Method for measuring the effective diameter of a ball screw and a screw grinding apparatus using the same method

Abstract

To make it easy to correct and process a ball screw effective diameter to a desired dimension by acquiring a relation with an absolute value relative to an output of a system described in an official gazette of a Japan Unexamined Patent Application Publication No.2021-117061.SOLUTION: In a ball screw effective diameter measurement system measuring an effective diameter of a screw groove processed to a ball screw shaft serving as a processing object, the ball screw effective diameter measurement system has: a first step of measuring and evaluating an outer diameter of the ball screw shaft; and a second step of measuring and evaluating a part corresponding to the effective diameter of the screw groove, By separating the first step and second step, and calculating a difference between the outer diameter and an evaluation of the part corresponding to the effective diameter about positions corresponding to a longitudinal direction of the ball screw, he ball screw effective diameter measurement system makes an evaluation of the effective diameter of the ball screw. In the ball screw effective diameter measurement system in a screw grinding device implementing the correction and evaluation using the evaluation of the effective diameter measurement system, the ball screw effective diameter measurement system mounts a function of measuring a gauge or workpiece serving as a reference of the effective diameter, and evaluates a deviation from a target dimension of the effective diameter from comparison between the deviation with the reference.SELECTED DRAWING: Figure 10

Description

Technical Field

[0001] The present invention relates to a method for measuring the effective diameter of a screw shaft constituting a ball screw and a screw grinding apparatus using the method. In particular, the present invention relates to a method for automatically measuring, evaluating, and correcting the effective diameter of a ball screw shaft during processing at a site where a ball screw shaft is produced using a screw grinding apparatus, and a screw grinding apparatus using the method.

Background Art

[0002] A ball screw is a mechanical element used in industrial machines, robots, etc. It includes a screw shaft having a screw groove on its outer peripheral surface, a nut having a screw groove on its inner peripheral surface, and a large number of balls (usually steel balls) accommodated in a rolling path formed by the screw groove of the screw shaft and the screw groove of the nut. It converts rotational motion into linear motion (or linear motion into rotational motion) with high power transmission efficiency and position accuracy. That is, a ball screw is composed of a screw shaft having a spiral ball rolling groove with a predetermined lead formed on its outer peripheral surface, and a nut member having a spiral ball rolling groove on its inner peripheral surface facing the ball rolling groove on the screw shaft side, and is configured such that the nut member moves in the axial direction of the screw shaft in response to the rotation of the screw shaft. Such a ball screw is used, for example, to move the feed table of a machine tool in sub-micrometer units. In order to ensure its feed accuracy, it is necessary to measure the effective diameter of the ball screw shaft. Conventionally, a method of measuring the effective diameter by bringing a touch probe into contact with the ball screw groove is known (see, for example, Patent Document 1).

[0003] Conventionally, various systems have been proposed for measuring the effective diameter of ball screws, typically evaluating the change in the effective diameter of the ball screw in the longitudinal direction of the workpiece. This method is classified as the radius method in terms of diameter measurement methods. While this method is effective for short objects, when measuring long objects, especially on a machining center, there is a problem in that the curvature of the object cannot be ignored when evaluating to the micrometer order, and depending on the configuration of the equipment, deformation due to gravity cannot be ignored either. Furthermore, when the measurement system is integrated with the machining center, the workpiece is held down with a device called a steady rest to suppress runout, but while this suppresses runout at the held position when the workpiece is rotated, it has the drawback of increasing curvature.

