Page 143 - Ship Construction.DJ Eyres 6Ed

Ch13-H8070.fm Page 132 Wednesday, October 18, 2006 6:56 AM
132 Ship Construction
furnace adjacent to the bending slabs. When at the right temperature it is
pulled out onto the slabs and fixed with dogs and pins against the set-bar, as
quickly as possible. Tools are available for forcing the frame round against
the set-bar, including a portable hydraulic ram, and the toe of the web may
require constant hammering to avoid buckling under compression.
As the frames fitted have their webs perpendicular to the ship’s centre
line, all except those immediately amidships will require bevelling. A bevel-
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ling machine is available which is placed in front of the furnace door, and as
the frame is removed it passes between its rollers which are controlled so
that the flange is bent at every point to an angle indicated by a bevel board
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
prepared by the mould loft.
Once bent the bar is put aside to cool, but is fastened down to prevent its
warping in the vertical direction. When cold it is checked against the frame
line drawn on the slabs. Meanwhile the set-bar is turned over and used to
bend the corresponding frame for the opposite side of the ship.

COLD FRAME BENDING It is now almost universal practice to cold bend
ship frames using commercially available machines for this purpose. The
frames are progressively bent by application of a horizontal ram whilst the
frame is held by gripping levers (Figure 13.4). Any type of rolled section can
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be bent in some machines with a limitation on the size of section. Obtaining
the correct frame curvature can be achieved by the ‘inverse curve’ method
or numerical control. With the ‘inverse curve’ method the inverse curve
information can be determined for each frame by the loftsman using a
CAD/CAM system. The inverse curve is drawn in chalk on the straight
frame and the frame bent until the chalk line becomes straight on the
curved frame (see Figure 13.4). A hydraulic cold frame bending machine
can be controlled by numerical control tapes prepared in a similar manner
to the numerically controlled flame profilers, the frame line being initially
defined from the computer stored faired hull.

SECTION PROFILERS Plate profilers produce very accurate plate com-
ponents for assembly into ship units and it is important that the supporting
stiffening members of the structure are produced with the same accuracy.
Section profilers accurately cut, scallop and bevel, as necessary, the profile
edge to be welded to the plate. Cutting is by oxy-fuel gas or plasma and in
modern systems is undertaken by a compact robot operating in a viewable
enclosed work station with integral fume extraction. Prior to cutting the
stiffening member can be moved through a marking cell where part identifi-
cation and where required, bending lines for inverse curve frame bending
(see Figure 13.4) can be marked.

ROBOTICS Robots have in recent years been provided with improved
control features which have made them more adaptable to the workshop