JPH09267132A - Method and device of manufacturing plate with folding flance shape - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve productivity, and to reduce a cost by bending a blank while holding it down at two places and pressing, and folding it to a flange shape, making it slide so that holding points on both sides approach each other to make forming complete speedily and smoothly. SOLUTION: The blank 21 is mounted on the front surface 47 of a lower clamper 24, punch 22 and upper clamper 23 are lowered, and the blank 21 is placed between the upper and lower clampers 23 and 24. The moment the upper and the lower clampers 23 and 24 are abutted, bending is started, and a downward bending tendency is given to the blank 21. Further, a press ram is lowered and the upper clamper 23 is pressed through an upper holder 26. The lower clamper 24 is moved in the width direction along the tilted supporting surface 49 of a lower holder 27. The lower clamber 24 reaches the bottom dead center of a press stroke, and when left and right clampers 23a, 23b, 24a and 24b approach further, the bending part of the blank 21 is allowed to closely adhere, and is completely folded, and a product plate with a flange shape is formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a plate having a folding flange shape using a press machine and a manufacturing apparatus for carrying out the method.

[0002]

2. Description of the Related Art Conventionally, for example, a roll forming method has been used to manufacture a plate having a folding flange shape which is a metal-worked plate used for automobile parts and the like. FIG.
As shown in FIG. 3, in this manufacturing method by roll forming, a raw material plate (coarse coil) 1 is sandwiched between a plurality of upper rollers 2 and 4 and lower rollers 3 and 5 each having an inclined peripheral surface. Then, it is bent and extended while being fed by the rotation, and is molded in two steps so as to form a W shape (a to b), and then the flat portion 1a is formed by another upper and lower rollers 6, 7, 8 and 9. Bend back at the stage (cd). Then, the portion 1b which becomes the central flange of the W-shape is brought into close contact with the rollers 10 sandwiching it to the left and right (e), and the flat portion 1a is finished again by the upper and lower rollers 11 and 12, and a plate having a folding flange shape (product It was intended to obtain a plate 13) (f).

[0003]

As described above, in the conventional manufacturing method, the dedicated forming rollers 2 ... 1 are used at each manufacturing stage.
Since 2 is required and the manufacturing process is performed through a large number of manufacturing processes (for example, 8 to 10 stages), there is a problem that it takes a long time and a lot of labor to complete the molding, resulting in an increase in cost. . Further, there is a problem in that versatility is lacking because the design needs to be changed each time depending on the number of folding flange shapes. In addition to the roll forming method, there is also a manufacturing method by bending, but it also has a number of bending steps, and has a problem that the applicable plate width is limited, which is not practical.

[0004]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, according to the present invention, the material plate is pressed at two places and a bending pressing force is applied between them to appropriately bend, and then the pressing points on both sides are Are slid so that they approach each other, and are folded into a predetermined flange shape. By this method, the molding can be completed extremely smoothly and quickly. Further, according to the present invention, a punch which abuts a raw material plate with a downward bending pressing force to perform initial bending, upper and lower clampers which sandwich and hold the raw material on both sides of the punch, and a pressing force of the clamper in a sliding direction. And a slide conversion mechanism for converting the clampers on both sides closer to each other. With this structure, the blank plate of the material is pressed by the clampers on both sides, and the punch is pressed between them to add a bending hump, and the slide conversion mechanism slides the clampers on both sides toward each other to bend the bent portion. Can be sandwiched and folded into a predetermined flange shape.

The slide conversion mechanism preferably comprises an inclined surface formed on the lower surface of the lower clamper, and a lower holder for supporting the lower clamper via a support surface which is in sliding contact with the inclined surface. Further, it is preferable that the punch and the upper clamper are collectively held by the upper holder that descends with a predetermined pressing force. The punch is preferably formed so as to engage with the upper clamper via the inclined surface and to rise when the upper clampers on both sides approach each other. Further, it is preferable that the clamper is provided with a biasing means that biases the clamper in a sliding direction away from each other. Further, it is preferable that the lower clamper is provided with a lock mechanism that holds the states in which they are close to each other at the stroke bottom dead center.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings.

