machine tool

Abstract

Machine tool (1, 21), comprising: a holding device (3) on which a shaft-shaped workpiece (5) is mounted; a tool (19) capable of moving relative to the workpiece (5) in a radial and axial direction of the workpiece (5); a machining control unit (10, 22) that performs a thread cutting process by repeating a tool pass in which the tool (19) cuts the workpiece (5) in the radial direction, moves in a direction parallel to an axis of the workpiece (5), and then moves away from the workpiece (5) in the radial direction while the workpiece (5) and the tool (19) are relatively rotated around the axis of the workpiece (5); a speed control unit capable of changing the speed based on a tool pass; and a rotational speed calculation section which calculates a relative rotational speed between the workpiece (5) and the tool (19) in each tool pass, the machine tool (1, 21) being characterized in that the speed control unit changes the speed between a predetermined high speed and a predetermined low speed, the machine tool (1, 21) further comprising: a specific speed calculating section (15) that decides whether the speed in a first tool pass in the thread cutting process should be the high speed or the low speed so that cutting in a specific tool pass is carried out at the high speed.

Description

[0001] The present invention relates to machine tools, such as lathes, capable of performing, for example, a thread cutting process.

[0002] When machining is performed with a machine tool (particularly machining a workpiece with low rigidity or machining using a tool with low rigidity), strong vibrations known as chatter are often generated during machining. Such chatter causes problems such as a chatter mark on the cutting surface, tool breakage, and so on. To solve such problems, a technique has been developed that suppresses chatter by varying the main spindle speed during a thread cutting cycle, as disclosed in Japanese Patent Application Publication No. 2004-209558 (JP 2004-209558 A).

[0003] However, it is known from experience that, as in the above prior art, in the case of alternately performing cutting at a low speed and cutting at a high speed, cutting at the high speed is less likely to cause chatter vibration. Accordingly, if a particular tool pass is known to be prone to generating chatter vibration, it is desirable that cutting be performed at the high speed in that particular tool pass. In particular, in view of cutting surface accuracy, it is desirable to perform cutting in the last tool pass at the high speed. However, in the case of changing the main spindle speed in each of multiple passes, it is difficult to know whether the main spindle speed of the first cutting should be the low speed or the high speed at the start of machining.Accordingly, cutting in the specific tool pass or the last tool pass may be performed at a low rotational speed, which may prevent effective suppression of chatter vibration or degrade cutting surface accuracy. Prior to the present application, the applicant invented a technique capable of suppressing an increase in cutting load. In this technique, the same lead-out angle is used in all passes when the main spindle speed is changed on a pass-by basis during a thread cutting cycle (Japanese Patent Application No. 2012-121107 (Publication No.: JP 2013-244576 A)). The present invention was developed in light of this technique.

[0004] It is an object of the present invention to provide a machine tool capable of reliably performing cutting in a specific tool pass at a high speed.

[0005] The object is achieved by a machine tool according to claim 1. Advantageous further developments are contained in the dependent claims.

[0006] To achieve the above object, a machine tool according to a first aspect of the present invention includes a holder, a tool, a machine control unit, a speed control unit, and a speed calculation section. A shaft-shaped workpiece is mounted on the holder. The tool is capable of moving in the radial and axial directions of the workpiece with respect to the workpiece. The machining control unit executes a thread cutting process by repeating a tool pass in which the tool cuts the workpiece in the radial direction, moves in a direction parallel to an axis of the workpiece, and then moves away from the workpiece in the radial direction while the workpiece rotates relative to the tool about an axis of the workpiece. The speed control unit is capable of changing a speed based on the tool pass.The speed calculation section calculates a relative speed between the workpiece and the tool in each tool pass. The speed control unit switches the speed between a predetermined high speed and a predetermined low speed. The machine tool further includes a specific speed calculation section that decides whether the speed in a first tool pass in the thread cutting process should be the high speed or the low speed, so that cutting is performed at the high speed in a specific tool pass.

[0007] According to a second aspect of the present invention, in the first aspect of the present invention, the special tool pass is a final tool pass in the threading process.

[0008] According to a third aspect of the present invention, the machine control unit in the first or second aspect of the present invention is capable of executing the thread cutting process in a plurality of types of cutting modes having different cutting methods of the tool, and a radial feed mode, a flank feed mode, and an alternate flank feed mode are set as the cutting modes.

