GB2040769A - Self-tapping screw - Google Patents

Abstract

A self-tapping screw has an elongated shank portion 10a which is tapered with thread spirals 161 and 162 on the surface thereof. A head portion 12 fixed to one end of the shank portion 10a has a recess 14 for the engagement and turning of the screw by means of a screw driver. The cross-sectional shape of the thread spirals is polygonal, specifically triangular with the sides of the triangular shape slightly convex in the radially outward direction. The screw is for fixing together two tape cassette halves, pieces of the case from the tapped hole remaining in the hole in the gaps B between the screw and the hole. <IMAGE>

Description

SPECIFICATION Self-tapping screw This invention relates to self-tapping screws, and provides a self-tapping screw which is conveniently utilised forfixing together the halves of a case for a cassette magnetic recording tape.

Generally speaking, the housing of a tape cassette consists of, as shown in Figure 1 of the accompanying drawings, a case or housing 1 having a plastics upper half-case 2 and a plastics lower half-case 3 held together by a plurality of screws 4, to 45, and reels (not shown) with a tape 5 wound thereon, which are held within the case 1. For connecting the two halves of the cassette case, screws with a spiral thread of self-tapping type, are utilised. With a self-tapping type screw, a nut is not necessary, as the screw engages directly with the plastics material of the case.

For assembling the two halves by the self-tapping type screws, those portions of one half-case which are to be tapped are provided with preliminary holes having a diameter which is slightly largerthan that of the root diameter of the thread but is slightly smaller than the outer diameter of the screw, while the portions of the mating half-case which correspond to the preliminary holes are provided with clearance holes the diameter of which is slightly larger than the outer diameter of the screws. The seif-tapping screw at first penetrates the large through hole until reaching the registering small preliminary hole. Then, as the screw is turned, the preliminary hole is tapped and the screw tightens up accordingly, so as to firmly fix together the two case halves.

The structure of the prior self-tapping screws 4, to 45 is shown in Figures 2A and 2B, in which Figure 2B is a cross-sectional view on the line A - A in Figure 1A. As shown in the drawing figures, a self-tapping screw has an elongated shank portion 10 which is substantially conical ortapered and is made of soft iron, and a head portion 12 fixed to one end of the shank portion 10. The head portion 12 has a recess 14for engagement by a screw driverforturning the screw. On the surface of the elongated shank portion 10 a spiral thread 16 is provided. The cross-sectional shape of the thread spiral is a circle both at the outside ofthe spiral and at the inside of the spiral as shown in Figure 2B. That is to say, the crosssectionai shape of the shank portion both at the root of the thread and at the outer crest of the thread is circular.

However, a prior self-tapping screw such as shown in Figures 2A and 2B has the disadvantage that microscopic shavings of the plastics material are produced by the tapping action and tend to move out of the tapped hole and to be deposited on the recording tape housed in the case, with a resultant deterioration of the electrical characteristics of the magnetic tape, and an increased number of dropouts. Also, when said shavings are deposited between the halves of the case, the outer thickness of the cassette magnetic tape case is changed.

It is an object, therefore, of the invention to overcome the foregoing disadvantages and limitations of a prior self-tapping screw by providing a new and improved form of self-tapping screw.

Another object of the invention is to provide a self-tapping screw which does not result in shavings being ejected from a tapped hole.

The above and other objects are attained by a self-tapping screw comprising at least an elongated shank portion which is substantially conical with a thread spiral on the surface thereof and a head portion fixed to one end of the shank portion, said head portion having a recess for engaging and turning the screw with a screw driver and the cross-sectional shape of said thread being polygonal.

The invention will now be further described with reference to the accompanying drawings which, in addition to said prior conventional self-tapping screw, illustrate by way of example a preferred embodiment of the invention. In the drawings: Figure 1 is a perspective view of a cassette tape case which is one application of the present invention; Figure 2A is a side view of the conventional self-tapping screw; Figure 2B is a cross-sectional view on the line A - A in Figure 2A; Figure SA is a side view of the blank of a self-tapping screw before forming of a spiral thread thereon; Figure 3B is a cross-sectional view on the line B - B of Figure 3A; Figure 4A is a side view of the embodiment of self-tapping screw according to the invention; Figure 4B is a cross-sectional view on the line C - C in Figure 4A; FigureS is an explanatory drawing of the effects of the self-tapping screw in accordance with the'inven- tion; and Figure 6 is a graph showing the relative sensitivity deviations of a cassette tape when the cassette case is assembled using the self-tapping screws of Figures 2A and 2B, and Figures 4A and 4B, respectively.

Figures 3A and 3B, and Figures 4A and 4B, show the manufacturing process and the form of the present self-tapping screw. First, the blank structure shown in Figures 3A and 3B is produced. In these figures, the reference numeral 10a indicates a bar or shank portion which is substantially conical or tapered, and the cross-section of which is substan- tially triangular as shown in Figure 3B. The polygonal cross-section, which in this case is triangular, is one of the most important features of the invention.

On the surface of the triangular-section shank portion 10a,a pair of spiral threads 161 and 162 are formed by a forging process as shown in Figure 4A.

