JP4681381B2 - Rotary draw bending control device, method, computer program, and computer-readable recording medium - Google Patents

Description

  The present invention relates to a rotary draw bending control apparatus, a method, a computer program, and a computer-readable recording medium for controlling a rotary draw bending process, which is one of bending methods for metal pipes such as automobile parts and pipes. .

  In recent years, metal pipes have been increasingly applied for the purpose of reducing the weight of automobiles, bicycles, etc., in addition to applications for passing liquids and gases inside plant pipes, gas / water pipes, boiler pipes, etc. . As the application range is expanded, the shape is also complicated. In this case, bending is often performed in advance in order to reduce costs and improve productivity and workability.

  There are various methods for bending metal pipes (see, for example, Non-Patent Document 1), and the most frequently used industrial method is a rotational pulling method. As shown in FIG. 2, this construction method is a method of rotating the rotary bending die in a state where a part of the metal tube is held by the rotary bending die and the clamping die. At that time, the mandrel fixed inside the metal tube prevents the metal tube from being flattened, and the wiper die fixed to the inner back of the bend prevents wrinkles. In addition, the position of the pressure die arranged outside the bending is conventionally fixed, but in a recent commercial apparatus, it is general to advance following the bending rotational speed. This bending method is basically referred to as rotational pulling or pulling because it is bent while being pulled, but depending on the apparatus, it can be bent while being pushed from the rear end of the metal tube with a booster die.

  Since this bending method is bent while being constrained by various tools as described above, it is difficult to cause processing defects such as wrinkles and buckling, and the reproducibility of the shape after bending (bending radius and cross-sectional shape). Therefore, it can be evaluated as a very advantageous method for industrial production.

“Tube Forming” pages 36-64 (issued by Corona on October 30, 1992) Nippon Steel Pipe Technical Report, Vol. 29 (1964), p. 28 Plasticity and processing, Vol.23, No.252 (1982), p.17 JIS Z 2254 "Plastic strain ratio test method for sheet metal materials" "Plastic Mechanics", p. 77 (published by Nikkan Kogyo on January 25, 1965)

  In the rotary bending method, axial tensile strain is generated outside the bend, and the wall thickness decreases. Therefore, when the tendency becomes remarkable, there is a risk of fracture on the outside of the bend. Moreover, even if it does not lead to breakage, there may occur a case where a predetermined thickness determined for the purpose of securing the strength of the product and securing the corrosion allowance cannot be satisfied. On the other hand, axial compressive strain occurs on the inner side of the bend and the wall thickness increases. However, when the tendency becomes significant, wrinkles and buckling may occur on the inner side of the bend.

  For the above reasons, if the axial strain and thickness direction strain generated during bending are within a predetermined range, processing failures such as breakage and wrinkles as described above can be prevented. Therefore, first of all, it is necessary to accurately estimate the strain after bending by the rotary pull bending method.

However, in the rotational pulling bending method, since it is bent while being restrained by various tools as described above, it is not possible to accurately estimate the strain from a simple geometric bending shape. In the examination examples so far, the thickness reduction ratio ζ (= (thickness t _{0 of the} raw tube) outside the bending as in the following formula (1) or the following formula (2) proposed by Oono et al. There is a calculation formula of wall thickness after bending t) / wall thickness t ₀ ).

  The above formula (2) is a calculation formula for thin wall and small bend radii, and the above formula (1) is a calculation formula to be applied to other cases, both of which are the outer diameter D (mm) of the raw pipe The thickness reduction ratio ζ on the outside of the bend is calculated only with the bend radius R (mm) at the center of the tube.

  However, the wall thickness after rotational pulling is greatly affected by the material properties of the base tube and bending conditions. In the above equation, the position of the pressure die is assumed to be fixed, but recently, the pressure die is often moved forward following the rotation of the rotary bending die. For the reasons described above, the above formula (1) and formula (2) cannot accurately estimate the thickness reduction ratio ζ outside the bending.

