Hard-material threaded connection

Abstract

A hard-material threaded connection includes a structural part, which has a hard material and a first threaded portion formed in the hard material, and a component having higher elasticity that has a second threaded portion cooperating with the first threaded portion. The first threaded portion and the second threaded portion have a different thread pitch.

Claims

1. A hard-material threaded connection, comprising: a structural part having a hard material selected from the group consisting of a cemented carbide, a cermet and a cutting ceramic and having a first threaded portion formed in said hard material; and a component having higher elasticity than said hard material and having a second threaded portion cooperating with said first threaded portion, said first threaded portion and said second threaded portion having different thread pitches, wherein a difference in said thread pitches amount to more than more than 1.5%.

2. The hard-material threaded connection according to claim 1, wherein a difference in said thread pitches amounts to between 2.1% and 8%.

3. The hard-material threaded connection according to claim 1, wherein said first threaded portion is an internal thread.

4. The hard-material threaded connection according to claim 1, wherein said first and second threaded portions define an internal threaded portion and an external threaded portion, said internal threaded portion has a higher pitch than said external threaded portion.

5. The hard-material threaded connection according to claim 1, wherein said first threaded portion is configured as an internal thread and is provided with a countersink such that said internal thread is applied so as to be recessed with respect to a surface of said structural part.

6. The hard-material threaded connection according to claim 1, wherein said structural part which has said hard material is an exchangeable cutter.

7. The hard-material threaded connection according to claim 1, wherein said hard material is a hard metal or a cermet.

8. The hard-material threaded connection according to claim 1, wherein a difference in said thread pitches amounts to more than 2%.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1: shows a partially sectional view of a tool with a hard-material threaded connection according to one embodiment;

(2) FIG. 2: shows a top view of the tool of FIG. 1;

(3) FIG. 3: shows an enlarged partially sectional view of a detail of the hard-material threaded connection in the tool of FIG. 1;

(4) FIG. 4: shows a more enlarged partially sectional view of a detail of the threaded portions in the hard-material threaded connection;

(5) FIG. 5: shows a graphical illustration to explain the load distribution in the hard-material threaded connection, as compared with a conventional hard-material threaded connection; and

(6) FIG. 6: shows a partially sectional view of a detail of a conventional hard-material threaded connection.

DESCRIPTION OF THE INVENTION

(7) An embodiment will be described below with reference to FIG. 1 to FIG. 4.

(8) In the embodiment, the hard-material threaded connection is implemented in a tool 1 with a structural part 2, which has a hard material, and with a component 3 having higher elasticity. The component 3 having higher elasticity may, for example, be manufactured, in particular, from steel. In the embodiment illustrated in the figures, the tool 1 is formed by a drill having an exchangeable cutter made from a hard material. In this case, the structural part 2 which has a hard material is formed by the exchangeable cutter which is designed as a cutting head. In the embodiment illustrated in FIG. 1 to FIG. 4, the component 3 having higher elasticity is formed by a separate threaded body which is connected to a tool body 4. However, it is also possible, for example, that the component having higher elasticity is formed directly by a tool body which is provided with a threaded portion.

(9) In the exemplary embodiment, the tool body 4 is manufactured in a way known per se from tool steel and is provided with spiral flutes. The structural part 2, which is implemented as an exchangeable cutting head, is designed in such a way that it can be fastened to the tool body 4 via a hard-material threaded connection which is described in more detail below. In the exemplary embodiment illustrated, the component 3 is firmly connected to the tool body 4, for example via a screw connection. The component 3 may in this case, in particular, likewise be formed from a tool steel.

(10) The structural part 2 (that is to say, the exchangeable cutter in the embodiment) is provided with a first threaded portion 2a, and the component 3 having higher elasticity (that is to say, the threaded body in the embodiment) is provided with a second threaded portion 3a. The first threaded portion 2a is formed in the hard material of the structural part 2. The second threaded portion 3a is formed in the material of higher elasticity of the component 3. The hard material, because of its high hardness, has relatively low elasticity, whereas the component 3 and the tool body 4 have a substantially lower hardness and a higher elasticity.

(11) In the embodiment illustrated, the first threaded portion 2a is designed as an internal thread made from the hard material. The second threaded portion 3a is designed as an external thread on the component 3. Although this configuration is preferred, it is, however, also possible, for example, to design the first threaded portion as an external thread made from hard material and the second threaded portion as an internal thread.

(12) The first threaded portion 2a and the second threaded portion 3a are designed to cooperate in order to form a hard-material threaded connection. In this case, however, the first threaded portion 2a and the second threaded portion 3a have a different thread pitch, as described in more detail below with reference to FIG. 3 and FIG. 4.

(13) In the exemplary embodiment, the second threaded portion 3a is designed as a standard thread (for example, as a metric ISO thread). By contrast, the first threaded portion 2a, formed in the hard material, is designed with a thread pitch deviating from the standard. The first threaded portion 2a has in this case a thread pitch deviating from the second threaded portion 3a (and therefore from the standard) by between 2.1% and 8%. In the exemplary embodiment, the first threaded portion 2a has a thread pitch which deviates from the thread pitch of the second threaded portion 3a by 3%. In this case, the first threaded portion 2a designed as an internal thread has a higher thread pitch than the second threaded portion 3a designed as an external thread.

(14) The effect achieved by this difference in the thread pitch is explained in more detail below with reference to FIG. 4. FIG. 4 shows the thread flights of the hard-material threaded connection in an enlarged illustration, so that the individual flanks of the internal thread and of the external thread can be seen. FIG. 4 is an illustration of the hard-material threaded connection in a fully screwed-in state.

