Patent Description:
When machining is performed by a numerically controlled machine tool, unwanted streak-like machining marks may appear on the machined surface, due to trajectory errors at quadrant glitches or steps that occur when the motion direction of the feed axis is reversed. Trajectory errors at quadrant glitches and steps can be reduced by appropriately setting the controller parameters of the controller. Patent Literature <NUM> discloses a method for adjusting numerical controller parameters based on measurement results of circular motion trajectory. Furthermore, Patent Literature <NUM> describes performing circular motion test, and adjusting parameters so as to minimize trajectory errors.

The present invention aims to provide a motion evaluation method for evaluating motion characteristics of a numerically controlled machine tool based on characteristics which are visible to a person, and to provide a workpiece machining method in which parameters are adjustable based on such evaluation.

In order to achieve the above object, the present invention provides a parameter adjustment method as defined in the appended claims.

According to the present invention, a method for evaluating an object surface based on visual characteristics to a person is provided. Furthermore, according to the present invention, it can be easily judged whether or not streak-like machining marks on a machined surface are visible to a person, the effect of which is significant.

Quadrant glitches and step-like machining marks occur when the motion direction of a feed axis is reversed such as along a cylindrical surface or in a circumferential groove using a machine tool comprising an at least three-axis feed device which machines a workpiece by moving a tool mounted on a spindle and a workpiece relative to each other. Non-Patent Literature <NUM> defines a circular motion test by numerical control accompanied by such feed axis reversal. The circular motion test results are evaluated by enlarging the radial error of the circular motion trajectory. Examples of circular motion trajectory measurement results are shown in <FIG>.

<FIG> shows step-like trajectory errors of about <NUM> micrometers which occur at the time of quadrant switching in which each feed axis is reversed. <FIG> shows protrusion-like trajectory errors (quadrant glitches) of about <NUM> micrometers which occur at the time of quadrant switching in which each feed axis is reversed. Conventionally, parameters are adjusted so as to minimize trajectory errors to the greatest extent possible. Specifically, the trajectory shown in <FIG> is more preferable than the trajectory shown in <FIG>.

These trajectory errors appear as streak-like machining marks on the machined surface. <FIG> shows the results of photography of a machined surface in which trajectory errors have occurred in the vicinity of <NUM>° when cylindrical machining is performed using a peripheral blade of a square end mill under conditions identical to those measured for the motion trajectories of <FIG>. With reference to <FIG>, while clear streak-like machining marks appear in <FIG>, in which the trajectory errors are small, machining marks cannot be observed in <FIG>. Thus, from the viewpoint of the appearance of the machined surface, the trajectory shown in <FIG> is more preferable than the trajectory shown in <FIG>. In the conventional evaluation method and adjustment method, the appearance of the machined surface is not considered in the evaluation results, and adjustment of the parameters does not necessarily reduce the appearance of defects.

The preferred embodiments of the present invention for solving such problems will be described below with reference to the attached drawings.

With reference to <FIG>, the parameter adjustment device <NUM>, as the object surface evaluation device of the present invention, comprises a motion evaluation device <NUM>, and a parameter change unit <NUM>. The motion evaluation device <NUM> comprises, as primary constituent elements, a circular motion trajectory data acquisition unit <NUM>, a trajectory analysis unit <NUM> including a normal direction change rate calculation unit <NUM>, a visible limit data storage unit <NUM> and a polar coordinate change unit <NUM>, and a display unit <NUM>. The trajectory analysis unit <NUM> can be constituted by a CPU, RAM, ROM, hard disk, SSD, bidirectional busses for connecting these components, and relevant programs. The display unit <NUM> can be constituted by a liquid crystal panel or a touch panel.

The circular motion trajectory data acquisition unit <NUM>, as will be described later, acquires circular motion trajectory data or coordinate values of the feed axes from the NC device of the machine tool <NUM> when spindle of the machine tool <NUM> undergoes in-plane circular motion. Alternatively, the circular motion data may be obtained by performing cylindrical machining on a workpiece and measuring the shape thereof using a roundness measurement instrument or the like.

Furthermore, the parameter change unit <NUM> changes the control parameters of the machine tool <NUM> in accordance with commands input by the operator via the input device <NUM>. The input device <NUM> can be, for example, a keyboard, a mouse, or alternatively, can be the touch panel constituting the display unit <NUM>.

In general, a person can visually recognize a shape change in portions in which the normal direction change rate of the object surface is large, and a person cannot visually recognize a shape change in portions in which the normal direction change rate is small. The limits of normal direction change rate at which a person can visually recognize shape change are stored in the visible limit data storage unit <NUM>. These limits of normal direction change rate at which a person can visually recognize shape change can be obtained by preparing a plurality of test pieces having a plurality of different known normal direction change rates, determining whether the shape change can be visually recognized by a plurality of observers, and averaging the normal direction change rates at that time.

