METHOD FOR THE MECHANICAL THERMAL CUTTING OF A WORKPIECE USING A PLASMA CUTTING TORCH

Abstract

Known methods for the mechanical thermal cutting of a workpiece using a plasma cutting torch include the steps of: a) igniting a plasma jet, b) producing a lead-in cut into a metallic, strip- or plate-type semi-finished product using the plasma jet and c) cutting a contour into the semi-finished product by guiding the plasma jet along a predefined contour line at a cutting speed in a cutting direction. Provided herein is such a method which further includes, after cutting the contour according to step c), guiding the plasma jet in the opposite direction to the cutting direction along at least a portion of the cut contour at a return speed, in order to achieve a high cut quality and high dimensional precision.

Claims

1. A method for the mechanical thermal cutting of a workpiece using a plasma cutting torch, comprising the method steps of: a) igniting a plasma jet, b) producing a lead-in cut in a metallic, plate- or strip-type semi-finished product using the plasma jet, c) cutting a contour into the semi-finished product by guiding the plasma jet along a predefined contour line at a cutting speed in a cutting direction, wherein after cutting the contour according to step c), the plasma jet is guided in the opposite direction to the cutting direction along at least a portion of the cut contour at a return speed.

2. The method according to claim 1, wherein after cutting the contour according to step c), the plasma jet is guided in the opposite direction to the cutting direction along the entire cut contour.

3. The method according to claim 1, wherein the lead-in cut is cut at a lead-in cut speed, wherein the lead-in cut speed is increased while cutting the lead-in cut until the cutting speed is reached, wherein the return speed is in the range of 150% to 400% of the cutting speed.

4. The method according to claim 1, wherein while the plasma jet is being guided in the opposite direction to the cutting direction along at least a portion of the cut contour, the return speed is reduced continuously.

5. The method according to claim 1, wherein a contour is cut into a semi-finished product made of aluminium or steel with a material thickness in the range of 5 mm to 100 mm.

6. The method according claim 1, wherein after cutting the contour according to step c) and before guiding the plasma jet in the opposite direction to the cutting direction, a further cut takes place in the cutting direction.

7. The method according to claim 1, wherein when cutting the contour according to step c), the position of the plasma jet is shifted to the right or left in relation to the contour line, depending on the cutting direction.

8. The method according to claim 7, wherein when the plasma jet is being guided in the opposite direction to the cutting direction, the position of the plasma jet is shifted from left to right or from right to left as appropriate, relative to the contour line.

9. The method according to claim 5, wherein the semi-finished product is made of stainless steel.

Description

EXEMPLARY EMBODIMENT

[0043] The invention will be described in more detail below with the aid of exemplary embodiments and drawings. The figures show the following:

[0044] FIG. 1: an illustration of the position of a plasma cutting torch nozzle over a workpiece surface during a cutting operation with a perpendicular lead-in path,

[0045] FIG. 2: an illustration of the position of a plasma cutting torch nozzle over a workpiece surface during a cutting operation with a semi-circular lead-in path,

[0046] FIG. 3: a first stainless steel workpiece with an incompletely cut, circular inner contour, which was produced by a plasma cutting machine using a conventional cutting method,

[0047] FIG. 4: a second stainless steel workpiece with an incompletely cut, circular inner contour, which was produced by a plasma cutting machine using a conventional cutting method,

[0048] FIG. 5: a first variant of a cutting method according to the invention with the method steps of: cutting a lead-in path (I), cutting a predefined contour (II) and guiding the plasma jet along a portion of the cut contour (III).

[0049] FIG. 6: a second variant of a cutting method according to the invention with the method steps of: cutting a lead-in path (I), cutting a predefined contour (II) and guiding the plasma jet along the entire cut contour (III).

[0050] FIG. 7: a third variant of a cutting method according to the invention with the method steps of: cutting a lead-in path (I), cutting a predefined contour (II), a further cut in the cutting direction (IIa), guiding the plasma jet along a portion of the cut contour (III), and optionally cutting a lead-out path (IV),

[0051] FIG. 8: a fourth variant of a cutting method according to the invention with the method steps of: cutting a lead-in path (I), cutting a predefined contour (II), a further cut in the cutting direction (IIa), guiding the plasma jet along the entire cut contour (III), and optionally cutting a lead-out path (IV),

[0052] FIG. 9: a comparison of an outer contour (B) of a workpiece that was obtained using a cutting method according to the invention, and an outer contour (A) of a workpiece as obtained by a conventional cutting method, and

[0053] FIG. 10: a comparison of an inner contour (B) of a workpiece that was obtained using a cutting method according to the invention, and an inner contour A, as obtained by a conventional cutting method.

