US20060068926A1

US20060068926A1 - Method, tool and device for the production of threads

Info

Publication number: US20060068926A1
Application number: US11/255,482
Authority: US
Inventors: Juergen Mann
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-04-22
Filing date: 2005-10-21
Publication date: 2006-03-30
Also published as: JP2006524139A; KR20060003898A; EP1497067A1; ATE352391T1; WO2004094094A1; DE10318203A1; EP1629919A2; EP1629919A3; DE502004002739D1; EP1497067B1

Abstract

A method for the production of threads, especially internal threads, by means of a rotationally driven thread former, with which the thread pitches are driven in and out of the surface of the workpiece in a chipless fashion by pressure forming, in particular in and out of the inner surface of a workpiece bore. In order to be able to produce threads of different nominal diameter especially economically and with improved joining quality in the area of the thread, the thread is formed such that a shank tool in the fashion of a thread miller, equipped with at least one profile projection, preferably two profile projections located at a constant distance from one another, where the profile projections at its forming head are constructed as continuous over the circumference and with radial extension varying over the circumference, is driven into the workpiece initially at a circumferential point of the workpiece bore, preferably is brought to the total thread pressing depth, and while substantially retaining the eccentricity set with respect to the axis of the workpiece bore, executes a relative circular movement running through 360° (circular movement) relative to the axis of the tool bore, while the forming head simultaneously executes a constant axial relative feed movement by the extent of the thread pitch to be produced.

