The invention relates to means for repairing female threads.

Repairing especially small female threads, or internal threads, 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 the 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, the damaged thread is cut by said tools to make the threads workable again. Obviously, cutting the damaged thread results in removing a portion of the 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 thus causing the forming jaws to move from each other, or to expand. The toothed portions made on the forming jaws come into 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. Besides, the tool does not allow precisely expand the forming jaws that is especially required for female threads with a small pitch.

The tool for repairing damaged threads in a blind hole according to US5281059A is very similar to that of JPS5040844U having the same disadvantage.

The closest prior art for the present invention is the tool according to AU612587B2 for 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 in use. Each finger is provided with the thread renewing formation extending axially at one end. The thread renewing formations cooperate with each other to restore or repair the thread upon rotation of the tool in use. The closest prior art 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, non-parallel movement of the movable fingers and, therefore, the thread renewing formations. Besides, 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, namely allows precise control over movement of the forming jaws while keeping movement thereof substantially parallel to each other, resulting in merely forming a damaged thread profile back rather than cutting thereof.

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 guide sleeve with an outer casing installed therein, a tool housing rotatably installed in the outer casing, at least two forming jaws installed in the tool housing and having toothed portions, and an expanding mandrel. The expanding mandrel is made movable along the forming jaws to provide a parallel movement of said at least two forming jaws from each other.

In the present invention, the forming jaws move essentially parallel to each other, to get in contact with a damaged female thread and further when repairing said thread, thanks to the construction of the claimed thread repairing tool. Besides, since the means for providing rotation of the forming jaws, namely the tool housing rotatably installed in the outer casing, and the means for repairing the thread, namely the forming jaws having the toothed portions, are split, it allows for a precise positioning of the forming jaws in respect to the female thread being repaired as well as adjusting the tool to a diameter of the female thread. All these features give significant advantages over the known prior art including the closest one.

It is preferable if the outer casing comprises a fixing means for fixing the tool housing therein.

Further, the expanding mandrel may be made oscillating so that it moves between the forming jaws forth and back with a required frequency up to 400 Hz.

The invention is further described in more detail with reference to the enclosed figures wherein: Fig. 1 provides a general view of the claimed thread repairing tool;

Fig. 2 explains how the forming jaws are expanded upon movement of the expanding mandrel.

As shown in Fig. 1, the claimed thread repairing tool (1) for repairing a female thread (2) being damaged comprises a guide sleeve (3), an outer casing (4), a tool housing (5), at least two forming jaws (6) each having a toothed portion (7), and an expanding mandrel (8).

The outer casing (4) is rotatably installed in the guide sleeve (3) for providing rotation to the tool housing (5), and therefore to the forming jaws (6) disposed in the tool housing (5), in respect to the female thread (2) when repairing the latter.

The tool housing (5) can be positioned in the outer casing (4) at a required position and then fixed by any useful fixing means (9), e.g. by a screw or the like.

The expanding mandrel (8) is positioned between the at least two forming jaws (6) so that the forming jaws (6) move to and apart from each other when the expanding mandrel (8) moves forth and back along the forming jaws (6). Essential is that the forming jaws (6) move (expand) parallel to each other, thus providing parallel movement of the toothed portions (8).

As will be apparent for a skilled person, the number of forming jaws (6) may be two, or three, or more, which can depend merely on a diameter of the female thread being repaired.

Positioning of the toothed portions (8) in respect to the female thread (2) as much parallel as possible when repairing the female thread (2) is necessary to avoid cutting the material the thread is made of. Repairing of the thread is carried out by re-forming, or bringing back, an original profile of the thread instead of newly forming, or threading, a new profile.

For this purpose, a shape of an outer surface of the expanding mandrel (8) and corresponding inner surfaces of the forming jaws (6) can be made as schematically shown in Fig. 1. Namely, the expanding mandrel (8) may comprise at least two portions (8.1, 8.2) spaced apart from each other and made conical or having cambers, while the respective areas of the inner surfaces of the forming jaws (6) have the corresponding profiles, e.g. conical. Then, movement of the expanding mandrel (8) along the forming jaws (6) to one side (say, to the left as shown by the arrow in Fig. 2) causes said portions (8.1, 8.2) to slide over said respective areas of the inner surfaces of the forming jaws (6), thus pushing the forming jaws (6) from each other, or making them expanded (as shown by the arrow in Fig. 2). Moving the expanding mandrel (8) back (i.e. to the right as shown by the arrow in Fig. 2) causes the forming jaws (6) to come closer to each other, or making them contracted (as shown by the arrow in Fig. 2). Since the portions (8.1, 8.2) of the expanding mandrel (8) are spaced, the forces pushing the forming jaws (6) to expand are applied to different points along each forming jaw (6), resulting in essentially parallel movement thereof.

It will however be apparent for a skilled person that other ways and means for providing parallel movement of the forming jaws (6) to and apart from each other can be utilized.

The claimed thread repairing tool (1) can be used for repairing the damaged female thread (2) as follows.

The expanding mandrel (8) is drawn back from the forming jaws (6) to allow the latter to enter a hole of the female thread (2).

The guide sleeve (3) is then fixed so that it is aligned with said hole. The tool housing (5) can be moved axial in the outer casing (4) to adjust the position of the toothed portions (7) in respect to the female thread (2).

The expanding mandrel (8) is then slowly inserted between the forming jaws (6) causing the forming jaws (6) to slightly expand, i.e. move (or spread) apart from each other, and the toothed portions (7) find their way into the grooves of the female thread (2).

Then the tool housing (5) is fixed in the outer casing (4) by fixing means (9).

Further movement of the expanding mandrel (8) causes the forming jaws (6) to further spread apart from each other, thus pushing the material of the female thread (2) back to the initial profile of the thread, i.e. re-forming said profile. This process is continuously repeated, and the outer casing (4) can be slowly rotated in the guide sleeve (3), so the entire female thread (2) is fully formed back by the toothed portions (7).

Additionally, the tool can be used to homogenize a structure of the material of the female thread (2), e.g. cast iron, by cracking large carbide islands within the thread into many small islands.

It has also been found useful if the expanding mandrel (8), apart from movement thereof to expand the forming jaws (6), is oscillated forth and back along the forming jaws (6) at a small amplitude but a relatively high frequency, e.g. up to 400 Hz. This option helps to better and faster re-form the initial profile of the thread and homogenize the structure of the material.

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