JP4779160B2 - Grinding method - Google Patents

Description

  The present invention relates to a grinding method, and more particularly, to a grinding method suitable for grinding the surface of hard and brittle material ceramics such as silicon and sapphire with a precision of a unit of submicron or less.

  A semiconductor substrate such as a sapphire wafer used for a white light emitting diode (white LED) using a GaN system is subjected to a required mirror finish through a grinding process, a lapping process, and a polishing process. When using a conventional grinding method designed to feed the work table at a constant speed, a damaged layer of several μm is formed on the sapphire wafer at the end of grinding and lapping, and this damaged layer is removed in the final polishing process. Since it takes about 1 hour per 1 μm, the polishing process is a bottleneck in LED production.

Therefore, in Patent Document 1, as shown in FIG. 10, a rotating grindstone (72) that is held in a predetermined position by a grindstone feeding means (not shown) and grinds the work (W) and a work (W) are supported. A work table (73) and a work table feed means (74) for moving the work table (73) in a direction parallel to the grinding surface of the work (W). It is a fixed-size feed means that moves up and down and controls the position of the rotating grindstone to approach the work.The work table feed means (74) is a hydraulic cylinder (75) that moves the work table (73), and hydraulic pressure is hydraulic An oil / pneumatic converter (not shown) that converts to a pneumatic pressure, and an electropneumatic regulator (not shown) that keeps the workpiece feed thrust constant by setting the air pressure supplied to the air pressure chamber of the oil / pneumatic converter Grinding equipment used as constant thrust feeding means A grinding method using the device (71) has been proposed.
JP 2000-317830 A

  According to the grinding method of Patent Document 1, since the force in the feeding direction acting on the work table is constant, when the work is disk-shaped, the contact length between the grindstone and the work changes during grinding. The workpiece is sent at a relatively high speed at the front end portion and the rear end portion of the workpiece with a short contact length, and the workpiece is sent at a relatively slow speed at the center portion with a long contact length. There is a problem that the front end side portion and the rear end side portion of the workpiece are easily damaged.

  The object of the present invention is to reduce workpiece damage when grinding hard and brittle materials such as sapphire. Therefore, it is possible to grind the surface with sub-micron accuracy. It is to provide a grinding method.

In the grinding method according to the present invention, a work table for supporting a workpiece is moved in a direction parallel to the grinding surface of the workpiece by a workpiece feeding device, and the disc-shaped workpiece is moved by a grindstone having a diameter larger than the workpiece diameter. In the grinding method of grinding parallel to the direction, the actuator of the workpiece feeder is ground by controlling the current of the workpiece feeder so that the surface pressure of the workpiece grinding surface is constant, and the grinding contact length l ( x) is determined in advance, and the surface pressure at which the workpiece is damaged is Po, the depth of cut is Δ, and the work force U in the feed direction acting on the work table is U <Po · Δ · l (x). In addition to controlling the control current of the feeding device, the control current of the workpiece feeding device is set to Pm, which is an allowable maximum surface pressure at which the workpiece is not damaged. The allowable load per unit contact length is Um, and the feed direction force U acting on the work table is controlled so that U = Um · l (x) with respect to Um = Pm · Δ. To do.

  As a workpiece to be ground by this grinding method, a hard and brittle material such as a sapphire wafer used for a white LED, a crystallized glass used for a hard disk, a silicon wafer, and an alumina sintered body is suitable.

  The surface pressure is obtained by dividing the force acting on the workpiece in the workpiece feeding direction by the area of the grinding surface of the workpiece (grinding contact length × cutting amount).

  The workpiece feeding device includes, for example, a linear motor as an actuator, a wire that connects the linear motor and the work table to transmit the generated thrust of the linear motor to the work table, a tension detection unit that detects the tension of the wire, and a linear Feed speed detection means for detecting the feed speed of the motor, and motor control means for controlling the thrust generated by the linear motor based on the tension value obtained from the tension detection means and the feed speed obtained from the feed speed detection means. It is supposed to be. In this case, the force in the feed direction acting on the work table is equal to tension. Linear motors can be replaced with other current controlled actuators, and the wires can be replaced with springs that couple the motor and worktable.

