Study of effect on parameters of bend quality for tubes

Tube bending is a metal forming process that is used to shape pipes or tubing. Bending machines take straight tube stock and create a shape involving single or multiple bends. This allows the tubing to be formed in any fashion to fit the application's specific needs. The process of tube bending involves using mechanical force to push stock material pipe or tubing against a die, forcing the pipe or tube to conform to the shape of the die. There are three basic bends that are commonly used: the 90° bend, the common offset, and the saddle. Wrinkles occur when the mandrel inside the tube is no longer able to provide enough counteractive force. Wipers are always used in combination with mandrels inserted inside the tube during bending. The mandrel's primary job is to control the shape on the outside radius of the bend. They are excessively used in everything from the automotive industry to aviation, shipbuilding, aerospace, oil and gas, etc. The automotive sector heavily relies on mandrel bending machines to minimize thin wall tubing's ovality when it's bent.


Introduction
Bending is a manufacturing process that produces a different degree and radius along a straight axis in ductile materials. Generally, bent tubes are used for structural purposes or as passageways carrying fluids or gases. Structural bent tubes: bicycle handlebars, furniture frames, grab bars, roll bars, etc. Passageways: hydraulic lines, fuel lines, exhaust pipes, water lines, etc. Industries typically using bent tube/pipes are automotive, aircraft, off-road and farm equipment, boiler, air conditioning, ship building, furniture, power generation, recreational vehicle, railroad, etc.

Materials and methods
When a tube is bent, the wall which forms the outside of the bend elongates and thins while the wall which forms the inside of the bend compresses and thickens. A common objective in tube bending is to form a smooth round bend. This is simple when a tube has a heavy wall thickness and it is bent on a large radius. To determine if a tube has a thin or heavy wall, its wall thickness to its outside diameter is compared. The result is called the tube's wall factor. The same type of comparison is made to determine if a bend radius is tight or large (D of bend). So, two ingredients -the wall factor and the D of bend -are used to determine the severity of a bend. As an example: a 2 in OD tube with a 0.200 in WT has a wall factor of 10. If the tube was bent on an 8 in centerline radius, the D of bend would be 4. In this case, an attractive bend can be formed with three basic tools: the bend die, around which the bend is formed; the clamp die, which grips the tube and holds it in position as the bend is formed; and the pressure die, which forces the tube into the bend die groove so it can be formed. Figure 1illustrates the basic tools. Unfortunately, all too often, bending requirements are not this simple. As the tube wall becomes thinner (the wall factor number becomes larger) and the bend radius tighter (D of bend number becomes smaller) a flat toned bend may result. This happens because the wall along the outside of the bend is not thick enough to support itself and collapses. To prevent this a mandrel is required. The mandrel is placed inside the tube and supports it during bending. Mandrels can be either a simple plug type or a segmented ball type.
In the latter type the ball segments extend into the area of tube which is to be bent and flex with it during the bending process. A tube with an outside diameter of 2 in. and a wall thickness of 0.100 in. has a wall factor of 20. When bent of a 5 in. radius (2.5 D of bend) a mandrel with three ball segments would be required to form a smooth round bend.

Figure 1 Positioning of tube bending
If bending conditions become even more severe, for example, a 2 in. OD tube with a 0.05 in. wall (40 wall factor) were bent on a 4in. radius (2 D),a fifth tool, called the wiper die, would be required. It was said in this article that the tube wall along the inside of the bend compressed and thickened; but, when the tube wall is thin and the bend radius tight, as in the example above, it will not compress evenly, but will instead wrinkle. The wiper die is made so that it can nest in the bend die groove with its very thin tip extending to the bend tangent point (the point where the tube will begin to bend). In doing so, it fills up the gap normally left by the bend die. Therefore, the tube is completely confined and does not have space in which to wrinkle. Elongation refers to the amount the material can stretch before it fractures. As was noted in the previous section, the tighter the bend radius, i.e. the smaller the D of bend, the more the material will be required to stretch. steel; therefore, it is much easier to bend on a tight radius. But, if the end product is a bicycle handlebar, stainless is too costly and; therefore, mild steel would be selected. The bend radius is also dictated by the end use, since it must create a shape which is functional and has aesthetic quality. Hopefully, the material which is selected and the bend radius which is chosen will be compatible.

