Machining of Steel Hatches 5-axis machining centre with a smaller footprint nearly doubles productivity

From MA Alexander Stark

For the ongoing production of a particular component for a defence industry customer, subcontractor PRV Engineering, Pontypool, has invested in a Spinner U5-630 trunnion-type, 5-axis, vertical-spindle machining centre (VMC). It was delivered in mid-2021 by Whitehouse Machine Tools, Kenilworth, sole sales and service agent for the German-built machines in the British and Irish markets.

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The fabricated steel hatch for an armoured tank is machined nearly twice as quickly on the Spinner U5-630 at Pontypool.
The fabricated steel hatch for an armoured tank is machined nearly twice as quickly on the Spinner U5-630 at Pontypool.
(Source: Whitehouse Machine Tools)

For the production of tank hatches, British subcontractor PRV Engineering has invested in a Spinner U5-630 5-axis machining centre. The U5-630 is devoted to producing a fabricated steel hatch measuring approximately 300 x 200 x 200 mm for an armoured tank in a cycle time of 5.5 hours, more than 20 percent faster than when the machine first arrived on the shop floor. The subcontractor confidently expects to reduce the single-setup metalcutting cycle by a further 20 minutes as it becomes more familiar with programming the new VMC.

The previous production route was to mill and bore the three-part fabrication on either of two much larger 5-axis machining centres having a 3,000 x 800 x 800 mm working envelope, a travelling column and a swivelling B-axis spindle head. Three setups were required and the total production time was 10 hours, which meant that it was not feasible to produce one hatch per day during a single shift.

Apart from raising the efficiency of production by nearly halving the cycle time, another reason for investing in a VMC dedicated to the job was a need to free up the larger capacity plant to fulfil an increasing amount of plate work. In any case, it is more difficult and therefore more time consuming to hold tolerance on a larger machine, as the heavier moving elements tend to cause vibration, added to which the B-axis providing one of the rotary axes is not as strong as a trunnion arrangement.

So PRV Engineering's managing director Simon Jones and works director Alun Cox set about researching the market for a 5-axis, trunnion-type VMC that was appropriately sized for this defence industry contract. The Spinner option was chosen partly due to its generous 630 x 530 x 465 mm working envelope in a footprint of just 2.75 x 2.53 metres. Other machines they looked at were much less compact.

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High quality German build and ex-showroom availability from Whitehouse Machine Tools' Kenilworth headquarters were other positive factors that influenced the decision. The good standard specification was also a bonus, as it includes a 21 kW / 135 Nm spindle motor ideal for machining S690QL and Hardox 400 steels, high-pressure coolant, a 32-station tool magazine, hydraulic clamping, and linear scales feeding back axis positions to a Siemens 840D sl touchscreen control with built-in ShopMill software.

The U5-630 has to be highly accurate, which pre-sales demonstrations in Kenilworth confirmed was achievable. Some tolerances are very tight on the hatch fabrication, which comprises three pre-machined and welded parts. The run-out of two holes bored from either side of the component after rotation through 180° has to be to within 0.04 mm TIR (total indicator reading), while their diameter tolerance is 25 µm total.

Better accessibility of the cutters to the workpiece on the smaller machine is a major benefit. CNC setter-programmer at the Pontypool factory, Darek Krochmalny, explained that it is possible to use shorter and therefore more rigid tools than on the larger machine, enabling faster feeds and speeds for higher productivity.

Furthermore it is now practicable to reduce the time needed to mill some features such as a locking pin pocket by interpolating all five axes simultaneously, whereas the production cycle on the B-axis machine was entirely 3+2. Cox concluded: “PRV relies on good service from machine tool providers to ensure maximum equipment availability so that we can be as competitive as possible. We have been particularly impressed with Whitehouse Machine Tools in this regard, a company we have not dealt with before.”