[0009] According to the present invention, the machine tool includes the specific speed calculation section that decides whether the speed in the thread cutting process should be the high speed or the low speed in a first tool pass, so that cutting in a specific tool pass is performed at the high speed. Accordingly, cutting in the specific tool pass can be easily performed at the high speed, which further simplifies the suppression of chatter vibrations, for example, when such a specific tool pass that tends to cause chatter vibrations is known.

[0010] According to the second aspect of the present invention, cutting in the final tool pass can be performed easily and reliably at high rotational speed. This can reliably prevent problems due to cutting at low rotational speed in the final tool pass, such as deterioration of cutting surface accuracy. Fig. 1 is a diagram showing an overall configuration of a lathe. Fig. 2 is a diagram showing an example of a tool pass in a thread cutting cycle using the lathe. Fig. Figure 3 is a diagram showing a manner in which the main spindle speed is changed during a thread cutting cycle. Fig. 4A to 4C are diagrams showing paths of the tool and width each affected by the preceding cutting surface in three types of cutting modes, namely a radial feed mode, a flank feed mode, and an alternate flank feed mode. Fig. 5 is a diagram showing a manner in which the main spindle speed is changed so that cutting in the last tool pass is performed at a high speed. Fig. 6 is a diagram showing the overall configuration of a lathe according to a modified embodiment.

[0011] A machine tool according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

[0012] First, the overall configuration of a lathe 1 is described with reference to Fig. 1. The lathe 1 includes a clamping device 3 having a claw 4 at the distal end of the main spindle 2, and capable of holding a shaft-shaped workpiece 5 by the clamping device 3. A motor 7, which rotates the main spindle 2, and an encoder 6, which detects the rotational speed of the main spindle 2, are contained in the headstock 8, which supports the main spindle 2, so that the main spindle 2 rotates.

[0013] A main spindle control section 9 monitors the rotational speed of the main spindle 2 through the encoder 6 and controls the rotational speed of the main spindle 2. A lathe control section 10 controls the entire behavior of the lathe 1 and is provided with the main spindle control section 9, an input unit 11, a storage section 12, a rotational speed calculation section 2 (hereinafter referred to as "a first calculation section"), a number-of-same-speed cutting passes calculation section 14 (hereinafter referred to as "a second calculation section"), and a first-pass rotational speed calculation section (which is "the specific rotational speed calculation section" 15 and hereinafter referred to as "a third calculation section"). The input unit 11 sets the cutting mode and sends an instruction to change the main spindle rotational speed. The memory section 12 stores a machining program, etc.The first calculation section 13 calculates the main spindle speed. The second calculation section 14 decides the number of same-speed cutting passes (i.e., the number of passes in which cutting is continuously performed at the same main spindle speed). The third calculation section 15 calculates the main spindle speed in the initial pass (first pass) so that cutting is performed at a high speed in the final tool pass. The lathe 1 controls the rotational speed of the workpiece 5 (i.e., the main spindle speed) via the main spindle control section 9. Further, the lathe 1 controls the machining operation so that a tool 19 cuts the peripheral surface of the rotating workpiece 5 and the workpiece 5 or the tool 19 is fed in the rotation axis direction and the radial direction using a well-known configuration.

[0014] An example of a tool pass in a thread cutting cycle as a machining form of the lathe 1 will be described with reference to Fig. 2 described.

[0015] The lathe 1 performs a thread cutting process by repeating a cycle consisting of multiple successive tool passes according to the machining program stored in the memory section 12 (in Fig. 2 by arrows). In the tool pass, the motor 7 is supplied with electric power under the control of the main spindle control section 9 to rotate the main spindle 2 at a predetermined main spindle speed, and the workpiece 5 rotates around the axis. Furthermore, the tool 19 attached to the tool holder 20, under the control of the lathe control section 10, cuts the workpiece 5 in the radial direction and is advanced in the longitudinal direction (the rotation axis direction) to machine a threaded portion A, and then moves away from the workpiece 5 in the radial direction.

[0016] During thread cutting cycle machining with the lathe 1, the main spindle speed is changed to a low speed or a high speed at a predetermined time. A manner in which the main spindle speed is changed during thread cutting cycle machining is briefly described below.

