Machining techniques and different configurations can assist in eliminating waste and improving efficiency in production

The Nissan Decherd powertrain assembly plant, some 128 kilometres south of Nashville, Tennessee, has the capacity to build 950,000 engines and 300,000 transaxles annually. It produces all the engines for the complete line of Nissan and Infiniti vehicles manufactured in the United States, including a 2.5 litre 4-cylinder, 3.5 litre 6-cylinder, 4.0 litre 6-cylinder, and a 5.6 litre 8-cylinder. Its 3.5-litre V6 VQ has made it onto the Ward’s Communications ‘Ten Best Engines’ list for the past 10 consecutive years.

The person entrusted with keeping the machine shop functioning smoothly and efficiently is Brent Gill, Director of Manufacturing at Decherd. His challenges will be familiar to all managers of machine shops – increasing overall efficiency, controlling scrap and ensuring the facility is flexible enough to meet a fluctuating product mix.

The first item that Gill mentions is scrap, something that is important to Nissan and high on its benchmarking priorities. “Our biggest challenge now is to keep our scrap low,” he confirms. “This is something that everybody is faced with. We look across the industry, especially internally at Nissan and then compare against other plants and try to keep waste as low as possible, but we see that we have got some opportunity there.”

Scrapping the waste

Keeping scrap low involves working closely with external suppliers to ensure the high quality and suitability of incoming parts. A key aspect of that input until now has been forgings, but with a new forging line due to start operating soon at Decherd, that will be within the plant’s own control. This will ensure that forgings are of a high quality, require as little machining as possible to maximise tool life as well as crucially being able to adapt to engineering changes.

“One good aspect of doing our in line forging straight into our machine line is that we are able to modify some of our forging dies to reduce some of the material going to them,” Gill explains.

“Since the process has moved in-house, similar dies are being examined to see if we can modify them to remove some of the excess material in forging. This will reduce the number of machines that are actually required in some cases,” he continues.

A more traditional measurement of productivity is overall equipment efficiency (OEE) and this is where Decherd has made huge strides. When the plant opened in May 1997, the benchmark was 70 OEE – a figure that was the norm at the time for Nissan globally. Productivity expectations have grown, and so too a higher expectation of OEE. Today the benchmark figure is 90. “Most of that is because of the growth in machining, we were able to get up to the eighties so quickly, to set the benchmark a bit higher in the nineties. I haven’t looked at it lately, but I think that most of the lines are running close to ninety now.”

Improving efficiency

The Decherd plant recently celebrated its tenth anniversary of engine making but there has been no significant investment in new equipment for the past four years. It’s had to concentrate on process improvement for efficiency and scrap gains. “We changed over the crank lines about four years ago and a few small machines were added,” Gill explains.

Block castings are currently brought into the plant, although some of these will be done in-house in the future.

“Our scrap rate really depends on our suppliers. When we start talking about block castings in terms of the porosity, the pinholes and those types of things, during our machine shop process it makes us remove that part from the process itself; all of that counts against our scrap. It also counts against the supplier scrap, but it is a part that we lost,” he continues.

Gill says the Decherd plant has improved its V8 blocks and 3.5 litre V6 block a great deal. “We still struggle with the 4.0 a bit because of the process involved. I think our impregnation has decreased substantially on the 3.5.” The plant has a fairly steady product mix, thus flexibility is not a major concern. What is a challenge is keeping production at the agreed level, even if a particular machine fails.

Moving between the lines

Gill reveals that although the company builds four different engines at Decherd, the product mix has not changed much.

“We do have volume swings between the products, but as long as we are not over capacity, we are still in fairly good shape, so we can move between lines and thus have that type of flexibility,” he says. “We haven’t gone to new models yet, other than the changes mentioned earlier. The crank lines were changed a bit to a super-micro finish, to get a better roughness. We haven’t faced the dilemma of being outside our capacity yet.”

Gill tells AMS that flexibility will allow the plant to shut down some of the lines this year to carry out certain changes for the new K3 model. It is also possible because of the plant’s flexible transfer lines (FTL). “As regards head and crank machining, we have multiple FTLs. We have the flexibility to shut down one line and modify it while maintaining production on the other lines. So we have flexibility from that point of view,” Gills explains.

A key factor for increasing the efficiency of the plant is to decrease cycle times, which involves working with the engineering department to analyse where bottlenecks are occurring in the operation. Gill explains that the teams examine the reasons for any bottlenecks – “Is it because there is a wait for parts? Can we speed up the robot? Can things be moved closer to the lines? Getting improvements in cycle time by eliminating a bottleneck will give you a higher OEE over the entire line,” he says.

“The other factor is that you might have eight drilling operations doing the same thing on the same line. One can be down and you can still have a reasonable OEE. But if you have a piece of equipment and it only performs one function, and there isn’t another piece of similar equipment, whether it is a pin milling or a line bore on a block, you have got to maintain the equipment by total productive maintenance (TPM),” he adds.

