GB2072781A - Woodscrew - Google Patents

Abstract

A woodscrew has a threaded portion 12 with a rolled thread and a conventional entry portion 13. An unthreaded shank portion 11 is of the same diameter as the threaded portion prior to thread rolling so that it is smaller than the crest diameter of the thread. The head 14 may have any conventional form. A raised portion 17 of the shank portion is provided immediately adjacent the head and may be formed during a forging operation which produces the head. The diameter of the raised portion is substantially equal to the crest diameter of the threads to facilitate location of the screw in a hole through which the threaded portion has passed. The raised portion is preferably circular in cross-section, but may also be triangular, lobular, or rectangular in cross-section. <IMAGE>

Description

SPECIFICATION Screws The invention relates to screws of a kind intended primarily for use with wood and which are hereinafter referred to as woodscrews. A typical woodscrew has a head, an unthreaded shank portion of uniform diameter adjacent the head, a threaded postion with a crest diameter equal to the shank diameter and an entry portion which is normally a tapered point with the screw thread running to the point.

The traditional method of manufacturing woodscrews has been to start with a length of wire of diameter equal to the shank diameter, to forge a head on one end (normally by a two-blow heading operation) and to cut the thread and the entry portion in a turning operation. The cutting of the thread imposes limitations on the speed at which screws can be manufactured. Also, with this manufacturing technique, approximately 30% of the material provided for screw manufacture becomes scrap. For these reasons the traditional screw manufacturing technique is not particularly economical and other ways of manufacturing woodscrews have become popular.

One alternative method which is in current production is to replace the thread cutting operation with a thread rolling operation which also provides the entry portion for the screw.

However the rolling operation produces a thread crest diameter greater than the diameter of the original wire so that in order to achieve a particular thread crest diameter, one starts with a smaller wire diameter. This leaves a shank with a smaller diameter than the thread crest diameter.

Such screws are known as scant shank screws.

One important requirement in many applications of woodscrews is that the unthreaded shank portion diameter should be equal to the thread crest diameter so that the unthreaded shank portion can act as a locating dowel in a hole through which the screw thread has passed.

Another problem which can arise with scant shank screws which incorporate a cruciform driving recess is that the recess can approach very closely to the neck joining the shank and the head and with a narrow shank a weak point can arise at this shoulder.

In order to achieve this large unthreaded shank diameter with a rolled thread, it has been proposed to start with a wire diameter equal to the unthreaded shank diameter, extrude a small diameter portion on which the thread is to be rolled and then roll a thread on the reduced diameter portion to provide a crest diameter equal to the unthreaded shank portion diameter. A problem of this procedure is that the extrusion dies necessary for extruding the small diameter shank portion are expensive and tend to wear out rapidly. This at least reduces the cost advantages associated with rolled threads. Another problem with this latter arrangement is that the junction between the extruded narrow diameter part and the larger diameter part can be a weak point at which the screw is likely to fracture.

An object of the present invention is to provide a woodscrew having a rolled thread but which does not suffer from the above disadvantages.

According to the present invention a woodscrew comprises a shank having a head at one end and a tapered entry portion at the other end, an unthreaded shank portion adjacent the head and a threaded shank portion extending from the unthreaded portion to the entry portion, the thread having been raised from a part of the shank of the same diameter as the unthreaded shank portion by a thread rolling operation so that the thread crest diameter is greater than the diameter of the unthreaded portion and is characterised by the provision of a raised location portion on the shank immediately adjacent the head conforming substantially to the crest diameter of the thread.

Such a screw has the advantages of simplicity and low cost associated with a scant screw shank screw while the raised location portion provides the practical advantages of a dowelling effect normally associated with a cut thread screw.

Preferably the raised location portion is in the form of an integral collar of the same diameter as the thread crest diameter. Preferably a taper is provided between the collar and the main portion of the shank to assist guiding the collar into a full diameter hole.

