Piercing method and piercing apparatus using hydroforming and hydroformed part and structure

The present invention was made with the object of eliminating the short pipe member when attaching a bolting nut to a hydroformed part, preventing an increase in the difficulty of hydroforming for attaching the nut, extending the nut length so as to enable sufficient strength at the time of bolting, and enabling application even to thin wall hydroformed parts and comprises inserting a metal pipe into a split mold having a pierce punch able to be moved perpendicular to an axis of the inserted metal pipe or in a slanted direction, having a front part narrower than a center part, and having around the front part a nut having an inside diameter larger than an outside diameter of the front part and smaller than an outside diameter of the center part; hydroforming it; then making the pierce punch advance to pierce part of the hydroformed part and pushing the nut by a center part arranged behind the nut and embedding the nut in the metal pipe.

TECHNICAL FIELD

The present invention relates to a method used for the production of exhaust system parts, suspension system parts, body system parts, etc. for automobiles which places a metal pipe in a mold, clamps the mold, then applies inside the pipe an internal pressure and a pushing force in the pipe axial direction so as to form a predetermined shape by hydroforming during which shaping using a pierce punch assembled inside the mold to pierce the metal pipe, to a mold for the same, to a hydroformed part worked by the same, and to a structure of worked parts joined together.

BACKGROUND ART

In recent years, hydroforming technology has been the focus of attention in the automobile field as one of the means for reducing the number of parts and thereby cutting costs, for lightening the weight, etc. In Japan as well, it began to be applied to actual cars starting in 1999. Since then, the parts which can be hydroformed have increased. The market has greatly expanded in size.

There are many advantages to hydroforming other than the above reduction of the number of parts and the lightening of the weight. For example, the fact that piercing of a metal pipe is possible at the same time as hydroforming may be mentioned. An outline of that technology is as shown inFIG. 1. In this technology, a pierce punch6assembled into the hydroforming mold (in the case of this example, the upper mold2) is being pushed in the direction of the mold cavity, so a hole is formed in the hydroformed part1. At that time, the high pressure internal pressure causes the hydroformed part1to be pushed against the mold2, so the edges of the hole will almost never droop down to the inner surface side and a good cut surface can be obtained. Further, the pierced metal piece7is sometimes completely punched out as shown inFIG. 1(a), but sometimes, as shown inFIG. 1(b), part is not cut and the piece left. Specifically, by partially making larger the chamfering of the edges of the front end of the pierce punch, cutting of that location is prevented.

There are many advantages to hydroforming as mentioned above, but as a defect, the point that joining with other parts is difficult may be mentioned. In the case of a conventional press formed part, it has been fastened to another part11by spot welding such as shown inFIG. 2(a) or bolting such as shown in (b). However, a hydroformed part is hollow, so spot welding was difficult. Further, attachment of a nut inside was also impossible. As shown inFIG. 3, the hydroformed part1may have a nut13welded to its outer surface side, but the nut13sticks out from the outer surface side of the hydroformed part1, so when joined with another part11, the parts cannot contact each other at their surfaces.

As an example of attachment of a nut to a hydroformed part, there is Japanese Patent Publication (A) No. 2002-45926. As shown inFIG. 4, this method performs the hydroforming by wrapping around the outer surface of the hydroformed part1a short pipe member61having a bag nut60attached to it.

Further, as another example of attaching a nut to the hydroformed part, there is Japanese Patent Publication (A) No. 2003-334625 as shown inFIG. 5. The main difference between the present method and the above-mentioned Japanese Patent Publication (A) No. 2002-45926 is the point that the nut13is sandwiched between the short pipe member61and the hydroformed part1.

Further, as other prior art, there is Japanese Patent Publication (A) No. 2005-297060. This method, as shown inFIG. 6, is a method which forms a burled part62at the inner surface side of the hydroformed part, takes the part out from the hydroforming mold, then cuts a tap63at the burled part and uses a bolt64to join the part with another part65.

DISCLOSURE OF THE INVENTION

However, in the technology of Japanese Patent Publication (A) No. 2002-45926, it is necessary to attach the bag nut and short pipe member by welding etc. before hydroforming. However, when hydroforming in the state with the short pipe member wrapped around the part, shaping of that location becomes extremely difficult and the danger of bursting, wrinkles, or other shaping defects occurring during the hydroforming rises. Therefore, this technology is also limited in applicable shapes of hydroformed parts. Conversely, to enable hydroforming, sometimes the length of the nut has to be shortened, so the fastening strength with the bolt sometimes might not be able to be sufficiently secured.

