METHOD FOR DETERMINING A MINIMUM WIDTH AND AN ATTACHMENT POSITION OF A MICROJOINT AND METHOD FOR MACHINING A WORKPIECE

Abstract

A method for determining a minimum width of a microjoint by which, when machining a workpiece, in particular a sheet-like workpiece, a workpiece part remains connected to a remaining workpiece of the workpiece. In the method, the minimum width of the microjoint is determined in dependence on at least one machining parameter which influences a relative position of the workpiece part in relation to the remaining workpiece during the machining of the workpiece. A further method determines an attachment position of such a microjoint and a still further method machines the workpiece.

Claims

1. A method for determining a minimum width of a microjoint by which, when machining a workpiece, a workpiece part remains connected to a remaining workpiece of the workpiece, which comprises the step of: determining the minimum width of the microjoint in dependence on at least one machining parameter which influences a relative position of the workpiece part in relation to the remaining workpiece during the machining of the workpiece.

2. The method according to claim 1, which further comprises: thermal cutting of the workpiece with a machining beam during the machining of the workpiece; and determining the minimum width of the microjoint in dependence on the at least one machining parameter in a form of a gas pressure of a cutting gas leaving a machining nozzle that acts on the workpiece part during a cutting free of the workpiece part from the remaining workpiece.

3. The method according to claim 2, wherein the minimum width of the microjoint at which a maximum upstanding height, by which the workpiece part stands up above the remaining workpiece, is not exceeded during a tilting of the workpiece part in relation to the remaining workpiece due to an effect of the gas pressure acting on the workpiece part.

4. The method according to claim 3, wherein the maximum upstanding height is not greater than a distance between the machining nozzle and the remaining workpiece, wherein the distance is less than 2 mm.

5. The method according to claim 1, wherein the machining of the workpiece includes a displacement of the remaining workpiece together with the workpiece part along a workpiece support, wherein the minimum width of the microjoint is determined in dependence on the at least one machining parameter in a form of an acceleration of the workpiece part during the displacement along at least one displacement direction.

6. The method according to claim 5, wherein the minimum width of the microjoint at which, during the displacement of the workpiece part together with the remaining workpiece, a flexural stress at the microjoint that does not exceed a maximum flexural stress is determined.

7. The method according to claim 6, wherein the maximum flexural stress at the microjoint is not greater than a yield strength of a material of the workpiece.

8. The method according to claim 6, wherein the minimum width of the microjoint is made up of the minimum width of the microjoint at which the maximum flexural stress is not exceeded and a safety factor.

9. The method according to claim 1, wherein the workpiece is a sheet-shaped workpiece.

10. The method according to claim 2, wherein the machining beam is a laser beam.

11. The method according to claim 4, wherein the distance is less than 1 mm.

12. The method according to claim 8, wherein the safety factor dependent on the minimum width of the microjoint at which the maximum flexural stress is not exceeded.

13. A method for determining an attachment position of a microjoint by which a workpiece part remains connected to a remaining workpiece when machining a workpiece, which comprises the step of: determining a minimum width of the microjoint in a case of multiple different attachment positions along an outer contour of the workpiece part, wherein the minimum width is determined in dependence on a machining parameter in a form of a gas pressure of a cutting gas leaving a machining nozzle that acts on the workpiece part during a cutting free of the workpiece part from the remaining workpiece; and selecting an attachment position along the outer contour for the machining of the workpiece for which a smallest minimum width of the microjoint has been determined.

14. A method for machining a workpiece, which comprises the steps of: machining the workpiece while thereby forming at least one microjoint by which a workpiece part remains connected to a remaining workpiece, the at least one microjoint is formed at an attachment position along an outer contour of the workpiece part, the attachment position is selected along the outer contour for the machining of the workpiece for which a smallest minimum width of the microjoint has been determined.

15. A non-transitory computer program having computer executable instructions for carrying out all of the steps of the method according to claim 1 when the computer program runs on a data processing system.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0033] FIG. 1 is a diagrammatic, perspective view of a machine tool in the form of a laser cutting machine for the separating machining of a sheet-like workpiece according to the invention;

[0034] FIGS. 2A and 2B are sectional views of a workpiece part which is connected to a remaining workpiece by way of a microjoint, during the tilting as a result of a gas pressure of a cutting gas;

[0035] FIG. 3 is a perspective view of the machine tool in a form of a combined laser and punching machine; and

[0036] FIGS. 4A and 4B are representations of the workpiece part which is connected to a remaining workpiece by way of a microjoint, during the displacement along a workpiece support.