Further, from the thickness reduction rate ζ on the outer side of the bend obtained from the above formula (1) or the above formula (2), the true strain ε _{t in the} thickness direction of the outer side of the bend (hereinafter, true strain is simply referred to as strain) by the following formula (3). Can be calculated.

  However, the strain in the axial direction and the circumferential direction cannot be obtained by the method of Omino et al. In addition, it is not possible to obtain the strain inside the bending in all directions.

  As an example of calculating the thickness or strain after the rotational pulling, there is an example obtained by elementary analysis of Sato et al. In this example, not only the thickness direction strain on the outer side of the bend but also the inner side of the bend and the axial / circumferential strain are calculated. In this calculation, the influence of the r value of the raw tube material and the pushing force from the rear by the booster die (hereinafter referred to as pushing force) is also taken into consideration.

However, in this calculation, a uniaxial tension state in the axial direction is assumed. However, from the examination results of the present inventors, the axial uniaxial tension state is not actually achieved. FIG. 3 shows an example of the FEM analysis result. This example shows a strain ratio β (= circumferential strain ε _θ / axial strain ε _φ ) when a steel tube of 94φ × 1.6 t is bent 90 ° with a bending radius (R / D = 2) of 188 (mm). Distribution. The strain ratio β ₀ in the axial uniaxial tension state is expressed by the following equation (4) from the r value of the raw tube.

In addition, r value is also called a plastic strain ratio, and is a material characteristic obtained by the method described in JIS "Plastic strain ratio test method for sheet metal material" (Non-Patent Document 4). As can be seen from the result of FIG. 3, the absolute value of the strain ratio β of the FEM result is generally smaller than the strain ratio β _{0 in the} axial uniaxial tension state. This is because, as shown in FIG. 4, the rotary pulling is bent while being constrained by various tools. That is, the range subjected to bending is limited to a very narrow region immediately after the start of bending, and the material does not flow freely in the circumferential direction, so that it approaches a plane strain state (β = 0) rather than uniaxial tension.

Further, although the influence of the wall thickness of the raw tube is ignored in the study by Sato et al., As shown in the FEM result example of FIG. The strain in the thickness direction varies greatly depending on t ₀ / D).

  For the above reasons, even after the study by Sato et al., It is not possible to accurately estimate the strain after rotational pulling. Moreover, in the primary analysis by Sato et al., The strain after bending is not expressed as a single simple expression, so it cannot be easily calculated.

  As another means, PAM-STAMP, a commercially available finite element method program, has a function called Simplified Bending, which can easily calculate the wall thickness after rotational bending. The input variables in this calculation are the outer diameter, the wall thickness, the r value, and the bending radius of the raw tube, and the pushing force from the rear is not considered. Since the calculation formula in this calculation method is not disclosed, details are unknown, but from the experience of the present inventors, the accuracy with respect to the actual wall thickness is not good. Further, the information obtained is only the thickness, and neither axial strain nor circumferential strain is obtained.

  There is also a method of creating and calculating a model in detail by a finite element method (FEM). If FEM is used, it is possible to accurately estimate the strain in consideration of various conditions. However, as described above, since many tools are used in the rotational bending, it takes time and labor to create a model.

As described above, conventionally, there has been no means for estimating the strain after rotational pulling that satisfies all the following conditions A to E.
A. B. Can estimate experimental values with high accuracy. B. Both bend outer side and inner bend can be estimated. Possible to consider the influence of the outer diameter, wall thickness, r value, bending radius, and pushing force of the tube. Estimate all strains in the thickness direction, axial direction and circumferential direction. Easy estimation in a short time

  Therefore, in the present invention, it is possible to estimate the distortion after rotational pulling, which has all of the above conditions, before actually bending, and to determine whether the product is acceptable or not using the distortion. And Moreover, when it is anticipated that it will fail, it aims at enabling the output of the value of pushing force which becomes a pass. It is another object of the present invention to make it possible to automatically correct a set value to a value of a pressing force that passes the pass.

