Abstract:
A nut consists of integral metal material and has a female screw threading; screw thread formed with a rolled tap in a screw hole that opens substantially perpendicularly to an end surface of one end of the nu. A concave lead-in is located at an opening end of the screw hole and has a shape including a concave curved portion and a linear tapered portion that are continuous with each other, and extend along the overall periphery of the opening end of the hole. Due to the special concave lead-in, the first thread formed by the rolled tap adjacent to the concave lead-in will not have a concave groove along its crest, although subsequent threads have such a concave groove along the thread crest as a characteristic of thread formation with the rolled tap. Such a thread structure avoids cross-threading when a bolt is engaged in the nut.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a nut and an intermediate product therefor, and more particularly, it relates to the structures of a nut having a concave lead-in shape suitable for internal threading with a rolled tap and an intermediate product therefor. 
     2. Description of the Prior Art 
     In general, a nut is internally threaded with the so-called cutting tap  52  shown in FIGS. 13A and 13B. In this threading with the cutting tap  52 , the inner peripheral surface of a prepared hole of an unthreaded intermediate product  51  for a nut is cut to form roots of a female screw threading while discharging chips through grooves  52   a  of the cutting tap  52  shown in FIG.  13 A. In such internal threading with the cutting tap  52 , the crest diameter of the worked female screw threading coincides with the inner diameter d of the unthreaded prepared hole and hence the accuracy of the female screw threading shape can be relatively readily ensured. However, parts of the chips tend to remain in the worked female screw threading to bite into a male screw engaging with the female screw threading and disadvantageously hinder proper fastening. 
     In order to solve such a problem, a nut is increasingly internally threaded with a rolled tap causing no chips. FIGS. 14A and 14B show a rolled tap  53  threading an intermediate product  51  for a nut. In such internal threading with a rolled tap  53 , a female screw threading is formed not by cutting but by plastic working. In other words, the inner peripheral surface of a prepared hole of the intermediate product  51  for a nut is depressed for forming roots of the female screw threading while the material extruded from the depressed portions is raised to form crests. The crest diameter of the worked female screw threading is smaller than the inner diameter d of the prepared hole of the unthreaded intermediate product  51 . 
     However, such conventional internal threading of the nut with a rolled tap has the following problem: 
     In general, an internally threaded screw hole of a nut is provided on its opening end with a tapered concave lead-in, called a countersink in general, in order to guide the forward end of a bolt engaging with the nut or facilitating discharge of chips resulting from internal threading with a tap. FIG. 15A shows an unthreaded intermediate product  131  for a nut provided with a concave lead-in  31   a  having such a shape, to be internally threaded with a rolled tap  53  from the side of the lead thread  31   a . As a consequence of internal threading with the rolled tap  53 , a nut  31  provided with a female screw threading having a concave portion CP 1  or CP 2  in each thread is formed as shown in FIG.  15 B. When the rolled tap  53  is screwed into the intermediate product  131  from the side of the concave lead-in  31   a , the material is plastically deformed along thick arrows appearing in FIG. 16, showing a detailed area enclosed with a circle XVI in FIG. 15B in an enlarged manner, in the vicinity of the concave lead-in  31   a . Thus, roots of the rolled tap  53  cannot be completely filled up with the material but crests of the threads of the formed female screw threading are disadvantageously depressed with concave portions or depression CP 1  and CP 2 . In particular, the root of the rolled tap  53  for forming a thread  31   b  adjacent to the concave lead-in  31   a  cannot be sufficiently filled up with the material in general due to the tapered shape of the concave lead-in  31   a . Thus, the thread  31   b  has a lower height a larger depression or concave portion CP 1  than the remaining threads. 