[0054] FIG. 1 shows the changes in position of a plasma cutting torch nozzle during a cutting operation relative to a workpiece surface, which is described by arrows x, y.

[0055] The plasma cutting torch, including nozzle, is mounted on a movable gantry and is movable relative to the workpiece surface. In the exemplary embodiment, the semi-finished product is a plate made of stainless steel with the following dimensions: length (L)=100 mm, width (B)=100 mm and height (H)=30 mm, into which a circular inner contour with a diameter of 38 mm is to be cut using the plasma cutting torch. The method for cutting the inner contour will be described in more detail below:

[0056] Before the inner contour is cut, the plasma cutting torch nozzle is first moved to a start position (A). The start position (A) is located in the unwanted material of the semi-finished product. While the plasma cutting torch nozzle is being positioned, the plasma cutting torch is not in operation. In FIG. 1, this method step is indicated by the broken line 101.

[0057] As soon as the plasma cutting torch nozzle has reached the start position (A), the plasma cutting torch is ignited. The plasma cutting torch nozzle is held in the start position A until it has pierced through the semi-finished product.

[0058] Once piercing has occurred, a lead-in cut (lead-in path) is firstly cut into the semi-finished product. To this end, the plasma cutting torch nozzle is moved in the direction of the arrow along the lead-in line 102 to a contour starting point while being accelerated from zero to a predefined cutting speed. The lead-in line 102 is selected such that it meets the eventual contour line 103 at an angle of 90°; it runs radially to the contour line 103.

[0059] From the contour starting point, the plasma cutting torch nozzle is guided at the predefined cutting speed of approx. 500 mm/min in an anti-clockwise direction on the predefined contour line 103, which is offset by approx. 3 mm to the left relative to the eventual inner contour of the workpiece. Such an offset of the contour line is necessary since the plasma jet produced by the plasma nozzle itself has a round cross-section with a mean diameter of approx. 6 mm. In this way, it is ensured that a hole is cut exactly with the predefined radius. The plasma cutting torch nozzle is guided in an anti-clockwise direction along the contour line 103 until it reaches the contour starting point again. Further steps can then be provided, e.g. guiding the plasma jet in the opposite direction to the contour line 103. To aid understanding and for reasons of clarity, these are not illustrated in FIG. 1. These method steps will be described below with the aid of FIGS. 5 to 8.

[0060] Finally, a lead-out path is cut into the unwanted material by guiding the plasma cutting torch nozzle along the lead-out line 104 until the end position (B) is reached. The plasma cutting torch is switched off during its travel to the end position B. As soon as the end position (B) has been reached, the plasma cutting torch nozzle is moved along the broken line 105 to a region such that it is no longer assigned to the workpiece surface.

[0061] FIG. 2 shows the sequence of an alternative cutting method. Here, in an x, y plot, the position of a plasma cutting torch nozzle over a workpiece surface during a cutting operation is illustrated. Compared to the cutting method of FIG. 1, in particular the shape of the lead-in path and the position of the lead-out path are modified in the cutting method according to FIG. 2.

[0062] Before the lead-in path is cut, the plasma cutting torch nozzle is brought along the line 201 to the start position (A) in the unwanted material. As soon as the plasma cutting torch nozzle has reached the start position (A), the plasma cutting torch is ignited. The plasma cutting torch nozzle is held in the start position (A) until it has pierced through the semi-finished product.

[0063] The lead-in path is then cut by guiding the plasma cutting torch along a semi-circular lead-in line 202 to a contour starting point 210 while accelerating it from zero to a predefined cutting speed. The position of the lead-in line 202 here is selected such that a change in direction of the plasma cutting torch nozzle at the contour starting point is not necessary; the lead-in line hits the contour line 203 at a tangent. This tangential meeting with the lead-in line 202 enables cut quality to be improved for circular inner contours in particular.

[0064] From the contour starting point 210, the plasma cutting torch nozzle is guided at the predefined cutting speed of 600 mm/min on the predefined contour line 203, which—as described for FIG. 1—is offset by 3 mm to the left relative to the eventual inner contour of the workpiece. The cutting direction runs anti-clockwise until the contour starting point 210 is reached again.

[0065] While the contour line is being cut, the cutting speed is kept constant. Once the contour starting point 210 has been reached again, a “further cut” 203a is provided in the cutting direction along the contour line 203 that has already been cut until a cut end position (B) is reached. During the further cut 203a, the speed is reduced down to zero at the end position (B).

