INCORPORATION BY REFERENCE
The disclosure of Japanese Patent Application No. 2013-015276 filed on Jan. 30, 2013 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a measuring method and a grinding machine.
2. Description of the Related Art
In a grinding process, the diameter of a workpiece is measured by a sizing device during grinding in order to obtain a desired diameter, and the grinding is stopped when the diameter of the workpiece reaches a target value. In order to maintain the accuracy of measurement by the sizing device at a high level, it is necessary to start the measurement after settlement of variations (drift) in the measured value during activation of the sizing device, the variations being due to heating caused by energization of the sizing device. Therefore, a long warm-up time is required during activation of the sizing device. In order to shorten the warm-up time by attenuating the impact of heat, in some sizing devices including a differential transformer, coils and components, which constitute the differential transformer, are made of special materials. Refer to, for example, Japanese Patent Application Publication No. 09-113203.
However, even if the sizing device is made of special materials, it is difficult to completely eliminate the impact of heat. Further, using the special materials increases the cost for manufacturing the sizing device.
SUMMARY OF THE INVENTION
One object of the invention is to provide a measuring method and a grinding machine that allow even a commonly-used measuring device to achieve a desired measurement accuracy without the need for warm-up or with a short warm-up time.
In a measuring method according to a first aspect of the invention: a physical quantity is measured by a measuring device in a main process that is repeated with a period of a process time having a prescribed length; and the measuring device is activated only for a measurement time that is shorter than the process time to measure the physical quantity.
In the measuring method according to the first aspect:
a relationship among t0, t1, t2 is expressed by a formula, t0=t1+t2
where to is the process time, t1 is the measurement time, and t2 is a deactivated time during which the measuring device is deactivated in the main process; and
where a temporal variation in a measured value of the physical quantity measured by the measuring device is a drift, a temporal variation in the measured value after activation of the measuring device is an activation drift, and a temporal variation in the measured value after deactivation of the measuring device is a deactivation drift,
the measuring method is executed using
a graph a indicating an activation drift characteristic of the measuring device,
a graph b indicating a deactivation drift characteristic of the measuring device, and
a prescribed allowable measurement error E, and
the measuring method may include
a step of determining a section in which the activation drift is equal to or smaller than the allowable measurement error E within a range of the measurement time t1, from the graph a,
a step of determining a section in which the deactivation drift is equal to or smaller than the allowable measurement error E within a range of the deactivation time t2, from the graph b, and
a step of adjusting a warm-up time tD from activation of the measuring device to start of measurement so that the process time t0 that is expressed by t1+t2 becomes equal to a prescribed value.
According to the measuring method in the first aspect, the ratio of the activated time of the measuring device to the process time of the main process is reduced. Therefore, it is possible to provide the measuring method that reduces the drift of the measured value obtained by the measuring device. In addition, according to the measuring method described above, the drift is equal to or smaller than a desired value and therefore measurement is carried out with a high degree of accuracy even if the warm-up time of the measuring device is shortened.
A grinding machine according to a second aspect of the invention includes:
a grinding wheel;
a feed device that feeds the grinding wheel relative to a workpiece so that the grinding wheel cuts into the workpiece;
a sizing device that measures a size of the workpiece; and
a controller that warms up the sizing device for a prescribed warm-up time, and then repeatedly executes a grinding process having a prescribed grinding process time and including a plurality of infeed steps that vary in a feed speed at which the grinding wheel is fed relative to the workpiece,
wherein the controller controls the feed device and the sizing device so that
in the grinding process, each of the infeed steps ends at an infeed position that is computed based on a measured value obtained by measuring a processed size of the workpiece in the infeed step at a prescribed time with use of the sizing device that is activated only for a sub-measurement time, an infeed position of the infeed device at the prescribed time, and a grinding condition for the infeed step, and
in a final infeed step, feed of the feed device ends when a measured value measured by the sizing device that is activated only for a final measurement time reaches a desired value, and
wherein the grinding machine measures the size of the workpiece by the measuring method according to the first aspect.
