Patent Description:
Thread forming screws can produce a thread in a material as the screw is driven into the material. These screws are used with existing/pre-formed (non-threaded) holes, and form threads by displacing material without removing it.

<CIT> discloses Zn-Ni as a coating layer on self-drilling screws.

It may be desired to make a thread forming screw of stainless material. However, stainless screws typically have a too soft surface to be used for thread forming, and typically get "jammed" if one tries to form threads with them.

It is an object of the present invention to overcome this problem, and to provide an improved thread forming screw.

According to a first aspect of the present invention, this and other objects are achieved by a (thread forming) screw, comprising: a (thread forming) screw body made of stainless steel and comprising a head, a threaded portion, and a tip; and a coating applied on at least the threaded portion, wherein the coating is a zinc-nickel coating and/or has a hardness in the range from <NUM> to <NUM> HV.

The present invention is at least partly based on the understanding that by applying a zinc-nickel coating having a hardness in the range from <NUM> to 450HV on a stainless steel thread forming screw body, the surface of the screw gets sufficiently harder and the coating builds a layer so that the above-mentioned "jamming" when forming threads can be avoided. Hence, a thread forming screw made of stainless material can be realized. Although it is known that zinc-nickel offers corrosion resistance, the (main) purpose of the zinc-nickel coating is here to achieve a harder surface of the screw and to create a barrier layer to prevent "jamming". The corrosion resistance of the zinc-nickel is in fact really not needed here, because the thread forming screw body is made of stainless steel.

The present coating on stainless steel could also be applied to screws that are not thread forming, such as a screw for a threaded hole. A technical effect of such a screw is that galling, which may occur when a conventional stainless steel screw (without the aforementioned coating) is mounted in a threaded hole, may be prevented due to the coating layer.

It can be noted that <CIT> discloses a screw, wherein the shank is integrally formed of an austenitic or other stainless steel and has a galvanically applied thin coating at least in the tip region, with the coating being harder than a non-coated region. However, the screw in <CIT> is self-drilling rather that thread-forming. Furthermore, the coating in <CIT> is harder (<NUM> to <NUM> HV0. <NUM>) and also made from a different material (chromium) compared to the present invention. The inventors of the present invention have realized that a hardness in the range from <NUM> to <NUM> HV is sufficient for a thread forming screw.

The present coating will typically be applied on the entire (thread forming) screw body, i.e. on the head, threaded portion, and tip. Alternatively the coating could be applied on only the threaded portion (and possibly also on the tip).

The coating may be formed by electrolytic surface treatment of the (thread forming) screw body, for example electroplating. The present coating may hence be an electroplated coating.

The coating may have a thickness in the range of <NUM>-<NUM>.

The present screw is thread forming but not self-drilling. In other words, the screw is devoid of a (self-)drilling tip or point and/or a (self-)penetrating/piercing tip or point. The screw may also be devoid of any flute.

The tip may be flat, or at least have a flat portion (specifically a flat end portion perpendicular to the longitudinal axis of the screw). The tip could also be (somewhat) pointed, such as a type CA point or an ogival point.

The threaded portion may comprise at least one ISO metric screw thread, for example a single ISO metric screw thread (single start) or dual ISO metric screw threads (double start/twin-lead). Reference standards for ISO metric threads include ISO <NUM>-<NUM>, IOS <NUM>, and ISO <NUM>.

The screw is adapted to be screwed into a pre-formed hole, for example a drilled or punched or cast hole.

According to a second aspect of the present invention, there is provided a method (of screwing a screw into a hole), comprising: providing a screw according to the first aspect; and screwing the screw into an existing hole in a material, for example sheet metal or (other) goods. This aspect may exhibit the same or similar feature and technical effects as the first aspect, and vice versa. The screw may be a thread forming screw, wherein the existing hole is an existing non-threaded hole. Alternatively, the screw is not thread forming, wherein the existing hole is an existing threaded hole.

According to a third aspect of the present invention, there is provided a method of manufacturing a screw, comprising: providing a thread forming screw body made of stainless steel and comprising a head, a threaded portion, and a tip; and applying a coating on at least the threaded portion, wherein the applied coating is a zinc-nickel coating and has a hardness in the range from 400HV to <NUM> HV, wherein the screw is not self-drilling. This aspect exhibits the same or similar feature and technical effects as the first and/or second aspects, and vice versa.

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention.

<FIG> and <FIG> illustrate a thread forming screw <NUM> for a pre-formed, non-threaded hole <NUM> in a material <NUM> according to an embodiment of the present invention.

The screw <NUM> is thread forming in that it is adapted produce a(n internal) thread <NUM> in the material <NUM> as the screw <NUM> is driven into the material <NUM> at the hole <NUM> by displacing some of the material <NUM> without removing it. The screw <NUM> is not thread-cutting. A thread-cutting screw does remove material when the thread is produced. The screw <NUM> is also not self-drilling. A self-drilling screw is not used with a pre-formed hole, but drills its own hole as the screw is screw into a material. As such, the present thread forming screw <NUM> is devoid of a drilling tip/flute.

The thread forming screw <NUM> comprises a (thread forming screw) body <NUM>. The body <NUM> is made of stainless steel. Stainless steel is an alloy of steel with chromium and possibly another element (such as nickel or molybdenum) that is practically immune to rusting and ordinary corrosion. Stainless steel may comprise iron (><NUM> %), at least <NUM> % Chromium (Cr), and Carbon content of max <NUM> %, with or without other alloying elements.

The thread forming screw body <NUM> comprises a head <NUM>, a threaded portion <NUM>, and a tip <NUM>. The body <NUM> is preferably made in one piece.

