JPH09178585A - Work strain measurement method - Google Patents

Abstract

(57) Abstract: When the workpieces Wa and Wb are clamped by a clamp unit, the strain of the workpiece can be accurately measured from the pressure reaction force detected by a detection means 35 such as a load cell provided in the clamp unit. To SOLUTION: One clamp unit provided with a detection means 35 is closed to clamp the works Wa and Wb, and a detection value of a pressure reaction force detected by the detection means 35 at this time is set to a reference value. Next, the plurality of clamp units are closed to clamp the work, and the amount of strain of the work at the clamp location is calculated from the deviation of the detected value of the pressure reaction force detected by the detection means 35 of each clamp unit from the reference value. To do.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention clamps two or more works set on a jig having a plurality of clamp units mounted thereon by these clamp units, and when the works are joined together in this state, The present invention relates to a method for measuring the strain of workpieces prior to joining.

[0002]

2. Description of the Related Art Conventionally, according to Japanese Patent Laid-Open No. 1-153387, a clamp unit is made movable, and at first, the clamp unit is positioned at a predetermined reference position determined from design data, and the workpieces are clamped to each other. Combine,
Next, the accuracy of the product assembled by this connection is optically measured, the position of the clamp unit is corrected based on this measurement result, and the next work is connected and the accuracy is measured. It is known to repeat the above process until the product is obtained.

[0003]

When assembling a product having a closed cross-sectional shape by joining two works Wa and Wb as shown in FIG. 1, for example, a strain δ as shown in FIG. 2b, the work Wa is forcibly bent by the amount of strain δ during clamping, and when the works W are assembled by assembling the works in this state, the product W is unclamped. As shown in FIG. 2C, a springback strain δ ′ occurs.

The prior art described above measures the strain δ'of such a product, corrects the position of the clamp unit in the direction of reducing the strain, and joins the next work. However, with such a feedback-type correction, it is not possible to obtain a highly accurate product from the first time, and there is much waste.
Also, in addition to the variation between lots, there is an inherent variation in each work within the same lot.If the strain of this work is different from the strain of the previous work, it can be obtained by combining the previous work. The correction based on the measured data of the product is not suitable for the work this time, and eventually the conventional feedback type correction cannot deal with the variation of each work.

By the way, as the clamp unit, a reference clamp unit that clamps a specific portion of the work piece at a fixed position and a movable clamp unit that clamps other portions of the work piece and is movable in the clamping direction are provided. The work is clamped by, and the work is clamped by the movable clamp unit while the movable clamp unit is positioned at a predetermined reference position in the clamping direction, and the work Wa is deflected by a strain δ as shown in FIG. 2B. Then, the movable clamp unit is moved to further bend the works Wa and Wb by a predetermined correction amount A as shown in FIG. 2D, and when the works are joined together in this state, the product W is corrected during unclamping. The product A is springbacked by the amount A, and no distortion remains in the product W.

A close relationship should be established between the correction amount A and the work strain amount δ. Therefore, if the strain amount δ of the work can be measured, the correction amount A can be obtained from the strain amount δ and correction by the feedforward method can be performed, and a highly accurate product can be obtained from the first time, and the work individual Variations can be dealt with, and productivity can be significantly improved.

In this case, it is possible to optically measure the amount of strain of the work, but this requires a large amount of measuring equipment and increases the cost. By the way, when a work having a strain is clamped at the reference position, the pressure reaction force of the work deviates from the reference value of the pressure reaction force by the force required to bend the work by the strain amount δ. Therefore, if the movable clamp unit is provided with a detection means such as a load cell for detecting the pressure reaction force of the work, the work is clamped by the reference clamp unit and the movable clamp unit is positioned at a predetermined reference position in the clamping direction. In this state, the work can be clamped by the movable clamp unit, and the strain of the work can be measured based on the deviation of the detection value of the pressure reaction force detected by the detection means from the reference value during this clamping.

However, due to the secular change of the detecting means and the clamp unit, the detected value of the pressure reaction force detected by the detecting means may deviate from the reference value when a work without distortion is clamped. The work strain cannot be measured correctly.

In view of the above points, it is an object of the present invention to provide a method capable of accurately measuring the strain of a work based on the pressure reaction force of the work.

