Page 142 - Ship Construction.DJ Eyres 6Ed

Ch13-H8070.fm Page 131 Wednesday, October 18, 2006 6:56 AM
Plate and Section Preparation and Machining 131
Shorter pyramid full circle rolls are also used in shipyards, these being
very useful for rolling plates to a full circle. This may be done to obtain
large mast and derrick post sections for example, or bow thruster tunnel.
Arrangements are made for removing the rolled full circle plate by releas-
ing the top roller end bearing. Vertical rolls are also available and may be
used to roll plates full circle, but can be much more useful for rolling heavy
flats used as facing bars on transverses of large tankers, etc.
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HEAT LINE BENDING The ‘heat-line’ bending procedure is a widely
used technique to obtain curvature in steel plates for shipbuilding purposes.
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It is, however, a process that until recently relied on highly skilled personnel
and did not guarantee constant accuracy of shapes formed.
Heat is applied in a line to the surface of a plate by a flame torch, and then
immediate cooling is obtained by air or water. The narrow heated line of
material is prevented from expanding in the direction of the plate surface
by the large mass of cold plate, and therefore expands outwards perpendicular
to the plate surface. On cooling contraction will take place in the direction
of the plate surface, causing the plate to become concave on the side to which
heat was applied (see Figure 13.3). An experienced operator is able to make
a pattern of such heat lines on a plate producing controlled distortion to
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obtain a required shape. Heat line bending can be more time consuming than
using rolls or presses but it has an advantage in that the plate holds its
form more accurately when stiffening and other members are added later in
the fabrication process. This is an important consideration since shape inaccu-
racy can be critical at the erection stage in terms of lost production time.
In recent years fully automated heat line bending systems have been
developed and installed in shipyards. These numerically controlled heat-line
bending machines permit highly accurate, reproducible thermal forming of
any steel plate using the data originating from the shipyards CAD system.
More than double the productivity of the traditional manual process is
achieved.

Frame Bending

The traditional system of bending side frames may still be in use for repair
work, and is described as follows. A ‘set-bar’ which is a flat bar of soft iron is
bent to the scrieve line of the frame on the scrieve board and then taken to
the frame bending slabs. On these solid cast-iron slabs pierced with holes the
line of the frame is marked, and modified to agree with the line of the toe
of the frame. As the heated frame on cooling will tend to bend, the set-bar
is sprung to allow for this change in curvature before it is fixed down on the
bending slabs by means of ‘dogs’ and pins inserted in the slab holes. Whilst
the set-bar is being fixed the frame section is heated in a long oil-fired