  The workpiece feeder also has a magnetic screw as an actuator consisting of a rotatable screw shaft and two nut bodies magnetically coupled to the screw shaft in a non-contact state, and moves in the axial direction as the screw shaft rotates. And a relative displacement measuring means for measuring the distance between the two nut bodies to be moved, and a control means for controlling the feed of the screw shaft of the magnetic screw. A movable table that is supported so as to be retractable in a direction parallel to the workpiece grinding surface and advanced in a direction parallel to the workpiece grinding surface, and a movable control table that is driven by an electromagnetic force according to the current value when a required control current is passed. An electromagnetic force clamping means as an actuator for coupling the work table, a relative displacement detecting means for measuring the relative displacement of the work table with respect to the moving table, and a displacement amount obtained by the relative displacement detecting means. Sometimes and a control means for controlling the control current of an electromagnetic force clamping means Te.

  As the rotating grindstone, for example, a grindstone using diamond, cubic boron nitride or the like as abrasive grains is used. The rotating grindstone feeding means is a fixed dimension feeding means, for example, configured such that the grinding head is moved up and down by a head feed motor, and the position of the rotating grindstone is controlled to approach the workpiece. The rotating grindstone is stopped at a predetermined position according to the grinding thickness by the fixed-size feeding means. A magnetic bearing device is used to control the position of the grindstone shaft of the rotating grindstone, and the grinding wheel feeding means and the workpiece feeding device are controlled using the axial direction control current and the radial direction control current of the magnetic bearing device. Also good.

  The current control of the actuator is performed by calculating a control thrust in the controller based on a tension value representing a force in the feed direction and outputting the thrust instruction value to the actuator. In the current control, the mode is switched such as thrust control at the time of grinding, position control at the time of feeding until the start of machining, and return after machining.

  In order to perform grinding with a constant surface pressure, for example, the grinding contact length l (x) with respect to the feed direction position x of the workpiece is obtained in advance, the surface pressure at which the workpiece is damaged is Po, the cutting amount is Δ, The control current of the work feeding device may be controlled so that the force U in the feeding direction acting on the work table satisfies U <Po · Δ · l (x).

  When the surface pressure of the grinding surface is P, there is a relationship of U = P · Δ · l (x). When the work table is driven at a constant moving speed, the feed force U acting on the work table is the grinding force. It appears with the same profile as the contact length l (x). Therefore, the surface pressure of the grinding surface can be kept constant by controlling the feed force U acting on the work table in accordance with the grinding contact length l (x). On the contrary, if the surface pressure of the grinding surface is kept constant when no error element is included, the moving speed of the work table appears as a constant speed as a result.

  The control current of the workpiece feeder is Pm as the maximum allowable surface pressure at which the workpiece is not damaged, and Um as the allowable load per unit contact length with respect to the cutting depth Δ, and Um = Pm · Δ. More preferably, the acting feed direction force U is controlled to be U = Um · l (x).

  Moreover, it is preferable to offset the center of the workpiece with respect to the center of the grindstone so that the force acting on the grinding surface of the workpiece from the grindstone has a component opposite to the workpiece feeding direction.

  When the center of the workpiece is offset with respect to the center of the grindstone, the tangential direction of the grindstone at the center of the contact length is not perpendicular to the workpiece feed direction. Therefore, the force acting on the workpiece from the grindstone has a component (sweeping force) in the workpiece feeding direction. The offset is applied so that the force acting on the grinding surface of the workpiece from the grindstone has a component opposite to the workpiece feeding direction. In this way, when the thrust of the feeder becomes zero due to a power failure or failure of the workpiece feeder, a force in the direction away from the grinding stone acts on the workpiece from the grinding stone, so that the workpiece is prevented from being pulled in, Damage is prevented.

  According to the grinding method of the present invention, since the workpiece feeding device is controlled and ground so that the surface pressure of the grinding surface of the workpiece is constant, the workpiece feeding device is controlled so that the thrust acting on the workpiece is constant. Grinding the surface of hard and brittle materials such as sapphire with sub-micron accuracy while eliminating damage at the tip and rear edges of the workpiece, which is a problem with grinding methods Is possible. In addition, grinding is completed quickly while the sharpness of the grindstone is good.If the sharpness decreases, the grinding time increases, and before abnormal surface pressure is applied, grinding does not proceed and safe grinding that does not damage the workpiece. Is possible.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows a model of the mechanism of the grinding method of the present invention. A grinding apparatus (1) for carrying out this grinding method includes a rotating grindstone (2) for grinding a workpiece (W) such as a sapphire wafer, and the like. Grinding wheel feed means (not shown) for moving the axis (2a) of the rotating grindstone (2) in a direction perpendicular to the grinding surface of the work (W), a work table (3) for supporting the work (W), and a work table A workpiece feeder (4) that moves (3) in a direction parallel to the grinding surface of the workpiece (W).