DETAILED DESCRIPTION OF THE INVENTION

[0037] In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

[0038] Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown a CO.sub.2 laser cutting machine 1 for laser cutting with a CO.sub.2 laser resonator 2, a laser machining head 4 and a workpiece support 5. A laser beam 6 generated by the laser resonator 2 is guided by means of a beam guide 3 from deflecting mirrors (not shown) to the laser machining head 4 and focused therein and also aligned with the aid of mirrors that are likewise not depicted perpendicularly to a surface 8a of the workpiece 8, i.e. the beam axis (optical axis) of the laser beam 6 extends perpendicularly to the workpiece 8.

[0039] For the laser cutting of the workpiece 8, first the laser beam 6 is used for piercing, i.e. the workpiece 8 is melted or oxidized at a location in the form of a point and the melt thereby produced is blown out. After that, the laser beam 6 is moved over the workpiece 8, so as to form a continuous kerf 9, along which the laser beam 6 cuts through the workpiece 8.

[0040] Both the piercing and the laser cutting can be assisted by adding a gas. Oxygen, nitrogen, compressed air and/or application-specific gases may be used as cutting gases 10. Which gas is ultimately used is dependent on which materials are being cut and which quality requirements the workpiece 8 has to meet. Particles and gases produced can be suctioned off with the aid of a suction device 11 from a suction chamber 12. A schematically represented programmable numerical control device 13 controls all of the essential functions of the laser cutting machine 1, for example the movement of the laser machining head 4, when a machining program is performed on it.

[0041] FIGS. 2A and 2B show the separating machining of the workpiece 8, to be more precise a rectangular workpiece part 14, which is separated from a remaining workpiece 15 (residual cut-out sheet) along a cutting contour 9. During the separating machining, the workpiece part 14 remains connected to the remaining workpiece 15 at its outer contour P by way of a microjoint 17. In the case of the example shown in FIGS. 2A, 2B, the microjoint 17 is at a microjoint position m along the outer contour P in the XY plane (the plane of the workpiece) which does not correspond to a cutting-free position f along the outer contour P that forms the end of the cutting during the separating machining along the cutting contour 9. At the moment at which the cutting contour 9 is closed at the cutting-free position f, a gas pressure p of the cutting gas 10 that leaves a machining nozzle 18 of the laser cutting machine 1 acts on the workpiece part 14 (cf. FIG. 2B).

[0042] If the laser cutting head 4 or the active area of the cutting gas nozzle 18 that is subjected to pressure (cf. FIG. 2B) is moved once again over a region of the workpiece part 14 connected by a microjoint 17, the microjoint 17 should attach the workpiece part 14 specifically in this region. In this way, the force introduced by the gas pressure p has the smallest lever with respect to the microjoint 17, and consequently also creates the smallest stresses.

[0043] Depending on how the workpiece part 14 is arranged in relation to the supporting workpiece supporting elements 5, there may be regions along the cutting contour 9 or along the outer contour P of the workpiece part 14 at which the gas pressure p of the cutting gas 10 causes the workpiece part 14 to become tilted in relation to the remaining workpiece 15 at the cutting-free position f.

[0044] In this case, the width B.sub.MJ of the microjoint 17 must not go below a minimum width B.sub.MJ,.sub.min at which the upstanding height of the tilted workpiece part 14 reaches a predetermined maximum upstanding height h.sub.max, shown in FIG. 2B. In the example shown in FIGS. 2A, 2B, the maximum upstanding height h.sub.max coincides with the distance A between the machining nozzle 18 and the remaining workpiece 15 or the workpiece 8. By specifying the maximum upstanding height h.sub.max as such, a collision of the upstanding workpiece part 14 with the machining nozzle 18 of the laser cutting head 4 can be prevented. In the example shown, the distance A between the end face of the machining nozzle 18 and the upper side 8a of the workpiece 8 is less than about 2 mm, generally 1 mm or less.

