Methods for CNC lathes

Abstract

The present invention uses a nominal rake angle (γ n The present invention relates to a turning method for a CNC lathe, comprising at least two machining steps, in which the cutting elements (1, 11) are repositioned between the machining steps such that the cutting element diameter (D) is changed, whereby the useful tool life can be extended and a high quality of the machined surface and a stable cutting process can be achieved even with worn tools. The present invention also relates to a system and a computer program for implementing the method.

Description

[Technical field]

[0001] The present invention is in the technical field of metal cutting, more particularly in the field of turning. [Background technology]

[0002] In metal cutting, turning is a common machining operation. CNC lathes are commonly used. When machining particularly hard materials, it is common to use turning tools with cutting elements made of cubic boron nitride (CBN) or ceramics. Even if such materials have a high wear resistance, for example compared to carbide cutting tools, wear of the rake and flank faces of the cutting elements will eventually limit the usability of the cutting elements and even lead to tool failure such that the cutting elements have to be replaced. For efficiency and economic reasons, it may be desirable to use the cutting elements for as long as possible. Therefore, efforts have been made to reduce the wear of the cutting elements. As an example, EP2015881 discloses a CBN tool component with a secondary layer on the rake face that has a higher resistance to crater formation. However, even if the tool life may be somewhat extended, such a layer does not prevent tool wear from eventually degrading the cutting properties of the CBN tool.

[0003] Therefore, there is a need to further increase the tool life of cutting elements used in turning processes. Summary of the Invention

[0004] The object of the present invention is to alleviate the drawbacks of the prior art and to provide a method for extending the tool life, i.e. the time over which a cutting element can be used for machining. A further object is to provide a method which consistently results in a high quality machined surface. A further object is to obtain a stable cutting process.

[0005] Thus, according to a first aspect, the invention relates to a turning method for a CNC lathe, said method comprising: - providing a workpiece that is rotationally rotatable about its axis of rotation; - providing a turning tool extending along a tool axis, the turning tool comprising a cutting element including a rake face, a clearance face and a cutting edge formed at an interface between the rake face and the clearance face, the cutting element being positionable in different orientations relative to a workpiece, each orientation being determined by a nominal rake angle relative to a surface of the workpiece, the cutting element having, at a point of contact between the cutting edge and the workpiece, an effective rake angle and an effective clearance angle that depend on wear of the cutting element; - positioning a cutting element relative to a workpiece in a first orientation determined by a first nominal rake angle resulting in a first effective rake angle and a first effective clearance angle; - machining a workpiece with the cutting element in a first orientation in a first machining step; After the first machining step, - repositioning the cutting element relative to the workpiece, or relative to another workpiece to be machined, in a second orientation determined by a second nominal rake angle resulting in a second effective rake angle and a second effective clearance angle, the second nominal rake angle being different from the first nominal rake angle; - machining the workpiece or another workpiece with the cutting element in a second orientation in a second machining step; Includes.

[0006] Thereby, the effective tool life can be extended, and a high quality of the machined surface and a stable cutting process can be achieved even with worn tools. Surface quality can refer, for example, to surface roughness or surface integrity, which is related, for example, to microstructural changes caused by the machining process.

[0007] The turning method is for a CNC lathe, i.e. a computer or computerized numerically controlled lathe, i.e. any CNC machine suitable for turning, such as, for example, a rotary lathe, a multitask machine, a turn-mill machine, or a sliding head machine. The workpiece may be a metal workpiece including an outer surface that is a radially outer surface. The radially outer surface faces away from the axis of rotation. The turning method may be used for turning the radially outer surface, i.e., external turning. The workpiece extends between a first end and a second end.

[0008] The workpiece may be of hardened steel, for example 40 HRC or higher. Alternatively, the workpiece may be a superalloy, such as a heat resistant superalloy (HRSA), for example a nickel-based alloy.

[0009] The workpiece may be clamped by a clamping means. The clamping means holds the workpiece and is at least partially controlled and driven by the motor or spindle. The clamping means may be in the form of a collet chuck, face driver, or three-jaw chuck and may include a tailstock. The headstock end of the machine is preferably located at the first end of the workpiece. A second end of the workpiece opposite the first end of the workpiece may be a free end. Alternatively, the second end is in contact with the tailstock or second chuck.

[0010] The turning tool includes a front end and an opposite rear end in the form of a linkage that is connected to a CNC lathe, and more particularly to a machine interface of the CNC lathe, such as a machine spindle or a tool revolver turret or tool post.

[0011] The connection part may have a square or rectangular cross section. The connection part may be conical or substantially conical, preferably according to ISO standard 26623-1, etc. In this context, a truncated cone is a cone-like shape. In this context, the tapered part forming the rear part of the connection of ISO standard 26623-1 is a cone-like shape. The connection part extends along the connection axis. The connection part is preferably conical such that said cone or cone-like shape is symmetric or substantially symmetric about the connection axis. In this context, three-fold symmetry is considered to be symmetric. The cross-sectional area of ​​said cone or cone-like shape preferably decreases in the rearward direction. The connection part may be in the form of a hollow tapered shank, such as HSK, according to DIN 69893.

