CNC automatic lathe rolls serrations too

By combining the rolling of serrations on steering shaft with metal cutting operations on a CNC automatic lathe, a manufacturer is saving over GBP 90,000/year.

UK vehicle steering systems specialist Pailton Engineering is saving over GBP 90,000/year by combining five operations into one on a Star SV32 sliding head automatic lathe, including the rolling of serrations on steering shaft using a LMT Fette F3K rolling head.

The company has thereby achieved greater control over safety critical components quality.

Pailton Engineering's senior production engineer, Jim Rawbone, said he 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 led to an investment in manufacturing of some GBP 600,000 in the first half of 2007.

Managing director John Nollett added: "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".

* Turnkey installation - 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 CNC sliding head automatic lathe.

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/h to 60/h, lead time was dramatically reduced over the previous individual job routing.

These actions enabled the company to progressively increase output parts from 1,000 to 1,500/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, according to a report sent to Manufacturingtalk.

With the help of LMT (UK), working on a 4-axis Biglia B501MB turning centre, 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.

* Rolling serrations, splines or threads - 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%.

With the process, the compressed microstructure of the material as it is displaced, rather than removed, and so develops a stronger form.

The burnishing effect on the flanks of the cold formed material improves resistance to wear and corrosion.

LMT Fette said that it also 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 up 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 2mm 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 0.025mm tolerance in order to ensure the final rolled size is maintained.

One shaft is produced out of 25mm bar, 400mm 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 21.51/21.54mm by 26.5mm long.

This is machined at 450 rev/min and 3.4m/min feed.

A steering shaft produced out of 25mm material by 275mm long has 48 serrations, 20mm long at one end and 16mm long at the other.

Each is rolled to an 18.42/18.44mm tolerance at 450 rev/min and 2.9m/min feed.

A lower shaft produced from 19.5mm bar by 485mm long has 36 serrations at each end rolled to 15.22/15.24mm diameter at 550 rev/min and 2.4m/min feed.

One serration is 35mm long the other 9mm.

On all operations neat oil is used to lubricate th tooling.

The Star sliding head automatic is 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.

Cell leader Steve George said 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 GBP 275 a set against GBP 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, George said this would only be a problem if the bar material was significantly undersize.

* Bevel gear shafts - on the Biglia lathe, the bevel gear shaft is turned complete and serrations produced at 400 rev/min with a feed rate of 2.7m/min.

Here 6% a soluble oil mix is used as coolant.

Concluded 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".

* Star and Biglia - Star CNC sliding head lathes are distributed in the UK by Star Mirconics GB.

Star Micronics and the Italian machine tool builder, Biglia, are registered on Manufacturingtalk Web site.

* About Pailton Engineering - originally founded in 1969 to provide a precision toolmaking service and subcontract machining operation, Pailton Engineering now employs 152 people including six apprentices, seen as key people for future expansion.

There is also a night shift of 28 at Coventry.

Since 1982 the company has progressively developed new products with the introduction in that year of its own range of universal joints and sliding shafts.

During 1985 the company moved into design and manufacture of adjustable steering columns for the truck industry and by 1988 this was expanded into the bus sector.

A major acquisition in 1992 brought the Lockhead Steering Division of Automotive Products into the fold, expanding production into ball joints, draglinks, tie rods and anti-roll bars.

By 1999 offices in America and Germany secured new business and two years later, the acquisition of SLT Steering Division of Sachsenring added a 20,000ft2 manufacturing facility in Germany that currently employs 22 people.

Now with a group turnover of some GBP 11.5 million, the Coventry, UK, 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.

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