First, a manufacturing apparatus for a plate having a folding flange shape according to the present invention will be described with reference to FIGS. This manufacturing apparatus comprises a punch 22 that comes into contact with a raw material plate 21 with a downward bending pressing force p1 and a punch 22.
Upper clampers 23a and 23b and lower clampers 24a and 24b for sandwiching and pressing the base plate 21 on both sides of 2.
And the pressing force of the clampers 23 and 24 is converted to the slide direction so that the clampers 23a, 23b, 24a and 24b on both sides are changed.
And a slide conversion mechanism 25 for bringing them closer to each other. The punch 22 and the upper clamper 23 are supported by an upper holder 26 that descends by driving a hydraulic drive unit (press ram), and the lower clamper 24 is supported by a lower holder 27 that is fixedly installed immediately below the upper holder 26. That is, the upper holder 26, the punch 22 and the upper clamper 23, and the lower holder 27 and the lower clamper 24 constitute the upper die and the lower die of the press machine, respectively.

The upper holder 26 is formed with a housing portion 28 that opens downward at the center in the left-right direction in FIG. 1, and the punch 22 is formed in the upper portion of the housing portion 28, and the diameter of the punch 22 is expanded slightly below. The left and right upper clampers 23a and 23b are held respectively. The punch 22 is composed of a block whose cross section is a substantially downward triangle, and its apex angle is formed at an angle θ suitable for bending, for example, about 110 degrees. Portions 29 at both ends in the width direction of the punch 22 are vertically movably guided by both side walls of the accommodating portion 28, and the accommodating portion upper wall of the upper holder 26 is interposed by the springs 30 arranged side by side. It is suspended on 31. As shown in FIG. 2, the left and right springs 30 are arranged in the longitudinal direction of the punch 22 at predetermined intervals, and the upper and lower ends of the springs are formed in the upper wall 31 of the accommodating portion and the upper surface 32 of the punch, respectively. It is attached to 33 and 34. The upper clamper 23 is a block having a substantially rectangular cross section and a length equal to that of the punch 22. The upper clamper 23 moves to the upper holder 26 in the left-right direction in FIG. 1 via a keeper 35 provided at the center in the longitudinal direction. It is held freely.

The keeper 35 has a cross section T as shown in FIG.
The block is formed of a relatively small block and is provided on a support portion 36 formed at the center of the upper holder 26 in the longitudinal direction. The support portion 36 projects from the accommodation portion upper wall 31 of the upper holder 26 into the accommodation portion 28 and extends in the width direction of the punch 22. In the support portion 36, a groove 37 that opens to the upper surface of the upper holder 26 and extends to the left and right with a substantially rectangular cross section, and a slit 38 that penetrates from the bottom surface to the housing portion 28 along the groove 37.
Are formed. And the keeper 35 has the slit 3
8 is movably fitted in the extension direction thereof, and in the state where the keeper head 35a is locked to the bottom surface of the groove 37, the lower end 35b
Is slightly projected from the slit 38. The central portion 39 of the upper clamper 23 is attached to the protruding lower end 35b. The upper surface of the central portion 39 of the upper clamper slidably contacts the lower surface of the support portion 36 of the upper holder 26, and the pressing force P of the pressing machine from the upper holder 26 is transmitted to the base plate 21 appropriately. A pressing force p2 for pinching is generated. At the position where the support portion 36 is formed, the punch 22 is divided in the longitudinal direction at a predetermined interval. In other words, the support portion 36 and the keeper 35 are provided using the space at the division position of the punch 22. The division position of the punch 22 is not limited to the center in the longitudinal direction and may be any position.

In the section other than the center in the longitudinal direction, as shown in FIG.
As shown in FIG. 5, an engaging surface 41 having the same inclination angle as the inclined surface 40 of the punch 22 is formed at an upper corner portion on the center side in the width direction of the upper clamper 23.
The punch 22 is supported from both sides via. Then, in the standby position where the upper die is raised, the left and right upper clampers 23a and 23a are driven by the weight of the punch 22 (biasing force of the spring 30).
b is pushed out, and the surfaces 42 on the left and right sides are spread out.
It is designed to be locked to the side of the. Further, the horizontal upper surface 43 of the upper clamper 23 comes into contact with the step portion 44 which is a boundary between the lower side and the upper side of the housing portion 28. In this standby state, the lower end (pressing point) 45 of the punch 22 is
A predetermined small distance S from the lower surface 46 of the upper clamper 23
It is formed so that it is located only below.

