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Steel Designers' Manual - 6th Edition (2003)
960 Fabrication
the workshops of those fabricators who specialize in plate girder work for bridges,
power stations and other heavy steelwork fabrications. For the more commonplace
range of smaller plates and sections, there is a range of equipment available which
is suitable for cutting to length or shaping operations. These machines feature a
range of shearing knives which can accept the differing section shapes. Shearing can
be adjusted so that angled cuts may be made across a section. This is particularly
useful for lacings of latticed structures. One version of the shear is a ‘notcher’ which
can cut shaped notches. Special dies are made to suit the notch dimensions, and it
is possible to obtain dies to cut the ends of hollow sections in preparation for
welding together.
Cold sawing
When, because of either specification or size, a section cannot be cut to length by
cropping or shearing, then it is normally sawn. All saws for structural applications
are mechanical and feature some degree of computer control. Sawing is normally
carried out after steel is shot blasted as the saw can be easily incorporated within
the conveyor systems associated with shot blast plants.
There are three forms of mechanical saw: circular, band and hack. The circular
saw has the blade rotating in a vertical plane which can cut either downwards or
upwards, though the former is the more common. The blade is a large milling wheel
approximately 5 mm thick. The diameter of the saw blade determines its capacity
in terms of the maximum size of section which can be cut. Normally, steelwork
contractors will have saws capable of dealing with the largest sections produced by
Corus. For increased productivity, sections may be nested or stacked together
and cut simultaneously. Some circular saws allow the blade to move transversely
across the workpiece, which is useful for wide plates. Most circular saws can make
raking cuts across a section of any angle, though some saws are restricted to single
side movement. The preferred axis of cut is across the Y–Y axis of the section. In
the case of beams, channels and columns, this is with the web horizontal and the
flange toes upwards. Depending on the control exercised, a circular saw can make
a cut within the flatness and squareness tolerances necessary for end bearing of
members.
Band saws have less capacity in terms of section size which can be cut. The saw
blade is a continuous metal band edged with cutting teeth which is driven by an
electric motor. The speed of cut is adjustable to suit the workpiece. Band saws can
make mitre cuts and can cut through stacked sections. Cutting accuracy is dependent
on machine set-up but produces results similar to circular saws.
Hack saws are as the name implies mechanically driven reciprocating saws. They
have normal format blades carried in a heavy duty hack saw frame. Hack saws have
more limited cutting capacity than band saws and have the capability to produce
mitre cuts.
All saws feature computer-controlled positioning carriages for accurate length
set-up; most also have computer sensing for angle cuts.