Self-tapping screw and fastening structure using same

Abstract

Provided are: a self-tapping screw with which fastening is easy and which is resistant to loosening; and a fastening structure using the self-tapping screw. A double start self-tapping screw 10 includes a first screw thread 13 and a second screw thread 14, and is screwed into and fastened in a pilot hole 21 of a mating material 20. The outer diameter of the second screw thread is equal to or less than the inner diameter of the pilot hole of the mating material.

Description

Tapping screw and fastening structure using same

[0001] The present invention is a tapping screw used for attaching parts of precision equipment such as automobiles, home appliances, electronic equipment parts, office equipment such as copying machines, optical equipment such as digital cameras, and mobile phones.

[0002] Conventionally, tapping screws, particularly those used for materials such as soft metals and synthetic resins, include, for example, multiple-start threads for preventing loosening (see Patent Document 1). When the multiple-start thread for preventing loosening is threaded into a mounting hole 11 in a workpiece 10, the first thread portion 1 and the second thread portion 2 each cut a spiral groove (female thread) on the inner circumferential surface of the mounting hole 11, and then enter the mounting hole 11 in the workpiece 10, completing the fastening process.

[0003] Patent No. 5455404

[0004] However, as is clear from Figures 3 and 4 of Patent Document 1, a portion of the workpiece 10 extruded from the inner circumferential surface of the mounting hole 11 in the workpiece 10 into the first thread portion 1 does not fully fill the valley formed between the first thread portion 1 and the second thread portion 2. This results in a large gap between the workpiece 10 and the multiple-start thread for preventing loosening. As a result, there is no sufficient contact area between the multiple-start thread for preventing loosening and the workpiece 10, which results in a problem of insufficient anchoring effect and the desired loosening prevention effect. In view of the above problems, an object of the present invention is to provide a tapping screw that is easy to tighten and difficult to loosen, and a fastening structure using the same.

[0005] In order to solve the above-mentioned problems, the tapping screw according to the present invention is a double-thread tapping screw that has a first thread portion and a second thread portion and is fastened by being threaded into a prepared hole in a mating material, and is configured so that the outer diameter of the second thread portion is equal to or smaller than the inner diameter of the prepared hole in the mating material.

[0006] According to the present invention, a portion of the mating material extruded into the first thread portion fills the valley formed between the first thread portion and the second thread portion and comes into contact with the second thread portion. This increases the contact area between the support member and the tapping screw, increasing the frictional force and making the screw less likely to loosen.

[0007] In an embodiment of the present invention, the flank surface of the first thread portion may be bent in two stages. According to this embodiment, if the mating material is, for example, a resin material, the flow of the resin material is improved and the base of the first thread portion is thicker, making it less susceptible to twisting and improving mechanical strength.

[0008] In another embodiment of the present invention, a fine groove may be provided along the crest of the second thread portion. According to this embodiment, the surface area of ​​the second thread portion is increased, and the contact area with a part of the mating material is increased, so that the frictional force is increased and the tapping screw becomes less likely to loosen.

[0009] In another embodiment of the present invention, a portion of the mating material extruded into the first thread portion may be filled so as to penetrate between adjacent first thread portions and come into contact with the second thread portion. According to this embodiment, the portion of the mating material that penetrates between adjacent first thread portions comes into contact with the second thread portion, thereby increasing the frictional force and making the tapping screw less likely to loosen.

[0010] In another embodiment of the present invention, of the gap formed between the valleys located between adjacent first thread portions and the inner circumferential surface of the pilot hole of the mating material, 65% or more of the gap may be filled with a portion of the mating material extruded into the first thread portion. According to this embodiment, the portion of the mating material comes into contact with the second thread portion, thereby increasing the contact area and the frictional force, making loosening more difficult.

[0011] The fastening structure of the tapping screw according to the present invention is configured such that the above-mentioned tapping screw is used to thread into a prepared hole in a mating member, thereby fastening a member to the mating member.

[0012] According to the present invention, a portion of the mating material extruded into the first thread portion fills the valley formed between the first thread portion and the second thread portion and comes into contact with the second thread portion. This increases the contact area between the support member and the tapping screw, increasing the frictional force and making the screw less likely to loosen.

