EP0788851A2 - Method and device for inspecting a drawing die - Google Patents

Abstract

The method uses a laser beam which is passed through the bore of the die, which is rotated by a stepper motor, and the resulting diffraction pattern is recorded by a camera. Defects in the diffraction pattern may be related to faults in the bore of the die. The laser (14) beam is directed on the bore of the die (10), which is held in a fixture attached to a stepper motor (22), and the resulting circular diffraction pattern is focused by a lens (26) on to a charge-coupled device camera (16) as the die is rotated through pre-determined angles by the motor. The results are analyzed by a computer (18) and displayed on a screen (28). Defects in the bore roundness, smoothness, etc. distort the diffraction pattern.

Description

Technical field

9. The invention relates to a method and a device for optically assessing a bore within a drawing die according to the preamble of claims 1 and 9, respectively.

Such a drawing die is used to pull fine wires, especially tungsten wires; The assessment is based on quantitative (e.g. diameter, ovality) and / or qualitative criteria (e.g. drawing die damage due to scoring, roughness, cracks).

For the production of so-called fine wires, i.e. wires with a diameter of less than 300 micrometers, such as tungsten wires, it is essential to assess the drawing hole through which the wire is drawn. A set of several (often eight) drawing dies connected in series is usually used, the drawing dies having to adhere to relative gradations in diameter that are defined among themselves. With the dimensions in the tenths of a millimeter range, an accurate, high-precision determination of the draw hole is essential, since there are smaller tolerances than in the millimeter range for wire production.

State of the art

A method for assessing drawing dies is known, which requires a multi-stage process. The pull is used by the operating personnel optically inspected with the aid of a light microscope and, based on experience, classified for any drawing die damage, such as scoring or breakouts on the drawing wood edges. The problem here is that with small diameters, in particular less than 100 micrometers, the intensity of the transmitted light decreases to values which make an exact assessment very difficult. Furthermore, the assessment in these dimensions requires the highest possible precision, which is not the case with the known method.

Up to now, the diameter of the drawing hole could only be determined using an indirect method. Copper wire was cold drawn through the drawing hole; Based on the predetermined parameters of the copper wire (weight, length, etc.), it was possible to draw indirect conclusions about the condition of the drawing hole, in particular its diameter, after the cold drawing process. For a comprehensive assessment of the drawing die, the parameters (drawing die damage, diameter, ovality of the drawing die, etc.) had to be determined in different and complex additional procedures. Due to the enormous amount of work, time and therefore also cost, a drawing fetch assessment in the prior art could only be carried out in 10 to 20% of the drawing dies to be assessed. Furthermore, it is a prerequisite in the prior art that the personnel are trained and experienced at the light microscope.

Presentation of the invention

It is therefore an object of the present invention to provide a method for the optical assessment of drawing dies and drawing die inserts which detects all the necessary parameters for the assessment of the drawing die in a measuring method and evaluates them automatically, precisely and minimizing the influence of further sources of error, and further to create a device that allows a simple, quick quality assurance of the drawing die without special previous knowledge of the operating personnel.

This object is achieved according to the invention by a method and a device with the features of claims 1 and 9, respectively. Particularly advantageous configurations can be found in the dependent claims.

In the context of the method according to the invention, a laser beam is emitted onto a hole to be assessed in the drawing die, which is used to pull fine wires, in particular tungsten wires. A diffraction pattern is generated by diffraction of the laser beam in the drawing die, at least a part of the diffraction pattern of the diffracted laser radiation is detected and signals are generated therefrom which define the detected diffraction pattern. Finally, the diffraction pattern is evaluated by transforming the signals obtained. The hole within the drawing die is assessed according to quantitative (e.g. diameter, ovality) and / or qualitative aspects (e.g. drawing die damage due to scoring, roughness, cracks).

Due to the detection and processing of the parameters according to the invention for the assessment of the drawing die from diffraction signals, this solution offers the advantage that previous interference, such as incorrect entries and misinterpretations, are kept to a minimum. While - as already described - it is a prerequisite in the prior art that the personnel be trained and experienced at the light microscope, this is no longer necessary in the method according to the invention because of the simple handling of the device.

A further advantageous embodiment of the method of the invention consists in that the inner surface of the drawing hole, which is equipped with diamond, is preferably blackened with graphite or black glass ink. This suppresses radiation components that are generated by transmission of the diamond-tipped parts of the drawing die and overlay the diffraction pattern. The result of this drawing die preparation is that the diffraction pattern is less disturbed, which in turn increases the quality of the assessment.

