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Combined turning and milling at AGM Michell

The ability to cut seven set-ups to two saves specialist bearing manufacturer seven hours per unit

Posted on 25 Aug 2020 and read 663 times
Combined turning and milling at AGM MichellAGM Michell invented the tilting pad bearing; and the company he founded in 1920 — Michell Bearings — today manufactures a wide range of white metal- and PTFE-lined bearing products. Customers, which are to be found world-wide, typically make pumps, motors, turbines and generators for the commercial marine, naval and industrial markets.

The mainstay of production on the shopfloor at the company’s South Shields factory is a pair of ‘highly specified’ five-axis mill-turn machining centres built by the German company Hermle and supplied by Kingsbury, the sales and service agent for the UK, Ireland and Middle East.

Chris Kemp, the manufacturing engineering manager at Michell Bearings, said: “Until 2017, we produced all bearing parts that fell within a 600mm cube in-house and sub-contracted the rest.

“However, it was becoming increasingly difficult to find machinists that could produce good quality parts in low volumes and at competitive prices.

“We therefore decided to invest in equipment that could machine cylindrical components up to 1,200mm in diameter, along with prismatic parts of an equivalent size.”

Mr Kemp explained that a five-axis mill-turn machine was the preferred option, as this style of production centre offered a single-platform solution for producing all main parts that go into a vertical bearing, as well as four principal constituents of a horizontal bearing; and because ones and twos are generally machined, cutting out set-up time is especially important for cost-effective production.

The traditional process for machining a vertical casing formerly took seven operations, but these are cut to two set-ups on a Hermle mill-turn centre, eliminating five re-clampings and saving around 7hr per component.

Mr Kemp was familiar with the Hermle brand, as many such machines are in use at a nearby aero engine factory that he has visited; and while other potential suppliers of mill-turn machines were also approached, a Hermle C 60 UMT with its 1,200 x 1,300 x 900mm work envelope was selected due to its robust construction and the high precision machining of which it is capable.

This level of performance is crucial as bearings include tight tolerances, such as 25µm maximum total runout over 400mm for the perpendicularity of a face to the bore.

Surface finish

During cutting trials, the machine was also found to be capable of turning a 0.4 Ra surface finish on bearing faces, saving the time and expense previously involved in grinding and polishing them.

Furthermore, with regard to overall machine capability, Mr Kemp said: “It is rare to find machines that are equally good at turning and milling, but Hermle machining centres are.

“That is the case even when roughing, due to the ability of the turning table’s 4,000Nm torque drive to handle high forces at speeds up to 400rev/min without stalling.”

Previously used by the Nuclear AMRC in Rotherham, this Hermle machine was transferred by Kingsbury to Michell Bearings’ factory, and within three weeks it was in production and has been running 24/7 ever since.

Mr Kemp said he had never seen a machine of this size installed in a factory so quickly. “Part of the reason is its one-piece construction with triple guideway system above the work zone for the Y-axis gantry carrying the X-axis slide.

“This allows the machine to be craned in and placed on a foundation without having to be fixed to the floor. Furthermore, the fact that major machine elements are not bolted together further enhances the milling and turning accuracy.”

The high performance and reliability of the Hermle C 60 UMT led Michell Bearings back to the same source in 2019 when a second mill-turn centre was needed to meet capacity demand. This time a new but slightly smaller C 52 UMT was selected; this has a 1,000 x 1,100 x 750mm work envelope.

An important part of the machine packages supplied by Kingsbury is the after-sales service provided by the company and its principal.

Indeed, Mr Kemp said that on the rare occasion there has been a failure, a telephone call to Kingsbury in Gosport has invariably solved the problem. If not, a ‘diagnostic dump’ from the control has been e-mailed to Kingsbury, and if necessary forwarded to Hermle for a more in-depth analysis.

“Furthermore, if three parts are suspected of possibly causing an issue, the Kingsbury engineer will bring all three, usually the next day, allowing the correct part to be fitted so that the machine is quickly back in production.”