[0004] Therefore, the inventor has developed and filed a patent application for a technology that accurately measures the effective diameter of a ball screw without being affected by bending or vibration damping, by separating the process of measuring and evaluating the outer circumference excluding the groove from the process of measuring and evaluating the groove bottom, and calculating the difference between the evaluation of the outer circumference and the groove bottom at corresponding positions in the longitudinal direction of the ball screw, and has published this technology (see Patent Document 2). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Utility Model Publication No. 2-135804 [Patent Document 2] Japanese Patent Publication No. 2021-117061 [Overview of the project] [Problems that the invention aims to solve]

[0006] By using the evaluation of the longitudinal distribution of the effective diameter of a ball screw based on the technology described in Patent Document 2, it is possible to machine a ball screw with a desired effective diameter distribution on the order of a few micrometers by performing corrective machining. When performing this corrective machining, the longitudinal distribution of the effective diameter is obtained using the evaluation system shown in Patent Document 2 in the machined state before the target finished diameter is reached. This is shown in Figure 5, which will be described later. However, the output of the evaluation system shown in Patent Document 2 shows the distribution, but the absolute value of the dimension is not known, as shown in the example in Figure 6, which will be described later.

[0007] This invention was made in light of the circumstances described above, and its purpose is to provide a technology that makes it easier to correct the effective diameter of a ball screw to a desired dimension by obtaining a relationship between the output of the system described in Japanese Patent Publication No. 2021-117061 and the absolute value. [Means for solving the problem]

[0008] To achieve the above objective, the inventors conducted extensive research and discovered that by measuring and comparing gauges, they could obtain a relationship between the system output and its absolute value. For this purpose, by setting a comparison reference coaxially with the workpiece being ground within the machine, it is possible to obtain a relationship between the system output and its absolute value as described in Japanese Patent Publication No. 2021-117061, making it easier to correct the effective diameter of the ball screw to a desired dimension.

[0009] That is, relating to the first aspect of the present invention The screw grinding machine is The subject of processing as work A ball screw effective diameter measuring system for measuring the effective diameter of screw grooves machined into a ball screw shaft. A screw grinding device having , The ball screw effective diameter measuring system is: A ball screw effective diameter measuring system comprising a first step of measuring and evaluating the outer diameter of the ball screw shaft, and a second step of measuring and evaluating the portion corresponding to the effective diameter of the screw groove, wherein the effective diameter of the ball screw is evaluated by calculating the difference between the outer diameter and the evaluation of the effective diameter at corresponding positions in the longitudinal direction of the ball screw, divided into the first and second steps. and , The aforementionedIn a screw grinding machine that performs corrective machining using an evaluation of the effective diameter measuring system, the machining A gauge of known diameter is placed coaxially with the target workpiece for comparison. The diameter is measured relative to the gauge, and the measured value itself, or if the measured value is not the effective diameter of the target groove, it is adjusted to account for the groove depth to obtain a value equivalent to the effective diameter, and this is used as a reference value. By comparing this reference value with the average value in the longitudinal direction of the measurement results of the groove portion, the deviation of the effective diameter from the target dimension is determined, and the corrective machining is performed according to this deviation. In addition, the ball screw shaft, which is the workpiece being machined, is machined, the longitudinal distribution of the diameter of the workpiece is measured and evaluated, and the remaining material removal is evaluated while performing the corrective machining, in order to finish to the target diameter, or if the distribution cannot be corrected to the target diameter, the operator is asked to make a judgment. It is characterized by the following:

[0010] Furthermore, screw grinding according to the second aspect of the present invention method The object to be processed is as work A ball screw effective diameter measuring system for measuring the effective diameter of screw grooves machined into a ball screw shaft. A screw grinding method using , The ball screw effective diameter measuring system is: The ball screw effective diameter measuring system comprises a first step of measuring and evaluating the outer diameter of the ball screw shaft, and a second step of measuring and evaluating the portion corresponding to the effective diameter of the screw groove, and evaluates the effective diameter of the ball screw by calculating the difference between the outer diameter and the evaluation of the effective diameter at corresponding positions in the longitudinal direction of the ball screw, divided into the first and second steps. The aforementioned Thread grinding that performs corrective machining using evaluation of the effective diameter measurement system. Method At A screw grinding method is provided in which the outer diameter of a part or the entire unmachined cylindrical portion of a ball screw shaft, which is the workpiece to be machined, is measured in advance, a reference value is calculated as the position of the difference from the average value of the measured value to determine the target value, the deviation of the effective diameter from the target dimension is determined by comparing the reference value with the average value in the longitudinal direction of the measurement results of the groove portion, and the correction machining is performed according to the deviation, wherein the priority item can be specified in advance as a coefficient whether to prioritize the local effective diameter distribution or the effective diameter. It is characterized by the following: [Effects of the Invention]

