Method for cutting a metal thread

Abstract

To improve the life of a tool by improving chip breaking or chip control.SOLUTION: In the machining method for forming a predetermined thread on a workpiece 19 for a CNC turning machine, a threading tool 1 is oscillated at a first frequency in the radial direction during a first pass P1. A method of processing comprising moving a threading tool between a first radial distance and a second radial distance, oscillating the threading tool radially at a second frequency during a second pass P2 so that the threading tool moves between a third radial distance and a fourth radial distance, and moving the tool radially without oscillation during a final pass P4.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to the technical field of metal cutting, and more particularly to the field of turning operations known as threading. [Background technology]

[0002] The present invention relates to a method according to the preamble of claim 1. In other words, the present invention relates to a machining method for a CNC lathe to form a predetermined thread in a workpiece, the method comprising the steps of providing a metal workpiece, providing a threading tool comprising a first cutting edge and a second cutting edge, rotating the metal workpiece about its axis of rotation, and moving the threading tool longitudinally through a set of passes, the longitudinal direction being parallel to or coincident with the axis of rotation, and oscillating the threading tool radially during the first pass at a first frequency so that the threading tool moves between a first radial distance and a second radial distance, the first radial distance being parallel to the axis of rotation. the first radial distance being greater than the second radial distance, oscillating the threading tool radially at a second frequency during a second pass such that the threading tool moves between a third radial distance and a fourth radial distance, and wherein the third radial distance is greater than the fourth radial distance and less than the first radial distance, the fourth radial distance being less than the second radial distance, and the third radial distance being greater than the second radial distance, and during a final pass, moving the tool radially without oscillation.

[0003] In threading, threads are formed. Threading may be performed in a turning operation. A metal workpiece is rotated. A threading tool is moved longitudinally relative to the rotating workpiece. The threads are gradually formed through one or many passes.

[0004] EP3241637A1 discloses a method for threading a workpiece in which the threading tool oscillates in the radial or X direction, which method is believed to reduce the problem of long chip strips.

[0005] The present inventors have discovered that there is a need for further improved methods for threading. Summary of the Invention

[0006] An object of the present invention is to improve chip breaking or chip control to improve tool life.

[0007] At least one of these objects is achieved by the method defined at the beginning, which method further includes setting the second frequency different from the first frequency, and arranging the second pass such that, for each oscillation of the second pass, the locus of the second pass intersects the locus of the first pass two or more times.

[0008] In this way, the chips can be shortened. The chips separate at least during the second pass as the trajectory of the second pass intersects with the trajectory of the first pass. In other words, the chips separate when the threading insert finishes cutting. For each oscillation, the chips break off as they finish cutting.

[0009] Chip breaking is further improved for the second pass because the first or leading cutting edge, positioned to cut the first thread flank, and the second or trailing cutting edge, positioned to cut the second thread flank, are selectively in the cut. More precisely, during the cut, the connecting cutting edge and the first cutting edge, which may be in the form of a convex cutting edge, or the connecting cutting edge and the second cutting edge, are in the cut. The connecting cutting edge should be understood as the part of the cutting edge that forms the root of the thread. This results in chips with a more favorable shape due to the chip cross-section. A method in which all passes have the same oscillation frequency results in chips whose cross-section can generally be described as V-shaped. In other words, both the first and second cutting edges, as well as the connecting cutting edge, are in the cut simultaneously. This described shape is particularly suitable for common thread profiles, such as V-profile 55- or 60-degree threads, or metric or UN60-degree threads. Such V-shaped chips are geometrically undesirable and result in poor chip control. Furthermore, V-shaped chips increase the risk of edge buildup on the rake face of the insert, resulting in reduced tool life. Such V-shaped chips are produced for oscillation pass number 2 shown in Figure 3 of EP 3241637 A1.

[0010] The inventors have found that with such a method, more than one point along the cutting edge enters and exits the workpiece during the second pass, which is preferable compared to methods that report one point along the cutting edge entering and exiting the workpiece, and the inventors have found that this point is more susceptible to wear.

[0011] During the second pass, the first and second cutting edges are selectively active, thereby spreading the wear along the cutting edges relatively evenly, which is favorable for tool life.

[0012] The machining method is a metal cutting method. A CNC lathe is any computer or computer numerically controlled machining tool suitable for turning. A thread or screw thread is a helical structure. The thread extends between a thread start and a thread end. The thread has a constant or substantially constant pitch, i.e., the distance along the axis of rotation that the thread travels per revolution, i.e., the number of threads per unit length. The thread can be straight, i.e., cylindrical, or tapered, i.e., conical. The thread can be external or internal. The thread can be right-handed or left-handed. The cross-sectional shape of the thread is called the thread form. The thread form or thread profile can have various shapes, such as ISO, trapezoidal, UNC, etc. The thread form preferably includes a first thread flank, a second thread flank, a crest, and a root.

[0013] The thread is preferably a single start thread. The metal workpiece is, for example, made of steel. The metal workpiece is preferably at least partially cylindrical or substantially cylindrical. The threading tool preferably comprises a tool body and a threading insert. The threading insert is preferably at least partially made of a wear-resistant material, such as, for example, cemented carbide. The threading tool, preferably the threading insert, comprises a cutting edge.

[0014] The cutting edges include a first cutting edge and a second cutting edge. The first and second cutting edges are preferably connected by a connecting cutting edge. The connecting cutting edge is preferably arranged to cut the thread root. The connecting edge may be convex in top view. The connecting edge may have a shape that is an arc in top view. The first cutting edge is preferably arranged to cut the first thread flank. The second cutting edge is preferably arranged to cut the second thread flank.

[0015] The shape of the cutting edge preferably corresponds or substantially corresponds to the thread form.