[0012] The articulation axis corresponds to the tool axis or central axis of the turning tool, which defines the longitudinal axis of the turning tool.

[0013] The turning tool comprises a cutting element. The cutting element is preferably made of a wear-resistant material, such as cubic boron nitride (CBN), polycrystalline cubic boron nitride (pCBN), ceramic or cemented carbide. The cutting element may be implemented as part of the turning tool, such as an integral part of such a turning tool, or as an exchangeable and / or indexable cutting insert or a turning insert attachable to the turning tool, for example in an insert pocket, in which the cutting insert may be fastened by any suitable fastening means, such as a screw or other means for holding the cutting insert securely in the insert pocket. The cutting element may also be considered as part of such a cutting insert. For example, the cutting element may be a CBN cutting tip brazed to a cemented carbide carrier, which together form an exchangeable cutting insert.

[0014] The cutting element comprises a rake face or an upper surface including a rake surface. The cutting element further comprises a clearance face or a clearance surface adjacent to the rake face. At the interface between the rake face and the clearance surface, a cutting edge is formed. The cutting edge, or a part of the cutting edge, generates a machined surface. The cutting edge may have a convex shape in top view, for example, the cutting edge may be a nose cutting edge, for example in the form of a circular arc. The cutting element, particularly when in the form of a cutting insert, may have a rhombus, triangle, octagon, square, circle or polygon shape in top view.

[0015] The upper surface may be flat. Alternatively, the surface may be non-flat or non-planar. For example, the surface may comprise one or more chip breaking means in the form of one or more protrusions and / or recesses. Furthermore, a chamfer may be formed on the upper surface such that a cutting edge is defined at the interface between the flank face and such chamfer. In such a case, for the purposes described in this disclosure, i.e. when considering the nominal rake angle and / or the effective rake angle, the chamfer corresponds to the rake face.

[0016] During machining, the workpiece is rotated in a rotational direction and the turning tool is controlled such that the cutting edges of the cutting elements engage the rotating workpiece. The machining performed during the first machining step includes machining the workpiece. The first machining step may also include machining a further workpiece.

[0017] The machining may include any method that results in a rotationally symmetric surface about which the workpiece rotates. Thus, the method may be used in single point turning operations as well as other turning operations, such as turning operations where the contact point or area between the cutting edge and the workpiece moves along the cutting edge during machining, for example where there is a linear feed movement transverse to the axis of rotation.

[0018] During machining, the cutting elements are subject to wear, in particular crater wear on the rake face and flank wear on the flank face. In particular, for CBN cutting elements used in turning hard parts, the microgeometry of the cutting element is radically altered according to the progression of wear during the life of the cutting element. As an example, when using a CBN cutting element with a chamfer, which has an initially negative rake angle, the effective rake angle eventually becomes positive due to the crater wear of the cutting element. Eventually, the cutting element will break or it will not provide acceptable machining results and must be replaced. It has been discovered by the inventors that the cutting element may assume a tribologically stable condition after a certain time of machining, i.e. a condition that is maintained for the remainder of the cutting element's life. For example, for a CBN cutting element with a chamfer, such a tribologically stable condition may appear after about 30% of the estimated tool life. In the tribologically stable condition, the effective rake angle and the effective clearance angle, as well as the cutting edge radius, are normalized. As a result, the wedge angle is also normalized and therefore does not change significantly during the stable tribological condition. Thus, even if the crater wear and flank wear may increase during this stable condition, the wear progression is such that the shape of the flank and crater does not change, i.e., the effective rake angle and the effective clearance angle remain substantially the same. It was found that the cutting edge line of the cutting element maintained good integrity even in the stable tribological condition, even though the effective rake angle was significantly changed compared to the original rake angle. However, it was also found that in the stable tribological condition, the effective clearance angle is often zero or close to zero, i.e., for a significant time of the tool life, which can potentially lead to various adverse effects. For example, the integrity of the machined surface may change due to increased contact between the flank face and the workpiece. The inventors recognized that if the nominal rake angle is changed before or just after the stable tribological condition is reached, the integrity of the machined surface may be improved (or maintained) and the effective tool life extended.

[0019] Stated differently, the inventors have discovered a way or method that allows cutting elements that were previously considered worn out to be used for a longer period of time. The way involves repositioning the cutting elements against the workpiece. The inventors have discovered that this method works particularly well when CBN or pCBN cutting elements, i.e., cutting or turning inserts comprising CBN or pCBN, are used to turn hardened steels or other hard metals, such as Ni-based superalloys.