A rotary pull bending control device according to the present invention includes a shell-shaped mandrel and a plurality of mandrel balls on the tip side connected in series to each other inside a metal tube, and the mandrel balls that can be bent flexibly. When inserting and bending a metal tube while holding a part of the metal tube with a rotary bending die and clamping die, the outer diameter of the tube, the wall thickness, the bending radius, the r value of the material, the pushing force, and after bending An input means for inputting a strain reference value which is a pass / fail judgment criterion of the product, and a strain in the thickness direction based on the outer diameter, the wall thickness, the bending radius, the r value of the material, and the pressing force before bending, Calculation means for calculating one or more of axial strain and circumferential strain, and one or more of the thickness direction strain, axial strain, circumferential strain and the strain before bending. Predict pass / fail of products based on reference values Characterized in that and means for.
In the rotational pull bending control method according to the present invention , a shell-shaped mandrel and a plurality of mandrel balls on the tip side are connected in series to each other inside a metal tube, and the mandrel balls are bent flexibly. When inserting and bending a metal tube while holding a part of the metal tube with a rotary bending die and clamping die, the outer diameter of the tube, the wall thickness, the bending radius, the r value of the material, the pushing force, and after bending Based on the input process in which the computer accepts the input of the strain reference value, which is a pass / fail criterion for the product, and the outer diameter, the wall thickness, the bending radius, the r value of the material, and the pressing force before bending. A calculation process in which a computer calculates one or more of thickness direction strain, axial direction strain, and circumferential direction strain, and one type of thickness direction strain, axial strain, and circumferential strain before bending. Based on two or more Computer Te is characterized in that it has a step of determining acceptability of a product.
The computer program according to the present invention includes a shell-shaped mandrel and a plurality of mandrel balls on the tip side connected in series to each other in a metal tube, and a plurality of mandrel balls that are flexibly bent are inserted and rotated. When bending a metal tube while holding a part of the metal tube with a bending die and a clamping die, the outer diameter of the tube, the wall thickness, the bending radius, the r value of the material, the pressing force, and the pass / fail of the product after bending An input process that accepts an input of a strain reference value as a judgment criterion, and a thickness direction strain and axis based on the outer diameter, thickness, bending radius, material r value, and pushing force of the tube before bending. Based on the calculation process to calculate one or more of directional strain and circumferential strain, and one or more of the thickness direction strain, axial strain and circumferential strain before bending. Processing to determine pass / fail and Characterized in that causing a computer to execute.
The computer-readable recording medium according to the present invention is characterized in that the computer program of the present invention is recorded.

  According to the present invention, it is possible to estimate the strain after rotational pulling, which has been difficult in the past with simple and accurate estimation, before actually bending, thereby predicting the pass / fail of the product before bending. It becomes possible to do. As a result, if it is predicted that the product will be rejected, the product's acceptance ratio is output by outputting a value of the pressing force that passes or automatically correcting the value of the pressing force. Can be increased. As a result, defective products can be reduced and the yield is improved.

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a diagram for explaining rotational draw bending control to which the present invention is applied. In the figure, 1 is a metal tube, 2 is a rotary bending die, 3 is a clamping die, 4 is a pressure die, 5 is a wiper die, 6 is a mandrel (cannonball), 7 is a mandrel (ball), and 8 is a rod for fixing a mandrel. , 9 is a booster die, which bends the metal tube 1 while holding a part (bending tip) of the metal tube 1 with the rotary bending die 2 and the clamping die 3.

