WO2007134361A1

WO2007134361A1 - Clearance hole cutting screw

Info

Publication number: WO2007134361A1
Application number: PCT/AU2007/000568
Authority: WO
Inventors: Christopher John Mallet; David Collinson
Original assignee: Itw Australia Pty Ltd
Priority date: 2006-05-19
Filing date: 2007-05-01
Publication date: 2007-11-29
Also published as: MY143094A; TW200801352A; CN101443562B; NZ572315A; CN101443562A; TWI342926B

Abstract

A screw for fastening semi-rigid sheet material to a substrate has a cutter arrangement integrally formed with the screw shank for cutting in the sheet a clearance hole relative to the screw shank so as to accommodate thermal movement between the sheet and the shank in use. The cutter arrangement comprises a plurality of wings spaced around the axis of the shank, each wing having a cutting edge extending generally in the longitudinal direction of the shank and directed forwardly in the direction of driving rotation of the screw.

Description

CLEARANCE HOLE CUTTING SCREW

The present invention relates to a screw for fastening sheet material to a substrate and more particularly to a screw for fixing roof sheeting and wall cladding of polycarbonate, fibre reinforced plastics, other plastics materials and non-plastics materials having a relatively high coefficient of thermal expansion.

Semi-rigid plastics sheeting such as polycarbonate sheeting or fibre-reinforced plastics sheeting as used in building construction such as for roofing and wall cladding has a significantly higher coefficient of thermal expansion than corresponding steel sheeting. When fastening such a sheet to underlying substrate such as battens or purlins of wood or metal by means of screws it is recommended that the shank of the screw passes through the sheet with a clearance sufficient to take up expansion and contraction of the sheet to avoid buckling and possible damage to the sheet by interference with the screw. While the sheet can be pre-drilled with a clearance hole of sufficient diameter prior to application of the fastening screw, for speed of installation it is more desirable to use a self-drilling screw with an in-built facility to provide a clearance hole of sufficient diameter to accommodate the thermal movement of the sheet. There have been a number of prior proposals for the incorporation of an integral cutter on the shank of the screw in order to cut the clearance hole. These prior proposals have not been entirely satisfactory in terms of a product which can easily be manufactured, can cut a clean round hole of the correct diameter, and easily permits assembly of a rubber-like sealing washer onto the shank of the screw to sit between the head of the screw and the outer surface of the sheeting around the hole.

According to the present invention, there is provided a screw for fastening semi-rigid sheet material to a substrate, the screw having a cutter arrangement integrally formed with the screw shank for cutting in the sheet a clearance hole relative to the screw shank so as to accommodate thermal movement between the sheet and the shank in use, the cutter arrangement comprising a plurality of wings spaced around the axis of the shank, each wing having a cutting edge extending generally in the longitudinal direction of the shank and directed forwardly in the direction of driving rotation of the screw. Advantageously the screw is a self-drilling screw.

In the preferred embodiment, the wings are formed in a part of the shank at the head end portion of the shank beyond the thread. Each wing is of substantially planar form and is so shaped that its cutting edge progressively diverges outwardly from the shank in a direction towards the head of the screw. Advantageously, the plane of the wing and thereby its cutting edge is inclined relative to the axis of the shank to produce a cutting action which tends not to cause flattening of the sheet during cutting. Preferably, the inclination is within the range of approximately 2.5° to 20° to the axis of the shank.