[0020] As used herein, the "nominal rake angle" refers to the angle of the original rake face relative to a plane perpendicular to the machined surface, without taking into account variations in the geometry of the cutting element (such as variations caused by wear). In this context, the nominal rake angle may also be considered the "system rake angle", i.e., determined by the original geometry of the cutting element in a particular machining setting (e.g., the orientation and position of the CNC lathe, tool holder, etc., relative to the workpiece). Correspondingly, the "nominal clearance angle" refers to the original, unworn clearance, i.e., the desired clearance, relative to the surface of the workpiece, without taking into account tool wear. In contrast, the "effective rake angle" as used herein is the true rake angle, i.e., the actual cutting angle, effective during machining, which depends on local variations in the geometry of the rake face. Correspondingly, the "effective clearance angle" refers to the actual clearance of the clearance relative to the machined surface. Both the effective rake angle and the effective clearance angle are affected by the wear of the cutting element. For example, the effective rake angle may increase compared to the original rake angle due to crater wear occurring on the rake face, and the effective clearance angle may decrease due to flank wear. Thus, when machining with unworn cutting elements, the effective rake angle and the effective clearance angle initially correspond to the nominal rake angle and the nominal clearance angle, respectively. However, as soon as wear occurs and starts to affect these angles, the effective angles deviate from the corresponding nominal angles.

[0021] Thus, during the first machining step, the wear of the cutting elements affects the actual geometry of the rake and flank faces. This can increase the effective rake angle and decrease the effective clearance angle. To compensate for such wear, the nominal second rake angle differs from the first nominal rake angle. In particular, reorienting the cutting elements to the second orientation, i.e., changing the nominal rake angle, can increase the effective clearance.

[0022] To achieve this, the second nominal rake angle used in the second machining step may be smaller than the first nominal rake angle used in the first machining step.

[0023] By reducing the nominal rake angle, the effective rake angle is also obviously reduced, at least when compared to the effective rake angle present immediately before rearranging the cutting elements, however, the second effective rake angle may still be larger than the first effective rake angle since wear caused by the first machining step causes an increase in the effective rake angle compared to the first effective rake angle.

[0024] It is also assumed that the first and second effective rake angles are the same, i.e. the change in the effective rake angle caused by wear from the first machining step corresponds to the difference between the first and second nominal rake angles. However, this may often not be the case if the first machining step involves initially using new unworn cutting elements, since the increase in the rake angle due to crater wear is often higher than it would be desirable to compensate for when repositioning the cutting elements. On the other hand, if the first machining step starts with already worn cutting elements repositioned from a previous state in which stable tribological conditions were reached, and the first machining step proceeds until new stable tribological conditions are reached, it is highly likely that the first effective rake angle corresponds to the second effective rake angle, i.e. the effective rake angle is restored by the change in the nominal rake angle. However, it may not be desirable or possible to restore the original effective rake angle of the unworn cutting elements.

[0025] By reducing the nominal rake angle, the effective clearance is increased and therefore the contact between the flank of the cutting element and the workpiece is reduced. Increasing the contact area between the flank of the cutting element and the workpiece changes the distribution of passive and feed forces, which results in an increase in the thickness of the white layer of the machined component, which is associated with undesirable microstructural changes on the machined surface. Therefore, by increasing the clearance, a more stable machining process is obtained, the formation of the white layer is reduced, and therefore an improvement in the quality of the machined surface is realized.

[0026] The second effective clearance angle may correspond or substantially correspond to the first effective clearance angle. In other words, the difference between the first nominal rake angle and the second nominal rake angle may correspond to a change in the effective clearance angle caused by the first machining step. For example, the first machining step may have caused flank wear of the cutting element such that the effective clearance angle is reduced to 0, i.e., friction now occurs between at least a portion of the clearance face and the workpiece. Then, by making the difference between the first nominal rake angle and the second nominal rake angle correspond to a desired clearance angle, e.g., the clearance angle present for an unworn cutting element, the desired (e.g., original) effective clearance angle may be restored by repositioning the cutting element. In other words, the original effective clearance may be restored by changing the nominal rake angle.

[0027] The second nominal rake angle may differ from the first nominal rake angle by 2-10 degrees. In many applications, this is sufficient to restore clearance while maintaining good cutting performance, and by not significantly changing the nominal rake angle, the cutting element may be repositioned multiple times, further extending the overall life of the cutting element. The second nominal rake angle may differ from the first nominal rake angle by 4-8 degrees, such as by only 6 degrees, which may be beneficial for at least some applications.

[0028] The step of repositioning the cutting element may be performed when the cutting element is out of cut. As used herein, "out of cut" refers to a condition in which the cutting element is not engaged with the workpiece. Such a condition may occur between the machining of two different workpieces or between two different passes of a turning operation on a workpiece.