[0011] According to the present invention, by obtaining a relationship between the output of the system described in Japanese Patent Publication No. 2021-117061 and its absolute value, it is possible to provide a technology that makes it easier to correct the effective diameter of a ball screw to a desired dimension. [Brief explanation of the drawing]

[0012] [Figure 1] This figure shows the basic configuration of a screw grinding apparatus according to the first embodiment of the present invention, where (A) is a plan view and (B) is a side view. [Figure 2] This figure shows a screw grinding apparatus according to the first embodiment of the present invention, focusing on the probing device and its rotation mechanism. [Figure 3] This is the first figure (without correction) illustrating a method for measuring the effective diameter of a ball screw according to a second embodiment of the present invention, and shows the operation flow when machining is performed in multiple stages such as rough machining and fine machining. [Figure 4]FIG. 2 is a second diagram for explaining a method for measuring the effective diameter of a ball screw according to the second embodiment of the present invention, showing details of the cutting process in FIG. 3. [Figure 5] FIG. 1 is a first diagram for explaining a method for measuring the effective diameter of a ball screw shown in JP-A-2021-117061. (A) shows the diameter variation at the bottom of the groove, that is, the variation in the measured values when the bottom of the groove of the ball screw is measured, and (B) shows the diameter variation at the outer peripheral portion, that is, the variation in the measured values when the outer periphery of the ball screw is measured. (C) is a diagram for explaining the coordinate system in the present application including this figure. [Figure 6] FIG. 2 is a second diagram for explaining a method for measuring the effective diameter of a ball screw shown in JP-A-2021-117061, and is a diagram showing an example of evaluation in the measurement method. (A) shows a case where the diameter variation is evaluated only by measurement at the bottom of the groove, (B) shows a case where the diameter variation is evaluated only by measurement at the outer periphery, and (C) shows the difference between the variation evaluation at the bottom of the groove and the variation evaluation at the outer periphery. Note that the average values of (A) and (B) are adjusted to be 0, respectively. [Figure 7] FIG. 3 is a third diagram for explaining a method for measuring the effective diameter of a ball screw according to a modified example of the second embodiment of the present invention. Even when the size of the tip ball of the stylus is different from the ball used as the ball screw, assuming that the cross-sectional shape of the screw groove is as designed, a simulation of the contact between the ball and the cross-section is performed to calculate the effective diameter. [Figure 8] FIG. 4 is a fourth diagram for explaining a method for measuring the effective diameter of a ball screw according to a modified example of the second embodiment of the present invention, and is a diagram for explaining a case where the bottom diameter of the groove is measured and a case where the diameter of a portion corresponding to the effective diameter is measured depending on the size of the tip ball of the stylus and the size of the ball used as the ball screw. [Figure 9] When the axial distribution of the effective diameter is obtained using the method for measuring the effective diameter of a ball screw shown in JP-A-2021-117061, the operation flow in the case of performing correction machining at the end of automatic operation (a method that pays attention only to the axial distribution of the effective diameter) is shown. [Figure 10]FIG. 5 is a fifth diagram for explaining a method of measuring the effective diameter of a ball screw according to the second embodiment of the present invention, and shows an operation flow in the case of performing correction machining at the end of automatic driving.