[0010]

In order to solve the above-mentioned problems, the present invention clamps two or more works set on a jig having a plurality of clamp units, and clamps the works in this state. A method for measuring the strain of workpieces prior to joining the workpieces when joining them together.As a clamping unit, a reference clamp unit that clamps a specific part of the workpiece at a fixed position and another part of the workpiece. A movable clamp unit that can be controlled to move in the clamp direction is provided, and the movable clamp unit is provided with detection means for detecting the pressure reaction force of the workpiece. The work is clamped by the movable clamp unit while it is positioned at the predetermined reference position In the one in which the strain of the work is measured based on the deviation of the detected value of the pressure reaction force detected by the detecting means at the time of this clamping, the work is positioned at the reference position before the work is clamped by the reference clamp unit. The work is clamped by the movable clamp unit, and the detection value of the pressure reaction force detected by the detection means at the time of this clamping is set to the reference value.

If the reference value clamp unit is unclamped, when the work is clamped by the movable clamp unit, even if the work is distorted, the work moves freely by the amount of the distortion, and the pressing reaction force of the work is distorted. Will be the same as if there is no. Therefore, if the detection value of the pressure reaction force detected by the detection means at this time is set to the reference value, the reference value is the pressure reaction force when there is no distortion in the work regardless of the secular change of the detection means and the clamp unit. Therefore, the strain of the workpiece can be accurately measured from the deviation between the reference value and the detected value of the pressure reaction force when the workpiece is clamped by the reference clamp unit and the movable clamp unit.

When a plurality of movable clamp units are provided, if the workpieces are simultaneously clamped by these movable clamp units, the free movement of the workpieces at the clamp points of the movable clamp units is obstructed, and the detection means of each movable clamp unit detects the movement. The applied pressure reaction force may deviate from the pressure reaction force when there is no distortion. Therefore, when a plurality of movable clamp units are provided, each movable clamp unit is closed to one to clamp the work before clamping the work with the reference clamp unit, and the detected value of the pressure reaction force at the time of clamping is It is desirable to set the reference value of the pressure reaction force of each movable clamp unit.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment in which the present invention is applied to assemble a product W having a closed cross-sectional shape by joining the works Wa and Wb shown in FIG. 1 by spot welding, arc welding, adhesion or the like will be described below. To do.

Referring to FIG. 3, reference numeral 1 denotes a jig, on which a pair of reference clamp units 2 and 2 for clamping one end of the works Wa and Wb at a fixed position, and a work. W
a pair of first movable clamp units 3 ₁ and 3 ₁ that clamps the middle part of a and Wb and that can control vertical movement, and 1 that clamps the other ends of the workpieces Wa and Wb that can control vertical movement A pair of second movable clamp units 3 ₂ and 3 ₂ are provided.

Each movable clamp unit 3 ₁ , 3 ₂ is shown in FIG.
As shown in FIG. 5, a lower fixed clamp piece 31 fixed to the bracket 30 and an upper movable clamp piece 32 pivotally attached to the bracket 30 so as to be vertically opened and closed are provided. Mount the piston rod 33a of the pressure cylinder 33 that is freely attached to the movable clamp piece 3
When the movable clamp piece 32 is closed by the operation of the pressurizing cylinder 33, the workpieces Wa and Wb are clamped between the contact tips 31a and 32a attached to the clamp pieces 31 and 32, respectively. There is.

The above configuration is similar to the reference clamp units 2, the movable clamp units 3 _1, 3 _2, further a moving mechanism 34 for moving the bracket 30 in the clamping direction serving vertical direction, the clamper arms 31 , 32 at least one clamp piece, in the illustrated example, a fixed clamp piece 3
Detection means 35 provided in No. 1 for detecting the pressure reaction force of the work
And

The moving mechanism 34 has an elevating rod 341 having a mounting base 341a for the bracket 30 at its upper end, which is inserted into the casing 340 erected on the jig 1 so as not to rotate and vertically move, and the lower end of the elevating rod 341. A nut 342 which is screwed to a screw rod 341b attached to the casing 340 and which is axially supported by a holder 342a on the lower inner circumference of the casing 340, and a belt 342 on a pulley 342b connected to the lower end of the holder 342a.
3a and a servo motor 343 which is a drive source having a pulley 343b on the output shaft, and the bracket 30 moves up and down via the elevating rod 341 by rotating the nut 342 forward and backward by the servo motor 343. Be moved. A spline groove (not shown) is formed on the outer peripheral surface of the elevating rod 341, and a ball spline sleeve 344 containing a ball engaging with the spline groove is fitted into the casing 340 with a key 344a to prevent rotation. I am inserting it.