[0029] Thus, the first machining step may include a first set of one or more machining operations and the second machining step may include a second set of one or more machining operations, where the step of repositioning the cutting elements may be performed between these sets of machining operations, e.g., between machining two different workpieces, where the machining operations in the second set do not overlap with the machining operations in the first set.

[0030] In this context, a machining operation may be considered as an operation performed on a workpiece and may include one or more passes with a turning tool.

[0031] As mentioned above, the step of repositioning the cutting elements may be performed during two different passes of the machining operation. In this case, the first machining step and the second machining step may each include different portions of a single turning operation. The first machining step may then include, for example, one or more complete machining operations (e.g., machined components) and a first portion of a particular machining operation, and the second machining step may include a second portion of a particular machining operation as well as a subsequent machining operation and / or a portion of a subsequent machining operation.

[0032] It is also envisioned that repositioning of the cutting elements occurs during machining, such as during a pass of a turning operation. Thus, repositioning of the cutting elements may include stepwise or continuous repositioning performed during machining. This may be useful, for example, when turning large HRSA components, where the cutting elements wear in a single pass, which may potentially cause the surface integrity of the component to be different at the start of the cut compared to the end of the cut.

[0033] The duration of each machining step may be selected based on a predefined time period during which the cutting elements have been in cut. Thus, repositioning of the cutting elements may occur after a certain predefined time period, e.g., a certain number of minutes during which the cutting elements have been active (i.e., "time in cut"). Thus, the first machining step may involve machining for a certain time that does not take into account the actual number of machining operations performed. A machining step may also be determined as including a machining operation or a pass of machining operations during which the predefined time period has ended, i.e., the current machining operation or pass is finished before the cutting elements are repositioned. Instead of a predefined time, the duration may alternatively be selected based on a predefined cut length. As a further alternative, each machining step may relate to a fixed number of machining operations, i.e., the machining step may be determined as including the machining of a predefined number of workpieces. Thus, repositioning of the cutting elements may occur after a certain number of components have been machined.

[0034] Regardless of which parameters are used to determine when repositioning of cutting elements should occur, each of the machining steps may involve machining a number of different workpieces.

[0035] Positioning and repositioning the cutting element between the first and second orientations may be accomplished in a variety of ways. According to some embodiments, the relative position between the workpiece and the tool axis may be adjusted. For example, the engagement point between the cutting element and the workpiece may be adjusted by moving the tool axis relative to the workpiece, transverse to the axis of rotation, from a first tool axis position to a second tool axis position, such that the tool axis at the second tool axis position is parallel to but not coincident with the tool axis at the first tool axis position.

[0036] Thus, the direction of engagement of the cutting element relative to the surface of the workpiece may be adjusted by changing the orientation of the tool axis relative to the workpiece, in particular by translation of the tool axis. In particular, considering a Cartesian coordinate system for the lathe, in which the Z axis points along the axis of rotation of the workpiece, the X axis points radially (i.e., toward or away from the axis of rotation) relative to the workpiece, and the Y axis points perpendicular to the plane defined by the Z and X axes, translation may include movement along the Y axis. Such an embodiment therefore requires the use of a CNC lathe with the ability to move the turning tool along the Y axis, for example, certain types of multitasking or mill-turn machines. Thus, when using such a CNC lathe, the step of repositioning the cutting element may be accomplished via the machine control system. Such repositioning of the cutting element may be performed accurately and relatively easily, as it does not require specially designed turning tools or tool holders.

[0037] According to some embodiments, the tool axis is moved a first distance in a first direction from a first tool axis position to a second tool axis position.

[0038] Considering the coordinate system determined above, the first direction may be along the Y axis. Thus, if the tool axis is oriented along or parallel to the X axis, the first direction may correspond to a direction transverse to both the tool axis and the axis of rotation of the workpiece. In particular, the first direction may correspond to or substantially correspond to the direction in which the workpiece rotates, i.e., the direction of tangential motion of the workpiece at the point of contact between the cutting element and the workpiece. Stated differently, the first direction may be along the Y axis and away from the direction in which the rake face or top surface of the cutting element faces.

[0039] In other words, movement of the tool axis may have a component along the Y axis, and the movement may be directed along or substantially along the direction of rotation of the workpiece at the point of contact between the cutting element and the workpiece, i.e., away from the direction in which the rake face or top surface of the cutting element faces.

[0040] The cutting element is thereby repositioned relative to the workpiece to a position where the nominal rake angle is reduced and the effective clearance angle is increased.

[0041] The tool axis may be moved along the X axis as well as along the Y axis to compensate for changes in the depth of cut caused by movement along the Y axis. For example, if the tool axis is moved a certain distance along the Y axis to change the nominal rake angle, the tool axis may also be moved a certain distance in the X axis direction to maintain the same depth of cut.