Further, the technology of Japanese Patent Publication (A) No. 2003-334625 has similar problems to Japanese Patent Publication (A) No. 2002-45926 in the point that a short pipe member is required, the point that hydroforming becomes difficult, and the point that there is a possibility of the nut length becoming shorter.

Further, when using the technology of Japanese Patent Publication (A) No. 2005-297060, cutting a tap is necessary after the hydroforming process. This is not efficient as a production process. Further, the present method directly cuts a tap at the hydroformed part, so cannot be applied when the shaped part is thin walled.

The present invention has as its object the provision of a working method and mold which, as explained above, are designed to eliminate the short pipe member required in the past when attaching a bolting nut to a hydroformed part, prevent an increase in the difficult of hydroforming due to the attachment of the nut, increase the length of the nut so as to secure sufficient strength at the time of bolting, and also enable application to a thin wall hydroformed part and to a worked part and a structure obtained by the same.

To solve this problem, the gist of the present invention is as follows:

(1) A piercing method using hydroforming comprising inserting a metal pipe into a split mold having a pierce punch able to be moved perpendicular to an axis of the inserted metal pipe or in a slanted direction, having a front part narrower than a center part, and having around the front part a nut having an inside diameter larger than an outside diameter of the front part and smaller than an outside diameter of the center part; applying to the metal pipe an internal pressure and pipe axial direction pushing force or an internal pressure for hydroforming; making the pierce punch advance to pierce part of the metal pipe by a front part of the pierce punch; then making the nut advance while pushing the center part arranged behind the nut; and pushing the surroundings of the pierced hole to the inner surface side of the metal pipe and embedding the nut in the metal pipe.

(2) A piercing device in a hydroforming apparatus having a split mold to which a metal pipe is attached and an internal pressure imparting means and axial pushing means,the hydroforming piercing apparatus characterized by having a pierce punch able to be moved perpendicular to an axis of the inserted metal pipe or in a slanted direction, having a front part narrower than a center part, and having around the front part a nut having an inside diameter larger than an outside diameter of the front part and smaller than an outside diameter of the center part, the pierce punch having the function of piercing the metal pipe by its front part together with advancing motion, then pushing the surroundings of the pierced hole by the nut to the inner surface side of the metal pipe and embedding the nut in the metal pipe.

(3) A hydroforming piercing apparatus as set forth in (2) characterized by having a secondary punch around a front part of the pierce punch and at an intermediate part between the nut and the pierce punch.

(4) A hydroformed part having an opening part at a side wall of a metal pipe, the hydroformed part characterized in that the opening part has a burled part projecting out to an inner surface side of the metal pipe and in that the burled part has a nut embedded in it at its inner side.

(5) A hydroformed part as set forth in (4) characterized in that the nut has a horizontal cross-sectional shape of a polygonal or elliptical shape or the nut has a horizontal cross-sectional shape of a contour of a combination of lines and curves or a combination of curves.

(6) A hydroformed part as set forth in (4) or (5) characterized in that the nut has a horizontal cross-sectional shape differing in an axial direction of the nut.

(7) A hydroformed part as set forth in any one of (4) to (6) characterized in that the nut as a whole is embedded at an inner surface side of the pipe from the outer surface of the metal pipe.

(8) A hydroformed part as set forth in any one of (4) to (7) characterized in that the burled part has a hole at its front end of a size smaller than the outside diameter of the nut and in that the burled part covers up to the edges of the nut at the inner surface side.

(9) A hydroformed part as set forth in any one of (4) to (8) characterized in that the nut has concave or convex dimples at its side surface.

(10) A hydroformed part as set forth in any one of (4) to (9) characterized in that the nut and metal pipe are welded together.

(11) A structure characterized by being integrally bolted with a hydroformed part as set forth in any one of (4) to (10).

