forming & fabrication
In-cycle roll-forming provides a
new direction for steering
system maker.
Heads
will roll
A saving of over £90k pa has been achieved, with
greater control over quality in safety critical components, at
Pailton Engineering. The vehicle steering systems specialist
has focused on investment-driven improvements. For
example five operations are now combined into one for
steering shaft production, a turnkey installation from NC
Engineering giving a complete production cycle from bar
to assembly-ready parts using a Star SV32 sliding head auto.
The new method brings cutting-off, turning, spline
hobbing, serration rolling and cross hole drilling operation
into a single one-hit cycle. Output is up from 25 to 60
completed shafts per hour, lead time is dramatically
reduced over the previous individual job routing and the
company can accommodate a progressive increase in
demand of parts from 1,000 to 1,500 per month.
In combining the operations, LMT (UK) supported NC
by providing 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 process, using
an LMT Fette F3K rolling head, replaced separate manually
operated hydraulic powered rolling operations on a
dedicated machine. The success of this operation prompted
similar re-engineering of production of a bevel gear shaft
with 36 serrations for 900 bevel steering boxes. Based on use
of a 4-axis Biglia B501MB turning centre, number of ops
performed on the part was cut from four to two – the
second op being the dedicated shaping of the bevel gear.
The process of producing serrations, splines or threads in
a single rolled or ‘chipless’ cycle can reduce the process time
for such features by up to 90%. 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 -
important in safety critical components.
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: 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
Steering shafts are
produced in a single
cycle on a Star sliding head
auto using an LMT Fette
F3K serration rolling head;
and Pailton is also rolling
36 serrations on its bevel
steering box gear shafts incycle
on a Biglia B501MB
turning centre
read more about
forming & fabrication
at www.mwponline.com
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92 MWP november 2007
heads assembled with the rolls and simply swap each
complete head as required.
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.
Three shafts are produced from EN8 bright bar and each
serration is pre-turned to a 0.025mm tolerance 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 at the other to a
tolerance of 21.51/21.54mm by 26.5mm long. This is
machined at 450revs/min and 3.4m/mm feed. A steering
shaft produced from 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
450revs/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 550revs/min and
2.4m/min feed. Serrations are 35mm and 9mm long.
On all operations neat oil is used, with the slider 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, George
maintains this would only be a problem if the bar material
was significantly undersize.
On the Biglia, the bevel gear shaft is turned complete
and serrations produced at 400revs/min with a feed rate of
2.7m/min. Here 6% soluble oil mix is used as coolant. ‘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’ says senior production engineer
Jim Rawbone. It really is a great motivator to continue our
investment programme.’ www.LMT-tools.com
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