Common Bending Styles of operation
There are several types of tube bending machines available today, each of which has its own particular advantage. Basically, three types are the 'work-horses' of the bending process.

Press-type bending
The press-type bending machine is similar to a vertical press machine used in the sheet metal forming industry. Press bending is one of the oldest forms of tube bending. As with most vertical presses, power is transferred through a vertical ram cylinder to which a bend former dismounted. The bend former is 'rammed' into the wing dies which then give way to the force of the ram and wrap the tube around the bend former. Press bending was very popular in the automotive exhaust pipe bending industry. In many instances an exhaust pipe manufacturer would set up a dedicated line of press benders.
Each bender would perform one bend of a multi-bend part and then be passed to the next machine. This process is still being used; however, due to the time necessary to perform a tool setup on a multiple press line, the cost of tooling and new faster CNC bending equipment, this type of manufacturing is becoming less and less economical. A disadvantage of the press bender is that a mandrel cannot be used. This has limited the machine to applications where out of roundness is not a critical factor.

Compression-style bending
Compression benders were also widely used in manufacturing exhaust pipes. This type of bender resembles the draw type benders with the exception of the roller or 'wipe shoe' used to roll or wipe the tube around the forming die. The machine had limited success with mandrel bending. The compression-style bending machine clamps the tubing to the stationary bend form and a rotating arm pushes the material around the bend form.

Draw-style bending
Several manufacturers offer draw style bending machines. This type of machine offers mandrel or compression bending. The machine clamps the tubing to the bend former which then rotates, 'drawing' the material around the former.

Tooling definition
The types and designs of tooling commonly used in bending machines are shown below. The position the tools occupy in the bending machine are also shown.

Bend die
The forming tool which is used to make a specific radius of bend is called a bend die. The bend die usually consists of two separate pieces called the insert and the bend radius. The insert is used for clamping the tube to the bend die before forming. The bend radius forms the arc of the bend as the tube is drawn around the die. The bend radius is normally sized to two times the tube diameter. Thus, a one inch bend on a two inch radius can be referred to as a 2D bend. The insert used for clamping the tube normally has a 2D clamp length. Thus, one inch tube with a two inch insert will have a 2D grip length.

Clamp die
The clamp die works in conjunction with the bend die to ensure it clamps the tube tithe bend die. The clamp die will move in and out to allow feeding of the tube.
The pressure die is used to press the tube into the bend die and to provide the reaction force for the bending moment. The pressure die will travel with the tube as it is being formed. The pressure die boost cylinder is attached to the pressure die. The boost cylinder can assist the tube through the bend to prevent tube breakage, wall thinning and ovality.

Mandrel
The mandrel is used to keep the tube round while bending. The major components of the mandrel are the shank and balls. Mandrel balls are required when bending thin wall tube. Thicker wall tubes may be bent with compression tooling (elliptical type) or bent using a plug mandrel.

Wiper Die
With mandrel bending, it is sometimes necessary to use a wiper die. This is used when a mandrel alone will not prevent wrinkling while bending a tube. The wiper die "wipes" wrinkles from the tube. It mounts directly behind the bend die.  Required type of former and die setting are arranged.  Test piece is first taken for bending, so that our Job not get spoiled.  Test piece is shown to the quality person for checking ovality, flatness and wrinkles.  After QC person approval we start the bend on jobs.  Job is marked as per the design with the help of chalk/ knife.  Bending is done as per the drawing.  Operator check the length; dimension and all other thing are matching as per drawing or not.  Quality person is again called for the inspection for checking the required parameter is matching or not like length, drawing degree, ovality, flatness and wrinkles.  Now, bending is over and job is ready.

Specification of Bending Materials
 Diameter of the tube  Wall thickness of the tube  Length of the tube  Nature of the Material or material type

Study of Pines Machine
Pines Machine is one of the Bending Machine which can bend these types of materials: -

Grade C material
This type of material is used as economizer in the Boiler. As we know economizer uses exhaust gases of flue used to heat the water in to steam. Thus, same flue is used to heat the economizer and thus this process is economical. For Grade C Material Dia. 44.5 Rad. 133.5,  If more pressure is applied flatness is more  More pressure will result in breaking of M/c center bolt  Right pressure can only give quality bends without any flatness.  Flatness and ovality must not go beyond 10% of the dia. Otherwise the job cannot be accepted.