According to the present invention, it is possible to eliminate the short pipe member which became necessary in the past when attaching a bolting nut to a hydroformed part, to prevent an increase in the difficulty of hydroforming due to the attachment of a nut, to lengthen the length of the nut to enable a sufficient strength to be secured at the time of bolting, and enable application to a thin wall hydroformed part. Due to this, bolting with other parts after hydroforming becomes easy, and the range of auto parts to which hydroforming can be applied is expanded. As a result, automobiles becomes lighter in weight which leads to an improvement in fuel economy and also contributes to improvement of the global environment. Further, the conventionally required short pipe member can be eliminated, so this can also contribute to a reduction of the costs.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, first the usual hydroforming is performed. The “usual hydroforming” is the method of mounting a pipe material between an upper mold and a lower mold, filling the inside of the pipe material with water or another pressurizing medium, raising the pressure, and simultaneously if necessary pushing in the material in the axial direction from the pipe end by an axial pushing punch so as to form the pipe material into a shape following the inner surfaces of the upper mold and lower mold.FIG. 7shows the process of using the usual hydroforming to shape a hydroformed part1, then embed a nut17from the lower mold3. In this example, a nut17having an inside diameter larger than an outside diameter of a front part18of a later mentioned pierce punch16and smaller than an outside diameter of a center part is embedded in the hydroformed part1from the direction of the lower mold3. Just that location is shown enlarged. After this, the figure will be used to explain the details of the present invention. Note that in the following explanation, the “top surface” means the surface at the top of the figure, while the “bottom surface” means the surface at the bottom of the figure. That is, when piercing from the top to the bottom of the paper surface, the top surface and bottom surface become reversed.

Inside the lower mold3, a primary pierce punch16is assembled. The primary pierce punch16is structured, from the top in the figure, by a small diameter front part18, a large diameter center part19, and a small diameter rear end20. Further below the rear end20, a cylinder is set (not shown). The cylinder can be used to raise the primary pierce punch16in this structure. Note that in this example, a small diameter rear end20was provided, but it is also possible to omit this and directly push the lower surface of the larger diameter center part19by the cylinder.

The shape of the hole provided at the lower mold3is structured changing in inside diameter in three stages. From the top of the figure, the front part21, center part22, and rear part23become narrower in inside diameter the further downward. The inside diameter of the center part22and the outside diameter of the above-mentioned center part19of the primary pierce punch become substantially equal. The primary pierce punch16rises from the reference location. Note that the center part22has a depth larger than a height of the center part19of the primary pierce punch. Further, the inside diameter of the rear part23is designed to be larger than the outside diameter of the above-mentioned rear end20of the primary pierce punch.

The state ofFIG. 7(a) shows the middle of hydroforming. This is the state where the inside of the pipe is subjected to a high pressure by the pressurizing medium5and the hydroformed part1is stuck to the surface of the lower mold3. At this point of time, the bottom surface of the center part19of the primary pierce punch and the upper surface of the rear part23of the lower mold hold contact each other. Even in a state where hydroforming results in internal pressure being applied, the primary pierce punch16is designed not to descend lower than that position. Note that the front part18of the primary pierce punch is arranged passing through the center hole part of the ring-shaped embedded nut17, while the top surface of the front part18of the pierce punch becomes the same height as the surface of the lower mold3.

The outside diameter of the embedded nut17is made smaller than the inside diameter of the front part21of the lower mold hole, while the height is made equal to the depth of the front part21of the lower mold hold. Further, the inside of the embedded nut17is formed with a tap24. The peaks of the tap24have an inside diameter set larger than the outside diameter of the above-mentioned front part18of the primary pierce punch. If setting such dimensions, in the state ofFIG. 7(a), the embedded nut17becomes placed at the position of the top surface of the center part22of the lower mold hole and, further, the position of the top surface A of the embedded nut17becomes equal to the positions of the surface of the lower mold3and the top surface of the front part18of the primary pierce punch.

Next, as shown inFIG. 7(b), while holding the inside of the hydroformed part1at a high pressure as it is, the front part18of the primary pierce punch16is made to rise (advance) through the embedded nut17. The edges25of the top surface of the front part18of the primary pierce punch are not chamfered, but are left sharp. For this reason, the metal piece7where the hydroformed part1was pierced is separated. The so-called “piercing” explained inFIG. 1(a) is therefore performed.

