SCREW HAVING A MILLING SECTION EMBEDDED IN THE THREAD

Abstract

A screw having the following successive functional sections: a threaded tip (150) shaped as a cone (A) having a double thread; a milling section (B) having a plurality of milling ribs (160) with outer diameter D.sub.F; a cylindrical shank section having the main thread with diameter D.sub.N; a thread-free shank section (D); optionally a holding thread section (E); optionally a short thread-free underhead section (F); and a head section (G) having a force engagement. The main thread (110) is guided continuously from the tip section (190) of the cone (A) via the milling section (B) to the head end of the thread-bearing shank section (C) and the ribs of the milling section (B) are arranged recessed in the thread base (140) of the continuous main thread such that the thread tips of the continuous main thread with nominal outer diameter D.sub.B project beyond the milling ribs, so that D.sub.F<D.sub.B.

Claims

1. A screw, comprising: functional sections that merge or are adjacent to each other as follows: a) a threaded tip shaped as a cone having a double thread, the double thread comprising a main thread and a secondary thread; b) a substantially cylindrical milling section having a plurality of milling ribs formed as a steep thread and having an outside diameter D.sub.F measured across the ribs; c) a substantially cylindrical shank section carrying only the main thread which has a thread outside diameter D.sub.N on the substantially cylindrical shank section; d) a substantially cylindrical, smooth, thread-free shank section; e) a head section with a force engagement; f) the main thread extends continuously and with a constant pitch from the tip section of the cone through the milling section to a head end of the thread-bearing shank section; and g) the ribs of the milling section are arranged recessed in a thread base of the continuous main thread such that thread tips of the continuous main thread with a nominal outer diameter D.sub.B in the substantially cylindrical milling section project beyond the milling ribs with D.sub.F<D.sub.B.

2. The screw according to claim 1, further comprising a holding thread section arranged between the thread-free shank section and the head section, and the holding thread section is configured as a short holding thread with 1 to 3 turns followed by a short thread-free underhead section.

3. The screw according to claim 2, wherein the holding thread comprises a double thread and a pitch of the holding thread corresponds to 1.6 to 1.9 times the constant pitch of the main thread.

4. The screw according to claim 2, wherein a length of the thread-free underhead section and the holding thread section is between 2 and 5 mm, respectively.

5. The screw according to claim 1, wherein the main thread is configured symmetrically with a flank angle of 60°±3°.

6. The screw according to claim 1, wherein the secondary thread starts in the tip section of the cone, and has a thread height that increases continuously and decreases again after half a length of the cone and runs out in a transition area of the cone and the milling section.

7. The screw according to claim 1, wherein the outer diameter D.sub.N of the main thread in the shank section is larger than the outer diameter D.sub.B of the main thread in the substantially cylindrical milling section.

8. The screw according to claim 1, wherein a flank height of the main thread at a transition of milling section to the cone, starting from the outer diameter D.sub.B, continuously decreases over the cone as the cone tapers and runs out in the tip section of the cone.

9. The screw according to claim 1, wherein an apex angle of a conical envelope formed by tangents at thread tips of the main thread on the cone, which form a common intersection with a longitudinal axis of the screw, is 35°±5°.

10. The screw according to claim 1, wherein a projection of a respective thread crest of a milling rib onto a longitudinal axis of the screw forms an angle of 30°±10° herewith.

11. The screw according to claim 1, wherein the screw comprises a bimetal screw, and substantially cylindrical, smooth, thread-free shank section, the substantially cylindrical shank section and the head section are formed of stainless steel, and the cone and the substantially cylindrical milling section of the threaded tip are formed of carbon steel and a transition point between the stainless steel and the carbon steel is located in the substantially cylindrical shank section carrying the main thread.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 shows a side view of a basic version of a screw according to the invention.

[0033] FIG. 2 shows a side view of a screw according to the invention extended by additional shank sections.

[0034] FIG. 3 shows an enlarged detail of the section “Z” from FIG. 1, namely the thread tip with adjacent milling section.

[0035] FIG. 4 shows detail Y of FIG. 1 with a cross-section of the main thread.

DETAILED DESCRIPTION

[0036] FIG. 1 shows a first embodiment of a screw 100 with the basic components G, D, C, B, A. G represents the head section with force application 130. Here, the force application 130 is shown as a hexagon in side view, which has a stop surface or stop underside 135 at its end facing the shank. This is followed, in the picture to the right or towards the tip, by a smooth, thread-free shank section D, which is cylindrical in design. The length of this section as well as of the following shank section C with the main thread is determined by the intended use and application and is therefore designed and manufactured as required. The thread-bearing shank section C passes over the milling section B, which in turn passes into the taper A. Both B and A are explained in more detail in FIG. 3.

