Truss fabrication machines are known in the art but typically such machines are constructed so that they can only produce building panels of a single particular width. If panels having a variety of dimensions are to be made, the machine generally has to be extensively modified. This is clearly undesirable as it results in the loss of production facilities. The alternative is to have a number of machines.

A prior art apparatus for manufacture of a wire truss is disclosed in W096/31314. The apparatus comprises two chains which are driven round preformed tracks by a chain drive. Each chain supports pivotally mounted arms which swing out into contact with the wire to bend the wire into a zigzag shape. The dimensions of the zigzag are determined by the size and spacing of the arms and hence the distance between the chains.

The apparatus cannot therefore be easily modified to produce to produce trusses having different dimensions. The only practical solution to provide a machine for each size of truss desired.

Another prior art apparatus for manufacture of a wire truss is disclosed in US 4,291,732. The zigzag wire forming means comprises a rear clamp, a transverse displacement die and a pair of blocking dies. Once the zigzags have been formed the wire is fed into a wire guide formed of two sections fixed to the machine bed. This arrangement again produces zigzag wire having fixed dimensions.

US 4,372,350 discloses a machine for manufacturing welded lattice girders.

The zigzag wire forming means comprises two bending levers which pivot to place the wire on wire deflector pins carried by two endless chains. Although girders of different heights may be produced on the same machine, no means are provided to change the dimensions of the zigzag wires themselves.

The present invention therefore seeks to provide a machine for making trusses, the dimensions of which trusses can easily be changed.

According to the invention, there is provided an apparatus for making a truss, comprising wire supply means adapted to supply a plurality of wires, first wire feeder means, which first feeder means is adapted to feed a wire to wire forming means, which wire forming means is adapted to form said wire into a zigzag shape, wire incrementation means being provided to draw said wire through said wire forming means, characterized in that the wire forming means comprises a swinging pin arrangement driveable by pneumatic means, the apparatus further including control means, which control means are adapted to control the frequency of the pin oscillations such that the dimensions of the zigzag are alterable.

The apparatus according to the invention advantageously facilitates the production of trusses in a variety of dimensions, by controlling the frequency of the pin oscillations, in contrast to the known machines which require substantial modification before the dimensions can be altered. The more cost effective production of trusses having different dimensions reduces the cost of such trusses and enables their economic use in a wider range of building specifications than has been possible.

Preferably the truss incrementation means comprise one or more reciprocating means and associated clamp means adapted to clamp the or each wire at least during a forward stroke of the reciprocating means.

Preferably the machine is driven via a motor driven camshaft, which camshaft may be provided with sensors to detect the position of the cams, wherein the output of the sensors is input to a control unit, which control unit sequences the various driving means and switches.

Preferably, the machine further comprises welding means adapted to weld second and third wires supplied by said wire supply means to said first wire, wherein the position of at least some of the welding means is adjustable to cope with changes in truss dimensions.

An exemplary embodiment of the invention will now be described with reference to the drawings in which Fig. 1 shows a wire straightening arrangement Fig. 2 shows a wire forming arrangement Fig. 3 shows part of a truss incrementation device and part of a welded portion of a truss.

The apparatus is constructed in two halves which are mirror images of each other. The wire supply means comprises two powered multi decoiler units on each of which three coils of wire can be mounted. The wire supply means is provided with a control system to ensure that the wire 1,2,3 can be decoiled without any snatching. A sensor on each wire supply detects whether the particular wire supply has been exhausted or not. Each wire 1,2,3 is fed to a wire straightener through a hardened location hole 4a-c, which hole helps to prevent any wire entanglement. A series of mutually spaced discrete holes 4a-f is provided to enable the width of the truss to be varied.

The wire straightener comprises twin-plane roller straightener arrangements formed of adjacent vertical 7a-c and horizontal groups 8a-c of rollers. The vertical and horizontal planes of each arrangement have seven rollers arranged in two banks of three and four rollers, respectively. The position of the rollers is adjustable to allow for re-positioning of the wires for varying the width of the truss. In use the wires are fed through the straightener arrangements which also serve to maintain the tension in the wires. The tensioning also facilitates the tight forming of the zigzag wire. After passing through the straighteners the wires are fed through hardened location holes. A number of hardened location holes are provided to facilitate the production of the different width trusses.

One of the wires is drawn through the wire straightener and fed to the wire former 10 by means of a reciprocating unit. This unit comprises a first pneumatic clamp mounted on a reciprocating plate. A further pneumatic clamp, which is fixed to the machine bed, is provided to act as a backstop. This backstop clamp is actuated when the reciprocating unit is on the return stroke to ensure that the wire does not return to the straightener or become entangled in any way. In use the first pneumatic clamp will clamp the wire on the forward stroke and hence draw it through the straightener. At the end of the forward stroke, the first clamp is released and the backstop clamp is actuated to prevent any return of the wire.

The wire former 10 comprises a pneumatically operated swinging pin device 11 which also assists in drawing the wire fed in by the reciprocating unit. The swinging pin device comprises a pin 12 mounted on a forming arm 13, which forming arm is pivotally mounted on the zigzag forming unit (all part of the second reciprocating carriage). A pneumatically operated cylinder 14 is connected to the forming arm and is adapted to drive the pivotal motion of the arm. In use the pin 12 is reciprocated between two positions in one of which positions it applies a folding force to the wire 2 to form the wire 2 into a zigzag. In the other position it is clear of the wire and returns to its home position.