As shown inFIG. 7(c), further, if making the primary pierce punch16rise, since the center part19of the primary pierce punch is larger in size than the front part18of the primary pierce punch, the top surface of the center part19of the primary pierce punch pushes the embedded nut17upward. However, the edges26of the top surface (inner surface side) of the embedded nut17are chamfered, so even if pushed into the hydroformed part1, it will not be pierced. This is the same result as explained inFIG. 1(b). This is because if the chamfering of the edges of the front end of the pierce punch is large, the hydroformed part1will not be sheared. As a result, along with the rise of the embedded nut17, the hole of the hydroformed part1formed by the above-mentioned step (b) will be pushed wider and the hydroformed part1will be formed with a burled part27at the inner surface side.

Further, if, as shown inFIG. 7(c), setting the final height by which the primary pierce punch16rises so that the top surface of the center part19becomes exactly the same height as the surface of the lower mold, the bottom surface B of the embedded nut17becomes the same position as the outer surface of the hydroformed part1.

Finally, if, as shown inFIG. 7(d), retracting the primary pierce punch16downward, only the embedded nut17will remain in the burled part27. Note that there are several types of timings for retracting the primary pierce punch16. For example, if maintaining the internal pressure while retracting the primary pierce punch16, the inside water will automatically leak and the internal pressure will drop. This method is preferable in terms of cycle time, but there is a problem with splattering of water etc. On the other hand, if making the internal pressure drop, then retracting the primary pierce punch16, the water will not easily splatter, but the cycle time will become longer. Further, it is also possible to take out the hydroformed part1from the lower mold3, then make the primary pierce punch16retract. In this case, caution is necessary that the tap24of the embedded nut17not be damaged when taking out the hydroformed part1. However, in the case of the present invention, the drive direction of the primary pierce punch16need only be upward. When making the punch retract, it is possible to push it in manually, by a robot, etc. after taking out the hydroformed part1. In particular, when continuously working parts, it is necessary to set the embedded nuts17for the next working operation, so the primary pierce punch16also may be pushed in downward at that time.

The above series of explanations given usingFIG. 7are for the case where the only pierce punch is the primary pierce punch16. However, the primary pierce punch16has to pierce the first hydroformed part1, so material-wise often uses tool steel or another extremely hard material. On the other hand, for the nut, usually soft steel is used. Therefore, the bottom surface of the embedded nut17may be pushed in by the top surface of the center part19of the primary pierce punch and become concave. To prevent this, as shown inFIG. 8, it is sufficient to add a secondary pierce punch28between the embedded nut17and the center part19of the pierce punch. The secondary pierce punch28has an outside diameter substantially equal to the inside diameter of the front part21of the hole of the lower mold. Its height, together with the height of the embedded nut17, is made the same as the depth of the front part21of the hole of the lower mold. The material is preferably hard and will not become concave by being pushed in by the top surface of the center part19of the primary pierce punch. Further, the secondary pierce punch28and embedded nut17contact each other over a wide area, so the risk of a concave part forming in the nut17is reduced. Further, if preparing several types of heights of secondary pierce punches28, even if the height of the embedded nut17changes, this can be easily handled by changing the primary pierce punch16or the shape of the hole of the lower mold3.

If using the above piercing apparatus for working, for example, as shown inFIG. 9(a), the hydroformed part1is formed with a burled part27at its inner surface side and that location may have an embedded nut17attached to it. That is, without attaching the short pipe member required in the prior art of Japanese Patent Publication (A) No. 2002-45926 or Japanese Patent Publication (A) No. 2003-334625, a hydroformed part in which a nut is embedded can be obtained. Further, in the present invention, the hydroforming is completed before embedding the nut, so the difficulty of hydroforming will not be increased due to embedding the nut.

If fastening another part11to the above obtained hydroformed part in which the nut is embedded by a mounting bolt29, the structure as shown inFIG. 9(b) is obtained. In the present invention, as explained above, the difficulty of hydroforming is not increased, so the embedded nut can be freely changed in height. Accordingly, to increase the bolting strength, it is easy to increase the height of the embedded nut so as to increase the effective thread length. Further, at the location where the embedded nut17is attached, a hole is formed passing through the hydroformed part1, so the mounting bolt29can be inserted passing through to the inside of the hydroformed part1. Accordingly, in terms of the effective thread length by which the mounting bolt29and embedded nut engage as well, the invention is more advantageous than the prior art of Japanese Patent Publication (A) No. 2002-45926 and Japanese Patent Publication (A) No. 2003-334625. Further, in the present invention, the thickness of the embedded nut17can also be freely changed, so the invention can also be applied to a thin walled hydroformed part to which the prior art of Japanese Patent Publication (A) No. 2005-297060 could not be applied.