[0016] The threading tool, preferably the threading insert, preferably comprises one or more teeth.

[0017] The metal workpiece is rotated in one direction about its axis of rotation, where the axis of rotation preferably coincides with the longitudinal axis of symmetry of the metal workpiece.

[0018] The method includes moving a threading tool longitudinally in a set or series of passes. The thread is formed through the set of passes. The number of passes is preferably 3 to 25, and even more preferably 4 to 20. For each pass, material is removed from the metal workpiece. The final pass produces the final shape of the thread, i.e., the predetermined thread.

[0019] This method can be used with radial infeed, flank infeed, or with alternating flank infeed.

[0020] The longitudinal direction is parallel to the axis of rotation of the metal workpiece. For each pass, the threading tool moves longitudinally in one direction to remove metal from the metal workpiece through metal cuts. At the end of each pass, the threading tool finishes the cut by retracting the threading tool, i.e., moving the threading tool in one direction away from the metal workpiece. Before the subsequent pass, the threading tool moves longitudinally in the opposite direction without making a cut.

[0021] In each pass, the threading tool moves from the thread start to the thread end, or from a position near the thread start to a position near the thread end. In other words, the threading tool enters the cut near the thread start and finishes the cut near the thread end. Near in this context is understood to mean within one revolution of the thread start.

[0022] If the thread is an external thread, these passes gradually approach the axis of rotation. In other words, during the first pass, the threading tool is, on average, at a greater radial distance from the axis of rotation than during the second pass. During the last pass, the threading tool is, on average, at a shorter radial distance from the axis of rotation than during the penultimate pass.

[0023] Moving the threading tool longitudinally, i.e. along the Z-axis, is called the longitudinal feed. The longitudinal feed is preferably constant on average during all passes, i.e. has the same mean value. This mean value is preferably equal to the thread pitch. The longitudinal feed during the last pass is constant, i.e. has a constant value.

[0024] During the first pass, the threading tool oscillates radially at a first frequency. This oscillation is preferably a periodic oscillation having a periodic waveform, such as a triangular or sinusoidal waveform. This waveform includes peaks and valleys. This oscillation of the threading tool oscillates between peaks and valleys. If the thread is an outer thread, the peaks are radially outward and the valleys are radially inward.

[0025] During the first pass, the threading tool makes a forward movement between a first radial distance and a second radial distance, and a rearward movement between the second radial distance and the first radial distance, the forward movement being toward the axis of rotation if the thread is an external thread, or away from the axis of rotation if the thread is an internal thread.

[0026] The first frequency is preferably constant. The frequency means one oscillation per unit time or time unit. For example, if one oscillation takes 2 seconds, the frequency is 0.5 seconds. -1 is.

[0027] Preferably, during the first pass, the threading tool oscillates between once for every revolution of the metal workpiece, ie, one period, and once for every four revolutions of the metal workpiece.

[0028] Even more preferably, during the first pass, the threading tool oscillates once for every two revolutions of the metal workpiece, i.e., one cycle, where one cycle is understood to start at a first radial distance, move to a second radial distance, and return to the first radial distance.

[0029] The time for one revolution of the metal workpiece about its axis of rotation is preferably equal to the time for one oscillation, or a multiple thereof, or equal to the time for one oscillation divided by an integer.

[0030] The oscillation of the threading tool is radial, such that the threading tool moves during a first pass between a first radial distance from the last pass and a second radial distance from the last pass. The first radial distance corresponds to a crest, and the second radial distance corresponds to a valley. These distances are from the last pass. The amplitude between the crests of the oscillation during the first pass is equal to the difference between the first radial distance and the second radial distance. Regardless of whether the thread is straight or tapered, these radial distances are perpendicular to the axis of rotation of the metal workpiece. The oscillation during each oscillation pass causes the threading tool to repeatedly engage and disengage from the metal workpiece, producing chips.

[0031] Oscillating during the first pass means that the depth of cut or infeed changes during the first pass. The active portion of the cutting edge of the threading tool changes during the first pass.

[0032] Preferably, if the thread is an external thread, the first radial distance is greater than the radius of the metal workpiece, in other words, preferably the locus of the first pass intersects the circumferential or outer surface of the metal workpiece.

[0033] The first pass is therefore preferably an interrupted cut, i.e. the cutting edge is intermittently active, i.e. it cuts the metal intermittently, the effect of which is to make the chip shorter during the first pass.

[0034] The threading tool motion during the first pass therefore includes two components: one longitudinal component in which the threading tool moves in one direction along or parallel to the longitudinal Z axis, and one radial component in which the threading tool oscillates radially, perpendicular to the axis of rotation.

[0035] After the first pass, the threading tool moves in one longitudinal direction without making a cut, opposite the longitudinal direction of the first pass.

[0036] During the second pass, the threading tool is oscillated radially at a second frequency different from, and even more preferably higher than, the first frequency, thereby further reducing the chip length during the second pass, the second frequency preferably being a multiple of the first frequency.

[0037] Preferably, during the second pass, the threading tool oscillates once for each revolution of the metal workpiece, ie, one cycle.

[0038] The oscillation of the threading tool is such that the threading tool moves between a third radial distance and a fourth radial distance from the last pass. During the second pass, the threading tool moves longitudinally in a direction along the Z axis, which is the same direction as for the first pass.

[0039] The third radial distance is greater than the fourth radial distance, and preferably is less than the outer or circumferential surface of the metal workpiece if the threads are external threads.

[0040] The first radial distance is greater than the third radial distance, and the second radial distance is greater than the fourth radial distance. In other words, the second pass, if the thread is an outer thread, is at a smaller diameter, on average, than the first pass.