[0042] The tool does not necessarily move linearly between the first and second orientations, or in two successive linear movements (e.g., first in the Y-axis direction and then in the X-axis direction), but may also be moved along an arc, for example along the periphery of the workpiece.

[0043] The first distance moved in the first direction, or the distance moved along the Y axis as a result, may be 0.03r to 0.18r, such as 0.07r to 0.14r, where r is the radius of the workpiece. A movement of 0.03r to 0.18r may result in a change in the nominal rake angle of only 2 to 10 degrees, at least if performed near the center of the workpiece, i.e., in the vicinity of the XZ plane. As an example, the first distance, or the distance moved along the Y axis as a result, may be about 0.1 times the radius of the workpiece, such as 0.09r to 0.11r, which may correspond to a change in the nominal rake angle of only about 6 degrees.

[0044] An alternative method for repositioning the cutting elements, which does not require translation of the tool axis, may be to mount the cutting elements in a different turning tool or tool holder, depending on which orientation is desired. For example, prior to a first machining step, the cutting elements may be mounted in a turning tool body that is designed such that the cutting elements obtain a first orientation relative to the workpiece when the turning tool is mounted in a CNC lathe. To reposition the cutting elements prior to a second machining step, the cutting elements may be mounted in another tool body that is designed such that the cutting elements obtain a second orientation relative to the workpiece when the turning tool is mounted in a CNC lathe.

[0045] It is also envisioned that the step of repositioning the cutting element in the second orientation may include changing the direction of engagement of the cutting element with respect to the surface of the workpiece by tilting the tool axis, for example by a rotational movement of a tool spindle to which the turning tool is connected, or by controlling the angle of a tool turret to which the turning tool is connected. Alternatively, the cutting element may be tilted with respect to the tool axis. For example, when using a tiltable or otherwise adjustable turning tool or tool holder, the positioning and repositioning of the cutting element may be achieved by such means. For example, a tool holder that is tiltable, or a tool whose part including the cutting element is tiltable, may be used. Such adjustment may be achieved by manual means or may be controlled by some kind of actuator, for example a remote control actuator. The repositioning of the cutting element by tilting the tool axis or by tilting the cutting element with respect to the tool axis preferably includes tilting by an angle corresponding to the difference between the first nominal rake angle and the subsequent nominal rake angle. If several steps of repositioning, for example in a third orientation, are performed by such tilting, the tilt angle for each step is preferably the same or substantially the same, i.e., within + / - 5 degrees. For each step, the nominal rake angle becomes progressively smaller or more negative.

[0046] According to some embodiments, the turning method further comprises, after the second machining step: - repositioning the cutting element relative to the workpiece, or relative to another workpiece to be machined, in a third orientation determined by a third nominal rake angle resulting in a third effective rake angle and a third effective clearance angle, the third nominal rake angle being different from each of the first nominal rake angle and the second nominal rake angle; - in a third machining step, machining the workpiece or another workpiece with the cutting element in a third orientation; Further includes:

[0047] Thus, the repositioning of the cutting elements may be repeated multiple times. The second nominal rake angle may be smaller than the first nominal rake angle, and the third nominal rake angle may be smaller than the second nominal rake angle. In other words, the third nominal rake angle may be smaller than both the first nominal rake angle and the second nominal rake angle. The difference between the first nominal rake angle and the second nominal rake angle may be the same as the difference between the second nominal rake angle and the third nominal rake angle. That is, the nominal rake angle may be changed by the same amount each time the cutting elements are repositioned. The repositioning may be repeated more than two times, for example three, four or five times, or even more. Each time the cutting elements are repositioned, the effective clearance angle may be restored to that at the beginning of the machining step. Thereby, the useful life of the cutting elements may be extended even further while maintaining the quality of the machined surface.

[0048] According to some embodiments involving multiple repositioning of the cutting element, where the step of repositioning the cutting element in the second orientation includes moving the tool axis a first distance in a first direction from a first tool axis position to a second tool axis position, the step of repositioning the cutting element in a third orientation may include moving the tool axis a second distance in the first direction from the second tool axis position, away from the first tool axis position to a third tool axis position.

[0049] By moving the tool axis away from the first and second tool axis positions, the nominal rake angle is further reduced and the effective clearance angle is increased as compared to the effective clearance that existed immediately prior to repositioning the cutting element at the third orientation. For example, the effective clearance may be restored to that immediately prior to repositioning the cutting element at the second position. In other words, the third effective clearance angle may correspond to the second effective clearance angle and / or the first effective clearance angle.

[0050] The second distance may be the same as the first distance or may be less than the first distance.

[0051] The change in nominal rake angle caused by tool axis translation depends on the distance along the Y axis from the center of rotation of the workpiece. For example, closer to the center, a greater translation along the Y axis is required to achieve the same nominal rake angle change as a tool axis translation performed farther from the center.