“You have to maintain that equipment because its up time is crucial. If it goes down, it directly impacts on the OEE.

So we would do extra things with the TPM. The maintenance department is aware that a particular piece of equipment has to continue running, so they will examine it during down hours to make sure it is ready to go. By implementing these types of activities and improving our cycle times with the help of the engineering department, the OEE has gone up,” Gill tells AMS.

Recycling the cutting fluid

Something that Nissan has been implementing globally in its machining operations is minimum quantity lubrication (MQL) and recycling cutting fluid. Engineers at Decherd visited Nissan’s plant in Britain to see these systems in action. When Nissan Motor Manufacturing United Kingdom (NMUK) installed its fluid recycling system in March 2004, the initial target was to reclaim between 60 and 80 per cent of its production cutting waste fluids. Within months, the plant was reclaiming levels of 90 to 95 per cent.

The results continue to surprise with reclaim efficiency levels of 100 per cent being recorded in recent months. This has helped NMUK’s control of on-site waste and ISO 14000 environmental objective, in particular its on-site waste management volumes.

Demand on the system has grown in line with NMUK’s increased production of cylinder heads at its Sunderland plant. This resulted in its swarf-centrifuging system extracting upwards of 1,000 litres of machining fluids per day. This water miscible system is specifically configured to remove tramp oil and particulate matter from coolant, and wash fluids down to eight microns or less. The unit is equipped for use on high temperature and high pH fluids and, being mobile, it is able to process fluids in different locations within NMUK’s engine parts manufacturing facility.

Increased tool life increases OEE

Gill confirms that the Decherd facility has had success with MQL. “We have it on our number two crank line and have not had any problems,” he says. “That type of technology is going to help reduce costs, and help the environment too, because very little coolant has to flow through it. It uses exactly what you need, so there is no waste. I’m not sure whether that would help us on the line, but it would help us on tool life and reducing costs.”

He explains that where it has helped, is with the tooling. “We have been able to do trials on new types of tooling, because if you increase tool life, you increase OEE as you are able to run that tool longer. You are able to get more cuts out of that tool; instead of getting 1,000 cycles you get 1,500 cycles, thus you are not down for tool changes. That also increases your OEE.”

Ford’s Bridgend engine plant has enjoyed much success recently, prompting a decision to increase camshaft production by 40 per cent to 1.4 million units a year. The carmaker wanted to achieve this without investing in extra machinery, so its engineers undertook a detailed analysis and identified cam lobe grinding as one process where improvements could be made.

Because of the complexity of the process, Ford called in Tyrolit, its grinding wheel supplier, to suggest ways to increase throughput and, at the same time, improve the quality and accuracy of the cam surfaces. Tyrolit formed a working party with Ford engineers to define the project targets and collect data on the existing process for analysis.

The supplier then submitted a detailed cost reduction plan in which savings were broken down into making cuts in consumables, set-up and machining costs. Floor-to-floor time was reduced from 124 to 112 seconds and the increase in grinding machine capacity was more than 12 per cent, allowing production of 1.4 million camshafts in a year without building a new facility.

Central to achieving the optimisation targets was the adoption of a newly developed Tyrolit CBN grinding wheel, which draws less spindle power, as it is 60 per cent lighter than conventional products. Also, the new tool absorbed vibration better, allowing a harder bond to be used. Together with improved dressing, this resulted in longer wheel life, and a significant reduction in cost per part. Overall, the saving in cam lobe grinding was calculated at over £300,000 ($599,000) per year over the lifetime of the project.

“The structured approach to this project was key to its success,” Mike Jones, Ford’s Senior Manufacturing Engineer, says. “It is a good example of our philosophy of working closely with partners and suppliers. The successful outcome proves how extraordinary results can be achieved when innovative minds and production tools are at work.”

Transfer vs parallel lines

Walking the divide between a process that is flexible and one that is highly productive is a tough act for machine shop managers at automotive OEMs, but one that they face on a daily basis. “Our job in the manufacturing technology industry is to find new approaches to keep up with, or get a step ahead of, the torrent of market fluctuations and capital location challenges that our customers face,” explains Ron Quaile, Vice President – Proposal and Estimating, MAG Industrial Automation Systems.

As regards component machining systems, OEMs and part suppliers need the powerful productivity advantages of traditional highvolume machining systems, but they also need greater levels of flexibility to juggle niche vehicle production and market shifts, all while cutting lead time to shorten the time-to-market. “These factors drive us to creatively find ways to build new system configurations, using proven elements that minimise risk,” Quaile continues.

“Consolidation of vehicle making adds to the pressure. As outsourcing moves to lower tiers, capital equipment budgets get smaller, and so do lead times for delivery of the equipment. Given that equation, the choices made are not always what you might assume,” he adds.

