JP5065735B2 - Self-tapping screw - Google Patents

Description

  The present invention is a self-tapping screw that is screwed in while forming a female screw in a pilot hole that is formed in advance in a workpiece. Concerning suppressed self-tapping screws.

  In automatic assembly work with many screw tightening operations, many self-tapping screws are used in which screws are screwed directly into prepared holes formed in the workpiece. By screwing such a self-tapping screw into the pilot hole of the workpiece, the female screw is screwed into the pilot hole while being formed by plastic deformation. However, a relatively large contact resistance is generated between the female screw of the pilot hole and the thread of the screw. As a result, a so-called seizure phenomenon occurs frequently in the middle of screwing in a screw thread in which the cross section perpendicular to the axis of a normal leg is circular. Further, in this screw, the thread of the incomplete thread at the tip of the leg is as shown in the thread development view of FIG. 10, and thus the thread crest angle (α) is constant and the width of the thread base (L). Since the screw thread becomes narrower and the height of the screw thread becomes lower as it reaches the tip of the leg part, the screw thread enters the work piece when forming the female thread, so that relatively long and large shavings are likely to be generated. In order to solve these problems, the self-tapping screw, which is widely used at present, has a triangular cross section perpendicular to the axis in order to reduce the contact resistance with the workpiece as much as possible when forming an internal thread and to enable screwing into the pilot hole with low torque. Some have become.

  In such a screw, as shown in FIG. 11, the tip height of the thread 110 is simply slightly lower than the diameter of the pilot hole so that the pilot hole 121 can be easily inserted at the beginning of screwing, and this is screwed in. Therefore, the contact resistance generated between the thread 110 and the workpiece 120 is not sufficiently reduced and the contact between the thread 110 and the pilot hole 121 is stable. do not do. For this reason, the screw 101 is tilted and screwed in the tilted state. Depending on the material of the workpiece 120, a seizure phenomenon may occur, and the screw may be screwed only halfway. Therefore, there is a problem that a large amount of molding waste of the work 120 is generated.

Therefore, a self-tapping screw as shown in Japanese Patent No. 3046746 is being used in order to solve such a problem as much as possible. As shown in FIG. 12, the cross section perpendicular to the axis of the leg 203 has a hexagonal shape, and a circular thread is formed following the head, followed by a special non-circular thread. Furthermore, a special non-circular taper threaded portion 213 is formed continuously therewith. The special non-circular threaded portion and the special non-circular tapered threaded portion 213 have a trough 211 having a circular cross section over the entire region, and the trough 211 of the special non-circular tapered threaded portion 213 has its diameter reaching the tip. It is getting smaller. Moreover, as shown in FIG. 13, the outer diameter of the screw thread has a so-called hexagonal cross-sectional shape in which six highest portions 214 appear in one winding of the screw, and such a tapping screw is screwed into the pilot hole. Therefore, the initial screwing torque is reduced, and the work can be easily tapped.
Japanese Patent No. 3046746

  However, when such a tapping screw is screwed into the pilot hole of the workpiece, the part that first contacts the pilot hole in the initial stage of screwing is the outer peripheral part of the screw thread, and this part is the ridge line part having a cross-sectional angle shape. When screwing with an automatic screwing machine, the screw tends to be inclined with respect to the driver bit. In particular, when the screw thread has a rough pitch, the inclination of the screw is prominent and oblique tightening occurs. In addition, since the tapped blade is formed at the highest part of this thread, the workpiece's pilot hole is plastically machined and scraped, so a lot of molding and shavings of the workpiece are generated. When used for assembling parts, electronic devices, etc., there is a risk that these scraps fall on the wiring of a printed circuit board, etc., the wiring is short-circuited, and thereby the device is ignited. In addition, since there is a partial cutting action in this way, depending on the material of the workpiece, there are many cases of relatively long streaky scraps and large chips, and there is a risk of short-circuiting the wiring. It is increasing. Furthermore, since this thread valley has a circular cross section at right angles to the axis, the locking of the screw is only due to the contact resistance between the flank surface of the screw and the flank surface of the female thread caused by the axial force of the screw. When the force decreases even a little, the tapping screw is loosened, and there are problems such as a decrease in fastening force.

  An object of the present invention is to obtain a self-tapping screw which eliminates such problems and reduces the inclination of the screw to be screwed in and eliminates the generation of long and large molding waste and shavings.

