FIELD

The invention relates to a tapping tool and a method for producing a threaded bore.

BACKGROUND

In a so-called one-shot tapping process, both a core bore and also an internal thread cut are carried out in one common tool stroke using a one-shot tapping tool. The one-shot tapping tool has a main cutting edge on its drill tip and a thread profile trailing in a tapping direction and having at least one thread cutting tooth. In the method, firstly the tapping stroke takes place and subsequently a reversing stroke opposite thereto. In the tapping stroke, on the one hand, the tool main cutting edge produces the core hole bore and, on the other hand, the tool thread profile produces the internal thread on the inner wall of the core hole bore until reaching a usable target thread depth. For this purpose, in the tapping stroke, the tapping tool is operated at a tapping feed rate using tapping speed synchronized thereto. In the subsequent opposing reversing stroke, the tapping tool is led out of the threaded bore in a reversing direction, specifically at opposing reversing feed rate and reversing speed synchronized thereto. It is thus ensured that the thread profile of the tapping tool is moved out of the threaded bore in the thread turn of the internal thread without load.

In the above method, chips are produced in the tapping stroke, which are conveyed out of the threaded bore in a chip removal direction opposite to the tapping direction. In this case, the chips moving in the chip removal direction collide with the thread flanks of the internal thread facing toward the chips. Therefore, material abrasion can occur on the thread flanks of the internal thread facing toward the chips, which results in flaws in the internal thread. Such flaws can in turn negatively affect the setting behavior of a screw element screwed into the internal thread.

A combined tool for drilling a hole and for tapping is known from DE 38 80 394 T2. Firstly a core hole bore is produced using the tapping tool. Subsequently, the tapping tool is moved with its tool axis in a circular path around the bore axis, specifically with rotation of the tapping tool, whereby the thread profile produces an internal thread in the core hole bore. Essentially the same method is also known from DE 39 39 795 T2 and from U.S. Pat. No. 5,678,962. Further tapping tools are known from U.S. Pat. No. 6,499,920 B2 and from DE 28 52 906 A.

The object of the invention is to provide a tapping tool and a method for producing a threaded bore in a workpiece, using which a durable operationally-reliable screw connection is ensured.

SUMMARY

The invention is based on the fact that in the tapping stroke, the chips to be removed collide with chip-facing thread flanks of the internal thread and can possibly damage them. Against this background, according to the characterizing part of claim1, in the tapping stroke, the chip-facing thread flanks of the internal thread are not yet produced to a finished dimension, but rather are produced having a flank allowance. In this way, a collision contour is provided on the chip-facing thread flanks, with which the chips to be removed collide.

The flank allowance is only removed from the chip-facing thread flanks of the internal thread down to the finished dimension in a finishing step following the tapping stroke. This finishing step takes place in the reversing stroke, during which the tool thread profile led out of the threaded bore in the reversing direction removes or forms the flank allowance from the thread flanks facing toward the chips down to the finished dimension. The material is removed from the thread flanks using at least one reversing tooth, which is formed in the thread profile of the tapping tool and the geometry of which is described later.

In the tapping stroke, the tapping feed rate and the tapping speed synchronized with it are matched with one another in such a way that the produced thread turn of the internal thread has a predefined tapping thread pitch. Similarly thereto, in the reversing stroke, the reversing feed rate and the reversing speed synchronized with it are matched to one another in such a way that a reversing thread pitch results. The reversing thread pitch can, depending on the setting of the above-mentioned parameters, be identical to the tapping thread pitch or, if necessary, can also be different from it. For example, a first pitch (i.e., tapping thread pitch) can be applied to the internal thread in the tapping stroke, while a different second thread pitch (i.e., reversing thread pitch) is applied to the internal thread in the reversing stroke. The reversing thread pitch and the tapping thread pitch can be matched to one another in such a way that the overall result is a load-optimized internal thread profile design.

