Steel Fabrication

Metal fabrication :- Metal fabrication is the fabrication of metal by cutting, bending, and assembling (by  welding, binding with adhesives,riveting, threaded fasteners,etc.) processes.

    Structural steel  and sheet metal are the usual starting materials for fabrication, along with the welding wire, flux, and fasteners that will join the cut pieces.

1.FABRICATION PROCEDURES

1. Workshop Layout

Fabricators range from small general firms to large specialized producers with different facilities at their disposal. In either case the fabrication must always be organized in such a way that the material will pass through a one-way system from receipt to final dispatch. A flow cart, as indicated in the main areas of activity in a modern fabrication shop; the specific activities for a simple steel beam can also be organized as a production line.

Most fabrication shops are equipped with overhead travelling cranes, sometimes remotely controlled from the shop floor. Mechanized conveyor systems are common in the larger shops. They can greatly reduce handling costs.

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2. Material Handling and Preparation

Material is taken into temporary stock in such a way that it can be easily identified and moved. Some companies stack the material for easy access and move it by using cranes equipped with chains and hooks. Other companies use a high degree of automation in their material handling, using cranes on conveyors with magnetic lifting devices; Slide 4, for example, shows a travelling Goliath Magnet Crane with the capacity to lift both plates and sections. Computerised records hold details of member sizes, lengths, weights and steel quality, all related to an identification mark.

3. Templates and Marking

Steel may be marked directly by hand with scribe lines and hole centres; nowadays, however, in most shops pre-programmed automatic plant is in use. Traditionally, full-sized templates, made of timber or heavy cardboard, were used to mark the steel for cutting and for centre popping where holes were to be drilled.

4. Sawing Line and Rolled Sections

The rolled sections are in most cases sawn to length, the other options being mechanical cutting or flame burning. Three types of saws are available to the fabricator:

• Circular saw;
• Band saw;
• Motor operated hacksaw.

5.Drilling and the Beam Line System

The traditional method of drilling involves three operations:

• Marking the position of the holes to be drilled;
• Moving the member to the drill by crane, by conveyor, or by other means;
• The actual drilling of the hole, using for instance, a radial drilling machine (radius about 1,5 metre).

Like the sawing line, this system is controlled by computer programs; some machines are equipped with multiple drilling heads enabling them to drill several holes simultaneously in each axis.

New twist drills are currently available which are capable of higher speeds and greater efficiency as follows:

• Coolant fed drills, giving a threefold increase in drilling speed.
• Titanium nitride coated drills, enabling a six-fold speed increase.
• Carbide tipped drills with exceptionally high cutting speeds.

6.Punching

Punching holes in steelwork is much faster, and therefore less costly, than drilling; its use, however, is generally limited to predominantly statically loaded structures with limited thickness, or to secondary members, unless HSFG bolted connections are used or the holes are reamed out to a larger size. The maximum thickness where punching is applicable depends on the material grade and quality.

7.Pressing and Forming

For the modern fabricator the most important application of plate forming and pressing is to add to the available range of rolled sections. The trapezoidal shaped trough used to stiffen bridge decks, is a very good example. Other examples are the circular sections of larger than standard dimensions.

8.Methods of Welding

Three welding processes are most commonly used in modern fabrication shops:

• Manual Metal-Arc Welding for fittings and for some profile and positional welding.
• Metal Active Gas Welding (MAG) and Cored Wire Welding with and without gas.
• Submerged Arc Welding for fully automatic processes; particularly useful for heavy welding in the flat or horizontal-vertical position and for the long-run welds in plate and box girders.
• uses laser welding, etc..

9.Machine Operations

Most fabrication shops are equipped with facilities for edge planing, for end milling and for surface machining of plate.

Unacceptable levels of hardness at the edge of the plate, often caused by burning, can be removed by planing.

End planing of members is used to get a higher standard of squareness than can be achieved by sawing. Optical laserbeam methods are used to align the axis of the member to the cutting head.

Surface machining is only necessary for special bearing surfaces and sometimes for the slab base plates of columns.

10. Fabrication Tolerances

Modern fabrication shops have accurate dimensional control over fabricated sections and have no problems in cutting the rolled material to length. The main problem is coping with the deviations in the sections and plates received from the steelmills. Euronorm (CEN) and ISO standards give dimensional tolerances for rolled sections, plates and flats, hollow sections and angles respectively. The fabricator will use bending rolls to straighten the material and to "square" flanges of beam sections at critical connection points. As already mentioned, the control of distortion due to The details and the connections must be designed in such a way that the tolerances will be met within the limits of good workmanship.

11.Inspection and Quality Control

Quality Control should commence with the designer and continue through the preparation of drawings and material procurement; maintaining the quality during the entire production process will depend heavily on the fabrication details and on the material obtained.

The larger fabricators have their own Quality Control Department, which will create and maintain a QC-manual, describing the method of operation throughout the fabrication process. The Quality Control Department will liaise with the shop management to make sure that all workers have the skill required for the job on hand and that welders are qualified to undertake the prescribed welding procedures.

Regular checks are necessary to ensure that:

• All materials can be checked against specifications.
• Material is checked for laminations.
• Welding electrodes are identifiable.
• Welding electrodes are stored in the required conditions.
• Welding procedures are being followed.
• Welding is being inspected during the process.
• Correct procedures are in operation for tightening HSFG bolts.
• Identification marks are clear and visible.
• All equipment is maintained correctly.

 The End