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THE AMERICAN UNIVERSITY IN CAIRO
DEPARTMENT OF ENGINEERING
MECHANICAL ENGINEERING UNIT
MENG 339- Materials and Manufacturing
 
 

Technical Report on Term Project
Manual Hydraulic Tube and Pipe Bending Machine

Presented to:
Dr. Nahed El-Mahallawi

Presented by:
Fadi Naaman Abi-Nader
Moataz Mohammad Attallah
Mohamed Abd-Allah Borhan
Sherif Samy Shenouda

SPRING '99




Bending:

Tube and pipe bending is one of the important manufacturing processes; yet it requires special tooling to avoid any of the possible drawbacks or limitations of the processes. Tube bends have many applications, either in plumbing operations and domestic plumbing installations, or even in some machine manufacturing. They are available for example in bicycles, metal furniture, water or other fluids pipes.
There are some concerns about any bending process. First, is the possibility of buckling or folding. Buckling is the deformation that happens to a tube surface—supposed to be straight after being bent, the tube undergoes a non-uniform deformation. Buckling is not the only concern; among the major concern is the possibility of fracture of the component being bent or reaching the ultimate stress of the material. Fracture or cracking is the major concern in any bending process. Hence, it requires special treatment.
Some methods were introduced to avoid the previously mentioned problems of bending. The commonest method is to fill the tube with loose particles, mostly sands, which is then shaken out the tube after finishing the process. Still, the tube should be fixed or clamped to avoid any buckling that may result from any deflection on the radial direction.
In many engineering applications, it is required that tube should have internal details. These details can be shaped by plugging the tube with special mandrels, made of rubber or any flexible material. These mandrels can have the shape of a normal plug, balls, cable or even laminated.
The thickness of the tube is an important factor in the bending process. Tubes with large thickness can be bent without the fear of fracture or cracking; yet the force required to perform the process would be higher. On the other side, tubes with small thickness must be filled with sand or loose particles to avoid buckling or cracks.
Another important concern about tube bending is the so-called the spring-back effect. It is known that the material experiences an elastic recovery upon the removal of the load. In bending, this recovery is the spring back. Spring back occurs in pipes and tubes, as well as flat sheets and plates. The theory section will include some relations for this effect.
There are four different methods for compensating the spring back. The most easier one is overbending. Here the work piece is bent with an angle smaller than the desired one (e.g. bending with 87 degrees for a 90 degrees bending radius)  so that when the tube restores its elastic recovery, the angle that remains after the spring back is the desired angle. Another method to overcome spring back is stretch bending. In this case, the work piece is subjected to tensile forces while being bent. In this way, the bending moment required to induce certain angle will decrease as the combined stresses between bending and tension will work on it. Another way to decrease the spring back is to carry out the process on elevated temperatures (hot bending), hence decreasing the spring back.



Bending Machines:

Any bending machine should have some components that help it perform its function without producing any defects or deformation into the work-piece. These components are:

  1 -  Source of force: There should be a source of force or moment that will bend the tube. The required force is the one that is capable of producing a stress that exceeds the elastic limit to produce permanent deformation into the tube. The force is produced with different methods. The force can be produced by a physical force…i.e. one has to push a certain arm by his hand. In order to produce the enough force, hence a large force arm should be considered. In other cases, this force is supplied by a hydraulic jack that is capable of producing a large force. This hydraulic jack can be manually or mechanically driven. Yet, the length of the jack stroke is a very important factor that determines the angle of bend to which the tube can reach.
  2 -  Tube Fixation: The tube should have a special fixation; a fixation that allows motion of the tube as it elongates and prevents its rotation about its center line to avoid buckling. Sometimes the tube is clamped to avoid the latter motion. In some cases, the tube is fixed between two rollers to allow the tube to move. An additional advantage to the rollers is that since they roll during bending, the tube does not experience any friction, opposing to what could have happened when the rollers are fixed.

  The tube position after bending relative to before bending
 
  3 -  Impression Die: The die or the shoe of the machine is the  component on which the tube is supposed to rest. The die should have slot on which the tube will rest. In some benders, the die is kept fixed while the tube approaches it, and in other machines—the produced model—the die approaches the tube.
The depth of the die slot depends on the tubes that will be bent using the die. Tubes with cross-section thickness can be bent using specific dies. This prevents buckling and other possible deformations.


