Page 138 - Ship Construction.DJ Eyres 6Ed

Ch13-H8070.fm Page 127 Wednesday, October 18, 2006 6:56 AM
Plate and Section Preparation and Machining 127
the draftsman to perform parts programming and nesting of plate parts and
the loftsman to add the cutting information and automatically generate the
NC tape. The tape can be read into the director which produces command
signals to the servo-mechanism of the profiling machine. The overall
arrangement is shown in Figure 13.1.
These machines may be provided with accessories for marking plates.
(b) A full size template or drawing may be used to control a cutting
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machine, and can be very useful where a single small item is to be cut in large
numbers. The size of the item is obviously restricted, and the location from
which the item is cut in a large plate is selected by the operator. Where a
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template is used a mechanical follower may be employed, and where a
drawing is used an electronic scanning device may follow the outline. Profilers
of this type have a limited application in shipbuilding, but can be efficient
for cutting batches of say, identical beam knees or stiffener brackets.

PLANING MACHINES Profiling machines are essentially for use where
a plate requires extensive shaping with intricate cuts being made. Many of
the plates in the ship’s hull, particularly those in straightforward plate panels,
decks, tank tops, bulkheads, and side shell, will only require trimming and
edge preparation, and perhaps some shallow curves may need to be cut in
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shell plates. This work may be carried out on a planing machine, usually a
flame or plasma-arc planer.
A flame planer consists basically of three beams carrying burning heads
and running on two tracks, one either side of the plate working area. One
beam carries two burning heads for trimming the plate sides during travel,
whilst the other two beams have a single burning head traversing the beam
in order to trim the plate ends. Each burning head may be fitted with triple
nozzles and is used to give the required edge preparation. Plate, beam, and
burning head positioning is manual, but cutting conditions are maintained
once set.
Mechanical planing machines were in existence prior to the flame planer,
and have an advantage where they are able to cut materials other than
steels. Higher cutting speeds than that obtained with flame cutting may
be obtained for vertical edges with a rotary shearing wheel on a carriage,
where the plate is held by hydraulic clamping. For edge preparations, older
machines with a conventional planing tool require a large number of passes
and are much slower than flame planers. There are however merits in
mechanically machined edge preparations where the superior finish is
advantageous for critical welds. The use of plasma-arc planers which also
provide a superior and faster cut tend to make the use of mechanical planers
obsolete.

DRILLING MACHINES Some plates or sections may need to have holes drilled
in them, for example bolted covers and portable plates. Drilling machines