The normal direction change rate calculation unit <NUM> calculates the normal direction change rate of the circular motion trajectory of the machine tool <NUM> based on the circular motion trajectory data from the circular motion trajectory data acquisition unit <NUM>. The normal direction change rate will be described with Reference to <FIG>. The circular motion trajectory data from the circular motion trajectory data acquisition unit <NUM> includes two-dimensional coordinate values. The example shown in <FIG> is partial cross-sectional view in which a cylindrical workpiece W has been cut along the plane (XY plane) perpendicular to the Z-axis. Normal vectors can be set at predetermined intervals on the surface of the workpiece W. The workpiece W is cut at predetermined intervals along the XY plane. By setting normal vectors at predetermined intervals in each cutting plane, it is possible to evaluate the entire surface of the workpiece W.

Set points <NUM> are set at predetermined intervals along the machined surface of the workpiece W. Next, at the set points <NUM>, normal vectors ni perpendicular to the surface inclination are set. The normal vectors ni are normal vectors of the ith set point <NUM>. Angles θi with respect to the normal direction can be set for the normal vectors ni. The angle relative to the Y-axis is set as the normal direction angle θi.

In <FIG>, the coordinate values of the ith set point <NUM> and the (i+<NUM>)th set point <NUM> are known. A vector ai can be set based on the coordinate values of these two set points <NUM>, <NUM>. The vector ai is the vector from set point <NUM> toward set point <NUM>. The vector perpendicular to vector ai can be set as the normal vector ni. The normal direction angle θi at this time can be calculated from the following formula (<NUM>). The normal direction angle θi for the ith set point of the machined surface can be calculated in this manner. [Formula <NUM>]<MAT> θi is the normal direction angle at the ith set point.

The normal direction change rate calculation unit <NUM> calculates the normal direction change rate at the set point <NUM>. The normal direction change rate is the rate of change of the angle of the normal direction of mutually adjacent set points. An example thereof is the change rate from the normal direction angle θi to the normal direction angle θi+<NUM>. The normal direction change rate can be calculated from the following formula (<NUM>). The following formula (<NUM>) represents the normal direction change rate at the ith set point <NUM> of the design shape. The normal direction change rate of the evaluation target shape can also be calculated by the same method. Note that it is geometrically clear that the normal direction change rate is the same as the change rate in the direction tangential to the machined surface. [Formula <NUM>] <MAT> dθi /dx is the normal direction change rate.

The polar coordinate change unit <NUM> changes the normal direction change rate obtained in this manner to polar coordinates, and transmits the change rate to the display unit <NUM> along with the visible limit values of the normal direction change rate stored in the visible limit data storage unit <NUM>. <FIG> shows the calculation results of the normal direction change rate calculation unit <NUM> displayed on the display unit <NUM>.

In <FIG>, the visible limit values of the normal direction change rate are represented by dashed lines. <FIG> correspond to <FIG> and <FIG>. Note that the normal direction change rate of the trajectory is obtained by extracting only spatial frequency components which are visually recognizable by a person from a geometric normal direction change rate of the trajectory. The range of spatial frequency components which are visually recognizable by a person may be determined based on an ophthalmologic contrast sensitivity curve or may be determined using a shape separately prepared for evaluation.

Further, in the drawings, the separately evaluated human normal direction change rate visual recognition limit is also represented by a dashed line. When <FIG> are compared with each other, a greater normal direction change rate occurs in <FIG>, and even if the error of the circular motion trajectory shown in <FIG> is smaller than that of the circular motion trajectory shown in <FIG>, specifically, even if the machining accuracy is high, clear machining marks occur as shown in <FIG>. Thus, according to the motion evaluation device <NUM>, the motion evaluation method of the present invention enables motion evaluation corresponding to the appearance of the machined surface by causing the machine tool <NUM> to perform circular motion and acquiring the trajectory data thereof prior to machining.

Furthermore, the operator of the machine tool <NUM> refers to the normal direction change rate displayed on the display unit <NUM>, and when there is a normal direction change rate which is equal to or greater than the visually recognizable limit, the operator corrects the control parameters of the machine tool via the input device <NUM> and the parameter change unit <NUM>, and repeats this process until the normal direction change rate is equal to or less than the visually recognizable limit. The adjustment target control parameters include position loop gain, speed loop gain, speed loop integral gain or time constant, friction compensation parameters, and backlash correction parameters.

