The shaped metals. II. Bending A large press

The Hull Construction workshop also called SWC is one of
the main workshop in Dockyard premises. The main task of the SWC is to
manufacture the hull of the ships.

Function of the hull construction workshop can be
classified as follows.

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1.    
Steel cutting
using CNC Plasma machine

2.    
Metal Forming

a.     
Bending

b.    
Pressing

c.     
Line Heating

3.    
Unit fabrication
and Pre fabrication

4.    
Lofting –
Templates were marked and cut in this section.

 

The
Hull workshop is there to manufacture and construct new hull parts for the new
ships build by Colombo Dockyard PLC. The materials were brought to the workshop
and fabricated into many units. The metal sheets were brought from KRY (Kelani
River Yard) with the Mill certificate.

Metal Forming

            The
plates which are cut into various shapes and sizes are then formed using three
methods. Those are rolling, bending and line heating.

     
I.         
Rolling

A large rolling machine with three rollers used for
this. Metal sheets were sent through these rollers and made some shaped metals.

   
II.         
Bending

A large press bending machine is used for this. The
metal plate is placed underneath this and pressed to bend plates at various
curvatures. The maximum weight force capacity is 500tons.

 

 

 
III.         
Line Heating

The metal plates are subjected to heating
and cooled rapidly. This expands the metal and is subjected to bending with no
physical external forces. Bending using rolling and bending machine may not
fully accurate, at these circumstances this method can be used to get the most
accurate shape of metal referencing the template. This line heating can be used
as a correction method for distortion metals due to welding. Because of weld
there can be distortion and bending. So we can use line heating method for this
type of problems.

 

 Metal
Cutting

            In
the workshop there were used various kinds of plate with various thicknesses.
Therefore, to cut these metal plates there were used several kinds of machines
in the hull construction workshop.

·      
CNC Plasma cutting
machine

·      
CNC Gas cutting

·      
Gas cutter(Hand)

 

 

Mill Certificate

            Mill
certificate means the quality inspection of raw materials. The quality of raw
material is a very important factor in production. This is the most important
process in production system. The inspection is done by comparing the raw
material with the mill test certificate that sent by supplier to the company
and compare it with required standards.

Certificate number, Production number, Tensile
strength, Chemical compositions and size are some of the contents in the mill
certificate.

 

 

01.CNC Plasma Cutting Machine

            Plasma
cutting is a process that cuts through an electrically conductive material by
means of an accelerated jet of hot plasma. This machine able to cut steel,
stainless steel, aluminum, brass and copper. This machine is very useful for
industrial construction operations. Plasma cutting machine is widely used in ship
industry due to high speed and precision cuts with low cost.

A CNC plasma cutter is used to get desired shapes from
the metal sheet.

Plasma cutter nozzle was connected with 05 lines

1.    
Coolant inlet

2.    
Coolant outlet

3.    
Oxygen

4.    
Compressed air

5.    
Pilot arc

The machine has a safety mechanism; while in
operation, there are sensors to detect any obstacles. Then the machine will
automatically stop.

 

Normally 350-400A current is flown through the current
path.

This machine`s nozzle should be changed according to
the thickness of plates.

 

 Procedure

·      
Firstly, a large
of steel plate was kept on the bed of the plasma cutter.

·      
The machine was
turned on and the drawing document was inserted to the computer of the machine.

·      
The numerical
codes were input to the machine by converting from the CAD drawing.

·      
Plate marking
process was started using LP gas to the different nozzle.

·      
Then the accurate
height, start point, endpoint for nozzle was set.

·      
Then the process
initiated by the computer and the nozzle moved to the start point of the plate,
the nozzle exerts pressurized oxygen and air at high pressure.

·      
After the
operation was done the machine was turned off.

·      
Finally, a hand
held cutter was used to remove the final metal pieces because of doesn`t fully
separate the required pieces.

 

 

 

 

 

 

 

 

 

 

 02.Line
Heating (Bending a Bulb bar)

            Plates
bending using rolling machine and bending machine may not fully accurate and
this method can be used to obtain the most accurate shape of metal referencing
a template.

A bulb bar is a straight steel bar having a unique
cross section.

h-Height

t-thickness

b-width

r-corner
radius

 

 

 

 

The bended bulb bars are used in large tanks for the
purpose in strengthening mechanism. Bulb bars were bent towards the less
thickness side.

·      
The bulb bar was taken
and fixed to the iron bed having holes with the use of two L shaped rods.

·      
Then the lesser
thickness side was to be bent according to the plywood template.

·      
Bending was done
by high temperature heating with the combination of two gases, oxygen and LP. Then
rapidly cooled by the use of flowing water.

·      
Heating was taken
place from one end and a bell shaped portions were heated using a heating
torch.

·      
The bulb bar gets
contract as due to cooling with compresses and bends.

·      
A plywood template was used to check the accurate
shape and this was done for the entire length of the bulb bar.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

03.Bending
of a metal sheet using a Hydraulic Press Machine

Hydraulic
press machine is used to bent a straight sheet of metal to obtain a circular
portion, mostly a less curvature products. The maximum force which can be
applied is 500 tons.

Normally
in the workshop the force would be used 20-50 tons.

·       The
desired plate was brought after being cut to the necessary shape.

·       Then
the plywood template was kept on the sheet surface and two points either two
ends were marked.

·       After
that parallel lines were drawn 50mm apart using a marked strip and a chalk.

 

 

 

 

 

 

·       One
end of the sheet was hold by a crane and the other end was insert into the
press machine.

·       Then
the operator aligned the pointed edge of the machine to the marked parallel
lines.

·       After
that this operation was carried out for the next 3-4 lines and checked the
curvature using the plywood template.

 

 

 

 

 

 

 

 

 

Problems
and solutions

The
curvature of the bended sheet was too high and to overcome this, a metal block
was inserted on top of the bent area. Then applied the press on the metal
block.

 

 

 04.Rolling a
metal sheet to gain a cylindrical shape using a rolling machine

            In this operation firstly this was done
by using a hydraulic press bender and then introduced to a roller machine.
Because the sheet should be pre pinched before rolling operation. Pre pinched
is done before, the plate length should be greater than to the sum of the
radius of the two rollers. If it is not the plate will slip away. That is why
the plate bend curved in press bender.

 

Plate
Rolling Diagram

 

·       The
metal plate was kept on the floor and the lines were marked parallel on either
side using a chalk.

·       Then
the sheet was introduced to the press bender to bent referencing to the plywood
template.

·       This
operation was done for small portions to gain the desired radius in the edges
of the sheet.

·       When
bending using the press bender, the crane was used to lifted up the sheet.

·       After
that the edge bended sheet was brought to the rolling machine.

·       Then
the rolling operation was done by checking the curvature using the same plywood
template as used it before.

·       After
rolling is done according to the shape, check whether both ends of the plate
have been correctly aligned.

·       Check
whether the plate has perfectly bended by keeping the templates on the round
surfaces to make sure accuracy.

·       After
the cylindrical shape was gain, the final product was sent for welding the
seam.