[0045] The calculation or the determination of the minimum microjoint width B.sub.MJ,.sub.min that the width of the microjoint must not go below in order to prevent a collision of the upstanding workpiece part 14 with the machining nozzle 18 is carried out in the case of the example shown in FIGS. 2A, 2B as shown below.

[0046] The configuration of supporting bars S refers to the set of all points in the XY plane that are given by the tips of the supporting bars 5, which are represented in FIG. 2A by dotted lines extending in the X direction. Also given are the outer contour P to be cut of the workpiece part 14, the microjoint position m and the cutting-free position f along the outer contour P to be cut.

[0047] The region I, hatched in FIG. 2A, represents the cut set of the interior of the outer contour P with the configuration of supporting bars S, combined with the microjoint position m. The supporting polygon A, represented by dashed-dotted lines, represents the convex envelope of I. D denotes the side of the supporting polygon A that is arranged closest to the cutting-free position f. The distance between the side D and the cutting-free position f is denoted by d. The distance between a position q, which lies on the other side of D with respect to the cutting-free position f and is at the greatest distance from D, and D is denoted by e. The force acting on the interior of the outer contour P at the cutting-free position f, produced by the gas pressure p of the cutting gas 10 leaving the machining nozzle 18, is designated hereinafter by F.

[0048] On the basis of the variables described above, the minimum width B.sub.BJ,.sub.mi.sub.n of the microjoint 17 can be determined: If the force F induced by the gas pressure p and described further above acts on the cut-free workpiece part 14, the latter tilts about the axis D.

[0049] In first approximation, the tilting angle α of the workpiece part 14 about the axis D is directly proportional to F * d, and so, with a material-dependent constant c.sub.0, it is the case that the maximum tilting angle α.sub.max in degrees is W(P,S,f,m) .sub.= max(90; C.sub.0 * F * d).

[0050] It has also been ascertained experimentally that 1 / α is directly proportional to the third power of the width B.sub.MJ of the microjoint 17 at the microjoint position m. Consequently, for a material-dependent constant c:

[00002]$\text{W}\left(\text{P,S,f,m}\right)=\max \left(90;\text{c*}\text{F*}{\text{d/B}}_{\text{MJ}}{}^3\right),$

where B.sub.MJ denotes the width of the microjoint 17 at the point m.

[0051] For a given tilting angle α, according to the invention it should be ensured that the upstanding height

[00003]$\text{H}\left(\text{P,}\alpha \right)=\sin \left(\alpha \right)\text{e}$

is less than a predetermined value h, which is allowed as the maximum tilting height h.sub.max, that is to say that

[00004]$\text{H}\left(\text{P,}\alpha \right)<{\text{h}}_{\max }.$

[0052] This condition is satisfied if [0053] sin(α) e < h.sub.max, that is to say [0054] (sin(W(P,S,p,m)) e < h.sub.max, that is to say [0055] (sin(max(90; c* F*d/B.sub.MJ.sup.3)) e < h.sub.max.

[0056] If e < h.sub.max, that is to say that in principle the workpiece part 14 may stand up too high, the following is therefore obtained as a condition: [0057] sin(c*F*d/B.sub.MJ.sup.3) < h.sub.Max/ e, which applies specifically whenever [0058] c* F*d/B.sub.MJ.sup.3 < arcsin(h.sub.Max/e), which applies specifically whenever [00005]$B_{MJ}>\sqrt[{}^3]{cFd/arcsin(\frac{h_{max}}{e})}$

[0059] The width B.sub.MJ of the microjoint 17, and consequently the minimum width B.sub.MJ,.sub.min of the microjoint 17, is specified by this inequation.

[0060] The minimum width B.sub.MJ,.sub.min of the microjoint 17, which has been determined in the way described further above, is used in a programming system for creating a control program for machining the workpiece 8 in order to create a machining program which runs on the numerical control device 13 during the machining of the workpiece 8.