  The work table (3) is a slide table of an air slide device, and is supported by a pair of guide shafts (5) of the air slide device so as to be movable in the longitudinal direction.

  The workpiece feeder (4) connects the linear motor (11), the front end of the work table (3) in the direction of travel, and the rear end of the linear motor (11) in the direction of travel. A steel first wire (12) transmitted to the table (3), a tension sensor (tension detection device) (13) for detecting the tension of the first wire (12), and a front end of the linear motor (11) in the traveling direction; A steel second wire (biasing means) (15) for connecting the rear end of the work table (3) in the traveling direction and applying a force opposite to the first wire (12) to the work table (3); From a plurality of first wire guide rollers (17) for guiding the first wire (12) and a feed speed detecting means for detecting the tension value obtained from the tension sensor (13) and the feed speed of the linear motor (11). Motor control means for controlling the generated thrust of the linear motor (11) based on the obtained feed speed.

  The motor control means for controlling the thrust generated by the linear motor (11) has a motion controller (21) for applying a control current to the linear motor (11) through a current amplifier (22) as shown in FIG. The feed amount obtained from the linear encoder (19) constituting the feed speed detection means, the feed speed, and the wire tension value obtained by the tension detection device (13) such as a tension sensor are fed back to the controller (21). Based on this, the control current of the linear motor (11) is controlled. The controller (21) calculates the control thrust by analog input from the tension detection device (13) and outputs the thrust instruction value to the current amplifier (22), thereby the feed direction force acting on the work table (3). Is controlled directly.

  In FIG. 1, each alphabet sign is x: position of the workpiece in the feed direction, p: position of the mover of the linear motor, Ua: tension of the first wire, Ub: tension of the second wire, fn: feed resistance force by grinding , F: linear motor generated thrust, r: friction resistance of the linear motor, respectively.

In the model shown in the figure, M is the mass of the work table, αw is the acceleration of the work (work table) (= d ² x / dt ² ), η is the resistance coefficient (val), and l (x) is the contact between the grindstone and the work. Arc length (hereinafter referred to as “contact length”), v: work (work table) moving speed (= dx / dt), m: mass of linear motor mover, q: movement of linear motor mover Speed (= dp / dt), αm: linear motor mover acceleration (= d ² p / dt ² ), Kt: motor thrust constant, I: motor control current, K: wire spring constant, workpiece feed With the direction as the positive direction in the x direction, the following equation holds.

M · αw = Ua−fn−Ub (1)
fn = η · l (x) · v (2)
m · αm = fr−Ua (3)
r = μ · q (4)
f = Kt · I (5)
Ua = K (p−x) (6)
For low speed feed (v ≦ several mm / s or less), αw and αm are approximately zero, and Ub is a constant term. Therefore, it can be seen from Equation (3) that the tension Ua is measured as a value obtained by subtracting the frictional resistance r from the motor generated thrust f. Since l (x) is known as will be described later, if driving is performed with v = dx / dt = constant, the tension Ua (= fn + Ub = η · l (x) · v + Ub) is l (x). Appear with the same profile. That is, in order to keep Ua constant, v draws a 1 / l (x) profile, and in order to keep the surface pressure of the grinding surface constant, Ua may be controlled in accordance with l (x). If Ua is determined, the linear motor generated thrust f is obtained from the equation (3), and the control current I of the linear motor (11) is controlled based on the equation (5). Such constant surface pressure control makes it possible to suppress the pressure applied to the grinding surface below the pressure at which damage occurs, and to eliminate grinding damage.

  As shown in FIGS. 3 and 4, the workpiece (W) is ground with its center offset from the center of the grindstone (2).

  In FIG. 3, the distance from the center of the grinding wheel to the center of the workpiece is L, the position of the workpiece center in the x direction is x, the position in the y direction is yc (yc is an offset amount), and the angle formed by the contact length. Assuming 2a (reference to the center of the grindstone), the radius of the grindstone as Rg, and the radius of the workpiece as Rw, the following equation holds.

L ² = x ² + yc ²
cosa = (Rg ² + L ² −Rw ² ) / (2 · Rg · L)
l (x) = (2a / 2π) · (2π · Rg) = 2a · Rg
That is, the contact length is obtained by l (x) = 2a · Rg. However, when the radius of the workpiece (W) is Rw, and (Rg−Rw) <L and (Rg + Rw)> L, l (x) = 0.