[0013] 1. A perspective view showing a first embodiment of the tapping screw according to the present invention. 1. A perspective view of the tapping screw shown in FIG. 1, seen from a different angle. 2. A partially enlarged perspective view of the tapping screw shown in FIG. 1. 3. A partially enlarged longitudinal sectional view of the tapping screw shown in FIG. 1. 4. A partially enlarged transverse sectional view of the tapping screw shown in FIG. 1. 5. A perspective view of a second embodiment of the tapping screw according to the present invention. 6. A perspective view of the tapping screw shown in FIG. 7, seen from a different angle. 7. A partially enlarged perspective view of the tapping screw shown in FIG. 7. 8. A partially enlarged longitudinal sectional view of the tapping screw shown in FIG. 7. 9. A partially enlarged transverse sectional view of the tapping screw shown in FIG. 7. 10. A partially enlarged sectional view of the tapping screw shown in FIG. 7. 11. A partially enlarged sectional view of a modified version of the tapping screw shown in FIG. 7. 12. A partially enlarged sectional view of another modified version of the tapping screw shown in FIG. 7. 13. A partially enlarged perspective view for explaining a method for measuring make-up torque and loose-tightening torque according to an embodiment. 14. A longitudinal sectional view of the partial perspective view shown in FIG. 15. 15. A graph for explaining the relationship between the measurement method of make-up torque using the measurement method shown in FIG. 15 and the measurement results. 16. A graph for explaining the relationship between the measurement method of loose-tightening torque using the measurement method shown in FIG. 15 and the measurement results. 27 is a partial perspective view for explaining a method for measuring make-up torque and loose-tightening torque according to an example. FIG. 28 is a longitudinal sectional view of the partial perspective view shown in FIG. 19. FIG. 29 is a graph for explaining the relationship between the measurement results and the method for measuring make-up torque using the measurement method shown in FIG. 19. FIG. 29 is a graph for explaining the relationship between the measurement results and the method for measuring loose-tightening ...30 is a photograph showing samples of Example 1 and Comparative Example 1. FIG. 31 is a photograph showing the measuring device for Example 1 and Comparative Example 1. FIG. 32 is a table showing the results of a screw-in test according to Example 1 and Comparative Example 1. FIG. 33 is a cross-sectional photograph showing the results of a screw-in test according to Example 1. FIG. 34 is a partially enlarged cross-sectional photograph of the cross-sectional photograph shown in FIG. 27. FIG. 35 is a cross-sectional photograph showing the results of a screw-in test according to Comparative Example 1. FIG. 36 is a partially enlarged cross-sectional photograph of the cross-sectional photograph shown in FIG. 36. FIG. 37 is a partially enlarged cross-sectional photograph of the cross-sectional photograph shown in FIG. 36. FIG. 38 is a photograph showing the measuring device for Example 2 and Comparative Example 2.1 is a table showing the results of a screw-in test according to Example 2 and Comparative Example 2. FIG. 2 is a table showing the results of a tightening / unfastening test according to Example 2 and Comparative Example 2. FIG. 3 is a cross-sectional photograph showing the results of a screw-in test according to Example 2. FIG. 4 is a cross-sectional photograph showing the results of a screw-in test according to Comparative Example 1. FIG. 5 is a table showing the results of a screw-in test according to Example 3 and Comparative Example 3.

[0014] Embodiments of a tapping screw and a fastening structure using the same according to the present invention will be described with reference to the accompanying drawings of Figures 1 to 14. As shown in Figures 1 to 6, the tapping screw 10 according to the first embodiment is a double-start screw consisting of a head 11 and a shank 12, with the shank 12 having a first thread portion 13 and a second thread portion 14.

[0015] The shape of the first thread portion 13 may be, for example, a triangular cross section, an equilateral triangle cross section, an isosceles triangle cross section, or a roughly triangular cross section with two inclined surfaces as shown in FIG. 6 . The thread angle of the first thread portion 13 may be, for example, 30 to 50 degrees, preferably 45 degrees. If the angle is less than 30 degrees, the cross-sectional area of ​​the first thread portion becomes thin and is prone to fracture due to shear forces generated during the fastening operation. If the angle is greater than 50 degrees, the pushing force against the mating material becomes too large, which may destroy the mating material. In particular, the angle of the base of the first thread portion 13 may be 70 to 120 degrees, preferably 90 degrees (see FIG. 6 ). If the angle is less than 70 degrees, a large gap is likely to form, making it difficult to obtain the desired contact area. If the angle is greater than 120 degrees, the resistance of the mating material increases, requiring a large fastening force during the fastening operation. It goes without saying that the base of the first thread portion 13 does not have to be straight and may be curved. This allows the extruded mating material to flow more smoothly. The pitch of the first thread portion 13 should be 30 to 45% of the nominal diameter, preferably 34 to 42%. If it is less than 30%, a sufficient cross-sectional area cannot be secured for the female thread formed in the mating material, making the female thread prone to breakage. If it exceeds 45%, it is difficult to obtain the desired fastening force, and the thread is prone to loosening.