[0041] The difference between the first radial distance and the second radial distance, i.e., the radial distance, is preferably different from the difference between the third radial distance and the fourth radial distance.

[0042] The locus of the second pass intersects the locus of the first pass. Preferably, for each period of the second pass, i.e., for each oscillation of the second pass, the locus of the second pass intersects the locus of the first pass twice. In such a case, the locus of the second pass is such that the sequence of entering and leaving each cut corresponds to the backward movement of the threading tool during the first pass or the forward movement of the threading tool during the first pass. In other words, both of each adjacent pair of intersections are on the same side along the longitudinal or Z-axis with respect to the nearest or adjacent point along the Z-axis corresponding to the second radial distance.

[0043] The trajectory of the second pass preferably crosses once during the forward motion and once during the backward motion, where backward and forward are opposite directions relative to the axis of rotation, with one crossing representing the end of the cut or the start of the cut.

[0044] Preferably, the time or distance that the trajectory of the second pass is radially below the trajectory of the first pass is greater than the time or distance that the trajectory of the second pass is radially above the trajectory of the first pass if the thread is an outer thread, where, for an outer thread, radially above means radially outward or a greater radial distance, and radially below means radially inward or a lesser radial distance.

[0045] During the last pass, the threading tool moves linearly, i.e., radially without oscillation. This linear motion is parallel to the axis of rotation if the thread is a cylindrical thread. The longitudinal feed during the last pass is equal to the pitch of the thread. The last pass forms the final shape of the thread.

[0046] According to one embodiment, the method further comprises setting the second frequency to be twice or substantially twice as large as the first frequency.

[0047] In other words, the second frequency is twice the first frequency.

[0048] This results in shorter chips during the second pass because chip length decreases with increasing frequency. Chips break every time the insert completes a cut. During the second pass, the insert completes one cut and enters one cut with each oscillation. In other words, the trajectory of the second pass intersects with the trajectory of the first pass twice with each oscillation. This results in shorter chips during the second pass because chip length decreases with increasing frequency if the chip breaks with each oscillation.

[0049] According to one embodiment, the method further includes setting a phase of the oscillation during the second pass such that the fourth radial distance or distances coincide with the first and second radial distances along the Z axis.

[0050] In such a manner, the wear of the insert is more evenly distributed to both the first and second cutting edges during the second pass, thereby increasing tool life.

[0051] Coincidence in this context can be understood as coincidence along the Z axis, i.e., longitudinal coincidence. In other words, the peaks of the second pass defined by the third radial distance are midway longitudinally between adjacent peaks and valleys of the first pass, while the peaks of the first pass are defined by the first radial distance and the valleys of the first pass are defined by the second radial distance.

[0052] According to one embodiment, the method further comprises: during a third pass, oscillating the threading tool radially at a third frequency such that the threading tool moves between a fifth radial distance and a sixth radial distance, wherein the fifth radial distance is greater than the sixth radial distance and less than the third radial distance, and wherein the sixth radial distance is less than the fourth radial distance, setting the third frequency different from the second frequency, setting the fifth radial distance greater than the fourth radial distance; arranging the third pass such that for each oscillation of the third pass, the locus of the third pass crosses the locus of the second pass two or more times; and setting the frequency of the third pass to be less than the frequency of the second pass or less than the frequency of the first pass.

[0053] In such a manner, the oscillation frequency can be set relatively low compared to methods where the frequency is continuously increased, thereby avoiding machine limitations or problems with the high frequency control and / or mechanics of the CNC machine.

[0054] In such a way, the chip can be interrupted during the third pass.

[0055] During the third pass, the threading tool oscillates radially at a third frequency. This third frequency is preferably different from the second frequency. Preferably, the third frequency is equal to or substantially equal to the first frequency. Preferably, the third frequency is less than the second frequency. The third frequency is preferably half the second frequency. The oscillation of the threading tool is such that the threading tool moves between a fifth radial distance and a sixth radial distance from the last pass. The third pass may or may not be the penultimate pass.

[0056] During the third pass, the threading tool moves longitudinally in a direction along the Z axis, which is the same direction as for the first and second passes.

[0057] The fifth radial distance is greater than the sixth radial distance.

[0058] The fifth radial distance is less than the third radial distance.

[0059] The sixth radial distance is less than the fourth radial distance.

[0060] The locus of the third path intersects with the locus of the second path. Preferably, for each period of the third path, the locus of the third path intersects with the locus of the second path twice. Preferably, the time or distance that the locus of the third path is below the locus of the second path is longer than the time or distance that the locus of the third path is above the locus of the second path.

[0061] The threading tool has a forward movement between a fifth radial distance and a sixth radial distance and a rearward movement between the sixth radial distance and the fifth radial distance.

[0062] According to one embodiment, the method further comprises setting the final oscillation pass to have a frequency greater than those of all previous oscillation passes.

[0063] In this way, chips near the root of the thread can be shortened, which is preferable because chip problems are more likely to occur near the root.

[0064] The last oscillating pass is the penultimate pass, during which the threading tool oscillates radially between an outer radial distance and an inner radial distance from the last pass, where the inner radial distance is zero or less than half the difference between the first radial distance and the second radial distance.

[0065] In other words, during the penultimate pass, the threading tool oscillates radially between two radial distances from the last pass, where the radial distance of said radial distance radially closest to the last pass is zero, i.e., intersects with the last pass, or is less than half the difference between the first and second radial distances. Preferably, this radial distance is less than 0.10 mm, even more preferably less than 0.06 mm.

[0066] In other words, preferably during the penultimate pass the threading tool oscillates radially such that the trajectory of the penultimate pass intersects or substantially intersects the trajectory of the last pass, where substantially in this context should be understood as within 0.10 mm, and even more preferably within 0.06 mm.

