**METHOD FOR BENDING PIPES**

CARLINI, Tiziano (Via Michelangelo 88, Ispra, I-21027, IT)

**B21D7/12**US3821525A | ||||

US3812694A | ||||

DE4436442A1 | ||||

DE3900990A1 | ||||

DE4312556A1 |

CLAIMS
1. Method for bending pipes or similar elongated elements in a pipe bending machine said machine comprising a seat for positioning the pipe to be bent, a forming roller of predetermined radius around which the pipe is bent and appropriated means for seizing and moving the pipe itself, characterised in that it comprises the following steps: a. providing a test pipe having the same characteristics as the actual pipe to be bent in such seat, b. performing on such test pipe, a pair of bendings setting the predetermined theoretical angles (ATI, AT2) , on the machine and measuring the resulting bending angles (ARl, AR2) , c. calculating - starting from such set theoretical and measured actual bending values on the test pipe - a proportional springback (RP) defined as the value of the angle to be added to each degree of theoretical angle (AT) set to obtain the correct bending angle of the pipe to be set in the machine (API) , d. calculating - starting from such calculated proportional springback and from such theoretical and measured actual bending values on the test pipe - the fixed springback (RF) defined as the value of the angle to be added to the theoretical bending degrees to obtain the correct bending angle of the pipe to be set in the machine (API) , e. calculate such correct bending angle of the pipe to be set in the machine (API) adding - to the theoretical bending angle (AT) - the value of the fixed springback (RF) and the proportional springback (RP) multiplied by the theoretical bending angle (AT) , f . providing the actual pipe to be bent in the pipe bending machine, g. setting such correct bending angle of the pipe in the machine (API) , h. performing the bending of the pipe around such forming roller of the bending machine.
2. Method according to claim 1, wherein in order to perform subsequent bendings the following steps are performed:
• calculating the radius of curvature (RA) of the pipe starting from the radius of the roller (RR) on which the bending has been performed and from the proportional springback (RP) ,
• recalculating the length of the rectilinear section between the two curves replacing the radius of the forming roller (RR) with the radius of the pipe (RA) calculated in the previous step, • calculating a backward movement factor (Y) defined as the circular arc whose radius is such radius of the forming roller (RR) and whose angle is the fixed springback (RF) ,
• setting on the pipe bending machine the actual forward movement calculated by subtracting such backward movement factor (Y) from the recalculated forward movement value (X) of the pipe,
• repeating the steps e) to h) of claim 1 to perform further bendings .
3. Method according to claim 1, wherein such calibration measurements are performed starting from theoretical bending angles one smaller and one greater than 90°, in such a manner to simulate a bending of an acute angle and an obtuse angle .
4. Method according to claim 1, wherein such proportional springback (RP) is calculated according to the following formula:
RP = ((AT1-AT2) - (AR1-AR2) ) / (AR1-AR2), where ATI and AT2 are such predetermined theoretical angles and ARl and AR2 are the bending angles resulting from the test bendings .
5. Method according to claim 1, wherein such fixed springback (RF) is calculated according to the following formula:
RF = AT - AR(I + RP) , where AT is used to indicate a set theoretical angle and AR is used to indicate a measured actual angle resulting on the pipe upon setting such theoretical angle on the pipe bending machine.
6. Method according to claim 2, wherein the bending radius of the pipe (RA) is calculated according to the following formula:
RA = (1+RP) * RR, where RR represents the radius of the roller of the pipe bending machine and RP is the proportional springback . 7. Method according to claim 6, wherein such backward movement factor (Y) is calculated according to the following formula:
Y = RA * RF 2π/360, where RA is the bending radius of the pipe and RF is the fixed springback.
8. Method according to claim 1, wherein the test angles are respectively 30° and 120°. |

METHOD FOR BENDING PIPES

The present invention refers to a method for bending pipes.

In particular, the present invention regards a method for bending pipes in a numerical control machine for bending pipes .

The method according to the present invention can be applied to all operations providing for the transformation of a manufactured product through bending (rods, sheets, bars etc..) .

According to the prior art, the pipe bending machines are used according to a "step-by-step" method i.e.: after having set the pipe bending machine according to the data indicated in the drawing defining the grooving parameters, the operator manufactures the first piece and controls the results .

Due to the elasticity effect of the pipe, subjected to bending, the result obtained is usually different from the one desired complying with the tolerances required according to the drawing.

The bending of a pipe occurs by means of plastic machining therefore after having subjected the pipe to bending, according to the indications of the drawing, such pipe is released but due to the springback effect the pipe loses part of the imparted bending.

In order to meet the requirements of the project, the operator - according to his experience - usually resets the machine with suitably modified data and, averagely after three attempts, he is able to obtain the required bending. When manufacturing a prototype pipe this method of operating implies a waste of about 75%, while when manufacturing a series pipe there is a waste of about 15%. The Applicant thought of how to overcome this type of drawback, that is having to reset (enter different bending parameters several times) the machine to obtain a correct bending in subsequent steps. Furthermore, the Applicant aims at being able to control the springback of the pipe and thus, increasing the bending angle, being able to manufacture the pipe according to drawing at the first operation.