[0016] The shape of the second thread portion 14 may be, similar to the first thread portion, for example, a triangle in cross section, an equilateral triangle in cross section, an isosceles triangle in cross section, or a roughly triangle with two inclined surfaces. The thread angle of the second thread portion may be, for example, 30 to 90 degrees, preferably 45 degrees. If it is less than 30 degrees, the mechanical strength of the thread portion of the male thread will be reduced and it will be prone to fracture. If it is more than 90 degrees, it will be difficult for the second thread portion to contact the mating material, the desired contact area will not be obtained, and the anti-loosening effect will not be achieved. The outer diameter of the second thread portion 14 is preferably equal to or smaller than the inner diameter of the pilot hole of the mating material. If the outer diameter of the second thread portion 14 exceeds the inner diameter of the pilot hole of the mating material, the crest of the second thread portion 14 will bite into the inner circumferential surface of the pilot hole, increasing the screw-in torque. The outer diameter of the second thread portion 14 may be 73% to 83%, preferably 76% to 80%, of the outer diameter of the first thread portion 13. If it is less than 73%, a portion of the mating material pushed out by the make-up operation of the first thread portion 13 will be less likely to come into contact with the second thread portion 14, making it difficult to obtain the anti-loosening effect. If it exceeds 83%, a portion of the mating material will come into contact with the second thread portion before the make-up operation is completed, increasing the screw-in torque and making the make-up operation more difficult. The surface area of ​​the second thread portion 14 may be increased by making the surface uneven by blasting or the like.

[0017] The shape and outer diameter of the second thread portion 14 are preferably designed so that, when the first thread portion 13 bites into the inner circumferential surface of the pilot hole, a portion of the mating material 20 extruded by the first thread portion 13 fills the space formed between the first thread portion 13, the second thread portion 14, and the inner circumferential surface of the pilot hole. This is because the contact area between the extruded portion of the mating material and the first thread portion 13 and the second thread portion 14 increases, increasing the frictional force and making the screw less likely to loosen. As a result, there is an advantage in that a fastening structure that requires the same tightening torque as a simple single-start tapping screw and is more resistant to loosening can be obtained. More specifically, it is preferable that the volume of the portion of the mating material extruded by the first thread portion 13 from the inner circumferential surface of the pilot hole be equal to (100%) or 65% or more of the volume of the gap between the inner circumferential surface of the pilot hole and the surface of the shank 12. If the ratio is less than 65%, a portion of the extruded mating material will not be in sufficient contact with the second thread portion, resulting in a small frictional force and the desired anti-loosening effect not being obtained. For this reason, for example, for a tapping screw with a nominal diameter of 4 mm, the appropriate diameter of the prepared hole in the mating material made of synthetic resin is 3.25 mm ± 0.05 mm.

[0018] The mating material is, for example, a synthetic resin material such as ABS resin, but is not limited to a synthetic resin material alone and may be a synthetic resin material to which a reinforcing material such as carbon fiber has been added.Furthermore, the mating material is not limited to a synthetic resin material and may be a soft metal material such as aluminum or copper.

[0019] The inner diameter of the pilot hole of the mating material is determined by the outer diameter of the tapping screw 10, the shape of the first thread portion 13, etc., but may be 70 to 90%, preferably 75 to 87%, of the nominal diameter of the tapping screw 10. If it is less than 70%, the roots of the tapping screw will come into contact with the inner peripheral surface of the pilot hole, requiring a large tightening force for the tightening operation, and if it exceeds 90%, the desired anti-loosening effect cannot be obtained.