[0067] This allows for further improved chip control for both the penultimate and final passes.

[0068] The penultimate pass may be the third pass. Alternatively, the penultimate pass may be the fourth pass, fifth pass, or even higher pass. The penultimate pass is the pass immediately before the last pass.

[0069] Preferably, the second radial distance is greater than the fifth radial distance.

[0070] Preferably, the oscillation frequency during the penultimate pass is no more than eight times the oscillation frequency during the first pass. Even more preferably, the oscillation frequency during the penultimate pass is no more than five times the oscillation frequency during the first pass. This is particularly advantageous when the number of oscillation passes is large, e.g., three or more or five or more oscillation passes, especially when compared to exponentially increasing oscillation frequencies. As such, this method may be used with more types of CNC machines, since not all CNC machines are capable of multi-frequency radial oscillation. The inventors have recognized that maintaining a relatively low oscillation frequency reduces the risk of damage to moving parts of the CNC lathe.

[0071] The peak-to-peak amplitude for all of the oscillation passes is preferably less than 1.0 mm. In other words, the difference between the first radial distance and the second radial distance is preferably less than 1.0 mm. Preferably, all of the oscillation passes are all passes except the last pass.

[0072] The inventors have recognised that a relatively low amplitude between peaks reduces the risk of damage to the moving parts of the CNC lathe.

[0073] According to one embodiment, the method further includes setting the frequency for all oscillation passes subsequent to the first pass to no more than four times the frequency of the first pass.

[0074] In such a way, the maximum frequency can be set relatively low, which reduces the risk of damaging the moving parts of the CNC lathe.

[0075] According to one embodiment, the method further comprises oscillating the threading tool during the penultimate pass between an outer radial distance and an inner radial distance from the last pass, wherein the inner radial distance is zero or less than half the difference between the first radial distance and the second radial distance.

[0076] Such a method improves chip control during the final pass.

[0077] The penultimate pass may be the third pass or a higher order pass, where the fifth radial distance is greater than the sixth radial distance, the fifth radial distance is less than the third radial distance, and the sixth radial distance is less than the fourth radial distance.

[0078] According to one embodiment, the method further includes setting the frequency for the penultimate pass to be greater than the frequency of the first pass and greater than the frequency of the second pass.

[0079] Such a method results in shorter chips during the penultimate pass, which is preferable because the chances of chip problems are higher near the root of the thread.

[0080] According to one embodiment, the difference between the first radial distance and the second radial distance is different from the difference between the third radial distance and the fourth radial distance.

[0081] Such methods may further improve chip breaking or chip control and / or tool life.

[0082] In other words, the amplitude of the oscillation during the first and second passes is different. Preferably, the amplitude of the oscillation during the first pass is longer than that during the second pass. This is preferable to increase the chances of the workpiece exiting during all of the oscillations during the first pass. The outer surface may have a non-uniform diameter.

[0083] In other words, the peak-to-peak amplitude of the oscillation during the first pass is different from the peak-to-peak amplitude of the oscillation during the second pass. Preferably, the difference between the first radial distance and the second radial distance is less than the difference between the third radial distance and the fourth radial distance. In other words, preferably, the peak-to-peak amplitude of the oscillation during the first pass is less than the peak-to-peak amplitude of the oscillation during the second pass.

[0084] Preferably, the difference between the fifth radial distance and the fourth radial distance is less than the difference between the third radial distance and the fifth radial distance.

[0085] According to one embodiment, the difference between the first radial distance and the third radial distance is smaller than the difference between the third radial distance and the second radial distance.

[0086] Such methods may further improve chip breaking or chip control and / or tool life.

[0087] According to one embodiment, the third frequency is the same or substantially the same as the first frequency.

[0088] Preferably, the first and third paths are in phase, ie, there is no phase shift.

[0089] According to one embodiment, the second radial distance and the sixth radial distance are coincident or substantially coincident in the longitudinal direction.

[0090] Such methods may further improve chip breaking or chip control and / or tool life.

[0091] In other words, the valleys of the oscillations during the first pass are aligned longitudinally with the valleys of the first pass.

[0092] Preferably, the first pass and the third pass have the same or substantially the same frequency. Preferably, there is no phase shift between the first pass and the third pass, i.e., the phase shift is zero or substantially zero. The longitudinal direction is the Z direction.

[0093] According to one embodiment, for every pass, the threading tool is moved longitudinally at a longitudinal feed rate of 0.5 to 3.0 mm / rev.

[0094] Such a method allows threads having a pitch of 0.5 to 3.0 mm to be processed more efficiently.

[0095] Preferably, the longitudinal feed rate is the same for all passes. In other words, the longitudinal feed rate preferably has a constant value that is the same for all passes. This value is equal to the thread pitch. The longitudinal direction is along or parallel to the Z axis.

[0096] According to one embodiment, a threading tool includes a threading insert and an insert seat, wherein the threading insert includes a top surface and a bottom surface, the top surface and the bottom surface being connected by a side surface, a first cutting edge and a second cutting edge being formed at the interface between the top surface and the side surface, and the bottom surface including an engagement means for contact with a corresponding structure formed in the insert seat.

[0097] Such a method results in a significant improvement in the surface quality of the machined surface. Such a method reduces the risk of the insert moving. For all oscillating passes except the first pass, the first and second cutting edges are selectively in the cut, resulting in a variation in the direction of the cutting force. This increases the risk of the insert moving and the risk of poor surface quality. Engagement means on the bottom surface of the insert reduce this risk. Other engagement surfaces, such as flanks, may be present. It has been found that threading inserts with flat bottom surfaces and / or contact surfaces located only on the flanks significantly increase the risk of poor surface quality of the threads when using the method described herein.