[0052] Thus, as an example considering this embodiment, if a first machining step is performed with cutting elements located in the XZ plane, the nominal rake angle change is the same for two subsequent repositionings of the cutting elements only if the second distance is smaller than the first distance.

[0053] However, for many applications and workpiece dimensions, it may be possible to ignore this difference, i.e., it may be sufficient to move the tool axis the same distance along the Y axis when repositioning the cutting element. In other words, the second distance may be the same as the first distance. This may also be the case, for example, when a first machining operation is performed above the center of the workpiece, a second machining operation is performed at the center, and a third machining operation is performed below the center. In that case, both translations along the Y axis may be of the same magnitude to achieve the corresponding nominal rake angle change.

[0054] According to a second aspect of the invention, there is provided a system comprising a CNC lathe, a processor and a turning tool including a cutting element, the system configured to perform the method according to the first aspect of the invention. The processor may be an integral part of the CNC lathe or operably coupled thereto and may be configured to execute a computer program that triggers at least some of the method steps to be performed. The system may further comprise a memory for storing such a computer program.

[0055] Therefore, according to a third aspect, the present invention relates to a computer program comprising instructions which, when executed by a system according to the second aspect of the invention, cause the system to carry out a method according to the first aspect of the invention.

[0056] Thus, the methods described herein may be embodied by one or more computer programs that may exist in various forms. For example, they may exist as software programs composed of program instructions for carrying out some of the steps of the method and may be embodied on a computer-readable medium. In other words, the relocation of the cutting elements may be performed according to instructions determined in the computer program. These instructions may include parameters, such as, for example, the intervals for relocating the cutting elements and the extent of each such relocation. The optimal values ​​of these parameters may be determined based on experience machining similar components and may be entered by an end user, for example found in a look-up table or the like. Alternatively, an algorithm may be used to calculate the optimal parameters based on input from the user, for example information regarding the type of insert used, the type of component to be manufactured and the "normal" wear when manufacturing such components, for example input related to the expected tool life when not using the method according to the invention. The expected tool life may be determined, for example, as machining time or number of components manufactured before the cutting insert is replaced. Such algorithms may be based on prior knowledge of wear propagation when manufacturing different components and materials. [Brief description of the drawings]

[0057] [Figure 1] 1 shows a cutting insert having CBN cutting elements. [Diagram 2] 1 shows a cross-sectional view of an unworn cutting element during machining. [Diagram 3] 1 shows worn cutting elements assuming stable tribological conditions. [Figure 4] 1 illustrates diagrammatically the effect of varying the nominal rake angle of a cutting element; [Diagram 5] 3 is a flow chart showing steps of a turning method according to the present invention; [Figure 6A-C] 1 illustrates a schematic diagram of an embodiment of a turning method. [Figure 7] 13 is a graph showing the change in effective rake angle in a test according to another embodiment of the turning method. [Figure 8] 8 is a graph referring to the same test as in FIG. 7, showing the progression of flank wear of the cutting element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] All figures are schematic and not necessarily to scale and generally show only the parts necessary to elucidate the respective embodiment, while other parts may be omitted or merely suggested. Unless otherwise indicated, similar reference symbols refer to similar parts in different figures.

[0059] Figure 1 shows a cutting insert 7 with a CBN cutting element 1 brazed thereto. The cutting insert is diamond shaped in this example and has fastening holes 8 that facilitate mounting of the insert in an insert pocket of a turning tool (not shown). A portion of the cutting element is shown in an enlarged view showing a top surface 6 including a chamfer 3 corresponding to the rake face of the cutting element and a clearance 4. Between the chamfer 3 and the clearance 4 a cutting edge 5 is formed.

[0060] 2 shows a cross section of a cutting element 1 when machining a metal workpiece 2, more precisely during a turning operation in which a chip 9 is formed and thus material is removed from the workpiece. The cutting element is arranged in a turning tool mounted on a CNC lathe and has a nominal rake angle γ n_1 and nominal clearance angle α n_1 The cutting element 1 is oriented relative to the workpiece at an effective rake angle γ e and the effective clearance angle α e are the nominal rake angles γ n_1 and nominal clearance angle α n_1 As can be seen, the initial rake angle of this cutting element is negative.

[0061] Figure 3 shows a worn cutting element 1' after assuming a stable tribological condition or state. In this condition, the effective rake angle and the effective clearance angle do not change significantly even if the wear propagates further. However, as can be seen, the effective rake angle γ e increases (now positive) due to crater wear, and the effective clearance angle α e has been reduced to 0 due to flank wear. The original unworn cutting element 1 is shown in dashed line. This steady state can be assumed quite early, for example after 30% of the total expected tool life, and continues for the remainder of the tool life. However, the integrity of the machined surface is affected, in part due to the increased contact between the flank and the workpiece.

