4-axis machining centre for demanding positional tolerances

The German-built machine, supplied by sole UK agent Geo Kingsbury (www.gkholdings.com), is of unusual design in that it is based on a Hermle C 20 U vertical-spindle, 5-axis machining centre, but without the rotary table or trunnion. Instead, the left-hand trunnion drive is employed to provide a fourth axis, which together with a fixed machine table makes it easier to achieve very tight tolerances, without compromising productivity.

Ken Jolley, Reliance Precision's Operations Manager, says: "We started making the part on a 3-axis machining centre fitted with a 2-axis tilting rotary table. Three holes on a PCD (pitch circle diameter) have to be within 10 microns true position relative to the OD (outside diameter), which was difficult to achieve consistently.

"True position imposes very different demands on a manufacturing process and to be in control of it, a machine with positional capability better than 5 microns is desired. "We were producing the parts satisfactorily, but with a significant amount of manual intervention to achieve the required quality. Now it is achieved routinely on the Hermle, faster and with far less operator attendance.

"It is a highly capable process, due in part to the machine's inherent rigidity and accuracy, combined with a System 3R quick-change pallet system and in-process workpiece gauging using a Blum probe in the spindle."

Machining is completed in three separate operations using pallets with bespoke fixtures permanently mounted to minimise loading and unloading times. First, a 50mm diameter by 25mm long pre-turned blank of precipitation hardening steel is loaded into the 4th axis. Pockets are milled around the periphery and three holes on a PCD are drilled to a semi-finished state.

The part then goes for heat treatment before Op2 is carried out in a fixture on the table, involving jig boring the holes to within 10 microns true position relative to the OD datum. It is during this, most critical operation that in-process gauging is included in the cycle. Op3 is completed in an adjacent fixture after the part has been inverted. The efficient manual loading and unloading arrangement could easily be automated, and while it is not envisaged at present, it is an option if future requirements dictate.

Consistent component quality

Reliance Precision is benefiting from greater control and more consistent component quality as a result of implementing the new production process. It is also helping to lower unit manufacturing cost, as overall machining time has been cut by nearly 30 per cent and inspection time has also been reduced. Whereas previously after Op2, every part was sent to a co-ordinate measuring machine in the metrology room to undergo a 4-minute cycle to verify that it was within tolerance, in-process gauging gives Reliance Precision the confidence to send significantly fewer parts to the CMM now.

Alex Greenhalgh, Estimating Manager, was part of the team at Reliance Precision that reviewed the various options for streamlining production of the defence component. He comments: "We used a weighting system to evaluate machines from four possible suppliers, with accuracy at the top of the list.

"A 5-axis CNC machine was unnecessary to produce the component, as we cannot manufacture it in one hit anyway due to the need for hardening before Op2. So we went for the Hermle with integrated 4th axis and Heidenhain control, rather than a 3-axis machine with bolt-on rotary table.

"Cutting trials were successfully completed on a full 5-axis machine at Hermle's factory in Gosheim, so we knew a 4-axis version of the C 20 U would only be as good or better in terms of accuracy, and so it has proved."

The machine was supplied with an enhanced specification guaranteeing 5 microns positional accuracy in X, Y and Z. Linear scales are fitted as standard to ensure high resolution feedback and the three linear guideways are above the working zone, keeping them free from swarf. The table does not move, contributing to rigidity, and the machine is thermally symmetrical around the X-axis, so compensation only needs to be applied in the Y-axis.

In another first on the shop floor in Huddersfield, the machine has been fitted with an HSK-A63 spindle, providing stable face-and-taper contact with the toolholder. Accuracy in the Z-axis is improved, especially at higher spindle speeds.

Ken Jolley adds: "The installation, commissioning, applications support and training provided by Geo Kingsbury have been exemplary. All queries were dealt with quickly and professionally, which gave us complete confidence. The machine's performance has exceeded our expectations. There is minimal room for error now and all our operators have been very positive about the new production process.

"It has really opened our eyes as to what is possible with milling and has resulted in a quantum step forward in our technical ability, helping us to build on our reputation for top quality and timely delivery."

In conclusion, Andrew Wright, Managing Director, says: "We have seen consistent growth in our business and expansion of our customer base since the last recession, especially in the defence industry. With a strong order book and significant recent contract awards, we have set out a major investment plan over the next few years, of which the Hermle installation is a part, to expand and develop our core precision manufacturing and assembly capabilities."

Please visit www.reliance.co.uk for further information about the Hermle 4-axis machining centre.