Description of the drawings

Fig. 1

eine Übersicht über die erfindungsgemäße Vorrichtung,

Fig. 2

eine übersichtsartige Darstellung des erfindungsgemäßen Verfahrens,

Fig. 3

eine diagrammartige Darstellung eines Intensitätsverlaufs eines Beugungssignales einer Winkelzeile, aufgetragen über den Ort bei einem Ziehhol mit einem Durchmesser von 80 Mikrometern,

Fig. 4

ein Beugungsbild bei einem guten Ziehstein mittlerer Laufzeit,

Fig. 5

ein Beugungsbild bei einem Ziehstein mit höherer Rauheit und Riß und

Fig. 6

eine Schnittansicht eines Ziehsteins mit einer Ziehsteinbohrung.

The present invention is explained in more detail below on the basis of preferred exemplary embodiments in conjunction with the figures. Show:

Fig. 1

an overview of the device according to the invention,

Fig. 2

an overview of the method according to the invention,

Fig. 3

2 shows a diagrammatic representation of an intensity curve of a diffraction signal of an angular line, plotted over the location at a drawing hole with a diameter of 80 micrometers,

Fig. 4

a diffraction pattern with a good drawing die of medium duration,

Fig. 5

a diffraction pattern with a drawing die with higher roughness and crack and

Fig. 6

a sectional view of a die with a die hole.

In order to obtain clarity about the basic shape and structure of a drawing die 10, reference is now made to FIG. 6. The arrow indicates the wire drawing direction, i.e. from right to left in the figure. The bore of the drawing die 10 comprises several sections. The section with the smallest diameter defines the drawing hole 12, which is essentially cylindrical. The drawing hole 12 defines the later diameter of the wire and its surface quality. In the wire drawing direction there is an undercut 13 behind it, the deformation part 11 in front of it.

2 schematically shows an overview of the method for assessing a drawing die 10. A laser beam, primarily a semiconductor laser 14, is drawn into a drawing die 12, with a diameter of, for example, less than 300 micrometers, diffracted in that the laser beam is directed as far as possible onto the center of the drawing die 10.

The diffraction behavior of the laser beam on the drawing stone 12 is used in the further process as the basis for the assessment of the drawing stone 12. The diffraction image, which is specific depending on the condition of the drawing die 10, is recorded by a camera 16 and converted into electrical signals which are evaluated in the further process.

The signals are fed to a processor 18, which processes them in such a way that they can be used to determine the diameter of the drawing block 12 and to assess the ovality and quality.

In comparison with the quality determination on the light microscope and the indirect determination of the diameter by means of the weighing technology in the prior art, the advantages of a very fast determination with high accuracy, which is approximately one tenth of a micrometer, and the possibility of testing all result from the method according to the invention for the assessment of drawing dies 10, compared to previous 10% - 20% testing, as well as the advantage of easy handling, which is important in practice.

In an alternative embodiment of the invention, the entire diffraction pattern is not recorded, but rather only a part of the diffraction pattern, which part corresponds to a radial section in relation to the center of the diffraction pattern.

A CCD (Charge Coupled Device) line camera 16 records the diffraction pattern line by line. 1 shows the measuring station structure, in particular the arrangement of the laser 14, the drawing die 10 and the camera 16. In this embodiment, the drawing die 10 is mounted in a rotatable receptacle which is driven by a stepper motor 22 in small predetermined angular steps. In this embodiment, a diode laser 14 with a wavelength of 830 that can be regulated in its power is preferably used Nanometer used, which achieves a maximum luminance of 1.2W / cm ² through suitable parallelization optics.

The CCD line camera 16 is arranged behind the drawing die 10 in such a way that it can no longer detect the main beam (intensity maximum of the 0th order) of the beams diffracted on the drawing die 10.

The aperture 24 ideally serves to shield the effects of stray light, in particular of the laser. The lens 26 (approximately in the form of a converging lens) is positioned between the drawing hole 10 and the CCD camera 16 at a distance from the focal length of the lens in front of the CCD camera. This creates a Fraunhofer diffraction pattern of the pull on the CCD camera. As a result, a more flexible detection of, for example, different drawing die diameters is made possible without time-consuming moving of the CCD camera.

Possible focal lengths are e.g. 50 mm and 100 mm for draw hole 12 with a diameter of less than 100 micrometers. With an array length of 2048 pixels (picture elements) with a pixel size of 13 μm x 13 μm, the line camera 16 permits a complete and precise detection of the diffraction image.

