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Steel Designers' Manual - 6th Edition (2003)
958 Fabrication
thousand holes per hour. Obviously, these machines can be obtained with different
capacities so that if the work of the fabricator is at the light end of the section range,
it is not necessary to purchase machines with large capacity.
For larger sections, where drilling and cutting are required, there are two basic
machines available. One is a drilling and plate-cutting system which is used for heavy
plates; the other machine is of a larger capacity and is used for drilling rolled sections
of all sizes. Modern drills normally have three-axis numeric control and aircooled
drills. These machines have sensors which can detect the position of the web
to ensure that hole patterns are symmetrical about the web centreline.
Where components only need a small number of holes, mobile drills are used.
These are normally magnetic limpet drills hand operated by the fabricator. The
holes are formed with rotary-broach drill bits, which have a central guide drill
surrounded by a cylindrical cutter.
The fabricator will always carry out hole-forming operations prior to any further
fabrication as the presence of any stiffeners or cleats on a bar would severely disrupt
the input of NC machines.
32.5 Cutting
The fabrication process of cutting has become highly automated. Universa1 beams,
columns and the larger angles, tees and channels are normally cut to length by saws.
Hollow sections are also treated in this way. Small sections of joist, channel, angle
and flat are cut to length by shearing, either as a separate operation or as part of a
punching and cropping operation which is computer controlled. Large plate sections
may be sheared but this involves specialist plant and equipment which is not available
to all fabricators.
32.5.1 Cutting and shaping techniques
Flame cutting or burning
This technique produces a cut by the use of a cutting torch. The process may be
either manual or machine controlled. Manual cutting produces a rough edge profile,
which can be very jagged and may need further treatment to improve its appearance.
Edges of plates cut manually require greater edge distances to holes for this
reason. Notches or holes with square corners should not be hand cut unless the
corner is first radiused by a drilled hole, Fig. 32.5.
When flame cutting is carried out under machine control, the cut edge is smooth
and therefore the restrictions on edge distances to holes are relaxed. Notches which
are machine cut need radiusing either by a predrilled hole or by the control of the
cutting machine. Unradiused notches are to be avoided due to their stress-raising