An object of the present invention is to provide a self-tapping screw in which a leg is screwed into a pilot hole formed in a workpiece, and the height of the thread of the leg gradually decreases as the tip of the leg is reached. The width of the base portion of the thread portion becomes wider as it reaches the tip of the leg portion, the width of the ridge line of the screw thread is constant from the rear end to the tip of the leg portion, and the flank surface of the screw thread is formed in a curved surface. This is accomplished by providing a self-tapping screw characterized in that

Further, the width of the ridge line of the thread may be formed so as to become narrower from the rear end to the front end of the leg portion. Furthermore, it is desirable that the ridge line of the thread has a peripheral surface with a predetermined width.

Further, it is desirable that the leg portion has a substantially triangular shape in a cross section perpendicular to the axis thereof. Furthermore, it is desirable that the ridge line of the thread is equal to the outer peripheral surface of the valley at the tip of the leg.

  According to the present invention, in the initial stage of screwing when the tapping screw is screwed into the pilot hole of the workpiece, the part that first contacts the pilot hole is the screw thread outer peripheral part, but the ridge line part that forms the screw thread outer peripheral part is Since the flank slope is relatively flat and the slope of the flank surface decreases as it reaches the tip of the leg, and the base of the flank surface that forms the thread is wider toward the tip of the screw, contact with the pilot hole When the area is increased and the screw is fitted in the pilot hole, the inclination of the screw is reduced with respect to the center of the pilot hole, and the screwing operation is facilitated because the screw is positioned substantially on the same line with respect to the pilot hole. In particular, in the assembly work of many parts, when screwing continuously with an automatic screw tightening machine, the screwing torque is stable, and even if the screw thread has a rough pitch, the inclination of the screw is greatly reduced and the oblique screwing is eliminated. . In addition, even if this tapping screw is used for assembling electrical parts or electronic devices, no large molding waste or shavings are generated in the screwing operation of this tapping screw. This eliminates the danger of short-circuiting the wiring and igniting the device. In addition, since the internal thread is not cut and formed, it is suitable for use in electronic and electrical equipment. Moreover, since the apex angle on the screw tip side is obtuse and gradually becomes sharper toward the fastening part located on the screw head side in this way, the material of the material to the screw apex part at the time of thread rolling is processed. The flow becomes smooth and the thread itself is not easily cracked.

  Furthermore, since the thread trough portion has a substantially triangular cross-section at right angles to the axis, the crest portion of the internally deformed female screw is deformed even if the axial force generated at the end of the tapping screw is lowered. Since it slightly bulges between the apexes of the valleys of the screw, even if a loosening action occurs on the tapping screw, a sufficient loosening prevention action is maintained. Moreover, the width of the base of the screw thread is wider at the tip of the screw, and the peripheral surface of the thread ridge line has an arc-shaped cross section, so that it can be easily re-fitted to the same female screw at the beginning of screw re-threading. It scoops and does not form double female threads. In addition, since the width of the peripheral surface of the thread ridge line becomes narrower as it reaches the tip of the leg portion, it is relatively easy to bite into the prepared hole of the workpiece. In addition, since the peripheral surface of these thread ridge lines is shaped to be equal to or close to the outer peripheral surface of the valley at the tip of the leg, it is unique in that the inclination of the screw is more reliably eliminated. The effect is obtained.

  Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 3, reference numeral 1 denotes a self-tapping screw (hereinafter referred to as a tapping screw) comprising a head 2 and a leg 3 integral with the head 2 and having a thread 10 on the outer periphery. The head 2 of the tapping screw 1 has a cross-conical groove-shaped drive hole that constitutes a drive part that can be engaged with a driver bit (not shown) that is positioned on the extension of the center line of the leg 3. 4 is formed. This drive part is a drive hole 4 in the shape of a cross-conical groove, but in addition to this, for example, a square, hexagonal shape or other groove or hole shape may be used. It is not limited to.

  Further, as shown in FIG. 3, the leg 3 formed integrally with the head 2 has a smaller diameter than the head 2 and an axially perpendicular cross section formed in a substantially triangular shape. Molded with a predetermined pitch. The screw thread 10 is formed with an acute angle in the cross section perpendicular to the lead wire, the pitch of the screw thread 10 is relatively large, and the interval between the valley portions 11 of the tapping screw 1 is a normal small screw. It is formed so as to be relatively wider than the interval. The leg portion 3 is configured such that a fastening portion 12 is formed on the head side and a fastening guide portion 13 is formed on the distal end side to guide the fastening portion 12 to be screwed into a prepared hole 21 formed in advance on the workpiece 20. The outer periphery of the fastening portion 12 is parallel to the axis on the plane. On the other hand, the fastening guide portion 13 continuing to the fastening portion 12 has a lower thread 10 as it reaches the tip of the leg portion 3 as shown in FIG. That is, as shown in FIG. 9, the tip is formed so that the thread height becomes lower as it reaches the tip so as to easily enter the pilot hole 21 of the workpiece 20 when screwed.