[0098] The threading insert is preferably made at least in part of a wear-resistant material, such as, for example, cemented carbide. The threading insert comprises a cutting edge. The shape of the cutting edge preferably corresponds or substantially corresponds to the thread form. The top surface comprises a rake face. The cutting edge is formed at the interface between the top surface and the side surface. The side surface comprises a flank face. The top surface preferably comprises chip-breaking or chip-forming means, preferably in the form of one or more protrusions and / or recesses.

[0099] The bottom surface of the threading insert includes corresponding mating means, i.e., mating means for contacting or cooperating with corresponding structure formed in the insert seat, also known as the insert pocket, which prevents movement, particularly rotation, of the threading insert when seated in the insert seat.

[0100] The threading insert is preferably mountable to the insert seat using a clamping member, such as a screw, bolt, or top clamp, and may preferably include a through hole for the screw or bolt, where the through hole extends between the top and bottom surfaces of the threading insert.

[0101] The mating means formed in the bottom surface of the threading insert may be in the form of one or more cavities, such as, for example, one or more grooves. Alternatively, the mating means may be in the form of one or more protrusions. Alternatively, the mating means may be in the form of one or more ramps.

[0102] If the mating means formed in the bottom surface of the threading insert is in the form of a cavity, such as a groove, the corresponding mating means formed in the insert seat is preferably in the form of a protrusion, such as a ridge.

[0103] The threading tool, in particular the bottom of the threading insert and the insert seat, may preferably be arranged according to that shown and described in EP 1 935 539 A1, which is incorporated herein by reference.

[0104] According to one aspect of the invention, there is provided a computer program having instructions which, when executed by a CNC lathe, cause the CNC lathe to perform the steps according to any of the preceding methods.

[0105] This computer program or computer program product may be comprised in a CAM software product, i.e. software for computer aided manufacturing. This computer program may be in the form of a computer readable medium such as a USB stick, a CD-ROM or a data stream.

[0106] The method may include more passes than those described above. For example, according to one embodiment, during the fourth pass, the threading tool oscillates radially at a fourth frequency such that the threading tool moves between a seventh radial distance and an eighth radial distance from the last pass, where the seventh radial distance is greater than the eighth radial distance, the seventh radial distance is less than the fifth radial distance, and the eighth radial distance is less than the sixth radial distance.

[0107] Such methods further improve chip breaking or chip control and / or tool life, and may allow threads with wider cross sections to be machined in a less risky and / or more economical manner.

[0108] Preferably, the difference between the seventh radial distance and the eighth radial distance is smaller than the radial distance between the third radial distance and the fourth radial distance. In other words, preferably, the amplitude between the peaks of the fourth pass is smaller than the amplitude between the peaks of the second pass. Preferably, the difference between the seventh radial distance and the eighth radial distance is smaller than the difference between the first radial distance and the second radial distance. In other words, preferably, the amplitude between the peaks of the fourth pass is smaller than the amplitude between the peaks of the first pass. Preferably, the difference between the seventh radial distance and the eighth radial distance is larger than the difference between the fifth radial distance and the sixth radial distance. In other words, preferably, the amplitude between the peaks of the fourth pass is larger than the amplitude between the peaks of the third pass. Preferably, the fourth frequency is the same or substantially the same as the second frequency. Preferably, there is no phase shift between the oscillations during the second pass and the oscillations during the fourth pass.

[0109] Preferably, the seventh radial distance is greater than the sixth radial distance. Preferably, the locus of the fourth path intersects with the locus of the third path.

[0110] The method may include more passes. According to one embodiment, during the fifth pass, the threading tool oscillates radially at a fifth frequency such that the threading tool moves between a ninth radial distance and a tenth radial distance from the last pass, where the ninth radial distance is greater than the tenth radial distance and the tenth radial distance is less than the eighth radial distance.

[0111] Such methods further improve chip breaking or chip control and / or tool life, and may allow threads with wider cross sections to be machined in a less risky and / or more economical manner.

[0112] Preferably, the difference between the ninth radial distance and the tenth radial distance is smaller than the difference between the third radial distance and the fourth radial distance. Preferably, the difference between the ninth radial distance and the tenth radial distance is smaller than the difference between the first radial distance and the second radial distance. Preferably, the difference between the ninth radial distance and the tenth radial distance is greater than the difference between the fifth radial distance and the sixth radial distance. Preferably, the difference between the ninth radial distance and the tenth radial distance is greater than the difference between the seventh radial distance and the eighth radial distance.

[0113] The fifth frequency is preferably different from the fourth frequency.

[0114] If the fifth pass is the penultimate pass, the fifth frequency is preferably greater than the first frequency. If the fifth pass is the penultimate pass, the fifth frequency is preferably greater than the second frequency.

[0115] If the fifth pass is not the penultimate pass, the fifth frequency is preferably equal to or substantially equal to the first frequency.

[0116] The present invention will be described in more detail below with reference to embodiments of the invention and the accompanying drawings. [Brief explanation of the drawings]

[0117] [Figure 1] 1 is a perspective view of a metal workpiece and clamping jaws showing the X and Z axes. [Figure 2] FIG. 1 is a side view of the threading method, showing four passes. [Figure 3]FIG. 3 is a perspective view of the threading tool in FIG. 2. [Figure 4] 1 is a schematic diagram of a first embodiment of the present invention, showing the path traced by a threading tool as viewed in the Z-axis direction. [Figure 5] FIG. 2 is a schematic diagram showing the position of a threading tool in relation to a workpiece in a first embodiment of the present invention. [Figure 6] FIG. 10 is a schematic diagram showing the position of a threading tool in relation to a workpiece in a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0118] Reference is made to FIG. 1 , which shows a metal workpiece 19 connected to a spindle (not shown) of a CNC lathe (not shown) by clamping means 22 in the form of clamping jaws. The clamping means 22 is connected to the spindle and contacts a peripheral surface 21 of the metal workpiece 19. Rotation of the spindle causes rotation of the metal workpiece 19. The clamping means 22 may be of other configurations. The metal workpiece 19 is cylindrical in this case. However, the metal workpiece 19 may have other shapes. An X-axis and a Z-axis are shown. Depending on the type of CNC lathe, such as the CNC lathe on which the bar feeder equipment is configured, the metal workpiece 19 may be movable in the Z-axis. For other CNC lathes, the metal workpiece 19 may not be movable in the Z direction.