[0017] S0 represents a reference speed, and W represents a main spindle speed change. These values ​​S0, W are input in advance by the operator via the input unit 11. The first calculation section 13 calculates a high speed SH and a low speed SL based on these parameters using the following equations (1), and the main spindle speed is changed to either the high speed SH or the low speed SL in each pass ( Fig. 3). This can suppress chatter vibration increase compared to the case where cutting is performed at a constant main spindle speed in all passes. SH=(1+W200)S0SL=(1−W200)S0 SH High-speed side main spindle speed [min -1 ] SL Low-speed side main spindle speed [min -1 ] S0 Reference speed [min -1 ] W Main spindle speed change [%]

[0018] A decision by the second calculation section 14 regarding the number of cutting passes at the same rotational speed will be described. Three types of cutting modes, namely a radial infeed mode, a flank infeed mode, and an alternate flank infeed mode, can be set as one embodiment of the cutting mode (i.e., a cutting method of the tool pass) in the lathe 1. Fig. 4A to 4C are diagrams each showing paths of the tool and the width affected by the previous cutting surface (hereinafter referred to as “regenerative width”) in the three types of cutting modes.

[0019] The Fig. The radial feed mode shown in Figure 4A is a cutting mode in which cutting is performed with cutting edges on both the right and left sides (right and left sides in Fig. 2) the blade tip of the tool 19. The smallest V-shaped surface in Fig. 4A corresponds to the path of the tool of the first pass. As machining continues, the path of the tool gradually changes to a larger V-shaped surface, and the largest V-shaped surface corresponds to the path of the tool of the tenth pass. Accordingly, in the radial feed mode, cutting of the nth pass is only affected by the cutting surface of the (n-1)th pass. When cutting the tenth pass, Fig. 4A the regenerative width from one pass before bd1a.

[0020] The Fig. 4B is a cutting mode in which cutting is performed with the cutting edge on the left side of the blade tip of the tool 19. The small V-shaped surface in Fig. 4B on the far right corresponds to the tool path of the first pass. As machining progresses, the tool path gradually changes to a V-shaped surface containing an inclined surface to the left, and the large V-shaped surface containing the inclined surface on the far left corresponds to the tool path of the tenth pass. Accordingly, in the flank feed mode, a cut of the nth pass is also only affected by the cutting surface of the (n-1)th pass. In the cutting of the tenth pass, the regenerative width from one pass before is bd1b in Fig. 4B.

[0021] The Fig. The alternating flank feed mode shown in Figure 4C is a cutting mode in which cutting is performed by alternately using the left and right cutting edges of the tool 19 in each pass. The smallest V-shaped surface in Fig. 4C corresponds to the path of the tool of the first pass. As machining progresses, the path of the tool alternately changes to a V-shaped surface containing a right or left inclined surface. The V-shaped surface containing the largest right and left inclined surface corresponds to the path of the tool of the tenth pass. Accordingly, in the alternating flank feed mode, a cutting of the nth pass is influenced by the cutting surface of the (n-1)th pass and the cutting surface of the (n-2)th pass. In the cutting of the tenth pass, Fig. 4C the regenerative width from one pass before bd1c, and the regenerative width from two passes before is in Fig. 4C bd2c. Regarding the relationship between the regenerative width and chatter, the larger the regenerative width, the more chatter is likely to be generated due to the influence of the previous cutting surface. In the alternating flank feed mode, chatter is more likely to be generated due to the influence of the cutting surface from two previous passes, since the regenerative width of two previous passes is larger than that of one previous pass.

[0022] In view of the above, the second calculation section 14 in the lathe 1 determines the cutting mode of the thread cutting process based on the set machining program, etc., and sets the number of cutting passes at the same speed to “2” when the alternate flank feed mode is set as the cutting mode, and sets the number of cutting passes at the same speed to “1” when the radial feed mode or the flank feed mode (namely, any cutting mode other than the alternate flank feed mode) is set as the cutting mode.

[0023] A calculation of the main spindle speed of the first pass through the third calculation section 15 is described. Fig. 5 is a diagram showing a manner in which the main spindle speed is changed so that cutting in the last tool pass is performed at the high speed.

