TITLE

Means for locating a datum point.

FIELD OF INVENTION

A preferred form of this invention relates to the determination of a datum point for work to be done on a workpiece. BACKGROUND

In the context of manufacturing, it is known to form or repair products or parts of products, often referred to as workpieces. This may involve cutting a workpiece or performing other operations on it. The process may require the location of a datum point, or in other words, a point of reference to guide a machine as it engages the workpiece. Setting a datum point may help avoid wasting time and improve the accuracy of work done on the workpiece.

Various means exist to locate the straight edge of a workpiece. Electrical discharge machines may do so by slowly approaching and sensing the edge electrically and then displaying its position. Milling machines can be fitted with a wiggler device which enables an operator to locate the edge of a workpiece. If the workpiece has two intersecting straight edges that are parallel or perpendicular to a machine axis, it is relatively simple to locate each edge and establish a datum point using X and Y movement features of the machine. It is more difficult to establish a datum point when the workpiece has, for example, one straight edge and one radiused edge, or if one of the edges is a curved spline and not a true radius, or if the workpiece edges are not parallel or perpendicular to one another or to the machine axis.

It is an object of a preferred form of the invention to go at least some way towards addressing one or more of the above issues. While this object applies to the preferred embodiment, it should not be seen as a limit on the scope of the invention. The object of the invention per se is simply to provide the public with a useful choice. SUMMARY OF THE INVENTION

According to one aspect of the invention there is a method for determining a datum point for a workpiece, comprising the steps of:

a) obtaining a tool that has an arm and a hub, the hub having an exterior radial edge and an aperture, the axis of the radial edge being at the centre of the aperture;

b) placing the tool against intersecting edges of a workpiece such that the arm abuts a straight edge of the workpiece, and the radial edge of the tool abuts a curved or further straight edge of the workpiece;

c) determining the centre of the interior aperture and assigning it as the datum point; and

d) causing a machine to work on the workpiece with reliance on the datum point. The hub's exterior radial edge may or may not be part of a longer curved edge that has non-radial parts.

Optionally the aperture of the hub is circular. Optionally the arm has a straight edge and that edge abuts the straight edge of the workpiece.

Optionally the straight edge of the arm is tangential to the exterior radial edge of the hub.

Optionally the datum point is inset from the straight edge of the workpiece and is offset from the curved edge of the workpiece, the degree of insetting and offsetting being the same. Optionally the datum point is inset from the straight edge of the workpiece and is offset from the curved edge of the workpiece, the degree of insetting and offsetting being different. Optionally at step c) the centre of the aperture is determined by placing an insert in the aperture, the insert having a radial edge, and determining the central axis of the insert's radial edge.

Optionally at step c) the centre of the aperture is determined by placing an insert in the aperture, the insert having a symmetrical projection, and determining the centre of the symmetrical projection.

According to a further aspect of the invention there is a tool for determining a datum point for a workpiece, the tool having:

· an arm adapted to align evenly with a straight edge; and

• a hub having:

o an exterior radial edge; and

o an aperture;

the tool being such that the radial axis of the exterior curved edge is at the centre of the aperture.

Optionally the arm has a straight edge.

Optionally the straight edge of the arm is aligned tangentially with respect to the exterior radial edge.

Optionally the hub's aperture is circular.

Optionally the radial dimension of the exterior curved edge is indicated on the tool.

BRIEF DESCRIPTION OF THE DRAWINGS

Some preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, of which: Figure 1 is a plan view of a tool for determining a datum point;

Figure 2 is a plan view illustrating the tool in use with a workpiece;

Figure 3 is a series of plan views illustrating the tool in use with workpieces of alternative shapes; Figure 4 is a plan view illustrating an alternative tool for determining a datum point in use with a workpiece.

Figure 5 is an isometric view of a the tool of figures 1 , incorporating a pillar shaped insert;

Figure 6 is a plan view of an alternative version of the tool;

Figure 7 is an isometric view of an alternative version of the tool, incorporating a square aperture and an insert that is square in transverse cross section;

Figure 8 an view illustrating an alternative form of the tool; and

Figure 9 is an isometric photographic view of a tool having an insert corresponding to that of figure 5,, but with a diagonal slit down its hub.

