METHOD OF FORMING AN INTERNAL THREAD IN A HOLLOW BODY

Abstract: For the manufacture of a nut, a piece of plate material which is bent to form a tube (9) can be used. This tube can be placed over a mandrel (8) and clamped together around the mandrel, first by means of a pair of spring-loaded dowels (5) and thereafter by two clamping jaws (1, 2). When the mandrel is then withdrawn, a thread-forming tap (12) during rotation in the direction of the thread can be pressed in through the tube to produce an internal thread by deformation. When the tap has passed right through the tube, the direction of rotation is reversed and the tap is screwed out again, after which the nut is finished. For cooling and lubrication, cutting oil is supplied to the tap before insertion into the tube and possibly to the inside of the tube. There can also be used a tap with a shank (13) which functions as the mandrel (8), and at one end of which extends the threading piece (14). In this way the thread is formed when the tap is turned against the direction of the thread, i.e. turned out of the tube, whereby the necessity of having to feed the tap through the tube more than once is eliminated.

Description: METHOD OF PORMING AN INTERNAL THREAD IN A HOLLOW BODY This invention relates to a method for the forming of an internal thread in a hollow body, principally of metal plate which is bent around the longitudinal axis to form a tube slotted along its length. Hitherto known methods of cutting internal threads in articles of a certain length are effected by machin- ing by means of a tap. This demands a certain wall thickness in order for the article to retain the ad¬ equate strength and, moreover, calls for considerable cleaning of tools and components after the cutting of the thread. A further disadvantage is the long pro- duction time, which again in turn means a high price of manufacture. Tools for the forming of an internal thread by means of a thread-forming tap, which is turned in through a hole, are already known within the art. However, these methods are limited to relatively thin work- pieces such as plates, in which there is formed a so-called trumpet hole. As soon as the thickness of the workpiece exceeds " the diameter of the thread, use must normally be made of thread-cutting taps. The object of the present invention is to overcome the drawbacks and disadvantages of known methods of thread forming, and this is achieved by a method whereby the workpiece in the form of a tube is in¬ troduced over a mandrel which is positioned between a number of clamping jaws in which there is located a yielding dowel, said dowel being pressed in towards the mandrel for the fixing of the tube, after which the clamping jaws are moved in against the tube on the mandrel and clamp the tube completely together, this. aking place before or after the mandrel is drawn out of the tube, whereupon a thread-forming tap rotat- ed in the direction of the thread is turned into the tube to.produce the internal thread by deformation, after which the tap is screwed out and the clamping jaws separated to release the tube. There is- thus achieved the possibility of producing a thread by deformation in a tube having a length which can be up to several times the diameter of the thread. By this deformation there is formed a perfect thread while retaining the material's -structure and strength. Moreover, the workpiece is. hardened by the deformation, thus ensuring a considerable retention of shape even when said workpiece is produced of spring steel. This method enables tubular nuts to be produced both more quickly and inexpensively and with hitherto unknown strength characteristics and shape retention. In order to achieve the best possible operating con¬ ditions, it is expedient, as presented in claim 2, to intro ' duce cutting oil both to the front and pos¬ sibly to the inside of the tube, hereby also enabling the length of the workpiece to be considerably in¬ creased. Finally, as presented in claim 3, a method can be used by which the tube is fed in onto a spindle with a thread-forming tap which begins at the shank, so that the formation of the thread is effected by a single screwing out of the tap. This saves consider- able time, in that in opposition to the normal pro¬ cedure, where the tap must be turned in, its direc- .• tion of rotation reversed and then turned out again, the tap needs to be turned only one way. The invention will now be described in closer detail with reference to the accompanying drawing, where fig. 1 shows, the tube introduced into a tool before the formation of the thread, fig. 2 shows a section seen in the direction II-I ' I in fig. 1, and fig. 3 shows the tube introduced over a sec¬ ond embodiment of the thread-forming tap. In figs. 1 and 2 is shown an example of one embodi- ment of a tool for the execution of the method in accordance with the invention. It consists of two clamping jaws 1, 2, which can slide towards and away from each other while guided by a guide 3, said guide 3 comprising a spindle upon which the jaws can slide against the spring pressure from the springs 4. The inside faces of the clamping jaws are provided with a recess for the workpiece 9 which is to be securely clamped. Moreover, in .said recesses are located one or more dowels 5 which, against the spring pressure from the springs 6, can be pushed inside the clamping jaws. In the example shown the workpiece is produced by the bending and slotting of a piece of plate material which is provided in the middle with a number of tabs 10. These tabs serve to anchor the finished nut dur¬ ing its use. For the whole length of the tube there is a slot 11, which at the end is bent to form a lengthwise rib. The clamping jaws are further pro¬ vided with, recesses 16 to accommodate said tabs. Extending inside the tool is a mandrel 8 upon which the tube 9 can slide with the slot 11 facing down¬ wards. Opposite this mandrel there is mounted a thread -forming tap 12 which can be inserted during rota¬ tion. Finally, there is shown a pair of supply pipes 7 for high-pressure cutting oil, said pipes having their outlets at the front end of the tube and at one of the tube's side holes. The method will now be described in closer detail. After the plate has been prepared in punching tools, it is conveyed to the tool's mandrel 8, as shown in fig. 2, where the tube arrives sliding from the left. The clamping jaws 1 and 2 are then moved towards each other, in that the dowels 5 first contact the tube and secure it in the correct position on the mandrel. When the jaws have closed completely together, the tube is finally fixed and secured, after which the mandrel 8 is drawn out to the left in fig. 2. The thread-forming tap 12 is inserted from the right as seen in fig. 2, while at the same time it is made to rotate. The thread is formed during the introduction of the tap into the tube, and when the tap has been fed through the tube, its direction of rotation is reversed and it is turned out again. Simultaneously with this formation of the thread, a hardening of the workpiece takes place and it therefore now remains in this form. The jaws are now withdrawn from each other, and the tube - now in the form of a nut - falls out of the tool, after which the procedure can be repeated. By using the tap as shown in fig. 3, a considerable increase in the production speed can be achieved. The tap is secured to a spindle 15 which can rotate in the direction of the thread while at the same time moving axially, i.e. towards the right; in fig. 3. At the end of. the tap is the thread-forming piece .14 which extends from a shank 13, this shank 13 corres¬ ponding to the above-mentioned mandrel 8. when the tube is slotted, it can slide in over this shank, after which the jaws can close together around the tube. The tap is then withdrawn to the right in. fig. 3 during its rotation, and the formation of the thread takes place at_the same time that the tap is "screwed out". In this way the tap needs only to be turned in one direction and moved only ohce through the tube. In the example. shown, only two jaws are used, but it naturally falls within the scope of the invention to use more than two, in that in some cases advantages can be achieved by the use of three jaws. In certain cases, where there is no risk of the work- piece turning on the mandrel before the clamping jaws have fixed it in place, the dowels 5 can be omitted.

