The replacement of an ageing single-table horizontal-spindle machining centre (HMC) with a new German-built Burkhardt + Weber (BW) twin-pallet model has ‘revolutionised’ the machining of industrial gearbox casings at Renold Gears’ facility in Rochdale.

These casings are produced mainly from iron castings and fabrications, but also from steel and aluminium.

The resulting savings in floor-to-floor times — between one half and two thirds — are due to increased metal removal rates during cutter engagement, significantly reduced non-cutting times through faster axis movements, fewer operations due to better fixturing methods, and faster component change-over through off-line set-up on the second pallet.

Furthermore, the twin-pallet configuration means that if there is an issue during the machining of a component, it can be brought out of the working area for inspection, allowing production of the next part to commence.

That would have resulted in a lot of unproductive time on the previous machine.

Renold’s gearboxes are large prismatic components that can be more than 2m high and weigh up to three tonnes.

Around 20% of products are standard, the remainder being customer-specific designs that are manufactured in quantities ranging from one-off to 30-off per month.

Fast and ‘flexible’ machine tools are needed to produce such relatively low batch sizes cost-effectively and allow the manufacturer to compete in world markets.

When the company’s current CEO was appointed in 2013, he instigated ‘modernisation initiatives’ that started with the factory infrastructure.

This was followed by a programme of drive and control retrofits, as well as mechanical upgrades to what are fairly specialised machine tools, such as worm screw and wheel production centres.

When — three years later — the production staff were asked which machine on the shop-floor they would most like to replace, the large HMC was almost unanimously chosen; its unreliability by then was such that Renold was sometimes having to sub-contract out machining to meet production deadlines.

Successful tests

After several twin-pallet HMC options had been considered, a BW MCX 1400 with a 3,200 x 2,200 x 2,000mm work envelope and a B-axis NC table was purchased from the Gosport-based UK agent Kingsbury (www.kingsburyuk.com).

A senior manufacturing engineer at Renold said: “During trials, the BW machine proved capable of more than halving the cycle times on the old HMC, and it was more productive than the other four-axis machines with pallet changer that we considered.

“We gave two test parts to each potential supplier — a gearbox casing for a heat exchanger and one for an escalator drive.

“On average, they were machined around 15% quicker on the MCX 1400.”

Key elements of the machine specification that deliver such high productivity include: axis accelerations of up to 5m/sec² to rapid-traverse speeds of 60m/min; and a 60kW (3,500Nm of torque) 5,000rev/min spindle with an HSK-100 interface.

The engineer added that other factors favouring the chosen HMC were its availability on a short delivery and its ergonomic design; the latter “promotes safety when personnel approach the machine to access the working area or to carry out servicing while it is running.”

Additionally, the 180-pocket tool magazine — extendable to 330 — is helpful, as a large number of cutters are needed to cope with Renold’s wide range of gearbox casing sizes and materials; most of these tools can be permanently resident in the magazine.

Interpolation milling

Drawing tolerances are tight for such large components, down to 20µm total for some machined features like gear centres and shaft bores, some of which are produced by interpolation milling.

The reliability and repeatability with which this level of accuracy is achieved on the MCX 1400 mean that downstream benefits are gained in the metrology department.

For example, CNC inspection is faster, as it requires less-comprehensive routines, and fewer components need to be checked.

Kingsbury says it prides itself on providing a production solution rather than simply a machine tool; and while the BW installation at Renold Gears’ Rochdale facility was not what the supplier would class as a full turn-key project with tooling and fixtures — Renold provided these itself — it nevertheless entailed significant early support.

Initial test programs were converted directly into cycles for production parts covering two families of casing, while further programming support was provided, along with on-site operator training.

Service is carried out by Kingsbury’s own engineers via the company’s divisional LPM (Large Prismatic Machines) offices in Warwick.

The Renold engineer concluded: “We have also been migrating the machining of our custom gearboxes and some standard products across to the BW machine to take advantage of its high productivity.

“It already does the work of the old HMC and another machining centre, and we are looking to consolidate onto it jobs that we currently put on a third machine. Once a process is in place on the MCX 1400, it eats the work.”