The present invention relates to angular measurement devices of the type intended primarily for use by both amateur and professional carpenters. The device is intended primarily for use in the production of accurately fitting mitre joints in dado rails, comices and the like but may also be used with benefit for a great many other applications, such as picture framing, glass cutting, laying flooring, metalwork and the like.

Many mitre joints are at precisely 90° which means that the two members to be jointed together must both be cut at precisely 45°. This is relatively easy to achieve using a so-called mitre box which includes two upstanding parallel formations which define a channel and in which a number of slots extending at 45'are formed. The members to be jointed are positioned within the channel and their ends cut off at 45° using a saw located in the slots.

However, whilst mitre boxes are suitable for small scale amateur use, they can only accommodate relatively small members and they cannot accommodate thicker members which are to be jointed together and nor can they generally be used to produce mitre joints which are at any angle other than 90°. Professional carpenters use so-called mitre saws, which include a saw, typically an electrical circular saw, mounted above a worktable so as to be pivotable about the table through a limited angular range. The worktable carries an upstanding support against which a workpiece may be located and extending around a proportion of the periphery of the worktable is an angular scale, which is typically marked 0° in the centre and extends to 45° on each side. Connected to the saw is an

angular indicator which cooperates with the angular scale. When the saw is positioned so that the indicator is at 0°, it will form a cut in an elongate workpiece against the support which extends at right angles to the length of the workpiece. The precise angle of the cut formed in the workpiece is determined by positioning the saw such that the indicator is at the desired angular position on the scale. When forming a mitre joint at, say, 130°, it is usual for a profiling member to be placed around the corner in question to obtain an indication of the shape and angle of the corner. This is then transferred manually onto the members to be cut and the carpenter or the like then makes a visual estimation of the bisector of the angle and cuts the members along that bisector. This is not only relatively time-consuming but also frequently results in untidy joints because it is not possible reliably to determine the precise position of the angular bisector by visual examination alone.

It is, therefore, the object of the invention to provide an angular measurement device which will facilitate the production of mitre joints and, in particular, will reliably enable the bisector of the joint angle to be determined and will thus ensure that neat and accurate mitre joints are made.

According to the present invention, an angular measurement device, particularly for use in the production of mitre joints, includes a first member, which overlies and is pivotably connected to a second member about a pivotal axis, the second member carrying a substantially part-circular angular scale and having a first straight edge, and the first member having an angular indicator, which cooperates with the angular scale, and a second straight edge, the two straight edges intersecting at a position adjacent the pivotal axis, the angular indicator being situated on the angular scale at a position on the scale indicative of the

value of the bisector of the angle subtended by the two straight edges. Thus the measurement device in accordance with the invention includes two straight edges carried by respective pivotally connected members. In use, the two straight edges are placed into contact with the surfaces of the wall or the like to which two mitre jointed pieces of wood or the like are to be connected by appropriate relative rotation of the first and second members. The angular indicator on the upper member will indicate an angular value on the angular scale on the lower member and this angular value is indicative of the value of the bisector of the angle subtended by the two straight edges. A mitre saw may then be used to cut the two pieces of wood to the desired complementary shapes by placing the angular indicator to the indicated angle on one side of the 0° mark and cutting one of the pieces of wood. The angular indicator on the mitre saw is then placed to the same angular value on the other side of the 0° mark and the other piece of wood is cut. The two pieces of wood will then fit together precisely to form a mitre joint whose angle corresponds precisely to that of the wall or like to which the jointed pieces of wood are to be connected.

As mentioned above, the first and second straight edges intersect at a position adjacent to the pivotal axis. It would be convenient if they were to intersect on the pivotal axis but this is not readily possible and nor is it essential. If the two straight edges themselves were actually to intersect, the measurement device would have an apex or sharp corner at this point and this is undesirable. It is therefore preferred that the first and second straight edges merge into radiussed portions of the second and first members respectively, i. e. that the pivotal axis is positioned adjacent to a radiussed corner of the first and second members. In this event, it will of course be the projection of the two straight edges which intersect at a position adjacent to the pivotal axis.

The first and second members may have a variety of different shapes but it is preferred that the first member has a third straight edge, which intersects the second straight edge and a first substantially part-circular edge, which intersects the second and third straight edges and projecting from which is the angular indicator. Similarly, it is preferred that the second member has a fourth straight edge, which intersects the first straight edge, and a second substantially part- circular edge, which intersects the first and fourth straight edges and adjacent to which is the part-circular angular scale.

The angle subtended by the two surfaces of the wall or like could, at least theoretically, be as low as 22. 5° or even 20° or so and as high as 350° or so. It is in practice not possible to provide a single measuring device capable of measuring angles of such a large range and if it is desired to be able to measure over the entire range a number of different measurement devices, preferably four, will be required.

In one embodiment of the invention, the first and second members are of generally semicircular shape, the second and third straight edges being substantially colinear and the first and fourth straight edges also being colinear.

Such a device is suitable for measuring angles in the range of approximately 180° to approximately 350°.

In a further embodiment of the invention, the first and second members generally have the shape of a quadrant of a circle, the second and third straight edges being substantially perpendicular and the first and fourth straight edges also being substantially perpendicular. Such a device is suitable for measuring

angles in the range of approximately 90° to approximately 180°.

