Background art It is widely known, that tools frequently used to prepare threads into bore-holes are machine screwing taps, manually operated screwing taps (brace taps) and machine threading taps. The conventional thread screwing and threading taps can be divided into two portions.

A working head provided with entering and controlling portions is situated in the first end of the tap. As a rule, the second end opposite to the first end of the tool is formed as a shaft having a smooth cylindrical shape, having a square chucking portion being able to be bitten by a die chuck or tooihoider.

Further common feature of the different taps is having a so called neck part connecting the tool shaft and the working head.

Contrary to the screwing taps chipping does not occur during operation of a threading tap, thus the method using a threading tap is a plastic deformation process. The working head of the screwing tap has a screw-thread having cutting edges interrupted by one or more longitudinal or helical grooves.

During thread shaping operation cutting is made by cutting edges.

Cutting operation made by screwing taps may be accomplished by means of machine tools or by manually operated taps. Brace taps are available in sets, since this cutting operation consists of several different stages (pre-cutting, re- threading and finishing). The machine screwing taps are formed as finishing tools with different (long or medium or short) entering edge lengths.

As mentioned above, contrary to the screwing taps chipping does not occur during operation of a threading tap. Such a threading tap can be known from the document US 6 514 148 B2, where the threading tap has a single working head and a tool shaft.

Main advantages of the threading tap as compared to screwing taps are: the higher productivity, longer lifetime, and favourable material quality obtained on the thread surface. Its only disadvantage is the very high tolerance requirement at the drilling operation of the bore-hole to be threaded.

With both screwing taps and threading taps according to the state of the art the cutting operation is achieved actually by means of 1/20 part of the entire tool length, including only the 3-5 coarse cut threads and the following 1-2 finishing threads of the working head. This is an unfavourably bad operating rate resulting in a large quantity of waste material and increased production costs. Enhancing this rate the major part of the base material may be saved.

Regarding to that fact, that no little threading taps are used in different fields of the industry, no need to insist on the importance of economy obtained by reduced input of costs, material, time and energy.

FR 2 787 048 discloses a tap having a shank carrying a head with bearing grooves which ho) d the three tap wheets at a siight ang) e so that the circumferential ridges screw themselves into the bore, rolling the threads to shape as they do so. The three tap wheels have ridges which successively increase in diameter to form the threads in easy stages. By this solution a reduced material requirement can be obtained in case of machine threading, however, it is time consuming to change the tap wheels in relation to the operation time of the machine.

With threading by brace taps the above mentioned exceptionally uneconomic base material requirement may be gathered, since two different taps (coarse cut and finishing) are required in this case.

Therefore, it is a need in the art to create an indexable machine, as well as brace thread shaping or threading tap having longer lifetime and an operating rate better than above rate, and to create a thread shaping tap being able to prepare much more threads than a conventional tap and not increase or

even lower production costs, and to diminish also the quantity of waste material arising at the plants using thread shaping taps, and principally to create an indexable tool for forming inner threads, which has complex function like a set of taps.

Description of the invention With the indexable threading tap having multiple working head placed both ends of the tool shaft according to the invention, the number of threads possibly prepared by a single thread shaping tap can be increased significantly and even so the production costs of a thread shaping tap do not increase, yet, there is a possibility to form and finish inner threads by means of the same tool.

The indexable threading tap for forming inner threads in a workpiece according to the invention has a longitudinal tool body and at least two threaded working heads arranged on the tool body for forming inner threads, and a tool shaft being to be bitten by a die chuck, and the tool body is provided with at least two working heads, and the tool shaft is arranged between said two working heads.

The indexable threading tap advantageously has neck parts, which are arranged between the tool shaft and the respective working head.

The at least two working heads preferably have identica) thread profiles, and a square chucking portion is formed on the tool shaft.

In a preferred embodiment the indexable threading tap according to the invention is provided with two kind of working heads, where a first type working head is formed with coarse cut thread profile, and a second type working head is formed with finishing thread profile, and further, the first type working head formed with coarse cut thread profile is provided with a guidepin portion.

In a preferred embodiment of the invention both working heads are provided with guidepin portions, and the chucking portion is provided on the centre part of the tool shaft.

Preferably, the chucking portion is provided on a part of the tool shaft adjacent the first type working head formed with coarse cut thread profile.

Advantageously, chucking portions are provided on both ends of the tool shaft, and each working head is formed with coarse cut thread profile.

The invention will be presented in the fullest detail in accordance of attached drawings.

Description of the drawings Figure 1 is a side elevation view of an indexable thread shaping tap according to the invention showing the at least two working heads formed on both ends of a tool, Figure 2 is a side elevation view of a thread shaping tap according to the invention showing the at least two working heads formed on both ends of a tool with neck parts formed between the working heads and the tool shaft, Figure 3 is a side elevation view of a thread shaping tap according to the invention showing four working heads formed on ends of a tool.