Once the zigzag has been formed the wire 2 is engaged by a series of pins 15 fixed to a second reciprocating carriage 16, which carriage then transports the wire to the welding unit. The reciprocating carriage is provided with a series of 25mm vertically spaced tapped holes, into which half of the clamping unit can be repositioned, thereby facilitating the transport of a variety of different widths. The dimensions of the zigzag are determined by the amount of wire fed in by the first reciprocating unit (adjustable by an eccentric pin on the unit's motor) and the dimensions of the forming arm. Control means are provided for the pneumatically operated cylinder driving the forming arm, which control means control the frequency of the pivotal motion and hence the dimensions of the zigzag.

Once formed each part of the zigzag 2 is successively engaged by one of a series of pins 15 fixed to the second reciprocating carriage 16. This second reciprocating carriage thus carries the zigzag wire along to the next stage of the machine, the welding station. Each of the pins is adjustable in position to facilitate the take up of different truss sizes. The second reciprocating unit 16 is also provided with a clamping unit to clamp the line wires and hence draws the line wires forward into the welding station. Each line wire 1,3 has a respective clamping unit. The clamping unit is formed in two parts, each part being provided with a hardened pad. One part is position fixed to the reciprocating carriage and the other is adjustable, so that different truss sizes can be accommodated. Within the clamping unit there are several hardened pads that clamp the line wires. One side of each pair of pads is fixed into position. The gap that the line wires goes through can be adjusted by means of a screw as the second pad is free in one plane only. When the clamp pressure is released the thickness of the wire pushes the free pad away from the fixed pad. The adjusting screw has a lock nut to secure its position.

The welding station comprises four pneumatically operated electrical resistance units, two for each truss. Each pair of welding units is employed at each edge of the zigzag wire/truss substantially simultaneously. Each of the welding units is separate and is individually fixed to the machine bed. Each welding head is also provided with its own transformer.

The welding station is provided with a cooling system to reduce the prospect of any overheating. The cooling system comprises pumping water from a reservoir through the transformers and welding heads, and then back to a reservoir.

The second reciprocating unit 16 draws the line wires 1,3 forward and moves the nodes of the zigzag wire 2 over the welding heads. The node of each zigzag 2 lies adjacent to but not completely co-axial to the respective line wire 1,3. Thus in the completed truss the node will protrude slightly from the plane of the line wires. The welding head then welds the zigzag wire 2 to each of the respective line wires 1,3. This welding action also acts as a clamping action thereby permitting the clamping unit 18 on the second reciprocating carriage to release the line wires and to return to its datum, where it can clamp the line wires again.

Once the wires have been welded together the truss is moved forward to a guillotine. A separate guillotine is provided for each truss and has twin blades wide enough to cut any size of truss. Each of the twin blades is provided with four cutting edges. The guillotine is hydraulically operated so that high tensile strength wire can be cut.

After being cut by the guillotine the truss is removed either by hand or collected in a tote bin.

The main movements of the apparatus, e. g. the wire former and the incrementation of the wires through the machine are controlled by a pair of motor driven camshafts. The various pneumatic clamping units are also driven from the PLC via pneumatic actuators. Sensors detect the positions of the reciprocating carriage and the cams and input this to a PLC, which then sequences the pneumatic powered functions and in particular synchronises the switching of the pneumatic valves for the main forming movements of the zigzag wire. The sensors comprise two proximity sensors located on the machine bed, which are able to detect when the reciprocating carriages are at their home positions. The camshafts are driven by an AC induction motor with a variable drive for speed variation. This speed variation together with the control of the pneumatic valves enables the dimensions of the trusses to be altered. The motor is driven from a 3-phase supply and so as the motor rotates, the amount of supply drawn will vary. This variation enables an accurate calculation of how far the motor has rotated in each cycle. If for example, a single revolution of the motor relates to a single cycle of the truss forming process, the control computer can translate the information on how far the motor has revolved to determine how far the whole machine is into the cycle. To make finer adjustments to each truss type, it is necessary to physically adjust the position of the eccentric cam.

The zigzag forming unit, the welding unit, the truss clamping unit and the guillotine clamping unit are all split into two halves. The machine edge side is position fixed, whilst the side facing the machine centreline is adjustable. A series of tapped holes is provided at typically 25mm intervals, through which one of the wires is fed, thereby enabling the width of a unit to increased or decreased to accept the different width trusses. Therefore two of the hardened location holes on the fixed side of the adjustable units are always used, being fed by the zigzag wire and one line wire. One of the other available hardened location holes is thus used to vary the size of the truss being produced.

The apparatus of the invention advantageously also permits the use of a number of different materials for the wires. Thus each wire may be either untreated mild steel, galvanised electroplated steel or stainless steel. Additionally each wire may have a diameter of 2,3 or 4 mm, except the zigzag wire which is only 2 or 3mm.

Exemplary specifications of the trusses producible by the apparatus are as follows: Mmimum Maximum Truss length 50mm infinie Truss width 75mm 150mm Truss pitch 100mm 200mm Typically increments in the truss length are 50mm and in the width 25mm.

The apparatus of the invention will typically produce a standard truss of 2.450m at the rate of 375 an hour, although 150mm wide trusses may be produced at up to 440 an hour.

The construction of the machine in two halves, each half being a mirror image of the other, brings a number of advantages. At a simple level it enables the output to be doubled. However, it also enables certain size trusses to be produced. When the panel parts are assembled, the truss node positions should alternate. If the panel length is a multiple of the truss node pitch (e. g. for a pitch of 100mm a truss length of 4.1 m, 4.2 m etc.), one side of the machine would produce exactly the same truss profile. To obtain an alternating node pattern, every other truss would need to be physically rotated as it came off the machine to give the desired effect. By producing a mirrored truss, the alternating pattern can be automatically produced and the trusses are simply stacked as they come out of the machine.

Although the cylinder etc have been described as pneumatically operated, it would also be possible to use an hydraulically operated system.