart from each other in the direction of rolling, characterized in that
   the rolling profile of each rolling die comprises a number of curved non-parallel depressions (58),
   wherein the mean slope P₂₁ of the depressions (58) in a first region (66) at the second end (56) of the rolling die (52) is smaller than the mean slope P₂₂ of the depressions (58) in a second region (70) at the second end (56) of the rolling die (52), and wherein the following applies: $${\mathrm{P}}_{\mathrm{21}}\mathrm{/}{\mathrm{P}}_{\mathrm{11}}\mathrm{<}{\mathrm{P}}_{\mathrm{22}}\mathrm{/}{\mathrm{P}}_{\mathrm{12}}\mathrm{,}$$

   

   
   wherein P₁₁ and P₁₂ denote the mean slopes in the regions (64, 68) at the first end (54) of the rolling die (52), which, when viewed in the direction of rolling, are opposite the above-mentioned first and second regions (66, 70) of the second end (56), respectively.
3. The method according to claim 2, in which the rolling die (24, 52) comprises a first and a second end (30, 32; 54, 56), which ends are spaced apart from each other in the direction of rolling, wherein the direction of rolling points from the first end towards the second end of the rolling die (24, 52),
   and in which the depressions (34, 58) in the region of the second end (32, 56) are designed in such a manner that the finish-rolled thread (28) in a region with a smaller thread pitch has a more acute flank angle than in a region with a larger thread pitch.
4. The method according to claim 2 or 3, in which the depressions (34, 58) in a first region at the second end (32, 56) of the rolling die (24, 52), in which the mean thread pitch is smaller than in a second region at the second end (32, 56) of the rolling die (24, 52), are narrower than in the second region.
5. The method according to any one of claims 2 to 4, wherein the rolling die (24, 52) comprises a first end and a second end (30, 32; 54, 56), which ends are spaced apart from each other in the direction of rolling, and the direction of rolling points from the first end (30, 54) towards the second end (32, 56),
   and wherein those depressions (34, 58) whose centre lines (34') in the region of the first end (30, 54) have a larger slope, are deeper in the region of the first end (30, 54) than those whose centre lines (34') in the region of the first end (30, 54) have a smaller pitch.
6. The method according to claim 5, in which the depression in the region of the first end (30, 54) of the rolling die (24, 52) is V-shaped in cross section and its depth is proportional, at least within ±10%, to the slope of the centre line (34').
7. The method of any one of the preceding claims, in which the pitch of the thread changes continuously.
8. A rolling die (24) for manufacturing a screw with a continuous thread with a variable thread pitch,
   wherein in the rolling die a rolling profile is formed that comprises a host of depressions (34),
   wherein as a result of a virtual displacement in the direction of rolling by a constant distance (T) the centre lines (34') of adjacent depressions (34) can be aligned, and wherein the slopes of the centre lines (34'), being defined as the quotient of the changes in the position of the centre line (34') in directions transverse and parallel to the direction of rolling, respectively, are identical at the respective intersections of the centre lines (34') with a line (40) that is parallel to the direction of rolling,
   wherein the rolling die has a first and a second end (30, 32; 54, 56), which ends are spaced apart from each other in the direction of rolling, wherein the direction of rolling points from the first end towards the second end of the rolling die (24, 52), characterized in that the rolling profile comprises a number of curved non-parallel depressions (34, 58),
   wherein the depressions (34, 58) in the region of the second end (32, 56) are designed in such a manner that the finish-rolled thread (28) in a region with a smaller thread pitch has a more acute flank angle than in a region with a larger thread pitch, and/or those depressions (34, 58) whose centre lines (34') in the region of the first end (30, 54) have a larger slope, are deeper in the region of the first end (30, 54) than those whose centre lines (34') in the region of the first end (30, 54) have a smaller slope.
9. A rolling die (52) for manufacturing a screw comprising a continuous thread with a variable thread pitch,
   wherein in the rolling die (52) a rolling profile is formed that comprises a host of depressions (58),
   wherein the rolling die (52) has a first end and a second end spaced apart from each other in the direction of rolling, characterized in that
   the rolling die comprises a number of curved non-parallel depressions (58), wherein the mean slope P₂₁ of the depressions (58) in a first region (66) at the second end (56) of the rolling die (52) is smaller than the mean pitch P₂₂ of the depressions (58) in a second region (70) at the second end (56) of the rolling die (52), and wherein the following applies: $${\mathrm{P}}_{\mathrm{21}}\mathrm{/}{\mathrm{P}}_{\mathrm{11}}\mathrm{<}{\mathrm{P}}_{\mathrm{22}}\mathrm{/}{\mathrm{P}}_{\mathrm{12}}\mathrm{,}$$

   

   
   wherein P₁₁ and P₁₂ denote the mean slopes in the regions (64, 68) at the first end (54) of the rolling die (52), which when viewed in the direction of rolling, are opposite the above-mentioned first and second regions (66, 70) of the second end (56), respectively.
10. The rolling die (24, 52) according to claim 9, which has a first and a second end (30, 32; 54, 56), which ends are spaced apart from each other in the direction of rolling, wherein the direction of rolling points from the first end towards the second end of the rolling die (24, 52),
    and in which the depressions (34, 58) in the region of the second end (32, 56) are designed in such a manner that the finish-rolled thread (28) in a region with a smaller thread pitch has a more acute flank angle than in a region with a larger thread pitch.
11. The rolling die (24, 52) according to claim 10, in which the depressions (34, 58) in a first region at the second end (32, 56) of the rolling die (24, 52), where the mean thread pitch is smaller than in a second region at the second end (32, 56) of the rolling die (24, 52), are narrower than in the second region.
12. The rolling die (24, 52) according to any one of claims 8 to 11, which has a first and a second end (30, 32; 54, 56)spaced apart from each other in the direction of rolling, wherein the direction of rolling points from the first end (30, 54) towards the second end (32, 56),
    and wherein those depressions (34, 58) whose centre lines (34') in the region of the first end (30, 54) have a larger slope are deeper in the region of the first end (30, 54) than those whose centre lines (34') in the region of the first end (30, 54) have a smaller pitch.
13. The rolling die (24, 52) according to claim 12, in which the depression in the region of the first end (30, 54) of the rolling die (24, 52) is V-shaped in cross section and its depth is proportional, at least within ±10%, to the slope of the centre line (34').
14. The rolling die (24, 52) according to any one of claims 8 to 13, in which the slopes of the centre lines (34') of the depressions (34) vary continuously.

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