As described above, since many tools are used in the rotary pull bending method, it is considered that there are many condition factors affecting the strain after processing. In addition, it is considered that there is an influence of the size and material characteristics of the raw tube. The following 14 types of condition factors can be considered.
1) Bending radius-outer diameter ratio R / D
2) Thickness-outer diameter ratio t ₀ / D of the metal tube 1
3) Strength of the metal tube 1 4) n value of the metal tube 1 5) r value of the metal tube 1 6) Pushing force F (kN) by the booster die 9
7) Speed ratio between the rotating bending die 2 and the pressure die 4 8) Friction coefficient between the wiper die 5, the mandrels 6, 7 and the metal tube 1) 9) The rotating bending die 2, the pressure die 4, the clamping die 3 and the metal tube 1 10) Number of mandrel balls 7 11) Axial position of mandrels 6, 7 12) Gap amount between mandrels 6, 7 and metal tube 13) Gap amount 14 between wiper die 5 and metal tube 1) Clearance between pressure die 4 and metal tube 1

These 14 kinds of conditional factors were changed independently and subjected to FEM analysis, and the influence of each conditional factor on the strain after bending was investigated. Specifically, when each condition factor was changed independently, a condition factor having a thickness direction strain Δε _t exceeding 0.05 was extracted. As a result, the extracted condition factors are the following four types.
1) Bending radius-outer diameter ratio R / D
2) Thickness-outer diameter ratio t ₀ / D of the metal tube 1
5) r value of metal tube 1 6) Pushing force F (kN) by booster die 9

Therefore, if the above four types of conditional factors are used, it is possible to accurately and easily obtain the strain after rotational pulling. That is, the thickness direction strain ε _t , the axial direction strain ε _φ , and the circumferential direction strain ε _{θ on the} outside and inside of the bend were formulated as the following equations (5) to (10) (subscript (o) Indicates the outside of the bend, and (i) indicates the inside of the bend).

In the equation, σ _F is a value obtained by dividing the pressing force F by the cross-sectional area of the tube, and means an average axial stress (MPa). Specifically, it is calculated by the following formula (11).

In addition, the constants k _{1 to} k ₁₂ in each formula vary slightly when the material characteristics of the pipe material, bending conditions, and the like change, but are approximately in the following ranges.
k ₁ = 0.4 to 0.6
k ₂ = 0.01~0.03
k ₃ = 0.3~0.6
k ₄ = 0.01~0.02
k ₅ = 0~200
k ₆ = 1-2
k ₇ = 0.4 to 0.6
k ₈ = 0.01~0.03
k ₉ = 0.3~0.6
k ₁₀ = 0.001 to 0.005
k ₁₁ = 400 to 800
k ₁₂ = 1-2

By using the above equations (5) to (11), the outer diameter D (mm) of the tube, the thickness t ₀ (mm), the bending radius R (mm), the r value of the material, and the pressing force F (kN) Therefore, it is possible to estimate the thickness direction strain ε _t , the axial direction strain ε _φ , and the circumferential direction strain ε _{θ on the} outside and inside the bend.

Using the above strain, the product pass / fail is predicted before bending. For example, when the minimum thickness is used as a pass / fail criterion from the viewpoint of product strength and corrosion allowance, the absolute value of the strain ε _{t (o) in} the thickness direction outside the bend is less than a certain value (eg, ε _t-cr ). Judge that it is acceptable. In addition, in the case of a product in which wrinkles are important, the absolute value of the axial strain εφ _(i) inside the bend may be accepted at a certain value (for example, εφ _-cr ) or less. In addition, various criteria can be considered depending on the use and purpose of the product. For example, a case where an equivalent strain ε _eq as in the following expression (12) described in Non-Patent Document 5 is used as a criterion is also conceivable.

Further, even if it is predicted that any of the above criteria will be rejected before bending, there is a possibility that the pass range can be entered if the pressing force F is changed. For example, the case of FIG. 1 is an example of a product in which the thickness on the outer side of the bending is important, and the reference value ε _{t-cr of the} thickness direction strain as shown in the following formula (13) is determined as a pass / fail judgment criterion. I think.