For example, in the tapping stroke—with the exception of the thread flanks of the internal thread facing toward the chips—the internal thread geometry (i.e., the thread flanks of the internal thread facing away from the chips, the radially inner thread inner vertex of the internal thread, and the radially outer thread base of the internal thread) can already be produced at a finished dimension. Only then can the chip-facing thread flanks of the internal thread be produced at the finished dimension in the reversing stroke.

In one preferred embodiment, the tapping stroke is not immediately followed by the reversing stroke, but rather a groove forming step in which a circumferential groove adjoining the internal thread is formed without thread pitch, in which the thread profile of the tapping tool can rotate without load. In this way, the tapping speed can be reduced to 0 without the tool breaking or the thread profile breaking out due to excessive cutting edge load.

The thread profile of the tapping tool can have thread profile teeth described later and/or the above-mentioned reversing tooth. Both the thread profile teeth and the reversing tooth can each be designed as a forming tooth (having corresponding forming edges) or as a cutting tooth (having corresponding chip-removing cutting edges) or as a combination thereof.

As mentioned above, the thread profile of the tapping tool can rotate without load in the circumferential groove without a thread pitch produced in the groove forming step. The provision of the circumferential groove additionally makes it possible for the tapping tool to produce a circumferential thread countersink in the bore opening of the bore using a cutting edge. The circumferential thread countersink can therefore be produced during the above groove forming step.

In a technical implementation, the tapping stroke in the tapping direction can be immediately extended with a groove forming stroke. In this case, the tapping tool is moved beyond the target thread depth until a target bore depth is reached, namely at a groove forming feed rate and a groove forming speed, which are not synchronized with one another and/or are different from the tapping feed rate and the tapping speed.

It is preferred if, at the end of the groove forming step, the thread profile, viewed in the axial direction, can rotate completely in the circumferential groove of the threaded bore without load. The circumferential groove is produced during the groove forming stroke with the aid of the main cutting edge and the tool thread profile on the tapping tool.

When the target bore depth is reached, the groove forming feed rate is reduced to 0. At the same time, the groove forming speed is also reduced to 0 in order to enable the reversal of the direction of rotation required for the reversing stroke.

At the start of the reversing stroke, the tapping tool is controlled in such a way that the tool thread profile cannot be moved into the thread turn runout, which opens into the circumferential groove, without load, but rather under chip-removing load. The tapping tool is then led out of the threaded bore in a reversing direction opposite to the tapping direction, specifically at a reversing feed rate and a reversing speed synchronized therewith, whereby the tool thread profile can be rotated out of the threaded bore with material removal and/or material forming (i.e., with finishing of the thread flanks facing toward the chips to the finished dimension).

During the execution of the tapping stroke, the groove forming stroke, and the reversing stroke, the core bore longitudinal axis and the axis of rotation of the tapping tool preferably remain coaxially aligned to one another continuously.

A tapping tool for carrying out such a method can preferably have a clamping shaft and an adjoining tapping body. At least one flute can extend along its longitudinal axis up to a frontal main cutting edge on the drill tip. A rake face delimiting the flute and a frontal free surface of the drill tip converge on the frontal main cutting edge. Viewed in the tool circumferential direction, the flute can be delimited by at least one drill web. The rake face of the flute can merge into an outer circumferential drill web rear face with the formation of a secondary cutting edge. The thread profile can be formed having at least one thread cutting tooth on the outer circumferential drill web rear face. The tooth height of the cutting tooth is dimensioned in the radial direction in such a way that the cutting tooth protrudes outward beyond the main cutting edge in the radial direction by a radial offset. If necessary, the cutting tooth can extend the main cutting edge outwards in the radial direction surface flush. Alternatively and/or additionally, the cutting tooth can be arranged behind the main cutting edge by an axial offset viewed in the axial direction.

