The Hull Construction workshop also called SWC is one ofthe main workshop in Dockyard premises.
The main task of the SWC is tomanufacture the hull of the ships.Function of the hull construction workshop can beclassified as follows.1. Steel cuttingusing CNC Plasma machine2.
Metal Forminga. Bendingb. Pressingc. Line Heating3. Unit fabricationand Pre fabrication4. Lofting –Templates were marked and cut in this section. TheHull workshop is there to manufacture and construct new hull parts for the newships build by Colombo Dockyard PLC. The materials were brought to the workshopand fabricated into many units.
The metal sheets were brought from KRY (KelaniRiver Yard) with the Mill certificate.Metal Forming Theplates which are cut into various shapes and sizes are then formed using threemethods. Those are rolling, bending and line heating. I. RollingA large rolling machine with three rollers used forthis. Metal sheets were sent through these rollers and made some shaped metals. II. BendingA large press bending machine is used for this.
Themetal plate is placed underneath this and pressed to bend plates at variouscurvatures. The maximum weight force capacity is 500tons. III. Line HeatingThe metal plates are subjected to heatingand cooled rapidly. This expands the metal and is subjected to bending with nophysical external forces.
Bending using rolling and bending machine may notfully accurate, at these circumstances this method can be used to get the mostaccurate shape of metal referencing the template. This line heating can be usedas a correction method for distortion metals due to welding. Because of weldthere can be distortion and bending. So we can use line heating method for thistype of problems. MetalCutting Inthe workshop there were used various kinds of plate with various thicknesses.Therefore, to cut these metal plates there were used several kinds of machinesin the hull construction workshop.· CNC Plasma cuttingmachine· CNC Gas cutting· Gas cutter(Hand) Mill Certificate Millcertificate means the quality inspection of raw materials. The quality of rawmaterial is a very important factor in production.
This is the most importantprocess in production system. The inspection is done by comparing the rawmaterial with the mill test certificate that sent by supplier to the companyand compare it with required standards.Certificate number, Production number, Tensilestrength, Chemical compositions and size are some of the contents in the millcertificate.
01.CNC Plasma Cutting Machine Plasmacutting is a process that cuts through an electrically conductive material bymeans of an accelerated jet of hot plasma. This machine able to cut steel,stainless steel, aluminum, brass and copper. This machine is very useful forindustrial construction operations.
Plasma cutting machine is widely used in shipindustry due to high speed and precision cuts with low cost.A CNC plasma cutter is used to get desired shapes fromthe metal sheet. Plasma cutter nozzle was connected with 05 lines1.
Coolant inlet2. Coolant outlet3. Oxygen4. Compressed air5.
Pilot arcThe machine has a safety mechanism; while inoperation, there are sensors to detect any obstacles. Then the machine willautomatically stop. Normally 350-400A current is flown through the currentpath.This machine`s nozzle should be changed according tothe thickness of plates. Procedure· Firstly, a largeof steel plate was kept on the bed of the plasma cutter.· The machine wasturned on and the drawing document was inserted to the computer of the machine.· The numericalcodes were input to the machine by converting from the CAD drawing.
· Plate markingprocess was started using LP gas to the different nozzle.· Then the accurateheight, start point, endpoint for nozzle was set.· Then the processinitiated 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 theoperation was done the machine was turned off.· Finally, a handheld cutter was used to remove the final metal pieces because of doesn`t fullyseparate the required pieces.
02.LineHeating (Bending a Bulb bar) Platesbending using rolling machine and bending machine may not fully accurate andthis method can be used to obtain the most accurate shape of metal referencinga template.A bulb bar is a straight steel bar having a uniquecross section.h-Heightt-thicknessb-widthr-cornerradius The bended bulb bars are used in large tanks for thepurpose in strengthening mechanism.
Bulb bars were bent towards the lessthickness side.· The bulb bar was takenand fixed to the iron bed having holes with the use of two L shaped rods.· Then the lesserthickness side was to be bent according to the plywood template.· Bending was doneby high temperature heating with the combination of two gases, oxygen and LP. Thenrapidly cooled by the use of flowing water.· Heating was takenplace from one end and a bell shaped portions were heated using a heatingtorch.· The bulb bar getscontract as due to cooling with compresses and bends.
· A plywood template was used to check the accurateshape and this was done for the entire length of the bulb bar. 03.Bendingof a metal sheet using a Hydraulic Press MachineHydraulicpress machine is used to bent a straight sheet of metal to obtain a circularportion, mostly a less curvature products. The maximum force which can beapplied is 500 tons.Normallyin the workshop the force would be used 20-50 tons.· Thedesired plate was brought after being cut to the necessary shape.
· Thenthe plywood template was kept on the sheet surface and two points either twoends were marked.· Afterthat parallel lines were drawn 50mm apart using a marked strip and a chalk. · Oneend of the sheet was hold by a crane and the other end was insert into thepress machine.· Thenthe operator aligned the pointed edge of the machine to the marked parallellines.
· Afterthat this operation was carried out for the next 3-4 lines and checked thecurvature using the plywood template. Problemsand solutionsThecurvature of the bended sheet was too high and to overcome this, a metal blockwas inserted on top of the bent area. Then applied the press on the metalblock. 04.Rolling ametal sheet to gain a cylindrical shape using a rolling machine In this operation firstly this was doneby using a hydraulic press bender and then introduced to a roller machine.Because the sheet should be pre pinched before rolling operation.
Pre pinchedis done before, the plate length should be greater than to the sum of theradius of the two rollers. If it is not the plate will slip away. That is whythe plate bend curved in press bender. PlateRolling Diagram · Themetal plate was kept on the floor and the lines were marked parallel on eitherside using a chalk.· Thenthe sheet was introduced to the press bender to bent referencing to the plywoodtemplate.· Thisoperation was done for small portions to gain the desired radius in the edgesof the sheet.
· Whenbending using the press bender, the crane was used to lifted up the sheet.· Afterthat the edge bended sheet was brought to the rolling machine.· Thenthe rolling operation was done by checking the curvature using the same plywoodtemplate as used it before.· Afterrolling is done according to the shape, check whether both ends of the platehave been correctly aligned.· Checkwhether the plate has perfectly bended by keeping the templates on the roundsurfaces to make sure accuracy.
· Afterthe cylindrical shape was gain, the final product was sent for welding theseam.