The present invention relates to a stapler for driv¬ ing staples into an object, such as a bundle of paper sheets, said stapler having a frame, a magazine arranged in the frame and containing substantially U-shaped sta¬ ples, and a drive element adapted to push a staple out of the magazine and drive its legs through the object. Prior-art staplers have a fixed anvil which is so positioned that the free leg ends of a staple discharged from the magazine will strike, when the staple is driven into the object, against the anvil which then bends the staple legs driven through the object, against the object in a direction towards each other. The staple thus bent exhibits, in the bent portion of each leg, a curve or bulge making the staple considerably thicker at its ends, as seen in the driving-in direction of the staple, than in its central portion where the staple legs engage the rear side of the object. Such a thickness increase is of course not desirable. For example, it is inconvenient when seve¬ ral bundles of an optional number of paper sheets stapled together are inserted in a binder, since the thickness of the collected bundle becomes considerably greater at the upper left-hand corner where the staples are usually placed when stapling paper sheets.

To overcome this problem, it has been proposed to replace the fixed anvil with a clinching mechanism which bends (clinches) the staples in such a manner that they will have a uniform thickness over their entire length. Known clinching mechanisms rely on one movable anvil mem¬ ber for each staple leg.

A known clinching mechanism will now be described with reference to Fig. 1, illustrating only those parts of a known stapler which are essential to the present description. In Fig. 1, the stapler is shown in a position in which its drive element 1 is driving a U-shaped staple 2 into a bundle 3 of paper sheets placed in the stapler.

The clinching mechanism of the stapler comprises two anvil members 4 which are arranged substantially symmetrically with respect to the staple 2 and which each cooperate with one leg 2a of the staple 2. Each anvil member 4 is pivot- able about a shaft or pin 5 perpendicular to the plane of the staple 2. As illustrated, the shafts 5 are disposed in the upper portion of the anvil members 4 on each side of the staple 2, i.e. outside its legs 2a. When the staple 2 is driven into the bundle 3 of paper sheets, its free leg ends will encounter, as shown, an oblique surface 6 of the respective anvil member 4, such that the legs 2a are bent slightly inwards, as the staple 2 is driven further into the bundle 3.

When the staple 2 has been driven completely into the bundle 3 of paper sheets, i.e. when its web portion engages the top side of the bundle, the anvil members 4 are pivoted by means of an operating slide 7, which is moved upwards, about the shafts 5 in the direction of the arrows indicated within the anvil members, to bend the legs 2a of the staple 2 into abutment against the under¬ side of the paper bundle 3. In this manner, the staple 2 will be bent without any thickened portions of the type described above occurring at the ends of the bent staple. Clinching mechanisms of this type are disclosed, e.g. in WO 90/08015, US 4,593,847 and US 4,449,661.

Since, as is often the case, the motion of the ope¬ rating slide 7 is synchronised with the motion of other components, such as the drive element, included in the stapler, and the slide 7 therefore is mechanically con- nected to these components, it is difficult to design the clinching mechanism of Fig. 1 in such a manner that the upward displacement of the slide 7 stops in a well-defined turning position, which is necessary for achieving optimum bending results. The difficulties in this respect are caused by tolerance variations of the different compo¬ nents, and by the components "settling" when the stapler has been used for some time.

The object of the present invention therefore is to provide a stapler having a clinching mechanism, by means of which this problem can be easily overcome and by means of which so-called post-compression, reducing the spring- back of the staple legs after bending, can be done close to the ends of the bent staple and, hence, becomes more effective.

According to the present invention, this object is achieved by means of a stapler for driving staples into an object, such as a bundle of paper sheets, said stapler having a frame, a magazine arranged in the frame and con¬ taining staples, a drive element adapted to push a sub¬ stantially U-shaped staple out of the magazine to drive its legs through the object, and an anvil device having two anvil members which are arranged substantially sym¬ metrically with respect to the staple driven into the object, and which are each pivotable about an axis sub¬ stantially perpendicular to the plane of the staple, said anvil members being pivotable between a starting position, in which they are so positioned that the free leg ends of the staple will engage the respective anvil member when the staple is driven into the object, and an end position, in order, when pivoting from the starting position to the end position, to bend the projecting end portions of the staple legs driven through the object, against the object in a direction towards each other, said stapler being cha¬ racterised in that the axis of each anvil member is so positioned that the anvil member, when pivoting from the starting position to the end position, is swung towards the other anvil member at the side of the anvil axis fac¬ ing the staple.

