BACKGROUND OF THE INVENTION

1) Field of the Invention The present invention relates to a self drilling screw for fixing metal roof sheeting or other cladding to a range of different substrates.

2) Description of the Invention

Type 17 self drilling screws, which have been widely available for many years, are primarily timber screws for fixing into soft and hard timber. While Type 17 screws are sometimes used to fix metal roof sheeting to underlying timber battens, the drilling tip of the screw has difficulty in penetrating the metal sheeting and sometimes fails to do so. Accordingly, while fastening of roof sheeting in this way using the Type 17 screws can be achieved, nevertheless it is quite time consuming and can result in substantial wastage of screws which were unable to properly penetrate the sheeting. Type 17 screws are not, however, suitable for fastening metal roof sheeting to steel battens without pre-drilling and alternative screws are available for that purpose. Screws are also currently available for driving into both timber and steel battens but tend to be limited as to the thickness of steel batten into which they can be driven; moreover, although these screws tend to have good performance when driving into relatively soft timbers, pine for example, they are not suitable for use into hard timbers.

SUMMARY OF THE INVENTION

The present invention relates to a self drilling screw having improved versatility for fixing metal roof sheeting and other cladding to a wider range of materials including both thin and thick steel battens and timber from soft to very hard.

According to the present invention there is provided a screw for fastening sheet metal roofing material or other cladding to a support of wood or metal, the screw having a drill portion at its forward end and a threaded shank portion rearwardly of the drill portion, the drill portion having opposed cutting edges at its outer end and flutes extending generally axially rearwardly from the cutting edges, the threaded shank portion being formed with a sharp thread (as herein defined) commencing at the end of the drill portion remote from the cutting edges and rising progressively over a plurality of turns to full thread height, the drill portion being of a substantially constant diameter inwardly of the cutting edges and the root diameter of the threaded shank portion being of substantially constant diameter which is substantially the same as that of the drill portion, the thread being a buttress thread in which the trailing flank of the thread is inclined at a greater angle to the axis of the shank than is the leading flank.

By "sharp thread" there is meant a thread having a crest which when considered in transverse cross-section has a width of no more than substantially 0.25mm. In this respect, although on casual inspection many thread forms may seem to terminate in a sharply pointed crest, detailed inspection will usually reveal that not to be the case. In current forms of roofing screw the thread crest tends to be flattened or rounded rather than pointed with a width in transverse cross-section typically of the order of 0.75 to 1.00mm. A crest of that form cannot act in the manner of a sharp thread to produce the effects described in detail herein.

In a preferred embodiment of the invention the trailing flank of the thread is inclined to the axis of the shank at an angle of from substantially 70 to 85°, an inclination of around 75° being especially preferred.

Advantageously the thread pitch of the screw is lOtpi (turns per inch) to 13tpi, with 12tpi being especially preferred.

Further according to the invention, there is provided a screw for fastening sheet metal roofing material or other cladding to a support of wood or metal, the screw having a drill portion at its forward end and a threaded shank portion rearwardly of the drill portion, the threaded shank portion being formed with a sharp thread of which the crest has a width of no more than substantially 0.25mm, the thread rising progressively over a plurality of turns to full thread height, the root diameter of the threaded shank portion being of substantially constant diameter which is substantially the same as the diameter of the drill portion, wherein the thread is a buttress thread in which the trailing flank of the thread is inclined at a greater angle to the axis of the shank than is the leading flank, and the pitch of the thread is 10 to 13tpi.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a side view of a screw in accordance with the invention; Figure 2 is an enlarged detail of zone A in Figure 1; Figure 3 is an enlarged detail of zone B in Figure 2; and Figure 4 is a schematic cross-section through the thread.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The screw shown in the accompanying drawings comprises a driving head 2 at one end, a drill portion 4 at the other end and a threaded shank portion 6 of constant root diameter extending from the inner end of the drill portion 4. Although as shown, the head 2 has a conventional hexagonal driving formation, the head may be of other configuration. The drill portion 4 has at its outer end inclined cutting edges 8 and a pair of flutes 10 intersecting the cutting edges and extending generally axially into the threaded shank portion 6. Apart from its outer end at which the cutting edges 8 are formed, the drill portion 4 is of constant diameter throughout its length. The thread commences at the end of the drill portion 4 remote from the cutting edges 8 and rises progressively to maximum height over several turns, three to four turns in the preferred embodiment.

The root diameter of the threads in the threaded shank portion 6 is substantially the same as the diameter of the drill portion 4. While the diameter of the two may be exactly the same, as a result of manufacturing tolerances and different manufacturing processes needed to produce the drill portion and the threaded portion (processes which are well known per se) slight variation may arise between the diameter of these two portions; that variation may be of the order of plus or minus 0.15mm and a difference of that order will not adversely effect the performance of the screw. Accordingly, the reference herein to the diameters of the thread root and drill portion being substantially the same is intended to encompass the possibility of slight variation between the two.

In use, when driving into a steel or timber batten or other substrate, the drill portion 4 bores a hole with swarf being cleared from the hole via the flutes 10. As the root diameter of the thread is substantially the same as that of the hole formed by the drill portion, the progressively rising thread will engage and tap the hole over the entire thread height.

The screw of the preferred embodiment is designed for use into all timbers ranging from soft to ultra hard, for thin gauge metal up to 1.2mm and for thicker metal up to 1.9mm. An important factor in achieving this versatility is the design of the thread. The thread itself is a sharp thread (as previously defined) thereby providing what will be termed as a sharp crest. The sharp crest is important because as the drill portion drills into the substrate the tapered thread progressively taps into the wall of the hole and the sharpness of its crest enables this action to occur even in ultra hard timbers. The performance of ultra hard timbers in this respect is more like metal and the action which has been described here also takes place when driving into thick steel battens, an action likewise enabled by the sharpness of the thread crest.

The thread is also of buttress type in which the trailing flank 12 of the thread (the flank facing the head end) is inclined at a greater angle to the axis x-x of the screw than is the leading flank 14 (the flank facing the drilling end); this is shown in Figure 4. The use of a buttress thread is important because it provides good resistance to pullout when driven into thin gauge metal battens. This effect is described in our prior Australian patent 729658 and this is effective even under cyclonic loading as discussed in that patent. Whereas the screw described in that patent was specifically designed for driving into thin metal battens, the screw of the preferred embodiment is designed for a much wider range of uses and the buttress thread is subject to rather different design considerations. Whereas the angle α of the leading flank 14 is substantially 60° (a broadly conventional angle), the angle β of the trailing flank 12 is from 70 to 85°, preferably around 75°. A trailing flank angle in that range while still of a magnitude which provides good pullout strength in thin gauge metal provides added strength to the thread so that the thread will not deform under load and will give increased holding strength when driven into hard timbers and thicker metals.

The thread pitch itself is an important factor in the performance of the screw when driven into hard timbers. Screws of the type under consideration for use in building construction are usually 13 to 15 gauge. The thread pitch of the screw of the preferred embodiment is lOtpi to 13tpi (turns per inch). With a pitch of that order, when the thread taps into hard wood an adequate volume of wood remains between adjacent threads to provide the required anchorage and hence the high pullout strength associated with the hard wood. Although tests have indicated that thread forms of either lOtpi or 13tpi are effective when used with hard wood, 12tpi is preferred as it has been determined that this provides better performance when the screw is used with thin metal battens.

The embodiment has been described by way of example only and modifications are possible within the scope of the invention.