In a further embodiment of the invention, the first and second members generally have the shape of a segment of a circle, the second and third straight edges and also the first and fourth straight edges defining an angle in the range of 40° to 50°, preferably about 45°. This device is suitable for measuring angles in the range of about 45° to 90°.

In a further embodiment of the invention, the first and second members again generally have the shape of a segment of a circle, the second and third straight edges and also the first and fourth straight edges defining an angle in the range of 20° to 25°, preferably about 22°. This device is suitable for measuring angles in the range of about 22° to about 45°.

The invention thus also embraces a set of four angular measurement devices which are capable between them of measuring angles of all sizes which are in practice likely to be encountered, the set including one device of each of the four types referred to above.

If the angle of the joint to be made is e. g. 40°, the angular indicator on the measuring device will indicate 20° and the two 20° settings on a mitre saw are then used to cut the pieces of wood or the like to produce the joint. If, however, the angle of the joint is to be, say, 130°, whereby the value of the angular bisector is 65°, it will be appreciated that this is in fact equivalent to cutting the pieces of wood at 25° and the two 25° settings on the mitre saw are then used.

If the angle of the joint is to be, say, 300°, the value of the angular bisector is 150° which may be achieved by using the two 30° settings on the mitre saw. As

mentioned above, the angular scale on mitre saws typically covers only 90°, that is to say with 0° in the centre rising to 45° on each side. It is therefore preferred that the angular scale on the second member carries numerical angular values, all of which are between 0° and 45°.

Further features of the invention will be apparent from the following description of four specific embodiments of angular measurement device in accordance with the invention which is given by way of example with reference to the accompanying drawings, in which: Figure 1 is a plan view of a first device in accordance with the invention for measuring angles in the range of 180° to 350° ; Figures 2 and 3 are plan views of the lower member and upper member respectively of a second embodiment of the invention suitable for measuring angles in the range of 90° to 180° ; Figures 4 and 5 are views corresponding to Figures 2 and 3 of a third embodiment suitable for measuring angles in the range of 45'to 90' ; and Figures 6 and 7 are further views corresponding to Figures 2 and 3 of a fourth embodiment suitable for measuring angles in the range of 22° to 45°.

Referring firstly to Figure 1, the device comprises a lower member 2 of substantially semicircular form extending around the outer circular edge of which is an angular scale which starts at 0° and rises to 45° and then sinks again towards 0, though in practice it does not actually reach 0 but terminates on the

left-hand side, as seen in Figure 1, at about 10° because it will in practice never be desired to make a mitre joint whose joint angle is greater than 350°.

Overlying the lower member 2 is a substantially semi-circular upper member 4 which is pivotally connected to the lower member 2 by a rivet or the like 6 which passes through mating holes in the two members 2 and 4. The axis of the holes is very close to the straight edge of the two members, typically between 3 and 15 mm from it, more preferably between 4 and 10 mm from it. The diametral dimension of the upper member 4 is slightly less than that of the lower member 2. Projecting from the upper member 4 at one end of its diameter is an angular indicator 8, which is positioned to cooperate with the angular scale. The lower and upper members have respective straight edges 10 and 12. The pivot 6 is positioned substantially symmetrically with respect to the semicircular lower member 2 but somewhat offset to the right from the lateral centre of the upper member 4, whereby, as the upper member 4 rotates with respect to the lower member 2, the angular scale on the lower member 2 is progressively exposed and the angular value indicated by the indicator 8 may always be read. In the specific embodiment of Figure 1, the lower member 2 is truly part-circular but the upper member 4 is not. Thus as may be seen in Figure 1, the left-hand portion of the curved edge of the upper member 2 is part- circular but a slightly different radius of curvature and is slightly offset centre to the part-circular right-hand portion. This feature coupled with the slightly varying length of the angular scale lines ensures that the relative angular position of the two members may be reliably read from the scale at all angular positions.

As illustrated in Figure 1, the two straight edges 10 and 12 extend at an angle to one another of 140°. The angular indicator 8 thus indicates an angle on the

angular scale of 20° and this is the value on the angular scale of the mitre saw which must be used to produce a mitre joint whose angle is 140°.

The drawings of the second to fourth embodiments show the upper and lower members separately from one another. Figures 2 and 3 show the lower and upper members 2'and 4'and the holes 14 and 16 formed in them for accommodation of the pivotal rivet 6 may be seen. The lower member has a straight edge 10'and a further straight edge 18 extending at right angles to it and connected to it by a radiussed portion 20 adjacent to the pivot hole 14. The upper member 4'seen in Figure 3 is of very similar construction and has a straight edge 12'and a further straight edge 22 extending at right angles to it and connected to it by a radiussed portion corresponding to the radiussed portion 20. In use, the angular indicator 8 will indicate a value on the angular scale indicative of the value of the bisector of the angle subtended by the straight edges 10'and 12'. This embodiment is used to measure angles in the range of about 90° to about 180°.

The construction and operation of the embodiment illustrated in Figures 4 and 5 is substantially the same and will therefore not be described in detail. In use, the angular indicator 8 will again indicate an angle on the annular scale indicative of the value of the bisector of the angle subtended by the straight edges 10'and 12'. This embodiment is used to measure angles in the range of about 45° to 90°.

The embodiment illustrated in Figures 6 and 7 is again very similar and will therefore not be described again. It is used to measure angles on the range of about 22° to 45°.