When the anvil members are each pivotable about one axis, the distance between the anvil axes is preferably less than or equal to the distance between the legs of the staple, and when the anvil members are pivotable about a common axis, this axis is preferably located midway between the legs of the staple driven in.

In a preferred embodiment, the anvil members are pivotable from the starting position to the end position by operating means cooperating with a first cam provided on the respective anvil member. The anvil members may also be pivotable from the end position to the starting posi¬ tion by said operating means which then also cooperates with a second cam provided on the respective anvil member. Alternatively, resilient means may be provided for urg¬ ing the anvil members to the starting position, which is defined for each of the anvil members by a stop lug, against which the anvil member abuts in the starting posi¬ tion.

The invention will now be described in more detail with reference to the accompanying drawings, in which: Fig. 1 shows parts of a stapler equipped with the clinching mechanism described above;

Fig. 2 is similar to Fig. 1 but shows parts of a stapler according to the invention equipped with a new type of clinching mechanism; and Figs 3-8 show another stapler according to the inven¬ tion in different stapling positions.

The stapler shown in Fig. 2 differs from that shown in Fig. 1 only in that it is equipped with a new type of clinching mechanism, and like parts therefore have like reference numerals in Figs 1 and 2. The basic difference between the new clinching mechanism used in the stapler of the invention and the clinching mechanism described above will now be explained with reference to Fig. 2.

In Fig. 2, the stapler is shown in the same position as in Fig. 1, i.e. the position in which its drive element 1 is driving a U-shaped staple 2 into a bundle 3 of paper sheets placed in the stapler. The clinching mechanism in the stapler of Fig. 2 also has two anvil members 4 which are arranged substantially symmetrically with respect to the staple 2 and which each cooperate with a respective one of the two legs 2a of the staple 2 and are each pivot¬ able about a shaft 5 perpendicular to the plane of the

staple 2. In the stapler according to Fig. 2, the shafts 5 are however located in the lower portion of the anvil mem¬ bers 4, and not in their upper portion, and the distance between the shafts 5 is smaller, and not greater, than the distance between the legs 2a of the staple 2. When the staple 2 is driven into the bundle 3 of paper sheets, its free leg ends strike on an oblique surface 6 on the respective anvil member 4, such that the staple legs 2a are bent slightly inwards as the staple 2 is being driven further into the bundle 3.

When the staple 2 has been driven completely into the bundle 3 of paper sheets, the anvil members 4 are swung by a respective operating member 7', which is moved upwards, about the shafts 5 in the direction of the arrows indicat- ed within the anvil members, for bending the legs 2a of the staple 2 into abutment against the underside of the paper bundle 3. Also in this case, the staple 2 is bent without any curves or bulges occurring at the ends of the bent staple. The basic difference between the known clinching mechanism of Fig. 1 and the new clinching mechanism of Fig. 2 thus resides in that the pivot shafts 5 in the known mechanism are disposed in the upper portion of the anvil members 4, whereas in the new mechanism they are disposed in the lower portion of the anvil members, that the distance between the pivot shafts in the known mecha¬ nism is greater than the distance between the staple legs, whereas in the new mechanism this distance is smaller than (or optionally equal to) the distance between the staple legs, and finally in that the anvil members in the known mechanism are pivoted towards each other below the pivot shafts and will thus engage the staple legs from below during the bending operation, whereas in the new mechanism they are pivoted towards each other above the pivot shafts so as to engage the staple legs from the side.

As appears from Fig. 2, the operating members 7' cooperate during their upward displacement with a cam 8 provided on the respective anvil member 4, for pivoting the anvil members. Each anvil member 4 will have reached its end position when its operating member 7' engages the outer side of the cam 8 with its vertical inner side, i.e. its side facing the pivot shaft 5. Continued upward dis¬ placement of the operating member 7' then produces no fur¬ ther pivotal movement of the anvil member 4. The operating member 7' can therefore be moved upwards a certain dis¬ tance past the position of displacement in which it has brought the anvil member to its end position, and thus need not be stopped in any well-defined turning position, provided it is situated above said position of displace- ment. The above-discussed problem encountered in the known clinching mechanism has thus been overcome.