Embodiments for Carrying Out the Invention

[0013] First, a screw grinding apparatus according to the first embodiment of the present invention will be described. FIG. 1 is a diagram showing a basic configuration of the screw grinding apparatus according to the present embodiment, (A) is a plan view thereof, and (B) is a side view thereof. As shown in FIGS. 1(A) and 1(B), a screw grinding apparatus (screw grinding machine) 100 according to the present embodiment includes a grinding wheel table 200, a grinding wheel rotating device (motor) 202 held by the grinding wheel table 200, and a grinding wheel 204 attached to the rotation shaft of the grinding wheel rotating motor 202 on a column 102. The grinding wheel table 200 has an X linear motion device 206 and an X linear motion guide 208. The grinding wheel table 200 moves in the X direction shown in the figure by the X linear motion device 206 and the X linear motion guide 208, and the workpiece 212 placed on the workpiece table 210 is cut in the X direction by the rotating grinding wheel 204. Further, the grinding apparatus 100 according to the present embodiment has a workpiece table 210, and the workpiece table 210 moves in the Z direction shown in the figure by a Z linear motion device 214 and a Z linear motion guide 216 to grind the workpiece 212. Note that the grinding apparatus 100 according to the present embodiment has an A-axis turning device 218, a C-axis (workpiece) rotating device 220, and a tailstock (workpiece support device) 222.

[0014] In this embodiment, the screw grinding apparatus 100 has a probing device 800 attached to the rear of the housing of the grinding wheel rotating device 202 via a rotatable V-shaped member, the probing device swivel mechanism 804, as shown in Figure 2. This probing device 800 is equipped with a stylus 802, which is a contact element. In this embodiment, the screw grinding apparatus 100 rotates the probing device swivel mechanism 804, causing the probing device 800 to swivel and retract, making it possible to grind the workpiece 212 with the grinding wheel 204. On the other hand, the probing device swivel mechanism 804 rotates in the opposite direction, causing the probing device 800 to swivel and set in the opposite direction, and it is also possible to bring the stylus 802 of the probing device 800 (see also Figure 8, which will be described later) into contact with the workpiece 212 in order to perform on-machine measurements.

[0015] In the screw grinding apparatus 100 of this embodiment, which has the above configuration, as shown in Figure 1(B), the A-axis rotation device 218 rotates around the A-axis, thereby adjusting the twist angle between the axis of the workpiece 212 to be ground and the rotation axis of the grinding wheel 204 (this is referred to as the A-axis). Furthermore, the grinding wheel axis is positioned so as to be parallel to the longitudinal axis of the workpiece 212 when the A-axis command is set to 0 degrees. In addition, the grinding apparatus 100 according to this embodiment can rotate the workpiece 212 to be ground around its longitudinal axis using the C-axis (workpiece) rotation device 220 (this is referred to as the C-axis). In the grinding apparatus 100 according to this embodiment, the workpiece 212 to be ground can be moved linearly in the longitudinal axial direction using the Z-linear motion device 214 and the Z-linear motion guide 216 (this is referred to as the Z-axis). The grinding wheel 204 is rotated via the grinding wheel shaft by the grinding wheel rotation device (motor) 202, and can also be moved in a direction perpendicular to the longitudinal axis of the workpiece 212 to be ground (this is referred to as the X-axis). The pivot point of the A-axis is configured to pass through the center of the lines on both sides of the rotation center of the grinding wheel shaft, or the center of the thickness, when viewed from a direction perpendicular to the axial direction of the grinding wheel shaft, as far as possible (with an accuracy of the order of 0.1 mm). Here, for explanatory purposes only, the direction perpendicular to both the X-axis and the Z-axis will be referred to as the Y-axis.

[0016] In the ball screw effective diameter measurement method according to this embodiment, screw grinding without correction is basically performed. That is, in this normal grinding, multiple processes corresponding to the grinding shown in Figure 4, which will be described later, are combined. In other words, the settings are read into the control device and grinding is performed many times. It is also conceivable to divide the process into multiple stages, such as rough grinding and fine grinding. Figure 3 is the first diagram for explaining the ball screw effective diameter measurement method according to the second embodiment of the present invention, and shows the operation flow in the case of normal grinding that is divided into rough grinding and fine grinding stages. That is, in the operation flow when the process is divided into rough grinding S300B and fine grinding S300C stages, when it starts (S306), in the rough grinding flow S300B, the processes of grinding (S307), grinding (S308), ... grinding (S309) are executed. Next, in the precision machining flow S300C, the cutting (S310), cutting (S311), ... cutting (S312) processes are executed. This advances the cutting of the workpiece and then finishes (S313). Then, as shown on the left side of Figure 3, in each of these cutting processes, in the cutting flow S300A, cutting is started (S301) and the depth of cut setting is read (S302). As a result, the grinding wheel advances according to the depth of cut setting (S303) and the workpiece is fed (S304), and the cutting of the workpiece advances and finishes (S305).