“We’re finding that in the vehicle powertrain component business, manufacturing in a low-labour-cost market doesn’t dictate an emphasis on using lots of manual labour in favour of material-handling automation, nor does it presume less importance on investing in fast, top-productivity machines. We’ve found that Chinese and Mexican facilities will invest in automatic machine load/unload capability but, in many cases, prefer to move the work in process from machine to machine manually. The priority is on the machines themselves, with no compromise to accuracy or machine reliability.”

Complete machining using a hybrid system

At a heavy-duty truck manufacturer in China, cast-iron truck engine blocks are completely machined by a hybrid system. Fixtures on the 12 four-axis, 800mm-cube horizontal MEGA machining centres, equipped with pallet swappers, are manually loaded. Four-second chip-to-chip time, 80m/min velocity, and fast-acceleration rates keep productivity high during heavy rough machining on these large parts. Parts are moved between the HMCs by powered conveyor. Then, fi nish milling and boring operations are performed on a 12-station in-line transfer machine. Fifteen thousand parts per year are made on the expandable system that uses various levels of automation.

A Wuling-based automotive producer has mastered high-volume engine production, machining blocks with 56 XS-series horizontal machining centres, and cylinder heads with 48 HMCs. These machines automatically clamp the parts after they are moved by operators between machines on simple roller conveyors. Output is 300,000 parts per year (150,000 multiplied by two modules). At yet another Chinese truck manufacturing facility, however, fully automatic overhead material handling systems move cast-iron truck engine blocks between machining stations within each of four transfer lines, and between the transfer lines. Also, 13 800mm-cube horizontal Magnus machining centres, grouped into three agile cells, complete the machining.

An engine producer in Mexico uses 27 STAR 500 horizontals for premachining (cubing) operations on cylinder heads. Besides qualifying the part, taking the gates and riser off, and adding manufacturing holes, the system in fully-automatic overhead material-handling systems are equipped to move cast iron engine blocks between machining stations at a truck manufacturing plant in China.

“In the United States, there are OEMs who are spending the budgets where they count – in high performance machines with automatic machine load/unload, but opting to move parts between machines manually,” Quaile says, “applications include transmission cases, converter housings, valve bodies, and others.” He explains that achieving the right agility factor is the trick in the automotive industry.

“There are many applications where a parallel process – consisting of one or more cells of multi-operation programmable machines – is best, while other types of applications call for a sequential process (associated with the traditional transfer line). A hybrid system that includes both approaches is now a very common solution for engine blocks and other powertrain components,” Quaile tells AMS.

Transfer lines are still favoured by the industry when the part is less susceptible to near-term design changes, and when heavy-duty metal removal or high-precision machining must be per formed. The individual machining operations are divided into stations that are arranged in one sequence and are connected by a rigid transport system. Each station performs relatively few machining operations (often only one) compared with programmable HMCs. The system is convertible, but not as flexible as parallel processing.

“Transfer lines work well with a closed family of parts, but if you’re looking for significant variations, it’s not a good way to go,” Quaile says. “The sequential process – transfer lines – can have a quality-assurance advantage, as there is only one stream of variation to trace in case of quality problems. This is as opposed to a parallel process, in which the same operation or set of operations are simultaneously performed by several identical machines in a cell.

Parallel process is more agile

“The advantage of the parallel process is that it is more agile, is easily expandable, and, if one machine shuts down, it doesn’t shut down an entire process. However, without stringent SPC procedures, the flexibility advantage that arises from the multiple paths available for any given part through the machines can be a disadvantage in terms of streams of variation and traceability. There is typically a higher rate of design changes in cylinder heads than in cylinder blocks, and a parallel process, CNC-based cell concept is typically preferred.”

Quaile says, “We have had good success with hybrid systems for cylinder blocks. A transfer line is favoured for roughing and fi nishing operations on features that don’t change. But a set of fl ex ible CNC machines (a parallel process) will machine features subject to design changes or variations such as four or six cylinders, different transmission mounting faces, and engine accessories.”

The parallel process can be optimised to achieve 80 to 85 per cent overall operational effectiveness (OOE) levels because of the use of individual machines and duplicated functions. This translates to machine operating efficiency in the high 90s. Transfer lines have traditionally had lower OOE levels, but we’re working on closing the gap and are getting close to matching the efficiency of the flexible cells in some applications.

An alternative to the single, high-cycle-rate transfer line designed to produce 600,000 parts/year, is to create three slower, highly reliable lines that each produce 200,000 parts/year. This approach offers higher OOE than traditional transfer lines and allows part variants to be fed into only one of two lines, simplifying tool management, inventory management, and other factors.

According to Quaile, the most innovative processing idea to come along in many years is embodied in a new transfer centre machining configuration that combines the machining centre, transfer machine, and turning machine approach in a single unit.