[0119] Referring now to FIG. 2, a threading tool 1 and a metal workpiece 19 are shown. The threading tool 1 includes a threading insert 2 and a tool body 9. The threading insert 2 includes a cutting edge 10. The metal workpiece 19 rotates in one direction 20 about its axis of rotation A1. The CNC lathe (not shown) includes a spindle (not shown). The metal workpiece 19 is clamped in clamping means 22 in the form of clamping jaws. A helical thread is formed on the circumferential or outer surface 21 of the metal workpiece 19. In FIG. 2, the circumferential surface 21 is the radially outward surface. In other words, the thread is a male thread. Alternatively, the thread may be a female thread. The thread form includes a first thread flank 15, a second thread flank 16, a crest or thread crest 17, and a root or thread root 18.

[0120] The thread is formed through multiple passes P1 to P4. The number of passes can vary. In this case, the fourth pass P4 is the final pass. During each pass P1 to P4, the cutting edge 10 removes metal from the metal workpiece 19 by metal cutting. The threading tool 1 is moved in the same longitudinal direction during all passes P1 to P4. In other words, the threading tool 1 is moved along the rotation axis A1 and / or along the Z axis. The threading tool 1 is preferably moved at the same or substantially the same longitudinal feed rate, i.e., along the Z axis, during all passes. This feed rate is equal to the thread pitch. During all passes except the last pass P4, the threading tool 1 oscillates along the X axis. During the last pass P4, the threading tool 1 does not oscillate. Between passes, the threading tool 1 is retracted to its starting position, i.e., the position along the rotation axis A1 where the thread begins. The arrows in FIG. 2 for passes P1 to P3 are horizontal. However, in detail, these arrows are not horizontal but have a slightly wavy shape, which is explained in Figures 5 and 6.

[0121] The CNC lathe (not shown) is controlled by a computer program. This computer program includes readable and executable code. This code includes information regarding the relative movement of the threading tool 1 with respect to the metal workpiece 19 and with respect to the rotation of the metal workpiece 19 about its axis of rotation A1. In other words, the computer program includes instructions for, for example, the number of passes, the radial oscillation, the longitudinal feed rate, and the rotation of the workpiece.

[0122] FIG. 3 shows the threading tool 1 in FIG. 2. The threading tool 1 includes a tool body 9 and a threading insert 2. An insert seat 6 is formed in the tool body 9. The threading insert 2 includes three teeth. Only one tooth is mounted in an active state, i.e., the tooth is mounted so that it can cut the thread. The threading insert 2 is clamped to the insert seat 6 using a screw 14. The threading insert 2 is indexable, so that the next tooth can be set to the active position by rotating it 120° or 240°. Each tooth includes a cutting edge. Each cutting edge includes a first cutting edge 11 and a second cutting edge 12 connected by a connecting cutting edge 13. The connecting cutting edge 13 is arranged to cut the thread root.

[0123] The bottom surface of the threading insert comprises engagement means in the form of a groove 7. The insert seat comprises ridges 8. The shape of one groove 7 corresponds or substantially corresponds to the shape of this ridge 8.

[0124] FIG. 4 is a schematic diagram showing paths P1 to P4 traced by the threading tool 1 on the workpiece 19, viewed in the Z-axis direction. The cutting conditions are adapted so that the number of oscillations of the threading tool 1 is one for two spindle revolutions for the first and third passes P1 and P3, and one for one spindle revolution for the second pass P2. One oscillation is for the first pass P1, from D1 to D2 and back to D1. One oscillation is for the second pass P2, from D3 to D4 and back to D3. One oscillation is for the third pass P3, from D5 to D6 and back to D5. There is no oscillation for the fourth pass P4. After the second pass P2, the metal workpiece, in cross section, i.e., when viewed in the Z-axis direction, has a shape close to an ellipse, or more precisely, an ellipse with two cutouts.

[0125] During the first pass P1 and the third pass P3, there is one air cut for every two revolutions of the metal workpiece 19. The air cut occurs from when the threading tool finishes cutting until when the threading tool enters cutting. The trajectory of the first pass P1, i.e., the path traced by the threading tool 1 during the first pass P1, intersects with the peripheral surface 21 of the metal workpiece 19.

[0126] FIG. 5 shows a graphic illustrating the position of the cutting tool in how a thread is formed through four passes P1 to P4, as in FIG. 2, for example. The vertical axis in FIG. 2 is the X-axis of the CNC lathe, i.e., the radial direction. The horizontal axis in FIG. 2 is the Z-axis of the CNC lathe. The horizontal axis can also be understood as time. As can be seen, all passes P1 to P3 except for the final pass P4 are wavy or corrugated. The vertical movement of the threading tool during passes P1 to P4, i.e., movement along the Z-axis, is represented by the extension along the horizontal axis relative to the line representing passes P1 to P4. The oscillation of the threading tool in the radial direction, i.e., the X-axis direction, during passes P1 to P4 is represented by the extension along the vertical axis in FIG. 5, i.e., the X-axis of the CNC lathe. The peripheral surface 21 is shown as a line. The radial distance from the peripheral surface to the final pass P4 is shown as D11. This radial distance D11 can be understood as the radial distance between the thread root and the peripheral surface 21.