In the present invention, the embedded nut17is attached to the hydroformed part1under a high internal pressure. Even with this alone, the embedded nut17and the hydroformed part1are strongly fit together. However, depending on the part, sometimes a stronger mounting strength is demanded, so below the method for increasing the mounting strength will be explained.

First, the strengthening method for preventing the embedded nut17from turning at the time of screwing in the bolt and after attachment with the other part will be explained. If the embedded nut17has a circular horizontal cross-section, it would easily turn, so embedded nuts17of the horizontal cross-sectional shapes shown inFIG. 10would be effective. (a) of the figure is an example of a hexagonal embedded nut30, but in addition to a hexagonal shape, an octagonal or other polygonal shape is also possible. However, when embedding and burling a hexagonal embedded nut30, there is a danger of the hydroformed part1breaking at the locations of the hexagonal corners31, so it is desirable to chamfer the hexagonal corners31by a very large amount. Further, in the case of a hexagonal embedded nut30, it is possible to use a commercially available hexagonal nut as it is or one chamfered somewhat more at the corners, so this is cost advantageous.

(b) of the figure is an example of an embedded nut32of an elliptical horizontal cross-sectional shape. In the case of an elliptical shape, there are no corners like the case of a polygon, so there is the advantage of resistance to breakage at the time of burling. However, the cost of fabricating the nut becomes higher.

As opposed to this, (c) of the figure is an example of a nut33with a horizontal cross-sectional shape of a circular cross-section partially cut away. The effect is substantially the same as the case of an elliptical shape. Further, the cost of fabrication of the nut is also cheaper than the case of an elliptical shape. In this way, the contour of the horizontal cross-sectional shape may also be a shape of a combination of lines and curves.

Further, (d) of the figure is an example of an embedded nut34of a horizontal cross-sectional shape of a combination of curved contours. If the shape becomes complicated, this becomes disadvantageous in terms of the nut fabrication costs or breakage at the time of burling, but the effect of preventing the nut from turning becomes higher.

Next, an effective method for preventing the embedded nut17from detaching at the outer surface side or inner surface side of the hydroformed part1will be explained. As shown by the vertical cross-sectional shapes of nuts inFIG. 11, the horizontal cross-sectional shape of an embedded nut may be made a shape which is not constant in the axial direction. Specific examples will be explained below. (a) is an embedded nut35tapered so that the pipe inner surface side becomes larger in diameter than the pipe outer surface side. At the initial stage of embedding the nut35, the burled part27is spread to a large diameter, but a high pressure is applied, so as the nut is embedded further to the inner surface side, the root part of the burled part27is constricted to a small diameter. As a result, the nut35is embedded by the shape such as in (a) and the nut35becomes resistant to detachment at the pipe outer surface side.

Conversely, if using an embedded nut36tapered so that the pipe inner surface side becomes smaller in diameter than the pipe outer surface side, the nut is structured to be resistant to detachment at the pipe inner surface side as shown in (b) of the figure.

Shapes provided with both the advantages of (a) and (b) are (c) and (d). (c) is an example of a barrel shaped embedded nut with a center part in the axial direction larger in diameter than the two end parts in the axial direction, while (d) is an example of an hourglass shaped embedded nut38with a center part of the embedded nut38smaller in diameter. In each case, the nut is structured to be resistant to detachment at the pipe inner surface side and outer surface side.

Further, the embedded nut need not continuously change in cross-section. As shown in (e), it may also be an embedded nut39with a step difference. In this example, a flange40is attached at the pipe inner surface side. The front end of the burled part27is structured to catch at the flange40. The nut is therefore structured to be resistant to detachment at the pipe outer surface side.

Further, as shown in the example of (f), an embedded nut41provided at its side surface with grooves42at one location or a plurality of locations would also be effective. Naturally, not only grooves, but also projections can be expected to have similar effects.

Next, the method of preventing detachment at the pipe outer surface side by the position of the embedded nut17will be explained. As shown inFIG. 12(a), the embedded nut17is embedded until the surface B of the nut at the pipe outer surface side is positioned at the pipe inner surface side from the outer surface of the hydroformed part1. If doing so, the internal pressure acts to provide a cut-in part43at the root of the burled part27, so the nut is structured to be resistant to detachment at the pipe outer surface side. Further, as shown by (b) of the figure, if using a mounting bolt29to join with another part11, when fastening the mounting bolt29, the cut-in part43is crushed, so the nut is structured to become further resistant to detachment.