Next, an application example of the parameter adjustment device <NUM> of the present invention will be described with reference to <FIG>. In the example shown in <FIG>, the circular motion trajectory data acquisition unit <NUM> is constituted by the NC device of the machine tool <NUM>. In the machine tool <NUM> of <FIG>, the parameter adjustment device <NUM> is combined with the machining device <NUM>. The machining device <NUM> comprises, as primary constituent elements, a bed <NUM> as a base secured to the floor of a factory, a table <NUM> which is attached to the upper surface of the bed <NUM> and on an upper surface of which the workpiece W is secured, a spindle head <NUM> which supports a spindle <NUM>, on a tip of which a tool T facing the workpiece W secured to the bed <NUM> is mounted, so as to be rotatable around a vertical axis of rotation O, a drive mechanism <NUM> for reciprocally driving the spindle head <NUM> in the X-axis, Y-axis, and Z-axis orthogonal directions relative to the bed <NUM>, and an NC device <NUM> for controlling the servomotors of the drive mechanism <NUM>.

The drive mechanism <NUM> comprises, for example, X-axis, Y-axis, and Z-axis ball screws (not illustrated), nuts (not illustrated) for engagement with the ball screws, X-axis, Y-axis, and Z-axis drive motors Mx, My, and Mz consisting of servomotors connected to one end of each of the X-axis, Y-axis, and Z-axis ball screws for rotationally driving the X-axis, Y-axis, and Z-axis ball screws. Furthermore, in addition to the three orthogonal feed axes of X-, Y-, and Z-axes, the machine tool <NUM> may include one or more rotational feed axes such as an A-axis for rotationally feeding about the X-axis in the horizontal direction, or a C-axis for rotationally feeding about the Z-axis in vertical direction. In such a case, in addition to the X-axis, Y-axis, and Z-axis drive motors Mx, My, and Mz, the drive mechanism <NUM> may include servomotors for the rotational feed axes such as the A-axis and C-axis.

The machining device <NUM> is provided with digital scales (not illustrated) for detecting the positions of the X-, Y-, and Z-feed axes, and the position of each of the feed axes is fed back to the NC device <NUM>. The circular motion trajectory data acquisition unit <NUM> of the motion evaluation device <NUM> receives trajectory data from the NC device <NUM> when the spindle <NUM> of the machining device <NUM> undergoes circular motion in the XY plane.

Next, another application example of the parameter adjustment device <NUM> of the present invention will be described with reference to <FIG>. In the example shown in <FIG>, the machine tool <NUM> comprises a measurement instrument <NUM> such as a ball bar gauge or a cross-grid scale. In the example of <FIG>, the circular motion trajectory data acquisition unit <NUM> receives trajectory data from the NC device <NUM> when the spindle <NUM> of the machining device <NUM> undergoes circular motion in the XY plane.

In the configurations of <FIG> and <FIG>, the parameter adjustment device <NUM> can be incorporated as a part of the control program of the machine controller (not illustrated) of the machining device <NUM> or the NC device <NUM>. In this case, the display unit <NUM> and the input device <NUM> can be constituted by the touch panel (not illustrated) provided on the control panel (not illustrated) of the machining device <NUM>.

<FIG> shows an example in which the adjustment method according to the present invention is applied. <FIG> is circular motion trajectory display results according to the prior art, and <FIG> is display results according to the present invention. As shown in <FIG>, according to the present invention, the normal direction change rate of the trajectory is equal to or less than the human visually recognizable limit represented by the dashed line. The normal direction change rate of the trajectory is obtained by extracting only spatial frequency components which are visually recognizable by a person from a geometric normal direction change rate of the trajectory.

<FIG> shows the results of photography of a machined surface in which trajectory errors have occurred in the vicinity of <NUM>° when cylindrical machining is performed using a peripheral blade of a square end mill under conditions identical to those measured for the motion trajectories of <FIG>. According to <FIG>, there are no visible streak-like machining marks on the machined surface, and it can be seen that a machined surface without visual defects can be obtained using the parameter adjustment method according to the present invention.

Claim 1:
A parameter adjustment method for evaluating a motion characteristic of a numerically controlled machine tool (<NUM>) using a circular motion test and adjusting control parameters of the machine tool (<NUM>), wherein the control parameters include position loop gain, speed loop gain, speed loop integral gain or time constant, friction compensation parameters and backlash correction parameters, and wherein the method comprises the steps of:
(<NUM>) by a normal direction change rate calculation unit (<NUM>), calculating a normal direction change rate of a trajectory from a circular motion trajectory obtained from a circular motion trajectory data acquisition unit (<NUM>);
(<NUM>) by a polar coordinate change unit (<NUM>), changing the normal direction change rate into polar coordinates;
(<NUM>) by a display unit (<NUM>), displaying the normal direction change rate of the trajectory as polar coordinates along with a normal direction change rate limit at which a shape change can be visually recognized by a person;
(<NUM>) by a parameter change unit (<NUM>) and associated input device (<NUM>), adjusting the control parameters of the machine tool (<NUM>) when the normal direction change rate is equal to or greater than the normal direction change rate limit; and
(<NUM>) repeating steps <NUM> to <NUM> until the normal direction change rate is equal to or less
than the normal direction change rate limit.