SCREW THREADED FASTENER

Abstract: A screw threaded fastener particularly intended for use in fastening corrugated sheet material to metal purlins comprises a drilling point (1), a head (2), a first self-tapping screw threaded portion (3) adjacent the drilling point (1), a second screw threaded portion (4) adjacent the head (2), the second screw threaded portion (4) having a larger crest diameter than the first screw threaded portion (3), and an unthreaded portion (5) extending between the first and second screw threaded portions. The diameter of the part of the fastener carrying the second screw threaded portion (3) is generally larger than that carrying the first screw threaded portion and the fastener also includes a transition portion (6) between its portions of smaller and larger diameter. The transition portion (6) includes forming portions (8 Figure 4) projecting outwards from the axis of the fastener no further than the diameter of the larger diameter portion. The forming portions (8) in use, enlarge the diameter of a hole drilled in the sheet material by the drilling point (1) to allow passage of the larger diameter portion of the fastener through the sheet material. Such a fastener is formed by a forward extrusion step which reduces its diameter and which, simultaneously, forms the forming portions (8) on a transition (6).

Description: Our earlier patent specification GB-A-1270545 describes and claims a screw threaded fastener which is particularly arranged for fixing a corrugated metal roofing sheet onto underlying metal purlins. The fastener includes a self-drilling point, a first self-tapping screw thread adjacent the drilling point, an unthreaded region, a head and a second screw threaded portion adjacent the head. In use, the drilling point firstly drills a hole in the corrugated sheet material and the first screw threaded portion forms a thread through the sheet metal portion. Then, with the sheet adjacent the unthreaded portion the drilling point drills a hole through the underlying purlin. Once this has been completed the first screw threaded portion cuts a thread in the underlying purlin and, as the fastener is screwed into the purlin the second screw thread picks up the roofing sheet and urges it upwards to clamp it between the second screw threaded portion and the head of the fastener and of course any waterproof sealing washer interposed between the top of the roofing sheet and the head of the fastener. Such fasteners have been successful and used widely. Our earlier patent specification GB-A-1277044 describes an improvement of the fastener described in specification GB-A-1270545 by making the fastener by a process including two forward extrusion steps using three different punch tools so that the diameter of the fastener adjacent the head is greater than the diameter adjacent the drill point. This enables the second screw threaded portion to have a larger diameter so that the crest diameter of the second screw thread is much greater than that of the first. This provides a more certain pick up of the roofing sheet onto the second screw threaded portion and so forms a more reliable seal between the head of the fastener and the roofing sheet. To provide clearance for the larger diameter portion of the fastener the process also includes a subsequent forging step in which wings are formed on the shank of the fastener above the first screw thread. The wings firstly ream out the screw thread formed in the roofing sheet by the first screw threaded portion and provide an enlarged hole which allows clearance for the larger diameter portion of the fastener, and secondly, when and if they come into contact with the upper face of the purlin, they prevent overdriving of the fastener. Forming the blanks of the screw thread to form the portions of larger diameter and also the subsequent forging operation to produce the upstanding wings leads to the production process of these fasteners being expensive with the result that whilst they are technically superior they are more expensive to manufacture which makes it more difficult for them to compete in the market place with fasteners made in accordance with patent specification 1270545. The US specification 4621963 describes a screw threaded fastener for securing two parts in spaced relation such as a sheet metal roof panel to a roof frame. In use, the first screw thread is arranged to pass through the top sheet and engage the roof frame, drawing the flared portion through the hole to enlarge the latter and roll over the metal around the edge of the hole. Tapping takes place before the flared portion touches the top sheet. According to a first aspect of this invention a screw threaded fastener for use in fastening corrugated sheet material to metal purlins comprises a drilling point, a head, a first self-tapping screw threaded portion adjacent the drilling point, a second screw threaded portion adjacent the head, the second screw threaded portion having a larger crest diameter than the first screw