The invention also relates to a method for positioning a roof truss jig on a base and for detaching it from the base as defined in the preamble of claim 1.

Roof trusses are industrially produced on a fitting table. The fitting table jig is formed with the aid of fitting tables and grids placed on these. When model of the roof truss model changes, the shape of the jig has to be changed accordingly, and in that case it is preferable to be able to change the jig shape relatively fast without fixing the fitting table solidly to the steel base underneath the table. Electric magnets are currently used to fasten fitting tables, locking the fitting table removably to a steel base underneath the table. The magnets are located in base plates in the lower portion of the fitting table, and 2 to 4 magnets are usually provided for each fitting table. However, the use of electric magnets involves the drawback of requiring electric conductors, which tend to hamper or at least complicate working around the fitting table.

In fastening a positioning device to a steel base, the use of a permanent magnet would be a notably better choice than an electric magnetic in terms of working conditions. Nonetheless, the use of permanent magnets has been impeded either by their power being inadequate to retain the fitting table or by their removal from the fitting table being onerous, requiring a change in the magnetic properties of the permanent magnet. The magnetic properties of a permanent magnet have been modified either by mechanical means by shifting the magnet rods relative to one another, this being a relatively laborious operation, however, or by means of an electric field, which, in turn, has required the use of electric conductors, which again hamper other working operations.

Positioning devices operating on the principle last described have been disclosed i. a. by EP patent specifications 211 329 and 245 939, in which the permanent-magnet base is magnetised and demagnetised as required. In addition, in the solutions of these publications, the intensity and the shape of the magnetic field of the base are matched with the shape of the tool to be retained or the body to be machined. In these patent specifications, the properties of the permanent-magnet bases are altered

by modifying the magnetic fields of the individual rods forming the permanent magnet by means of the electric field.

EP Patent Application 576 699, in turn, discloses a magnetic picking device intended for picking relatively small magnetised metal objects from narrow places.

The picking device disclosed by this patent application has a distinctly different structure than the magnetic positioning device of the invention due to its handle, elongated picking arm, support structure and shaped, non-magnetic cover plate. The picking device described in this patent application is not suitable for use in the positioning of roof trusses.

The basic idea of the invention is to develop a magnetic positioning device which does not entail the problems appearing the state of the art mentioned above.

The primary object of the invention is thus to provide a positioning device which does not comprise electric conductors hampering working operations.

The second object of the invention is to provide a permanent-magnet positioning device whose magnetic field or magnetic flux density does not need to be altered by the electric field or by mechanical means in any working step.

The third object of the invention is to develop a magnetic positioning device which can be readily fixed to and removed from its mounting base without lateral movements of the positioning device.

The permanent-magnet positioning device now developed fulfills the objects mentioned above. The invention relates to a permanent-magnet positioning device as defined in claim 1. The invention also relates to the method for positioning a jig on the base and for removing it from the base as defined in claim 5.

The basic idea of the positioning device of the invention is that the restraint action of the device is based on a permanent magnet, which can be fastened to and removed from the base by arranging the plate-like permanent magnet support with progressive spring relief. The spring force and thus the force required for removing the magnet are adjustable by varying the maximum distance of the support to the base with a vertical stopper.

The positioning device of the invention comprises a frame consisting of a plate-like frame plane and a skirt under this. Below the frame plane, a plate-like support for one or more permanent magnets has been fastened. One or more permanent magnets

have been fastened under the support. The support and the magnet have been arranged underneath the frame plane by means of one or more joining members.

The joining members consist of a rod portion and an expansion located at the lower end of the rod portion and having a thickness substantially greater than the remainder of the rod portion shape. A progressive spring is provided in the upper part of the rod portion of the joining member, on top of the plate-like frame plane.

At least one joining member is provided for each permanent magnet.

The spring accumulates spring energy when the magnet and its support are pressed into lower position. The spring then strives to lift the support and the magnet upwards. The spring is progressive with regard to the spring force, meaning that the spring energy it accumulates increases sharply as the compression ratio of the spring increases. The design of such a spring-releaved support and the permanent magnet fastened to it has the advantage of allowing the permanent magnet to be raised from its lower position to its upper position with relatively small additional energy. The positioning device is thus easy to detach and to refasten to the mounting base without having to alter the magnetic properties of the permanent magnet or the properties of its magnetic field. Nor do the magnet rods of the permanent magnet of the positioning device need to be shifted with regard to one another.

The expansion of the joining member, in turn, acts as a stopper of the vertical movement of the magnets and its/their support (s), its position being vertically adjustable in the lower portion of the rod-like portion. The vertical movement stopper is at a location in the lower portion of the rod-like portion such that it provides a given play for the magnet and its support to shift from upper position into lower position. In addition, the vertical movement stopper should be able to be blocked or otherwise immobilised at the location in the lower portion of the rod-like portion of the joining member at which it has been fitted.