Figure 4 is a side elevation view of a brace tap according to the invention, having a coarse cut and a finishing working head formed opposite ends of a tool.

Detailed description of the invention Figure 1 shows a preferred embodiment of an indexable threading tap 1 with multiple working head A, B according to the present invention for forming inner threads. Inner threads are formed on the inner periphery of bore-holes in <BR> <BR> a workpiece. Indexable threading tap 1 has a longitudinal tool body 2 and at least two threaded working heads A, B arranged on the tool body 2 for forming inner threads, and a tool shaft C being to be bitten by a die chuck of a chucking machine or a handcrank, as against the taps according to the state of the art having working heads on the only one side of the tool shaft and the rest of the too body 2 forms the tool shaft C. Figure 1 shows two working heads A, B formed on either side of the tool shaft C placed therebetween.

As a rule, forming threads takes place by means of screw cutting or thread shaping. The threading tap 1 according to the invention must be formed as a screwing tap in the firs case, while the latter case it is formed as a thread shaping tap. Since screw cutting is a chipping operation, the working heads A, B are provided with entering portion consisting of first or coarse screwing threads and a controlling portion consisting of finishing threads, and flutes for

chips. When threading tap according to the invention is formed as a thread shaping tap, its working heads A, B must be provided with entering portion consisting of first or coarse shaping threads and a controlling portion consisting of finishing threads, both threads being able to plastically deform the material of the bore-hole periphery. Indeed, flutes for chips are omitted in this case. Since the threading tap 1 according to the invention is able to be formed either screwing tap or thread shaping tap, different embodiments according to the threading methods are not denotated in figures, because this is obvious to a person skilled in the art.

Using the threading tap according to the invention, the tool shaft will be chucked in a chuck of a machine tool or a handcrank, respectively, to begin thread shaping or screwing. During thread shaping the first working head A will work firstly. When the first working head A will have been damaged the user will simply turn the tool. Consequently, the threading tap 1 will be ready again to work by the working head B.

Figure 2 shows a second embodiment of the threading tap 1 according to the present invention. In this embodiment the threading tap 1 has also at least two adequate working heads. Between each working head A, B and the too) shaft C a neck part D is formed resulting in the tool shaft C diameter to be different from the diameter of the working head A, B. By this neck part D it is possible to prevent the chips from clogging, when the threading tap 1 is formed as a screwing tap.

Figure 3 shows a preferred embodiment of the threading tap 1 according to the present invention. In this embodiment the threading tap 1 has four adequate working heads A1, A2, B1, B2. The working heads A1, A2, B1, B2 have an interspace G located therebetween. Therefore, when the first working head A1 will have been damaged the user will simply detach it from the threading tap 1 by means of a cutting device applied between working head A1 and A2 or B1 and B2. Consequently, the threading tap 1 will be ready again to work by the working head A2 or B2. In this case the turning of the threading tap 1 may be advantageous, when the given side has no longer any useful working head A1, A2, B1, B2. The threading tap 1 may also be formed asymmetrically

indeed, therefore it can be provided with more or less, i. e. only three working heads A1, A2, B1.

Figure 4 shows a further, brace tap embodiment of the threading tap 1 according to the present invention for cutting handmade threads, which is similar to above second embodiment with exception that its first working head A is provided with threads for coarse cut, and the other working head B is provided with finishing threads. After coarse cutting has been terminated, the threading tap 1 might be turned about to finish the coarse thread prepared previously by working head A, without dechucking the threading tap 1 from the handcrack, and finishing operation can be accomplished by working head B.

The working head A formed with coarse cut thread profile is provided with a guidepin portion F to prevent the tool driven by hand from both lateral displacement and angular feeding in the bore-hole. Guidepin portion F is free of cutting portions or forming threads. This embodiment is essentially a set of brace taps formed in one tool, having both coarse cut and finishing working heads A, B. On the tool shaft C of the threading tap 1 for forming handmade threads a polygonal, e. g. a quadrangular chucking portion E is provided on a part of the tool shaft C, e. g. adjacent the first type working head A formed with coarse cut thread profile. This chucking portion E is for fitting of a handcrank to the threading tap 1. In a preferred embodiment the tool shaft C might be provided more than one chucking portion E. Advantageously, the chucking portion E is close about the working head A with coarse cut thread profile to minimise lateral displacement and angular feeding mentioned above.

Therefore, principal advantages of threading tap 1 according to the present invention are as follows : -reduces overall base material needs, - reduces the quantity of waste material accrued from machining, - reduces needs of energy used for production of tools, - increases significantly the number of threads prepared by a single tap, - reduces the operation time of threading, and - it can be formed as a machine thread shaping tap and as a brace tap, too.