Conversely, an example of leaving the front end44of the burling at the pipe inner surface side of the embedded nut17, making the size of the hole at the front end of the burled part smaller than the outside diameter of the nut, and using the burled part to cover the edges26of the nut at the inner surface side is shown inFIG. 13. If made such a structure, the embedded nut17will become resistant to detachment at the pipe inside surface side. Further, compared with the case of expanding the hole size of the front end of the burled part to the outside diameter of the embedded nut17, the rate of expansion of the pierced hole becomes lower, so this is advantageous when the rate of hole expansion of the material of the hydroformed part1is low.

In the above way, to prevent the front end44of the burling from being spread to the outside diameter of the embedded nut17, it is sufficient to lower the height of the embedded nut17. However, in this case, the fastening strength of the thread also ends up falling, so a method for obtaining the structure shown inFIG. 13while leaving the height of the embedded nut17high will be explained next. As shown inFIG. 14(a), at the point of time of hydroforming before piercing, the top surfaces A of the primary pierce punch16and embedded nut17are set to a state sticking out from the inner surface of the lower mold3to the pipe inner surface side. Such a sticking out state can be achieved by for example setting the depth of the front part21of the hole of the lower mold smaller than the height of the embedded nut17, setting the lengths of the front part18of the primary pierce punch and secondary pierce punch28high, etc. In that state, the part is pierced as shown in (b), then is burled as shown in (c) to embed the embedded nut17in the hydroformed part1. In the step of (c), the rising stroke of the pierce punch can be set smaller, so the rate of expansion of the pierced hole becomes smaller. As a result, as shown in (d), the structure as shown inFIG. 13is obtained even when the height of the embedded nut17is high.

Further,FIG. 15shows an example of provision of dimples at the side surface of the embedded nut. (a) is an example of an embedded nut45given concave dimples46, while (b) is an example of an embedded nut47given convex dimples48. In both cases, if providing the nut side surface with dimples, rotation of the embedded nut can also be suppressed and detachment to the inside surface side and outside surface side of the pipe can also be suppressed.

If using such a means, the embedded nut17will become resistant to detachment from the hydroformed part1, but to further firmly fasten it, as shown inFIG. 16, after being embedded, it is effective to weld together the embedded nut17and hydroformed part1. The welding may be performed by circular welding as illustrated, but welding at just several points is also effective. The welding method may also be MIG, TIG, or other arc welding or may be laser welding.

EMBODIMENTS

For the base pipe, steel pipe of an outside diameter of 63.5 mm, a wall thickness of 2.3 mm, and a total length of 490 mm was used. For the steel type, STKM13B of carbon steel pipe for machine structures was employed. The mold used for the hydroforming has a shape for enlarging the pipe to the long cross-sectional shape as shown inFIG. 17. The pierce punch was assembled at the center of the lower mold3. This structure is shown inFIG. 18. The tests were run for two types of cases of the case (a) of not using a secondary pierce punch28and the case (b) of using one. In both cases, the outside diameter of the front part18of the primary pierce punch was made 10 mm, the outside diameter of the center part19was made 15 mm, the inside diameter of the front part21of the hole of the lower mold was made 20 mm, and the inside diameter of the center part was made 15.10 mm. Further, the stroke of the primary pierce punch16from the initial position to the bottom surface of the embedded nut17or secondary pierce punch28was made 8 mm. The depth of the front part21of the hole of the lower mold was made 7 mm in the case of (a) of no secondary pierce punch28and was made 20 mm (=7 mm+secondary pierce punch height 13 mm) in the case of (b) with a secondary pierce punch28. Further, the stroke after contacting the bottom surface of the embedded nut17or secondary pierce punch28was made 7 mm. That is, at the final point of time, both in the case of (a) and (b), the bottom surface of the embedded nut17is set to become exactly equal to the height of the outer surface of the hydroformed part1. Note that as the steel type of the mold, for the upper mold2and lower mold3, S50C was selected, for the primary pierce punch16, SKH51, and for the secondary pierce punch28, SKD11.

