FMS And Quick-Change Fixtures Boost Subcontractor's Output And Profitability

In less than four years, the prismatic machining capability of Lancashire subcontractor Clitheroe Light Engineering has progressed dramatically from manned vertical machining centres to include horizontal machining centres (HMCs) working largely unattended, 16 hours a day, seven days a week. A stand-alone, 7-pallet HMC started operation in 2004 and in October last year (2006), a 36-pallet flexible manufacturing system (FMS) comprising three more HMCs was installed.

"The change has been unbelievable," said Chris Wilkinson, a director and one of five family members working at Clitheroe Light. "The investments have greatly increased production output and made us more competitive, without the expense of putting on a third shift and moving to 24-hour operation."

When the subcontractor originally installed its first two horizontal-spindle machines with twin pallet changers (2xAPCs) from Daewoo, it achieved dramatic reductions in labour costs and lead-times as well as improved cash flow, as parts could be invoiced more quickly. A couple of years ago, Clitheroe Light moved ahead with automation by retrofitting a seven-pallet pool to one of the Daewoo machines, an HM500.

Now the company has taken things a major step further by investing in an automated Fastems system for storing and retrieving machine pallets, and two Mori Seiki NH5000 2xAPC HMCs, which together with the original HC400 2xAPC Daewoo and a load/unload station forms the three-machine FMS. The other major system component is a Zero Point sub-plate system from Wixroyd that avoids having to buy 36 expensive machine pallets and also makes the FMS more versatile.

Around 95 per cent of Clitheroe Light's business is supplying multinational companies in the hydraulic and pneumatic component industries. It is a highly price-sensitive sector, with relentless cost-down clauses every time the company pitches for new contracts. Materials machined range from bronzes, brasses and aluminium through to steel, stainless, duplex and monel.

To allow it to make a profit on tight margins and cope with an increased workload, the subcontractor considered adding a third, 8-hour shift from 10.00 pm to 6.00 am. However, it would have been nearly impossible to find eight local staff to replicate the day shift teams on nights, good quality setter / operators being especially hard to find.

So Mr Wilkinson and the other family members decided to go down the lean manufacturing route, having calculated that the double-shifted FMS, without the cost of the pre-existing Daewoo machine, would pay for itself within two years based on labour and overhead savings from not adding a night shift. If the two new Mori Seiki machines are taken out of the equation as well, the automation-enabling Fastems and Wixroyd equipment will be amortised in just one year.

To keep down the cost of installation, Clitheroe Light took responsibility for co-ordinating the supply of all components of the FMS, rather than giving turnkey responsibility to one of the vendors. All suppliers understood their individual roles and co-operated well, so the installation went smoothly, took just 10 days to commission and was ready a week ahead of schedule.

The Fastems store houses 36 pallets on three levels. Again with cost cutting in mind, the subcontractor chose not to buy 24 Mori Seiki pallets and 12 Daewoo pallets for around £95,000. Instead it went to Wixroyd, which supplied six receiving plates each fitted with four Zero Point fixturing pots. The plates are mounted on the machine tool pallets and stay permanently in the three 2xAPC machines. The receiving plates accept any one of 36 sub-plates carrying a fabricated tombstone fixture on top and four Zero Point pull studs on the underside that locate in the pots.

Total cost of this solution was £17,000 per machine, ie a little over half that of buying the proprietary machine pallets. It has the added advantage that components fixtured on any of the 36 sub-plates can be directed to either of the Mori Seiki machines, whereas Daewoo machine pallets could not have been accommodated. Even though the Daewoo HMC can only accept 12 designated sub-plates owing to tombstone height restriction in the machine, the overall set-up is much more flexible than it otherwise would have been.

Based on a reliable, collet-type locking system rather than a ball and cage that clamps at only six points, Zero Point dampens vibrations to promote good component accuracy and surface finish. Each pot has 1.5 tons of holding pressure and with four pots per plate, total clamping force is 6 tons for this system. (Wixroyd supplies systems rated at up to 3 tons per pot for heavier duty applications).