[0024] In Fig. 5, C represents the number of cutting passes at the same speed, and N represents the total number of cutting operations. C = 2 and N = 6 in the Fig. 5 shown processing. In Fig. 5, P represents the period with which the main spindle speed is changed, (n) represents the order of cutting passes in the period P, and s(n) (n = 1, 2, ..., P) represents the main spindle speed in the cutting pass in the period P. In this example, the main spindle 2 is rotated at the high speed SH in s(1) to s(C), while the main spindle 2 is rotated at the low speed SL in s(C+1) to s(P). In this case, the third calculation section 15 calculates the main spindle speed ss in the first pass using the following equations (2) to (4). That is, the third calculation section 15 decides whether the high speed SH in s(1) to s(C) or the low speed SL in s(C+1) to s(P) should be the main spindle speed ss in the first pass. P=2×C nm=(N−1)mod P ss=s(P−nm+1)

[0025] The total number of cutting operations N can be calculated by dividing the total cutting amount by the cutting amount per pass.

[0026] The lathe 1 includes the second calculation section 14, which determines the number of cutting passes at the same speed. The second calculation section 14 determines the cutting mode of the thread cutting process based on the set machining program, etc., and automatically determines the optimal number of cutting passes at the same speed according to the cutting mode. Thus, even inexperienced operators can easily suppress chatter vibrations and use the machine tool with ease.

[0027] Lathe 1 includes the third calculation section 15. The third calculation section 15 automatically decides whether the main spindle speed in the first pass should be the high speed SH or the low speed SL, so that the cutting of the last pass is performed at the high speed. Therefore, cutting of the last tool pass at the low speed can be reliably prevented.

[0028] The machine tool according to the present invention is not limited in any respect to the form of the above embodiment, and not only the overall configuration of the machine tool but also the manner in which the main spindle speed is changed, etc., can be appropriately modified as necessary without departing from the spirit and scope of the invention.

[0029] For example, although the cutting mode is manually set in the lathe 1 of the above embodiment, a cutting mode more likely to suppress vibrations may be automatically set in response to detection of vibrations generated on the main spindle. A lathe 21 according to such a modified embodiment is based on Fig. 6 described. In Fig. 6 are the same components as those in Fig. 1 shown lathe 1 is designated by the same reference numerals.

[0030] The lathe 21 has a configuration similar to that of the lathe 1, but a vibration sensor 18 that detects vibrations generated on the main spindle 2 is included in the spindle box 8 near the main spindle 2. A vibration direction determining section 16 and a cutting mode deciding section 17 are connected to the lathe control section 22, which controls the overall behavior of the lathe 21. The vibration direction determining section 16 determines the direction in which vibrations are most likely to be generated according to the output of the vibration sensor 18. The cutting mode deciding section 17 decides the cutting mode based on this direction.

[0031] In the lathe 21, the vibration sensor 18 detects vibrations generated on the main spindle 2 in both the feed direction and the axial direction. The vibration direction determining section 16 determines that the direction of larger vibrations is the direction in which vibrations are more likely to be generated. If vibrations are more likely to be generated in the feed direction, the cutting mode deciding section 17 selects the flank feed mode or the alternate flank feed mode. In either the flank feed mode or the alternate flank feed mode, the cutting width in the feed direction is smaller, and therefore the cutting cross-sectional area is less likely to change, and the contribution of the feed direction component (thrust force) to the combined cutting force is smaller compared to the radial feed mode.The cutting mode decision section 17 then changes the cutting mode from the radial infeed mode to the flank infeed mode or the alternate flank infeed mode to continue machining. If vibration is more likely to be generated in the axial direction, the cutting mode decision section 17 selects the radial infeed mode. In the radial infeed mode, the cutting width in the axial direction is smaller, and therefore, the cutting cross-sectional area is less likely to change, and the proportion of the axial component (infeed force) to the combined cutting force is smaller compared to the flank infeed mode and the alternate flank infeed mode. The cutting mode decision section 17 then continues machining without changing the cutting mode.When the cutting mode is changed, the third calculation section 15 substitutes the number of remaining cutting operations for N rather than the total number of cutting operations. Then, after the cutting mode is changed, the third calculation section 15 calculates the main spindle speed ss in the first pass using equations (2) to (4). Whether the flank feed mode or the alternate flank feed mode is selected when the cutting mode is changed can be preset by the operator or can be automatically decided based on the machining program, etc.

[0032] Advantages similar to those of the lathe 1 can be expected from this lathe 21. For example, even an inexperienced operator can easily suppress chatter vibrations, and cutting in the last tool pass at the low speed can be reliably prevented.