Next, the lower clamper 24 is connected to the lower surface 4 of the upper clamper.
6, a block having an upper surface 47 having a width substantially equal to or slightly smaller than the width of the upper surface of the lower surface 4.
6 and the upper surface 47 are butted against each other so that the blank plate 21 is sandwiched therebetween. Upper surface 4 of lower clamper 24
Surface treatment such as knurling is applied to 7. The side surface of the lower clamper 24 on the center side in the width direction is formed to be sufficiently larger than the size of the folding flange shape to be formed. On the lower side of the lower clamper 24, inclined surfaces 48 are formed which are inclined from the outer side surface toward the center so as to be narrowed as they go downward, and these inclined surfaces 48 cross the central portion in the width direction of the lower holder 27 in a cross section V. The support surface 49, which is divided into a letter shape, is in sliding contact with both sides. Therefore, these inclined surface 48 and support surface 49
When the lower clamper 24 receives the pressing force P of the lower press machine via the upper clamper 23, the slide conversion mechanism 25 is constituted by the lower holder 27, which is the lower end of the support surface 29 in a state of overlapping these left and right. It is designed to be centered in the width direction of. The crossing angle (internal angle α) of the support surface 49 is, for example, about 90 degrees smaller than the tip angle θ of the punch 22 (α <θ).

Further, as shown in FIGS. 2 and 3, a cushion plate 50 and a cushion pin 51 for supporting the lower clamper 24 are provided at the longitudinal center of the lower holder 27.
Is provided. The cushion plate 50 is a plate that extends horizontally in a length equivalent to the left-right width of the lower holder 27, and is engaged with the central portion 52 of the lower clamper 24. The lower clamper central portion 52 is formed to have a rectangular cross section, and the lower surface thereof abuts the upper surface of the cushion plate 50. There is no inclined support surface 49 in the central portion of the lower holder 27 in the longitudinal direction, and the concave portion 5 having a shape corresponding to the central portion 52 is formed.
3 are formed. Both width ends of the cushion plate 50 have hook-shaped engaging portions 5 formed in the lower clamper central portion 52.
4 is supported by restraint. That is, lower clamper 2
4 is supported so that it can slide in the left-right direction in FIG. As shown in FIG. 3, the three cushion pins 51 are juxtaposed in the longitudinal direction of the cushion plate 50 at appropriate intervals, and penetrate through the recess lower wall 55 of the lower holder 27 to stand upright. The cushion pin 51 is provided with an urging means (not shown) that exerts an appropriate upward elastic force f, and supports the cushion plate 50 so that it can be lowered until it abuts the lower wall of the lower holder recess 55. There is. That is, the left and right lower clampers 24a and 24b are allowed to move along the support surface 49 of the lower holder 27, and after the upper holder 26 and the upper clamper 23 are lifted, the lower clamper 24 is lifted up to its original position. It is designed to let you. A guide post 56 penetrating both ends of the cushion plate 50 is erected on the lower holder 27 to guide the lifting and lowering of the cushion plate 50 and the lower clamper 24. The cushion pin 51 is provided with a lock mechanism (not shown) so that the lower clamper 24 can be held in a state where they are close to each other at the stroke bottom dead center. As the lock mechanism, locking (fixing in a cushion down state) provided in the press machine may be used. A pair of springs 57 is provided between the central portions 52 of the lower clampers 24, and the left and right lower clampers 24 are provided.
The a and 24b are urged in a direction in which they are separated from each other (a direction in which they are divided into left and right). The biasing force of this spring 57 is
The force is set to be smaller than the sliding force (closer force) of the lower clamper 24 generated by the pressing force P from the upper holder 26.

Next, an embodiment of a method of manufacturing a plate having a folding flange shape according to the present invention will be described as a step of manufacturing a plate by the above-mentioned manufacturing apparatus.