[0062] FIG. 4 shows diagrammatically the effect of changing the nominal rake angle of a cutting element. When or before a stable tribological condition is assumed, the nominal rake angle may be changed to restore the effective clearance angle. In FIG. 4, the original unworn cutting element is shown in dashed lines, the worn cutting element 1′ is shown in dotted lines, and the repositioned worn cutting element 1″ is shown in solid lines. In this example, the nominal rake angle change Δγ n The repositioning that results is accomplished by tilting the cutting elements. However, as will be described below, repositioning of the cutting elements can be accomplished by other means.

[0063] In the following, the turning method for a CNC lathe is described with reference to FIG. 5, which is a flow chart showing the steps of the turning method, and with reference to FIGS. 6A-6C, which are schematic illustrations of one embodiment of the method.

[0064] In step 501, a workpiece 2 rotatable in a rotation direction R about its axis of rotation is provided.

[0065] In step 502, a turning tool 10 is provided extending along a tool axis L, in this case parallel to the X-axis of the lathe. The turning tool 10 comprises a cutting element 11 in the form of a CBN cutting insert. In contrast to the cutting element 1 shown in Figures 1-4, the cutting element 11 shown in Figures 6A-6C does not have a chamfer formed between the cutting edge and the upper surface of the cutting element. The cutting element can be positioned in different orientations relative to the workpiece, each orientation being determined by a nominal rake angle γ with respect to a line perpendicular to the surface of the workpiece at the point of contact between the cutting edge and the workpiece. n is determined by.

[0066] In step 503, the cutting element is rotated at a first nominal rake angle γ n_1 and nominal clearance angle α n_1 The cutting element 11 is positioned relative to the workpiece in a first orientation, as shown in FIG. 6A, determined by: If the cutting element 11 shown in FIG. 6A is an unworn cutting element, the nominal rake angle and nominal clearance angle would initially correspond to the effective rake angle and effective clearance angle, respectively. FIGS. 6B and 6C show subsequent machining steps where wear may have occurred on the rake and clearance faces such that the effective angles may differ from the nominal angles. However, for improved visibility and ease of understanding, the wear and effective angles are not shown in FIGS. 6A-6C.

[0067] In step 504, the workpiece is machined with the cutting element in a first orientation.

[0068] In step 505, the cutting element 11 is repositioned relative to the workpiece in another orientation, as shown in FIG. 6B, or relative to another workpiece to be machined. Thus, the cutting element 11 is repositioned relative to the workpiece in another orientation, as shown in FIG. 6B, with a different nominal rake angle γ n_2 and different nominal clearance angles α n_2 In this example, the new nominal rake angle γ n_2 is negative, i.e., the previous nominal rake angle γ n_1The repositioning of the cutting element 11 is accomplished by a translational movement of the turning tool 10 relative to the workpiece 2. This movement has a Y-axis component, i.e., it displaces the tool axis L in the Y-axis direction by a distance ΔY1. As a result, the nominal rake angle is decreased and the nominal clearance angle is increased (i.e., γ n_2 <γ n_1 , α n_2 >α n_1 ).

[0069] In step 506, the workpiece or another workpiece is machined with the cutting elements in the relocated orientation.

[0070] As indicated by arrow 507 in FIG. 5, the steps of repositioning the cutting element in a different orientation and machining the workpiece (or another workpiece) with the repositioned cutting element may be repeated several times.

[0071] 6C, the cutting element 11 has again been repositioned by translating the tool axis L in a direction having a component in the Y direction such that the tool axis L is displaced in the Y direction by a distance ΔY2. Thereby, the cutting element 11 is now repositioned relative to the previous nominal rake angle γ n_2 A nominal rake angle γ that is smaller or more negative than n_3 and the previous nominal clearance angle α n_2 Nominal clearance angle α larger than n_3 and are arranged as follows.

[0072] FIG. 7 shows how the effective rake angle of a cutting element can change during machining and how the effective rake angle is affected by multiple repositioning of the cutting elements. The graph shows the effective rake angle (γ n_1 , γ n_2 , γ n_3 and γ n_4FIG. 1 shows the results of a test in which four differently designed tool holders (providing a tool holder for each cutting element) were used to reposition the cutting element at different orientations relative to the workpiece. The cutting elements used in this test were CBN cutting elements with chamfers (i.e., similar to the cutting element shown in FIG. 1). The cutting elements were initially aligned at a first nominal rake angle γ n_1 (Therefore, the initial effective rake angle γ e ) was -36 degrees and the clearance angle (not shown in the graph) was 6 degrees. After 15 minutes of machining, the cutting elements were repositioned such that the nominal (and effective) rake angle was reduced by 6 degrees. Such repositioning was done again after 30 minutes of machining, and then again after 45 minutes of machining. Each of these repositionings, shown in dashed lines in FIG. 7, resulted in an immediate reduction in the effective rake angle by 6 degrees. As can be seen in the figure, the first repositioning was done before a stable condition was reached, and the second and third repositionings were done after such a stable condition was reached. In this example, the stable condition, where the effective rake angle was about 15 degrees, was fairly quickly reassumed after each of the repositionings done during 30 and 45 minutes of machining.