The plate-like frame plane of the positioning device is supported by the mounting base through a skirt surrounding the frame plane. The skirt should be high enough to allow for sufficient play between the upper and lower position of the magnet by means of a stopper. Such a frame construction has the advantage of the magnet being lowered to the mounting base in its lower position, the attractive force between the base and the magnet then efficiently preventing any movements of the positioning device. All the same, in its upper position, the magnet is at a distance from the mounting base, at the level of the lower portion of the positioning device frame skirt such that it does not notably impede the movements of the positioning

device on the mounting base when one wishes to shift the positioning device in the lateral direction.

The advantage of the positioning device of the invention described above over conventional positioning device solutions based on the use of an electric magnet is that the force retaining the jig of the device is based on the use of a permanent magnet, so that the positioning device does not require any electric conductors which hamper working.

The advantage of the positioning device of the invention over prior art solutions, in which the intensity of the permanent magnet, the magnetic flux density or other properties of the magnet have to be changed before the magnet is removed from the base, is that the attractive force between the permanent magnet and the base can be modified by purely mechanical means in the solution of the invention.

Other advantages of the positioning device of the invention to be mentioned is that the positioning device has a straightforward design and low production costs, because its feature of being attachable to and removable from the base is based on purely mechanical solutions. The adjustable vertical stopper provides the benefit of allowing the same positioning device to be used for positioning fitting tables of various sizes and volumes.

The positioning device of the invention has the additional advantage of being usable in the lifting of bodies when a protective plate with alternating non-magnetic and magnetising material has been placed under the magnet. The use of a protective plate involves the additional advantage that such a positioning device usable as a lifting device is easy to clean.

The invention is described in greater detail below with reference to the accompanying figures.

Figure 1 is a perspective view of the positioning device of the invention.

Figures 2A and 2B show longitudinal cross-sections of the joining member in plane II-II of figure 1. Figure 2A shows the spring in compressed position, figure 2B shows the same spring in rest position.

Figure 3 is a bottom view of the positioning device viewed in direction III of figure 1.

Figure 4 shows a cross-section of the positioning device of figure 1 viewed in plane IV-IV with the magnet and its support in lower position, i. e. touching the base.

Figure 5 shows the positioning device of figure 4 with the magnet and its support in upper position. The figure also shows an optional structure of the positioning device, in which a protective plate has been placed under the magnet (the portion indicated with a broken line in figure 5).

Figure 6 is a bottom view of the protective plate of figure 5, i. e. viewed in direction VI.

The main components of the figures are briefly discussed below.

Figure 1 illustrates the structure of the joining member 5 extending on top of the frame 2 and of the frame plane 2,2a.

Figures 2A and 2B show structural details of the joining member 5. Figure 2A shows a situation in which the progressive spring 7 is in compressed state and figure 2B a situation in which the same spring is in rest position.

Figure 3 is a bottom view of the positioning device. The figure shows the positioning of the magnet 4 and the magnet support 3 underneath the frame plate 2, 2a.

Figures 4 and 5 show the magnet 4 and its support 3 in their lower and upper positions, when the progressive spring 7 located in the upper portion 6,6a of the rod-like portion of the joining member is pressed into compressed position and released into rest position, respectively.

Figure 6 shows the structure of the protective plate 14 in greater detail.

Figure 1 shows the magnetic positioning device 1 of the invention, comprising a frame 2. The frame comprises a plate-like rectangular frame plane 2,2a and a skirt 2,2b solidly or removably fastened to the edges of the frame plane, the positioning device being supported by the base through the skirt. A plate-like support 3 for the permanent magnet 4 is provided underneath the frame plane 2,2a. The support has two parts (figures 4 and 5), consisting of a spacer plate 3,3a and an elevating plate 3,3b. The support acts as a support and a mounting base for the permanent magnet 4. The permanent magnet support 3, the permanent magnet 4 and the frame plane 2, 2a have concentric openings, through which the rod-like portion 6 of the joining member 5 is inserted. The upper portion 6a of the rod-like upper portion of the

joining member is located above the frame plane and the lower portion 6b of the rod-like portion is located under the magnet when the positioning device is viewed from above the frame plane. The positioning device illustrated in the figure comprises four joining members.

At a suitable location of the lower portion 6b of the rod-like portion of the joining member, a vertical movement stopper 11 is provided to prevent the support located underneath the positioning device frame and the associated magnet from moving underneath the stopper. In the arrangement of the figures, the vertical movement stopper 11 consists of an elongated nut, which is retained in position by splitting the nut on the side facing the shorter side of the positioning device frame plane 2,2a.

The rotation of the movement stopper is prevented by providing a suitable revolution stopper 12, such as the flat bar shown in figure 3. A progressive spring 7 has been solidly or removably fastened in the upper portion 6,6a of the rod-like portion of the joining member, i. e. on the side opposite to the positioning device frame plane 2,2a relative to the magnet 4. The location of the vertical movement stopper in the lower portion of the rod-like joining member depends on the intensity of the permanent magnet, the spring force of the spring and the volume of the fitting table to be aligned. The vertical movement stopper is fitted at a location of the lower portion 6,6b of the rod-like portion of the joining member such that the permanent magnet touches the base when the progressive spring 7 is pressed into its lower position (figure 4).

A permanent magnet strong enough for the positioning device of the invention is used as the magnet. The embodiment of the permanent magnet is shown in figure 2.