[0061] The minimum width B.sub.MJ,.sub.min of the microjoint 17 may be determined not only in dependence on the cutting gas pressure p as a machining parameter but also in dependence on other machining parameters that influence a relative position of the workpiece part 14 with respect to the remaining workpiece 15 during the machining of the workpiece 8. This is the case for example during the manipulation, to be more precise the displacement, of a workpiece 8, as described below on the basis of a combined laser and punching machine 20, which is represented in FIG. 3.

[0062] The machine tool 20 configured as a laser and punching machine has as machining tools for the separating machining of the sheet-like workpiece 8 in the form of a metal sheet a conventional punching head 21 with a punch 21a and a laser machining head 4. The workpiece 8 to be machined is mounted on a workpiece support 5 in the form of a machining table during the machining of the workpiece. By means of a conventional holding device 22, which has clamps 23 for securely holding the workpiece 8, the workpiece 8 can be displaced with respect to the punch 21a and the laser machining head 4 in the X direction of the plane of the workpiece (XY plane of an XYZ coordinates system) by means of a conventional linear drive 23a, indicated by an arrow. The workpiece 8 can be moved in the Y direction of the plane of the workpiece by the workpiece support 5 being displaced together with the holding device 22 relative to a substrate 24, on which the workpiece support 5 is mounted, by means of a conventional linear drive 23b, indicated by an arrow.

[0063] The workpiece 8 can in this way be displaced in the X and Y directions with respect to the punch 21a and the laser machining head 4, and so the region of the workpiece 8 that is to be machined in each case can be positioned in a fixed machining region 25 of the punch 21a or a fixed machining region 26 of the laser machining head 4. Positioned in the machining region 25 of the punch 21 is an (interchangeable) punching die 27, which has an opening 27a for the engagement of the (likewise interchangeable) punch 21a. Correspondingly arranged in the fixed machining region 26 of the laser machining head 4 is a laser die 28, which serves as an opening delimitation for a substantially circular suction opening 26a in the workpiece support 5. The subregion of the workpiece support 5 in the X direction at which the machining regions 25, 26 are formed is fixed here, and is not displaced in the Y direction in relation to the substrate 24. The laser machining head 4 may thereby perform a movement in the X and Y directions that is limited by the suction opening 26a. The machine tool 20 shown in FIG. 3 has a control device 13, which serves for controlling the linear drives 23a, 23b in the X direction and in the Y direction of the machine tool 20.

[0064] FIGS. 4A, 4B show a workpiece part 14, which is held on a remaining workpiece 15 by way of a microjoint 17. During the displacement of the workpiece part 14 held by the microjoint 17 on or along the workpiece support 5 in the X direction, the force of the weight F.sub.G acts on the workpiece part 14 in the Z direction when the workpiece part 14 passes over an unsupported region of the workpiece support 5. In addition, the workpiece part 14 is bent in the XY plane about the microjoint 17. The minimum width B.sub.BJ,.sub.m.sub.in of the microjoint 17 is therefore also dictated by the requirement that the bending does not become so much that the workpiece part 14 slips under or over the remaining workpiece 15.

[0065] The calculation of the minimum microjoint width B.sub.BJ,.sub.m.sub.in is dependent on the position m of the microjoint on the workpiece part 14.

[0066] The microjoint 17 is advantageously attached to the workpiece part 14 at a location (microjoint position or attachment position m) at which the main axis of inertia of the workpiece part 14 intersects with the outer contour P (for example at an axis of symmetry of the workpiece part 14 - as a departure from the representation shown in FIGS. 4A, 4B). In this way, there is no longer any torsional loading of the microjoint 17 by the force of the weight F.sub.G. If the microjoint 17 is additionally at a position m along the outer contour P obtained by projection of the center of gravity S of the workpiece part 14 in the direction of the relative movement between the workpiece part 14 and the workpiece support 5, there is no longer any further flexural loading by the acceleration force and friction force in a second axial direction.

[0067] Moreover, the microjoint 17 should be attached at the point of intersection of the main axis of inertia with the outer contour P that is at the smallest distance from the center of gravity S of the workpiece 14, or in the axial direction (X or Y) in which the greatest acceleration acts on the workpiece part 14.