[0017] Figure 4 is a second diagram illustrating the method for measuring the effective diameter of a ball screw according to the second embodiment of the present invention, and is a diagram for further explanation of the cutting flow shown in Figure 3 above. That is, in a normal cutting operation (without correction), as shown in Figure 4, in the preparation flow S200A, preparation begins (S201), and is completed by inputting grinding conditions (S202) (S203). The depth of cut is set according to the input grinding conditions (S204), and the grinding wheel coordinates are set according to the progress of the machining (S205). After this, in the cutting flow S200B, cutting begins (S206), and the depth of cut setting is read (S207). As a result, the grinding wheel advances according to the depth of cut setting (S208), the workpiece is fed (S209), and the cutting of the workpiece progresses and is completed (S210). In this way, in the method for measuring the effective diameter of a ball screw according to the present embodiment, the machining of the workpiece is completed based on the pre-set grinding conditions and the depth of cut setting at each stage.

[0018] Figure 5 is the first diagram illustrating the method for measuring the effective diameter of a ball screw as shown in Japanese Patent Application Publication No. 2021-117061. (A) shows the diameter variation at the bottom of the groove, i.e., the variation in the measured value when measuring the bottom of the groove of the ball screw, and (B) shows the diameter variation at the outer circumference, i.e., the variation in the measured value when measuring the outer circumference of the ball screw. (C) is a diagram illustrating the coordinate system in this application, including this figure. Figure 6 is the second diagram illustrating the method for measuring the effective diameter of a ball screw as shown in Japanese Patent Application Publication No. 2021-117061. It shows an example of evaluation in that measurement method. (A) shows the case where the diameter variation is to be evaluated only by measuring at the bottom of the groove, (B) shows the case where the diameter variation is to be evaluated only by measuring at the outer circumference, and (C) shows the difference between the variation evaluation at the bottom of the groove and the variation evaluation at the outer circumference. The average values ​​of (A) and (B) are adjusted to be 0. In this way, by correcting (subtracting) the evaluation of the diameter variation of the outer circumference (B) for the diameter variation of the groove bottom (A), we obtain (C), and by using this as the evaluation of the longitudinal variation of the effective diameter, the problems of the radius method are resolved, and even when measuring the effective diameter of a relatively long ball screw, it becomes possible to measure accurately without being affected by bending of the ball screw or by the vibration damper. Note that the evaluation of the longitudinal distribution of the outer diameter of the ball screw (evaluation of the cylindricity of the outer circumference) may be performed by a separate system. Also, in the case of a nut instead of a ball screw, it goes without saying that the outer diameter is replaced by the inner diameter.

[0019] Figure 7 is a third diagram illustrating a method for measuring the effective diameter of a ball screw according to a modified example of the second embodiment of the present invention, with the scanning direction in the case of helical measurement indicated by an arrow. As shown in Figure 7, even when the tip ball 803 of the stylus 802 is different in size from the ball 806 used when it is used as a ball screw, as shown by the dashed line, the effective diameter is calculated by simulating the contact between the tip ball 803 and the cross-section, assuming that the cross-sectional shape of the screw groove of the screw (workpiece) 212 to be measured is as designed.