As shown in FIG. 2, the diffraction signals detected by the camera 16 are fed to a processor 18 of a computer 28, stored and further processed. These signals are subjected to an inverse Fourier transformation and are used to determine the diameter and ovality of the drawing hole 12. The relationship between diffraction and Fourier transformation is described in "Hecht, Zajac: Optics, Addison-Wesley, Reading / MA, 1974, pages 411 - 414, 462-471 ".

Quality assurance in tungsten wire production plays an important role in making optimal use of the service life of expensive drawing die inserts. It can be improved by using an optical measuring method Classification of drawing dies 10 is provided, which quickly and accurately provides the diameter and ovality of the drawing die 12.

So far, because only one drawing hole (preferably the one with the smallest diameter) within a drawing block insert of eight drawing stones, for example, was assessed and found to be good due to the high effort involved, damage often occurred because another drawing hole did not meet the requirements (diameter, surface quality). corresponded.

An advantageous alternative embodiment of the invention, in particular of the evaluation device 28, therefore consists in the fact that, by means of additional processing of signals within the method, drawing die damage (e.g. cracks in the drawing die 10 or breakouts at the drawing die edges) within a fine pull cascade (sequential arrangement of drawing dies) with decreasing diameter of the drawing hole 12) is possible. In this way, the diffraction signals are stored per line in order to generate a two-dimensional diffraction image by rotating the drawing die around its bore axis.

With known methods of image processing and pattern recognition, in addition to determining the diameter and ovality (quantitative aspects), qualitative features (quality, etc.) can also be "diagnosed" automatically in a conclusion system. Alternatively, the detected and processed signals can also be passed on directly to the user via the interface unit 30 in order to subject them to a (qualitative) assessment by the personnel that is not supported by the method and the device. This semi-automatic assessment has the advantage that the experience of the staff can always be incorporated if necessary.

1 shows, the device according to the invention comprises a computer 28 with an interface unit 30. The control unit of the computer 28 controls the stepper motor 22 as a function of the number of angular lines recorded; the number of angular lines in this exemplary embodiment is 22. Furthermore, the control unit of the computer 28 controls the exposure setting of the camera 16.

For the highest possible signal dynamics, the exposure is automatically set per line scan in a manner known per se. An evaluation device, which is formed in the computer 28 in particular by at least one image processing card and a corresponding software module, receives data from the laser 14 (laser frequency), from the stepper motor 22 (rotary position of the drawing die 10) and the diffraction signals from the camera 16 as line information, from which the spatial frequencies determined and the diameter is determined iteratively per angular line for the entire circumference of the drawing hole 12.

3 shows an intensity curve of the diffraction signal. A line of the drawing block 10 that passes through the center of the drawing hole 12 is captured by the camera 16. The course of the curve, plotted over the location (symbolized here by pixel numbers), provides information about the nature of the drawing hole 12 on this line.

In the case of approximately 100 to 200 angular lines, depending on the desired fineness, the 100 to 200 intensity curves are again integrated into an image by the computer 28 and enable an overall statement to be made about the draw hole 12.

4 and 5 each show a diffraction image in gray value representation, which was recorded with an area camera. The intensity curve of the diffraction signal can be seen from the different shading of the rings. As an alternative to the line camera mentioned above, the area scan camera captures both dimensions of the diffraction image in one measurement pass however, the disadvantage that it can capture less accurately than the line scan camera.

It is therefore a particularly advantageous embodiment of the invention to rotate the drawing die 10 over, for example, 100 angular steps and to detect one high-resolution angular line for each angular step.

In the practice of wire production, as already mentioned in the introduction to the description, a set of several sequentially arranged drawing dies 10, a fine drawing cascade, is usually used, a specific reduction in diameter from drawing die 10 to drawing die 10 being established. The more uniform the cross-sectional change from drawing die to drawing die, the longer the service life for the complete drawing die insert and the better the wire quality produced. If the degree of deformation of the wire is too high, e.g. the risk of embrittlement of the wire. For this reason, the gradations between the drawing dies 10 are particularly important. For this it is necessary to assess all drawing dies 10 in a set. A random assessment is not sufficient, for example, if a draw hole 12 is at the upper limit of the tolerance range, its successor must also be at the upper tolerance limit so that the percentage deformation from draw hole 12 to draw hole 12 remains as constant as possible.