T91 Material
 If more pressure is applied flatness is more  More pressure will result in breaking of M/c center bolt  Right pressure can only give quality bends without any flatness.  Flatness and ovality must not go beyond 10% of the dia. Otherwise the job cannot be accepted.

SS Material
 If more pressure is applied flatness is more.  More pressure will result in breaking of M/c center bolt  Right pressure can only give quality bends without any flatness.  Flatness and ovality must not go beyond 10% of the dia. Otherwise the job cannot be accepted. If all this conditions are fulfilled, BEND is OK.

Flatness check
Flatness problem is checked if it is below 10% bend flatness or not. If it is less than 10% , it is OK. More than percentage of flatness , ovality is more and it is not accepted. So, this can not be accepted.

FOT (First of Trial Bend)
FOTs are to be carried out for a bending process having a change in any of the following parameters than the existing qualified FOTs.
 Type of machine  Material specification  Dia of the tube  Thickness of the tube  Radius of the bend  Angle of bend  Type of heating (if applicable)

However the following exemption is permitted.
 Existing FOT qualification on a bend qualifies for any bend angle less than the qualified angle.  FOT carried out on a combination of material, outside diameter (OD), bend radius and for minimum and maximum required thickness shall qualify for all intermediate ranges. Also, if FOT for lower R/D qualifies, then the higher R/D also qualifies if there is no change in machine, material specification, OD, thickness.

Sample inspection
 All bends shall be cut transverse to tube axis as detailed below.  For bend angle up to and including 90 degrees, one section shall be cut at the middle of the bend. The cut sections face should be parallel to the bend radial axis  For bend angles above 90 degrees, three sections shall be cut one at the middle and the other two sections shall be at an angle of 45 from the middle. If, maximum reduction in OD found visually anywhere in the bend, this section also to be cut and consider as a sample. The cut sections face should be parallel to the bend radial axis The following checks shall be done.
 Minimum thickness at the cut sections.  Maximum and Minimum diameters (OD &ID) at the cut section.  Imprints of the cut sections (For other than Squeezed bends).  Visual examination of the cut sections of the bend.
After recording the data, the following calculations shall be made to ensure conformance to specifications.

Acceptance
3.15.1 Unless otherwise stated in the contract, the following are the requirements to be met for accepting the FOT. The minimum required outside diameter at any part of the bend is given by the formula. Where  R is the mean radius of bend to the center line of the tube (in mm)  D is the Nominal outside diameter of the tube (in mm)  T1 is the thickness measured at the end of the tube after bending, by drawing a line parallel to tube bend axis from T2.  T2 is the minimum thickness observed in the tube after bending.
3.15.6 Tube bends shall not have flat areas in excess of 12.5mm wide running longitudinally with the centre line of the tube.
If the requirements of 3.15.2 & 3.15.6 are not met (or) 3.15.3 to 3.15.6 is not met, the bends are to be rejected.
If the minimum available OD at any point is less than that calculated as per clause 6.1, the bend can be accepted if the requirements of clauses 3.15.2 to 3.15.5 are met.
All the 3 samples must conform to the above requirements for successful qualification of the FOT.
If one of the three samples fails to qualify, two more samples shall be taken and the conformance if established in both samples, this shall qualify the FOT.
If more than one sample fails, the FOT shall be repeated after incorporating changes as required to eliminate the cause of nonconformance FOT -(First of Trial)

Conclusion
The Study of effect on bending parameters on bend quality for tubes for pressure parts of Super Critical Boiler has improved the quality of products, avoided rework and wastage of materials. In this study manufacturing cost are reduced. Simultaneously it ensured Safety of operation along with Quality of the job also improved.

Scope of future study
The Boiler Components are generally repetitive in nature. The New method of process can be used for upcoming or future pressure parts products of Tower type Super Critical Boiler.

Acknowledgments
Authors are thankful to my project guide Mr. P. Vijayakumar, for his valuable and invariable suggestion and encouragement in carrying out this project successfully.

Disclosure of conflict of interest
There is no conflict of interest among authors.