[0127] During the first pass P1, the threading tool oscillates radially between a first radial distance D1 and a second radial distance D2 from the last pass P4. The difference or radial distance between these first and second radial distances D1, D2 is the peak-to-peak amplitude of the oscillation during the first pass P1. This oscillation has a first frequency F1. The period or period of the first pass P1 is shown to be B1. During the first pass P1, the threading tool is shown to exit and enter cutting once per oscillation. In other words, the locus of the first pass P1 intersects the peripheral surface 21 of the metal workpiece twice for each oscillation, i.e., twice per period. The time that the line representing the first pass P1 is above the line representing the peripheral surface 21 is the time that the cutting edge is inactive during the first pass P1, i.e., air time.

[0128] During the second pass P2, the threading tool oscillates radially between a third radial distance D3 and a fourth radial distance D4 from the last pass P4. The difference or radial distance between the third radial distance D3 and the fourth radial distance D4 is the peak-to-peak amplitude of the oscillation during the second pass P2. The period or duration of the second pass P2 is B2. The period B2 represents one rotation of the metal workpiece. The period B2 is different from the period B1. More specifically, the period B2 is shorter than the period B1. In other words, the frequency of the oscillation during the second pass P2 is greater than the frequency of the oscillation during the first pass P1. The trajectory of the second pass P2 intersects with that of the first pass twice for each period. The trajectory of the second pass P2, i.e., the path traced by the threading tool 1 during the second pass P2, does not intersect with the peripheral surface 21 of the metal workpiece 19. The time that the line representing the second pass P2 lies above the line representing the first pass P1 is the time the cutting edge is inactive during the second pass P2, i.e., air time. In other words, there is one air cut for each revolution of the metal workpiece 19 during the second pass P2. During the third pass P3, the penultimate pass, the threading tool oscillates radially from the last pass P4 between a fifth radial distance D5 and a sixth radial distance D6. The sixth radial distance D6 is zero or less than half the difference between the first radial distance D1 and the second radial distance D2, preferably less than 0.10 mm. Reaching a depth of cut that is zero or near zero improves the likelihood of chip shortening during the subsequent and final pass, i.e., the fourth pass P4. The difference or radial distance between the fifth radial distance D5 and the sixth radial distance D6 is the peak-to-peak amplitude of the oscillation during the third pass P3. The third pass P3 has a period or duration B3. The period B3 is different from the period B2. More specifically, the period B3 is longer than the period B2. In other words, the frequency of the oscillation during the second pass P2 is greater than the frequency of the oscillation during the third pass P3. The period B3 is the same as or substantially the same as the period B1. The trajectory of the third pass P3 intersects the trajectory of the second pass P2 twice for each period. The time that the line representing the third pass P3 is above the line representing the second pass P2 is the time that the cutting edge is inactive during the third pass P3, i.e., air time.In FIG. 5, it can be seen that the third radial distance D3 is greater than the fourth radial distance D4, the first radial distance D1 is greater than the third radial distance D3, the second radial distance is greater than the fourth radial distance D4, the fifth radial distance D5 is greater than the sixth radial distance D6, the fifth radial distance D5 is less than the third radial distance D3, and the sixth radial distance D6 is less than the fourth radial distance D4. During the first pass P1, the first and second cutting edges are simultaneously in the cut, resulting in a chip with a V-shaped cross section. During the second pass P2, if the feed direction is right to left, the first cutting edge is in the cut from the lowest point of the second pass P2 to the left of that lowest point until it finishes cutting. Upon entering the cut, the second cutting edge is in the cut until the second pass reaches its lowest point, corresponding to D4, at which point the first cutting edge enters the cut. Thus, as long as the turning tool is in cutting during the second pass P2, the first and second cutting edges are selectively active or in cutting. During the third pass P3, the first cutting edge is in cutting from a point to the right corresponding to D6 to a point along the trajectory of the third pass P3 that is perpendicular to the right crest of the trajectory of the second pass P2, where the second cutting edge enters the cut. The turning tool then ceases cutting. Upon entering the cut, the first cutting edge is active until a point along the trajectory of the third pass P3 that is perpendicular to the left crest of the trajectory of the second pass P2, where the second cutting edge enters the cut.

[0129] Now, reference is made to FIG. 6. FIG. 6 shows a similar graphic representation to FIG. 5, except that while only four passes are shown in FIG. 5, here there are six passes P1 to P6. The first three passes P1 to P3 are substantially the same as those in FIG. 5. During the fourth pass P4, the threading tool oscillates between a seventh radial distance D7 and an eighth radial distance D8, and these radial distances are the radial distances from the last pass, which is the sixth pass P6. The radial distance between the seventh radial distance D7 and the eighth radial distance D8 is the peak-to-peak amplitude of the oscillation during the fourth pass P4. The locus of the fourth pass P4 crosses the locus of the third pass P3 twice for each period. The period of the fourth pass P4 is B4, which is different from the period B3 of the third pass P3. The period B4 of the fourth pass P4 is equal to or substantially equal to the period B2 of the second pass P2. During the fifth pass P5, the penultimate pass, the threading tool oscillates radially between a ninth radial distance D9 and a tenth radial distance D10 from the last pass P6. The tenth radial distance D10 is zero or less than half the difference between the first radial distance D1 and the second radial distance D2, preferably less than 0.10 mm. Reaching a depth of cut of zero or near zero during the penultimate pass, i.e., the fifth pass P5, improves the likelihood of chip shortening during the subsequent and final pass, i.e., the sixth pass P6. The difference or radial distance between the ninth radial distance D9 and the tenth radial distance D10 is the peak-to-peak amplitude of the oscillation during the fifth pass P5. The period or duration of the fifth pass P5 is B5. Period B5 is different from period B4. More specifically, period B5 is shorter than period B4. In other words, the oscillation frequency during fifth pass P5 is greater than the oscillation frequency during fourth pass P4. The trajectory of fifth pass P5 intersects the trajectory of fourth pass P4 twice for each period. The time that the line representing fifth pass P5 is above the line representing fourth pass P4 is the time that the cutting edge is inactive during fifth pass P5, i.e., air time.