The detecting means 35 includes a fixed clamp piece 31.
It consists of a load cell buried in the contact tip 3
1a is fixed to the clamp piece 31 and the shaft portion 31 of the tip 31a
b, the shaft portion 31b is inserted so as to be vertically movable.
Is brought into contact with the load cell 35, and the pressure reaction force of the work is received by the load cell 35 via the contact tip 31a. The contact tip 31a has a shaft portion 31b.
It is secured to the fixed clamp piece 31 by a circlip 31c attached to the tip of the fixed clamp piece 31, and the tip insertion portion 31d of the fixed clamp piece 31 is detachably attached to the main body portion of the clamp piece 31 to separate the tip insertion portion 31d. In this state, the contact tip 31a can be replaced by removing the circlip 31c.

The detection signal of the load cell 35 is transmitted to the personal computer 4 as shown in FIG. 3, and the computer 4 calculates the strain amount of the work based on the detected value of the pressure reaction force of the work, and the strain is calculated. Based on the amount, the correction amount of the work necessary to prevent the distortion from remaining when the works are joined is calculated, and the correction amount data is transmitted to the control panel 5 for the jig 1, The servomotor 343 of the moving mechanism 34 of each movable clamp unit 3 ₁ , 3 ₂ is operated by a command signal from the control panel 5, and each movable clamp unit 3 ₁ , 3 ₂ is moved by its bracket 30 to a predetermined vertical direction. The work pieces are joined together by a welding robot or the like (not shown) while being displaced from the reference position by the correction amount.

The method of calculating the distortion amount and the correction amount will be described below. Work Wa, Wb is the reference clamp unit 2
The state of being clamped by can be considered as a cantilever as shown in FIG. 5 in which the clamped portion by the reference clamp unit 2 is the fixed end C0. In the figure, C1 is a clamp location by the first movable clamp unit 3 ₁ , and C2 is a clamp location by the second movable clamp unit 3 ₂ . The work has a strain amount of δ1 between C0 and C1,
It is assumed that the strain amount of δ12 is between C2 and C2. Note that C
2 is originally displaced by β · δ1 due to the influence of strain at C1, so the apparent strain amount δ2 at the reference position at C2 is δ2.
Becomes δ2 = δ12 + β · δ1. However, β is the distance between C0 and C1, L1, C
If the distance between 1 and C2 is L2, β≈ (L1 + L2)
/ L1. The strain amount is positive when the strain direction is upward and negative when the strain direction is downward.

When the work is clamped at the reference position, the work is forcibly bent by the amount of strain δ, and the pressing reaction force received by the lower fixed clamp piece 31 by the amount of force necessary for bending the work. Decrease. If the force required to bend the work unit length at the clamp location is α, the force required to bend the work by the strain amount δ is δ · α, and the reference value of the clamp force is Ps and the load cell 35 If the detected pressure reaction force is P, then P = Ps−δ · α δ = (Ps−P) / α. Thus, the strain amount δ can be calculated from the deviation ΔP (= Ps−P) of the detected value P of the pressure reaction force from the reference value Ps.

When C1 and C2 are clamped as in the present embodiment, the pressure reaction force at C1 and the pressure reaction force at C2 influence each other and the first movable clamp unit 3
If the deviation from the reference value of the detected value of the load cell 35 provided in the ₁ Delta] P1, deviation and ΔP2 the reference value of the detected value of the load cell 35 provided in the second movable clamp units _{3 2, ΔP1 = δ1 · α11} + δ2 · α21 ... (1) ΔP2 = δ1 · α12 + δ2 · α22 (2) However, α11 ... α22 are coefficients obtained by clamping the master works of the works Wa and Wb and performing the bending test. That is, when the first movable clamp unit 3 ₁ is displaced by a unit length while the second movable clamp unit 3 ₂ is fixed at the reference position, it is detected by the load cell 35 provided in the first movable clamp unit 3 _1. The amount of change in the pressure reaction force is α11, and the amount of change in the pressure reaction force detected by the load cell 35 provided in the second movable clamp unit 3 ₂ is α12, and the first movable clamp unit 3 ₁ is fixed at the reference position. When the second movable clamp unit 3 ₂ is displaced by a unit length in such a state, the change amount of the pressure reaction force detected by the load cell 35 provided in the first movable clamp unit 3 ₁ is α21, the second movable clamp. The amount of change in the pressure reaction force detected by the load cell 35 provided in the unit 3 ₂ is α22.

