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On-site machining service stacks up

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A rapid response was needed when a busted slew ring put an iron ore stacker out of service earlier this year.

Furmanite mobilised four specialist technicians to site to ensure the machining work was completed in-situ in just four days.

The challenging conditions (the distance between the top and bottom faces for surveying and machining was less than 400mm) and strict tolerances to an accuracy of within 0.10mm per linear metre meant sophisticated machining skills and technology was needed to complete the work and achieve the required results.

The company brought specialist low profile circular self-levelling machine (CSLM) technology on site that can machine circular structures up to 24m in diameter to high levels of accuracy, to workshop tolerances and with exceptional overall flatness.

The 3.9m upper and lower slew bearing mounting faces were visually inspected and surveyed using laser technology. This revealed the upper face was out of specification, with the total flatness envelope exceeding the maximum tolerance of 0.40mm over 180º.

Furmanite Australia national sales manager Jim Friel says the CSLM consists of a wheel-mounted milling box/cutter that rolls on the face being machined as it rotates round a central pivoting system.

A datum ring mounted around the central pivot is used as a reference and, with the use of electronic probes, the milling head is continuously adjusted during machining for high-level accuracy.

“The machine centre spider was installed and bucked in to plane of best fit so as to minimise the amount of metal to be removed from the upper face,” Friel says.

“The machine was then installed with the milling cutter fitted and the hydraulics and electronics plumbed and connected. The upper face was machined flat, with level checks carried out during the process using a box machine level accurate to 0.10mm per metre. Post-machining surveys showed the face to be well within the 0.40mm over 180º specification.”

Cross checks with the lower face, which should have corresponded with the upper face, revealed a discrepancy found on further investigation to have a fall of 0.75mm from inner to outer circumference.

The cutting head was turned over and positioned to machine the lower face. Post-machining surveys, using an independent dial test indicator and machine level checks, confirmed the face was well within specification.

A post-machining graph showed the final flatness to be less than 60% of the allowable 0.40mm total flatness over 180º.

“This is typical of the service we are able to offer,” Friel says.

The company’s achievement has effectively extended the life of the stacker by avoiding the premature bearing failure likely if a new bearing was installed on out-of-tolerance surfaces.

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