[0066] Further steps can then be provided, e.g. guiding the plasma jet in the opposite direction to the contour line 203. To aid understanding, these are not illustrated in FIG. 2. These method steps will be described more precisely below with the aid of FIGS. 5 to 8.

[0067] The cutting of a lead-out path into the unwanted material is optionally possible (not illustrated). In the present exemplary embodiment, the plasma cutting torch is switched off at the end position (B) and the plasma cutting torch nozzle is moved along the broken line 204 to a region such that it is no longer assigned to the workpiece surface.

[0068] In conventional methods for cutting a contour with a plasma cutting torch, cut surface damage is often observed in the lead-in and/or lead-out region. Such cut surface damage is particularly undesirable when cutting small holes with a diameter of less than 20 mm, since cut surface damage to these holes has a particularly marked impact on the effective hole diameter. In FIGS. 3 and 4, examples of such cut surface damage are shown, as can often be observed particularly when cutting workpieces made of high-alloy steels (stainless steel).

[0069] FIG. 3 shows a workpiece 300 made of stainless steel that has undergone a conventional cutting method for producing a hole-type inner contour 301. The inner contour 301 is circular in form; the circle diameter is 36 mm. The thickness (height) of the workpiece 300 is 20 mm.

[0070] The cutting method comprised the method steps of: a) positioning the plasma cutting nozzle, starting from a starting position, at a position above the material of the inner contour, the so-called unwanted material, b) operating the plasma cutting torch, c) piercing the unwanted material, d) cutting a lead-in path 302 running perpendicular to the inner contour 301, e) cutting the circular contour 301, f) switching off the plasma cutting torch and g) moving the plasma cutting torch nozzle to the starting position.

[0071] Apart from the fact that the cut is not complete, FIG. 3 shows that cut surface damage can occur in particular in the region where the lead-in path 302 meets the contour 301 (see arrow 305). The smaller the diameter of the contour 301, the greater the impact of this cutting damage on the effective diameter of the contour 301.

[0072] FIG. 4 shows a workpiece 400 made of stainless steel, which has likewise undergone the cutting process explained with reference to FIG. 3. The inner contour 401 is circular in form; the circle diameter is 38 mm. The thickness (height) of the workpiece 300 is 20 mm.

[0073] When cutting the contour (method step e) according to FIG. 4, the arc has “jumped” at the end of the contour cut in the region where lead-in cut and lead-out cut cross, such that a micro-bridge 405 has remained. This phenomenon is often observed when cutting high-alloy steels and in particular when cutting stainless steel.

[0074] With the aid of FIGS. 5 to 8, four variants of the method according to the invention will be described in detail. To simplify the illustration of the method steps, each of the method variants is shown in a plurality of drawings (I, II, III or I, II, IIa, III), with the drawings representing different method stages in a time sequence. The current method steps of a method stage in each case are indicated by continuous, black lines. Method steps that have taken place previously and orientation lines are illustrated by broken lines. Where the same reference numerals are used in FIGS. 6, 7 and 8 as in FIG. 5, they denote method steps that are the same as or equivalent to those explained with reference to FIG. 5.

[0075] FIG. 5 shows a schematic diagram of the sequence of method steps of a cutting method that is used in particular for processing semi-finished product material thicknesses in a range of 5 mm to 100 mm.

[0076] Firstly, the plasma cutting torch nozzle is moved along the broken line 500 to the start position A. As soon as the plasma cutting torch nozzle has reached the start position A, the plasma cutting torch is ignited and held in the start position A until it has pierced through the semi-finished product. Finally, a lead-in path 501 is cut into the semi-finished product. FIG. 5-I shows a semi-circular lead-in path 501 as described above e.g. with reference to FIG. 2, which tangentially meets the contour line 503 to be cut. Naturally, the shape and course of the lead-in path 501 can, in principle, be selected at will. While the lead-in path is being cut, the plasma cutting torch nozzle is accelerated to cutting speed. At the contour starting point 510, the plasma jet is guided through the lead-in path 501 up to the contour starting point 510 in such a way that no change in direction is necessary.

[0077] Moreover, the plasma jet is already at cutting speed when it reaches the contour starting point 510, so that there is likewise no need for a change in speed.