When machining at a pallet station is complete and it has been rotated to the outer position, hydraulic pressure at 70 bar is applied to the receiving plate to release the pull studs holding down the sub-plate, whereupon it can be removed by the FMS crane and returned into the store. Retention of the plate is fail-safe, however, as it is mechanically clamped after the hydraulic pressure is removed.

To satisfy himself that Wixroyd was a reliable and professional supplier, Mr Wilkinson spoke with two existing Zero Point users, boiler manufacturer Baxi Potterton in nearby Bamber Bridge and Isle of Man subcontractor, RLC Ronaldsway, before making the final decision. The system can be used on virtually any machine tool including grinding machines and turning centres. It is used by many blue chip engineering companies such as BAE Systems, Rolls Royce and Smiths Aerospace along with progressive medium sized engineering companies.

The Fastems store at Clitheroe Light houses 36 sub-plates (400 mm x 400 mm) with tombstones on which components are fixtured. The racking is 17 metres long and runs approximately one-third the way along the industrial unit. It could therefore be trebled in length and further machining centres and load / unload stations added as production demand dictates. Mr Wilkinson anticipates the first extension within the next two years.

Running along the length of the store is a 3-axis CNC stacker crane travelling on rails. The mast of the crane carries a load handling device that docks to a repeatability of ± 2 mm to effect sub-plate transfer into and out of each storage position and onto and off the receiving plates. Entry of workpieces to the system and their subsequent exit after machining is carried out at a load / unload station equipped with a pallet table capable of being indexed in 45-degree increments. An operator removes completed components as a sub-plate emerges and fixtures the next parts on the tombstone, which is taken into the store by the stacker crane to await processing.

The Fastems MMS v3.0 software that orchestrates FMS operation runs on a standard, Windows-based PC, its main tasks being to control the stacker crane and schedule automatic sub-plate transfers between the load stations, store and machine tools.

The efficiency with which a mixed schedule of work is presented to the three spindles means that small batches can be produced economically. For example, Clitheroe Light used to machine a batch of 120 manifolds for a customer that wanted 10 per month and then deliver them in 12 instalments throughout the year, absorbing the financial cost of the consignment stock.

Now it is economical to run off 10 components and deliver them; even one-offs are cost-effective. Repeat jobs for which programs exist are in production very quickly, and the company is so confident of repeatability that first-offs are not even sent to inspection unless they are safety-critical.

A manufacturing route defines the progress of work through the FMS. The route contains information about the NC program to be used as well as the component, fixture and tools. If the intended machine is not available, for example it already has work waiting on the second pallet or all the required tools are not in the magazine, the sub-plate is rerouted to another machine or is transported into store until a suitable destination device becomes available, creating a buffer for unattended running. As cycle time per pallet can be as much as four hours, it is theoretically possible for Clitheroe Light to benefit from a full, 8-hour lights-out shift utilising all three machines within the FMS.

To automate the often difficult and time-consuming task of managing tools, a mirror image of the 120-tool magazine in the Daewoo HMC and the 240-tool magazines in each of the Mori Seiki machines is held in the Fastems control, which includes data on the remaining life of each cutter. Available tools as well as those that need to be added and removed for any given job or series of jobs can be evaluated. Even tools stored outside the magazines can be included, which is a useful facility for Clitheroe Light as the HMC magazines are nearly full.

Performance data and statistics of stacker crane operation are available, while the production reporting interface generates information such as total machining time of a particular order. The operational modes of the machine tools are also tracked in a database, providing data covering on-line/off-line status, cycle on, feed on and errors/alarms, which are automatically sent as text messages to the mobile ‘phones of relevant staff.

As a postscript, Mr Wilkinson commented on the ease of use of the FMS, despite its apparent complexity. He said that an engineering student on work experience from Imperial College London was able to operate the system with minimal supervision after one week's training. In another example, an engineering apprentice that had been working in the CAD department of a local conservatory company, after having being made redundant and joining Clitheroe Light, was conversant with the FMS within a couple of weeks.

SOURCE: Fastems Ltd and Wixroyd Productivity Systems Ltd