[0068] The following assumptions apply for the interpretation described below of the minimum necessary microjoint width B.sub.BJ,.sub.mi.sub.n: [0069] Acting in the Z direction is the force of the weight F.sub.G of the workpiece part 14 that acts at the center of mass (center of gravity S). [0070] Acting on the microjoint 17 in the X and Y directions are an (axial) acceleration a.sub.x, a.sub.Y and a static friction. [0071] The forces act at the center of gravity S; in this case, a small lever (.sub.= distance between center of gravity S and attachment position m of the microjoint 17) is favorable. This establishes the preferred attachment position m of the microjoint 17 on the workpiece 14. [0072] The microjoint 17 lies on one of the main axes of inertia. [0073] During the laser cutting, the microjoint 17 is set at the end of the cutting, and so the gas pressure plays a secondary role and can be ignored.

[0074] The following variables are required for the calculation of the minimum necessary microjoint width B.sub.BJ,.sub.mi.sub.n on the basis of the following assumptions: [0075] geometrical properties of the workpiece 14: [0076] center of gravity S of the workpiece part 14 [0077] attachment point m of the microjoint 17: optimally lies on one of the main axes of inertia of the workpiece part 14, which corresponds to a respective axis of symmetry of the workpiece part 14 (if present) [0078] material properties: [0079] sheet thickness d [0080] permissible stress B.sub.ges [0081] modulus of elasticity [0082] density (weight or mass m) [0083] friction value or friction coefficient .Math. with the workpiece support 5 [0084] axial parameters of the machine tool 20: [0085] acceleration ax, a.sub.Y in the X and Y directions.

[0086] The microjoint 17 is assumed hereinafter as a bending beam on which the following moments act:

[0087] moment in the direction of gravitational force (about the X axis):

[00006]$M_{\text{x}}=F_{\text{G}}\ast {\text{h}}_{\text{y}}\text{with}{F}_{\text{G}}=m\ast g$

moment in the X and Y directions (about the Z axis):

[00007]$M_{\text{z}}=\left(F_{\text{ax}}+F_{\text{R}}\right)\ast {\text{h}}_{\text{y}}+\left(F_{\text{ay}}+F_{\text{R}}\right)\ast {\text{h}}_{\text{x}}\text{with}$

[00008]$F_{\text{ax}}=m\ast a_{\text{x}}\text{and}{F}_{\text{ay}}=m\ast a_{\text{y}}{\text{and F}}_{\text{R}}={\text{F}}_{\text{G}}\ast \mu ,$

[0088] With the following designations: m = mass of the workpiece part, g = acceleration due to gravity h.sub.x = distance between center of gravity S and the attachment point m of the microjoint 17 in the X direction, h.sub.y = distance between the center of gravity S and the attachment point m of the microjoint in the Y direction, a.sub.x = acceleration in the X direction, a.sub.y = acceleration in the Y direction, .Math. = friction coefficient between the material of the workpiece part 14 and the workpiece support 5.

[0089] In the case of the example shown in FIGS. 4A, 4B, in which the workpiece part 14 is only displaced in the X direction, there is no longer the friction force F.sub.R in the Y direction. The moment about the Y axis no longer occurs as a result of the simplification that the microjoint 17 lies on one of the main axes of inertia.

[0090] Determination of the moment of resistance W.sub.x, W.sub.Y of the microjoint 17:

[00009]$W_{\text{x}}=I_{\text{x}}/\left(d/2\right)with{I}_{\text{x}}=\left({\text{B}}_{\text{MJ}}\ast {\text{d}}^3\right)/12$

[00010]$W_{\text{z}}=I_{\text{z}}/\left({\text{B}}_{\text{MJ}}/2\right)with{I}_{\text{z}}=\left(\text{d}\ast {\text{B}}_{\text{MJ}}{}^3\right)/12$

(D=workpiece thickness, B.sub.MJ = microjoint width)

[0091] This allows the flexural stress B.sub.ges on the microjoint 17 to be calculated:

[00011]${\text{B}}_{\text{x}}={\text{M}}_{\text{x}}/\text{Wx}$

[00012]${\text{B}}_{\text{z}}={\text{M}}_{\text{x}}/{\text{W}}_{\text{z}}$

[00013]$.Math.{\text{B}}_{\text{ges}}={\text{B}}_{\text{x}}+{\text{B}}_{\text{z}}\left(\text{vectorial}\text{addition}\right).$