[0033] The lathe 21 is equipped with the vibration sensor 18 that detects vibrations generated on the main spindle 2, the vibration direction determining section 16 that determines the direction in which vibrations are more likely to be generated, and the cutting mode deciding section 17 that decides the cutting mode based on the direction in which vibrations are more likely to be generated. Thus, according to the direction in which vibrations are more likely to be generated, the cutting mode less likely to cause chatter vibrations is decided, and machining is performed in that cutting mode.

[0034] For example, the above embodiment and the modified embodiment are configured such that cutting is performed at the high speed in the last tool pass. However, another modification may be configured such that cutting is performed at the high speed in a specific tool pass rather than in the last tool pass because chatter vibrations tend to be caused, etc. In this case, N represents the pass in which cutting is desired to be performed at the high speed (or the number of remaining cutting operations until that pass), and the main spindle speed ss in the first pass (or the first pass after the change of the cutting mode) is calculated using equations (2) to (4).

[0035] The main spindle speed can be changed according to other patterns different from those in the embodiment and the modified embodiment, based on the cutting mode or other machining conditions. For example, the main spindle speed can be changed every three passes, or cutting can be performed in two passes at the high speed SH and in one pass at the low speed SL.

[0036] Controlling the thread cutting process in the above embodiment and the modified embodiment is applicable to both an internal thread cutting process of a cylindrical portion, etc., and an external thread cutting process.

[0037] In the above embodiment and the modified embodiment, the three types of cutting modes, namely the radial feed mode, the flank feed mode, and the alternate flank feed mode, can be set as one cutting mode. However, other cutting modes can also be set in the present invention. Alternatively, only two types of cutting modes including the alternate flank feed mode can be set in the present invention.

[0038] In the case where the cutting mode can be manually input, and was manually input in the modified embodiment, machining may be started by the input cutting mode rather than the radial feed mode, or the cutting mode may not be changeable.

[0039] Although the vibration sensor 18 is included in the spindle box 8 in the modified embodiment, the vibration sensor 18 may be placed on the tool holder 20 to detect vibrations generated on the workpiece 5.

[0040] The embodiment and the modified embodiment are described with respect to the lathe that rotates a workpiece as one embodiment of the machine tool. However, the present invention may be another machine tool, such as a machining center, that performs the same machining by relatively rotating a tool and a workpiece through a feed shaft rather than by rotating the main spindle. It is explicitly stated that, regardless of the arrangement of features in the embodiments and / or the claims, all features disclosed in the specification and / or the claims are intended to be disclosed separately and independently of each other for the purpose of original disclosure as well as for the purpose of limiting the claimed invention.It is explicitly stated that all ranges of values ​​or indications of groups of instances disclose every possible intermediate value or intermediate instance for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, in particular as limits of ranges of values.

Claims

[1] Machine tool (1, 21), comprising: a holding device (3) on which a shaft-shaped workpiece (5) is mounted; a tool (19) capable of moving relative to the workpiece (5) in a radial and axial direction of the workpiece (5); a machining control unit (10, 22) that performs a thread cutting process by repeating a tool pass in which the tool (19) cuts the workpiece (5) in the radial direction, moves in a direction parallel to an axis of the workpiece (5), and then moves away from the workpiece (5) in the radial direction while the workpiece (5) and the tool (19) are relatively rotated around the axis of the workpiece (5); a speed control unit capable of changing the speed based on a tool pass; and a speed calculation section which calculates a relative speed between the workpiece (5) and the tool (19) in each tool pass, wherein the machine tool (1, 21) characterized by is that the speed control unit changes the speed between a predetermined high speed and a predetermined low speed, the machine tool (1, 21) further comprising: a specific speed calculating section (15) that decides whether the speed in a first tool pass in the thread cutting process should be the high speed or the low speed so that cutting in a specific tool pass is carried out at the high speed. [2] Machine tool (1, 21) according to claim 1, wherein the special tool pass is a last tool pass in the threading process. [3] The machine tool (1, 21) according to claim 1 or 2, wherein the machining control unit (10, 22) is capable of executing the thread cutting process in a plurality of types of cutting modes including different cutting methods of the tool (19), and a radial feed mode, a flank feed mode, and an alternate flank feed mode are set as the cutting modes.

Images