With the grinding machine according to the second aspect, it is possible to grind the workpiece with a desired dimensional accuracy, even if the warm-up time of the sizing device is shortened.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:
FIG. 1 is a plan view illustrating the overall configuration of a grinding machine according to embodiments of the invention;
FIG. 2 is a graph illustrating drifts in a measuring device;
FIG. 3 is a graph illustrating fluctuations in the drift when the measuring device is activated and then deactivated;
FIG. 4 is a graph illustrating fluctuations in the drift in a first embodiment of the invention;
FIG. 5 is a graph illustrating the relationship between the activated time and the deactivated time in a second embodiment of the invention;
FIG. 6 is a graph illustrating fluctuations in the drift in the second embodiment;
FIG. 7 is a graph illustrating a method of obtaining a correction value in a third embodiment of the invention;
FIG. 8 is a graph illustrating a grinding process in a fourth embodiment of the invention;
FIG. 9 is a flowchart illustrating the grinding process in the fourth embodiment; and
FIG. 10 is a block diagram illustrating control in a modified example of the embodiments.
DETAILED DESCRIPTION OF EMBODIMENTS
First, characteristics regarding the accuracy of measurement by a commonly-used measuring device during activation will be described. When the measuring device is activated, components of the measuring device are energized and the temperatures of the components are increased due to heating by electric resistances. The output characteristics of the measuring device vary as the temperatures of the components increase. As a result, the measured value varies with the lapse of time after the activation of the measuring device. The variations are usually referred to as “drift”.
An embodiment of the invention will be described based on an example in which the diameter of a workpiece is measured by a sizing device that serves as a measuring device in a grinding machine. As illustrated in FIG. 1, a grinding machine 1 includes a bed 2, and a grinding head 3 (radial feed device) and a table 4 that are arranged on the bed 2. The grinding head 3 is able to reciprocate in an X-axis direction, and the table 4 is able to reciprocate in a Z-axis direction orthogonal to the X-axis direction. The grinding head 3 supports a grinding wheel 7 such that the grinding wheel 7 is rotatable, and is provided with a wheel spindle motor (not illustrated) that rotates the grinding wheel 7. A main spindle 5 and a tailstock 6 are arranged on the table 4. The main spindle 5 holds and supports one end of a workpiece W such that the workpiece W is rotatable, and the main spindle 5 is driven to be rotated by a main spindle motor (not illustrated). The tailstock 6 supports the other end of the workpiece W such that the workpiece W is rotatable. The workpiece W is supported by the main spindle 5 and the tailstock 6, and is driven to be rotated during grinding. A sizing device 10 that measures the diameter of the workpiece W is arranged on the table 4 at such a position that the sizing device 10 is opposed to the grinding wheel 7 across the workpiece W.
The grinding machine 1 is provided with a controller 30. The controller 30 includes, as functional units, an X-axis control unit 301 that controls the feed of the grinding head 3, a Z-axis control unit 302 that controls the feed of the table 4, a grinding wheel control unit 303 that controls the rotation of the grinding wheel 7, a main spindle control unit 304 that controls the rotation of the main spindle 5, a sizing device control unit 305 that controls the sizing device 10, a storage unit 306 that stores programs and data, and a computation unit 307 that executes various computations.
FIG. 2 illustrates drift characteristics that are obtained when the diameter of the same portion of the workpiece W is measured by the sizing device 10 continuously for a certain time. The sizing device 10 continuously measures the same diameter, which does not vary, with a prescribed measurement sampling period. The measured value is supposed to be a constant value (because FIG. 2 is used to obtain the drift characteristics, each curve exhibits a value obtained by subtracting the diameter from the actually measured value. Therefore, the constant value is supposed to be zero). However, in actuality, the measured value varies in a curve under the influence of the drift. A curve a exhibits the drift that indicates temporal variations of the measured value after the sizing device 10 is activated and energized, and that is referred to as “activation drift”. The activation drift becomes larger with the lapse of time. The rate of increase in the activation drift is decreased with the lapse of time, and the measured value is saturated at a substantially constant value after the lapse of a certain time. The sizing device 10 is deactivated with the measured value saturated, and the sizing device 10 is then activated for a short time to carry out measurement after the lapse of a certain time. If the activation and deactivation of the sizing device 10 is repeated while the lapse time is changed, “deactivation drift” indicated by a curve b in FIG. 2 is obtained. The deactivation drift b is a curve having a shape obtained by vertically inverting the curve a.
If the sizing device 10 is deactivated after being activated for a certain time, the drift characteristics illustrated in FIG. 3 are obtained, that is, the measured value obtained by the sizing device 10 varies so as to be increased as indicated by the curve a during activation of the sizing device 10, and decreased as indicated by the curve b after the sizing device 10 is deactivated.
