Automated guided vehicles deliver parts to a U-shaped production cell with heijunka boards, angled parts delivery and a traffic light system to show which part needs picking when. Will Stirling visits the Lean Factory Group’s latest demonstration.
The Lean Factory Group is a consortium of seven suppliers of production efficiency equipment and services to the manufacturing sector, formed in Germany five years ago. In the UK they comprise of ADM Automation, Bosch Power Tools, Bosch Rexroth, K. Hartwall, Spitfire Consultancy, Sick UK and SSI Schaefer. Collectively, they offer all of the products and services needed to implement efficient lean manufacturing and assembly systems.
The objective was to prove that production time of a standard assembly could be reduced using this equipment configured in a U-shaped cell with six main operator stations. The main product – in this case a trolley – is fed around the cell on a rolling system. Components are arranged in trays angled towards the operator using aluminium structural framing units, designed at the optimum height and to avoid needing to reach to the back. Parts are sensed using automation light grids and a red/green traffic light tells the operator which parts to pick in order.
Kanban cards and a heijunka board display clearly what products the operator team should produce next, and what they need to pick in order. The kanban system, if operated properly, can control and limit stock. Self-labelling provides control of parts organisation – the operator punches holes in label blanks to identify the parts and can control this in batches. ‘Reject’ labels at each station minimise defects.
The demo was designed to show labour flexibility. The six-station cell was set up to make one finished part every 90 seconds, including packaging. Demo One was six personnel and made three and a half products in six minutes, off a target of four. Demo Two, with three staff, finished three products (one was unboxed) proving that productivity in this cell with fewer staff could be equal to or higher than a fully-manned cell.
Is the U-shaped cell limited to smaller products? “It does lend itself to small to medium assembly of complete parts, but it also works for sub-assembly for bigger production environments too,” says Bosch Rexroth’s Ross Townshend, who led the demonstration. “The U-shaped cell format does not suit every type of production, but the principles of ergonomics, parts supply, poke-yoke etc can just as easily be utilised for individual stations, small sequence build or larger machine production.”
Visual management within any production area is a key tool to quickly recognise issues proactively. Simple traffic light rollers are used here to immediately show a depletion of stock to this operator, prompting replenishment from his logistics team.
As a means to ensure a no faults forward build sequence, the first operation employs both Pick to Light technology and integrated Tightening control via a DC spindle.
To eliminate excessive inventory, the cell works on a ‘Pull’ principle to build to order only. This is controlled by the Heijunka cards which are fed into the day’s schedule and control the variant of parts built.
The sub-assembly area utilises the benefits of a Kanban card system to ensure that only the parts needed are produced. This approach prevents wastage from overproduction and waiting times for parts on the main cell.
Elimination of faults during production is essential to prevent wasted labour and reworking. A Pick To Light system is used here to ensure the correct parts are fitted to each order using a barcode to activate each sequence.
When fully populated with six operators, this U-shaped cell layout is capable of a 90-second Takt time for each operation, and allows for small fluctuations in build speeds between operators while maintaining this rate.