RENISHAW PROBED CITIZEN MACHINE TECHNOLOGY TO ACHIEVE PRODUCTIVITY TARGETS

To Mark Buckingham, a principal manufacturing engineer in the Manufacturing Services Division of Renishaw plc., each of the 12 Citizen M32-III CNC sliding head automatic turn-mill centres installed by the company since 2003 remind him of hedgehogs because they bristle with tools! And since up to 80 tools can be mounted on every one of the 13-axis machines, the Citizens play a key role in Renishaw’s continual quest to maximise machine utilisation and productivity while minimising changeover times.

Indeed, as part of this quest Renishaw has developed an adaptive guide bush to reduce setting times and in-cycle probing techniques to measure features and update tool offsets both at the start of a batch and during production, a feature that is said to possess a level of sophisticated inspection that has not been realised on machines of this type before. In the near future this system will also be offered to other Citizen users as an affordable option.

Other high productivity elements used by Renishaw are common tools between different components on the 10- station, driven tool turret of the Citizen M32, which enables full use of the machine’s intermediate, or half turret indexing capability to be made that can enable up to eight tools to be applied to the job from each turret face. Although not used by Renishaw, Citizen also supply special rotary tools for cross angle machining or thread milling and slitting, which can be mounted via an array of special toolholders.

In addition to the machine turret, the vertical gang tool post can accommodate a further five turning tools and four driven tools, while the back machining station also has the option of three driven tool positions.

Productive developments from the Renishaw production engineering team centre on the adaptive guide bush that replaces the normal fixed head bush and so reduces any need for adjustment to account for any variation in bar size. This development allows unmanned bar changing without having to pre-grade or select material due to any variance in the diameter of the bars. It also minimises the time taken to set up the raw material, allows a lower specification, and therefore cheaper, bar to be used as the guide bush adjusts for any discrepancies along the length of the material.

Chris Higgins, production engineer, explains that although one of the M32s has run a body component continuously for three months, this is unusual: “We tend to changeover machines every 24 hours. In fact, up to 800 machine changeovers can be performed in some months on batch sizes that vary between 50 and 2,000 parts.”

To further improve productivity, by alleviating longer set up times normally associated with very complex prismatic features on their components, the world-famous metrology maker has also recently developed an in-cycle probing solution based on its LP2 LD low force touch probe. This is mounted on the M32’s main platen to accommodate both X- and Y-axis travel around the part and also enable features to be measured such as the centre point of a hole. The stylus configuration also allows a single probe to measure multiple features produced at the main or subspindle while the machine control software automatically updates tool offsets both at the start of a batch and during production.

In other machining areas of the company in-cycle probing has been used to great effect for many years to improve productivity, automatic job set up and reduce reliance on traditionally skilled setters to maintain effective production.

Altogether, the £200 million turnover company has installed 33 Citizens as its chosen CNC sliding head turn-mill centre, at its Gloucestershire production facilities of New Mills, Wooton-Under-Edge and Stonehouse – where the company employs 1,100 people. The more recently constructed 100,000 ft2 facility in Stonehouse must be one of the most impressive and organised production machine shops in the world achieved as a result of significant refurbishments and capital reinvestments following the purchase of the site in 2004.

Adds Mr. Buckingham: “Our policy is to standardise when possible on a particular make of machine and for up to 32 mm diameter, single cycle turn-milling, Citizen has provided the capability we require. As a result, we benefit from factors such as concurrent engineering, common tooling and toolholders, machine service and programming knowledge with the added advantage of creating a very close working relationship with Citizen UK.”

All M32s are fitted with Citizen’s CoolBlaster, the 2,000 psi high pressure system which “significantly helps swarf management, consistency and tool life predictability while also providing high confidence for limited manning and unmanned running”, says Mr. Higgins. While production runs across three shifts, the Citizens tend to be left for up to four hours in totally unmanned mode.

Renishaw has also developed a very effective pre-production philosophy which restricts any new job being sent directly to the full production machine shop at Stonehouse. Instead, each component progresses through an intense three-stage prove-out regime embracing feasibility engineering, design for manufacture and cost–down processes all unhindered by day-to-day production demands.

Initially, design and programming is carried out using GibbsCAM on a prototype machine divorced from the production area in the development machine shop at New Mills. This is followed by pre-production development by a different manufacturing engineer to provide a ‘fresh look’ prior to final cutting trials on a production machine to asses process capability. Proven programs are then held in the network with the ‘job pack’ of background information drawn down from a central hub that supports up to 10 machines in an area for production.

Renishaw’s Roger Burleigh is very complimentary over the consistency of production from the Citizens following the progressive development of each process. “The machines will hold tolerance and surface finish requirements all day,” he claims, and explains that these sizes can be as tight as 0.03 mm on diameter but, mostly, general requirements tend to be around 0.1 mm.

Materials vary from EN31 to 174 PH stainless steel and with the 2,000 psi high pressure coolant, this has enabled one particular threaded sleeve component to be produced by rough turning at a 2 mm depth of cut and a 16 mm hole drilled from solid. To produce the finished part, 17 tools are required of which 10 are driven, and the smallest hole is drilled and tapped at M1.6. A key diameter of 11.02/11.00 mm is held well within SPC requirements without any attention, according to Mr. Higgins, which is the same achievement in maintaining the surface finish of 0.4 RA.

Other medium to high complexity parts of varying sizes include probe extension bars, and probe styli, all produced in single cycle sequences. Whenever possible, families of parts are grouped together to further reduce demands on setting and to improve utilisation.