[0073] The vertical axis shows the average width of flank wear VB B The graph in Fig. 8 showing the propagation of flank wear 81 (indicated by dots) of the cutting elements in the test described above. The reference flank wear 82 (indicated by triangles) of the corresponding cutting elements that were not repositioned is also shown in the graph. As can be seen in Fig. 8, when using the method according to the invention, the average width of flank wear VB B approaches 200 μm after 60 min of machining, whereas this level of flank wear is already obtained after 36 min of machining without the use of the method.

Claims

1. A turning method for a CNC lathe, comprising: - providing a workpiece (2) rotatable in a rotational direction (R) about its axis of rotation; - providing a turning tool (10) extending along a tool axis (L), said turning tool comprising cutting elements (1, 11) including a rake face (3), a flank face (4) and a cutting edge (5) formed at the interface between said rake face (3) and said flank face (4), said cutting elements (1, 11) being positionable in different orientations relative to said workpiece (2), each orientation defining a nominal rake angle (γ ) relative to a surface of said workpiece (2); ｎ ) and the cutting elements (1, 11) have an effective rake angle (γ ) at the contact point between the cutting edge (5) and the workpiece (2), which depends on the wear of the cutting elements (1, 11). ｅ ) and effective clearance angle (α ｅ providing a - first effective rake angle (γ ｅ＿１ ) and the first effective clearance angle (α ｅ＿１ ) resulting in a first nominal rake angle (γ ｎ＿１ positioning the cutting element (1, 11) relative to the workpiece (2) in a first orientation determined by - in a first machining step, machining said workpiece (2) with said cutting elements (1, 11) in said first orientation; After the first machining step: - Second effective rake angle (γ ｅ＿２ ) and the second effective clearance angle (α ｅ＿２ ) resulting in a second nominal rake angle (γ ｎ＿２ ) and repositioning the cutting element (1, 11) relative to the workpiece (2) or to another workpiece to be machined in a second orientation determined by the second nominal rake angle (γ ｎ＿２ ) is the first nominal rake angle (γ ｎ＿１ a rearrangement step, different from - in a second machining step, machining said workpiece (2) or said further workpiece with said cutting elements (1, 11) in said second orientation; A turning method comprising:

2. The turning method of claim 1 , wherein the second nominal rake angle is smaller than the first nominal rake angle.

3. 3. The turning method according to claim 1, wherein the second effective clearance angle is equal to the first effective clearance angle.

4. 4. The turning method according to claim 1, wherein the second nominal rake angle differs from the first nominal rake angle by 2 to 10 degrees.

5. 5. The turning method of claim 1, wherein the step of repositioning the cutting element is performed when the cutting element is not engaged with the workpiece.

6. 6. A turning method according to claim 1, wherein the duration of each machining step is selected based on a predetermined period of time that the cutting element has been cutting.

7. 7. The turning method of claim 1, wherein the step of repositioning the cutting element at a second orientation comprises moving the tool axis relative to the workpiece in a plane perpendicular to the axis of rotation from a first tool axis position to a second tool axis position, the tool axis at the second tool axis position being parallel to but misaligned with the tool axis at the first tool axis position.

8. The turning method of claim 7 , wherein the tool axis is moved a first distance in a first direction from the first tool axis position to the second tool axis position.

9. After the second machining step: - repositioning the cutting element relative to the workpiece, or relative to another workpiece to be machined, in a third orientation determined by a third nominal rake angle resulting in a third effective rake angle and a third effective clearance angle, the third nominal rake angle being different from each of the first nominal rake angle and the second nominal rake angle; - in a third machining step, machining the workpiece or the further workpiece with the cutting element in the third orientation; The turning method according to claim 1 , further comprising:

10. 10. The turning method according to claim 9, wherein the step of repositioning the cutting element in the third orientation includes moving the tool axis a second distance in the first direction from the second tool axis position, away from the first tool axis position, to a third tool axis position.

11. The turning method of claim 10, wherein the second distance is the same as or less than the first distance.

12. 12. The turning method of claim 1, wherein the cutting elements comprise cubic boron nitride or polycrystalline cubic boron nitride.

13. 13. A turning method according to any one of claims 1 to 12, wherein the workpiece is made of hardened steel or heat-resistant superalloy having a hardness of 40 HRC or higher.

14. - CNC lathe, a processor; - a turning tool including cutting elements; 14. A system comprising: a processor configured to execute a method according to any one of claims 1 to 13.

15. A computer program comprising instructions which, when executed by a system according to claim 14, cause said system to carry out the method according to any one of claims 1 to 13.

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