The head <NUM> may be arranged at a first end of the screw <NUM>. The head <NUM> may comprises a tool interface, also referred to as a (screw) drive. The tool interface may be external, for example an external hex tool interface <NUM> (as in <FIG>) or an external Torx. Alternatively, the tool interface could be internal (see <FIG>), such as a hexalobular socket, internal hex, etc. The head <NUM> may also comprise a flange <NUM>.

The tip <NUM> may define a second end of the screw <NUM> opposite said first end. The tip <NUM> may for example be flat, as shown in <FIG>. In other words, the screw <NUM> can have a flat point. The tip <NUM> could also be a so-called dog point (see <FIG>) or a CA point (which is somewhat pointed, but with a flat portion; see <FIG>), for example. In any case, the tip <NUM> is not self-drilling, as discussed above.

The threaded portion <NUM> may generally be present between the head <NUM> and the tip <NUM>. The threaded portion <NUM> may comprise at least one thread <NUM>, namely at least one external thread <NUM>. The at least one thread <NUM> may for example be a one thread (single start), as in <FIG>, or two threads (double start/twin-lead). The at least one thread <NUM> is preferably at least one ISO metric screw thread. As illustrated in <FIG>, the at least one thread <NUM> may extend from/start at the second end of the screw <NUM>. The at least one thread <NUM> may extend (substantially) from the second end of the screw <NUM> also in case the tip <NUM> is (somewhat) pointed, see for example <FIG>. In case the tip <NUM> is a dog point, the at least one thread <NUM> will start some distance from the second end of the screw, which distance corresponds to the (non-threaded) length of the dog point tip, see <FIG>. The at least one thread <NUM> may extend substantially all the way to the head <NUM>, as in <FIG>. Alternatively, a non-threaded shank portion could be present between the head <NUM> and the threaded portion <NUM>.

The thread forming screw <NUM> further comprises a coating <NUM>. The coating <NUM> is applied at least on the threaded portion <NUM> of the body <NUM>, but will typically be applied over the entire thread forming screw body <NUM>. As such, the coating <NUM> may form an external/outer surface of the thread forming screw <NUM>.

According to the present invention, the coating <NUM> may be a zinc-nickel coating <NUM> having a hardness in the range from <NUM> to <NUM> HV. By applying such a coating <NUM> on the stainless steel thread forming screw body <NUM>, the surface of the screw <NUM> gets sufficiently harder and any "jamming" when forming threads in the material <NUM> can be avoided. Hence, a thread forming screw <NUM> made of stainless material can be realized.

The zinc-nickel (Zn-Ni alloy) coating <NUM> may comprise a zinc-nickel layer composed of approximately <NUM>-<NUM>% nickel or <NUM>-<NUM>% nickel (and the rest is zinc). The zinc-nickel layer/coating <NUM> may for example consist of approximately <NUM> wt% nickel and <NUM> wt% zinc. At least one additional layer could be provided over the zinc-nickel layer, such as wax or other coating to provide low and stable friction.

As mentioned above, the coating <NUM> may have a hardness in the range from <NUM> to <NUM> HV (Vickers hardness). Specifically, the coating <NUM> may have a microhardness in the range from <NUM> to <NUM> HV, wherein 'micro' means that the load used (in kgf) is ≤<NUM>, preferably in the range from <NUM> to <NUM> (i.e. HV0. Expressed otherwise, the coating <NUM> may have a hardness in the range from <NUM> to <NUM> HV0. <NUM>-<NUM>. The coating <NUM> may for example have a (micro)hardness of approximately 415HV, or the hardness of the coating <NUM> may be a microhardness <NUM>-<NUM> HV. The Vickers method for measuring the hardness is described in ISO <NUM>-<NUM>, the content of which herein is concorporated by reference.

Furthermore, the coating <NUM> may have a thickness in the range from <NUM> to <NUM>, specifically in the range from <NUM> to <NUM>. Typically, the thickness of the coating <NUM> is substantially uniform throughout. Furthermore, the structure of the coating <NUM> may be even; the Sa roughness of the coating <NUM> may be below <NUM>.

The coating <NUM> may be formed by electrolytic surface treatment of the thread forming screw body <NUM>, for example electroplating. The coating <NUM> may hence be an electroplated coating. Accordingly, a method of manufacturing the screw <NUM> may comprise: providing the thread forming screw body <NUM>; and applying the coating <NUM> on at least the threaded portion <NUM> of the body <NUM>, preferably by electrolytic surface treatment such as electroplating.

Exemplary process equipment/parameters etc. of/for the present method may include one or more (or all) of the following:.

In use, the thread forming screw <NUM> is screwed using a suitable tool (not shown) into existing/pre-formed non-threaded hole <NUM> in material <NUM>, see <FIG> and <FIG>. The hole <NUM> may for example be a drilled or punched or cast hole. The hole <NUM> could be a through hole (<FIG>) or it could have a bottom (<FIG>). The material <NUM> may for example be sheet metal (<FIG>) or thicker goods (<FIG>). When the screw <NUM> is screwed in the hole <NUM> in material <NUM>, the screw <NUM> forms at least one thread <NUM> in the material <NUM> by displacing some of the material <NUM> without removing it. The screw <NUM> may be used to attach an element <NUM> (also having a pre-formed hole <NUM>) to the material <NUM>. Specifically in <FIG>, the screw <NUM> is used for assembly of metal sheets <NUM>, <NUM>.

The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above.

Claim 1:
A thread forming screw (<NUM>) for a pre-formed, non-threaded hole (<NUM>), comprising:
a thread forming screw body (<NUM>) made of stainless steel and comprising a head (<NUM>), a threaded portion (<NUM>), and a tip (<NUM>); and
a coating (<NUM>) applied on at least the threaded portion, wherein the coating is a zinc-nickel coating and has a hardness in the range from <NUM> to <NUM> HV,
wherein the screw is not self-drilling.