Types of Tube Bending methods:

  1 -  Three point bending: Type of bending in which the bending force is applied only on one point, and the tube is fixed on two other points. These force produces a maximum moment at the midpoint between the two supports.
  2 -  Four point bending: The bending force is applied on two points, and the tube is fixed on two supports at two other points.

Three point bending has an advantage over the four point bending is that in the first, the maximum moment happens only on one point, while on the second, it takes place on a range. Therefore, there is a greater probability for defects and imperfections to be present in the volume of material between the loading points in the 4-points bending than in the much smaller volume under the single load in the 3-point test.

 
3 point bending
4 point bending

Theory:

The value of maximum stress due to a bending moment is given as:
 
In this case the stress should exceed the yield stress.
On the other side, the minimum bend radius of a given sheet or pipe depends on the R/T ratio, which is the ratio between the bending radius to the thickness (T) of the piece. The relation is given as:

Where r is the final radius of the tube after bending.
Another factor is the Spring back factor Ks , which is determined as follows:

Where Ri is the initial bend radius and Rf is the final bend radius.
A spring back factor K=1 means that no spring back will occur. If K=0, it indicates complete elastic recovery after the removal of the load.



Different Types of Bending Machines:
 
Ensley Tools Bending MachineEnsley Tools companyAn exact model for the assigned machine. The force is applied using a hydraulic jack that is mounted vertically on the work piece. The dies contains slots for the tube to be inserted in. In addition, it has two rollers for the tube to slide freely on them.
Manual Hydraulic Bender: Horizontal JackJesan Kovo company
Manual Mechanical Bending Machine:The source of force is the physical power derived from the moving of the handle, which creates a motion and a moment that bends the pipe. This model may have some limitations for the possible limits of bending.
Manual Draw Bender:Draw bending machines let you bend pipe and tube at extremely low cost. The compact design permits a great deal of mobility, as well as flexibility of location. They are light-weight and require less than half square meter of bench space--including the bend arm sweep. The limitation of this machine is tube size limitation; it can work for all cross-sections.


SELECTED MACHINE: Design, Description, Components:

The machine that was selected for manufacturing is the first model, based on the Ensley tool machine. A large part of the project was done using Reverse Engineering. First, using the internet and getting manuals for similar machines. Next, a similar model for the machine was studied thoroughly and its measurements was taken. Finally, a new design was implemented to overcome the problems of the large price and other problems of the original design.

The machine is composed of the following main components:

  1 -  Bender Frame: These are two identical plates between which the tube is supposed to rest. It should be strong and rigid so as not to move during the bending process, causing any of the previously mentioned defects.
  2 -  Rollers: They represent two points of the three points bending. They tube should rest on these rollers. Therefore, they should have additional mobilities (translation and rotation along its center line) to allow the sliding of the tube over them, else the tube surface will deform due to friction between it and the rollers. The slot engraved inside the rollers should fit the cross section of the largest tube section that machine is supposed to bend.
  3 -  Die: The die is the point of force application. It is circular sector with a slot engraved in it for the tube to rest on. The machine should have a special die set for each tube section.
  4 -  Hydraulic Jack: This is the source of the force. The used on is a 20 ton jack, however it should be used on a vertical direction. It total stroke is around 26 cm.
  5 -  Others: Table, fixations between the bender and the table, fixation between the jack head and the die.



OPERATION:

As the jack moves upwards, the die moves until it meets the work piece. The three points in contact with piece; therefore bending starts. The jack gives a small feed of motion (2 mm per time). This is important because if its minimum feed was high, it could have caused sudden fracture. The small feed is an advantage because it gives a chance for very reliable and precision bending.



IMPLEMENTED DESIGN:

The implemented design uses the vertical jack setting: Ensley Tools Machine: This offers:

  1 -  Maximum Stability: By comparing Ensley tools model which is fixed on a table to the one of Jesan Kovo, it is clear that the tripod fixation is not as stable as being fixed to a table. This is because when a force is applied on the jack arm, there will be a tendency for the whole machine to rotate over one of the tripod struts. Accordingly, by avoiding the tripod, more stability was achieved.
  2 -  Less Price: The price of the horizontal jacks in comparison to vertical jacks is very high (around 2000 L.E. for the horizontal to just 90 L.E. for the vertical jack with the same capacity—20 tons). Therefore, it is more economical to use the vertical bender.