Description

The invention relates to a method and a tool for the production of threads, especially internal threads, according to the preamble of claim 1 or 12. The invention further relates to a device for carrying out the method.
Various methods for producing so-called female threads, i.e., internal threads, are known. Commonly used is thread cutting where a tapping tool equipped with cutting lands and cutting grooves is driven into a workpiece bore having a pre-determined core size with the feed predetermined by the pitch of the tool and synchronized speed. In this case, cutting machining of the workpiece takes place.
In order to achieve a higher strength especially in the bearing thread flanks, the so-called thread forming method is used where a thread former or thread ridging machine, also known as thread bulging machine, is used and with which the material is cold formed without the so-called “fiber course” in the material being interrupted as in thread cutting. An important advantage compared with thread cutting is furthermore that no chips are formed during thread forming.
During thread forming a helical thread portion provided with a polygon for the formation of pressing lands is screwed into the pre-bored workpiece with a uniform feed corresponding to the pitch of the thread. In this case, the thread profile presses stepwise into the material of the workpiece via the start of the thread portion whereby the stress in the compression zone is so high that the compression limit is exceeded and the material is plastically deformed. The material yields radially, flows along the thread profile into the free gullet and thus forms the core diameter of the female thread. The degree of deformation of the thread can be controlled by means of the pre-bore diameter.
The particular advantage is that the improvement in the structure that can be achieved has the result that the loading capacity of the thread is still sufficient even at 50% bearing depth. Since the lubrication is of decisive importance for this method, lubricating grooves must be formed in the tool between the pressing lands.
A disadvantage of this method is that a separate forming tool corresponding to the thread diameter is required for each thread, the geometry of the start for controlling the performance of the thread former is relatively difficult to optimize, and the cutting speeds and feed values cannot be selected independently of one another.
Finally it is known to mill internal threads. In this case, the thread is produced by juxtaposing the cutting lines of the thread milling cutter. The thread pitch is produced by the machine and corresponds to the so-called “pitch” of the thread milling cutter, i.e., the axial spacing of the rows of milling teeth.
An important advantage of this method is that one and the same milling cutter can produce threads having different diameters and the same pitch. Compared with cut threads, there is the further advantage that the milled thread is fully formed over almost the entire tool length used. A disadvantage with this method of production is that as a result of the cutting treatment, the structure is quasi-disturbed in the area of the cut teeth as a result of which the bearing force of the thread remains limited.
It is thus the object of the invention to provide a method, a tool, and a device for the production of threads, especially internal threads with which threads of different nominal diameter can be produced particularly economically and with improved structure quality in the area of the thread.
This object is solved with regard to the method by the features of claim 1, with regard to the device by the features of claim 27 and with regard to the tool by the features of claim 12.
According to the invention, the thread is as it were “hammered” into the inner surface of the workpiece bore by means of a newly shaped tool. The tool has a forming head which combines the shaping features of a thread milling cutter with those of a thread former and specifically such that the forming head is equipped with at least one, but preferably at least two profile projections or teeth constructed in the fashion of a thread milling cutter and positioned at a constant distance from one another, which are constructed as continuous over the circumference but with radial extension varying over the circumference so that at least one pressing land is formed in the area of each profile projection over the circumference as in the case of a thread milling cutter.
It has surprisingly been found that the radial forces acting on the shank of the tool in this type of chipless machining with milling kinematics can easily be absorbed without excessive radial deflection of the forming head and even if the forming head has a plurality of profile projections and thus is capable of itself producing relatively long threads with a circular movement. This is because the forming work to be undertaken by a pressing land can simply be controlled merely by means of the speed of the forming head and the speed of the circular movement of the tool which is independent thereof and thus kept sufficiently small so that the shank of the tool need not be excessively stressed. Further possibilities for limiting the radial forces acting on the forming head involve varying the number of pressing lands per profile projection and/or displacing the pressing lands of adjacent profile projections in the circumferential direction.
In addition to ensuring an improved bearing force of the thread as a result of the better structures, the concept for producing threads according to the invention has the additional advantage that the machine control provides the possibility of influencing the diameter tolerances in the range of the nominal and/or the flank diameter during production. A further important advantage of the concept for producing threads according to the invention is that no chips are produced.