Claims: C L A I M S 1. Method of forming an internal thread in a hollow body, principally of metal plate bent around the longitudinal axis to form a tube slotted along its length, c h a r a c t e r i z e d in that the 5 tube (9) is introduced over a mandrel (8) which is positioned between a number of clamping jaws (1, 2) , wherein there is provided a yielding dowel (5) which presses against the tube (9) on the mandrel (8) to fix the position of said tube (9) , after which the 0 clamping jaws (1, 2) are closed in against the tube (9) on the mandrel (8) and clamp the tube (9) com¬ pletely together, this taking place before or after the mandrel (8) is drawn out of the tube (9) , where¬ upon a thread-forming tap (12) is turned into the 5 tube (9) to produce the internal thread by deforma¬ tion, ' after which the thread-forming tap (12) is screwed out and the clamping jaws (1, 2) separated to release the tube (9) . 0 2. Method according to claim 1, c h a r a c t e r ¬ i z e d in that cutting oil is supplied to the thread -forming tap (12) immediately in front of the tube (9) and possibly also to the inside of the tube (9) . 5 3. Method according to claims 1 and 2, c h a r a c ¬ t e r i z e d by the change that the tube (9) is fed in over a shank (13) , the one end of which extends to form the thread-forming piece (14) , after which the shank (13) and the thread-forming piece (14) are Q screwed out during the formation of the internal thread, whereupon the jaws (1, 2 ) are opened to re-, lease the tube (9) . --"- " "•• <«.

Inventors: BREDAL TORBEN, BREDAL, TORBEN
Applicants: HOLM J H HOLDING, AKTIESELSKABET J.H. HOLM HOLDING
Application year: 1983
Main classification: B23G7
IPC codes: B21H3, B23G1