  The fastening portion 12 and the fastening guide portion 13 have a substantially triangular cross section at the axis, and the trough portion 11 is also formed in a substantially triangular shape. The trough portion 11 is an axial direction along the center line of the leg portion 3. In the same cross section, the distance (t) from the center of the leg 3 to the outer periphery of the trough 11 has a substantially constant distance over the entire length thereof. Further, as shown by a two-dot chain line in FIG. 4, the thread of the fastening portion 12 is inclined along the thread lead line direction and parallel to each of the leading flank surface 14 and the follower flank surface 15. The summit angle (α) is also constant.

  On the other hand, the thread 10 of the fastening guide 13 located on the distal end side of the leg 3 is spirally formed with the ridge line of the thread 10 toward the distal end of the leg 3 with respect to the valley 11. It is formed so as to gradually become lower. Specifically, as shown in FIG. Furthermore, the width (L) of the base portion of the screw thread 10 of the fastening guide portion 13 is wider than the width (L1) on the fastening portion 12 side, and the width (Ln) on the distal end side of the leg portion 3 is wider. In other words, the width (L) of the base portion increases as the tip of the leg 3 is reached, and the base of the flank surfaces 14 and 15 sandwiching the valley 11 between the adjacent screw threads 10 and 10 is the tip of the leg 3. As shown in FIG. 5, the bases of the flank surfaces 14, 15 with the thread 10 being low and the valley 11 sandwiched between the bases of the legs 3 are formed. As you reach, you will approach each other. For this reason, as shown by a two-dot chain line, the inclination angle of the flank surfaces 14 and 15 becomes loose, and the screw thread apex angle (α) gradually increases and becomes an acute angle to an obtuse angle in the middle. That is, the apex angle (α) on the head side is large as (α1)... (Αn), and the base width (L) is as (L1). It is getting wider. For this reason, in the re-screwing of the screw, it is easy to follow the already formed female screw, and the generation of chips is reduced.

  On the other hand, a flat peripheral surface 16 having a predetermined width is formed in the ridge line portion of the screw thread 10 from the fastening portion 12 to the fastening guide portion 13 as shown in FIGS. Further, the peripheral surface 16 is not limited to a flat surface. Instead, as shown in FIG. 6, the cross section perpendicular to the lead wire of the screw thread 10 may be an arc shape. By doing so, even if the tapping screw 1 is once loosened and re-screwed, the crushing of the female screw is further reduced. In this case, the width of the peripheral surface 16 of the fastening guide portion 13 may be constant. Alternatively, the peripheral surface 16 of the fastening guide portion 13 is added to the surface shape as shown in FIG. It is good also as a structure which makes this width | variety narrow gradually as it reaches the front-end | tip of the leg part 3, and makes the width | variety (L) of the base part of the screw thread 10 gradually wide like (L1) ... (Ln). . By doing so, the initial biting into the pilot hole 21 of the workpiece 20 becomes smooth, and the material of the workpiece 20 is gradually expanded in the female screw formation in the screwing work, and the screwing torque is rapidly increased. Can be suppressed. And the peripheral surface 16 of this thread ridgeline is formed so that it may become equal to the outer peripheral surface of the trough part 11 in the front-end | tip of the leg part 3, and this can eliminate the inclination of the tapping screw 1 in the initial stage of screwing more reliably. It has become. Further, depending on the diameter of the pilot hole, the peripheral surface 16 of the thread ridge line may only be close to the outer peripheral surface of the valley portion 11, and a sufficient effect can be obtained even if it is not always equal to the outer peripheral surface.

  In such a configuration, as shown in FIG. 9, when the tapping screw 1 is screwed into a prepared hole 21 formed in advance in the work 20, the tip of the fastening guide portion 13 enters the prepared hole at the initial stage of screwing. At this time, as shown in FIG. 1, the tip of the fastening guide portion 13 of the screw has a height of the screw thread 10 substantially equal to the valley portion 11. In a state where the 10 flat peripheral surfaces 16 are close to surface contact and in some cases, some of the corners of the pilot holes 21 come into contact with some of the leading flank surfaces 14 of the thread 10. In addition, since there are a maximum of three contact points, the driver bit that engages with the drive hole 4 of the screw 1, the tapping screw 1, and the pilot hole 21 can be substantially collinear.