In a modification each wing includes at least one notch which extends through its cutting edge to receive the edge portion of the sheet around the hole being cut in circumstances where the cutter arrangement has pushed that edge portion away from the head of the screw during cutting. This is of particular benefit for use with polycarbonate sheeting which is somewhat softer and more flexible than fibre-reinforced plastic sheeting.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a perspective view of a screw in accordance with the preferred embodiment of the invention;

Figure 2 is an enlarged view showing a drilling portion of the screw; Figure 3 is an enlarged view showing the array of cutting wings at the head end portion of the screw; Figure 4 is a side view of the screw of Figure 1 but with the sealing washer absent;

Figure 5 is a schematic enlarged view to show the inclination of a cutting wing; Figure 6 is an enlarged fragmentary section on line D-D of Figure 4; Figure 7 is a sectional view through a cutting wing to show its cutting edge in greater detail; Figure 8 is a view of the screw during installation into sheeting;

Figure 9 is a view similar to Figure 8 but showing the screw when installed; Figure 10 is a fragmentary view showing a modified form of cutter arrangement; and

Figure 11 shows a detail of Figure 10.

The screw as shown in the accompanying drawings is a self-drilling screw. In the embodiment shown the drilling portion 2 of the screw is of the form described in our patent no. 719725 and is suitable for fixing to timber or relatively thin metal battens. Other embodiments designed for fixing to metal purlins of approximately 1.5mm thickness or greater will have a fluted metal drilling portion of conventional form. In the embodiment shown, and as described in our patent no. 719725, the screw thread extends over the fluted zone of the drilling portion 2 whereas in an embodiment having the conventional metal drilling portion the screw thread commences adjacent the inner end of fluted section. It is to be understood that the invention is not restricted to the use of one or other of these forms of drilling portion and other drilling portions suited for the intended application could alternatively be used, for example a type 17 drilling portion.

The thread extends part way along the shank of the screw and terminates short of the head. An array of cutting wings 4 is formed on the shank and lie adjacent the proximate end of the thread. The wings 4 serve to cut in the sheeting a clearance hole of larger diameter than that of the screw shank. The wings 4 are equally angularly spaced around the axis of the shank and in the form shown there are two such wings diametrically opposed although other arrangements are possible. Each wing is of substantially planar shape and advantageously its plane is inclined to the axis of the shank such that its outer end (its end adjacent to the thread) leads its inner end (its end closer to the head) in the direction of driving rotation of the screw. This is shown schematically in Figure 5. Preferably the inclination is within the range of approximately 2.5° to 20° with an inclination of approximately 8° being particularly preferred. The wing is also shaped to progressively increase in depth substantially from zero at its outer end to a maximum at its inner end. The wing is thereby provided with an outer edge which inclines radially outwardly in a direction towards the head of the screw and this edge is shaped to provide a sharp cutting edge 6 which extends forwardly in the direction of driving rotation of the screw. The cutting edge 6 is shown in the enlarged cross-sectional views of Figures 6 and 7. This form of cutting edge which is directed forwardly of the leading face of the wing provides a sharp, clean cut in the sheet as the screw is rotated. It is to be noted that this is in contrast to certain prior designs in which so-called cutting wings actually wear or rub a hole in the roofing sheet which can result in a ragged and uneven clearance hole which may not be of the recommended size to accommodate the range of thermal movement of the sheet.

The rate at which the wings 4 cut into the sheeting is determined by the rate at which the screw is driven into the underlying batten or purlin and this is determined by the pitch of the thread. In order to ensure that the wings cut through the sheet rather than punch or tear a hole in the sheet, the rate of penetration of the screw needs to be controlled and this necessitates a relatively fine thread, typically a thread having at least 10 turns per inch (or the metric equivalent of that). In practice there will be between 10 and 20 turns per inch with approximately 14 turns per inch providing an acceptable balance between speed of installation of the screw and controlled rate of penetration of the wings during cutting.

The wings are pressed out of the screw shank during manufacture of the screw and a screw thread of 14 gauge (or its metric equivalent) should provide sufficient material in the shank to permit forming of the wings. With a screw thread of smaller gauge the shank would need to be of increased diameter in the portion beyond the thread to permit formation of the wings, and while this would be feasible, in practice it is not preferred as it would complicate manufacture somewhat. The particular design of the sharp cutting edge as illustrated in Figures 6 and 7 is also advantageous in terms of its manufacture as, by appropriate design of the press tooling, the cutting edge can be produced without producing flashing which requires subsequent removal.