     When a bolt  32  is to engage with the nut  31  internally threaded with the rolled tap  53  in the aforementioned manner from the side of the concave lead-in  31   a  as shown in FIG. 17A, a thread on the forward end of the bolt  32  hits against the depression on the crest of the thread  31   b  of the nut  31  adjacent to the concave lead-in  31   a  to cause the so-called cross-thread, as shown in a part enclosed with a circle A in FIG.  17 B. Consequently, the bolt  32  incorrectly engages with the nut  31 , to result in an improper fastening state. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a nut having a concave lead-in shape causing no cross-thread in engagement with a bolt also when the is internally threaded with a rolled tap. 
     In order to attain the aforementioned object, a nut according to a first aspect of the present invention consists of integral metal material and has a female screw threading formed in a screw hole opening substantially perpendicularly to an end surface of one end, and a concave lead-in defining an opening end of the screw hole for the end surface has a concavely depressed shape along the overall periphery thereof. 
     In a preferred embodiment, the concave lead-in includes a concave bent or curved portion in an arbitrary longitudinal section including the central axis of the screw hole. The concave lead-in preferably has such a shape that the concave bent portion is continuously combined with a linear tapered portion, and in this case, the concave bent portion further preferably starts from the inner peripheral side of the opening end to be continuous with the linear tapered portion on an outer prescribed position. 
     A nut according to a second aspect of the present invention comprises a shaft portion and a flange portion outwardly extending from a first end of the shaft portion, consisting of integral metal material. The shaft portion is in the form of a hollow cylinder, has a portion slated or adopted for caulking in a second end opposite to the first end and is formed with a female screw threading on the inner peripheral surface excluding the portion slated for caulking. The portion slated for caulking has an inner diameter larger than that of the portion formed with the female screw threading, and a concave lead-in defining an end of the portion of the shaft portion formed with the female screw threading adjacent to the portion slated for caulking has a concavely depressed shape along the overall periphery. In this nut, the present invention is applied to the so-called rivet type tee nut having a predeterminate caulked portion, and the shape of the portion formed with the female screw threading is common to the nut according to the first aspect of the present invention. 
     The present invention includes an unthreaded intermediate product for a nut consisting of integral metal material and having a screw hole opening substantially perpendicularly to an end surface of one end, slated for internal threading with a rolled tap at least in a prescribed region, and a concave lead-in defining an end of the prescribed region of the screw hole slated for internal threading has a concavely depressed shape along the overall periphery. 
     The nut according to the first or second aspect of the present invention is formed by working a female screw threading in the screw hole of such an intermediate product for a nut with a rolled tap. In the intermediate product for a nut according to the present invention, the concavely depressed shape of the concave lead-in is preferably so formed that the crest of a thread of the female screw threading adjacent to the concave lead-in has no concave groove but a convex shape and particularly a line edge crest formed along an intersection of the two sloping flanks of the screw thread ridge. Consequently, no concave groove is formed on the crest of the thread of the female screw adjacent to the lead thread when the intermediate product is threaded with the rolled tap, whereby it is possible to prevent such an inconvenient phenomenon that a thread on the forward end of a bolt engaging with the nut hits against the thread of the female screw threading to cause cross-thread. Further, the nut maintains such a characteristic of the countersink that the diameter of the concave lead-in enlarges toward the opening end, whereby the female screw threading can be worked without damaging the function of the countersink facilitating discharge of chips resulting from internal threading with the tap. 
     The concavely depressed portion of the concave lead-in is shaped as a concave bent or curved portion, not to damage another function of the conventional tapered countersink properly guiding the forward end of the bolt engaging with the nut for automatically aligning the central axes of the nut and the bolt. Even if the bolt for engagement is inserted into the nut in a slightly inclined state, the central axes of the nut and the bolt are automatically aligned due to the guiding function of the concave lead-in, so that the nut and the bolt properly engage with each other. 
     The nut according to the present invention is also applicable to the so-called rivet type tee nut in which a predeterminate caulked portion of a shaft portion has an inner diameter larger than a first inner diameter and a uniform outer diameter substantially identical to a first outer diameter or another rivet type tee nut in which a predeterminate caulked portion of a shaft portion has an inner diameter larger than a first inner diameter and an outer diameter larger than a first outer diameter. 
