Electric heating – Metal heating – By arc
Reexamination Certificate
1999-10-22
2001-05-22
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121390, C219S121580, C219S121480, C219S121370, C266S067000
Reexamination Certificate
active
06236013
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the field of installations and automatic processes for plasma-jet marking and for cutting or welding workpieces, in particular made of metals or plastics, such as plates or sheets of steel.
Automatic machines or installations For plasma-jet cutting have been used for many years to plasma-jet trim or cut geometrical or other shapes in metal workpieces, such as plates or sheets, in particular made of ferrous and non-ferrous materials.
Plasma cutting is based on the combined thermal and kinetic effects of a plasma jet, that is to say an electric arc created in a strongly constricted ionized-gas medium established between, on the one hand, an emissive element of the plasma torch, forming the cathode, and on the other hand, the sheet or workpiece to be trimmed, forming the anode, these two electrodes, known as the cathode and the anode, being respectively connected to the negative and positive terminals of a DC electricity source, such as a current generator.
Processes and installations for plasma cutting are described, for example, in documents EP-A-0040925, JP-A-02108464, U.S. Pat. No. 5,760,363 and EP-A-0599709.
In view of the progress made both as regards the characteristics of the plasma jets employed (composition of the plasma gas, structure of the electrodes of the plasma torch, power and shape of the plasma jet, etc.) and as regards the cutting machines themselves (numerical control, off-line programming centres, shaft actuators, mechanical structure for guiding and driving the shafts, etc.), it is now possible to integrate this type of plasma-cutting installation in an overall manufacturing line or process, with a view to improving productivity and/or production efficiency.
However, the machines or installations for automatic plasma-jet cutting which currently exist are for the most part usable only for trimming materials. In other words, the plasma jet delivered by a plasma torch with which a cutting installation is equipped will produce melting through the full thickness of the metal workpiece to be machined, so as to obtain trimming of this workpiece with a predefined geometry.
In a manufacturing process, however, at least three types of additions to the trimming operation often prove necessary, namely drawing geometrical markers on the workpieces, identifying the cut workpieces using a number or standardized code, and marking with a standardized code to provide traceability or tracking of the manufactured workpieces.
In general, these additional operations follow the operation of plasma cutting the workpiece, but it is not inconceivable that they may be, or may have also to be carried out before the said plasma-cutting operation in certain cases.
More precisely, the operation of drawing geometrical markers on workpieces which have been trimmed or are to be trimmed, may consist in drawing or marking, in particular:
either continuous or discontinuous positioning lines on workpieces or elements which are intended to be welded or assembled later;
or centre points or centre-locating axes to facilitate subsequent operations of mechanically drilling holes of predefined diameter and position on the workpieces;
or lines or markers for the positioning of tools for subsequently shaping the trimmed workpieces, for example tools for folding the said workpieces.
Furthermore, the identification of workpieces which have been cut or are to be cut by a number or standardized code is also used to label various workpieces in order to identify them more easily and avoid any confusion between these various workpieces during subsequent operations. In fact, identification has, in particular, the advantage that it makes it possible to carry out relatively complex automatic programmes for plasma-jet trimming metal workpieces of highly varied geometries within a standard sheet-metal format, by fitting the said workpieces to be cut around one another in order to minimize loss or waste of metal and therefore optimize production costs.
Furthermore, marking workpieces with a standardized code guarantees that these workpieces are traceable, which is almost essential for companies receiving certification, such as a standard, confirming the quality of their manufacturing, who must be able to monitor the manufactured products that need to comply with this certification.
There are currently several marking systems which are or can be used according to their individual characteristics, the nature of the work to be carried out and the nature of the material or of the workpiece to be identified, namely, on the one hand, so-called “cold” markers, such as ink-jet marking, felt-pen labelling or using a pneumatic punch or electric graver, and, on the other hand, so-called “hot” markers, such as flame-heating the workpiece to be marked while spraying a marking powder, for example zinc powder, or marking the workpiece using a laser beam.
However, depending on the case, these various marking processes present drawbacks having relatively negative repercussions in industrial terms.
Thus, marking a workpiece with an ink jet or felt-pen is not ideal because this marking technique cannot be applied to certain materials, in particular to materials on which the ink pigments adhere poorly or not at all.
It has also been found in practice that this type of marking may, in certain cases, rub off fairly rapidly over the course of time, and that the workpieces then no longer have any identification marking, which poses problems of differentiating between these various workpieces in the subsequent steps of the production or machining process.
Similarly, marking using a pneumatic punch or electric graver may cause problems by deforming the workpiece, and these become all the more serious as the workpiece becomes thinner.
Furthermore, the technique of marking by spraying metal powder cannot be applied to all materials, given that, in particular, the problem arises of the marking powder's compatibility with the material of which the workpiece to be marked is made.
In addition, these marking techniques are often limited to a given material or family of materials, for example steels, and cannot in general be applied to workpieces made of very different types of material, for example metal workpieces and workpieces made of polymers or plastics.
On the other hand, document EP-A-865857 describes a process for marking sheet metal coated with paint, in which marks are made on the sheet metal by burning the paint locally with a plasma jet sheathed in a jet of cooling water. It will, however, be understood that this process is limited to marking coated sheet metal and cannot be applied to all types of sheet metal, that is to say irrespective of whether they are coated. Furthermore, according to this process, the upper surface of the sheet metal is not affected by the plasma jet, that is to say neither discoloured nor partially melted through its thickness, given that only the paint coating is burnt.
Other documents also propose processes for marking sheet metal. In this regard, mention may be made of documents U.S. Pat. No. 5,773,788, JP-A-62238020 or U.S. Pat. No. 5,760,363.
Document U.S. Pat. No. 5,760,363 proposes a plasma torch which can either cut or mark metal workpieces according to requirements.
However, several problems have not yet been resolved to date.
There is thus an industrial need to increase the productivity of marking and cutting or welding operations. This is because, at present, when it is necessary to carry out both cutting and marking, for example, on a workpiece such as sheet metal, the cutting is generally carried out first using a cutting torch located on a first support frame, then the plasma marking is carried out next using a marking torch located on a second support frame, and it is therefore essential to transfer the cut workpiece from the first support frame to the second support frame.
It is easy to see that, on the one hand, this entails significant encumbrance owing to the use of two support frames, as well as an installation for transferring work
La Soudure Autogene Francaise
Paschall Mark
Young & Thompson
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