In-Cycle Roll-Forming By LMT Fette Gives Steering System Maker Pailton Engineering New Direction
Chatham, MA - Such is the focus of UK vehicle steering systems specialist Pailton Engineering on seeking productivity improvements through investment that by combining five operations into one, including the rolling of serrations on steering shaft production, a saving of over £90,000 a year has been achieved with greater control over quality in safety critical components.
Pailton Engineering's Senior Production Engineer, Jim Rawbone is committed to continuous improvement seeing, where practical, the economics of automation of a manufacturing process as the saving grace against off-shore competition. This has led to the spending of some £600,000 in the first half of 2007 to maintain the company's competitive edge. Adds managing director John Nollett: "Our business is geared to provide design, prototyping, manufacture, assembly and test. But we have also created a niche operation because it is critical when customers' vehicles are off the road that we are structured to respond to their needs within 24 hours whenever possible."
One of the key elements in the automation of steering shaft production was a turnkey installation giving a complete production cycle from bar to being ready for assembly using a Star SV32 sliding head auto. The new method brought previous cutting-off, turning, plus two separate operations of spline hobbing and serration rolling and a cross hole drilling operation into a single cycle. Not only did this ‘one-hit' cycle enable output to be improved from 25 completed shafts an hour to 60 an hour, lead time was dramatically reduced over the previous individual job routing and enabled the company to accommodate a progressive increase in demand of parts from 1,000 to 1,500 per month.
In combining the operations, LMT (UK) of Coventry was called upon to support the sliding head turnkey supplier to provide the capability to in-cycle roll 36 and 48 serrations at either end of two types of steering shaft and 48 serrations at one end of a splined shaft. This ‘in-cycle' process, via an LMT Fette F3K rolling head, replaced separate manually operated hydraulic powered rolling operations on a dedicated machine.
Such was the success of the operation that one of Mr Rawbone's production engineers Mark Cairns re-engineered production of a bevel gear shaft with 36 serrations for 900 bevel steering boxes. With the help of LMT (UK), working on a four-axis Biglia B501MB turning centre, Mr Cairns was able to halve the number of operations performed on the part from four to two – the second operation being the dedicated shaping of the bevel gear.
Now with a group turnover of some £11.5 million, the Coventry operation mainly produces commercial steering systems to an international customer base of over 160 companies. There are also certain well-respected and specialist sports and luxury car companies on the customer listing as well as the US Military. Altogether, some 3,500 different part numbers are on the MRP system of which 1,000 would be on the shopfloor in any month.
The process of producing serrations, splines or threads in a single rolled or ‘chipless' cycle is able to reduce the normal process time for these features by up to 90 per cent. With the process, the compressed microstructure of the material as it is displaced, rather than removed, creates a stronger form. The burnishing effect on the flanks of the cold formed material improves resistance to wear and corrosion with the added benefits of enhanced surface tension yield and improved shear strength – highly important in Pailton Engineering's safety critical components.
In order to produce the serrations at either end of the steering shafts, two LMT Fette heads are carried on the turret of the sliding head lathe to produce one at the main spindle; the other after the shaft is automatically transferred to the subspindle. Meanwhile, a single LMT Fette head is mounted on the turret of the Biglia lathe for the bevel gear shaft. Each rolling head carries three hardened rollers with the appropriate serrated form ground on the periphery. To speed resetting between batches, Pailton's setters leave the heads assembled with the rolls and simply swap each complete head as required, according to the part being machined.
In operation, the rolling head is not driven and the rollers are free to rotate. The machine is programmed to axially feed the head forward to engage the workpiece and create the form required direct to size. As the dimensioned length of the serration is reached, the machine feed is halted, the head continues its motion against a spring for about 2 mm and the rolls released. The head then retracts and is reset against a stop.
There are three shafts produced out of EN8 bright bar and each serration is pre-turned to a 0.025 mm tolerance in order to ensure the final rolled size is maintained. One shaft is produced out of 25 mm bar, 400 mm long. It has a hobbed nine tooth spline at one end and 48 serrations rolled by the LMT Fette head at the other to a tolerance of 21.51/21.54 mm by 26.5 mm long. This is machined at 450 revs/min and 3.4 m/mm feed. A steering shaft produced out of 25 mm material by 275 mm long has 48 serrations, 20 mm long at one end and 16 mm long at the other. Each is rolled to an 18.42/18.44 mm tolerance at 450 revs/min and 2.9 m/min feed. A lower shaft produced from 19.5 mm bar by 485 mm long has 36 serrations at each end rolled to 15.22/15.24 mm diameter at 550 revs/min and 2.4 m/min feed. One serration is 35 mm long the other 9 mm.
On all operations neat oil is used, with the sliding head machine fitted with an 80 bar high pressure coolant system. In order to prevent problems from swarf being imbedded in the rolled surface, a 50 micron filtration unit has been installed. According to cell leader Steve George, the life of a set of LMT Fette rolls is around 50,000 parts – the same as the rolls on the dedicated machine. However, replacement costs are £275 a set against £1,000 for the dedicated machine rolls. When questioned over any possibility of ‘push-back' into the machine due to axial forces as the rolls engage, Mr George maintains this would only be a problem if the bar material was significantly undersize.
On the Biglia lathe, the bevel gear shaft is turned complete and serrations produced at 400 revs/min with a feed rate of 2.7 m/min. Here 6 per cent soluble oil mix is used as coolant. Concluded Mr Rawbone: "The combination of these operations into single cycles has had a dramatic effect on the cost of production, and particularly lead time, which has made us a lot more competitive and responsive to customers. It really is a great motivator to continue our investment programme."
SOURCE: LMT Fette