This particular structure of the present self-tapping screw is obtained by preparing the forging die of the particular shape concerned. The formation that is provided by two spiral threads is also a preferred feature of the invention. Although the embodiment utilises the triangular cross-section described and illustrated, it should be appreciated that any polygonal cross-section can provide an effect similar to that of the triangular cross-section.

Because of the polygonal cross-section, a selftapping screw in accordance with the invention has the following advantages.

1) The tapping action orthe cutting of the plastics material is carried out by the corners or vertices of the polygonal self-tapping screw and, therefore, the tapping action is made stable. With a conventional self-tapping screw, the thread of the screw contacts uniformly with the inner surface of a hole provided in plastics material, and rubs the inner surface. Therefore, a correspondingly stable tapping action cannot be achieved with a prior self-tapping screw which has a circular cross-section. From this view-point, among various polygonal shapes the use of a shape that has fewer vertices is preferable, and the triangular shape is the best since it provides a smaller area in contact with the inner surface of the engaged hole in the plastics material.

2) The powdery shavings produced by the cutting action during tapping fill in spaces between the screw and the hole as shown in Figure 5. In Figure 5, the profile (c) shows the shape of the spiral thread of the self-tapping screw, and this shape is triangular as already mentioned. Preferably, each side of the section triangle is slightly convex in the radially outward direction as shown in Figure 5. The circle (b) shows the cross-section of a hole which is prepared in plastics material for the insertion of the selftapping screw. When screwing in the self-tapping screw having the profile (c) into the hole with the profile (b), three apical corners (d) of the screw cut the inside wall of the hole, and provide the circular trench shown by the circle (a). In this case, it will be appreciated that there are-provided gaps (A) and (B) between the self-tapping screw and the hole, because of the triangular cross-section of the screw.

The area (A) shows the gap between the wall of the hole and the surface of the spiral thread of the self-tapping screw, and the area (B) shows the gap which is dug by the self-tapping screw. That is to say, the shavings produced by the corners (d) of the screw can fill the spaces provided by the gaps (A) and (B). The shavings in these spaces form a detent, thus providing a self-locking action, against loosening of the screw. Also, such powdery shavings do not move out of the hole, so that there is no deposit ofthe shavings on the magnetic recording tape, and no shaving enter between the two halves of the cassette tape case.

It should be noted that a prior self-tapping screw which has a circular cross-section does not provide any space between the screw and the hole; thus, shavings must be ejected out of the hole.

3) With the present self-tapping screw the torque which is required for screwing in is small, since the area of the screw in contact with the inner surface of the hole is small. That is to say, only the corners (d) contact with the hole, while a prior self-tapping screw contacts a hole along the entire length of the engaged spiral thread. In this sense, a triangular cross-section is the best of polygonal shapes to choose. Furthermore, with the double threads as shown in Figure 4A, plastic case halves are not broken even when they are fastened with a large torque. In addition to this, since the necessary torque for inserting the self-tapping screw is small negligable friction is generated and thus the plastics material is not heated nor does it melt. Therefore, the present self-tapping screw can be used for fastening case halves made of acrylic resins, with which a prior self-tapping screw cannot be utilised, since acrylic resins are too hard and fragile to fasten with a priorform of self-tapping screw.

Figure 6 illustrates experimental results showing the deviation of the sensitivity of cassette magnetic tapes (over 100 samples). In Figure 6 the vertical axis shows the deviation of the sensitivity in dB, and the horizontal axis shows the number of samples. The curve (a) in Figure 6 shows the deviation with a cassette tape the case of which uses the present self-tapping screwforfastening the case halves together, whereas the curve (b) shows the deviation using a prior self-tapping screw with circular crosssection.

In the cassette magnetic tape employing the present form of screw, the average of the deviation of sensitivity is7= 0.18 dB, and the standard deviation of that average is a = 0.04dub. On the other hand, with the conventional self-tapping screws, x 0.25dB arid a = 0.60 d B. Accordingly, the cassette magnetic tape utilising the present self-tapping screws has a smaller sensitivity average and the smaller standard deviation. Some relevant data for the experiment providing the curve (a) in Figure 6 are, that the diameter of the screws is approximately 2 mm (made of carbon steel rod, Japanese Industrial Standard SWCH1 6A), and the size of the cassette case is approximately 100 mm x 63 mm x 9 mm (made of high impact styrene resin). Also, double spiral threads are provided with a lead angle (0) of 15" and a pitch (d) of 1 mm.

Claims (6)

1. A self-tapping screw comprising an elongated shank portion with a thread spiral on the surface thereof and a head portion fixed to one end of the shank portion, said head portion having a recess for engagement and turning of the screw with a screw driver and the shank portion having a cross-section of polygonal shape.

2. A self-tapping screw according to claim 1, wherein said polygonal cross-sectional shape is triangular.

3. A self-tapping screw according to either claim 1 or claim 2, wherein the screw is a double-thread screw and said thread spiral is one of two such spirals.

4. A self-tapping screw according to any one of the preceding claims, wherein each side of the polygonal cross-sectional shape of the shank portion is somewhat convex in the radially outward direction.

5. A magnetic recording tape cassette having a case formed in two halves fixed together by means ofself-tapping screws in accordance with any one of the preceding claims.

6. A self-tapping screw substantially as herein particularly described with reference to Figures 3 to 5 of the accompanying drawings,.