The invention relates to means for repairing female threads.

Repairing female threads, or internal threads, especially small ones, is known to be a more complicate task than repairing male threads. Not always a workpiece with a damaged, i.e. having imperfections, female thread can be replaced with a new one due to long lead times or high total costs. On the other hand, the already known repairing tools for female threads are quite complicate in use and usually applicable for a female thread of one given diameter.

Another concern of the known repairing tools is that these do not carry out repairing a damaged thread as such. Rather, a damaged thread is cut by said tools to make the thread workable again. Obviously, cutting the damaged thread results in removing a portion of a material the thread is made of, thus causing weakening the repaired thread, especially that one of a small pitch.

JPS5040844U discloses the simple thread repairing tool for repairing female threads comprising the tool body having three forming jaws and the expanding mandrel with the conical portion on the one end thereof and the threaded portion on the other end thereof. When screwed on the threaded portion, the expanding portion is moving inside the tool body, or retracting, thus causing all the forming jaws to move from each other simultaneously, to expand. The toothed portions made on the forming jaws come into a permanent tight contact with the female thread being repaired. Then rotation is applied to the tool body, to repair the female thread. The main drawback of the known repairing tool is that the forming jaws and therefore the toothed portions go not parallel when moving from each other, or expanding, resulting in at least partial cutting the material of the female thread when the tool body is being rotated, rather than restoring the female thread as such.

The tool for repairing damaged threads in a blind hole according to US5281059A is very similar to that of JPS5040844U having the same disadvantage of at least partial cutting the material of the female thread when the tool body is rotated, rather than restoring the thread.

The rethreading tool according to AU612587B2 is aimed at restoring or repairing a thread having at least a part of the thread which is damaged and a further part which is undamaged. The known tool comprises the body portion having at least two resiliently movable fingers (forming jaws) and the centrally located axial bore for receiving the operating means, or the expanding mandrel. Each finger is provided with the thread renewing formation, or the toothed portion, extending axially at one end. The thread renewing formations cooperate with each other to restore or repair the thread upon extending the operating means and rotation of the tool. The rethreading tool eliminates some disadvantages of the above mentioned simple thread repairing tools, though still causes the material of the female thread to be cut when restoring because of the same reason. Namely, the thread renewing formations come into a permanent tight contact with the female thread being repaired, and then the tool is rotated. Non-parallel movement of the movable fingers and, therefore, the thread renewing formations causes the female thread to be cut. The more the movable fingers are spaced apart from each other, the higher the effect of cutting of the thread being repaired. It limits use of the known tool to one thread diameter only.

The objective of the present invention is providing a universal thread repairing tool for repairing female threads that eliminates the disadvantages of the known tools, allowing not only precise control over movement of the forming jaws while keeping movement thereof substantially parallel to each other, but also affecting a damaged female thread so as to merely re-form a thread profile back rather than cutting.

The technical result of the invention is repairing a female thread upon re-forming a material of the thread and avoiding cutting thereof, while allowing repairing of female threads of different diameters.

For achieving said objective and technical result, a thread repairing tool for repairing a female thread comprises a tool housing; a base accommodated in the housing; at least four forming jaws extending from the base substantially along an axis of the thread repair tool; and an expanding mandrel extending between said forming jaws and having an expanding portion on a one end thereof with a cross section other than round. Each forming jaw has a toothed portion on a free end thereof. The expanding mandrel is arranged to reciprocally rotate in respect to the base and to move along said axis between a first position and a second position so as to, when in the second position, bring the expanding portions into contact with inner surfaces of said free ends. The latter causes the toothed portions of at least two of said at least four forming jaws to move apart from each other.

When the claimed tool is inserted into a damaged female thread and the toothed portions are brought into initial contact with the thread upon bringing the expanding mandrel to the second position, reciprocal rotation of the base and the expanding mandrel causes the opposite toothed portions, being pushed away from each other by the expanding portions, to come in a closer contact with the thread affecting thereon like hammering. If necessary, further gentle moving the expanding mandrel helps to precisely and almost completely repair the thread while avoiding cutting of a material the thread is made of.

As stated above, the expanding portion has the cross section other than round. Said cross section may be oval, ellipsoidal, rhombic e.g. with rounded comers, or of any other shape pushing the opposite toothed portions to move apart from each other when the base and the expanding mandrel are rotated in respect to each other, i.e. reciprocally rotated.

It is preferable, though not strictly necessary, if a number of the forming jaws is even, which helps to more accurately repair the thread. Then the forming jaws may extend from the base equally, in pairs one opposite the other.

Further, the forming jaws are preferably parallel to each other or slightly directed to each other with the free ends thereof when the expanding mandrel is in the first position. It helps to provide the opposite toothed portions substantially parallel to each other when repairing the thread.

The first position of the expanding mandrel may be an extended position, and then the second position is a retracted position. Alternatively, the first position may be the retracted position, then and the second position is the extended position.

For better control of the claimed thread repair tool, the housing may be provided with a handle.

It is preferable if the expanding mandrel may be moved along the axis of the thread repair tool with a frequency up to 400 Hz.

Also it is preferable if a frequency of the reciprocal rotation of the base and the expanding mandrel is up to 400 rpm.

The invention is further described in more detail with reference to the enclosed figure wherein a general view of a possible example of the claimed thread repair tool is provided.

The claimed thread repairing tool 1 for repairing a damaged female thread comprises a housing 2 that may be provided with a handle 3 and accommodates a base 4 with forming jaws 5 extending therefrom.

The housing 2 and the base 3 can be made as a single article or be connected with each other.