[0020] As shown in FIGS. 7 to 12 , the second embodiment is substantially similar to the first embodiment described above, except that the cross-sectional shape of the second thread portion 14 is trapezoidal and a narrow groove 15 is formed along the apex of the second thread portion 14. The same parts are designated by the same numbers and will not be described again. The thread angle of the first thread portion 13 according to this embodiment is 45 degrees. The second thread portion 14 according to this embodiment has narrow grooves 15 formed by juxtaposing threads with a thread angle of 45 degrees. Note that the second thread portion 14 may have narrow grooves 15 formed by juxtaposing threads with a thread angle of 60 degrees, as shown in FIG. 13 . Alternatively, the second thread portion 14 may have narrow grooves 15 formed by juxtaposing threads with a thread angle of 90 degrees, as shown in FIG. 14 . In the second embodiment, the first thread portion 13 may also have flank surfaces that are inclined in two stages, as in the first embodiment.

[0021] Next, with reference to Figures 15 to 18, various torques that occur when the tapping screw 10 is fastened into a pilot hole 21, which is a through-hole in a mating material 20, to fasten a workpiece 25, and various torques that occur when loosening the fastened tapping screw 10, will be described. In this embodiment, the mating material 20 refers to a material into which the tapping screw 10 is directly screwed to form a female thread. The inner diameter of the pilot hole refers to the diameter of the pilot hole 21 formed in the mating material 20 to form the female thread. The fastened material 25 refers to a material that is sandwiched between the head 11 of the tapping screw 10 and the mating material 20. The driving torque TD (Driving Torque) refers to the maximum value of torque required from the time the tapping screw 10 starts to form a female thread in the pilot hole 21 of the mating material 20 until it seats. Seating refers to the time when the head of the tapping screw comes into contact with the workpiece. The tightening breaking torque (TB) refers to the maximum torque generated before the female thread and / or male thread formed on the mating material 20 breaks. The tightening torque (TT) refers to the torque generated when the tapping screw 10 is screwed in. The appropriate tightening torque refers to the optimal tightening torque calculated from the results of a screwing test, and can be calculated, for example, using the following formula: appropriate tightening torque = TDmax + (TBmin - TDmax) x 0.5. The loosening torque (TL) refers to the maximum torque required to loosen the tapping screw 10 after it has been tightened with a specified tightening torque. The torque ratio refers to the minimum tightening breaking torque divided by the maximum screwing torque (= TBmin / TDmax). A large torque ratio allows for a wider torque setting range for the electric screwdriver, making work easier. In practice, a torque ratio of 2.5 or more is preferable. The loosening rate refers to loosening torque / tightening torque (= TL / TT) x 100. The thrust refers to the force that propels the tapping screw 10 in the axial direction when the tapping screw 10 is fastened. The clamp force CL refers to the force that tightens the fastened member 25 when the tapping screw 10, which has been extended in the axial direction, tries to contract. The fracture axial force (fracture CL) refers to the axial force at which the male and / or female threads are broken.

[0022] As shown in Figures 19 to 22, the various torques that occur when fastening a tapping screw 10 into a pilot hole 21, which is a blind hole, of a mating material 20 to fasten a workpiece 25, and the various torques that occur when loosening the fastened tapping screw 10 are the same as when fastening a tapping screw into a pilot hole 21, which is a through hole, so the same torques will be denoted by the same letters and will not be described again.

[0023] Various embodiments have been described in detail above with reference to the drawings, and various aspects of the present invention will be described below. Note that in the following description, reference numerals will also be used as examples.

[0024] The tapping screw 10 of the first aspect of the present invention is a double-thread tapping screw 10 that has a first thread portion 13 and a second thread portion 14 and is screwed into a pilot hole 21 of a mating material 20 for fastening, and is configured such that the outer diameter of the second thread portion 14 is equal to or smaller than the inner diameter of the pilot hole 21 of the mating material 20.

[0025] The tapping screw 10 of the second embodiment according to the present invention is configured such that the flank surface of the first thread portion 13 in the tapping screw 10 of the first embodiment is bent in two stages.

[0026] The tapping screw 10 of the third aspect of the present invention is configured such that a fine groove 15 is provided along the top of the second thread portion 14 in the tapping screw 10 of the first or second aspect.

[0027] The tapping screw 10 of the fourth aspect according to the present invention is the tapping screw 10 according to any one of the first to third aspects, wherein a part of the mating material 20 extruded into the first thread portion 13 penetrates between the adjacent first thread portions 13, 13 and fills so as to come into contact with the second thread portion 14.