In one preferred embodiment variant, the tapping tool can have three drill webs. Each of these drill webs is formed having at least one thread cutting tooth. The thread cutting teeth are preferably not designed having the same cutting edge geometry, but rather are embodied differently. For example, a preliminary cutting tooth, a middle cutting tooth and a finishing tooth of different cutting geometry can be formed on the drill in succession in the drill circumferential direction. The cutting teeth are formed on the tapping tool offset from one another in the axial direction. Their offset dimensions are matched with the tapping speed and the tapping feed rate so that perfect thread cutting is ensured.

The tool thread profile has the above-mentioned reversing tooth so that in the reversing stroke, the flank allowance is removed from the chip-facing thread flank in an operationally reliable manner (i.e., without premature tool breakage). It can be formed having a thread flank cutting/forming edge. In the reversing stroke, the thread flank cutting/forming edge can remove the flank allowance preserved in the tapping stroke from the chip-facing thread flanks to the finished dimension.

The reversing tooth, like the thread profile tooth, is formed on the drill web rear face. In a technical tool implementation, the reversing tooth can project radially outward beyond the main cutting edge by a reversing tooth height. The thread flank cutting edge of the reversing tooth can merge into a reversing cutting edge at a radially inner cutting edge inside corner. In this case, the tapping tool can not only machine the chip-facing thread flank of the bore internal thread, but rather can also deburr its inner thread vertex at the same time. The reversing tooth and/or the reversing cutting edge can preferably be designed structurally in such a way that they are only active in the reversing stroke and are largely inoperative in the tapping stroke.

The above-mentioned reversing cutting edge can extend along the drill longitudinal direction. In this case, the outer circumferential drill web rear face and the rake surface of the flute can converge at the reversing cutting edge. Therefore, the reversing cutting edge and the secondary cutting edge are formed on drill web longitudinal edges that are opposite in the drill circumferential direction.

In order to form a stable thread profile on the tapping tool, it is preferable if, in the drill circumferential direction, a tooth web formed on the drill web rear face adjoins the at least one thread profile tooth and/or the reversing tooth. In this way, the thread profile tooth and/or the reversing tooth are protected from premature tool breakage in the tapping stroke and/or in the reversing stroke. The thread profile tooth and the reversing tooth can preferably be connected to one another via a tooth web formed on the drill web rear face. The tooth web can have end faces facing away from one another in the drill circumferential direction, which respectively form the thread profile tooth and the reversing tooth.

The tooth web can have a radially outer web vertex surface and a web flank surface facing toward the drill tip and a web flank surface facing away from the drill tip. To reduce the tool load during the tapping stroke and/or during the reversing stroke, the above-mentioned web surfaces can be formed at least partially as free surfaces, which are essentially inoperative in the tapping stroke and/or in the reversing stroke.

The web vertex surface of the above tooth web can merge at a first circumferential web edge into the web flank surface facing toward the drill tip. Moreover, the web vertex surface can merge at a second circumferential web edge into the web flank surface facing away from the drill tip.

Alternatively and/or additionally to the above aspects of the invention, the thread profile of the tapping tool can have a circumferential groove cutting edge according to independent claim9. The circumferential groove cutting edge is active in the groove forming step, which takes place between the tapping stroke and the reversing stroke. The circumferential groove cutting edge causes a reduced tool load during the production of the circumferential groove in the groove forming stroke.

In a preferred embodiment variant, at least one of the two above-mentioned circumferential web edges can be designed as such a circumferential groove cutting edge by means of which the circumferential groove adjoining the hole internal thread is formed in the groove forming stroke In the tapping stroke and in the reversing stroke, in contrast, the circumferential cutting edge can be essentially inoperative.

As can be seen from the above description, the circumferential groove can adjoin the internal thread of the threaded bore. This fulfills the following double function: First, the thread profile of the tapping tool can rotate in the circumferential groove without load during the thread production. Second, when a fastening screw is screwed, the circumferential groove forms a compensation space that compensates for screw length tolerances of the fastening screw. The screw length of such a fastening screw is subject to strong tolerances due to the production. With the aid of the circumferential groove, the tolerance-affected fastening screw can be screwed in a process-reliable manner without the thread depth of the threaded bore having to be increased, as would be required in the prior art.