     Further, the nut according to the present invention is also applicable to the so-called rivet type tee nut in which a predeterminate caulked portion of a shaft portion has an inner diameter larger than that of a portion formed with a female screw threading and is formed to be smaller in thickness than the portion formed with the female screw threading, or another rivet type tee nut in which a predeterminate caulked portion of a shaft portion has an inner diameter larger than that of a portion formed with a female screw threading and an outer diameter larger than that of the portion formed with the female screw threading, for example. 
     When applying the nut according to the present invention to a tee nut, the shaft portion preferably has a step portion in the region formed with the female screw threading, so that a region closer to the flange portion with reference to the step portion has a shape larger in inner and outer diameters than a region on an end opposite to the flange portion. The nut having such a step portion is formed by internally threading an intermediate product for a nut having a step portion with a rolled tap or the like. When the shaft portion of the unthreaded intermediate product for a nut has such a step portion, allowance for working accuracy can be attained by working only the screw hole of the prescribed region including the concave lead-in in the female screw threading forming region into a shape maintaining necessary accuracy while rendering the remaining region slightly larger in inner diameter than the prescribed region so that a female screw threading shape defined in a standard such as JIS can be accurately formed. It can be said that such a step portion is effective also for working the concave lead-in in the present invention in high accuracy. 
    
    
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates a central longitudinal section of a nut according to a first embodiment of the present invention, and FIG. 1B is a partially enlarged sectional view showing a part enclosed with a circle  1 B in FIG. 1A in an enlarged manner; 
     FIG. 2 is a perspective view showing the nut appealing in FIGS.  1 A and  1 B carried along a feed track; 
     FIG. 3A is a sectional view showing an intermediate product for the nut according to the first embodiment of the present invention to be internally threaded with a rolled tap, and FIG. 3B is a sectional view showing the intermediate product in the process of internal threading with the rolled tap; 
     FIG. 4 is a partially enlarged sectional view showing a part enclosed with a circle IV in FIG. 3B, for illustrating flow of material caused by plastic deformation in the process of internal threading on the intermediate product for the nut according to the first embodiment of the present invention with the rolled tap; 
     FIG. 5A is a sectional view showing the nut according to the first embodiment of the present invention to engage with a bolt, and FIG. 5B is a sectional view showing such a state that a thread on the forward end of the bolt starts to engage with a portion of a female screw threading of the nut adjacent to a concave lead-in; 
     FIG. 6A is a sectional view showing a state of working a concave bent or curved portion of the concave lead-in with a punch having a prescribed shape in a press step of molding the intermediate product for the nut according to the first embodiment of the present invention, and FIG. 6B is a sectional view showing a state of forming a linear tapered portion with another punch after working the concave bent portion of the concave lead-in; 
     FIG. 7A illustrates a central longitudinal section of a nut according to a second embodiment of the present invention, and FIG. 7B is a partially enlarged sectional view showing a part enclosed with a circle VIIB in FIG. 7A in an enlarged manner; 
     FIG. 8A illustrates a central longitudinal section of an unthreaded intermediate product for the nut according to the second embodiment of the present invention, and FIG. 8B is a partially enlarged sectional view showing a part enclosed with a circle VIIB in FIG. 8A in an enlarged manner; 
     FIG. 9A illustrates a central longitudinal section of a nut according to a third embodiment of the present invention, and FIG. 9B is a partially enlarged sectional view showing a part enclosed with a circle IXB in FIG. 9A in an enlarged manner; 
     FIG. 10A illustrates a central longitudinal section of an unthreaded intermediate product for the nut according to the third embodiment of the present invention, and FIG. 10B is a partially enlarged sectional view showing a part enclosed with a circle XB in FIG. 10A in an enlarged manner; 