When δ1 and δ2 are calculated from the equations (1) and (2), δ1 = (α22 · ΔP1−α21 · ΔP2) / (α11 · α22−α12 · α21) ... (3) δ2 = (α11 · ΔP2-α12 ・ ΔP1) / (α11 ・ α22-α12 ・ α21) ... (4) and the true strain amount δ12 at C2 is δ12 = δ2-β ・ δ1 = {(α11 + βα21) ΔP2- (α12 + β α22) ΔP1} / (α11 α22-α12 α21)… (5)

FIG. 6 shows the relationship between the amount of flexure and the pressure reaction force detected by the load cell 35 when the master work is bent by vertically moving the clamp unit while the master work is clamped. In the figure, line a is the relation line before the work pieces are joined together, and line b is the relation line after the work pieces are joined together. The horizontal axis represents the amount of deflection with the clamp unit as the reference position, that is, when the master work is deflected upward from the state where there is no deflection, the deflection amount is defined as negative when it is deflected downward, and the vertical axis. Is the deviation ΔP of the detected value of the pressure reaction force with respect to the reference value (pressure reaction force when the clamp unit is at the reference position), where the detection value is greater than the reference value as positive, and the deviation is less when it is negative. There is.

When the products are assembled by combining the work pieces in a bent state, in order to return the product to a state without bending, the force corresponding to ΔPb obtained from the b line corresponding to the bending amount is the bending direction. You need to add in the opposite direction. Here, the restoring force corresponding to ΔPa obtained from the a line corresponding to the amount of bending remains on the work, and when the force F1 corresponding to the deviation between ΔPb and ΔPa is applied to the product, the product is in a state without bending. Return. When the work is distorted, a bending reaction force (spring back force) F2 corresponding to the amount of strain δ is applied to the product. Thus, if F1 and F2 are equal, the product returns to a non-deflected state. , No distortion remains in the product.

In the bending test for obtaining the empirical formulas representing the lines a and b, the moving pitch of the movable clamp units 3 ₁ and 3 ₂ is set by the personal computer 4, and the movable clamp unit 3 is instructed by the control panel 5. 3 ₁ and 3 ₂ are moved up and down at a set pitch, and the detection data of the load cell 35 is read into the personal computer 4 every time it is moved by one pitch, and the computer performs regression processing or the like on the a line and the b line by the sampling data Calculate an empirical formula that represents Here, assuming that the amount of deflection is x, the equation for line a is ΔPa
= Fa (x), if the formula of the b line is ΔPb = fb (x), then F1 = fa (x) -fb (x) F2 = fa (δ), so that no distortion remains in the product. The deflection correction amount A of the work required for the following must satisfy the following formula: fa (A) -fb (A) = fa (δ). From this formula, find the correction amount A that matches the strain amount δ. You can

If the lines a and b are straight lines as shown and ΔPa = Ka · x ΔPb = Kb · x, then Ka · A−Kb · A = Ka · δ and α = Ka / As (Ka−Kb), A = α · δ.

When C1 and C2 are clamped as in this embodiment, the master work is clamped by the reference clamp unit 2 and the first movable clamp unit 3 _1, and the second movable clamp unit 3 ₂ is released from the work. In this state, the first movable clamp unit 3 ₁ is moved up and down, and the coefficient Ka1 representing the relationship between the amount of bending of the master work before joining C1 and the pressure reaction force and the bending of the master work after joining the C1 are combined. The coefficient Kb1 representing the relationship between the amount and the pressure reaction force is calculated, and the calculation coefficient α1 of the correction amount A1 at C1 is calculated.
Is calculated by the following equation, α1 = Ka1 / (Ka1-Kb1). Further, the master work is clamped at the reference position by the reference clamp unit 2 and the first movable clamp unit 3 ₁ , respectively, and the second movable clamp unit 3 ₂ is moved vertically while the master work is clamped to move the master work. The coefficient Ka2 that represents the relationship between the amount of flexure and the pressure reaction force in C2 before joining and the coefficient Kb2 that represents the relationship between the amount of flexure and the pressure reaction force in C2 after joining the masterwork are determined, and are calculated as C2. The calculation coefficient α2 of the correction amount A2 is calculated by the following equation, α2 = Ka2 / (Ka2-Kb2).

