automated fabrication

STEP 1: CADCAM Modelling

Prior to fabrication commencing, the structure or component to be fabricated is modelled in 3-D using a bespoke CADCAM computer software package.

The full scale model is then checked before the computer software prepares the CNC programmes that will drive each machine in the fabrication process.

Consequently, fabrication drawings are not used in the factory.

STEP 2: Pre Fabrication Butt Welding

An under cover automated facility for balanced welding is available to create component plates of up to 31m in length.

This avoids the higher expense of post fabrication section butt welding in the majority of cases.

All butt welds are ground flush and ultrasonically tested if required by the specification, as part of Fairfield-Mabey's Quality Control programme.

STEP 3: Plate Preparation

Mill scale and surface corrosion are removed by shot blasting before plates arrive in the plate preparation bay where all components of the structural sections are cut to shape and size by CNC plasma and oxy-propane plate profiling machines.

The fully integrated CADCAM system allows bolt holes to be pre-drilled in the plate form with due allowance for cutting/welding shrinkage. Excellent standards of fit are consistently attained via this method. The CNC plate preparation machines also powder mark onto the flanges and other plates the position of any fittings to be welded to them. This eliminates the need for hand measurement in the fabrication process and the possibility of human error.

The system allows the accurate fabrication of girders with curvature in both plan and elevation, as necessary, by cutting the relevant component to the final shape prior to assembly, thus avoiding rolling costs.

Following profiling, plate edges are ground by machine in preparation for protective treatment.

STEP 4: Section Assembly

T and I sections up to 4m deep and 1.5m wide are assembled using the semi-automatic girder assembly machine (T&I machine) which carries out the web to flange welding on both sides simultaneously using Submerged Arc Welding (SAW). This cuts the assembly time of a typical girder down from 18 hours (by hand) to 3 hours (by machine). The T&I machine can cope with vertical and plan curvature in the girder.

Box sections are assembled using a mixture of semi-automatic and traditional techniques. Welding to these sections is completed using semi-automatic SAW.

STEP 5: Section Stiffening

T and I sections are stiffened post assembly. Box section components are stiffened pre-assembly.

Stiffeners are positioned against the marks that are put on during the plate preparation stage and tack welded into position. Thus the possible error associated with hand measurement is avoided.

Stiffener welding is completed by robot or semi-automatic welding using a Metal Active Gas (MAG) process.

STEP 6: Trial Erection

Following fabrication and prior to application of protective treatment, the components enter the trial erection yard.

The 250m x 30m facility, served by overhead cranage with 100 tonnes lifting capacity, is used to trial erect complete or part structures, where required by the specification or client, prior to delivery to site, to provide complete peace of mind.

It is recommended that high risk structures, such as those crossing railway lines or motorways, where installation is required during very limited periods, are always fully trial erected prior to delivery.

STEP 7: Protective Treatment

Shear studs are fixed via the drawn arc process to the components as required.

The components are then grit blasted (to SA3), aluminium metal sprayed (if required) and painted to provide the required protective treatment system.

Highways Agency, Network Rail and other client specification requirements are met in full.

HSFG connections are masked for site
painting after assembly and final tightening of the bolts.

These works are carried out using automatic and semi-automatic machinery wherever possible, in a heated facility served with fume and dust extraction equipment.

STEP 8: Post Fabrication Butt Welding

Fairfield-Mabey always carry out as much welding work as possible in the factory controlled environment, and as little as possible on site.

The maximum length of girder that can be transported by road in the UK is generally 45m - 50m, depending on section size and weight. Girders of up to 31m in length are post fabrication butt welded end to end to form indivisible lengths of up to this maximum.