As shown in FIG. 1, with the upper mold such as the upper holder 26 raised and the press machine opened, the base plate 21 made of a flat plate-shaped material is placed on the upper surface 47 of the lower clamper 24. To do. Next, as shown in FIG. 4, the press ram is driven to lower the punch 22 and the upper clamper 23 supported by the upper holder 26, and the base plate 21 is moved to the upper and lower clampers 23, 24.
Sandwich it in between. At this time, since the position of the lower end 45 of the punch 22 is lower than the position of the lower surface 46 of the upper clamper 23, bending is started at the moment (immediately before) when the upper and lower clampers 23 and 24 are brought into contact with each other, and the lower plate 45 is lowered. Bending habits are attached. By thus performing the initial bending instantly, the bending direction can be stabilized downward, and the bending pressing force p1 can be small. The bending pressure p1 at this time is
This is due to the weight of the punch 22 and the downward biasing force of the spring 30. In addition, the pressing force p of the upper and lower clampers 23 and 24
2 has a size obtained by subtracting the amount of cushioning by the cushion pin 51 from the pressing force P of the press machine, and a sufficient clamping force for forming the initial bending can be obtained.

Subsequently, as shown in FIG. 5, the press ram is further lowered, and the upper clamper 23 is inserted through the upper holder 26.
Press. The pressing force p2 causes the lower clamper 24 to move toward the center in the width direction along the inclined support surface 49 of the lower holder 27. And the upper clamper 23
Follows the movement of the lower clamper 24 while the base plate 21 is clamped, and integrally moves to the center in the width direction. By this movement, the sandwiched base plate 21 gradually buckles such that it is folded at the position where it is bent by the punch 22. Then, the biasing force of the upper clamper 23 is converted by the inclined surface 40 and the engagement surface 41 into a force that raises the punch 22 that has finished the role of initial bending, and resists the biasing force of the spring 30 to move the punch 22 upward. It is retracted so as to be pushed back into the holder 26.

Then, as shown in FIG. 6 to FIG. 8, the left and right clampers 23a, 2a reach the bottom dead center of the press stroke.
When 3b, 24a, and 24b further approach each other, the blank 2
The bent portion of 1 is closely contacted and completely folded to form a product plate 58 which is a plate having a folding flange shape. During this leaning process, the upper clamper 23 separates from the step portion 44 of the upper holder 26, but the upper holder 2
6 is directly pressed by the support portion 36 of the punch 6 and the punch 2
The spring 30 that suspends 2 contracts and the recesses 33, 3
By being in the state of being housed in the housing 4, the pressing force P of the press machine is transmitted from the housing upper wall 31 through the punch 22.
At the bottom dead center, the cushion plate 50 is fixed by the central lower wall 55, so that the pressing force p2 from the upper clamper 23 is partially converted in the sliding direction without being buffered and acts on the lower clamper 24, A sufficient pinching force p3 is exhibited. At the bottom dead center of the press stroke, the left and right upper clampers 23a, 23b are in close contact with each other, and the lower clampers 24a, 24b are separated by the thickness of the flange shape. Further, the upper holder 26 and the lower holder 27 are formed so as not to come into contact with each other in the state of the bottom dead center.

When the folding flange forming is completed,
The lock mechanism of the press is operated to hold the lower clamper 24 in a state where the folding flange-shaped portion is sandwiched, and then the upper holder 26 is raised. The lower clamper 24 is now restrained at the bottom dead center, and the upper clamper 2
3 is separated from the surface of the product plate 58 and is pushed to the left and right by the weight of the punch 22 and the biasing force of the spring 30 to return to the standby state. When the upper clamper 23 is sufficiently separated from the lower clamper 24 and the pressing force p2 no longer acts, the locking is released and the cushion pin 5 is released.
The lower clamper 24 is lifted to the original position by the urging force of 1. At this time, the product plate 58 that has been molded is the lower clamper 2
4, the left and right lower clampers 24 are divided into both widthwise ends by the urging force of the springs 57, so that the product plate 58 is not constrained and can be easily taken out. For this removal, the product plate 58
It is also possible to install a lifter that lifts by the length of the folding flange shape. The product board 5
When further folding flange forming is to be performed on different parts of 8, the pitch is continuously shifted in the left-right direction, and the next forming process is performed in the same procedure.