A first embodiment of the invention will be described below. In a measuring method according to the first embodiment, a main process is repeatedly executed with a period of time t0, which is a duration of the main process, without warming up the sizing device 10. In each main process, the sizing device 10 is activated only for a measurement time t1 to carry out measurement. In this case, the drift characteristics of the sizing device 10 are as illustrated in FIG. 4. In the first main process, when the sizing device 10 is activated to start the measurement, the measured value is increased as indicated by an activation drift characteristic illustrated by the curve a in FIG. 2, and then reaches a drift Da1 after the lapse of time t1. Then, the sizing device 10 is deactivated for a certain time t2 (=t0−t1). A deactivation drift characteristic of the sizing device 10 is as indicated by the curve b in FIG. 2.
As illustrated in FIG. 2, the curve b of which the start point coincides with the elapsed time on the abscissa axis, which corresponds to the drift Da1 on the ordinate axis, indicates the deactivation drift characteristic. As indicated by a curve b1, which is a curve extending through a section from the start point to a time point at which the certain time t2 has elapsed since the start point, the drift is decreased. An amount of decrease in the drift is Db1.
In the next main process, when the sizing device 10 is activated for the time t1, the drift is increased as indicated by a curve a2 of which the start point coincides with the elapsed time on the abscissa axis, which corresponds to a drift Da1−Db1 on the ordinate axis. An amount of increase in the drift is Da2. Next, when the sizing device 10 is deactivated for the time t2, the drift is decreased as indicated by a curve b2 of which the start point coincides with the elapsed time on the abscissa axis, which corresponds to a drift Da1+Da2−Db1 on the ordinate axis. An amount of decrease in the drift is Db2. Because the curve a is a curve with an increasing rate that gradually decreases with the lapse of time, the drift Da1 is larger than the drift Da2 (Da1>Da2). Because the curve b is a curve with a decreasing rate that gradually decreases with the lapse of time, the drift Db1 is smaller than the drift Db2 (Db1<Db2).
FIG. 4 illustrates fluctuations in the drift characteristics when the main process is repeated twice. As is understood from FIG. 2, the absolute value of Da1 is larger than the absolute value of Db1. If the main process is repeated n times, the absolute value of a drift Dan expressed by a curve an becomes substantially equal to the absolute value of a drift Dbn expressed by a curve bn. That is, n represents the number of times at which the drift varies within a certain range. At this time, a cumulative value Dm of the drifts is expressed by Dm=Da1+Da2+. . . +Dan−Db1−Db2− . . . −Db(n−1). The value of Dm is small if the cumulative value obtained by cumulating the values from Da1 to Dan is small and the cumulative value obtained by cumulating the values from Db1 to Db(n−1) is large. If the time t1 of measurement by the sizing device 10 is set short while the deactivated time t2 is set long, the absolute value of Da1 becomes smaller but the absolute value of Db1 becomes larger. As a result, the cumulative value Dm of the drifts becomes smaller. At this time, an error of measurement by the sizing device 10 is smaller than or equal to the cumulative value Dm of the drifts. Therefore, by setting the measurement time t1 and the deactivated time t2 such that the cumulative value Dm falls within an allowable measurement error range, it is possible to achieve a prescribed degree of accuracy in the measurements up to n times even without warming up the sizing device 10.
A second embodiment of the invention will be described below. In a measuring method in the second embodiment, the sizing device 10 is warmed up for a certain time, and in the main process repeated with a period of time t0, the sizing device 10 is activated only for a measurement time t1 to carry out measurement. The concept of setting the time of warm-up will be described with reference to FIG. 5 and FIG. 6. In FIG. 5, a solid line indicates the curve a that represents the activation drift, a dotted line indicates the curve b that represents the deactivation drift, and the curve a and the curve b are superimposed on each other in the same graph. In the drawings, t0 denotes a process time of the main process, t1 denotes a time of measurement by the sizing device 10, t2 (=t0−t1) denotes a deactivated time of the sizing device 10, and E denotes an allowable measurement error that is the maximum drift acceptable to the sizing device 10.
