Robotic arms enable plasma torches, lasers and the like to be manipulated for cutting three dimensional contours in sheet material in particular.

Accuracy of such cutting requires careful establishment of datum point (s) and axes of the workpiece with respect to a robotic arm and its cutting head. This difficulty is aggravated if, as is conventionally the case, the robotic arm and a workpiece support are both mounted on a third structure, such as a concrete floor. At least, this floor will have a differential coefficient of thermal expansion. At worst, the floor may be inadequately strong, introducing a variety of deflection errors.

The object of the present invention is to provide an improved cutting machine, wherein the variable of the third structure is removed.

According to the invention, there is provided a machine for cutting workpieces with a cutting head, the machine comprising : * a robotic arm for carrying a cutting head at its distal end and * a C-shaped rigid chassis, of which: the top limb provides at the serif end of the C a mounting for the robotic arm, the upstanding limb provides an upstand from the bottom limb and * the bottom limb provides a workpiece support, the arrangement being such that a workpiece fixed to the workpiece support has a predetermined dimensional relationship with the mounting for the robotic arm irrespective of the installation of the machine.

The machine has the advantage of isolating the inter-relationship of the robotic arm to the workpiece from variables introduced by structures such as the floor on which the machine is installed. It has the further consequential advantages of being

movable from one installation to another with little if any floor reinforcement and/or stabilisation being required and being able to be installed in factories whose floors are otherwise inadequate.

Preferably the chassis is fabricated of welded steel plate. Conveniently, the three limbs are separate fabrications which are dis-assemblably connected together, typically by bolting. Further the limbs are preferably closed sections, filled with sound-deadening foam. Conveniently the bottom limb has one or more fume extraction ducts extend within it from the region of the workpiece support.

Normally the robotic arm is a six axis arm.

In accordance with a particular feature of the invention, the machine includes a turntable mounted on the bottom limb, providing a seventh axis. Usually the seventh axis will be a Z axis, i. e. a vertical axis in the normal orientation of the machine. However, it is envisaged that it could have an X or Y orientation.

Preferably where the seventh axis is a Z axis, it is offset from below the first, Z axis, of the robotic arm.

To help understanding of the invention, a specific embodiment thereof will now be described, with reference to the accompanying drawings, in which: Figure 1 is a perspective view from one side of a cutting machine according to the invention, without a robotic arm ; Figure 2 is a similar perspective from the other side with a robotic arm fitted and showing a jig and a workpiece, but with a seventh axis extension removed.

Referring first to Figure 1, the cutting machine there shown has a C-shaped rigid chassis 1. Its bottom part or limb is in the form of a fabricated steel plate box structure 2 having a top plate 3, side plates 4 and a bottom plate-not visible in Figure 1. The box structure is welded closed and filled with sound deadening foam. Onto the top plate at the front of the bottom limb, a further plate 5 with four clamping bars 6 provide a cutting bed, to which jigs and workpieces-not shown in Figure 1-can be fixed. The cutting bed plate 5 has three openings 7 to internal ducts 8 passing

through the box structure to a rear plenum chamber 9 and thence to an exhaust duct 10 attached to the plenum chamber. The ducts 8 have selection valves whereby the opening (s) 7 closest to the plasma exhaust being created during cutting can be drawn into the opening (s).

Additionally at the front of the bottom limb there is provided a removable extension 11 for a seventh axis platform 12 described in more detail below.

At the back of the top plate of the bottom limb, it has a joint face 14. Bolted 15 to the joint face 14 is an upright limb 16, having top and bottom joint faces 17, 18, the bottom one being abutted against the joint face 14. The upright limb 16 is a fabricated from steel plate in like manner to the bottom limb as a closed box with sound deadening foam inside. A top limb 21 also is similarly constructed. It has a joint face 22, via which it is bolted 23 to the top joint face of the upright limb.

The top limb, again fabricated of steel plate, tapers forwards to a bearing housing 24 for the first axis of a six axis robotic arm 25 shown in Figure 2 and equipped with a plasma cutting head 26. At the back of the machine, above the exhaust plenum chamber 9, a controller and a generator for the arm and the cutting head are housed 27.

As will be appreciated the C-shaped chassis, comprised of the three limbs bolted together, is a strong, self-contained structure and as such it is able to isolate a workpiece on the cutting bed 3 from dimensional variations with respect to the cutting head 26, which might otherwise be introduced by deflections of the floor or other structure on which it is installed.

For processing large workpieces, of a size that would for instance stretch the reach of the robotic arm to the point where inaccuracies are incipient due to large elbow angles, the seventh axis platform 12 can be installed at the front of the bottom limb with its axis 30 vertical. The workpiece W is clamped to turntable so as to be rotatable in the horizontal plane, in which it is most extensive. By rotating the workpiece W, about the vertical axis the robotic arm can be programmed to never have to reach to the remote side of the workpiece, which is instead rotated to be close

to the arm. It will be appreciated that this has advantage in speed of processing by reducing the length of traverse from one cut to another, if not the length of traverse during cutting, and in accuracy of cutting in not over extending the arm.

As the man skilled in the art will appreciate small changes of elbow angle close to full extension when the elbow angles are large give rise to comparatively large cutting head deflections. Thus small zero errors in elbow angles give rise to larger cutting zero errors at larger elbow angles than is the case with smaller elbow angles.

Installation with the seventh axis offset from the fifst axis has the advantage of being able to present more portions of the workpiece close below the first axis through the housing 24 with the arm little extended than other wise.

The removable extension 11 for the seventh axis platform 12 can be installed on the cutting bed 5, should this be more convenient for certain jobs. Whilst normally the seventh axis 30 will be vertical, it is conceivable that the axis could be arranged horizontally for certain types of workpiece.