Advantageous further developments are the subject matter of the dependent claims.
The further development of claim 2 has the advantage that the traveling-in movement can be used for centering the tool and that the forming of the thread can be completed using a singular movement.
If the radially outwardly directed movement of the forming head takes place along an arc-shaped curve, preferably in the form of a 180° run-in loop, so that on entry of the profile projections into the workpiece, the arc-shaped curve has a motion component in the direction of the following circulation motion, the tool loading during immersion into the workpiece will be the lowest.
If the profile projections i.e., the geometrical design of the polygon, each form a plurality of pressing lands over the circumference with the radial extension varying over the circumference, the efficiency of the tool is increased.
The pressing lands can be distributed uniformly over the circumference but also nonuniformly which has the advantage of reducing the tendency to vibration.
If, in accordance with claim 8, the pressing lands of adjacent profile projections are offset with respect to one another in the circumferential direction, for example, in the form of a helix, the total radial force which acts on the forming head during the pressing process of the internal thread can be further reduced.
In this case, it is advantageous from the production technology point of view if, according to claim 9, the axially adjacent pressing lands of the forming head each lie along a helix.
The process parameters can be optimized within broad limits according to the material to be processed. As a result of the kinematics of claims 10 and 11, in conjunction with alternating axial feed movement left- or right-hand threads can be produced using the same tool.
Advantageous embodiments of the tool are the subject matter of claims 12 to 26.
If the depth of the grooves between neighboring profile projections varies over the circumference, the flow behavior of the plastic material can be advantageously influenced.
For certain materials it can be advantageous to keep the depth of the grooves between adjacent profile projections substantially the same over the circumference.
If the tool, in accordance with claims 21, consists overall of a high-strength material, preferably of a hard material, especially a hard-metal material or another high-strength sintered material, the stability of the material is particularly high which has a particularly favorable influence on the bending deformation as a result of the overall radial acting force.
These tools could likewise be made of a combination of different materials. For example, strips made of a different material, can be inserted in suitable receptacles in a support made of a support material, for example, hardened or soft steel (or heavy metal), high-speed steel HSS or HSSE, aluminum alloys, hard metal or other sintered materials. For example, mention is merely made here of hard metals, cermets, PkD, CBN, ceramic etc. The pressing lands on the finished tool are formed using these materials.
The inserted strips can be executed as different shapes, e.g. cylindrical, conical, round etc. The strips can be joined to the support in different fashions, such as, for example, soldering, screwing, clamping, gluing or welding.
The forming head is preferably provided with a coating at least in the area of the pressing lands. All commonly used coatings which can achieve a reduction in friction and/or a reduction in wear can be achieved. Especially preferred is a hard material layer, such as of diamond, for example, preferably nanocrystalline diamond, TiN, TiAlN or TiCN or a multilayer coating. A lubricating layer, for example of MoS2 known under the name “MOLYGLIDE” would also be feasible.
Particularly good lifetimes and processing parameters are obtained using a coating according to claims 24 to 26.
A device for carrying out the method according to the invention requires the functions specified in claim 27 which can be realized for example using a triaxial control system of a 3D CNC machine tool.
In detail, hard materials, especially sintered materials can be used particularly economically in the tool according to the invention, solid hard metal but also so-called cermet materials being especially preferred. In this case, a particularly economical method of production is ensured. This is because the polygonal shape of the cross-section of the forming head can already be formed in the sintered blank so that subsequent machining can substantially be restricted to the area of the thread ridges.
Cermets can also be used, i.e. sintered materials which have titanium carbide and nitride (TiC, TiN) as essential hardness carriers and where nickel is predominantly used as the binding phase. In this material the criteria such as low chemical affinity to steel alloys, low heat conduction coefficient, higher hot hardness and fine-grained structure have a particularly advantageous influence of the field of application. The structural makeup of cermets has now been studied to such an extent that very fine-grained structures with high toughness can be prepared by suitably controlling the process parameters. It can be advantageous to incorporate titanium nitride into the material which has a low solubility in iron as a result of its high thermodynamic stability and thus positively influences the diffusion and friction behavior.