The invention also extends to a method of making a woodscrew including the steps of forging a head on a length of wire to provide a screw blank having a shank and a head, rolling a thread on a portion of the shank remote from the head and providing an entry portion at the end of the shank remote from the head whereby the screw has an unthreaded shank portion of smaller diameter than the thread crest diameter, characterised in that while forging the head, a raised location portion is also forged on the shank immediately adjacent the head and conforming substantially to the crest diameter of the thread to be rolled on the shank.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a side elevation of a countersunk slotted screw in accordance with the invention, Figures 2 to 6 show parts of screws with different types of head; and, Figure 7 shows 'a punch and die set being used in the manufacture of a screw according to the invention.

The screw shown in Figure 1 has an unthreaded shank portion 11 corresponding to the diameter of a length of wire from which the screw has been manufactured. Adjacent the unthreaded shank portion 11 is a threaded shank portion 12 with a thread form of the kind normally employed in woodscrews. This thread form incorporates steep angle flanks with an included angle between the flanks of for example 500. The typical woodscrew thread also has wide flat roots between the flanks of adjacent raised portions of the thread. The threaded portion 12 terminates in a tapered entry portion 13 of the screw thread intended to assist insertion of the screw into a hole.Over the length of the tapered entry portion, the core diameter of the screw tapers to a point and also the depth of the thread tapers during at least the part of the entry portion near the point.

At the end opposite the entry portion, the screw incorporates a conventional head. In this example, the head is a countersunk head 14 with the usual flat outer surface 1 5 and a conical under surface 16.

Due to the thread rolling action, the crest to crest diameter of the screw threaded portion 12 is grater than the diameter of the shank 11. However the shank 11 incorporates an enlarged diameter portion 17 which extends from the head 14 part way along the unthreaded shank portion 11 towards the screw thread. The enlarged diameter portion 17 extends without any taper for a distance equal to at least one third of the wire diameter and then has a conically tapered portion 18 joining the increased diameter portion 17 to the shank portion which remains at the diameter of the original wire. The diameter of the portion 17 corresponds to the crest to crest diameter of the threaded portion 12.

The head 14 is equipped with a conventional driving means, in this example, a diammetrical slot 20 for receiving a screwdriver.

In a typical use of the screw, it has to be passed through a drilled or similarly formed circular hole in one member and then screwed into a hole in a second member to be jointed to the first member.

The hole in the first member should be of such a diameter as to allow the screw thread to pass through it and it follows that this diameter is greater than the diameter of the wire from which the screw and the basic part of the unthreaded shank portion is formed. Thus, in the absence of the raised portion 1 7 of the shank adjacent the head, there would be a possibility of lateral displacement of the screw in its hole, allowing lateral movement between the two components to be jointed together. While the countersink may prevent lateral movement between the head of the screw and a member secured thereby, it may still be possible for the screw to bend in its hole in the second member so that relative displacement between the two members can still occur. The raised portion 17 solves this problem by acting as a dowel to locate the screw laterally with respect to at least one of the members.Where the first member is a thin metal sheet such as part of a hinge, the thin meta sheet is located laterally on the countersink and the raised portion 1 7 is located in the hole in the second member Figure 2 to 6 show alternative screw heads complete with raised shank portions corresponding to the shank portion 17 of Figure 1.

Figure 2 shows a countersunk head similar to that of Figure 1 but with a cruciform recess of the kind described in U.K. Patent No. 1,521,141 and known by the Registered Trade Mark SUPADRIV in place of the slot shown in Figure 1. An additional example of the raised portion 1 7 in the screw shown in Figure 2 is that it enables a suitable size of recess to be provided in the head without reducing the quantity of metal between the root of the recess and the outer lower edge of the head beyond a satisfactory amount for maintaining the strength of the screw between the head and the shank.

Figure 3 incorporates a screw with a conventional raised head and incorporating a SUPADRIV recess.

Figure 4 shows a round headed screw, that is one with a flat under surface to the head and a generally rounded top surface. The absence of any taper on the underside of the head makes lateral location by means of a raised portion even more important than in the previous embodiments.