     FIG. 11A is a partially enlarged sectional view showing a part, close to a concave lead-in, in a central longitudinal section of an unthreaded intermediate product for a nut according to a fourth embodiment of the present invention in an enlarged manner, and FIG. 11B is a partially enlarged sectional view showing the part close to the concave lead-in of the intermediate product shown in FIG. 11A formed with a female screw threading in a screw hole with a rolled tap; 
     FIG. 12A is a partially enlarged sectional view showing a part, close to a concave lead-in, in a central longitudinal section of an unthreaded intermediate product for a nut according to a fifth embodiment of the present invention in an enlarged manner, and FIG. 12B is a partially enlarged sectional view showing the part close to the concave lead-in of the intermediate product shown in FIG. 12A formed with a female screw threading in a screw hole with a rolled tap; 
     FIG. 13A is an explanatory diagram showing internal threading with a cutting tap, and FIG. 13B is an explanatory diagram showing the shape of an insert of the cutting tap in plane; 
     FIG. 14A is an explanatory diagram showing internal threading with a rolled tap, and FIG. 14B is an explanatory diagram showing the shape of an insert of the rolled tap in plane; 
     FIG. 15A is a sectional view showing an intermediate product for a conventional nut to be internally threaded with a rolled tap, and FIG. 15B is a sectional view showing the intermediate product in the process of internal threading with the rolled tap; 
     FIG. 16 is a partially enlarged sectional view showing a part enclosed with a circle XVI in FIG. 15B for illustrating flow of material caused by plastic deformation in the process of internal threading on the intermediate product for the conventional nut with the rolled tap; and 
     FIG. 17A is a sectional view showing the conventional nut for engaging with a bolt, and FIG. 17B is a sectional view showing the so-called cross-thread caused by a thread on the forward end of the bolt hitting a concave portion of a first thread of the female screw threading when the forward end of the bolt starts to engage with a portion of the female screw threading of the nut adjacent to a concave lead-in. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are now described with reference to the drawings. 
     (First Embodiment) 
     A first embodiment of the present invention is now described with reference to FIGS. 1A to  6 B. In the first embodiment, the present invention is applied to the so-called hopper feed tee nut provided with a flange portion  3  having pawls  4  on an end of a hollow cylindrical shaft portion  2 , which can be automatically fed along feed tracks  50  as shown in FIG.  2 . The feature of this nut  1  resides in the shape of a concave lead-in  5  thereof, i.e., the shape of a starting end of a female screw threading or portion on the upper end of the shaft portion  2  shown in FIG.  1 A. As most clearly understood from FIG. 1B showing a part enclosed with a circle IB in FIG. 1A in an enlarged manner, the concave lead-in  5  includes a concave bent portion  5   a  on the inner peripheral side and a tapered portion  5   b  continuous with the bent portion  5   a  on the outer peripheral side. The concave bent portion  5   a  is generally set in a substantially arcuate shape. 
     In order to form a female screw threading in the nut  1  having the aforementioned structure, a rolled tap  53  is screwed into an unthreaded intermediate product  101  for the nut  1  shown in FIG. 3A from the side of the upper-end thereof, i.e., the side of the concave lead-in  5 . A concave lead-in  105  of the intermediate product  101  for the nut  1  includes a substantially arcuate concave bent portion  105   a  provided on the inner periphery of the upper end of a hollow cylindrical shaft portion  102  in an arbitrary longitudinal section including the central axis of the shaft portion  102  and a linear tapered portion  105   b  continuous with the concave bent portion  105   a  on the outer peripheral sides. 
     The concave lead-in  105  has the aforementioned shape so that the shaft portion  2  of the nut  1  is provided with a female screw threading having a sectional shape shown in FIG. 3B when internal threading with the rolled tap  53  is completed. FIG. 4 illustrates a part enclosed with a circle IV FIG. 3B in an enlarged manner, to show flow of material caused by plastic deformation when the intermediate product  101  shown in FIG. 3A is internally threaded to obtain the nut  1  shown in FIG.  3 B. As shown by thick arrows in FIGS. 4, the original inner periphery from and or roots  26  of the shaft portion  102  is depressed due to formation of the female screw so that the material fills up spaces for defining threads to cause this flow. 