[0028] The tapping screw 10 of a fifth aspect according to the present invention is the tapping screw 10 according to any one of the first to fourth aspects, characterized in that, of the gap formed between the valley portion located between adjacent first thread portions 13, 13 and the inner circumferential surface of the pilot hole 21 of the mating material 20, 65% or more of the gap is filled by a part of the mating material 20 extruded into the first thread portion 13.

[0029] The fastening structure of the tapping screw 10 of the sixth aspect of the present invention is configured such that the tapping screw 10 described in any one of the first to fifth aspects is screwed into a pilot hole 21 of a mating material 20, thereby fastening a fastened member 25 to the mating material 20.

[0030] Using the tapping screw according to Example 1 shown in FIG. 23 as a sample, various torques were measured using the measuring device shown in FIG. 24. Three samples (total) of tapping screws with a bind head and a nominal diameter of 4 mm, an effective length of 25 mm, and a pitch of 1.46 mm were used for Example 1. More specifically, the outer diameter of the first thread portion was 4.1 mm, the root diameter was 2.8 mm, the under-neck length was 25.3 mm, the ridge angle of the first thread portion was 45 degrees, and the height of the entire first thread portion was 0.6 mm. However, the angle at the base of the first thread portion from the root to one-third of the height was 90 degrees. The outer diameter of the second thread portion was 3.2 mm, the angle of the second thread portion was 45 degrees, and the height was 0.2 mm. The samples were zinc-plated and then trivalent chromate-treated.

[0031] The mating member 20 was made of ABS resin with a thickness of 25 mm and a pilot hole with a diameter of 3.2 mm. The edge of the pilot hole was countersunk to a depth of 0.4 mm. The fastened member 25 was made of cold-rolled steel plate (SPCC) with a thickness of 1.2 mm and a through hole with a diameter of 4.6 mm.

[0032] The load sensor constituting the measuring device was an M4 load cell 30 (manufactured by Kyowa Electronics Co., Ltd.) with a thickness of 8 mm and a maximum load of 10 kN. The effective thread engagement length was adjusted to 8 mm by appropriately using eight 1 mm thick washers and two 0.8 mm thick washers as load adjustment materials. The screw-in test and loosening test were performed using an electric screwdriver (manufactured by Atlascopco, ETD-ST-10-10) with an adjustable tightening torque (not shown) that had a torque sensor of 10 Nm, a tightening rotation speed of 300 rpm, and a thrust of 30 N.

[0033] A plurality of washers, a load cell, a plurality of washers, and a fastened member 25 were stacked above the pilot hole of the mating member 20. Then, sample tapping screws (a total of three) were screwed into the mating member 20 via an electric nut runner, and the screwing torque TD, tightening breakage torque TB, and breakage axial force CL were measured. The measurement results are shown in Figure 25. Note that all breakages occurred in the female threads.

[0034] In addition, the samples (3 in total) were tightened with an electric screwdriver adjusted to the appropriate tightening torque, and the tightening torque TT and tightening axial force CL were measured. The tightened tapping screws were then loosened with a testing machine to measure the loosening torque TL. The measurement results are shown in Figure 26.

[0035] Furthermore, the tapping screw was fastened to the mating member 20 and then cut, and the cross section of the fastened state was photographed. The photographed results are shown in FIG.

[0036] Next, the gap remaining around the second thread portion was measured based on Fig. 28, which is a partially enlarged version of the photograph in Fig. 27. The measured area was 0.0309 mm ２ Based on the same photograph, the gap between the inner peripheral surface of the pilot hole (shown by the vertical line in Figure 29) and the shaft portion was measured. The measured area was 0.1395 mm ２ From this result, the filling rate of the gap by the part of the mating material pushed out by the first thread portion was 77.8%. Comparative Example 1

[0037] The samples of Comparative Example 1 were tapping screws (three in total) with a pitch of 1.46 mm, manufactured in the same manner as in Example 1, except that the second thread portion provided in the sample of Example 1 was not formed. Measurements were made by conducting the same screw-in test and loosening test as in Example 1. The measurement results are shown in Figures 25 and 26, respectively. Note that all fractures occurred in the female thread.

[0038] Furthermore, the tapping screw was fastened to the mating member 20 and then cut, and the cross section of the fastened state was photographed. The photographed results are shown in FIG.