MATERIAL SELECTION:

Steel was chosen for the machine parts for the following reasons:

  1 -  Low cost
  2 -  Easy to machine, in comparison to Aluminum or other materials.
  3 -  High strength that can withstand the loads.
  4 -  Long expected life-durable.



Introduction to Pipes and Tubes:

Steel pipes may be made from slabs or billets. The slabs usually are processed to make welded pipes. Seamless pipes are normally made from billets. The slabs are reduced to narrow strips called skelp. The skelp is as wide as the circumference of the pipe to be produced. A machine draws the skelp through a bell, or bell-like die, or through contoured rolls. The bell or rolls bend the strip of steel into the rounded shape of pipe or tubing. The seam is then electrically welded.
   In order to make large-diameter pipe, a steel plate is pressed by machines into the general contours of pipe. The seam is welded, and the ends are temporarily sealed. Water is then pumped into the pipe under pressure, and this expands the pipe hydraulically to the right diameter.
   Billets are reduced to bar-like lengths, which are called tube rounds, before they are made into seamless tubing or pipe. The tube round is first heated to a uniform temperature in a continuous oven. Then one end is fed between the piercing rolls of a mandrel mill. The rolls work and knead the round until an opening develops through the center. A mandrel, or piercer, is inserted into the hole of the tube round. The process is continuous. The round moves forward, opening into the shape of a pipe or tube as it passes over the long mandrel. After an opening is made through the entire length of the round, the mandrel is withdrawn. The pipe may be further shaped by elongating in a stretch-reducing mill; or it may be enlarged by being passed over a sizing mandrel, which operates in the same way as the piercing mandrel. While a mandrel is inside, the tubing or pipe may also be worked between rolls, or reelers. The pressure of the reelers thins the outside wall of the pipe or tubing.
   Tubing is sometimes cold-drawn through a die while a long mandrel is inserted inside the tubing. This technique enlarges the diameter of the tubing. The pressure of cold-drawing can also result in tubing that has a greater tensile strength and a smoother, tougher surface.
  


Main Manufacturing Processes:

Introduction to Machine tools

A machine tool is a power-driven tool that changes the size, shape, or finish of metal objects. A true machine tool does at least four things: It holds a workpiece, holds a cutting tool, moves one or both of these objects, and provides a feeding movement for the tool or the workpiece.
 The cutting tool makes a new part by shaving metal from an original piece. A machine tool carves a piece of metal in much the same way that a pocket knife is used to whittle a toy sailboat from a piece of wood.
 An important feature of a modern machine tool is precision. It can cut metal to an accuracy of millionths of an inch. This figure is approximately equal to 1/300 of the thickness of a human hair. Such precision makes it possible to produce two parts or thousands of parts that are exactly alike.
 There are more than 400 kinds of machine tools at work in industry today. They range from small bench devices to complicated machines that weigh hundreds of tons. Machine tools perform thousands of operations primarily on metal objects. They produce tiny screws and great turbine rotors with precision.
 In principle machine tools are simple. They perform some basic operations on metals. These operations are turning, planing, milling, drilling, power sawing, grinding, and metalworking.

In our project, several metal working operations were used. We were basically limited by the tools available at the AUC workshop. Some operations would have required more appropriate tools and manufacturing processes.

 The basic maunfacturing processes that we practiced during our work on the project are the following.

1- Turning
The lathe is the machine tool used to perform turning operations. It produces a cutting action by rotating a workpiece against a fixed tool having a single edge. The line of cut forms a cylindrical surface. The principle of the lathe has been known and used for hundreds of years. It probably developed from an early combination of the potter's wheel and a knife.

A lathe is the most versatile machine tool. It can machine a workpiece to a round, a concentric, an eccentric, or a tapered shape. It can cut plain or tapered screw threads inside or outside. With the proper attachments, a lathe can perform milling or grinding operations. It does all work with precision.
 