[0092] The microjoint width B.sub.BJ must be chosen such that the flexural stress B.sub.ges is at most as great as the yield strength R.sub.p0.2 for the material of the currently displaced workpiece 8:

[00014]${\text{B}}_{\text{ges,max}}\le {\text{R}}_{\text{p0}\text{.2}}$

[00015]$B_{mj1,2}=\frac{-b\pm \sqrt{b^2-4ac}}{2a}$

[00016]$\begin{array}{l}\text{with}a=-R_{p0,2};b=\frac{6\left|h_y\right|\cdot F_G}{d^2};\\ c=\frac{6\left(\left|h_x\right|\cdot F_{ay}+\left|h_y\right|\cdot \left(F_{ax}+F_R\right)\right)}{d}\end{array}$

and finally:

[0093] The minimum microjoint width B.sub.MJ,.sub.minB is then calculated for this predetermined limit value R.sub.p0..sub.2 of the stress B.sub.ges,.sub.max as follows:

[00017]$B_{MJ,minB}=\max \left\{B_{MJ1},B_{MJ2}\right\}$

[0094] The minimum microjoint width B.sub.BJ,.sub.minB is the maximum of the two values B.sub.MJ1, B.sub.MJ2, because the smaller of the two values is always negative as a result of the root used in the calculation.

[0095] An empirically ascertained safety factor c.sub.1, which takes into account the influence of external disturbances, such as for example vibrations during the punching process, sag of the workpiece part 14, deflection of the workpiece part 14 when it passes over supporting elements (for example balls or brushes), may be added to the calculated minimum microjoint width B.sub.BJ,.sub.minB, i.e. BBJ,min = B.sub.BJ,.sub.minB + C1.

[0096] Furthermore, the safety factor c.sub.1 may take into account the notch effect occurring at the attachment position m of the microjoint 17 as a result of the abrupt reduction in diameter, which leads to a reduction in the maximum permissible flexural stress B.sub.ges,max. The safety factor c.sub.1 is in this case ideally dependent on the calculated microjoint width (c.sub.1(B.sub.BJ,.sub.minB)), i.e. it is not an absolute value. In this way, the calculated minimum microjoint widths B.sub.BJ,.sub.mi.sub.n for the different workpiece parts 14 of a workpiece 4 change relatively and not absolutely, which prevents small workpiece parts 14 from being attached by an overdimensioned microjoint 17.

[0097] Both the method described in connection with FIGS. 2A, 2B and the method described in connection with FIGS. 4A, 4B for determining the minimum width B.sub.BJ,.sub.mi.sub.n of the microjoint 17 are typically carried out for a number of different attachment positions m along the outer contour P of the workpiece part 14. The attachment position m along the outer contour P for which the smallest minimum width B.sub.BJ,.sub.mi.sub.n of the microjoint 17 has been determined is selected for the machining of the workpiece 8. During the subsequent machining of the workpiece 8, the at least one microjoint 17 by which the workpiece part 14 remains connected to the remaining workpiece 15 is formed at the attachment position m selected in the way described above and with the minimum width B.sub.BJ,.sub.min determined in the way described further above.

[0098] The minimum width B.sub.BJ,.sub.min and the attachment position m of the microjoint 17 are used in a programming system for creating a control program or for creating control commands for machining the workpiece 8. The control program created in this way is executed by the control device 13 during the machining of the workpiece 8. Stored in the programming system are items of information concerning the workpiece and machining parameters for the machining of the workpiece 8 that are required for the determination of the minimum width B.sub.BJ,.sub.min of the microjoint 17, for example the cutting gas pressure p during the cutting-machining of the workpiece 8 or the axial accelerations a.sub.x, a.sub.Y during the displacement of the workpiece 8 along the workpiece support 5. It goes without saying that, as an alternative or in addition to the machining parameters described further above, other machine parameters that influence the relative position of the workpiece part 14 connected to the remaining workpiece 15 by way of the (at least one) microjoint 17 with respect to the remaining workpiece 15 or with respect to the workpiece support 5 may be used for the determination of the minimum width B.sub.BJ,.sub.min of the microjoint 17.