     The intermediate product  101  for the nut  1  according to this embodiment corresponds to that prepared by substantially arcuately depressing the region close to the inner peripheral end of the concave lead-in  31   a  of the intermediate product  131  for the conventional nut shown in FIG. 15A, formed by only a linear tapered portion, thereby forming the concave bent portion  105   a . The lead thread  105  of the intermediate product  101  has the concave bent portion  105   a  so that no concave part or concave groove is formed on a crest of a first thread  2   a  of the female screw threading adjacent to the concave lead-in of the completed nut  1  dissimilarly to the corresponding thread  31   b  of the intermediate product  131  for the conventional nut. In other words, the amount of depression of the concave bent portion  105   a  of the concave lead-in  105  of the intermediate product  101  and the ratios of the concave bent portion  105   a  and the linear tapered portion  105   b  are previously set so that no concave portion is formed on the first thread  2   a  of the female screw threading adjacent to the concave lead-in  105 . 
     The linear tapered portions  5   b  and  105   b  are left on the outer peripheral ends of the concave lead-in  5  and  105  for smoothly guiding the forward end of the rolled tap  53  into a screw hole for internally threading the intermediate product  101  or guiding the forward end of a bolt for engaging with the nut  1  formed with the female screw threading into the screw hole. When the concave bent portion  105   a  must be largely formed, however, the concave bent portion  105   a  may be provided along the overall region of the concave lead-in  105  of the intermediate product  101  for the nut  1  with no linear tapered portion  105   b . The angle of the linear tapered portions  5   b  and  105   b , i.e., the angle denoted by θ in FIG. 1, is preferably set at 118° to 120° slightly smaller than the angle formed by the forward end of the rolled tap  53 . 
     When a bolt  3  is inserted in the nut  1  according to the embodiment formed in the aforementioned manner as shown in FIGS. 5A and 5B, a thread on the forward end of the bolt  3  reliably engages with a first troush or root  2   b  of the female screw threading without hitting against the first thread  2   a  of the female screw threading having no concave part on its crust. Even if threads other than the thread  2   a  of the female screw threading adjacent to the concave lead-in  5  have concave parts CP 2 , therefore, crests of a male screw threading of the bolt  3  slide along the roots of the female screw threading of the nut  1  to be prevented from hitting against the concave parts, i.e., from cross threading. 
     Further, such a characteristic of the countersink is maintained that the diameter of the concave lead-in enlarges toward the opening end, whereby the female screw can be worked without damaging a function of the countersink facilitating discharge of chips resulting from internal threading with a tap. In addition, the concave lead-in  5  is shaped to define a substantially arcuate concave bent portion, not to damage another function of the conventional tapered countersink properly guiding the forward end of the bolt for engaging with the nut and automatically aligning the central axes of the nut and the bolt. Even if the bolt is inserted into the nut  1  in a slightly inclined state, the central axes of the nut  1  and the bolt are automatically aligned due to the guiding function of the concave lead-in  5 , so that the nut  1  and the bolt properly engage with each other. 
     The concave lead-in thread  105  of the intermediate product  101  for the nut  1  according to this embodiment can be readily formed through a press molding step by first molding the concave bent portion  105   a  with a punch  54  shown in FIG.  6 A and thereafter molding the linear tapered portion  105   b  with a punch  55  shown in FIG. 6B, for example. 
     While the nut  1  according to this embodiment has a structure obtained by applying the present invention to a full screw type hopper feed tee nut, the present invention is also applicable to a concave lead-in shape of a nut, such as a standard hexagon nut, other than the tee nut for similarly enabling internal threading causing no cross-thread by working with a rolled tap, as a matter of course. 