  FIG. 5A shows the relationship between the displacement amount in the x direction of the work table after contact and the contact length when the grindstone diameter is 305 mm and the workpiece diameter is 50.8 mm, using the offset amount yc as a parameter. As can be seen from FIG. 3, l (x) varies depending on the position x of the workpiece (W) (after gradually increasing, it gradually decreases). Therefore, if the generated thrust of the linear motor (11) is kept constant, the load (surface pressure) per unit length of the workpiece (W) will fluctuate, and control taking into account that l (x) will change It turns out that is preferable.

  Further, as shown in FIG. 4, when the force in the tangential direction of the grindstone is Fg, the force Fx in the direction opposite to the x direction at the position of the angle φ (based on an axis parallel to the x direction passing through the center of the grindstone) is Fx = Fg · sinφ. X-direction force applied to the entire contact length = x-direction force (push-out force) Fx created by grinding resistance, with the offset amount being θ (converted to an angle with the center of the wheel as a reference), If the force depends only on the grinding force in the tangential direction of the grindstone, it can be obtained as follows.

Fx = ∫ _θ−a ^{θ + a} Fg · sinφdφ = ² Fg · sinθ · sina
FIG. 5B shows the relationship between the displacement amount in the x direction of the work table after contact and Fx / Fg when the grindstone diameter is 305 mm and the workpiece diameter is 50.8 mm, using the offset amount yc as a parameter.

  The offset of the center of the workpiece (W) with respect to the center of the grindstone (2) is such that the pushing force Fx is positive, that is, the force Fg of the grinding surface of the workpiece (W) is opposite to the workpiece feed direction (−x direction) ) So as to have Fx as a component. Therefore, when the generated thrust of the linear motor (11) becomes zero due to power failure or linear motor failure, the force in the direction away from the grinding wheel (2) acts on the workpiece (W) from the rotating grinding wheel (2). The drawing of the workpiece (W) is prevented, and the workpiece (W) is prevented from being damaged.

FIG. 6 shows tension control by speed servo (FIG. 6 (a)) and tension control only by current (FIG. 6 (b)) as control methods for achieving constant surface pressure control. As shown in FIG. 6A, the tension control by the speed servo is performed with respect to the maximum permissible surface pressure Pm, contact length l (x), cutting depth: Δ, and cutting depth Δ, as shown in FIG. As permissible load per contact length: Um (= Pm · Δ), U _m · l (x) is set as the tension target value U * in the command value determining unit (23), and the controller (21) The speed target value v * is obtained and output to the speed control controller (24). The speed control controller (24) calculates a target control current I * based on the speed target value v * and outputs it to the current amplifier (22). The linear motor (11) is controlled by the amplified current I corresponding to the target control current I *. The work position x and the moving speed v are measured by an encoder, the speed is fed back to the speed control controller (24), and the work position x is determined by a command value determining unit for obtaining a command value Um · l (x). Feedback on (23). If the workpiece position x is used, the tension Ua acting on the workpiece table (3) can be obtained from the equation (6). The tension Ua thus determined is fed back to the controller (21) for determining the speed target value v * from the tension target value U *.

  The tension control based only on the current shown in FIG. 6B is obtained by removing the speed control controller (24) from the tension control system based on the speed servo. The controller (21) uses the target control current I from the tension target value U *. * Is calculated. The system after the current amplifier (22) is the same as that shown in FIG. 6 (a), and is different from that shown in FIG. 6 (a) only in that the servo is not effective for the speed. Although not shown, a correction function S (x) for thrust unevenness due to magnetic flux distribution and sliding unevenness may be inserted after the controller (21).

  In the control with the servo applied to the speed in Fig. 6 (a), even if a force that pushes the workpiece from the grinding wheel (2) to the workpiece (W) is applied due to the grinding stone (2) being caught, etc. Attempts to push the workpiece (W) with excessive pressure on the workpiece (W) may cause damage to the workpiece (W), and conversely, this is pulled from the grindstone (2) into the workpiece (W). Even when such a force is applied, the workpiece (W) may be damaged, but this can be prevented by offsetting the workpiece (W) with respect to the grindstone (2) by the method described above. Therefore, when the pushing force is applied, it is more preferable that the control is based only on the current, which is a control that does not resist this.