Figures 5 and 6 show portions of screws similar to those shown in Figures 3 and 4 but with slots in place of the cruciform recesses.

In the above described embodiments. the raised portion 1 7 immediately adjacent the head of the screw is completely circular in cross section.

This is a convenient shape for the raised portion and is likely to be the configuration used in practice. However, the basic function requirement of the raised portion is that is should be capable of positive location in a hole of a diameter corresponding to the crest diameter of the thread.

This could be achieved by other shapes than an enlarged circular section such as a triangular or lobular or rectangular shape. However, a circular shape is preferred because it provides good location and facilitates rotation of the screw.

In the examples shown, the screw thread is a two-start thread. The invention may be applied to screws with two-start threads or single-start threads or any other form of thread.

The method of manufacture of the screw thread and the entry end of the screw is conventional thread rolling procedure and so need not be described in any greater detail. The head is forged in a generally conventional two-blow heading operation but in view of the unique shape of the combined head and shoulder the way in which the head is formed in the two-blow heading operation will be described. It is conventional for the first blow to produce a mushroom like shape on the head of a screw and for the second blow to form the head completely, including a cruciform recess if any.

Figure 7 illustrates a typical die and first blow punch for manufaturing a head for a screw of the present invention, in this example a countersunk head. The die has the usual shank guide portion 21 of the same diameter as the wire, i.e., the shank 11. There is also the usual head forming portion conical recess 22 for the countersunk head. Between guide portion 21 and recess 22 is an enlarged diameter shank forming portion 23 conforming to the enlarged diameter shank 1 8 and taper 1 9 of the screw of Figure 1. The first blow punch has the usual spring loaded punch body 24 and positively driven punch pin 25. The punch pin diameter is slightly larger than the wire diameter.

The shape of the blank after the first blow is also shown in Figure 7. The blow by the punch pin shortens the blank and expands it laterally as shown. The lateral expansion is partially free but is partially controlled by the die and spring punch.

The blank almost fills the enlarged shank forming portion 23 as shown and also expands to the usual mushroom shape in the main head opening.

The second blow punch, not shown, conforms to the usual shape of a second-blow countersunk punch and completes formation of the head to the shape shown in Figure 1, apart from the slot 20 which is subsequently machined in the head.

Other punch and die shapes are used for other types of head. For example, for a round head, the head forming portion of the die is merely a flat end surface of the die.

Claims (5)

1. A woodscrew comprising a shank having a head at one end and a tapered entry portion the other end, an unthreaded shank portion adjacent the head and a threaded shank portion extending from the unthreaded portion to the entry portion, the thread having been raised from a part of the shank of the same diameter as the unthreaded shank portion by a thread rolling operation so that the thread crest diameter is greater than the diameter of the unthreaded portion, characterised by the provision of a raised location portion (17) on the shank 1) immediately adjacent the head (14) conforming substantially to the crest diameter of the thread.

2. A woodscrew according to Claim 1 characterised in that the raised location portion is an integral collar of the same diameter as the thread crest diameter.

3. A woodscrew according to Claim 2 incorporating a taper between the collar and the main part of the shank.

4. A method of making a woodscrew including the steps of forging a head on a length of wire to provide a screw blank having a shank and a head, rolling a thread on a portion of shank remote from the head and providing an entry portion at the end of the shank remote from the head whereby the screw has an unthreaded shank portion of smaller diameter than the thread crest diameter, characterised in that while forging the head, a raised location portion is also forged on the shank immediately adjacent the head and conforming substantially to the crest diameter of the thread to be rolled on the shank.

5. A die for manufacturing a head and raised location portion of a woodscrew according to Claims 1 comprising a wire guide portion corresponding to the unthreaded shank diameter a head forming portion corresponding to the required shape of the underside of the head, and an enlarged (as compared with the guide portion) shank forming portion between the guide portion and the head forming portion corresponding to the required shape of the raised location portion.