     Further, the shaft portions  2  and  101  of the nut  1  according to this embodiment and the intermediate product  101  therefor have step portions  6  and  106  having different inner and outer diameters, so that regions closer to the flange portion  3  with reference to the step portions  6  and  106  have shapes larger in inner and outer diameters than regions on ends opposite to the flange portion  3 . The unthreaded intermediate product  101  for the nut  1  has such a step portion  106  so that a female screw threading shape defined in a standard such as JIS can be accurately formed. In the screw hole of, the shaft portion  102 , only a prescribed region, including the concave lead-in  105 , of a region slated for internal threading can be worked in necessary accuracy while the remaining region can be slightly larger in inner diameter to attain allowance in working accuracy. It can be said that formation of the step portion  106  is also effective for working the concave lead-in  5  and  105  according to this embodiment in high accuracy. 
     (Second Embodiment) 
     The structure of a nut  11  according to a second embodiment of the present invention is now described with reference to FIGS. 7A to  8 B. Referring to FIGS. 7A to  8 B, components similar to those of the nut  1  according to the first embodiment are denoted by the same reference numerals, and redundant description is not repeated. 
     In this embodiment, the present invention is applied to the so-called rivet type tee nut already proposed by the inventor in U.S. Pat. No. 5,348,432. As shown in FIG. 7A, a hollow cylindrical shaft portion  12  of the nut  11  according to this embodiment includes a female screw threading forming portion  12   a  as well as a predeterminate caulked or caulkable portion  12   b  having a smaller thickness, dissimilarly to the nut  1  according to the first embodiment. The predeterminate caulked portion  12   b  has an inner diameter larger than that of the female screw threading forming portion  12   a  and an outer diameter substantially identical to that of the female screw threading forming portion  12   a . As shown in FIG. 7B, the nut  11  is provided on an end portion of the female screw threading forming portion  12   a  adjacent to the predeterminate caulked portion  12   b  with a concave lead-in  15 , which is formed by a substantially arcuate concave bent portion  15   a  and a linear tapered portion  15   b  in an arbitrary longitudinal section including the central axis of the shaft portion  12 , similarly to the first embodiment. 
     For the nut  11  having the aforementioned structure, a rolled tap  53  is screwed into an unthreaded intermediate product  111  from the side of the upper end of the nut  11 , i.e., the side of the concave lead-in  15  as shown in FIG. 8A, for forming a female screw threading. As shown in FIG. 8B, a concave lead-in  115  of the intermediate product  111  for the nut  11  includes a substantially arcuate concave bent portion  115   a  provided on the inner peripheral side of the upper end of a hollow cylindrical shaft portion  112  and a linear tapered portion  115   b  continuous with the concave bent portion  115   a  on the outer peripheral side in an arbitrary longitudinal section including the central axis of the shaft portion  112 . 
     With the concave lead-in  115  having the aforementioned shape, the intermediate product  111  is internally threaded with the rolled tap  53  in a mode basically similar to that described with reference to FIGS. 3A,  3 B and  4  in the first embodiment. The concave bent portion  115   a  of the concave lead-in  115  of the intermediate product  111  is worked into a predetermined proper shape so that the crest of a thread of the female screw threading, adjacent to the concave lead-in  15  has no concave groove when the intermediate product  111  is completely internally threaded with the rolled tap  53 . Consequently, when a bolt engages with the nut  11  from the side of the concave lead-in  15 , the nut  11  is prevented from cross-thread in a similar manner to the above description of the first embodiment with reference to FIGS. 5A and 5B. 
     The intermediate product  111  for the nut  11  according to this embodiment can be relatively readily formed without remarkably increasing the manufacturing cost by including a molding step with a punch described in relation to the first embodiment with reference to FIGS. 6A and 6B in the process of a progressive press molding step already proposed by the inventor in U.S. Pat. No. 5,503,596. 