An independent object of the invention is furthermore a sintered blank for the forming head of a thread former which has profile projections with pre-formed pressing lands located at uniform axial spacing with respect to one another so that the end processing of the forming head after the finish-sintering, usually the grinding to the final dimension, can be restricted to a minimum. These forming heads formed from sintered blanks can be obtained from the manufacturer as semifinished products. These forming heads are then advantageously ground with allowances of the order of magnitude of only 0.5 mm relative to the nominal dimension.
Further advantageous embodiments of the invention are the subject matter of the remaining dependent claims.
Exemplary embodiments of the invention are explained in detail hereinafter with reference to schematic drawings. In the figures:
FIG. 1 is a schematic side view of a tool for forming an internal thread according to the method according to the invention;
FIG. 2 is the schematic section along II-II in FIG. 1;
FIG. 3 is a schematic perspective view of a variant of the forming head used for the tool as a blank before incorporating thread profile projections;
FIG. 4 is a diagram to illustrate how the individual work steps for executing the method according to the invention are successively arranged; and
FIG. 5 is a schematic view to illustrate a preferred run-in loop for the forming tool.
Shown schematically in FIGS. 1 and 2 is a tool which can be used to carry out the method according to the invention for producing threads, especially internal threads.
This comprises a thread former 10 which can be rotatably driven about a shank axis 14 by means of which the thread pitches can be driven in a chipless fashion by pressure forming from the surface of the workpiece, namely from the inner surface of a workpiece bore which is indicated by the dot-dash line 16 in FIG. 1.
The tool 10 has a shank 12 and a forming head 18.
The forming head 18 has a plurality of profile projections 20 constructed in the fashion of a thread milling cutter and located at a constant axial spacing T (pitch) with respect to one another, which are continuous over the circumference as can be seen from FIG. 2. As can likewise be seen from FIG. 2, they have a radial extension which varies over the circumference, which lies between ERMAX and ERMIN so that in the area of each profile projection 20 at least one pressing land 22 is formed over the circumference (in the embodiment according to FIGS. 1 and 2 four pressing lands 22 are formed).
The profile projections 20 are axially offset with respect to one another by the dimension of the thread pitch to be produced, i.e. the dimension T corresponds to the thread pitch of the thread to be produced.
It can be seen from the diagram in FIG. 2 that the core cross-section of the forming head 18 has a polygonal shape and that the radial depth RT of the profile projections 20 remains substantially the same over the entire circumference. However, it should already be emphasized at this point that this detail is not essential. The circumferential contour of the core cross-section and/or the contour of the envelopes of the profile projections 20 can easily be varied so long as the function of the pressing lands 22 is maintained.
The forming head 18 has an axial extension EA which substantially corresponds to the length TG (see position 4.5 in FIG. 4) of the thread to be produced.
It can be seen from FIG. 2 that the pressing lands 22 are uniformly distributed around the circumference. However, they can also be nonuniformly distributed to reduce the tendency of the tool to vibrate.
The production process using the thread forming tool according to the invention is explained in detail with reference to FIG. 4:
The thread is formed by setting in rotational motion the shank tool 10 equipped in the fashion of a thread milling cutter with at least two profile projections 20 located at a constant spacing T with respect to one another and, as is shown in FIG. 4A on the left-hand side under position 4.1 positioning centrally above the workpiece bore 16. The tool 10 can be constructed such that a cutter 24 for application of a chamfer 26 is provided in the transition zone to the shank 12, as is indicated in position 4.2.
The tool 10 is then driven axially to such a distance from the center parallel to itself until the tool tip 28 has reached the desired dimension TG of the thread depth, as is indicated in position 4.3. The tool 10 is then fed radially and specifically in such a fashion that, as is shown in position 4.4, at a circumferential point SU it first reaches its radial position required for pressing the thread to the desired dimension of the flank and/or nominal diameter by plastic deformation of the workpiece. This dimension is usually determined empirically and depends, among other things, on the material parameters (flow behavior) of the workpiece and the parameters of the forming process.
The path on which the tool 10 is brought from the center of the workpiece bore 16 into the position SU is designated by the arrow ES. This curve as it were describes a type of run-in loop which will be discussed in detail at a later point.
After the tool 10 has thus been brought to the full processing or thread depth, while maintaining the eccentricity EX of the axis of rotation 14 set with respect to the axis 30 of the workpiece bore 16, it executes a circular movement running through 360° (circular movement BZ, see arrow in position 4.5) about the axis of the workpiece bore 16 while the forming head 18 simultaneously executes a constant axial feed movement BV by the dimension of the thread pitch P to be produced. In this case, the material of the workpiece is, as it were, subjected to a continuous pressure forming in the fashion of a hammer treatment by the pressing lands successively coming into engagement, which causes the material to flow so that it can penetrate into the grooves 42 between the profile projections 20 in a controlled fashion. In this way, the thread acquires a similar quality and geometry in the area of the thread ridges as in thread forming.