  Since the current (or power) of the motor that drives the grindstone fluctuates according to l (x), it can be used as a feedback signal for control instead of the tension Ua. However, the current of the motor that drives the grindstone cannot directly express the force applied to the workpiece like the tension Ua, and is susceptible to disturbances such as the influence of the coolant flow rate, and the influence of Fx cannot be reflected. Feedback is preferred.

  FIG. 7 shows an example of a timing chart of the grinding method of the present invention. As described above with reference to FIG. 3, after the start of grinding, the contact length l (x) gradually increases due to the movement of the workpiece (W), and accordingly, the grinding resistance increases. After approximately half of the grinding, the contact length l (x) is gradually reduced by the movement of the workpiece (W), and accordingly, the grinding resistance is also reduced. Therefore, in the thrust control considering the change of l (x), the thrust is increased / decreased in accordance with the increase / decrease of the contact length in the grinding stage. In the pre- and post-grinding stages, the thrust control is not used but the position control mode is selected. From the standby position to the grinding position, the work table (3), that is, the linear motor mover is fed at a high speed (rapid approach), and grinding is performed. After the end, it is moved again at a high speed to the standby position (rapid reverse). The feed rate during grinding varies depending on the state of the workpiece (W) and the grindstone (2). However, according to the constant surface pressure control, the feed rate is basically constant in the ideal state as shown by the solid line in FIG. In addition, when a constant tension control is performed instead of a constant surface pressure, the feed speed is distorted corresponding to l (x), and basically a gentle arc shape as indicated by a broken line in the figure. It becomes.

  8a and 8b (FIGS. 8 (a), (b), (c), and (d)) show the motor current, the motor speed, and the motor speed when grinding at a constant speed in order to verify the grinding method of the present invention. It shows how the tension changes. During normal grinding shown in FIG. 8a (a), the motor speed is controlled to be constant, and the tension changes in proportion to l (x). Since the motor current needs to be constant regardless of the magnitude of l (x), when l (x) is large, the current becomes large (large thrust). When the sharpness is reduced as shown in FIG. 8a (b), the motor speed is kept constant, but the tension and the motor current (thrust) increase. In FIG. 8b (c) when grinding continues, the tension further increases. As a result, the surface pressure of the workpiece exceeds the allowable value, and the workpiece is damaged. In the state where the grindstone shown in FIG. 8b (d) is not cut, the motor current reaches the set value (damage prevention limit value), and the workpiece cannot be fed further. The workpiece has already been damaged at the stage of FIG. 8b (c), and it is difficult to prevent the damage in advance.

  From the above tension actual measurement data when the speed is constant, the grinding force and the relationship between the grinding force and the tension are obtained. 8A and 8B, the above-described constant surface pressure control allows the tension to be controlled so that the surface pressure is constant even when the sharpness of the grindstone is reduced, and the workpiece is damaged. It is possible to prevent the surface pressure from being received and to ensure safety. Further, the force Fx in the x direction created by the resistance due to grinding in the tension control is small while the cutting of the grindstone is continued, but the cutting is reduced, or the workpiece upper surface and the lower surface of the grindstone after cutting are cut. When the grinding state becomes abnormal, such as rubbing, it becomes larger, and the tension profile appearing in the wire driving the table changes from l (x) to Fx. Therefore, the abnormality of the grinding process can be predicted from the change of the shape.

  FIG. 9 shows the operation during the constant surface pressure control grinding (the relationship between the workpiece feed direction position x, the workpiece velocity v, and the force U acting on the workpiece). In the constant surface pressure control using the above-described model, the speed is constant if the resistance force fn due to grinding ideally changes in proportion to the contact length l (x) between the workpiece and the grindstone. However, actually, the resistance force fn includes a sweeping force due to offset in addition to the surface pressure, and the resistance force fn and the contact length l (x) are not completely proportional. Further, depending on the degree of sharpness of the grindstone, the resistance force by grinding varies as seen in the constant speed grinding shown in FIGS. 8a and 8b. Therefore, the case where the grinding resistance force as shown in FIG. 8a (b) or FIG. 8b (c) occurs (when the grinding ability is reduced and the tension becomes larger than that during normal grinding) is maintained at a constant surface pressure. When the control is performed, the thrust is adjusted in order to adjust the tension. As a result, as shown by the thick solid line in FIG. 9, the work speed is reduced when the grinding resistance force indicated by the broken line is smaller than the target tension value indicated by the thin solid line. However, in a large area, the speed of the work becomes slow, and even if it does not completely match the model, proper control is performed.