In particular, the Applicant understood that the bending errors were usually due to the structural modifications to which the pipe is subjected in the rectilinear zone in immediate proximity to the bent zone, which remains rectilinear for a portion which, according to the project theoretical data, on the contrary should be already bent. Thus, the solution consists in correct positioning of the pipe taking into account such structural modifications upstream (hence before performing the

bending) calculating such movement backwards and positioning and thus bending the pipe in a correct manner .

An aspect of the present invention regards a method for bending pipes or similar elongated elements according to the attached claim 1.

Further objects and advantages of the present invention shall be clear from the following description and attached drawings, strictly provided for exemplifying and non-limiting purposes, wherein:

• figure 1 illustrates a project drawing of a pipe

• figure 2 represents a pipe during the bending step on a roller of a pipe bending machine,

• figure 3 illustrates a chart of the parameters calculated according to the present invention.

Usually a pipe bending machine comprises a seat for positioning the pipe T, a roller R around which it is bent and appropriated means for seizing and moving the pipe itself. From such seat, the seizure means see to picking the pipe and the movement means see to rotating the pipe, axially moving in a longitudinal direction and arranging the pipe itself around the roller. The bending parameters to be set in the pipe manufacturing machine are substantially three: bending angle around the roller (bending rotation) , distance between a curve and the subsequent one (axial forward movement) ,

degrees of rotation around the axis of the pipe (axial rotation) . The axial forward movement serves to position the pipe at the bend starting point. The axial rotation serves to impart the correct orientation on the bending plane to the pipe. The bending rotation serves to set the correct angle for bending the pipe . Given such three parameters it is possible to manufacture a pipe starting form a preset "three dimensional" drawing. An electronic processing unit (for example a PLC or a microprocessor card) of the pipe bending machine commands and controls the movement and seizure means according to a preset programme and it is also adapted to measure, through suitable sensors, the bendings performed. The method according to the present invention can be advantageously applied to metal pipes (such as for example titanium) used for example in aircraft hydraulic systems. The present invention is however applicable to all elongated elements (pipes as well as other similar elements such as for example rods, sections, drawn products etc..) which require bending according to such preset drawing.

In particular, shown in figure 1 is an example of such preset drawing, illustrated in which is the pipe highlighted in which are some critical points,

identified by letters, in proximity to the bending (nodal points) .

The origin of a reference system in the space (X, Y, Z) is fixed at point A of the beginning of the pipe and all the other points are given as value triads with respect to such system (observable in the table of figure 1 which also indicates the diameter of the roller, the length of the pipe to be bent and the distance between the starting and finishing point of the pipe) .

Starting from such data the processing programme stored in the pipe bending machine is capable of transforming the data present in the preset drawing of the pipe (which is in turn stored in a data base associable to the machine processing unit) into such bending parameters .

The calculation of such parameters in the methodaccording to the present invention provides for that in a preliminary step with respect to the actual start of the pipe bending steps, at least one pair of calibration bendings be performed with a test pipe having the same diameter as the pipe to be manufactured, in such a manner to be able to calculate two fundamental coefficients adapted to subsequently determine the correct bending parameters in the pipe bending machine.

As illustrated in figure 2 on a pipe section of known diameter performed is a first bending of a theoretical angle ATI (in the illustrated example of 120°); actually, the pipe - due to the elastoplastic behaviour of the material - is subjected to an actual bending angle ARl smaller than the theoretical bending angle. In particular, in case of theoretical bending of 120° the actual bending is calculated (manually or by means of such sensors) to be of 105.98. Analogously, performed is a second bending of a theoretical angle AT2 (in the illustrated example of 30°) ; actually the pipe - due to the elastoplastic behaviour of the material - is subjected to an actual bending angle AR2 smaller than the theoretical angle. In particular, in case of a theoretical bending of 30° the actual bending is calculated to be of 21.50°. Such calibration measurements are preferably performed starting from different theoretical bending angles and preferably one smaller and one greater than 90°, in such a manner to simulate a bending of an acute and an obtuse angle.

From such calibration measurements for a pipe of a predetermined diameter it is possible to calculate the value of the two fundamental bending parameters i.e. the so-called fixed springback RF and proportional springback RP .

According to the present invention the term proportional springback is used to indicate the value of the angle to be added to each degree of theoretical angle set to obtain the desired bending of the pipe. In particular, such proportional springback RP is obtained from the following formula:

RP = ((AT1-AT2) - (AR1-AR2)) / (AR1-AR2). According to the present invention the term fixed springback is used to indicate the value of the angle to be added to the theoretical bending degrees to obtain the desired bending of the pipe. In particular, such fixed springback RF is obtained from the following formula: RF = AT - AR(I + RP) ; where AT represents a set theoretical angle and AR represents a measured actual angle resulting on the pipe upon setting such theoretical angle on the pipe bending machine . Having calculated such two parameters as indicated beforehand, for a pipe of a known diameter according to the present invention determined can be the bending angle to be set actually in the machine (with respect to the theoretical angle indicated in the drawing of the pipe) to obtain the desired bending without errors. In particular, the correct bending angle API to be set in the machine is calculated as follows:

API = AT ( 1+RP) + RF ; where AT is the theoretical angle set according to the drawing .