[0020] Figure 8 is a fourth diagram illustrating a method for measuring the effective diameter of a ball screw according to a modified example of the second embodiment of the present invention. It illustrates the difference between measuring the groove bottom diameter and measuring the diameter of the portion corresponding to the effective diameter, depending on the size of the tip sphere of the stylus and the size of the ball when used as a ball screw. That is, if the size of the tip sphere of the stylus is extremely small compared to the size of the ball (sphere) 806 when in use (shown by a dashed line), although not shown in the diagram, and it fits into the groove bottom 104 for measurement, there is little problem. However, if the size of the stylus tip of the probing device is as shown by the solid line in Figure 8, compared to the size of the ball (sphere) 806 when in use (shown by a dashed line), a gap is created between it and the groove bottom 104, making it impossible to fit it into the groove bottom 104 for measurement. Therefore, in the modified example of the second embodiment of the present invention, in such cases, instead of measuring the groove bottom diameter, the diameter of the portion corresponding to the effective diameter, such as portion 106 that is in contact with the inner circumferential surface of the groove, is measured. In this case, it is preferable to use a contact ball with a diameter close to the intended ball diameter, to make contact between the ball and the groove at two points, and to configure the probing device to be sensitive only in the radial direction, or to be able to decompose the output into its radial component.

[0021] On the other hand, when making corrections, the longitudinal distribution of the effective diameter is obtained using the evaluation system shown in Japanese Patent Publication No. 2021-117061 in the machining state before reaching the target finished diameter. This correction is shown in Figure 9. Figure 9 shows the operation flow when making corrections at the end of automatic operation (a method that focuses solely on the axial distribution of the effective diameter) when the axial distribution of the effective diameter is obtained using the ball screw effective diameter measurement method shown in Japanese Patent Publication No. 2021-117061. As mentioned above, the output of the system shown in Japanese Patent Publication No. 2021-117061 shows the longitudinal distribution of the effective diameter, but the absolute value of the dimension is not known, as shown in the example in Figure 6 above. Therefore, the relationship between the system output and the absolute value is obtained by measuring the gauge and comparing it. For this purpose, a comparison reference is set up coaxially with the workpiece being ground inside the machine. A structure in which the comparison gauge rotates with the workpiece is preferable, but not necessarily required. It is also preferable that it has a screw groove. If that is not possible, that is, if it is difficult to prepare a special gauge, it is also possible to obtain the average of a part or the whole of the cylindrical portion of the workpiece in advance using an outer diameter evaluation method such as a micrometer, and use the workpiece itself as a substitute for the gauge. The average of the measurement results in the groove portion in the longitudinal direction is compared with the gauge. That is, in the ball screw effective diameter measurement method shown in Japanese Patent Application Publication No. 2021-117061, in the ball screw effective diameter evaluation system, if the longitudinal effective diameter variation is evaluated as shown in Figures 6(A) and (B), the lower evaluation (B) is corrected (subtracted) from the upper evaluation (A) to evaluate the longitudinal variation of the effective diameter. Here, Z is written in Figures 6(A) and (B), but the coordinate system is defined as shown in (C). That is, the Z axis is taken in the longitudinal direction of the workpiece 212, and the X axis is taken perpendicular to it. Then the rotation C axis is taken around the Z axis. The outer circumference is the part of the workpiece 212 without grooves, and the diameter evaluated in the outer circumference is called the outer diameter. This coordinate definition will be used from here on.

[0022] As mentioned above, it is preferable to have screw grooves, and the longitudinal average of the measurement results of the groove portion is compared with the gauge, but this is not necessarily required. In this case, a comparison with B in Figure 6 is obtained, and the relationship between B and A is geometrically calculated and used as a substitute. In other words, even when the tip ball 803 of the stylus 802 is of a different size than the ball 806 used when it is used as a ball screw, as shown in Figure 7 above, the effective diameter is calculated by simulating the contact between the tip ball 803 and the cross-section, assuming that the cross-sectional shape of the screw groove is as designed.

[0023] Furthermore, a single-directional movement operation that cuts a helical groove into the workpiece while maintaining a constant depth of cut is defined as one pass. The evaluation cycle for correction is performed not one pass before the final pass, but several passes before the target dimension. Based on this evaluation, the depth of cut is corrected (adjusted) in the longitudinal direction, and the average is recorded as the average depth of cut. After performing one or several corrected passes, the evaluation cycle is performed again. There, the average depth of cut and a coefficient of 1 for dimensional change are calculated. Furthermore, the effective diameter is measured before the final finishing pass, and the depth of cut and correction amount are adjusted so that the correction process is completed when the target diameter is reached. Monitoring is performed to ensure that the correction does not exceed the depth of cut required to finish to the target diameter.