The forming jaws 5 each has an elongated shape and extends from the base 4 along an axis of the thread repairing tool 1 (not shown in the figure but merely coincides with an arrow A). It is preferable to use four forming jaws 5 for the reasons explained below, though a greater number is also possible, preferably being even e.g. so that the forming jaws 5 extend from the base 4 equally, in pairs one opposite the other, to allow a more precise control over forces applied to the damaged thread.

On a free end of each forming jaw 5, a toothed portion 6 is formed or located. The toothed portions 6 are aimed at engaging a surface of the female thread being repaired, to repair the latter. A profile of the toothed portion 6 may correspond to a profile of the female thread in its original form. Further, the profile of the toothed portion 6 can be fully developed or just partially developed over a circumference of the toothed portion 6. Said partially developed profile is more preferable, though, to reduce a surface touching a material of the female thread to be re-formed and thereby increasing a pressure applied thereto.

Between the forming jaws 5, an expanding mandrel 7 extends along said axis of the thread repairing tool 1. The expanding mandrel 7, on a one end thereof adjoining the free ends of the forming jaws 5, is provided with an expanding portion 8, wherein a cross section of the expanding portion 8 is made other than round. For instance, said cross section may be oval, or ellipsoidal, or rhombic with rounded comers, or of any other shape that will be understood for a skilled person from the further description.

The expanding mandrel 7 is made movable forth and back along said axis in directions schematically shown by the arrow A, to expand from or retract to the thread repairing tool 1 and thus occupy an extended position or a retracted position, respectively. For example, the figure shows the extended position of the expanding mandrel 7.

The expanding mandrel 7 is made movable so that it can occupy a first position or a second position. In the first position, the expanding portion 8 does not or substantially does not contact the forming jaws 5, in particular, inner surfaces of the free ends thereof where the toothed portions 6 are substantially formed or located. In the second position, the expanding portion 8 is brought into contact with the forming jaws 5, in particular, with said inner surfaces of the free ends, to cause the toothed portions 6 of at least two forming jaws 5, preferably the opposite ones, to move apart from each other as schematically shown by arrows B.

The first position of the expanding mandrel 7 may correspond to the extended position thereof, as shown in the figure. Consequently, the second position will correspond to the retracted position. It is somewhat similar to the positions of the expanding mandrel of the above cited thread repair tools according to JPS5040844U and US5281059A. For a skilled person it will be obvious that the opposite case is also possible, i.e. when the first position of the expanding mandrel 7 corresponds to the retracted position thereof, while the second position corresponds to the extended position, similar to that disclosed in AU612587B2.

Thus, when the thread repair tool 1 is inserted into the female thread being repaired, moving the expanding mandrel 7 from the first position to the second position causes the toothed portions 6, preferably the opposite ones, adjoining protruding surfaces of the expanding portion 8 to expand, or move from each other, and get in contact with the respective surface areas of the damaged thread. A material of said respective surface areas of the thread is brought back, or re-formed, in accordance with the profile of the toothed portions, and consequently the thread becomes locally repaired but not cut. To repair a whole surface of the thread, the expanding mandrel 7 is further made rotatable in respect to the base 4. This is a kind of reciprocal rotation when the expanding mandrel 7 may be fixed while the base 4 is rotated or, alternatively, the expanding mandrel 7 may be rotated while the base 4 is fixed.

When the expanding mandrel 7 in the second position, said reciprocal rotation causes the protruding surfaces of the expanding portion 8 to sequentially come into contact with the inner surfaces of the free ends, preferably the opposite ones, of the respective forming jaws 5. Therefore, the respective toothed portions 6 sequentially get in contact with the respective surface areas of the thread, and upon a one or few full rotations the whole surface area of the thread becomes repaired by bringing back the material of the thread in accordance with the profile of the toothed portions.

The reciprocal rotation of the base 4 and the expanding mandrel 7 may be provided by hand, e.g. using the handle 3 if the housing 2 is firmly connected with the expanding mandrel 7 and free for rotation in respect to the base 4. Alternatively, the expanding mandrel 8 may be rotated by any known tool (a key, a drive etc.) if free rotation of the expanding mandrel 8 is allowed in the housing 2. For a skilled person it will be understood that other options to provide the reciprocal rotation of the base 4 and the expanding mandrel 7 are available.

Preferably, the forming jaws 5 extend parallel to each other when the expanding mandrel 7 is in the first position, i.e. when the forming jaws 5 are in a relaxed position. It allows merely re-forming the female thread rather than cutting thereof, especially for small threads.

For large threads it may be more preferable if the forming jaws 5 are slightly directed to each other with the free ends thereof when the expanding mandrel 7 is in the first position. In this case, an envelope of the profile of the toothed portion 6 is tapered, whereby teeth of the toothed portion 6 become tilted by an angle of the envelope when the expanding mandrel 7 is in the first position (the forming jaws 5 are in a relaxed position). When the expanding mandrel 7 is in the second position, the profile of the toothed portion 6 becomes parallel to the profile of the female thread to be repaired.

Both options provide for a wider range of thread sizes that can be efficiently repaired with no cutting the material the thread is made of.

Another important improvement of the claimed thread repair tool 1 is providing the expanding mandrel 7 with ability to move and/or rotate at certain frequencies. For instance, the expanding mandrel 7 may be made rotatable with a frequency up to 400 Hz, and/or movable with a frequency of up to 400 Hz. Both options, especially taken together, have shown to not only lead to a more accurate and precise repairing of the thread, even made of a hard material, but also help to homogenize a structure of the material of the female thread by cracking large carbide islands within the thread into many small islands.

Thus, the claimed invention provides for a universal robust tool for repairing damaged female threads allowing adjustment to a diameter of the thread while causing substantially re-forming of the thread rather than cutting thereof.