BACKGROUND Open jaw flat spanners have a fixed jaw which presents two parallel opposed faces which fit over two faces of a hexagon nut or bolt head and are provided in a range of standard sizes. Shifting spanners present similar parallel opposed faces but with a means, usually a worm screw, for adjusting the gap between the faces to provide for a range of nut and bolt head sizes. A typical shifting spanner is shown in SABS specification 1211-1984. Ring spanners have a fixed ring, which presents opposed faces to fit over all the faces of a hexagon nut or bolt head. Often the ring presents twelve faces in order to allow resetting the spanner every twelfth of a turn rather than every sixth; to do this the middle of each face is indented with the corners of the adjacent faces. This feature has been proposed for spanners with movable jaws in U. S. patents 2 537 838 and 2 618 996, for example.

A problem often encountered in work with spanners on hexagon nuts and bolt heads when large torque forces must be applied, frequently to unscrew a stubborn nut or bolt, is that the apexes of the hexagon formations become rounded. In bad cases the hexagon formations are lost to such an extent that a spanner cannot turn the nut or bolt head and then extreme difficulty often ensues.

THE INVENTION A spanner in accordance with this invention comprises opposed parallel faces for engaging a hexagon nut or bolt head, characterised in that the faces have a groove formed in the middle of each face, the groove exceeding the dimensions of an apex of a hexagonal shape and presenting two sharp corners where the groove meets the face.

Thus the groove shape and dimensions do not match the hexagon apex shape and dimensions, so that the groove does not fit onto portions of the faces of the hexagon apexes, but the sharp corners engage the hexagon faces, with a potential for digging into these faces if very large torques must be applied and hence allowing an enhanced grip.

Such a spanner gives the option of gripping a hexagon nut or bolt head not with the faces of the spanner, but with the corners of the grooves engaging on either side of apexes of the nut or bolt head.

This is a different approach from the art where faces are presented which match the hexagon shape of a nut or bolt head, albeit that where twelve faces are provided in a ring spanner the faces are indented with the corners of adjacent faces.

It has been found that the sharp corners of the grooves on the spanner faces give a grip that does not have an equivalent tendency to round off the corners of the hexagon nut or bolt head.

The groove may be semi-circular in each face. This is a convenient form, since it may be provided by drilling the faces when they are clamped together.

However, a very high quality drill must be used to cut the hardened steel usual in quality spanners. Other techniques such as laser cutting or spark eroding could be used, for example.

Alternatively the groove may be provided by appropriate inserts in the moulds or dies used in investment casting, die forming or forging techniques, for example.

THE DRAWINGS The invention will be more fully described by way of examples, with reference to the drawings. In the drawings:- figure 1 is a view of a shifting spanner according to an embodiment of the invention, figure 2 is a view of an open jaw flat spanner according to another embodiment of the invention, and figure 3 is a view of a ring spanner according to another embodiment of the invention.

THE PREFERRED EMBODIMENTS As shown in figure 1, the shifting spanner 1 comprises an open jaw of adjustable gap by means of a worm screw 2 which can move the movable jaw 3 along a slide relative to the fixed jaw 4. A handle 5 gives leverage for applying a torque to the spanner, in use.

Semi-circular grooves 6 and 7 are provided in the middle of each face 8 and 9 of each jaw, of semi-circular shape. As can be seen the grooves exceed the dimensions of apexes 10 and 11 of a hexagon nut or bolt head 12. The nut or bolt head is shown held in the jaws, gripped between the sharp edges of the grooves.

This grip is an alternative to a conventional grip between the flats of the faces of the spanner jaws.

Figure 2 is a view of an open jaw flat spanner 20 of a fixed size, these being provided in sets over a range of sizes, for mechanic's work.

As shown in figure 2, the spanner 20 comprises open jaws of fixed gap having a jaw 21 and an opposed parallel jaw 22. A handle 23 gives leverage for applying a torque to the spanner, in use.

Semi-circular grooves 24 and 25 are provided in the middle of each face 26 and 27 respectively of each jaw, of semi-circular shape. Again the grooves exceed the dimensions of apexes of a hexagon nut or bolt head (not shown) which will fit with its flats between the faces of the jaws.

Figure 3 shows a ring spanner 30 which is again of fixed size, such spanners being provided in sets over a range of sizes for mechanic's work.

The ring 31 presents six flats so that a nut or bolt head indicated by broken lines 32 will fit for normal work. Each flat has a semi-circular groove 33 in the middle of its face 34. A nut or bolt head 35 can be fitted with its apexes in the grooves for very hard work where large torques have to be applied.