  Then, from the above formulas (5) and (11), the condition of the pressing force F for the product to pass is obtained as the following formula (14).

However, in actual industrial production, F may be set to a value obtained by multiplying the coefficient a ₁ as shown in the following equation (15) with a certain margin. As the value of the coefficient a ₁ , for example, about 1.1 or 1.2 is appropriate.

  Thus, before actually bending, the computer calculates and outputs the value of the pressing force F using the above equation (15). If the set value of the pressing force F is changed to the value from the initial condition and bending is performed, a product that can clear the standard thickness can be obtained. Further, the setting may be automatically corrected to the value of the pushing force F calculated as described above.

Similarly, in order to prevent wrinkles inside the bend, when there is a reference value ε _φ-cr of axial strain as in the following equation (16) as a pass / fail criterion, from the above equations (8) and (11), The range of the pressing force F for passing is obtained as in the following formula (17).

Here, as in the previous case, considering actual industrial production, F may be set by multiplying by a coefficient a ₂ as shown in the following equation (18) with a certain margin.

Here, the value of the coefficient a ₂ is suitably about 0.9 or 0.8. Then, if the value of the pressing force F is output before bending and the setting value of the pressing force is changed to that value, or the setting value of the pressing force F is automatically corrected, The axial strain inside the bend falls within the acceptable range, and as a result, a product in which wrinkles are prevented can be obtained.

In addition, when the circumferential strain is used as a pass / fail criterion, it is generally small. However, if the circumferential strain inside the bend is equal to or less than the reference value ε _θ-cr , the pass criterion is acceptable. The range of the pressing force F for this is obtained from the above equations (10) and (11) as in the following equation (19).

Here, as in the previous case, considering actual industrial production, F may be set by multiplying by a coefficient a ₃ as shown in the following equation (20) in consideration of a certain margin.

Here, the value of the coefficient a ₃ is suitably about 0.9 or 0.8. Then, if the value of the pressing force F is output before bending and the setting value of the pressing force is changed to that value, or the setting value of the pressing force F is automatically corrected, It is possible to prevent the circumferential strain on the inner side of the bending from being in the acceptable range and preventing the yield as a defective product.

Example 1
First, an embodiment in the case where it is desired to secure the minimum wall thickness due to product strength restrictions will be described. The metal pipe 1 is a welded steel pipe having an outer diameter D = 60.5 (mm), a wall thickness t ₀ = 2.0 (mm), and an r value = 0.8, and a bending radius R = 181.5 (mm). Rotational pull bending process. Further, the reference value ε _t-cr of the thickness direction strain that is a pass / fail criterion is set to −0.1, and the initial setting value of the pressing force F is set to 0 (kN).

First, the thickness direction strain ε _{t (o)} on the outside of the bend was calculated using the above equations (5) and (11). At that time, it was input from a keyboard and calculated with a personal computer. The constants k _{1 to} k ₆ used the following values with high estimation accuracy in the welded steel pipe.
k ₁ = 0.51
k ₂ = 0.020
k ₃ = 0.41
k ₄ = 0.0118
k ₅ = 93
k ₆ = 1.36

As a result of the calculation, ε _{t (o)} = −0.122 and ε _{t (o)} <ε _t-cr , so the product is expected to be rejected. Therefore, the pressing force F that would pass the product was calculated using the above equation (15). At that time, the coefficient a ₁ for allowing a margin was set to 1.1. As a result, F = 39.1 (kN) was calculated. The value was input as a set value of the pressing force, and actual bending was performed.

As a result, when the thickness of the bent outer side of the actually bent steel pipe was measured with an ultrasonic thickness meter and converted into strain in the thickness direction, ε _{t (o)} = −0.095. Therefore, ε _{t (o)} > ε _t-cr and a product that passed was obtained.