A stapler according to the present invention will now be described in more detail with reference to Figs 3-8. Like components in Fig 3-8, on the one hand, and in Fig. 2 on the other hand, bear like reference numerals.

The stapler according to the invention shown in Figs 3-8 has a fixed frame 9, a magazine 10 arranged therein, a drive element 1 and an anvil device or clinching mecha¬ nism 11 which is of the type described with reference to Fig. 2.

The magazine 10 contains U-shaped staples 2 arranged close to each other and forming a horizontal row of sta¬ ples in the magazine. The drive element 1 is vertically displaceable to push the foremost staple 2 in the row of staples out of the magazine 10 to drive its legs 2a through a bundle 3 of paper sheets placed in the stapler.

The clinching mechanism 11 comprises a block 12 which is vertically displaceable in the frame 9 and on which the anvil members 4 described above with reference to Fig. 2 are mounted. The block 12 is vertically displaceable to permit setting its working position in the vertical direc¬ tion depending on the thickness of the bundle 3 of paper

sheets placed in the stapler. The block 12 is urged upwards by a spring leaf 13 and is shown in Figs 3-8 in a position in which it engages with its upper surface the underside of the paper bundle 3. The pivot shafts 5 of the anvil members 4 consist of circular-cylindrical pins integrally formed with the block 12 and engaging in a circular-arc-shaped recess in the respective anvil member 4. Two stop lugs 14 and a substan¬ tially rectangular spring holder 15 are also integrally formed with the block 12. The stop lugs 14 define the starting position of the pivotable anvil members 4, in which the anvil members are shown in Figs 3-6. A spring 16 in the form of a metal wire bent into an open rectangle is mounted on the spring holder 15. At the free ends of the metal wire, the spring 16 engages the anvil members 4 for urging them into abutment against the stop lugs 14, i.e. to the starting position.

As appears from Figs 3-6, the clinching mechanism 11 is symmetrical with respect to the staple 2 driven into the paper bundle 3, or rather with respect to a plane P (Fig. 6) perpendicular to the plane of the staple 2 and passing through the centre thereof.

The operating members 7' are synchronously displace¬ able so as to be displaced upwards during a stapling ope- ration from a lower position (Fig. 3), to an upper posi¬ tion (Fig. 8), thus pivoting the anvil members 4 from the starting position (Figs 3-6) to the end position (Fig. 8).

As the drive element 1 is driving the staple 2 into the bundle 3 of paper sheets, the staple legs 2a will encounter the anvil members 4 and be slightly bent inwards, as shown in Fig. 6, where the staple 2 has been completely driven into the bundle 3. During this driving- in of the staple 2, the operating members 7' are displaced upwards from the lower position (Fig. 3) to an interme- diate position (Fig. 7), in which they have just entered into engagement with the cam 8 of the respective anvil member 4. The operating members 7' are then further dis-

placed until they reach their upper position (Fig. 8), in which they engage with their vertical inner side the outer side of the respective cam 8 and in which the anvil mem¬ bers 4 are thus located in the end position. In this posi- tion, the legs 2a of the staple 2 engage the underside of the bundle 3 of paper sheets. Each of the anvil members 4 is so designed as to have an upper portion which, when the anvil member approaches the end position (Fig. 8), engages the staple leg 2a adjacent the end portion of the staple 2 in order, during the final phase of the pivotal movement of the anvil member 4, to bring about a post-compression of this staple portion. Post-compression in this portion minimises the spring-back of the staple legs 2a after bending. The stapler can be modified in various ways within the scope of the invention. For example, the spring 16 may thus be replaced by a second cam provided on each anvil member 4 and cooperating with the respective operating member 7' to swing the anvil members 4 from the end posi- tion to the starting position. Moreover, the two anvil members 4 may be pivotable about a common axis which is perpendicular to the plane of the staple 2 and is located in the plane P, i.e. midway between the legs 2a of the staple 2. In such an embodiment, one anvil member may have a part-circular recess, whose centre is located on the pivot axis, while the other anvil member may have a part- circular projection which is rotatably mounted in this recess and whose centre is thus also located on the pivot axis.