threaded portion, and an unthreaded portion extending between the first and second screw threaded portions, the diameter of the part of the fastener carrying the second screw threaded portion being larger than that carrying the first screw threaded portion and the fastener also including a transition portion between its portions of smaller and larger diameter, the transition portion being frusto-conical in shape and including forming means which project outwards from the axis of the fastener no further than the diameter of the larger diameter portion, the forming means being formed by a projection which projects outwards from the frusto-conical surface, characterised in that the forming means has the form of a chordal segment cut from a diametral slice through a hemisphere, and the transition portion including the forming means is arranged substantially adjacent the first screw threaded portion so that, in use, the first screw threaded portion passes through the corrugated sheet material and the forming means enlarges the diameter of the hole through the corrugated sheet material before the first screw threaded portion engages the metal purlin. According to a second aspect of this invention a method of forming a screw threaded fastener according to the first aspect of the invention for use in attaching a corrugated roofing sheet to an underlying metal purlin comprises the steps of forward extruding part of a fastener blank through a die, thereby reducing the diameter of the blank and simultaneously producing forming means on the transition portion between the smaller and larger diameter portions of the blank, forming the blank to produce a drill point at the free end of the smaller diameter portion of the blank and a head at the free end of the larger diameter portion of the blank, and thread rolling the blank thereby forming a first screw thread on the smaller diameter portion of the blank adjacent the drilling point and a second screw thread having a larger crest diameter than the first screw thread on the larger diameter portion of the blank adjacent the head. Preferably more than one such projection is formed around the transition zone and the projections are arranged equi-angularly around the transition zone. There can be a reduction in cross-sectional area as high as 70% between the smaller and larger diameter portions of the fastener if two dies are used and this can accordingly give rise to a corresponding increase in crest diameter of the second screw thread. However, more usually, a reduction of cross-sectional area of around 30% is provided between the larger and smaller diameter portions of the fastener and this can be achieved in a single die by a single cold forward extrusion process. It is preferred that the unthreaded portion is of the larger diameter and, in this case, the transition zone is formed at the intersection between the first screw threaded portion and the unthreaded portion. A particular example of a fastener in accordance with this invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a side elevation of the fastener; Figure 2 is an end elevation showing the head of the fastener; Figure 3 is a side elevation drawn to a larger scale of the transition portion; and, Figure 4 is a perspective view of the transition portion. The fastener comprises a drilling tip 1 formed at one end and a head 2 at the other end, a first self-tapping screw thread 3 formed adjacent the drilling point 1 and a second screw thread 4 formed adjacent the head. An unthreaded portion 5 extends between the screw threads 3 and 4 and a transition portion 6 is located between the end of the screw thread 3 and the unthreaded portion 5. The head 2, the second screw thread 4 and the unthreaded portion 5 are all formed from a portion of a screw blank having a larger diameter of, in this example 5.1 mm whilst the thread 3 and drilling point 1 is formed from a screw blank of diameter 4.55 mm. The transition portion 6 comprises a frusto-conical portion 7 extending between the larger and smaller diameter portions of the fastener and two forming portions 8. The forming portions 8 include shoulders 9 and rounded faces 10. The forming portions 8 include rounded outer faces 10 and correspond to chordal segments of a diametral slice through a hemisphere so that they are rounded both in a plane containing both segments and the axis of the fastener, as shown in Figure 3 and rounded in the circumferential sense. On use such a fastener is intended to be used for attaching a corrugated sheet metal covering to an underlying metal purlin. The drilling point 1 first drills a hole through the corrugated sheet and then the first screw thread 3 cuts a thread around the aperture. Further forward movement and rotation urges the forming means 8 against the aperture. These configurations of the forming means 8 rolls the corrugated sheet material downwards as the transition portion 6 moves through the hole. This firstly enlarges the hole to allow the larger diameter unthreaded portion 5 of