For comparison, when bending is actually performed with the initial push force setting value of 0 (kN), the thickness outside the bend is measured in the same manner and converted into the thickness direction strain, ε _{t (o)} = -0.119. Therefore, since ε _{t (o)} <ε _t-cr , the product was rejected.

(Example 2)
Next, an example in which it is desired to obtain a product having no wrinkles on the inner side of the bend due to restrictions on the appearance will be described. The conditions of the metal tube 1 and the bending radius were the same as in Example 1. The axial strain reference value εφ _-cr, which is a pass / fail criterion for preventing wrinkles, was set to −0.2, and the initial setting value of the pressing force F was set to 60 (kN).

First, the axial strain εφ _(i) inside the bend was calculated using the above equations (8) and (11). At that time, it was input from a keyboard and calculated with a personal computer. Moreover, the constant k ₇ to k ₉ are high estimation accuracy of the welded steel pipe were used the following values.
k ₇ = 0.50
k ₈ = 0.021
k ₉ = 0.46

As a result of the calculation, εφ _(i) = − 0.203 and εφ _(o) <εφ _-cr , so the product is expected to be rejected. Therefore, the pressing force F that would pass the product was calculated using the above equation (17). At that time, the coefficient a ₂ for allowing a margin was set to 0.9. As a result, F = 44.4 (kN) was calculated. The value was input as a set value of the pressing force, and actual bending was performed.

As a result, when measuring the axial strain inside the bent steel pipe, εφ _(i) = − 0.196. Therefore, εφ _(i) > εφ _-cr , and a product without wrinkles was obtained. The axial strain was obtained from a deformed shape obtained by transferring a 5 (mm) square scribed circle to the outer surface of the steel pipe in advance.

For comparison, when bending is actually performed with the initial pressing force setting value of 60 (kN) and the axial strain inside the bending is measured in the same manner, ε _{φ (i)} = −0.208 And _{εφ (i)} <εφ _-cr . In the actual product, fine wrinkles were generated.

  Each processing operation of the embodiment described above is specifically executed by a computer system or apparatus. Accordingly, a storage medium storing software program codes for realizing the functions described above is supplied to the system or apparatus, and a computer (or CPU or MPU) of the system or apparatus reads and executes the program codes stored in the storage medium. Needless to say, this can be achieved.

  In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the program code itself and the storage medium storing the program code constitute the present invention. As a storage medium for supplying the program code, for example, a flexible disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a nonvolatile memory card, a ROM, or the like can be used.

It is a figure for demonstrating the rotation draw bending process control to which this invention is applied. It is a figure for demonstrating a general rotation draw bending method. It is a figure which shows the example of the strain ratio distribution of the distortion which generate | occur | produces with a rotation drawing bending method. It is a figure for demonstrating the restraint state of the tool in a rotation drawing bending method. It is a figure which shows the influence of the initial thickness which acts on the thickness direction distortion | strain after rotation drawing.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Metal pipe 2 ... Rotary bending type 3 ... Tightening die 4 ... Pressure die 5 ... Wiper die 6 ... Mandrel (cannonball)
7. Mandrel (ball)
8 …… Rod for fixing mandrel 9 …… Booster die

Claims (9)