As described above, the initial bending is performed by pressing the base plate 21 at two points and applying the bending pressing force p1 between them, and then the pressing points on both sides having the pressing force p2 are brought close to each other. Since the base plate 21 is slid and folded by the pinching force p3, the forming can be completed in two consecutive steps, which is significantly faster than the conventional roll forming method or the bending method. It is possible to obtain a predetermined folding flange shape. That is, productivity is improved by reducing man-hours, and cost reduction is achieved. As a manufacturing apparatus, a punch 22 that gives the base plate 21 a bending behavior, and clampers 23 and 24 that sandwich the base plate 21 from above and below on both sides of the punch 22.
And the slide conversion mechanism 25 that causes the clampers 23 and 24 sandwiching the blank plate 21 to come closer to the center after the initial bending is performed. Therefore, only the downward pressing force P of the press machine causes the bending pressing force. The desired folding flange shape can be formed by generating p1, the pressing force p2, and the pinching force p3. That is, with a simple configuration,
Increased productivity is achieved. Further, since the initial bending and folding operations can be continuously performed by the press stroke of a normal press machine, it is extremely practical and versatile. The clamper 2 is attached to the base plate 21.
Since it is possible to perform flange forming as long as there is a flat surface that can be pressed by 3, 24, it is possible to easily form a product having a plurality of folding flange shapes on one plate, which is more versatile. Improvement is achieved.
In this case, since the base plates may be sequentially shifted according to the pitch (flange spacing), it is possible to cope with a wide range of pitches.

Further, as the slide conversion mechanism 25, the lower clamper 24 is moved along the inclined support surface 49 of the lower holder 27, so that the lower clamper 24 can be caused to move toward the side with a very simple structure. Flanging can be completed with one press stroke. Further, since the punch 22 and the upper clamper 23 are collectively provided on the upper holder 26, the upper holder 26
The bending pressing force p1 and the pressing force p2 can be exerted on them only by lowering the driving mechanism, and the driving mechanism can be simplified. And the punch 22 that was bent at the beginning
Is retracted upward in the folding process, so that the clampers 23 and 24 do not interfere with the leaning movement. Further, since the springs 30 and 57 for separating the clampers 23 and 24 from the leaning state to the left and right are provided, the clampers 23 and 24 can be immediately returned to the original position after the completion of molding, and continuous molding can be easily performed. In addition, the spring 57 of the lower clamper causes the clampers 23a, 23b, 24 on both sides to move in the folding process (FIG. 5).
By acting as a reaction force against the biasing force (sandwiching force p3) of a and 24b, the lowering speed of the lower clamper 24 due to the leaning is suppressed and the balance with the pressing force p2 by the upper clamper 23 is maintained, It is possible to maintain the state in which 21 is sandwiched with an appropriate force.

Further, since the lower clamper 24 is provided with a lock mechanism, it is possible to contribute to product quality maintenance. That is, after the bent portion is brought into close contact with the lower clamper 24 by the leaning, the lower clamper 24 is moved when the upper clamper 23 moves up.
If it is also raised, the remaining restraining force separates the product plate 58 into the left and right while sandwiching the product plate 58 in the upper and lower directions, resulting in opening the product plate 58 formed in the folding shape to the left and right. In this embodiment, as a countermeasure against this, locking of the cushion of the press machine is used to lock the cushion at the bottom dead center of the press stroke. Therefore, even if the upper holder 26 starts to move upward, The plate 58 is held in the folded shape and stopped at the bottom dead center, so that the force for restraining the product plate 58 immediately disappears. Then, when the locking of the press cushion is released at an arbitrary timing after that, the lower clamper 24 opens to the left and right as it rises. At this time, since the load acting on the lower clamper 24 is only the weight of the product plate 58, the opening is accompanied by this opening. The folded shape does not collapse. That is, the clampers 23 and 24 can quickly return to their original positions after the completion of molding without adversely affecting the quality of the product.

In this embodiment, the structure in which one folding flange shape is formed by one press stroke is shown, but the left and right movable parts (clampers 23, 2) are formed.
By providing the fixing portion between 4), folding flange shapes may be formed on both sides of the fixing portion. That is, it is also possible to perform folding flange forming at two places with one press stroke. Further, by making the left-right spacing (sliding width) of the clamper variable, the length of the flange shape to be molded can be changed, so that the versatility of the manufacturing apparatus can be further improved. Further, in carrying out the manufacturing method of the present invention, only the press stroke of the pressing machine is used in this embodiment, but in some cases, a mechanism for sliding the clamper is separately provided, and the initial bending is performed. It is also conceivable to slide the clamper to complete the molding after performing.