A sealing washer 10 of rubber-like material is installed on the unthreaded section of the shank between the cutting wings 4 and the head. Due to the divergent shape of the wings 4 the washer 10 is able easily to be assembled on the screw by pushing the washer 10 over the wings 4. The configuration of the screw with its sealing washer during installation and when installed is illustrated in Figures 8 and 9, respectively. The angular inclination of the wings, and thereby their cutting edges, relative to the axis of the shank (as illustrated in Figure 4) makes the cutting action somewhat more gradual than would be the case if the wings were to be directed in alignment with the axis of the shank. In turn this reduces the tendency of the wings to flatten the sheet during cutting. Flattening of the sheet, or more accurately flattening of the corrugation or ridge through which the screw extends, will result in a width change of the overall sheet and this can lead to alignment problems when fixing the sheet.

The screw also has utility not only with polycarbonate and fibre-reinforced plastics sheeting, but also other plastics sheeting and sheeting of material other than plastics and which features a high coefficient of thermal expansion and which requires a clearance hole to be produced, aluminium sheeting for example.

Polycarbonate sheeting is somewhat softer and more flexible than fibre-reinforced plastics sheeting and in some circumstances there may be a tendency for the cutting edges 6 to push downwardly the peripheral edge portion of the hole being cut in the sheet which means that the hole might not be cut to the required diameter and also might not be a smooth circular cut. In a modification to mitigate this possible effect, at least one notch 20 is formed in each cutting edge 6 and the adjacent part of the wing 4 as shown in Figures 10 and 11. The notch 20 is of such a depth as to accept the peripheral edge portion of the sheet around the hole deflected downwardly by the portion of the wings 4 beneath the notch so that when reaching the notch that deflected edge portion is able to flex into the notch. The upper or inner edge of the notch extends transversely to the axis of the screw whereby the peripheral edge portion of the cut sheet around the hole tends to be restrained within the notch during cutting by the part of the edge above the notch to produce a cleaner and rounder cut. The lower edge 20b of the notch is rather more obliquely inclined to the axis of the screw to facilitate entry of the edge portion of the cut sheet into the notch. Although as shown each of the two wings 4 is provided with a single notch 20, each wing may have two or more such notches distributed along its length. The embodiment has been described by way of example only and modifications are possible within the scope of the invention.

Claims

CLAIMS:

1. A screw for fastening semi-rigid sheet material to a substrate, the screw having a cutter arrangement integrally formed with the screw shank for cutting in the sheet a clearance hole relative to the screw shank so as to accommodate thermal movement between the sheet and the shank in use, the cutter arrangement comprising a plurality of wings spaced around the axis of the shank, each wing having a cutting edge extending generally in the longitudinal direction of the shank and directed forwardly in the direction of driving rotation of the screw.

2. A screw according to claim 1, wherein the wings are formed in a part of the shank at the head end portion of the shank beyond the thread.

3. A screw according to claim 1 or claim 2, wherein each wing is of substantially planar form and is so shaped that its cutting edge progressively diverges outwardly from the shank in a direction towards the head of the screw.

4. A screw according to any one of claims 1 to 3, wherein the wing is inclined relative to the axis of the shank such that the end of the wing closer to the head of the screw trails the other end of the wing in the direction of driving rotation of the screw.

5. A screw according to claim 4, wherein the inclination is approximately from 2.5° to 20° relative to the axis of the sheet.

6. A screw according to claim 5, wherein the inclination is approximately 8°.

7. A screw according to any one of claims 1 to 6, wherein the screw is a self-drilling screw.

8. A screw according to any one of claims 1 to 7, wherein each wing includes at least one notch which extends through its cutting edge to receive the edge portion of the sheet around the hole being cut in circumstances where the cutter arrangement has pushed that edge portion away from the head of the screw during cutting.

9. A self-drilling screw having a cutter arrangement substantially as hereinbefore described with reference to the accompanying drawings.