The Impress Group has a diversified portfolio of customers, served by a similarly diverse range of manufacturing technologies within the group’s five businesses units; these provide metal pressings, fabrication, laser profiling, high-volume turned parts, coded welding and general sub-contract machining.

The materials processed are equally varied, but with growth in the aerospace sector, the group is machining an increasing amount of light-alloy parts on its range of VMCs from XYZ Machine Tools.

To help meet this demand, Impress turned to the Sheffield-based tooling and work-holding company WNT (UK) Ltd (www.wnt.com) to improve productivity on a long-term contract for machining light-alloy brackets in relatively high volumes.

Company director Steven Young said: “Because we wanted to break into the aero-space sector, we quoted a very keen price for this job; and having won the contract, we knew we would have to work extremely hard — or clever — to make it profitable and ensure that we retained the work in the future.”

Having already built a good relationship with WNT, Mr Young asked technical sales engineer Rob Buxton to assess the job and propose a solution.

The high-speed (12,000rev/min) spindle on the XYZ 1060 HS VMC made WNT’s Centro P tool-holding system an obvious choice, as these tool-holders are designed for maximising the performance of solid-carbide milling cutters (Centro P allows cutters to run at high spindle speeds with run-out accuracy of 3µm; their design and high clamping forces also minimise vibration).

The Centro P collets were paired with WNT’s W-HPC Alu Ripper cutters for the roughing operations, the objective being to remove as much material as possible in the minimum time. Moreover, the HSS drills that had previously been used were replaced with WNT’s solid-carbide tools for further cycle-time gains.

Essential support

Mr Young said: “Throughout the development process, Mr Buxton stood with the operators ‘on the duckboards’ to ensure that everything went according to plan and was running to its optimum performance. Not only that, in these initial stages the trials of the tooling and work-holding were at zero risk to us because WNT was willing to make the investment in our processes on an ‘if it works you buy it’ basis.”

Having proved the machining strategy, the next step was to further rationalise the manufacturing process. This was achieved by the introduction of WNT’s ZSG mini-concentric vices, with a number of these low-profile units fixed to the machine table. This arrangement has a number of benefits, the first being that having several components available to be machined at the same time reduces ‘takt’ time by reducing the number of tool changes and workpiece loadings.

“With these vices in place and fully loaded, the machine can run without operator intervention for extended periods,” says Mr Young. “It is now monitored by any one of our operators, who can reload components once the cycle is complete.

Furthermore, each individual tool is presented to multiple components before the next tool change is required; this has a significant impact on the overall cycle required to complete the loaded components.

“If we take a simplified machining cycle of drilling, counter-sinking and tapping a hole in a component using a single clamping vice, this would require three tool changes to complete the operation of drilling the hole, counter-sinking the hole and tapping it. Moreover, while the actual machining time is only 15sec, the cycle takes 45sec to complete, taking a typical 10sec tool-change time into consideration.

“If you multiply the number of vices and clamp 10 workpieces in a single set-up, the individual component cycle time is reduced to 18.5sec, thanks to the elimination of multiple tool changes.”

In conclusion, Mr Young said: “The work that Mr Buxton and WNT did on this project has been invaluable. By looking at the project with a fresh pair of eyes, they have come up with a very viable solution, allowing us to maximise the machine and operator capacity that we have — and ensure that we remain competitive as we move forward into new market sectors.”