[0022]

In summary, according to the present invention, the following excellent effects are exhibited.

(1) According to the method described in claim 1, the molding can be completed extremely quickly and smoothly, and the productivity is improved and the cost is reduced.

(2) According to the configuration of claim 2, the molding can be performed only by the downward pressing force, and the practicality and the versatility are improved.

(3) According to the third aspect of the invention, the clamper can be slid with a very simple configuration, and the flange forming can be completed with one press stroke.

(4) According to the structure described in claim 4, the pressing force can be applied to the punch and the clamper only by lowering the upper holder, and the driving mechanism can have a simple structure.

(5) According to the configuration of claim 5, the punch that has been initially bent can be retracted upward, and the slide movement of the clamper is not hindered.

(6) According to the structure of claim 6, the punch and the clamper can be immediately returned to the original position after the completion of molding, and continuous molding can be easily performed.

(7) According to the configuration of claim 7, it is possible to prevent the collapse of the folded shape due to the separation of the clamper after the completion of molding, and to maintain the quality of the product.

[0030]

[Brief description of drawings]

FIG. 1 is a front sectional view showing an embodiment of an apparatus for manufacturing a plate having a folding flange shape according to the present invention.

FIG. 2 is a sectional view taken along the line AA in FIG.

FIG. 3 is a sectional view taken along the line BB in FIG.

FIG. 4 is a front sectional view for explaining an embodiment of a method for manufacturing a plate having a folding flange shape according to the present invention.

5 is a front cross-sectional view for explaining the next manufacturing step of FIG. 4. FIG.

FIG. 6 is a front cross-sectional view for explaining the next manufacturing step of FIG.

7 is a cross-sectional view taken along the line CC in FIG.

8 is a sectional view taken along the line DD in FIG.

FIG. 9 is a front view for explaining a roll forming method which is a conventional method for manufacturing a plate having a folding flange shape.

[Explanation of symbols]

 21 blank plate 22 punch 23 upper clamper 24 lower clamper 25 slide conversion mechanism 26 upper holder 27 lower holder 30 spring 40 inclined surface (for punch) 41 engaging surface 48 inclined surface (for lower clamper) 49 support surface 50 cushion plate 51 cushion Pin 57 Spring (biasing means) 58 Product plate P Press force of press machine p1 Bending press force p2 Pressing force p3 Pinching force

Claims (7)

[Claims] 1. After appropriately bending by pressing a raw material plate at two locations and applying bending pressure between them,
A method for manufacturing a plate having a folding flange shape, characterized in that pressing points on both sides are slid so that they come close to each other and are folded into a predetermined flange shape. 2. A punch for abutting a raw material plate with a downward bending pressing force for initial bending, upper and lower clampers for sandwiching and holding the raw material plate on both sides of the punch, and a pressing force of the clamper. And a slide conversion mechanism for converting clamps on both sides to approach each other by converting the plate into a slide direction, and an apparatus for manufacturing a plate having a folding flange shape. 3. The slide conversion mechanism according to claim 2, further comprising an inclined surface formed on a lower surface of the lower clamper, and a lower holder that supports the lower clamper via a support surface that is in sliding contact with the inclined surface. An apparatus for manufacturing a plate having a folding flange shape. 4. The apparatus for manufacturing a plate having a folding flange shape according to claim 2, wherein the punch and the upper clamper are collectively held by an upper holder that descends with a predetermined pressing force. 5. The folding flange shape according to any one of claims 2 to 4, wherein the punch is formed so as to engage with the upper clamper via an inclined surface and to be raised by the approach of the upper clampers on both sides. Plate manufacturing equipment. 6. The apparatus for manufacturing a plate having a folding flange shape according to any one of claims 2 to 5, wherein the clampers on both sides are provided with a biasing means for biasing the clampers in a sliding direction away from each other. 7. The apparatus for manufacturing a plate having a folding flange shape according to claim 2, wherein the lower clamper is provided with a lock mechanism that keeps the lower clamper and the lower clamper in a state where they are close to each other at the stroke bottom dead center.