Magnetically active and inactive parts alternate in the permanent magnet. The active parts are elongated steel bars 9 and the inactive parts are rubber-coated metal alloy or steel bars 10.

If a protective plate 14 (figure 5) is fitted underneath the lower portion 6,6b of the rod-like portion of the joining members of the positioning device 1, roughly at the level of the lower end of the skirt 2,2b, the device is usable both for aligning the jig and for lifting magnetising bodies. Magnetising zones 14,14a and non-magnetising zones 14,14b alternate in the protective plate 14. A magnetising zone is made of a magnetising steel quality, whereas a non-magnetising zone is made of a non- magnetising steel quality. The magnetising zones in the protective plate are aligned with the magnetically active parts 9 of the permanent magnet and the non- magnetising zones are aligned with the magnetically inactive parts 10 of the permanent magnet.

The positioning device has the following operation: when it is desirable to modify the jig shape, the locations of the desired fitting tables (not illustrated) are repositioned either by ocular approximation or by means of instruments such as a CAD-controlled laser meter. The positioning device magnets inserted in the holes in the plates provided in the lower portion of the fitting tables are brought into lower position (figure 5) by pressing the spring-releaved joining member or members by their head 13 (cf. figure 2, illustrating the operation of a spring-releaved joining member), the magnet being positioned on the mounting base at the level of the lower edge of the positioning device skirt. The spring force generated by the progressive spring on the joining member when the spring is compressed shall be adaptable to the properties of the base by adjusting the vertical position of the movement stopper in the lower portion of the rod-like portion of the joining member. In the positioning device of the invention, the spring force is easily controlled by screwing or otherwise shifting the movement stopper 11 in the upper portion of the rod portion. The positioning device inserted into the hole in the lower portion of the fitting table blocks any lateral rotation of the fitting table owing to the attractive force between the permanent magnet and the mounting base.

When it is desirable to modify the shape of the jig, the desired fitting tables are detached from the mounting base and are repositioned by lifting the spring-releaved joining member 5 upwards by its head 13 at the upper end of the rod-like portion of the joining member, using either a solid or a removable spanner wrench. In this situation, the spring force of the spring on the joining member forces the magnet located underneath the positioning device frame plane to rise into upper position (figure 5), i. e. close enough to the frame plane for the attractive force between the magnet and the mounting base to decrease to an extent allowing effortless displacement of the fitting table on the base.

Only one embodiment of the magnetic positioning device of the invention and its use for positioning a jig have been described above. It is obvious to a person skilled in the art that the invention can be performed in many other ways implementing the inventive idea defined in the claims.

Thus, there may be a varying number of joining members in one magnetic positioning device, depending primarily on the intensity of the magnet or magnets, but the magnet still acts in the same way with one single joining member. If the positioning device comprises a plurality of separate permanent magnets, at least one joining member provided with a progressive spring will be required for each permanent magnet. The shape of the plate-like frame depends on the design of the

fitting table and the position of the positioning device in its lower portion. Yet in most cases, the frame plane is rectangular as in the embodiments of the invention described above. The structure of the permanent magnet as such is not vital for the invention, but the permanent magnet used may have any structure of prior art solutions. The permanent magnet is usually a plate-like body consisting of a plurality of rod-like bars, part of which are provided in a magnetically inactive state and part in a magnetically active state. The inactive material is often a magnetic material, such as a magnetic metal, metal alloy or any other material made non- magnetic e. g. by coating it with rubber, and the magnetically active material is a magnetising material, usually a metal. The magnet may also consist of a plate-like body with alternating magnetic material and non-magnetic material.

The rod-like portion 6 of the joining member is a bar, screw, bolt or the like. The expansion of the rod-like portion of the joining member 5 is a pin, bushing, nut, ring or any similar fitting fastened vertically adjustably to the lower end 6 of the rod portion of the joining member.

The immobilisation of the movement stopper can be performed by other means than those described in the embodiment above. One way of doing this is to fasten the movement stopper to the rod portion with a separate carrier pin.

The material and the shape of the plate-like support does not have any special impact on the invention. The support is made up of one single or several parts. The spacer plate 3, 3a in the magnet support and any elevating plate 3, 3b provided under this, as well as the permanent magnet 4 under the elevating plate, have been fastened solidly or removably to one another. The way of fastening the magnet to the spacer plate does not have any special impact on the inventive idea, and thus this fastening can be performed e. g. with riveted joints, screw joints or any similar joints.

In addition to the components described above, the positioning device may include various parts fastening the positioning device to the fitting table and parts facilitating displacements of the fitting table. Such parts may be e. g. various parts attached outside the positioning device frame, parts inserted into the hole in the fitting table to prevent vertical movement of the positioning device, and various projections and claws attached to the frame. To facilitate displacements of the fitting table, e. g. rolls, wheels or similar components can be fitted accordingly on the lower side of the positioning device frame.

The problem of blocking any movements of the vertical movement stopper during the use of the positioning device can be resolved with the stopper design or by fastening the stopper to the rod portion with external fastening means, such as a pin or the like.