the fastener to penetrate the corrugated sheet and secondly provides an apparent thickening of the sheet material to ensure better engagement of the second screw thread 4 with the sheet material. Further rotation and forward movement of the fastener causes the drilling point 1, to drill through the purlin and then the screw thread 3 cuts a screw thread as it is screwed into the purlin at the same time as the thread 4 picks up on the thickened region of the corrugated sheet. The fastener is made by taking a blank of diameter 5.1 mm and subjecting this to a forward extrusion step to form the smaller diameter portion to carry the screw thread 3 and the drilling point 1. The blank is driven through a generally conical die having an input diameter of 5.1 mm and an exit diameter of 4.55 mm by a punch. The die includes a pair of diametrically opposed recesses having rounded bases formed by the cap of a sphere of diameter substantially 5.1 mm. Thus the die has a shape which is the inverse of the shape of the transition region 6 shown in Figure 4. Forward extrusion continues until the smaller diameter portion reaches an end stop. Further forward movement of the punch causes a further thickening of the larger diameter portion of the blank and partly forms the head 2. A second punch then completes the thickening of the larger diameter portion and the head 2 whilst the blank is still contained in the same die. The drilling point 1 is then formed at the free end of the smaller diameter portion of the blank and screw threads 3 and 4 are rolled onto the blank. The blank is subsequently case hardened to complete the fastener. Preferably the second screw thread 4 has an included flank angle of 30° to increase still further its crest diameter.

Claims: A screw threaded fastener for use in fastening corrugated sheet material to metal purlins comprising a drilling point (1), a head (2), a first self-tapping screw threaded portion (3) adjacent the drilling point (1), a second screw threaded portion (4) adjacent the head (2), the second screw threaded portion (4) having a larger crest diameter than the first screw threaded portion (3), and an unthreaded portion (5) extending between the first and second screw threaded portions, the diameter of the part of the fastener carrying the second screw threaded portion being larger than that carrying the first screw threaded portion (3) and the fastener also including a transition portion (6) between its portions of smaller and larger diameter, the transition portion being frusto-conical in shape and including forming means (8) which project outwards from the axis of the fastener no further than the diameter of the larger diameter portion, the forming means (8) being formed by a projection (9) which projects outwards from the frusto-conical surface (7), characterised in that the forming means has the form of a chordal segment cut from a diametral slice through a hemisphere, and the transition portion (6) including the forming means (8) is arranged substantially adjacent the first screw threaded portion (3) so that, in use, the first screw threaded portion (3) passes through the corrugated sheet material and the forming means (8) enlarges the diameter of the hole through the corrugated sheet material before the first screw threaded portion (3) engages the metal purlin. A screw threaded fastener according to claim 1, in which two projections (9) are formed opposite each other around the transition portion (6). A screw threaded fastener according to claim 2, in which more than two such projections (9) are formed around the transition portion (6) and in which the projecting (9) segments are arranged equi-angularly around the transition portion (6). A method of manufacturing a screw threaded fastener according to any one of the preceding claims for use in attaching a corrugated roofing sheet to an underlying metal purlin comprising the steps of forward extruding part of a fastener blank through a die, thereby reducing the diameter of the blank and simultaneously producing forming means (8) on the transition portion (6) between the smaller and larger diameter portions of the blank, forming the blank to produce a drill point (1) at the free end of the smaller diameter portion of the blank and a head (2) at the free end of the larger diameter portion of the blank, and thread rolling the blank thereby forming a first screw thread (3) on the smaller diameter portion of the blank adjacent the drilling point (1) and a second screw thread (4) having a larger crest diameter than the first screw thread (13) on the larger diameter portion of the blank adjacent the head (2). A method according to claim 4, in which a reduction of cross-sectional area of around 30% is provided between the larger and smaller diameter portions of the fastener and this is achieved in a single die by a single cold forward extrusion process.

Inventors: HEWISON, GEORGE DAVID BURNTHWAITE
Applicants: ITW LIMITED
Application year: 1988
Main classification: F16B25
IPC codes: F16B25