Lafarge Cement - Dunbar Raw mill table repair
Faced with the task of machining the 5.2 metre diameter base table on a raw mill at Lafarge’s Dunbar works,Coventry firm Metalock Engineering UK designed a special machining rig to restore the main surface that hadsuffered wear. This is the only cement works in Scotland and £35 million has been invested in the factoryresulting in a rise in production to one million tonnes a year.
The revolving table supports a baseplate that forms one of the crushing surfaces. Others come from two pairsof steel tyres(rollers) weighing 75 tonnes each that run in mating grooves on the baseplate. When in operationthe tyres are subjected to 130 bar pressure to effectively crush raw limestone to a fine powder for the manufactureof cement clinker. The pressures and vibration are enormous and severe fretting corrosion had taken placebetween the table and the baseplate since the mill had been installed in 1985. The mill had not been refurbishedbefore and the fretting corrosion was causing fixing bolts to shear due to substantial undesirable movementbetween the baseplate and the table.
Metalock was recommended by the mill’s chief mechanical engineer who had previous experience of workingwith the company when he was at the Llanwern steelworks in South Wales.
Metalock’s machine design comprised an outer support track, feed rack and feed assembly all of which waswelded to the circumference of the 5.2m diameter base table after the baseplate segments had been removed.The assembly was aligned by laser and revolved around a main bearing hub located in a 640mm diameter holein the centre of the table. A 3 metre long cross slide with its own feed and outer support mechanism, designedto rotate at 1 rpm, was attached to the hub. On the cross slide was an hydraulically driven milling head whichenabled Metalock to remove material more quickly and provide a better finish than if a single point tool hadbeen used.
To replace the material that had been machined from the surface of the base table, Metalock produced twelvenew 4mm thick stainless steel segments. These were predrilled to allow spotting through and subsequentdrilling and tapping of 21 holes for M6 countersunk screws to fix each segment.
The purpose of the stainless steel segments was so that, if necessary, they could be replaced in future ratherthan having to machine further material from the base table. Stainless steel was selected to better resist anyfuture fretting corrosion. The fixing bolt configuration for the crusher track plate was also changed.
The baseplate was changed just over 12 months after the Metalock operation, and no marks on the main tableindicated that the modification was successful.
Metalock’s machine had an outer supporttrack, feed rack and feed assembly, all ofwhich was welded to the periphery of the5.2 metre diameter base table. It revolvedaround a hub at the table’s centre.