Method and rolling die for producing a screw with a variable thread pitch

1. A method for manufacturing a screw comprising the steps of:

providing two rolling dies, wherein on each rolling die a rolling profile is formed that comprises a plurality of curved non-parallel depressions, and wherein each rolling die has a first and a second end 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 each rolling die, respectively, and

wherein as a result of a virtual displacement in the direction of rolling by a constant distance the center lines of adjacent depressions can be aligned, and

wherein the slopes of the center lines, being defined as the quotient of the changes in position of the center line in directions transverse and parallel to the direction of rolling, respectively, are identical at the intersections of the center lines with a line that is parallel to the direction of rolling; and

rolling a blank between the two rolling dies to form on the blank a continuous thread with a variable pitch, such that:

depressions in a region of the second end are designed in such a manner that a first region of the continuous thread has thread pitch T 1 and flank angle A 1 , and a second region of the continuous thread has thread pitch T 2 and flank angle A 2 , wherein T 1 <T 2 and A 1 <A 2 , or

a depression in a first region of the first end of each rolling die has depth D 1 and has a center line with slope S 1 , and a depression in a second region of the first end has depth D 2 and has a center line with slope S 2 , wherein D 1 >D 2 and S 1 >S 2 .

2. A method for manufacturing a screw comprising the steps of:

providing two rolling dies, wherein on each rolling die a rolling profile is formed that comprises a plurality of curved non-parallel depressions, and wherein each rolling die has a first end and a second end 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 each rolling die, respectively, and

wherein for one of the rolling dies the mean slope P 21 of the depressions in a first region at the second end of said rolling die is smaller than the mean slope P 22 of the depressions in a second region at the second end of said rolling die, and wherein the following applies:

P 21 /P 11 <P 22 /P 12

wherein P 11 and P 12 denote the mean slope in a first and a second region, respectively, at the first end of said rolling die, which, when viewed in the direction of rolling, are opposite the first and second regions of the second end, respectively; and

rolling a blank between the two rolling dies to form the screw with a continuous thread with a variable thread pitch.

3. The method according to claim 2 , wherein depressions in a region of the second end are designed in such a manner that a first region of the continuous thread has thread pitch T 1 and flank angle A 1 , and a second region of the continuous thread has thread pitch T 2 and flank angle A 2 , wherein T 1 <T 2 and A 1 <A 2 .

4. The method according to claim 2 , wherein the depressions in a third region at the second end of each rolling die, in which the mean thread pitch is smaller than in a fourth region at the second end of each rolling die, are narrower than in the fourth region.

5. The method according to claim 2 , wherein a depression in a first region of the first end of each rolling die has depth D 1 and has a center line with slope S 1 , and a depression in a second region of the first end of each rolling die has depth D 2 and has a center line with slope S 2 , wherein D 1 >D 2 and S 1 >S 2 .

6. The method according to claim 5 , wherein the depression in a region of the first end of the rolling die is V-shaped in cross section and its depth is proportional, at least within ±10%, to the slope of the center line.

7. The method according to claim 1 , wherein the pitch of the thread changes continuously.

8. A rolling die for manufacturing a screw with a continuous thread with a variable thread pitch, comprising:

said rolling die having a rolling profile that comprises a plurality of curved non-parallel depressions, and wherein said rolling die has a first and a second end, 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,

wherein as a result of a virtual displacement in the direction of rolling by a constant distance the center lines of adjacent depressions can be aligned, and

wherein the slopes of the center lines, being defined as the quotient of the changes in the position of the center line in directions transverse and parallel to the direction of rolling, respectively, are identical at the respective intersections of the center lines with a line that is parallel to the direction of rolling, and

wherein the depressions in the region of the second end are designed in such a manner that the continuous thread in a region with a smaller thread pitch has a more acute flank angle than in a region with a larger thread pitch, or

wherein those depressions whose center lines in the region of the first end have a larger slope, are deeper in the region of the first end than those whose center lines in the region of the first end have a smaller slope.

9. A rolling die for manufacturing a screw with a continuous thread with a variable thread pitch, comprising:

said rolling die having a rolling profile comprising a plurality of curved non-parallel depressions,

wherein said rolling die has a first end and a second end spaced apart from each other in the direction of rolling, and

wherein the mean slope P 21 of the depressions in a first region at the second end of said rolling die is smaller than the mean pitch P 22 of the depressions in a second region at the second end of said rolling die, and wherein the following applies:

P 21 /P 11 <P 22 /P 12

wherein P 11 and P 12 denote the mean slope in a first and a second region, respectively, at the first end of said rolling die, which when viewed in the direction of rolling, are opposite the first and second regions of the second end, respectively.

10. The rolling die according to claim 9 , wherein depressions in a region of the second end are designed in such a manner that a first region of the continuous thread has thread pitch T 1 and flank angle A 1 , and a second region of the continuous thread has thread pitch T 2 and flank angle A 2 , wherein T 1 <T 2 and A 1 <A 2 .

11. The rolling die according to claim 10 , wherein the depressions in a third region at the second end of the rolling die, in which the mean thread pitch is smaller than in a fourth region at the second end of the rolling die, are narrower than in the fourth region.

12. The rolling die according to claim 8 , wherein a depression in a first region of the first end of each rolling die has depth D 1 and has a center line with slope S 1 , and a depression in a second region of the first end has depth D 2 and has a center line with slope S 2 , wherein D 1 >D 2 and S 1 >S 2 .

13. The rolling die according to claim 12 , wherein the depression in a region of the first end of the rolling die is V-shaped in cross section and its depth is proportional, at least within ±10%, to the slope of the center line.

14. The rolling die according to claim 8 , wherein the slopes of the centre lines of the depressions vary continuously.

15. A method for manufacturing a screw comprising the steps of:

providing two rolling dies, wherein on each rolling die a rolling profile is formed that comprises a plurality of curved non-parallel depressions, and wherein each rolling die has a first and a second end 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 each rolling die, respectively, and

wherein as a result of a virtual displacement in the direction of rolling by a constant distance the center lines of adjacent depressions can be aligned, and

wherein the slopes of the center lines, being defined as the quotient of the changes in position of the center line in directions transverse and parallel to the direction of rolling, respectively, are identical at the intersections of the centre lines with a line that is parallel to the direction of rolling; and

rolling a variable cross-section blank between the two rolling dies to form on the blank a continuous thread with a variable pitch, wherein the 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.