[0037] FIG. 2 shows a variant 200 of the screw according to the invention, in which two further functional areas are inserted between the head section G and the thread-free shank section D. Adjacent to the head section G is a short thread-free underhead section F, followed in the direction of the tip by a support thread section or holding thread section E. The latter advantageously deviates from the thread geometry of the main thread.

[0038] A realization of the screw according to the invention as shown in FIG. 2 has a nominal diameter D.sub.N of 6.4 mm. It is manufactured in various lengths. In a typical example, the screw has an effective length (sections A to and including F) of 150 mm, the stainless steel portion of which is 132 mm. Sections D, E and F have a combined length of approximately 74 mm. For all screws of the same diameter, the length of the carbon steel section is approximately 18 mm. For all screws of this diameter, the length of section A is about 7 mm, of section B about 4 mm. This means that an approximately 7 mm long section of the shank section C, which is provided with the main thread, is also made of carbon steel before it is continued in stainless steel. For the above-mentioned screw, a pitch of the main thread of p=2.5 mm is provided, which applies to all sections A-C. The outer diameter in section B D.sub.N is approximately 5.8 mm.

[0039] FIG. 3 shows in detail an embodiment of the front section Z (detail from FIG. 1) of a screw according to the invention, for FIG. 1 as well as FIG. 2. In the drawing (from left to right), arranged along the longitudinal axis 180, a cut shank section C with man thread 110 is shown, followed by a milling section B, which merges into a cone A with thread tip 150. The main thread consists of a thread train with a flank angle of 60° and a pitch p (pitch) which, as usual, is given in mm per turn. FIG. 4 corresponds to the outline Y of FIG. 1 and shows a section of the main thread 110 in section C with the flank angle and pitch p indicated.

[0040] FIG. 3 shows an example of a thread on the shank section C with a thread base 140 and a thread flank 145. The nominal diameter of this screw D.sub.N is measured across the thread tips of the main thread 110 and is drawn in FIG. 3.

[0041] The shank section C, while retaining the pitch and flank angle, merges into the milling section B with a slightly reduced thread height D.sub.B of the main thread 110. There are two reasons for this reduction: Since the screw is manufactured by cold forming from a cylindrical blank, only the locally available base material is available for any thread forming. Although thickening can be achieved by upsetting, e.g. in the area of the head section G, this would be very costly in the middle of the shank area and would complicate thread rolling. After the milling ribs 160 are arranged in the milling section B in the thread base, less material is available for the main thread. Although it would be possible to achieve the same thread height in section B as in section C by reducing the flank angle of the main thread 110, this would compromise the stability of the main thread in the milling section in particular.

[0042] One of the inventive features, however, is the continuous passage of the main thread and the resulting seamless guidance of the screw from the displaced, grooved metal into the wood. Therefore, it is advantageous to reduce the thread height in the milling section B rather than the flank angle. Since during the setting process of the screw after passing the cone A through the metal plate(s), the screw temporarily has a maximum external (thread) diameter D.sub.B, this means that the main thread nevertheless grips snugly in the self-furrowed channel. At the same time, when the milling section B acts in the wood, the deformation resistance of the screw is lower because of the smaller outer diameter D.sub.B<D.sub.N, considering the portion of the main thread alone. As tests showed, the guidance of the screw in the grooved metal section is not affected; on the contrary, the thread tips of the main thread act as spacer elements keeping the milling ribs away from the hole edges. At the transition from section B to C, the hole or thread diameter grooved in the metal plate is widened to D.sub.N, but then the milling section B has already passed the metal plate(s).

[0043] FIG. 3 shows the milling ribs 160 arranged so that the milling ribs form essentially a steep left-hand thread, which increases the milling effect in the wood. The line with reference mark 330 marks the projection of the thread crest 320 onto the longitudinal screw axis 180. The drawing also shows the angle of intersection with the longitudinal screw axis 180.

[0044] In the area of the cone, it is shown how the main thread 110, while maintaining the pitch and the flank angle, slowly decreases from the diameter D.sub.B at the transition area of section B to A continuously to zero and tapers off in the tip section 190. Also marked is the secondary thread 120, which, starting in the tip section, continuously increases like the main thread and tapers off again at the transition area A to B. This achieves the double thread on the thread tip on the cone A, which is advantageous for forming in the metal. The tangents on the thread crests of the main thread with a common intersection 310 form the envelope 300, with the intersection 310 being on the longitudinal axis 180 of the screw 100 or 200.