  In particular, in order to position the tapping screw 1 substantially in the same straight line without inclining with respect to the driver bit and the pilot hole 21, the diameter of the pilot hole is slightly smaller than a standard size. Desirably, for example, in the plate-shaped workpiece 20 having a relatively small thickness, the diameter of the pilot hole is set to be slightly larger than the rotational locus of the three apexes, which is the highest portion of the substantially triangular valley portion 11 of the screw. Thus, the peripheral surface 15 at the tip of the screw thread 10 is likely to be in surface contact. As a result, the tip of the screw fastening guide portion 13 can come into contact with the inner peripheral surface of the pilot hole 21 in a state close to surface contact.

  Subsequently, when the driver bit rotates from this state, the tapping screw 1 is screwed in while forming a female screw in the prepared hole 21 of the workpiece 20. In this screwing operation, as shown in FIG. 7, the leg 3 has a substantially triangular cross section at right angles, so that the contact resistance generated during screwing gradually increases, and a sudden screw torque increase is avoided. Is done. On the other hand, at the end of the screwing operation, the deformed material generated at the time of plastic deformation of the female screw into the pilot hole 21 moves so as to be pushed out in the direction of the valley portion 11 between the threads of the leg portion 3. Fit in between. Thereby, even if a loosening action occurs in the tapping screw 1 after the screwing is finished, the loosening is limited.

  Further, by forming the peripheral surface 16 of the screw thread 10 in a circular arc shape and screwing the tapping screw 1 while forming the female screw in the pilot hole 21, there is a possibility of causing cracks in the valleys of the female screw at the time of plastic deformation of the female screw. And no slender thread-like debris is generated. In addition, when the tapping screw 1 once screwed is loosened and re-screwed, it can be smoothly fitted into the previously formed female screw, and the damage of the female screw can be reduced in the re-screwing. In this embodiment, the tapping screw 1 having a substantially triangular axial cross section has been described. However, the present invention is not limited to this, and the screw thread of the fastening guide portion 13 is not limited to this. A similar effect can be obtained if the apex angle of 10 increases in shape as it reaches the tip of the leg.

It is an enlarged principal part front view which shows embodiment of this invention. 1 is an overall front view of a self-tapping screw according to the present invention. FIG. 3 is a bottom view of FIG. 2. It is an expanded sectional view which shows the screw thread of a fastening part. It is an expanded sectional view which shows the screw thread of a fastening guide part. It is an expanded sectional view showing other embodiments of the present invention. It is an expanded-axis perpendicular | vertical sectional view which shows the state which screwed this invention in the workpiece | work. It is a screw thread development view of a fastening guide part showing a modification example of the present invention. It is a partial cross section front view which shows the state which engaged this invention with the pilot hole of the workpiece | work. It is a screw thread expansion view of the incomplete thread part which shows the prior art example of this invention. It is a partial cross section front view which shows the state which engaged the prior art example with the pilot hole of the workpiece | work. It is an expanded sectional view showing other conventional examples of the present invention. FIG. 13 is a development view per one screw thread of another conventional example shown in FIG. 12.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Self-tapping screw 2 Head 3 Leg 4 Drive hole 10 Screw thread 11 Valley part 12 Fastening part 13 Fastening guide part 14 Advance side flank face 15 Follow-up side flank face 16 Peripheral face 20 Workpiece 21 Pilot hole

Claims (5)

In self-tapping screws in which legs are screwed into pilot holes formed in the workpiece,
The thread height of the leg gradually decreases as it reaches the tip of the leg,
The width of the base of the thread becomes wider as it reaches the tip of the leg,
The width of the ridgeline of the thread is formed constant from the rear end to the tip of the leg,
A self-tapping screw, wherein the flank surface of the thread is formed into a curved surface .   2. The self-tapping screw according to claim 1, wherein a width of the ridge line of the screw thread is formed so as to become narrower from a rear end to a front end of the leg portion.   The self-tapping screw according to claim 1, wherein the ridge line of the thread has a peripheral surface with a predetermined width.   The self-tapping screw according to any one of claims 1 to 3, wherein the leg portion has a substantially triangular shape in a cross section perpendicular to the axis thereof.   The self-tapping screw according to any one of claims 1 to 4, wherein a ridge line of the screw thread is equal to an outer peripheral surface of the trough at the tip of the leg.

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