Principal Parts of the Lathe
 The bed is the frame that supports the tool and the workpiece. The headstock supplies mechanical power. It contains a cone pulley that provides variable operating speeds. The tailstock controls the dead center that supports the end of the workpiece. The carriage moves and controls the tool. A compound rest holds the tool post and the cutting tool. The cutting tool is made of tempered steel or of steel tipped with alloys such as the carbides of tungsten, boron, titanium, or tantalum.
In operation the workpiece may have a hole drilled in each end and be held rigidly between centers. The workpiece may also be held in chucks. In either case the workpiece rotates, and the tool is fed into it.
 Types of Lathes
 The engine lathe is the type most widely used. It has back gears to provide the low speed and high torque required to make heavy cuts on a large workpiece. An engine lathe is built with change gears and a lead screw for cutting threads. It also has powered feed in both longitudinal and cross directions.

 -  Turning was one of the processes that we used extensively during our labwork. The rollers were manufactured by turning. We used a special tool with an especially designed contour to work the rollers.
 
 -  Turning was also used a basic cleaning operation. We removed 2mm of the surface of our materials to remove the oxidation and dirt. Facial turning was also used, as well as longitudinal turning.
 
 -  The die was also manufactured by turning a large disc. The disc was latet sawed to obtain an arc-shaped die.
 
 -  Large bolts were machined to the appropriate size by turning. We also threaded them using the lathe, with great precision and improved surface finish.
 
 -  The die fixation problem was also solved by turning. We manufactured an especially designed part that enabled us to fix the die over the piston head.


 2- Shaping
Planing Operations
   Planing metal with a machine tool is similar to planing wood with a hand plane. The planer produces a flat surface by its cutting action. A single-edged tool is held in a toolhead on a rigid cross member called a rail. The workpiece is mounted on a table that is supported by tracks in a heavy bed. Cutting action takes place when the workpiece is moved back and forth against the stationary cutting tool.
 A shaper is a machine tool that is used to plane metal. A shaper's single-edged tool moves back and forth in a straight line, and the workpiece is fed into the cutting tool. The standard shaper consists of a base on which is mounted a column that supports the operating parts. A ram over the column carries the toolhead. It travels in ramways that control the tool's motion.
   Shapers are used for machining small flat or curved surfaces.

 -  Shaping was one of the operations that we used to manufacture our machine.
We used the shaper to improve the dimensional accuracy and surface finish of the plate’s borders. The plates were originally cut using an oxy-acetylene flame. The resulting surface finish was very bad. A shaping operation was needed to remove excess metal and to increase the dimensional accuracy of the workpiece.



3- Milling

A milling machine is used to form flat, curved, or irregular surfaces. The cutting action occurs when a workpiece is fed against one or more rotating tools called milling cutters, or mills. This operation is exactly opposite to that of a lathe, which removes metal by feeding a tool into a rotating workpiece.
 The general-purpose milling machine performs a variety of operations. Column-and-knee, manufacturing, and planer types are examples.
Special milling machines include the planetary type for hole and facing operations and double-end machines for milling the ends of rods and shafts.
 Principal Parts of a Milling Machine
 The column-and-knee type is a milling machine that has many industrial uses. It consists of a base that supports a column. The front of the column is the face. A part known as the knee projects from the column and moves up and down on the face. It supports a table that travels horizontally on tracks called ways. An overarm supports the toolhead. The milling cutter is mounted on an electrically driven spindle or on a part called an arbor held by the spindle.
In general a milling cutter has a cylindrical body that rotates on its own axis. It is designed with equally spaced cutting teeth that engage the workpiece in regular sequence. Plain-milling cutters have teeth on the perimeter only. They produce flat surfaces parallel to the cutting axis. An end-milling cutter has teeth on the end as well as on the perimeter. Face-milling cutters have teeth on one or both sides as well as on the end of the cutter. An angle-milling cutter is shaped like the frustum of a cone. It is used to make angular cuts such as grooves and V notches.
  The operation of plain, or surface, milling consists of machining a flat, horizontal surface with a plain-milling cutter. If the cut is much wider than the diameter of the cutter, the process is called slabbing. End milling is used for cutting slots, facing narrow surfaces, and making accurate holes. Face, or side, milling is the process of cutting vertical surfaces at right angles to the cutter's axis.

Milling operations are very slow and costy. However, the surface finish and dimensional accuracy is increased. In our project, milling was the least used manufacturing operation.
We used the milling machine to improve the surface finish and dimensional accuracy of a 9cm hole in a 0.9 mm thick plate. The hole was origoinally drilled with the lathe machine.



