A spike plate is usually rectangular in form and made of a thin steel sheet which has on one or both planar surfaces perpendicularly projecting spikes or prongs made from the plate blank by punching in such a manner that the spikes remain fastened to the plate by means of their bottom parts while being bent outwards from the plate.
The spike plate is generally used in connection with wood joints, for example in the making of ceiling lattices. In this manner, wooden parts can be joined together by placing the parts to be joined next to each other and by pressing the spike plate on top of them, a convenient manner being by means of a press, so that the spike plate is placed equally on both parts. Thus, the spikes of the plate penetrate into the wood so that the plate itself is against the surface of the wood. Another spike plate can be pressed on the opposite side of the joint.
The strength of the spike plate joint is determined partly according to the tension strength, compression strength and stiffness of the plate itself, partly according to the holding force caused by the friction between the spikes of the plate and the wood, and partly according to the strength of the wood material.
The frictional force between the spikes of the plate and the wood material is a very decisive factor. If it can be increased, the thickness of the plate material used can be decreased, and thus raw material cost, tool cost, and the like are reduced significantly. In addition, smaller spike plates can be used with the strength values remaining the same, the number and length of the spikes can be decreased, and other advantages can be realized. Thus the cost of the spike plates can be reduced indirectly, while the earlier, accepted strength values remain the same. In addition, other indirect benefits may be achieved.
In order to increase the gripping force of the spikes of the plate, it has been known previously to make the spikes in such a manner that on the edges of the spikes there are barbs or teeth which by penetrating into the wood are fastened more firmly, thus increasing the holding force of the spikes. This spike form has been introduced in the Finnish Patent Application No. 790210, for example, and in some older American patent publications. In these applications, the spikes of the spike plate are arrow-like in form, or they have several barbs on the edges. These spike forms are, however, vary hard to manufacture mainly due to the fact that the manufacturing tools are complicated. For this reason, the service life of the tools is short and their maintenance extremely laborious. Tool cost and manufacturing cost along with it are thus uneconomically high. In addition, due to their small cross-section the barbs on the edges of the spikes do not increase the holding force of the spikes significantly. At least not so much that the benefit achieved would be warranted by the increased manufacturing cost.
The purpose of the invention in question is therefore to introduce a new type of spike plate in which, compared with known structures, the holding force of the spikes has been increased by means of a special improved construction in accordance with the invention, so that correspondingly stronger spike plate joints are achieved with still less cost. Also, a purpose of the invention is to introduce a manufacturing method and device for making a spike plate in accordance with the invention.