[0024] The coefficient 2 of the longitudinal distribution B of the diameter correction is locally determined from the difference between the longitudinal distribution of the correction (excluding the average of the depth of cut) and the longitudinal distribution of the diameter in the evaluation result. Coefficient 2 is not a single number but a sequence or formula with a longitudinal distribution. These are used to calculate the next correction. That is, the number of passes required to reach the target dimensional tolerance is calculated from coefficient 1, and the system is automatically set to perform another evaluation cycle just before finishing. For the longitudinal distribution of the effective diameter, a depth of cut distribution is applied (corrected) in the longitudinal direction to reduce distribution B, similar to the system shown in Japanese Patent Publication No. 2021-117061. The correction machining and evaluation cycle are repeated. The system is adjusted so that the longitudinal distribution of the effective diameter converges to 0 first.

[0025] Figure 9 above shows the operation flow when corrective machining is performed at the end of automatic operation (a method that focuses solely on the axial distribution of the effective diameter) when the axial distribution of the effective diameter of a ball screw is obtained using the method for measuring the effective diameter of a ball screw shown in Japanese Patent Application Publication No. 2021-117061. In this machining flow, as shown in Figure 9, machining is started (S601), and rough machining is performed (S602). In the subsequent precision machining operation flow S600A, the processes of machining (S603), machining (S604), ... machining (S605) are performed. After this, the diameter of the ball screw is measured and evaluated (S606), and it is determined whether or not the diameter of the ball screw is within tolerance (S607). If it is within tolerance in S607 (Yes in S607), the precision machining is completed (S608). If the tolerance is not met in S607 (No in S607), a correction is created (S609), the depth of cut setting is updated (S610), and the grinding wheel coordinates are set according to the machining progress (S611). Then, machining is performed according to this correction (S612). After this machining, the process returns to S606, where the diameter of the ball screw is measured and evaluated (S606), and the same process is repeated until the diameter of the ball screw is within tolerance.

[0026] On the other hand, if the distribution of the effective diameter is too large before or during the iterative process and there is a strong probability that the target will not be met, the operator may be notified. In the above case, the operator can provide a coefficient to indicate whether to prioritize the distribution variation or the dimensional target, and proceed to the next machining operation. At that time, the priority ratio is calculated from coefficient 1 × operator's dimensional priority coefficient : coefficient 2 × operator's effective diameter variation priority coefficient. This will be explained in detail below using a formula. First, if there is a difference between the commanded depth of cut and the actual depth of cut (the difference between the evaluation of the diameter in that operation and the evaluation of the diameter in the previous operation), it is necessary to obtain the proportionality coefficient, which can be obtained by the following formula (1).

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[0027] Figure 10 is a diagram illustrating a method for measuring the effective diameter of a ball screw according to a second embodiment of the present invention, and shows the operation flow when corrective machining is performed at the end of automatic operation. In this operation flow, as shown in Figure 10, cutting is started (S701), and rough machining is performed (S702). In the subsequent fine machining operation flow S700A, the processes of cutting (S703), cutting (S704), ... cutting (S705) are performed. After this, the diameter of the ball screw is measured and evaluated (S706), and it is determined whether or not the diameter of the ball screw is within tolerance (S707). If it is within tolerance in S707 (Yes in S707), fine machining is completed (S708). If it is not within tolerance in S707 (No in S707), it is determined whether or not there is insufficient material removal (S709). If there is no material loss in S709 (No in S709), a correction is created (S710), the depth of cut setting is updated (S711), and the grinding wheel coordinates are set according to the machining progress (S712). Then, machining is performed according to this correction (S713). After this machining, the process returns to S706, where the diameter of the ball screw is measured and evaluated (S706), and the same process is repeated until the diameter of the ball screw is within tolerance. However, in the example shown in Figure 10, if it is determined that there is insufficient material loss in S709 (Yes in S709), the system displays a warning to the operator (S714). This allows the operator to take action to resolve the material loss. As an alternative example (modification), an OPTIONAL process (S700B) may be provided. In this case, even if it is determined in S709 that there is insufficient material removal (Yes in S709), if the operator instructs to continue (S715), the system will determine whether or not to continue (S716). If it does not continue (No in S716), the precision machining will be stopped and the process will end (S708). If it is decided to continue in S716 (Yes in S716), a correction will be created (S710), and the subsequent processes will be executed. [Explanation of symbols]