The comparison of the individual measurements with each other is necessary to put together an optimal die insert. This is achieved with an advantageous alternative solution according to the invention in that a device is made available (not shown) with which a set of drawing dies 10 can be assessed taking into account the individual assessments of the drawing dies 10 each building it up and the required relative diameter reduction. This, for example, carousel-like device thus offers the possibility of arranging the entire set of, for example, eight drawing dies 10 in the device, which are successively used there Clocking will be assessed. After performing the procedure on the entire set of drawing dies, an evaluative result can be obtained on it. This ensures that the quality of the entire wire production is improved and ensured.

A further variant for increasing the throughput can be a device with several simultaneously active measuring stations (transmission and detection devices).

It is positive in the solution of the task presented that the method is not restricted to a specific material of the drawing die 10 or its inner surface. For example, drawing dies 10 made of hard metal can also be assessed.

Furthermore, there is the significant advantage for the practice of wire production that crack and crack detection of the drawing die 10 is ensured with only one method, the detection of the diffraction signals according to the invention, which permits a qualitative rough classification of the drawing die 10 on the basis of the specific diffraction pattern.

Claims (18)

Method for the optical assessment of a hole (12) within a drawing die (10), which is used to pull fine wires, in particular tungsten wires, according to quantitative (e.g. diameter, ovality) and / or qualitative aspects (e.g. drawing die damage due to scoring, roughness, cracks) , characterized by process steps a) - d): a) emitting a laser beam onto the hole in the drawing die (10), b) generating a diffraction image by diffraction of the laser beam in the drawing die (10), c) acquiring at least a part of the diffraction image of the diffracted laser radiation and generating signals which define the acquired diffraction image, d) evaluating the diffraction pattern by transforming the signals of the diffraction pattern. Method according to Claim 1, characterized in that the diffraction image is evaluated by comparing the generated signals of the detected diffraction image with signals of a reference diffraction image. Method according to Claim 1 or 2, characterized in that the method steps of detecting and evaluating are applied to a plurality of drawing dies (10), relative gradations in diameter and tolerances of the individual drawing dies (10) being recorded and related to one another. Method according to at least one of the preceding claims, characterized in that the laser beam emits in the axial direction Direction is essentially directed to the center of the cylindrical bore (12). Method according to at least one of the preceding claims, characterized in that the detection of at least a part of the diffraction pattern is carried out line by line and the drawing die (10) is rotated in 360 ° about its bore axis in predetermined angular steps. Method according to Claim 5, characterized in that the diffraction image is evaluated iteratively for each angular line detected by comparing the signals recorded line by line until all signals of the diffraction image are evaluated. Method according to at least one of the preceding claims, characterized in that data generated in the method are processed by means of a software program. Method according to at least one of the preceding claims, characterized in that at least part of the data is subjected to a filtering process and then to an inverse Fourier transformation. Device for the optical assessment of a bore (12) within a drawing die (10), which is used to pull fine wires, in particular tungsten wires, according to quantitative (e.g. diameter, ovality) and / or qualitative aspects (e.g. drawing die damage due to scoring, roughness, cracks) , marked by: a) a laser (14), the beam of which is directed onto the bore of the drawing die (10), c) an evaluation device (28) for evaluating the diffraction image by transforming the signals of the diffraction image. Apparatus according to claim 9, characterized in that the evaluation device (28) is designed in such a way that the diffraction image is evaluated by comparing the generated signals of the detected diffraction image with signals of a reference diffraction image. Apparatus according to claim 9 or 10, characterized in that the evaluation device (18) is designed in such a way that the signals generated are assessed automatically or semi-automatically. Device according to at least one of claims 9 to 11, characterized in that the laser (14) is a diode laser with a wavelength of 830 nm. Device according to at least one of claims 9 to 12, characterized in that the detection device (14) is a CCD camera. Device according to at least one of claims 9 to 13, characterized in that the drawing die (10) is rotatably mounted in a receptacle (20). Device according to at least one of claims 9 to 14, characterized in that the evaluation device (28) has a control unit which is assigned a control program which defines the method according to at least one of claims 1 to 8. Device according to at least one of claims 14 or 15, characterized in that the mounting of the drawing die (10) in the receptacle (20) can be controlled. Device according to at least one of claims 9 to 16, characterized in that it comprises a receiving device for several drawing dies (10). Device according to at least one of claims 9 to 17, characterized in that the drawing die (10) is coated with an opaque material at least in the region of the cylindrical bore (12).

Images