[0039] Next, the gap remaining around the second thread portion was measured based on Fig. 31, which is a partially enlarged version of the photograph in Fig. 30. The measured area was 0.0846 mm ２ Based on the same photograph, the gap between the inner surface of the pilot hole and the shaft was measured. The measured area was 0.1942 mm ２ From this result, it was found that the filling rate of the voids caused by the part of the mating material pushed out by the first thread portion was 56.4%. From this result, it was found that Example 1 had a larger filling rate and a larger contact area between the mating material and the shank.

[0040] As is clear from Figure 25, Example 1 consistently had a greater breaking axial force and a larger torque ratio than Comparative Example 1. In particular, the female thread of Example 1 was less likely to break, which indicated that the axial force of Example 1, and in particular the mechanical strength of the female thread, was high. Furthermore, it was found that Example 1 allowed for a wider adjustment range of the electric screwdriver than Comparative Example 1, making it easier to use.

[0041] As is clear from FIG. 26, the loosening rate of Example 1 was greater than that of Comparative Example 1, and it was found that Example 1 was less prone to loosening.

[0042] 28 and 29 showing Example 1, a portion of the mating material 20 extruded into the first thread portion 13 is in contact with the second thread portion 14. On the other hand, in Figures 31 and 32 showing Comparative Example 1, it can be seen that a portion of the mating material 20 extruded into the first thread portion 13 has moved between the first thread portion 13 and the first thread portion 14. However, it was found that a larger gap remains in the valley between the first thread portion 13 and the first thread portion 14 than in Example 1, and a contact area similar to that of Example 1 cannot be obtained.

[0043] The same tapping screws as those used in Example 1 were used as samples (three in total). Each sample was treated in the same manner as in Example 1, except that a lubricant was applied to it. The mating material was an aluminum plate (A5052) with a thickness of 3.0 mm and a pilot hole with a diameter of 3.2 mm, which was approximately the same as the outer diameter of the second thread portion 14 of the tapping screw, as shown in Figure 33. The fastened member was a cold-rolled steel plate (SPCC) with a thickness of 1.2 mm and a pilot hole with a diameter of 4.6 mm.

[0044] The samples of Example 2 were also subjected to a screwing test and a loosening test in the same manner as in Example 1. The measurement results are shown in Figures 34 and 35. All of the fractures occurred in the female threads.

[0045] As in Example 1, the mating member was cut in the fastened state in Example 2, and a photograph of the cross section of the fastened state was taken. The photographed results are shown in FIG. Comparative Example 2

[0046] The samples of Comparative Example 2 were tapping screws (three in total) manufactured in the same manner as in Example 2, except that the second thread portion provided in the sample of Example 2 was not formed. Then, a screw-in test and a loosening test were performed in the same manner as in Example 2. The test results are shown in Figures 34 and 35. Note that all fractures occurred in the female thread.

[0047] As in Example 2, a photograph was taken of the cross section of Comparative Example 2 in a fastened state. The photographed results are shown in FIG.

[0048] As is clear from Figure 34 showing the results of the screwing test, by comparing Example 2 and Comparative Example 2, the screwing torque TD of Example 2 is almost equal to the screwing torque TD of Comparative Example 2. Furthermore, the tightening breakage torque TB of Example 2 is almost equal to the tightening breakage torque TB of Comparative Example 2. It was also found that Example 2 has a larger torque ratio than Comparative Example 2. Therefore, it was found that Example 2 allows for a wider setting range of the electric screwdriver than Comparative Example 2 and is easier to use. As a result, it was found that the appropriate tightening torques of Example 2 and Comparative Example 2 are almost equal.

[0049] As is clear from FIG. 35 showing the results of the loosening test, the loosening rate of Example 2 was greater than the loosening rate of Comparative Example 2, and it was found that Example 2 was less prone to loosening than Comparative Example 2.

[0050] 36 showing Example 2, it was found that as the first thread portion 13 bites into the inner circumferential surface of the mating material 20, a portion of the extruded mating material 20 penetrates between the first thread portions 13, 13 and comes into contact with the second thread portion 14, resulting in a large contact area. On the other hand, FIG. 37 showing Comparative Example 2 also shows that as the first thread portion 13 bites into the inner circumferential surface of the mating material 20, a portion of the extruded mating material 20 penetrates between the first thread portions 13, 13. However, it was found that a larger gap remains in Comparative Example 2 than in Example 2, and the same contact area as in Example 2 cannot be obtained. Therefore, it was found that even though the mating material 20 is an aluminum plate, Example 2 is not only as easy to fasten as Comparative Example 2, but also more difficult to loosen than Comparative Example 2.