First, a range in which the fluctuation of the drift becomes the allowable measurement error E is determined. Specifically, as illustrated in FIG. 5, two horizontal parallel lines with an interval that is equal to the width of the allowable measurement error E are set. The measurement time t1 is set to a time between two points at which the curve a intersects with the parallel lines. The deactivated time t2 is set to a time between two points at which the curve b intersects with the parallel lines. The vertical positions of the two parallel lines are adjusted such that the sum of the measurement time t1 and the deactivated time t2 is equal to the process time t0. Note that, if a straight line L that passes through the intersection between the curve a and the curve b is located between the two parallel lines, one set of the measurement time t1 and the deactivated time t2 is obtained. On the other hand, if the straight line L is not located between the two parallel lines, two sets of the measurement time t1 and the deactivated time t2 are obtained. When both the two parallel lines are located below the straight line L, the measurement time t1 is shorter than the deactivated time t2. When both the two parallel lines are located above the straight line L, the measurement time t1 is longer than the deactivated time t2. When both the two parallel lines are located below the straight line L, whether the measurement time t1 is longer than an actually required measurement time is determined. When the measurement time t1 is longer than the actually required measurement time, this set of the measurement time t1 and the deactivated time t2 is employed. When the measurement time t1 in a section below the straight line L is shorter than the actually required measurement time, the set of the measurement time t1 and deactivated time t2 when both the two parallel lines are located above the straight line L is employed. A warm-up time tD is a time from the origin of the curve a to the start point of the measurement time t1.
FIG. 6 illustrates fluctuations of the drift when the times are set as described above. By adjusting the length of the warm-up time tD as an initial condition, it is possible to set the process time t0, which is expressed by the sum of the activated time t1 and the deactivated time t2, within a desired time range. Note that the adjustment of the warm-up time tD means the adjustment to the vertical positions of the two parallel lines in FIG. 5. According to the present embodiment, it is possible to carry out measurement with a desired degree of accuracy without limiting the number of times of the measurements to a value equal to or smaller than n as in the first embodiment.
A third embodiment of the invention will be described below. In a measuring method according to the third embodiment, the sizing device 10 is not warmed up, and in the main process in the early stage, in which the drift increases and which is among the main processes repeated with a period of time t0, the measured value is corrected by a predicted drift, which is computed for each main process. In this way, a desired accuracy of measurement is ensured.
When the main process time t0 and the measurement time t1 for the sizing device 10 are set, the deactivated time t2 (=t0−t1) is automatically set. The predicted drift can be obtained in advance by the following method. When the measurement time t1 and the deactivated time t2 are determined, temporal drift fluctuation curves can be obtained, as illustrated in FIG. 7, with the use of the curve a of the activation drift and the curve b of the deactivation drift. In FIG. 7, in the case where the measurement is carried out at a point P that is reached after a time tS has elapsed since the sizing device 10 is activated, the predicted drift in a first main process is Dp1, and the predicted drift in a second main process is denoted by Dp2. In a similar manner, there are obtained the predicted drifts at the time of measurements in the subsequent main processes up to an n-th main process in which an amount of increase in the drift and an amount of decrease in the drift are equal to each other. The values of the predicted drifts from Dp1 to Dpn are stored in the storage unit 306, as the values of the predicted drifts for the main processes arrayed in time sequence. In each actual main process, the measurement is carried out after the time tS has elapsed since the sizing device 10 is activated, and the thus obtained actually measured value is corrected by the predicted drift stored in the storage unit 306 in association with the number of the main process. The thus corrected value is used as the measured value after correction. With this measuring method, it is possible to obtain an accurate measured value without warming up the sizing device 10.
A fourth embodiment of the invention will be described below. In a measuring method according to the fourth embodiment, the sizing device 10 is activated for a plurality of sub-measurement times to carry out the measurement in a grinding process that is repeated with a period of a grinding process time.
FIG. 8 is a cycle diagram that illustrates X-axis positions (infeed (cutting-in) positions of the grinding wheel 7) in time sequence in the grinding process. The grinding process includes an infeed (cutting-in) step and a spark-out step. The infeed step includes a rough grinding step, a precise grinding step and a fine grinding step in the descending order of feed speed. In the spark-out step, grinding is carried out with the cutting-in stopped in order to enhance the accuracy of roundness. The timing of changeover between the steps is determined on the basis of the diameter of the ground workpiece W. The reason why the values of the X-axis positions, which are the infeed positions of the grinding wheel 7, are not used to determine the timing of changeover between the steps, is that the diameter of the ground workpiece W varies even at the same infeed position due to thermal displacements of various portions of the grinding machine 1 and variation in sharpness of the grinding wheel 7. Conventionally, the sizing device 10 is constantly activated to measure the diameter of the workpiece W, and when the diameter of the workpiece W reaches a prescribed value, the feed speed is changed to make changeover between the steps. In this case, because the sizing device 10 is continuously activated, warm-up of the sizing device 10 is continued until the drift in the initial stage of the activation of the grinding machine becomes constant.