DETAILED DESCRIPTION

Referring to Figure 1 , the tool is for determining a datum point and has a straight edged arm 1 extending from a hub 2. The hub 2 has a central circular aperture 3 and an exterior curved edge 4. The curved edge 4 is radial from the centre 5 of the circular aperture 3. The straight edge 6 of the arm 1 is tangential with respect to the curved edge 4 . The radius of the curved edge 4 is indicated on the tool as at 7. In this case, it is 4mm although it could be any useful dimension.

Referring to Figure 2, the tool is shown engaged with a workpiece 8. The workpiece 8 has a straight edge 9 intersecting with a curved edge 10. The straight edge 6 of the tool's arm 1 abuts the straight edge 9 of the workpiece 8, and the curved edge 4 of the hub 2 abuts the curved edge 10 of the workpiece. The centre 5 of the circular aperture 3 defines a datum point for machine work to be done on the workpiece 8.

Referring further to Figure 2, the dashed line 1 1 is parallel to the straight edge 9 of the workpiece and inset from it by a distance equivalent to the radial dimension of the hub's curved edge 4. The dashed line 12 follows the curved edge 10 of the workpiece, and is offset from it by a distance equivalent to the radius of the hub's exterior curved edge 4. The datum point is located at the intersection of the two dashed lines 1 1 and 12. These lines 1 1 , 12 may be indicated on a drawing of the workpiece and the datum point used as the basis for commands given to a machine that performs cutting, milling or other operations on the workpiece.

As shown in Figure 3, the tool may also be used in similar fashion at the intersecting edges of alternative shaped workpieces 8a - 8f.

When the tool has been arranged at the intersecting edges of a workpiece, the datum point, or in other words, the centre 5 of the tool's circular aperture 3, may be determined using known techniques such as centre finding mechanisms on electrical discharge machines or with a wiggler tool or a 'dial test indicator' (also known as a 'clock gauge').

Referring to Figure 4, in an alternative embodiment of the invention, the tool may be formed so that the straight edge of its arm 13 is not tangential to the exterior curved edge 14 of its hub 15. In this case, the curved edge of the hub is spaced from the straight edge of the arm. This enables the location of a datum point to be more inset from the straight edge of the workpiece than it is offset from the curved edge of the workpiece. For example the datum point could inset 6mm from the line of the straight edge of the workpiece and offset 4 mm from the curved (or adjacent) edge of the workpiece.

Referring to figure 5, in an alternative embodiment of the invention the tool 16 has an insert in the form of a pillar 17 . The pillar 17 is placed snugly in the hub's circular aperture so that the pillar projects outwards. In this instance the pillar 17 is in the form of a symmetrical bar (circular in transverse cross section). The central axis 18 of the pillar is aligned with the centre 5 of the hub's circular aperture 3. The datum point, or in other words the hub's centre 5, can be therefore determined by reference to the radial outer edge of the pillar 17. Referring to figure 6, in still further embodiments of the invention the hub's aperture may be other than circular; for example square. In this case the centre 19 of the square aperture defines the datum point. The square's centre may be determined with or without a pillar-like insert 20 as shown in figure 7; the insert 20is square in transverse cross section to match the square aperture. The shape of the hub's aperture and the insert's pillar is not critical in that a number of symmetrical shapes can be used in a similar fashion to what is described above. Referring to figure 8, in some embodiments of the invention the part of the arm of the tool that contacts a straight edge of a work piece may be other than straight-edged, but is nonetheless able to align evenly with the straight edge of the workpiece. As shown, the arm has a series of lobes 21 that extend the same distance so that they can all contact the straight edge of a workpiece simultaneously. The workpiece is not shown, but the position its straight edge would occupy is indicated by the straight broken line 22. The curved broken line 23 indicates a tangential relationship between the line 22 and the radial exterior of the hub 24. The datum point is indicated at 25. In still further embodiments the radial part of the hub 24 may be lower down so that it is not tangential (eg in similar manner to the tool of figure 4). Further, instead of the lobes 21 some other shape may be adapted, for example a blunt evenly serrated profile or some other shape that allows the arm to be evenly laid against the straight edge of a workpiece.

Referring to figure 9, in some embodiments of the invention the tool may have a diagonal slit 26 to enable a centre locating wire to be more easily moved into the hub's central aperture, eg side-ways through the hub's side wall without an operator having to detach the wire from its supportive hardware. The fact that the slit 26 is diagonal means that the centre finding wire is not able to move strictly side-ways out of the aperture once aligned straight and inside the aperture doing its centre finding work.

While some preferred embodiments of the invention have been described by way of example it should be appreciated that modifications and improvements can occur without departing from the scope of the following claims.