[0051] Three tapping screws identical to the tapping screw of Example 1 shown in Figure 23 were used as samples, and various torques were measured using the measuring device shown in Figure 24. More specifically, the outer diameter of the first thread portion was 4.06 mm, the root diameter was 2.82 mm, and the under-neck length was 25.3 mm. The ridge angle of the first thread portion was 45 degrees, and the height of the entire first thread portion was 0.6 mm. However, the angle at the base from the root to one-third of the height of the first thread portion was 90 degrees. The outer diameter of the second thread portion was 3.19 mm, the angle of the second thread portion was 45 degrees, and the height was 0.2 mm. The samples were zinc-plated and then trivalent chromate-treated. The mating member 20 was an 8 mm thick glass fiber-reinforced PPS resin material with a pilot hole of 3.35 mm diameter. A 1.6 mm thick cold-rolled steel plate (SPCC) with a through hole of 4.6 mm diameter was used for the fastened member 25. The other conditions were the same as in Example 1, and the screw-in test was carried out. The measurement results are shown in Figure 38. All fractures occurred in the female threads. Comparative Example 3

[0052] In Comparative Example 3, a sample was fabricated in the same manner as in Example 3, except that the outer diameter of the second thread portion was set to 3.48 mm, just like in Example 3. As a result, the top of the second thread portion directly bites into the inner peripheral surface of the pilot hole. A screw-in test was conducted in the same manner as in Example 3. The measurement results are shown in Figure 38. All of the fractures occurred in the female thread.

[0053] From the measurement results in Figure 38, it was found that the screwing torque TD was smaller in Example 3, in which the second thread portion did not bite into the inner surface of the pilot hole, than in Comparative Example 3, and that Example 3 was easier to fasten than Comparative Example 3.

[0054] From the above test results, it was found that in Examples 1, 2, and 3, even when the mating material was, for example, ABS resin, glass fiber-reinforced PPS resin, or aluminum, the outer diameter of the second thread portion was equal to or smaller than the inner diameter of the pilot hole in the mating material. Therefore, the crest of the second thread portion did not contact the inner circumferential surface of the pilot hole during the make-up operation, and a large make-up torque was not required during the make-up operation. On the other hand, after the make-up operation, a portion of the mating material extruded into the first thread portion entered between adjacent first thread portions and came into contact with the first thread portion and the second thread portion, increasing the frictional force and making the screw less likely to loosen. As a result, it was found that a tapping screw that was easy to make and less likely to loosen was obtained.

[0055] The tapping screw according to the present invention can be applied to hard and soft synthetic resin materials, as well as soft metal materials including aluminum, and can also be applied to inorganic materials.

[0056] REFERENCE SIGNS LIST 10 tapping screw 11 head 12 shank 13 first thread portion 14 second thread portion 15 narrow groove 20 mating member 21 pilot hole 25 fastened member 26 through hole 30 load cell

Claims

1. A double-thread tapping screw that has a first thread portion and a second thread portion and is threaded into a pilot hole in a mating material for fastening, characterized in that the outer diameter of the second thread portion is equal to or smaller than the inner diameter of the pilot hole in the mating material.

2. The tapping screw according to claim 1, characterized in that the flank surface of the first thread portion is curved in two stages.

3. A tapping screw according to claim 1 or 2, characterized in that a fine groove is provided along the top of the second thread portion.

4. A tapping screw as described in any one of claims 1 to 3, characterized in that a portion of the mating material extruded into the first thread portion penetrates between adjacent first thread portions and is filled so as to come into contact with the second thread portion.

5. A tapping screw as described in any one of claims 1 to 4, characterized in that of the gap formed between the valley portion located between adjacent first thread portions and the inner surface of the pilot hole of the mating material, 65% or more of the gap is filled by a portion of the mating material extruded into the first thread portion.

6. A fastening structure for a tapping screw, characterized in that a fastening member is fastened to a mating member by threading the tapping screw described in any one of claims 1 to 5 into a prepared hole in the mating member.

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