(15) As can be seen in FIG. 4, near a free end 30 of the second threaded portion 3a (at the top in FIG. 3), the thread flanks 31 of the second threaded portion 3a which face away from the free end 30 bear against the corresponding thread flanks 21, facing the free end 30, of the first threaded portion 2a designed as an internal thread. In that region of the hard-material threaded connection which faces away from the free end 30, that is to say near the surface of the structural part 2 (at the bottom in FIG. 3), the thread flanks 32 of the second threaded portion 3a which face the free end 30 bear against the corresponding thread flanks 22 of the first threaded portion 2a which face away from the free end 30.

(16) As a result, in the screwed-together state, the external thread is subjected to slight prestress, so that the flanks of the external thread near the free end are subjected to tension and the flanks of the external thread remote from the free end are subjected to compression.

(17) As a result of this configuration, in the event of tensile load upon the hard-material threaded connection or upon the threaded portion designed as an external thread, in the case of low tensile load the thread flights lying well inside in the internal thread (that is to say, near the free end of the external thread) are first loaded and, with increasing tensile load and expansion of the body which has the external thread, the thread flights lying further out (remote from the free end of the external thread) are also loaded.

(18) For comparison, FIG. 6 illustrates a conventional hard-material threaded connection 100, in which both the first threaded portion 102a formed in the hard material 102 and the second threaded portion 103a formed on the component 103 having higher elasticity are designed as standard threads. In this conventional configuration, in the event of tensile load upon the hard-material threaded connection (illustrated diagrammatically by a double arrow P), mainly the thread flights remote from the free end 130 and near the surface of the structural part made from hard material 102 are loaded, this increasing with an increase in tensile load. There is therefore the risk that these thread flights are stripped off. In the case of high load peaks, this may lead, due to the formation of cracks, to chips or destruction of the entire structural part made from hard material.

(19) The advantages of the hard-material threaded connection according to the invention, as compared with a conventional hard-material threaded connection in which both the first threaded portion and the second threaded portion are designed as standard threads, are described in more detail below with reference to FIG. 5. FIG. 5 illustrates, by the example of a six-flight thread, what load fraction in % is attributable to the respective force-transmitting thread flights, specifically, on the one hand, for a conventional hard-material threaded connection 100 (unbroken line) and, on the other hand, in the hard-material threaded connection according to the embodiment (dashed line).

(20) The force-transmitting thread flights are in this case numbered, starting from the surface of the hard material, that is to say thread flight number 1 is located near the surface of the hard material and thread flight number 6 is located near the free end of the external thread.

(21) As can be seen in FIG. 5, in the conventional hard-material threaded connection (unbroken line) the first thread flights bear the most load and the thread flights in the direction of the free end of the external thread bear increasingly less load. In contrast to this, in the hard-material threaded connection according to the embodiment (dashed line), mainly the thread flights near the free end of the external thread bear the load.

(22) The advantages achieved by means of the hard-material threaded connection according to the embodiment also become clear from the following description of examples.

EXAMPLES

(23) Tests were conducted, in which internal threads with a different pitch were formed by means of a helical milling method in each case in an essentially cylindrical structural part made from hard material, in particular hard metal.

(24) In each case a threaded connection was made by means of a steel screw (screw quality 8.8 phosphated) with a metric M6 ISO thread (pitch 1.000 mm) and with a thread depth of 15 mm. The yield strength of the screw was reached at a tensile force of 12864 N and a break occurred at 16080 N.

(25) In sample 1, an unmodified M6 ISO thread (pitch 1.000 mm) was formed in the hard material.

(26) In sample 2, an M6 thread with a thread pitch (pitch 1.010 mm), increased by 1% as compared with the standard thread, was formed in the hard material.

(27) In sample 3, an M6 thread with a thread pitch (pitch 1.020 mm), increased by 2% as compared with the standard thread, was formed in the hard material.

(28) In sample 4, an M6 thread with a thread pitch (pitch 1.030 mm), increased by 3% as compared with the standard thread, was formed in the hard material.

(29) In sample 5, an M6 thread with a thread pitch (pitch 1.040 mm), increased by 4% as compared with the standard thread, was formed in the hard material.

(30) When the hard-material threaded connection was being formed, samples 1 to 3 could be screwed in by hand, but a tool was already required in order to screw in samples 4 and 5 because of the resulting prestress. All the samples were loaded to above the yield strength of the screw on a hydraulic press. It was observed under which load the first thread flights were stripped.

(31) Results

(32) In sample 1, cracking could be heard even under a tensile force of approximately 4000 N, and ring-shaped chopping occurred. In sample 2, destruction of the structural part made from hard material occurred at a tensile force of approximately 10000 N. In samples 4 and 5, by contrast, stripping of thread flights in the structural part made from hard material could no longer be observed until the breaking load of the screw was reached.

(33) Development

(34) According to a development of the embodiment, the structural part 2 is again provided with a first threaded portion designed as an internal thread. In contrast to the embodiment described above, in the development the first threaded portion is provided with an oblique countersink in such a way that the first thread flight of the internal thread does not start directly at the surface of the structural part made from hard material, but instead at a stipulated spatial distance from the surface. The internal thread is thus applied so as to be recessed with respect to the surface. In this way, the stability of the first threaded portion formed in the hard material can be additionally increased.

(35) Although, in terms of the embodiment, a structural part 2 has been described which is formed entirely from hard material, it is also possible, for example, that the structural part 2 is formed only in regions made from hard material. In this case, too, the first threaded portion 2a is formed in a region of the structural part 2 which is made from hard material.

(36) Although, in terms of the embodiment, it has been described that the hard-material threaded connection is used in the case of a tool in the form of a drill, other applications are also possible, in which a structural part made from a hard material is connected to a material of lower elasticity via a threaded connection, in particular (but not only) applications in the case of other tools, in particular cutting tools.