[0078] FIG. 5-II shows the actual contour cut, which immediately follows the cutting of the lead-in path 501. The cutting of the lead-in path 501 ends when the contour starting point 510 is reached. Starting from there, the contour 503 is cut at cutting speed until the contour end point A1, which is identical with the contour starting point, is reached.

[0079] According to FIG. 5-III, the plasma cutting torch is guided in the opposite direction to the cutting direction along the portion 511 of the contour 503 to the end position B at the return speed. During this process, the return speed of the plasma cutting torch is reduced in steps down to zero, so that it is unnecessary to cut an additional lead-out path. By reducing the return speed and the associated deceleration of the plasma torch cutting machine, the lag of the plasma jet is reduced. Since part of the contour 503 was cut again in the opposite direction, any bridges remaining in the region of the counter-cut are cut and bevels are straightened.

[0080] FIG. 6 shows a variant of the cutting method described for FIG. 5, which can likewise be employed for processing semi-finished product material thicknesses in a range of 5 mm to 100 mm.

[0081] The illustrations in FIG. 6-I and in FIG. 6-II correspond to those of FIGS. 5-I and 5-II. Accordingly, reference is made to the description of the latter figures.

[0082] FIG. 6-III shows that, at the end of the contour cut 503, the plasma jet produced by the plasma cutting torch is guided in the opposite direction to the previous cutting direction along the portion 512 of the contour 503, the portion 512 here being in the form of a full circle, so that the complete contour is cut in the opposite direction. This method is particularly suitable for small circular contours with a peripheral length of e.g. 60 mm. At the same time, a high cut quality is achieved. The plasma jet is guided to the end position B at the return speed. During this process, the return speed of the plasma cutting torch is reduced in steps down to zero at point B, so that it is unnecessary to cut an additional lead-out path. The positions Al and B are identical here. By reducing the return speed and the associated deceleration of the plasma torch cutting machine, the lag of the plasma jet is reduced. Since the contour 503 was cut again in the opposite direction, any bridges remaining in the region of the counter-cut are cut and bevels are straightened.

[0083] Instead of guiding the plasma jet in the opposite direction to the cutting direction, alternatively a contour repeat can be provided such that, following the first contour, a second contour is cut in the same direction. Expanding on this, a further cut could be provided after the first contour cut, followed by a contour repeat in the cutting direction. This has advantages if process parameters are to be modified after the further cut, such as the cutting speed or the position or inclination of the plasma in relation to the workpiece surface.

[0084] FIGS. 7 and 8 show a third and fourth variant of the method according to the invention, which are both provided for cutting comparatively thick semi-finished product material thicknesses in the range of 50 mm to 100 mm.

[0085] The illustrations in FIGS. 7-I, 7-II and in 8-I, 8-II correspond to those of FIGS. 5-I and 5-II. Accordingly, reference is made to the description of the latter figures.

[0086] In the method variant according to FIG. 7, it is provided in FIG. 7-IIa that the contour cut 503 is continued along the line 710 as far as the point A2 after passing round the full circle once. This has the advantage that the plasma jet is positioned behind any micro-bridge remaining at the position A1.

[0087] When the point A2 is reached, the plasma cutting torch is positioned again because of the imminent change of direction. The plasma jet produced by the plasma cutting torch is then guided in the opposite direction to the cutting direction along the portion 711 of the contour 503 to the end position B at the return speed. The plasma cutting torch is switched off before it reaches the point B.

[0088] The method of FIG. 8 differs from the method according to FIG. 7 essentially by the fact that, in contrast to the portion 711, the portion 811 is in the form of a full circle, so that the entire contour is re-cut. The positions A2 and B are identical. This method is particularly suitable for small contours with a peripheral length of up to 60 mm. As a result, a high cut quality is achieved.

[0089] The methods described above all describe the cutting of inner contours. They can also, of course, be applied to the cutting of outer contours.

[0090] FIG. 9 shows a comparison between an outer contour (B) of a workpiece that was obtained using a cutting method according to the invention and an outer contour (A) of a workpiece as obtained with a conventional cutting method.

[0091] The most notable differences are highlighted by circles. The kerf in FIG. 9A is formed unevenly and in particular has cut surface damage on the underside of the workpiece.

[0092] The kerf from FIG. 9B, on the other hand, has an even, tapering shape.

[0093] In FIG. 10, an inner contour (B) of a workpiece obtained using a cutting method according to the invention according to FIG. 6 and an inner contour A, as obtained with a conventional cutting method, are compared. While the hole in FIG. 9A shows cutting damage in the lead-in region (left), an almost circular contour was obtained by the method according to the invention as in FIG. 6.