4- Hole Making Operations:

Throughout our work on the project, several hole making oprations were performed. We had to make holes in different plates in order to implement the designs.
The hole making operations we performed ranged from 0.8 mm to 1.2 mm thick plates.
The holes ranged from 4mm to 22mm diameter holes.
The process of making a finished hole is one that may require several steps. The first operation produced a rough hole. It was then necessary to enlarge the hole in order to obtain greater precision. Finally, a special finish, such as a thread, was cut on the inside of the hole.
 
Drilling Operations:
 The simple cutting of holes in or through metal is called drilling. It is done by a rotating tool called a drill that has multiple cutting edges at its point. The power-driven machine that holds, rotates, and feeds the drill is called a drill press.
   There are two standard drill presses in the AUC workshop. The sensitive type is used for light work. The bench type is a small machine with a column for mounting on a bench. It is used on a short workpiece of medium size. The toolhead can be swung around the support column as well as toward or away from it. These two motions produce radial action.

To manufacture our machine, a 9cm diameter hole had to be made in an 0.9mm thick steel plate. For this process, the usual drilling machine was unappropriate. We had to use the center lathe with a proper tool to manufacture the hole. However, the surface finish and dimensional accuracy needed improvement, so the workpiece was taken to the milling machine (see previous section, "milling")
  



5- Sawing

In many manufacturing operations power sawing is the best method for cutting metal. It is an economical way in which to saw bar stock to the approximate size needed for further work. Contour cutting can be done accurately and quickly by power sawing.
To manufacture our machine, we went through several sawing processes. Various cylinders and angles bars were cut to appropriate dimensions for further machining. We also sawed 10mm plates.
We encoutered two different sawing machines during our work. One was at the AUC workshop and the other one during the steel buying process (in a Sabteyya workshop).
Circular Saw
 The circular saw consists of a large, power-driven blade with inserted teeth. Its action is similar in many ways to that of a milling cutter. The circular saw is used mainly to cut bar stock to a desired length. It runs at slow speeds but has great power.
 Reciprocating Saw
 A reciprocating, or power hack, saw cuts stock slowly but cheaply. A cranking device raises and lowers the blade after each stroke. This tool is provided with an automatic cutoff that stops the machine when the stock is severed. Several bars can be cut at one time.

Manual sawing was also used to manufacture our machine. Small parts that could not fit on the electric sawing machine had to be sawed manually with a handsaw. Manual sawing helps control the accuracy of the cutting and intricate cuts can be performed.



6- Shearing operations
 
   A shear consists basically of a power unit that drives and controls a set of sharp blades. The metal to be cut is located between the two blades. One blade presses the metal upward against a resisting edge. The cut is made when the second blade moves down and cuts through the rigidly held sheet of metal.
 
Shears that could cut relatively thick plates (up to 1.2mm) were not available in the AUC workshop. During our work on the project, we had to use external shears. We went to largers workshops in the Sabteyya region in order to cut our plates in the desired shapes and dimensions in order to implement the design.

We also sheared parts of the angle beams to manufacture the stand. Triangular sections were cut of the ends of the angle beams to weld them together.



7- Grinding
 
The process of grinding consists of removing metal by feeding a workpiece against a rotating abrasive wheel. It is a true cutting operation. The action of a grinding wheel is similar to that of a multitooth milling cutter. A wheel 2 inches (5 centimeters) wide and traveling at 5,000 feet (1,524 meters) a minute presents about 300,000,000 cutting edges to the workpiece in one minute. It produces dust-fine chips. Because of their high temperature the chips appear as sparks.
 
Grinding was originally a finishing operation. It has now developed to a point where it competes with other machine-tool operations in the rough machining of surfaces. Grinding is now considered the most accurate of all machine processes. A grinder can produce the full range of finishes from the roughest (snagging) to the finest possible surface (modern internal grinding).
 
 The surface of a grinding wheel consists of sharp abrasive particles embedded in a suitable binder. The natural abrasives in common use are emery, sandstone, and quartz. Among the most effective manufactured abrasives are aluminum oxide and silicon carbide. Binders commonly used for grinding wheels are clay, sodium silicate, and resins.
Lapping is a grinding operation that is used to produce highly polished surfaces. Lapping is done with a surface mixture of abrasive particles and oil. It corrects minor surface imperfections to provide a close fit for mating parts.