[0028] 100 Screw grinding device (screw grinding machine), 102 Column, 104 Groove bottom, 106 Diameter measuring section, 200 Grinding wheel base, 202 Grinding wheel rotating device (motor), 204 Grinding wheel, 206 X Linear motion device, 208 X Linear motion guide, 210 Workpiece table, 212 Workpiece, 214 Z linear motion device, 216 Z linear motion guide, 218 A axis swivel device, 220 C axis (workpiece) rotation device, 222 Tailstock (work support device), 800 Probing device, 802 Stylus, 803 Tip sphere, 804 Probing device swivel mechanism, 806 Ball for use as a ball screw (ball used in operation)

Claims

1. A screw grinding apparatus having a ball screw effective diameter measuring system for measuring the effective diameter of a screw groove machined on a ball screw shaft, which is the workpiece to be machined, wherein the ball screw effective diameter measuring system comprises a first step of measuring and evaluating the outer diameter of the ball screw shaft and a second step of measuring and evaluating the portion corresponding to the effective diameter of the screw groove, and the ball screw effective diameter measuring system evaluates the effective diameter of the ball screw by calculating the difference between the outer diameter and the evaluation corresponding to the effective diameter at corresponding positions in the longitudinal direction of the ball screw, divided into the first and second steps, and in a screw grinding apparatus that performs corrective machining using the evaluation of the effective diameter measuring system, a gauge of known diameter for comparison is placed coaxially with the workpiece to be machined. A screw grinding apparatus characterized by providing a gauge, measuring the diameter against the gauge and using the measured value itself, or if the measured value is not the effective diameter of the target groove, adjusting it to a value equivalent to the effective diameter by considering the groove depth and using this as a reference value, comparing this reference value with the average value in the longitudinal direction of the measured result of the groove portion to determine the deviation of the effective diameter from the target dimension, and performing the correction machining according to this deviation, as well as machining the ball screw shaft as the workpiece to be machined, measuring and evaluating the distribution of the diameter of the workpiece in the longitudinal direction, and performing the correction machining while evaluating the remaining material to finish to the target diameter, or, if the distribution cannot be corrected to the target diameter, requesting a judgment from the operator.

2. A screw grinding method using a ball screw effective diameter measuring system for measuring the effective diameter of a screw groove machined on a ball screw shaft, which is the workpiece to be processed, wherein the ball screw effective diameter measuring system comprises a first step of measuring and evaluating the outer diameter of the ball screw shaft and a second step of measuring and evaluating the portion corresponding to the effective diameter of the screw groove, and the ball screw effective diameter measuring system evaluates the effective diameter of the ball screw by calculating the difference between the outer diameter and the evaluation corresponding to the effective diameter at corresponding positions in the longitudinal direction of the ball screw, divided into the first and second steps, and using the evaluation of the effective diameter measuring system A screw grinding method that performs corrective machining, wherein the outer diameter of a part or the whole of the unmachined cylindrical portion of the ball screw shaft, which is the workpiece to be machined, is measured in advance, and a calculated value representing the position of the difference from the average value is determined as a reference value, and the deviation of the effective diameter from the target dimension is determined by comparing the reference value with the average value in the longitudinal direction of the measurement result of the groove portion, and the corrective machining is performed according to the deviation, wherein the screw grinding method allows specifying in advance, as a coefficient, whether to prioritize the local effective diameter distribution or the effective diameter.

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