Inside the metal tube, a shell-shaped mandrel and a plurality of mandrel balls on the tip side are connected to each other in series, and a mandrel that can be bent flexibly is inserted into the plurality of mandrel balls, and the metal is rotated and bent and clamped. When bending a metal tube while holding a part of the tube, the outer diameter of the tube, the wall thickness, the bending radius, the r-value of the material, the pressing force, and the strain criteria that are the acceptance criteria for the product after bending An input means for inputting a value;
Calculation means for calculating one or more kinds of strain in the thickness direction, strain in the axial direction, and strain in the circumferential direction based on the outer diameter, the thickness, the bending radius, the r value of the material, and the pressing force before bending. When,
And a means for predicting pass / fail of a product based on one or more of the strain in the thickness direction, the strain in the axial direction, and the strain in the circumferential direction and a reference value of the strain before bending. Rotating draw bending control device.   Based on the outer diameter of the tube, the wall thickness, the bending radius, the r value of the material and the reference value of the strain before bending, 1 of the thickness direction strain, axial strain and circumferential strain of the product. The rotation according to claim 1, further comprising means for calculating a pressing force such that a seed or two or more kinds pass, and an output means for outputting a value of the pressing force before bending. Drawing bending control device. Based on the outer diameter of the tube, the wall thickness, the bending radius, the r value of the material and the reference value of the strain before bending, 1 of the thickness direction strain, axial strain and circumferential strain of the product. Means for calculating a pressing force such that a seed or two or more seeds pass;
The rotary pull bending control apparatus according to claim 1, further comprising a correcting unit that automatically corrects the setting condition of the pressing force from an initial condition to a pressing force calculated by the calculating unit. In the calculation means, the outer diameter D of the pipe, the wall thickness t ₀ , Bending radius R, r value of material, pressing force F, constant k ₁ ~ K ₁₂ , And σ shown in the following equation (11) _F Using the thickness direction strain ε _t The axial strain εφ and the circumferential strain εθ are expressed by the following equations (5) to (10) (the subscript (o) indicates the outside of the bend, and (i) indicates the inside of the bend).

The rotation drawing bending process control apparatus according to claim 1, wherein the rotation drawing bending process control apparatus is calculated according to claim 1. Inside the metal tube, a shell-shaped mandrel and a plurality of mandrel balls on the tip side are connected to each other in series, and a mandrel that can be bent flexibly is inserted into the plurality of mandrel balls, and the metal is rotated and bent and clamped. When bending a metal tube while holding a part of the tube, the outer diameter of the tube, the wall thickness, the bending radius, the r-value of the material, the pressing force, and the strain criteria that are the acceptance criteria for the product after bending An input process in which a computer accepts input of values;
Before bending, the computer calculates one or more of thickness direction strain, axial strain, and circumferential strain based on the outer diameter, thickness, bending radius, r value of material, and pressing force. A calculation process to calculate,
And a process of determining whether the product has passed or not based on one or more of the strain in the thickness direction, the strain in the axial direction, and the strain in the circumferential direction before bending. Method. Based on the outer diameter of the tube, the wall thickness, the bending radius, the r value of the material and the reference value of the strain before bending, 1 of the thickness direction strain, axial strain and circumferential strain of the product. 6. The method according to claim 5 , further comprising: a step in which a computer calculates a pressing force such that a seed or two or more types pass, and an output step in which the computer outputs a value of the pressing force before bending. The rotation drawing bending process control method as described. A step of calculating a pushing force to pass the product strain based on the outer diameter of the tube, the wall thickness, the bending radius, the r value of the material, and the reference value of the strain before bending;
6. The rotational pull bending control method according to claim 5 , further comprising: a correction step in which the computer automatically corrects the pressing force setting condition from the initial condition to the pressing force calculated in the calculation step. . Inside the metal tube, a shell-shaped mandrel and a plurality of mandrel balls on the tip side are connected to each other in series, and a mandrel that can be bent flexibly is inserted into the plurality of mandrel balls, and the metal is rotated and bent and clamped. When bending a metal tube while holding a part of the tube, the outer diameter of the tube, the wall thickness, the bending radius, the r-value of the material, the pressing force, and the strain criteria that are the acceptance criteria for the product after bending Input processing that accepts input of values;
Before bending, calculate one or more of thickness direction strain, axial strain and circumferential strain based on the outer diameter, thickness, bending radius, r value of material, and pressing force of the tube. Calculation processing,
A computer program that causes a computer to execute pass / fail judgment processing based on one or more of the thickness direction strain, axial direction strain, and circumferential direction strain before bending. The computer-readable recording medium which recorded the computer program of Claim 8 .

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