As previously described and as indicated in figure 2 a parameter required to manufacture a pipe having at least a rectilinear section between two curves is the length of the rectilinear section itself. As a matter of fact, in a pipe requiring to be subjected to only one bending all that is required is to calculate the two abovementioned proportional and fixed springback parameters. The theoretical length between the two curves X can be obtained from the drawing (like the one illustrated for exemplifying purposes in figure 1) . The Applicant understood that, if set directly in the pipe bending machine without taking into account the behaviour of the pipe during the forming, such theoretical ^{' }
length X would lead, as a consequence, to poorly performed operations and the result would not be as desired, i.e. complying with the drawing. In particular, the theoretically calculated rectilinear section would be too long, in that it would not take into account the deformations which occur on the pipe upon the performance of the forming operations . The radius of curvature of the pipe depends on the radius of the roller RR on which the curve is formed and on the proportional springback and in particular

the bending radius of the pipe is calculated as follows:

RA = (1+RP) * RR.

According to the present invention, in order to obtain the actual forward movement to be performed on the pipe in the machine, it is necessary to take into account the radius of curvature of the pipe and of the fixed springback RF, which are the parameters conditioning the proper axial forward movement of the pipe to position the pipe at the point of starting the subsequent bending. The circular arc can be approximated to a rectilinear section in that upon terminating bending the pipe such circular arc - due to the elasticity effect - returns rectilinear and becomes an integral part of the rectilinear portion between the two curves. Such section is determined by the angle corresponding to the fixed springback RF on the circumference CI whose radius is the radius RR of the roller on which the pipe is curved and represents such factor (backward movement Y) to be further subtracted from the forward movement or theoretical distance or rectilinear section X between two curves. Consequently, once the radius of curvature has been obtained, it is obtained that the backward movement corresponds to:

Y = RA * RF 2π/360.

In the example illustrated in figures 2 and 3 using the two calibration bendings performed (30° and 120°) it is obtained that the fixed springback RF is of 4.80 degrees and the proportional springback RP is of 0.0909 per degree of curvature.

Consequently, in order to perform a curve of 90° it is necessary to set, on the pipe bending machine, a bending angle API = 102.98.

Starting from a radius of the forming roller of 57.15° obtained is a pipe radius of 62.34°.

In summary, the method according to the present invention provides for that the following steps be performed in sequence:

• providing a test pipe having the same characteristics as the pipe to be bent in a pipe bending machine,

• performing on a test pipe, having the same diameter as the pipe to be bent, a pair of bendings setting the predetermined theoretical angles (ATI, AT2) on the machine, and measuring the resulting bending angles (ARl, AR2) ,

• calculating - starting from such set theoretical and measured actual bending values - a proportional springback (RP) defined as the value of the angle to be added at each degree of theoretical angle (AT) set to

obtain the correct bending angle of the pipe to be set in the machine (API) ,

• calculating - starting from the calculated proportional springback and from such theoretical and measured actual values - the fixed springback (RF) defined as the value of the angle meant to be added according to the theoretical bending degrees to obtain the correct bending angle of the pipe to be set in the machine (API) , • calculating such correct bending angle of the pipe to be set in the machine (API) adding - to the theoretical bending angle (AT) - the value of the fixed springback (RF) and the proportional springback (RP) multiplied by the theoretical bending angle (AT) , • providing the actual pipe to be bent in the pipe bending machine,

• setting such correct bending angle of the pipe in the machine (API) ,

• performing the bending of the pipe around a roller, of a known radius, of the bending machine.

According to the present invention, the term "pipe having the same characteristics" , is used to indicate that the test pipe has the same diameter, thickness and material as the pipe to be bent . Subsequently, once the first bending has been performed - in cases where provided for in the drawing project of

the pipe are more curvatures in succession - it is necessary to calculate the rectilinear section between two subsequent curves in such a manner to move the pipe, through such movement means of the machine, by the correct distance forward movement.

Calculation of such correct distance is performed according to the following steps:

• calculating the radius of curvature (RA) of the pipe subsequent to the rectilinear section to be calculated starting from the radius of the roller (RR) on which the bending is performed and from . the proportional springback (RP) ,

• recalculating the length of the rectilinear section between the two curves replacing the radius of the forming roller (RR) with the radius of the pipe (RA) calculated in the previous step,

• calculating the backward movement factor (Y) defined as the circular arc whose radius is the radius of the forming roller (RR) and whose angle is the fixed springback (RF) ,

• setting - on the pipe bending machine - the actual forward movement calculated by subtracting such backward movement factor (Y) from the forward movement theoretical value (X) of the pipe obtainable from the project data.

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