Then, the correction amount A1 at C1 is calculated by the following equation, A1 = α1 · δ1 (6), based on the strain amount δ1 of the work at C1. Further, the correction amount A2 at C2 is equal to the correction amount A12 corresponding to the true strain amount δ12 of the workpiece at C2.
Displacement amount of C2 by correction amount A1 at 1 β · A1 (however,
It becomes a value obtained by adding β≈ (L1 + L2) / L1). Therefore, A2 is A2 = β · A1 + A12 = β · A1 + α2 · δ12 (7) = β · α1 · δ1 + α2 (δ2-β · δ1) = α2 · δ2 + β (α1-α2) δ1 ... (8)

When assembling the product W by combining the works Wa and Wb, the work Wa with the reference clamp unit 2 and the first and second movable clamp units 3 ₁ and 3 ₂ all positioned at the reference position. , Wb are clamped, and the detection data of the load cell 35 of each movable clamp unit 3 ₁ , 3 ₂ at the time of this clamp is read into the personal computer 4, and the computer 4 calculates the strain amount δ1 at C1 from the above equation (3). In addition to the calculation, the apparent strain amount δ2 at C2 is calculated from the above formula (4) or the true strain amount δ12 at C2 is calculated from the above formula (5). Then, by substituting the calculated value of δ1 into the above formula (6), the correction amount A1 at C1 is calculated,
Further, the correction value A2 at C2 is calculated by substituting the calculated value of δ12 into the above formula (7) or the calculated value of δ2 into the formula (8).

Next, the personal computer 4 transmits the data of A1 and A2 to the control panel 5, and the first and second movable clamp units 3 ₁ and 3 ₂ are temporarily unclamped by a command from the control panel 5. The workpieces Wa and Wb are again clamped by displacing the workpieces Wa and Wb from the reference position, respectively, and the workpieces Wa and Wb are joined in this state.

By the way, the secular change of the load cell 35,
The movable clamp units 3 ₁ , 3 ₂ are worn due to wear of the shaft attachment portions of the movable clamp pieces 32 of the movable clamp units 3 ₁ , 3 _2.
Load cell 3 when clamping a work piece without distortion
The pressure reaction force detected in 5 may deviate from the reference value, which causes an error in the strain amounts δ1, δ2, δ12 calculated by the above equations (3), (4) and (5). Will end up.

Therefore, in this embodiment, after the works Wa and Wb are set on the jig 1 (S1) according to the procedure shown in FIG. 7, first, the reference clamp unit 2 and the second movable clamp unit 3 ₂ are set. First, unclamped
The movable clamp unit 3 ₁ is positioned at the reference position to clamp the works Wa and Wb (S2), and at this time, the detection value P1 of the pressure reaction force detected by the load cell 35 of the first movable clamp unit 3 ₁ is set to the first value. Movable clamp unit 3 ₁
Is stored in the personal computer 4 as the reference value P1s of the pressure reaction force for use (S3), and then the second movable clamp unit is unclamped with the reference value clamp unit 2 and the first movable clamp unit 3 _1. 3 ₂ is positioned at the reference position and the workpieces Wa and Wb are clamped (S
4) At this time, the detected value P2 of the pressure reaction force detected by the load cell 35 of the second movable clamp unit 3 ₂ is set as the reference value P2s of the pressure reaction force for the second clamp unit 3 ₂ .
It is stored in the personal computer 4 (S5) and the reference value is updated once or every predetermined number of times.