When this circular movement (BZ) is completed, the thread is ready-formed. The tolerance of the flank and the nominal diameter can be specifically and continuously influenced during the forming process by correction interventions in the area of the control system for the circular movement, which benefits the service life of the tool 10.
When the tool 10 has again reached the position SU, it is driven radially inward on a curve AS, as shown in position 4.6, so that the profile projections 20 of the forming head 18 come out of engagement with the internal thread 34 which has been produced.
The method for “chipless thread milling” by means of a tool 10 which has an unchanged cross-section in the axial direction has been described with reference to FIGS. 1 to 4. The pressing lands 22 of all the profile projections 20 thus each simultaneously come into contact with the material of the workpiece to be expelled. To overcome the forces thus produced and to be absorbed by the shank 12 of the tool 10, the feed in the circular direction is matched to the speed of the tool so that the deformation work of the pressing land 22 located in engagement is as small as possible.
In order that these forces can be applied even more uniformly to the tool 10, in accordance with the further development from FIG. 3, the pressing lands 122 of neighboring profile projections can be offset with respect to one another in the circumferential direction. Advantageously the axially adjacent pressing lands 122 of the forming head 118 each lie along a helix which is indicated by the dashed line W in FIG. 3. In other words, FIG. 3 shows a pre-finished or pre-machined blank for a forming head 118 in which the profile grooves 142 are then incorporated to finish the forming head.
FIG. 5 finally shows a detail of the construction of the run-in loop ES on a slightly enlarged scale. It can be seen that the motion takes place along a circular movement guided through 180° which has a positive influence on the tool loading. However, especially if there is a fairly large difference in diameter between tool and thread, it is equally advantageous to select a quadrant run-in loop which is easier to program.
The tool described hereinbefore can equally well be used to produce right and left-hand threads where only the feed direction BV and, optionally the direction of the rotation of the tool need to be reversed in this connection so that deformation can take place concurrently and/or countercurrently.
For additional improvement of the processing quality the forming head can be provided, at least in the area of the most stressed sections, i.e., in the area of the pressing lands 22, 122, with a coating which is preferably constructed as a hard material coating. This hard material layer can, for example, be diamond, preferably nanocrystalline diamond, titanium nitride or titanium aluminum nitride. Particularly suitable among other things are a titanium aluminum nitride layer and a so-called multi-ply layer which is marketed under the name “Fire I” from Gühring oHG. This comprises a TiN/(Ti, Al)N multi-ply layer.
This can particularly preferably comprise a wear-protection layer which substantially consists of nitrides having the metal components Cr, Ti and Al and preferably a small fraction of elements for grain refining wherein the Cr fraction is 30 to 65%, preferably 30 to 60%, especially preferably 40 to 60%, the Al fraction is 15 to 80%, for example 15 to 35% or 17 to 25% and the Ti fraction is 16 to 40%, preferably 16 to 35%, especially preferably 24 to 35%, and specially each related to all the metal atoms in the entire layer. In this case, the layer structure can be single-layer with a homogeneous mixed phase or it can consist of a plurality of homogeneous layers per se which alternately consist on the one hand of (Ti_xAl_yY_z)N with x=0.38 to 0.5 and y=0.48 to 0.6 and z=0 to 0.04 and on the other hand consist of CrN wherein the uppermost layer of the wear-protective layer is formed by the CrN layer.
The forming head 18, 118 and/or the shank or the entire tool 10 of the exemplary embodiments show can consist of a wide range of materials, but a material which exhibits high stability and wear resistance is especially advantageous. The shank of the tool should have a particularly high bending strength so that the forces acting radially on the forming head 18, 118 can be effectively intercepted which benefits the manufacturing precision of the thread.
In the stressing profile here the criteria abrasive wear and hot hardness can also be of decisive importance so that cermet types such as, for example a cermet of the type “HTX” marketed by Kennametal-Hertel can well be used. Good results are also achieved using the types “SC30” from the manufacturer Cerasiv GmbH (Feldmühle) and “Tungaly NS530” from Toshiba Europa GmbH.
It is especially preferable to use as material for the tool a hard material such as, for example, a carbide, a nitride, a boride or a nonmetallic hard materials or a hard material system such as has become known for example in the form of mixed carbides, carbon nitrides, carbide boride combinations or mixed ceramics and nitride ceramics. In this case, those hard material which can be manufactured as sintered moldings can also be used.
