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Learning the drill Diamond drilling has been proving its worth on the Millennium Dome, reports Mike Walter.

The precise nature of diamond drilling equipment means that it can be used for a whole host of intricate tasks which require pinpoint accuracy. Take the Millennium Dome for example. Precision Cutting was recently subcontracted by Watson Steel to drill into the inclined thrust blocks which secure the dome's 12 masts.

Each mast sits on a quadrapod structure which is anchored back to four thrust blocks. Each block required six, 35.5mm diameter bolt holes to accommodate resin anchors. The quadrapod's thrust blocks were set at an inclined angle of 40.66.

The circular holes of 650mm depth were diamond drilled perpendicular to the axis using a Nimbus 750 rig. This was secured to each concrete block with a central fixing bolt and was operated using a fine feed attachment, a gearing system giving the operator a very tight control adjustment.

A 110V motor drove the drilling rig with a spacer plate fitted to increase the distance between the rig and the motor, allowing the greater manoeuvrability required due to the inclined angle. The choice of drills had to accommodate the fact that the C50 grade concrete was still curing to full strength.

'A balance had to be struck between hard and soft grades of diamond drills as the concrete was still green, two weeks after being poured. Our aim was to drill the holes as perfectly as we could, as accuracy in this job was paramount,' says Precision Cutting's John Hornby.

Contractor Watson Steel played a vital role in ensuring that the holes for each thrust block were correctly marked out. A template assembled from eight varying lengths of steel was bolted to four end plates which covered the inclined thrust block plinths. Each of these plates - fabricated at the same time as the quadrapods - marked out the holes, forming a 750mm Pitch Circle Diameter on the block.

The temporary structure weighed in excess of 5t and was kept in place purely by its own weight. Lowered using three hydraulic jacks, the template covered each base area of 113m2 and was used for every one of the 12 bases in turn.

This article was prepared for NCE by Barrett, Byrd Associates.

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