  In the above embodiment, the linear motor (11) is used as the driving means of the workpiece feeding device, but the driving means may be a motor other than the linear motor, for example, a rotary motor. However, in order to properly grind hard and brittle materials such as sapphire wafers, it is necessary that the feed thrust is several hundred gf or less, and the grinding speed is several μm to several mm / s and low thrust / low speed. If this is performed with a rotary motor, it is necessary to achieve a large reduction ratio, and it is difficult to achieve both low thrust and high thrust. According to the linear motor (11), thrust can be directly generated by current control, and both low thrust and low speed can be easily achieved. In addition, when the low thrust is applied, the second wire tension Ub acts, so that the operation of the linear motor can be used in the linear region instead of the low current nonlinear region.

  Further, the work table may be directly driven by a linear motor without using a wire. In this case, a dynamometer or the like is used as a sensor. Driving through a wire is preferable in that thrust can be detected without using an expensive sensor such as a dynamometer, and thrust errors (guide resistance, wiring resistance, etc.) caused by the drive system can be canceled. .

  As for the offset amount, a preferable value is determined by creating a graph as shown in FIG. 5 according to the grindstone diameter, the workpiece diameter, and other conditions. The offset is not always necessary. In the mechanism system shown in FIG. 1, since the tension (back tension) Ub from the second wire acts, the work is prevented from being drawn into the grindstone, and there is no problem even when the offset amount = 0. .

FIG. 1 is a view showing a mechanism of a grinding method according to the present invention. FIG. 2 is a diagram showing the configuration of the control system of the grinding method of the present invention. FIG. 3 is a diagram illustrating a grinding load model used in the grinding method of the present invention. FIG. 4 is a diagram for explaining the force acting on the workpiece in the grinding method of the present invention. FIG. 5A shows the contact length obtained by using the offset amount as a parameter, and FIG. 5B shows the pushing force (force in the reverse direction in the x direction) obtained by using the offset amount as a parameter. FIG. 6 is a block diagram showing the configuration of the control system. FIG. 7 is a diagram showing an example of a timing chart of the grinding method of the present invention. FIG. 8a is a diagram showing the relationship between motor current, motor speed and tension in the case of constant speed grinding, and shows the normal time (a) and the sharpness reduction time (b). FIG. 8b is a diagram showing the relationship between motor current, motor speed, and tension in the case of constant speed grinding. When grinding damage occurs due to further grinding after the sharpness is reduced (c) and the state where the grindstone is not cut (d) ). FIG. 9 is a diagram illustrating an operation of constant surface pressure control. FIG. 10 is a side view showing a conventional grinding method.

Explanation of symbols

(1) Grinding equipment
(2) Rotary grinding wheel
(3) Work table
(4) Work feeding device
(11) Linear motor
(12) First wire
(13) Tension detection means
(W) Workpiece

Claims (4)

Grinding a disc-shaped workpiece parallel to the workpiece feed direction with a grindstone having a diameter larger than the workpiece diameter while moving the workpiece table supporting the workpiece in a direction parallel to the workpiece grinding surface by the workpiece feeder. In this method, it is assumed that grinding is performed by controlling the current of the actuator of the workpiece feeding device so that the surface pressure of the grinding surface of the workpiece becomes constant, and the grinding contact length l (x) for the workpiece feeding direction position x is obtained in advance. The surface pressure at which the workpiece is damaged is Po, the cutting depth is Δ, and the control current of the workpiece feeding device is controlled so that the force U in the feeding direction acting on the workpiece table is U <Po · Δ · l (x). In addition, the control current of the workpiece feeding device is set to Pm as the maximum allowable surface pressure at which the workpiece is not damaged, and the allowable negative per unit contact length with respect to the cutting depth Δ A grinding method, wherein the load is Um, and the feed direction force U acting on the work table is controlled so that U = Um · l (x) with respect to Um = Pm · Δ . 2. The grinding method according to claim 1, wherein the current control of the actuator of the workpiece feeding device is performed by calculating an instruction value of the control thrust based on a force in the feeding direction and outputting the thrust instruction value to the actuator . 3. The grinding method according to claim 2, wherein an abnormality of the grinding process is predicted by changing the shape from l (x) in a normal tension profile to a force Fx in the x direction created by resistance caused by grinding .   The center of the work is offset with respect to the center of the grindstone so that the force acting on the grinding surface of the work from the grindstone has a component opposite to the feed direction of the work. Grinding method.

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