In the embodiment according to FIGS. 1 to 5, the forming head 18, 118 consists of solid hard metal but can also be made of another hard material, that is for example also from a cermet sintered molding into the geometry of the forming head, i.e., the polygonal shape of the core and/or the forming grooves are already incorporated before the sintering. This can be accomplished for example by suitable forming during the pressing process or also by suitable aftertreatment of the sintered blank before the sintering process. Thus, the sintered blank can be pre-fabricated with a small allowance relative to the nominal diameter. In this case, it is sufficient, for example to restrict the allowance to the range of about 0.5 mm relative to the final dimensions. It is even possible to preform the polygonal shape of the core section of the forming head in the area between the polygon edges more or less to the final dimension in the original molding process so that grinding treatment to the final dimension in this area can even be omitted after the sintering process. Grinding to the final dimension is thus merely required in the area of the forming teeth . . . 18.
As a result of the very high values of the compressive strength, bending strength and elastic modulus of hard materials, especially hard metal or cermet, the shank, when made of such materials, is deformed only very slightly even under very large radial forces which act over the entire length of the forming head so that the forming precision can be maintained at a very good level even when processing materials which are particularly difficult to deform.
It has been shown that the material cermet is particularly suitable for the forming head 18, 118. The material has a low chemical affinity to steel alloys, a lower heat conduction coefficient, a higher hot hardness and a relatively fine-grained structure so that small forming edge radii can be achieved. The heat conduction coefficient of cermet, which is a factor of 7 lower than that of hard metal has the effect that the heat of deformation produced does not flow directly into the forming head and thus lower operating temperatures are obtained at the tool which in turn makes it possible to achieve higher dimensional constancy over the entire forming process.
However, the invention is not restricted to a specific material for the forming head and/or shank or to a specific hard material such as, for example, cermet quality which the average person skilled in the art selects from the now widely compartmentalized range of hard materials depending on the area of application of the tool.
In particular, the type “HTX” has particularly long lifetimes, i.e. a very low volume wear compared with other types of cermet during the forming process, especially during forming without lubrication. The sintered material can however also be selected according to other criteria, such as, for example, the desired bending strength or according to the respective material to be processed. Thus, for example the material hard metal in the so-called K quality has proved to be particularly favorable with regard to the attainable useful life for the processing of cast iron and aluminum.
The material for the shank can be selected according to one variant so as to produce a low tendency to vibration. Thus, conventional heat-treatable steels and tool steels can also be used, for example the heat-treatable steel 42CrMo4 with tensile strengths 1000 N/mm²<sigma<1500 N/mm².
Naturally, deviations for the variants of the tool and the method described hereinbefore are also possible without departing from the basic idea of the invention.
Thus, for example the kinematics of the tool with respect to the workpiece can naturally be reversed so that for example, the workpiece executes the movement of the run-in loop and/or the circular movement.
The polygonal shape in the forming head need not necessarily be incorporated before a sintering process. It is equally possible to incorporate the blank mold of the head with its polygonal cross-sectional shape and/or the profile projections into the cutting head before the sintering process.
The forming head or the entire tool can also consist of other metals, advantageously from a high-strength material such as, hard metal for example, high-speed steel such as, for example, HSS, HSSE or HSSEBM, ceramic, cermet or another sintered metal material.
It is further possible to provide a coolant and lubricant supply, including internally.
The invention can also be used if the axial length of the thread to be formed exceeds the length of the forming head 18, 118. In this case, the motion kinetics of FIG. 4 is executed several times in succession, i.e. with axial staggering of the thread forming.
In principle, the forming principle can also be applied to the production of external threads.
The invention accordingly provides a method for the production of threads, especially internal threads, by means of a rotationally driven thread former, with which the thread pitches are driven out of the surface of the workpiece in a chipless fashion by pressure forming, in particular out of the inner surface of a workpiece bore. In order to be able to produce threads of different nominal diameter especially economically and with improved joining quality in the area of the thread, the thread is formed such that a shank tool in the fashion of a thread miller, equipped with at least two profile projections located at a constant distance from one another, where the profile projections at its forming head are constructed as continuous over the circumference and with radial extension varying over the circumference, is driven into the workpiece initially at a circumferential point of the workpiece bore, preferably is brought to the total thread pressing depth, and while substantially retaining the eccentricity set with respect to the axis of the workpiece bore, executes a relative circular movement running through 360° (circular movement) relative to the axis of the tool bore, while the forming head simultaneously executes a constant axial relative feed movement by the extent of the thread pitch to be produced.