For the shape of the embedded nut, the four types such as shown inFIG. 19were used. (a) is a basic shape comprised of a cylindrical shape with a 20φ circular cross-section, (b) is a cylindrical shape with a 20 mm×18 mm elliptical cross-section, (c) is a barrel shape with a bulging center in circular cross-section, and (d) is a shape the same as (a) and given 2.5φ dimples at its surface. In each case the nut height was 7 mm and an M12 tap was cut at the center M12.

The above base pipe and working device (mold) were used to run tests to embed various types of nuts into hydroformed parts. As the hydroforming conditions, the parts were formed by a maximum internal pressure of 200 MPa and axial pushing amounts at the two ends of 50 mm. After forming, while holding the pressure at 200 MPa as is, the primary pierce punch16was pushed inside through the hole part of each embedded nut17to embed the nut in a hydroformed part1. That is, first, the front part18of the primary pierce punch was used to pierce of a 10φ hole, the punch was raised as is by a stroke of 8 mm, then was raised together with the embedded nut17to widen the 10φ hole and embed each nut at the burling location. The rising stroke of the embedded nut17was made 7 mm, so finally the bottom surface of the embedded nut17and the outer surface of the hydroformed part1became equal in height.

As a result, the nut of each ofFIG. 19(a) to (d) could be embedded in a hydroformed part1and the nut would not detach even when the part is taken out from the mold. While embedding the nut, the internal pressure did not fall much at all, so the shape of the hydroformed part1around the embedded part was good and no cracks formed at the burled part27either. Note that tests were run for the case where there was no secondary pierce punch and the case where there was, but in both cases the nuts could be embedded. However, when there is no secondary pierce punch, the surface of the nut showed some dents due to the primary pierce punch after working, so in the case of a part where the appearance is of a concern, it can be said to be desirable to provide a secondary pierce punch.

Even if changing the horizontal cross-section of the nut to the shape shown inFIGS. 10(a), (c), and (d) or the vertical cross-section to the shape such as shown inFIGS. 11(a), (b), and (d) to (f), the nut could be similarly embedded.

Next, a test was run embedding a nut ofFIG. 19(a) by the same pierce structure as inFIG. 18(a) during which only the rising stroke of the embedded nut was changed from 7 mm to 10 mm. As a result, the surface B of the nut at the outer surface side of the steel pipe was embedded down to a position 3 mm above the outer surface of the hydroformed part1(seeFIG. 12(a)). Further, as shown inFIG. 12(a), the burled part cut inward at the part below the nut.

Further, a test was run embedding the nut ofFIG. 19(a) by a pierce punch structure such as shown inFIG. 20. In this case, the front part18of the primary pierce punch and the secondary pierce punch28were increased in length by 3 mm, so in the initial state of setting the nut, the top surface of the nut was positioned 3 mm above the surface of the lower mold3(inner side of pipe). If this extent, the part could be shaped with changing the hydroforming conditions. After shaping, the pierce punch was pushed in to embed the nut, but the stroke of the embedded nut was set to 4 mm, so the final position of the lower surface of the nut became the same height as the outer surface of the hydroformed part1. However, since only embedded by 4 mm, the hole was not widened completely to the nut size of 20φ and was stopped at 17φ. Accordingly, a structure was obtained in which the hole at the front end of the burled part was smaller than the outside diameter of the nut and in which the burled part covered the edges of the nut at the inner surface side and partially covered the top surface of the nut.

As explained above, in all cases, the nut could be embedded in the hydroformed part1without problem. Next, as an example of the other part, a steel sheet of a sheet thickness of 3 mm formed with a hole of 14φ was used and fastened with the above obtained hydroformed part with an embedded part by an M12×length 20 mm hexagonal bolt. As a result, in each above-mentioned case, the other part could be bolted without problem. Further, at one section of the hydroformed part with the nut ofFIG. 19(a) attached to it, the nut and hydroformed part were circularly welded by TIG, while other sections were spot welded manually. The shaped part could be bolted with the other part without problem.

INDUSTRIAL APPLICABILITY

The present invention performs hydroforming by inserting a metal pipe into a mold and shaping it into a predetermined shape during which shaping process it uses a pierce punch assembled inside the mold to pierce the metal pipe and is particularly useful when working a hydroformed part used for the production of for example exhaust system parts, suspension system parts, body system parts, etc. of an automobile etc.