Touch probe calibration method

Abstract

To provide a calibration method with which deviation of a sensor position of a touch probe can be calibrated on an NC-controlled machine tool.SOLUTION: A touch probe calibration method has a step of bringing a stylus of the touch probe into contact with an object at a predetermined feed rate, and on the basis of a trigger signal output upon contact of the stylus with the object, acquiring a skip coordinate value of the object; and retracting the stylus from the object to acquire an OFF coordinate value at which the trigger signal is turned OFF, and the method correcting an overshoot amount from a difference between the OFF coordinate value and the skip coordinate value.SELECTED DRAWING: Figure 1

Description

[[Technical Field]]

[0001] The present invention relates to a calibration method for a touch probe used when measuring the position of an object such as the end face of a work chuck portion, the position of various tools, and a reference surface of a work in a machine tool. [[Background Art]]

[0002] In machine tools, when machining a workpiece, it is necessary to measure and correct positions serving as machining references, positions of tools, and the like in advance using a position sensor such as a touch probe in order to achieve high-precision machining.

[0003] A stylus (contact) provided on a touch probe is advanced toward an object to be measured, and the position of the object is measured by a trigger signal output when the stylus contacts the object. However, when the stylus advances toward the object, there occurs a coasting distance (overshoot amount) from when the tip ball or the like of the stylus actually contacts the object to when it stops in response to the trigger signal. The overshoot amount is caused by many factors such as core deviation of the tip ball of the touch probe, sensor characteristics of the touch probe, differences in the contact direction of the touch probe, and changes in environmental temperature. Therefore, this calibration is preferably performed immediately before machining is performed.

[0004] In a conventional calibration method using a ring gauge, it is necessary to align the stylus axis with the center of the ring gauge, which requires adjustment work. In addition, the work of attaching and detaching the calibration ring gauge is also problematic. For example, Patent Document 1 discloses that by using a machine capable of positioning the positional relationship between a spindle and an object and controlling a rotating shaft to index a jig to be measured to a plurality of positions, the indexed position of the jig to be measured in a three-dimensional space is measured to calibrate a position measurement sensor. Even in this case, it is necessary to measure the initial position in advance using the jig to be measured. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2016-83729 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The present invention aims to provide a calibration method that can calibrate the sensor position deviation of a touch probe on an NC-controlled machine tool. [Means for solving the problem]

[0007] The present invention relates to a method for calibrating a touch probe, comprising the steps of: bringing a stylus on the touch probe into contact with an object at a predetermined feed rate; obtaining a skip coordinate value of the object based on a trigger signal when the stylus contacts the object; then moving the stylus back from the object and obtaining an OFF coordinate value when the trigger signal turns OFF; and correcting the amount of overshoot from the difference between the OFF coordinate value and the skip coordinate value. Here, any method is acceptable for retracting the stylus, as long as the amount of retraction that turns off the trigger signal can be obtained, such as repeatedly retracting it in the direction away from the target at a predetermined pitch, or gradually retracting it.

[0008] Here, the object is the end face of the chuck portion of a workpiece in an NC-controlled machine tool, a tool mounted on the machine tool, or a workpiece, and calibration can be performed automatically on the machine tool. [Effects of the Invention]

[0009] The calibration method for a touch probe according to the present invention involves contacting a stylus with the end face of the spindle or tool axis of a machine tool, the reference position of the tool length, the machining position of the workpiece, etc., and measuring the overshoot amount and reference position from the skip coordinate value obtained by a trigger signal, and then operating in the direction away from the stylus to obtain the OFF coordinate value when the trigger signal turns OFF. Since calibration can be performed automatically on the machine before machining as needed, it can easily respond to changes in the ambient temperature of the machine tool, time-series changes, etc. [Brief explanation of the drawing]

[0010] [Figure 1] A schematic diagram of the calibration method according to the present invention is shown. [Figure 2] The measurement results for the amount of overshoot are shown. [Figure 3] The graph shows the average value of the overshoot and the 2σ variability. [Modes for carrying out the invention]

[0011] Figure 1 shows an explanatory diagram of the calibration according to the present invention. The tip of the stylus is advanced relative to the workpiece or other object in the axial direction of the stylus, or perpendicular to it. A trigger signal is output when the tip of the stylus makes contact, causing the stylus to stop. The amount of overshoot is measured from the difference between this skip coordinate value A and the coordinate value B, where the stylus tip is gradually pulled away from the object and the trigger signal turns OFF.

[0012] The overshoot amount was measured using two methods: one using a shim and the other using a program based on the present invention. The results of this comparison are shown in Figures 2 and 3. As shown in Figure 2(b), five measurements were taken when the position-controlled stylus tip ball was brought into contact with one of the spindle end faces of a two-spindle opposing lathe in direction a, and when it was rotated by b=90°, c=180°, and d=270° relative to that direction, as well as when it was brought into contact with the end face in the stylus axis direction (Z axis direction). The stylus feed rate was set to 50 mm / min. The shim shown in Figure 2(a) and the measurement method in the program are as follows. Shim: Use a shim plate to measure the position where the stylus ball contacts the chuck end face, and define that position as Z0. The absolute value of the Z coordinate obtained by skipping forward becomes the amount of overshoot. Program: Starting from a state where the probe is in contact with the chuck end face using skip feed, gradually withdraw the probe (repeatedly in 1 μm increments) until the probe's trigger signal turns OFF. The difference between the skip coordinate value and the coordinate value when the trigger is turned OFF is the amount of overshoot. Figure 2(a) shows the measurement results (in mm) for n=10 trials, and the graph in Figure 3 shows the mean and the 2σ variability range. The amount of overshoot is somewhat affected by the stylus contact direction, but when the contact direction is kept constant, it was within 3 μm at 2σ. When comparing the overshoot amount measured using a shim plate with the overshoot amount obtained using the program (on-machine) according to the present invention, the difference is a maximum of 6 μm, which is sufficiently practical for general machining accuracy.

Claims

1. A method for calibrating a touch probe, The steps include: bringing the stylus of the touch probe into contact with the object at a predetermined feed rate, and obtaining the skip coordinate values ​​of the object based on the trigger signal that the stylus has come into contact with the object; Next, the stylus is moved back from the object, and the OFF coordinate value at which the trigger signal turns OFF is obtained. A method for calibrating a touch probe, characterized by correcting the amount of overshoot from the difference between the OFF coordinate value and the skip coordinate value.

2. The object is the end face of the chuck portion of a workpiece on an NC-controlled machine tool, a tool mounted on the machine tool, or a workpiece, and the calibration method for a touch probe according to claim 1 is characterized in that the calibration is performed automatically on the machine tool.

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