Claims

1. A method for the production of threads by means of a rotationally driven thread tool, with which the thread pitches are driven in and out of the surface of the workpiece in a chipless fashion by pressure forming, in particular in and out of the inner surface of a workpiece bore, wherein the thread is formed such that a shank tool in the fashion of a thread miller, equipped with at least one profile projection where each profile projection at its forming head is constructed as continuous over the circumference and polygonal with radial extension varying over the circumference, inserted in a workpiece bore having a larger diameter and moved along the inner surface of the workpiece bore with a pre-determined axial feed while turning, wherein after insertion into the workpiece bore, the shank tool is driven radially into the workpiece initially at a circumferential point of the workpiece bore, and while substantially retaining the eccentricity set with respect to the axis of the workpiece bore, executes a relative circular movement running through 360° relative to the axis of the tool bore, while the forming head synchronously executes a constant axial relative feed movement by the extent of the thread pitch to be produced.

2. The method according to claim 1, wherein the forming head has an axial extension which corresponds to the length of the thread to be produced.

3. The method according to claim 1, wherein the forming head is driven into the workpiece bore substantially centrally to the extent of the thread depth, is then driven radially outward while retaining the axial relative position to the workpiece bore, until a thread ridge is fully formed at a circumferential point between adjacent profile projections, then executes the circular movement extending over 360° with simultaneous axial feed and finally is driven radially inward so that the profile projections of the forming head come out of engagement with the internal thread produced.

4. The method according to claim 3, wherein the radially outwardly directed movement of the forming head takes place along an arc-shaped curve.

5. The method according to claim 4, wherein on entry of the profile projections into the workpiece, the arc-shaped curve has a motion component in the direction of the subsequent circular movement.

6. The method according to claim 1, wherein the profile projections with the radial extension varying over the circumference each form a plurality of pressing lands over the circumference.

7. The method according to claim 6, wherein the processing lands are nonuniformly distributed over the circumference.

8. The method according to claim 6, wherein the pressing lands of adjacent profile projections are offset with respect to one another in the circumferential direction.

9. The method according to claim 8, wherein the axially adjacent pressing lands of the forming head each lie along a helix.

10. The method according to claim 1, wherein the area of engagement with the workpiece the circumferential speed of the forming head is synchronized with the circular movement.

11. The method according to claim 1, wherein the area of engagement with the workpiece the circumferential speed of the forming head is oppositely directed to the circular movement.

12. A rotationally drivable tool for the production of threads by means of chipless pressure forming of the inner surface of a workpiece bore, comprising a forming head comprising at least two profile projections embodied in the fashion of a thread miller and located at a constant distance from one another, which are constructed as continuous over the circumference and with radial extension varying over the circumference, so that in the area of each profile projection, at least one pressing land is formed over the circumference, wherein the profile projections each form a plurality of pressing lands over the circumference with radially varying axial extension over the circumference, wherein the pressing lands of neighboring profile projections are offset with respect to one another in the circumferential direction.

13. The tool according to claim 12, wherein the profile projections are axially offset with respect to one another by the extent of the thread pitch to be produced.

14. The tool according to claim 12, wherein the forming head has an axial extension which substantially corresponds to the length of the thread to be produced.

15. The tool according to claim 12, wherein the pressing lands are nonuniformly distributed over the circumference.

16. The tool according to claim 12, wherein the axially adjacent pressing lands of the forming head each lie along a helix.

17. The tool according to claim 12, wherein the depth of the grooves between neighboring profile projections varies over the circumference.

18. The tool according to claim 12, wherein the depth of the grooves between neighboring profile projections remains substantially the same over the circumference.

19. The tool according to claim 12, wherein said tool consists of a high-strength material.

20. The tool according to claim 12, further comprising a tool carrier made of a support material which receives at least one tool strip of another material.

21. The tool according to claim 12, wherein at least in the area of the pressing lands, the forming head is provided with a coating.

22. The tool according to claim 20, wherein the coating is a hard material layer which consists of nitrides comprising the metal components Cr, Ti and Al and comprising the component C, wherein the Cr fraction is 30 to 65%, the Al fraction is 15 to 80%, and the Ti fraction is 16 to 40%, each relating to all the metal atoms in the entire layer.

23. The tool according to claim 21, wherein the structure of the entire layer consists of a homogeneous mixed phase.

24. The tool according to claim 21, wherein the structure of the entire layer consist of a plurality of homogeneous individual layers per se, which alternately consist on the one hand of (Ti_xAl_yY_z)N with x=0.38 to 0.5 and y=0.48 to 0.6 and z=0 to 0.04 and on the other hand consist of CrN wherein the uppermost layer of the wear-protective layer is formed by the CrN layer.

25. A device for carrying out the method according to claim 1, comprising a drive spindle for the rotationally driven thread former and a triaxial control system with which the feed movement along the tool axis, the driving-in and driving-out movements of thread formed into and out of engagement with the workpiece and the circular movement are executed in a synchronized fashion.

26. The device according to claim 25, wherein the triaxial control system is provided by a 3D CNC machine tool.