Tools – Wrench – screwdriver – or driver therefor – Having work engaging and force exerting portion inserted...
Patent
1997-09-12
1999-09-21
Eley, Timothy V.
Tools
Wrench, screwdriver, or driver therefor
Having work engaging and force exerting portion inserted...
81438, 81900, B25B 1348
Patent
active
059539690
DESCRIPTION:
BRIEF SUMMARY
FIELD OF BACKGROUND OF THE INVENTION
The present invention relates to a screwdriver, screwdriver bit or the like.
Such tools are made of steel. As manufacturing process, machining and forming and well as compressing are known. The tools have a working end which may be developed in cruciform shape if the tool is to be suitable for use with Phillips screws. The tool has working surfaces on its working end with which corresponding mating surfaces in the opening in the screw head are to be acted on. Various attempts have been made in the past permanently to shape such tools which are formed of steel. One known method is to coat the working end of the tool with a hard material, for instance titanium nitrite. In this way, a surface layer on the working end is harder than the core of the working end. However, it has been found that such layers applied by the CVD or PVD process from a material other than the material of the core can tear and chip off when the tool is used.
If the tip of a known tool, for instance a Phillips screwdriver, is inserted into the corresponding opening in a screw head, all the working surfaces do not act simultaneously on the corresponding opening surfaces in the screw head due to the tolerances in the tool and the opening in the screw head. If a torque is now exerted, then, in the case of a hard work end, only a punctiform load is exerted on individual engagement surfaces which would have the result that the surfaces are easily overstressed and then break. One theoretically conceivable solution for preventing this breaking-off of the engagement flanks would be maintaining the precise geometry. This is not feasible from the standpoint of manufacture.
An alternative for this are bits of a tough material so that the flanks of the working end deform upon being subjected to load and thus all four flanks come into engagement substantially uniformly. Such tools have a useful life. On the other hand, however, wear of the tool can in particular be noted if, in case of too high a torque, the working end slips out of the screw head and then continues spinning.
It is furthermore contemplated to develop the tools with a hard center and then, by a targeted heat treatment, have the hardness decrease towards the surface. Such tools show favorable properties in laboratory tests but in actual practice they wear in the same way as tools which are soft throughout. The above-mentioned coating of a soft tool with hard material led to the above-indicated problems. The layer of hard material, to be sure, results initially in a certain minimizing of the wear, but this layer tends to tear under the varying load and then peel off. The tool then wears down in the same manner as all other tools.
SUMMARY OF THE INVENTION
The object of the present invention is therefore to produce, starting from the known prior art, a tool which wears less.
As a result of the development in accordance with the invention a tool is obtained in which the layer of material lying directly below the surface is of greater hardness than the core. In this connection, to be sure, the hardness does not decrease suddenly as in the prior art but an intermediate layer is provided in which the hardness decreases continuously to the value of the core. In such screwdrivers, to be sure, the formation of a crack is also observed after a large number of alternations of load, but there is no enlargement of the crack or peeling-off of the harder layer, as is the case in the prior art. The hard surface layer, the intermediate layer and the core material are of identical material with only a different hardness or different structure. The resistance to wear of the tool has increased as a result of this development. The intermediate layer preferably has a thickness of 0.05 to 0.2 mm and has a hardness gradient of 1500 to 3000 HV 0.3/mm. The surface layer, which is the hardest layer and which the intermediate layer adjoins, has a thickness of 0.05 to 0.1 mm and a hardness of 900 HV 0.3 or 67-68 HRC. In this connection, the core hardness should lie below 70
REFERENCES:
patent: 2627192 (1953-02-01), Jenney, Jr. et al.
patent: 5704261 (1998-01-01), Strauch et al.
Lampman et al, "Metals Handbook 10th Ed. vol. 1, p. 793-800" 1990 ASM, Metals Park, Oh. US XP002009427 167750.
VDI Z. vol. 118, No. 21. 1976, Dusseldorf Germany, pp. 997-1003, XP002009426 K. Vetter, "Werkstoffeinflusse auf oberflachengehartete Bauteile".
Werkzeuge aus martensitaushartendem Stahl, Dr.-Ing. H. Berns, Dusseldorf, (1971) No. 6, p. 292.
Eley Timothy V.
Farber Martin A.
Skinner Sinclair
Wera Werk Hermann Werner GmbH & Co.
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