As described above, when the works Wa and Wb are clamped by the movable clamp units 3 ₁ and 3 _{2 at} only one position, even if any of the works is distorted, the works are restrained at the other clamp positions. Since it moves freely without moving, the pressure reaction force detected at that time is the same as when the work is not distorted. Therefore, each movable clamp unit 3 ₁ , 3 ₂
By setting the reference values P1s and P2s for use as described above, the reference values P1s and P2s coincide with the pressurizing reaction force when clamping a work without distortion. Thus, when the workpieces Wa and Wb are clamped by the reference clamp unit 2 and the first and second movable clamp units 3 ₁ and 3 ₂ , they are detected by the load cells 35 of the movable clamp units 3 ₁ and 3 _2. The deviations ΔP1 and ΔP2 of the detected values P1 and P2 of the pressurizing reaction force from the reference values P1s and P2s are the clamping points C1 and C2.
In accordance with the bending reaction force corresponding to the strain amount of the workpiece in Fig. 3, the strain amounts δ1, δ2, δ12 are accurately calculated by substituting ΔP1 and ΔP2 into the above equations (3), (4) and (5). it can.

[0035] Incidentally, it is also possible to omit the first movable clamp units 3 _1, this is likely to strain remains in the middle portion of the product W, To increase the accuracy of the product W, the present embodiment It is desirable to provide the first and second sets of movable clamp units 3 ₁ and 3 ₂ as described above. It is also possible to provide three or more sets of movable clamp units.

By the way, in the above-mentioned embodiment, the correction amount is calculated from the measured strain amount by using the empirical formula. However, various test works having different strains are prepared, and the optimum correction amount corresponding to the strain amount is prepared. It is also possible to experimentally obtain the correction amount, store this as a data table, and obtain the correction amount corresponding to the measured distortion amount by a table search.

After the works are joined together, the movable clamp units 3 ₁ and 3 ₂ are returned to the reference positions to re-clamp the product, and the product is detected by the deviation of the pressure reaction force detected from the reference value from the reference value. The presence or absence of distortion may be confirmed, and the correction coefficient calculation coefficient may be corrected by learning control.

[Brief description of the drawings]

FIG. 1 is a perspective view of a work.

2A to 2E are diagrams showing the principle of distortion generation and the principle of distortion removal of a product.

FIG. 3 is a perspective view of equipment used for implementing the present invention.

FIG. 4 is a sectional view of a movable clamp unit.

FIG. 5 is a diagram showing a distortion amount and a correction amount of a work.

FIG. 6 is a graph showing the relationship between the amount of flexure of a work and the pressure reaction force

FIG. 7 is a flowchart showing a procedure for setting a reference value of a pressure reaction force.

[Explanation of symbols]

Wa, Wb Work W Product 1 Jig 2 Reference clamp unit 3 ₁ , 3 ₂ Movable clamp unit 34 Moving mechanism 35 Load cell (Detection means) 4 Personal computer 5 Control panel

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location // B23K 31/00 B23K 31/00 K (72) Inventor Yutaka Asano 1 Shin-Sayama, Sayama City, Saitama Prefecture Chome 10-1 within Honda Engineering Co., Ltd. (72) Inventor Isao Satoh 1-10 Shin-Sayama, Sayama City, Saitama Prefecture Within Honda Engineering Co., Ltd.

Claims (2)

[Claims] 1. When two or more workpieces set on a jig having a plurality of clamp units are clamped by these clamp units and the workpieces are joined together in this state, the workpieces are joined prior to the joining of the workpieces. A method for measuring strain, in which a reference clamp unit that clamps a specific part of a workpiece at a fixed position and a movable clamp unit that clamps other parts of the workpiece and that can be controlled to move in the clamping direction are provided as clamp units. Then, the movable clamp unit is provided with a detection means for detecting the reaction force applied to the work, the work is clamped by the reference clamp unit, and the work is movable with the movable clamp unit positioned at a predetermined reference position in the clamping direction. Clamping by, and the pressure reaction force detected by the detection means during this clamping When measuring the strain of a workpiece based on the deviation of the detected value from the reference value, before clamping the workpiece with the reference clamp unit, the movable clamp unit positioned at the reference position clamps the workpiece, and the detection means is used during this clamping. A method for measuring strain of a work, wherein a detected value of the detected pressure reaction force is set to the reference value. 2. When a plurality of movable clamp units are provided, each movable clamp unit is closed to one before the workpiece is clamped by the reference clamp unit, and the workpiece is clamped, and the pressure reaction force at the time of this clamp is detected. The method for measuring strain of a work according to claim 1, wherein the value is set to a reference value of the pressure reaction force of each movable clamp unit.