     The shaft portions  12  and  112  of the nut  11  according to this embodiment and the intermediate product  111  therefor are provided with step portions  16  and  116  having different inner and outer diameters, so that regions closer to the flange portion  3  with reference to the step portions  6  and  106  have shapes larger in inner and outer diameters than regions on ends opposite to the flange portion  3 . The object and effect of the step portions  16  and  116  are similar to those of the step portions  6  and  106  in the first embodiment, while it can be said that the step portions  16  and  116  are more effective for the nut  11  according to this embodiment requiring higher working accuracy due to the predeterminate caulked portion  12   b  having a relatively small thickness. 
     (Third Embodiment) 
     The structure of a nut  21  according to a third embodiment of the present invention is now described with reference to FIGS. 9A to  10 B. Referring to FIGS. 9A to  10 B, components similar to those of the nut  1  according to the first embodiment are denoted by the same reference numerals, and redundant description is not repeated. 
     In this embodiment, the present invention is applied to a nut described in U.S. Pat. No. 5,618,144. While the nut  21  according to this embodiment is the so-called rivet type tee nut similarly to the nut  11  according to the second embodiment, the shape of a predeterminate caulked portion  22   b  thereof is different from that of the nut  11 . As shown in FIG. 9A, the predeterminate caulked portion  22   b  of the nut  21  according to this embodiment has an inner diameter larger than that of a female screw threading forming portion  22   a  and an outer diameter larger than that of the female screw threading forming portion  22   a . As shown in FIG. 9B, the nut  21  is provided on an end of the female screw threading forming portion  22   a  adjacent to the. predeterminate caulked portion  22   b  with a concave lead-in  25 , which is formed by a substantially arcuate concave bent portion  25   a  and a linear tapered portion  25   b  in an arbitrary longitudinal section including the central axis of a shaft portion  22 , similarly to the first and second embodiments. 
     For the nut  21  having the aforementioned structure, a rolled tap  53  is screwed into an unthreaded intermediate product  121  from the side of the upper end of the nut  21 , i.e., the side of the concave lead-in  25  as shown in FIG. 10A, for forming a female screw threading. A lead thread  125  of the intermediate product  121  for the nut  21  includes a substantially arcuate concave bent portion  125   a  provided on the inner peripheral side of the upper end of a hollow cylindrical shaft portion  122  and a linear tapered portion  125   b  continuous with the concave bent portion  125   a  on the outer peripheral side in an arbitrary longitudinal section including the central axis of the shaft portion  122 , as shown in FIG.  10 B. 
     With the concave lead-in  125  having the aforementioned shape, the intermediate product  121  is internally threaded with the rolled tap  53  also in a mode basically similar to that described with reference to FIGS. 3A,  3 B and  4  in the first embodiment. The concave bent portion  125   a  of the concave lead-in  125  of the intermediate product  121  is worked into a predetermined proper shape so that the crest of a first  2 B thread of the female screw threading adjacent to the concave lead-in  25  has no concave groove when the intermediate product  121  is completely internally threaded with the rolled tap  53 . 
     Consequently, when a bolt engages with the nut  21  from the side of the lead thread  25 , the nut  21  is prevented from cross-thread in a similar manner to the above description of the first embodiment with reference to FIGS. 5A and 5B. 
     The shaft portions  22  and  122  of the nut  21  according to this embodiment and the intermediate product  121  therefor are also provided with step portions  26  and  126  having different inner and outer diameters, so that regions closer to a flange portion  3  with reference to the step portions  26  and  126  have shapes larger in inner and outer diameters than regions on ends opposite to the flange portion  3 . The object and effect of the step portions  26  and  126  are similar to those of the step portions  6  and  106  in the first embodiment or the step portions  16  and  116  in the second embodiment. 