In the present embodiment, in order to shorten the activated time of the sizing device 10 in the grinding process, the sizing device 10 is activated for the short sub-measurement time to measure the diameter of the workpiece W in each of the rough grinding step and the precise grinding step, and the position at which changeover to the next step is made is determined, based on the measured value. A sub-measurement time t1a in the rough grinding step, a sub-measurement time t1s in the precise grinding step, a sub-measurement time t1b in the fine grinding step and various grinding data are stored in the storage unit 306. Note that, the sub-measurement time fib in the fine grinding step will be hereinafter referred to as “final measurement time t1b”. The total measurement time t1 is obtained from the formula, t1=t1a+t1s+t1b. The warm-up time tD obtained according to the method described in the second embodiment with the use of the measurement time t1 is stored in the storage unit 306. Then, after the grinding machine 1 is activated, the grinding wheel 7 is rotated. In addition, after the sizing device 10 is warmed up for the warm-up time tD, the grinding process is carried out. The grinding process will be described below in detail with reference to a flowchart in FIG. 9.
The workpiece W is placed on the grinding machine 1, and then the main spindle 5 is rotated (S1). The grinding head 3 is advanced at a fast feed speed until the grinding wheel 7 approaches the workpiece W (S2). The sizing device 10 is set on a measured portion of the workpiece W (S3). The grinding head 3 is advanced until the grinding wheel 7 reaches an X-axis position X1 in FIG. 8 at a rough grinding feed speed (S4). The sizing device 10 is activated to measure a diameter Dwa of the workpiece W, and then the sizing device 10 is deactivated. The sub-measurement time in this step is t1a (S5). A rough grinding end position Xa is computed according to the formula Xa=(Dwar (theoretical rough grinding diameter)−Dwa (actually measured diameter))/2+Xar (theoretical rough grinding end position) by the computation unit 307, and the thus computed value is stored in the storage unit 306 (S6). The grinding head 3 advanced at a rough grinding feed speed until the grinding wheel 7 reaches the position Xa (S7). The grinding head 3 is advanced at a precise grinding feed speed until the grinding wheel 7 reaches an X-axis position X2 (S8). The sizing device 10 is activated to measure a diameter Dws of the workpiece W, and then the sizing device 10 is deactivated. The sub-measurement time in this step is t1s (S9). A precise grinding end position Xs is computed by the computation unit 307 according to the formula Xs=(Dwsr (theoretical precise grinding diameter)−Dws (actually measured diameter))/2+Xsr (theoretical precise grinding end position), and the thus computed value is stored in the storage unit 306 (S10). The grinding head 3 is advanced at the precise grinding feed speed until the grinding wheel 7 reaches the position Xs (S11). The grinding head 3 is advanced at a fine grinding feed speed, and the sizing device 10 is activated to continuously measure the diameter of the workpiece W. When the measured value of the diameter of the workpiece W reaches a finished size (Dwe), the advance of the grinding head 3 at the fine grinding feed speed is stopped (S12). The sizing device 10 is deactivated after the final measurement time t1b has elapsed since the sizing device 10 is activated, and then the spark-out grinding is carried out for a certain time (S13). The grinding head 3 is retracted at a fast feed speed, and the sizing device 10 is also retracted (S14). The main spindle 5 is stopped and the workpiece W is taken out of the grinding machine 1 (S15).
By changing the feed speed during the grinding process according to the above-described method, the measurement time during which the sizing device 10 is activated in the grinding process is shortened. The lower the ratio of the time of measurement by the sizing device 10 to the time of the grinding process is, the shorter the warm-up time is. Therefore, by executing the grinding process as in the present embodiment, it is possible to provide a grinding machine that is able to carry out measurement with a desired degree of accuracy even if the warm-up time is short.
In the embodiments described above, the sizing device 10 is activated and deactivated as a whole. Alternatively, as illustrated in FIG. 10, in a sizing device 10 in which a differential transformer 101 is used, only the differential transformer 101 with a large drift may be activated and deactivated by an application control unit 102. In the above-described grinding process, the rough grinding end position and the precise grinding end position are obtained by the computation. Alternatively, the sizing device may be activated in a time zone around the time at which a prescribed grinding step is estimated to end, and the grinding process may end when an actually measured value of the diameter of the workpiece coincides with the theoretical value.
Patent Application
20140213147