Grinding was used extensively during our labwork. The dimensional accuracy and surface finish was improved my griding the edges of the plates. We used both manual and electric griding. Sharp edges as well as excess metal were removed by grinding.



8- Metal joining operations:

-Welding:
Welding, brazing, or soldering processes join two metallic surfaces by creating bonds between their constituent atoms. In welding we frequently use a filler material between the surfaces, and all three apply heat to melt either the local surfaces or the filler. During the process the surfaces are protected from the oxygen in air, which tends to combine with the metallic atoms thus decreasing the strength of the bond, by the use of a chemical flux, which cleans the surfaces, or by immersion in a protective atmosphere. Welding provides the greatest junction strength. In a proper weld the joint will be as strong as the parent material sometimes stronger.
  In welding, two metal sections are normally joined by bringing their surfaces into contact under high temperature, high pressure, or both, depending on the application. Although most welds are made between similar metals, different compatible metals may also be welded.

The principle method we used throughout our labwork is gas welding, using Oxyacetylene.
Gas welding uses a flame to melt the local material and the filler. The filler is usually a metal rod that is allowed to flow between the parts to be joined. The flame also provides a protective atmosphere that discourages accumulation of oxides. Gas welding is used primarily for repairs in areas where portable equipment is an advantage.
   Arc welding uses the intense heat of an electrical arc generated when a high current flows between the base metal and an electrode. Temperatures of up to 3,870  C are applied to melt the local base and filler materials. Shielded metal-arc welding uses electrodes made of a coated metal filler wire. The electrical arc breaks down the coating to provide a protective atmosphere that both stabilizes the arc and acts as a flux. In gas-tungsten arc welding a non-melting tungsten electrode holds the arc, and an inert gas, such as nitrogen or argon, provides the protective atmosphere. Meanwhile a filler wire is fed through the electrode holder. Since no separate flux is used, the quality of the resulting weld depends heavily on the cleanliness of the initial surfaces.
 
We used welding operations to manufacture our stand. 4 angle bars and a plate were welded to produce the stand, in addition to 4 other angle bars to form the legs. In this operation, oxy-acetylene welding was used.
We also used welding to join several plates to the bender frame. The piston "box" (the three plates that surround the piston) were fixed on the table by welding.
To fix the spring on the die, we welded a small bent steel part on the die as well as on the piston box.

During our labwork, we also performed temporary welding processes. The steel frame consists of two plates. In order for the two plates to be identical, we had to machine them together. We performed a temporary welding operation to join them and be able to machine them in a single step.

The oxy-acetylene process was also used to cut steel plates. In this process, the torch provides a higher oxygen to fuel ratio. The oxy-acetylene cutting process was used to cut our steel plates to the desired dimensions. It was also used to cut the arc-shaped part from the disc.

The oxy-acetylene cutting process has a very poor surface finish and dimensional accuracy. The cut part must undergo at least a further machining operation, such as shaping or milling.


RECOMMENDATIONS:

  1 -  The size of the plates used in the frames is heavy. Though it increases the machine stability, it should be considered to decrease the plates thickness when implementing similar model. This may result in decrease in price.



ECONOMICAL EVALUATION:

The price of a similar machine that was purchased by AUC maintenance (Plumbers) was around 2800 L.E. The produced model cost around 400 L.E., though some costs were spent on needless parts because of the forced changes in design. In general, it was an economically feasible machine when talking about a very small production. On mass production limit, the machine will definitely have less cost.
Another machine, but with only 12 tons jack capacity, costs 600 L.E.



REFERENCES:

Colliers Encyclopaedia. Manufacturing Processes. 1997.

Kalpakjian, Serope. Manufacturing Processes for Engineering Materials. Addision Wesley,
USA. 3rd Ed. 1997.

http://www.pvtnet.cz/www/jesankovo/j05.htm#XOTR29

http://www.ensley-tools.com/8.html

http://www.pro-tools.com/Page11.html

http://www.phi-tulip.com/benders/manbend.html



ACKNOWLEDGEMENTS:

We would like to express our deep appreciation to Dr. Hani A. Arafa (coz he is simply the best!) for his support for us during the project implementation in the work shop. He contributed to the design of the machine, and perhaps for all the 339 class, especially when we consulted him concerning the springs and some other challenges that we faced during the project.


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