     (Fourth Embodiment) 
     The structure of an intermediate product  141  for a nut  41  according to a fourth embodiment of the present invention and that of the nut  41  prepared by internally threading the intermediate product  141  with a rolled tap are now described with reference to FIGS. 11A and 11B. 
     As shown in FIG. 11A, a concave lead-in  145  provided in the vicinity of the forward end of a shaft portion  142  of the intermediate product  141  for the nut  41  according to this embodiment includes a concave depressed portion  145   a  having a bottom substantially horizontal along the overall periphery and a peripheral side perpendicular thereto and a linear tapered portion  145   b  continuous with the concave depressed portion  145   a  on the outer peripheral side. A female screw threading is formed in a screw hole of the intermediate product  141  with a rolled tap, to obtain the nut  41  according to this embodiment having the structure shown in FIG. 11B. A concave lead-in  45  in the vicinity of the forward end of a shaft portion  42  of the nut  41  according to this embodiment includes a concave depressed portion  45   a  having a bottom substantially horizontal along the overall periphery and a peripheral side perpendicular thereto and a linear tapered portion  45   b  continuous with the concave depressed portion  45   a  on the outer peripheral side. 
     Also when forming the concave depressed portion  145   a  of the lead thread  145  of the intermediate product  141  so that a corner part is substantially right-angled, the crest of a first thread  42   a , adjacent to the concave lead-in  45 , of the female screw threading formed with the rolled tap can be convexly formed to include no concave groove, as shown in FIG.  11 B. Thus, it is possible to prevent cross-threading caused by a thread of a bolt hitting against a concave groove of the crest of the threading of the nut  41 . Further, the female screw threading can be worked without damaging still another function of the countersink facilitating discharge of chips resulting from internal threading with the tap. 
     (Fifth Embodiment) 
     The structure of an intermediate product  151  for a nut  51  according to a fifth embodiment of the present invention and that of the nut  51  prepared by internally threading the intermediate product  151  with a rolled tap are now described with reference to FIGS. 12A and 12B. This embodiment corresponds to a nut prepared by replacing the concave lead-in of the rivet type nut according to the second embodiment shown in FIGS. 7A to  8 B with a concave lead-in having a shape similar to that in the fourth embodiment. 
     As shown in FIG. 12A, a concave lead-in  155  defining an end, closer to a portion  152   b  slated for caulking, of a portion  152   a  slated for formation of a female screw threading in a shaft portion  152  of the intermediate product  151  for the nut  51  according to the present invention includes a concave depressed portion  155   a  having a bottom substantially horizontal along the overall periphery and a peripheral side perpendicular thereto and a linear tapered portion  155   b  continuous with the concave depressed portion  155   a  on the outer peripheral side. A female screw threading is formed in a screw hole of the intermediate product  151  with a rolled tap, to obtain the nut  51  according to this embodiment having the structure shown in FIG. 12B. A concave lead-in  55  of the nut  51  according to this embodiment includes a concave depressed portion  55   a  having a bottom substantially horizontal along the overall periphery and a peripheral side perpendicular thereto and a linear tapered portion  55   b  continuous with the concave depressed portion  55   a  on the outer peripheral side. 
     Also when forming the concave depressed portion  155   a  of the concave lead-in  155  of the intermediate product  151  so that a corner portion is substantially right-angled, the crest of a first thread  52   a , adjacent to the concave lead-in  55 , of the female screw threading formed with the rolled tap can be convexly formed to include no concave groove, as shown in FIG.  12 B. Thus, it is possible to prevent cross-threading caused by a thread of a bolt hitting against a concave groove of the crest of the threading of the nut  51 . Further, the female screw, can be worked without damaging the function of the countersink facilitating discharge of chips resulting from internal threading with the tap. 
     While the concave lead-in has the linear tapered portion continuous only on the outer peripheral side of the concavely depressed portion in each of the aforementioned embodiments, a linear tapered portion continuous with the inner peripheral side of the concave depressed portion may be further provided on the inner peripheral end of the concave lead-in. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.