[0130] During the sixth and final pass P6, the threading tool is moved radially, i.e. in the X direction, without oscillation.

Claims

1. A machining method for forming a predetermined thread (3) in a workpiece (2) for a CNC lathe, comprising: Providing a metal workpiece (2); Providing a threading tool (1) comprising a first cutting edge (11) and a second cutting edge (12); Rotating the metal workpiece (2) around its axis of rotation (A1); moving the threading tool (1) in a vertical (Z) direction through a set of paths (P1 to P6), the vertical (Z) direction being parallel to or coincident with the axis of rotation (A1); oscillating the threading tool (1) in a radial (X) direction at a first frequency so that the threading tool (1) moves between a first radial distance (D1) and a second radial distance (D2) during a first pass (P1), the first radial distance (D1) being greater than the second radial distance (D2); oscillating the threading tool (1) in the radial (X) direction at a second frequency during a second pass (P2) so that the threading tool (1) moves between a third radial distance (D3) and a fourth radial distance (D4), wherein the third radial distance (D3) is greater than the fourth radial distance (D4) and less than the first radial distance (D1), the fourth radial distance (D4) is less than the second radial distance (D2), and the third radial distance (D3) is greater than the second radial distance (D2); During the last pass (P4, P6), the tool is moved in the radial (X) direction without oscillation; A processing method comprising: The method comprises: setting the second frequency to be different from the first frequency; arranging the second path (P2) so that for each oscillation of the second path (P2), the locus of the second path (P2) crosses the locus of the first path (P1) two or more times; The processing method further comprises:

2. The method of claim 1 , further comprising setting the second frequency to be twice or substantially twice as large as the first frequency.

3. 3. The method of claim 1, further comprising: setting a phase of the oscillation during the second pass (P2) such that the fourth radial distance or distances (D4) coincide with the first and second radial distances (D1, D2) along the Z axis.

4. oscillating said threading tool (1) in the radial (X) direction at a third frequency during a third pass (P3) so that said threading tool (1) moves between a fifth radial distance (D5) and a sixth radial distance (D6), said fifth radial distance (D5) being greater than said sixth radial distance (D6) and less than said third radial distance (D3), said sixth radial distance (D6) being less than said fourth radial distance (D4); setting the third frequency to be different from the second frequency; Setting the fifth radial distance (D5) to be longer than the fourth radial distance (D4); arranging said third path (P3) so that for each oscillation of said third path (P3), the locus of said third path (P3) crosses the locus of said second path (P2) two or more times; setting the frequency of the third path (P3) to be lower than the frequency of the second path (P2) or lower than the frequency of the first path (P1); The processing method according to any one of claims 1 to 3, further comprising:

5. 5. The method according to claim 1, further comprising setting the final oscillation pass (P3, P5) to have a higher oscillation frequency than all previous oscillation passes (P1 to P2, P1 to P4).

6. 6. The machining method according to claim 1, further comprising setting the vibration frequency for all oscillation passes (P2 to P3, P2 to P5) subsequent to the first pass (P1) to a frequency equal to or less than four times the vibration frequency of the first pass (P1).

7. 7. The method according to any one of claims 1 to 6, further comprising, during the penultimate pass (P3, P5), oscillating the threading tool (1) between an outer radial distance (D5, D9) and an inner radial distance (D6, D10) from the last pass (P6), wherein the inner radial distance (D6, D10) is zero or less than half the difference between the first radial distance (D1) and the second radial distance (D2).

8. 8. The method of claim 1, further comprising setting a frequency for the penultimate pass (P3, P5) higher than the frequency of the first pass (P1) and higher than the frequency of the second pass (P2).

9. 9. The method according to claim 1, wherein a difference between the first radial distance (D1) and the second radial distance (D2) is different from a difference between the third radial distance (D3) and the fourth radial distance (D4).

10. 10. The method of claim 1, wherein a difference between the first radial distance (D1) and the third radial distance (D3) is smaller than a difference between the third radial distance (D3) and the second radial distance (D2).

11. The processing method according to claim 1 , wherein the third frequency is the same as or substantially the same as the first frequency.

12. 12. The method according to claim 1, wherein the second radial distance (D2) and the sixth radial distance (D6) coincide or substantially coincide in the longitudinal (Z) direction.

13. 13. The method according to any one of claims 1 to 12, wherein for all passes (P1 to P6) the threading tool (1) is moved in the longitudinal (Z) direction with a longitudinal feed rate of 0.5 to 3.0 mm / rev.

14. The threading tool (1) includes a threading insert (2) and an insert seat (6), The threading insert (2) includes a top surface (3) and a bottom surface (4), The top surface (3) and the bottom surface (4) are connected by a side surface (5), The first cutting edge (11) and the second cutting edge (12) are formed at the boundary between the top surface (3) and the side surface (5); The bottom surface (4) includes mating means (7) for contact with corresponding structures (8) formed in the insert seat (6), The processing method according to any one of claims 1 to 13.

15. A computer program having instructions which, when executed by a CNC lathe, cause the CNC lathe to perform the steps of any one of claims 1 to 14.

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