automotive sector report
New manufacturing process uses a shoulder milling cutter for the rear axle tube of a Mercedes van.
Shoulder to the wheel
MERCEDES-BENZ Works in Kassel was
having trouble with the manufacturing of the
axle tube for a van in its current range. Trials
revealed that turning the axle tube as planned
was not a reliable process, and an alternative
solution was necessary. ‘Any downtime, even if
it seems inconsequential in itself, results in high
costs both in the long term and overall’ says
Markus Groppe, Milling Products Manager at
Sandvik Coromant Germany. ‘Solutions to
minimise downtime permanently have to be
tailor-made to match the situation.
Interruptions to the production process have to
be analysed in detail, and need everyone
involved to search for creative solutions.’
A wide variety of axle tubes are
manufactured on automatic production lines.
At the beginning of the process, the various
parts to be mounted on the axle tube are joined
to it, in this case by friction welding. This
produces a double-bulge weld bead, which has
to be made cylindrical in diameter for further
manufacturing steps. Initially, the Kassel
engineers tried to achieve this by conventional
turning. However, when the weld bead was
machined away, two circumferential burr rings
were produced. These burr rings were a
problem because an automated line cannot be
stopped in mid-operation for manual removal
of rings or chips. Process reliability and
unmanned operation would be severely
impaired, and it would no longer be possible to
guarantee the output of the plant. To make
matters more difficult, it was absolutely
essential not do damage the external shape of
the axle tube under any circumstances, as this
would be a fracture and would lead inevitably
to the part being rejected.
The purpose of optimising the machining
process was to prevent the burr rings from
forming, thereby reducing downtime. It was
then left to the responsibility of the Tool
Management team at the Mercedes-Benz Works
in Kassel. The team is a group of some 30
employees who manage everything to do with
tools: new planning, process development and
production optimisation to job organisation,
tool stock management and regrinding. Initial
attempts with all kinds of turning strategies,
and tools and inserts in all possible geometries,
failed to bring the desired result. ‘The best ideas
often come outside working hours’ says Carsten
Klinge, the team leader at Sandvik Coromant
responsible for Daimler and who belonged to
the project team on the tool manufacturers’ side.
july 2009
‘After a day of unsuccessful tests, the idea
occurred to me on the way home of attempting
turn milling.’
At that time Sandvik Coromant was carrying
out market testing on the new CoroMill 490
shoulder milling cutter, which had already
shown itself to be particularly easy-cutting
with low power consumption. ‘The CoroMill
490 is a particularly suitable milling cutter for
use with drive units on lathes and smaller
machining centres like the Montforts
MultiTurn RNC-500 used here,’ says Groppe.
However, to avoid burr rings completely, a
special process was necessary; each friction weld
bead was first machined down to the finished
dimension at two diametrically opposite places.
This means that the subsequent turn milling
produced two semi-circular rings that simply
fell off the work-piece. The turning/milling
operation required a detailed preliminary study,
since process parameters like the centre offset of
the milling cutter diameter in the Y-direction
had to be determined in order to obtain a
cylindrical shape of the required length.
Naturally, this also had to take into account the
interfering contour of the working space of the
machine and work-piece, in other words a kind
of simultaneous engineering.
Optimised machining
Tools with six GC1030 inserts and a Coromant
Capto C4 tool holder are used, which guarantee
the highest repeat positional accuracy when
changing tools, and short tool changing times.
A cutting speed of 275m/min turned out to be
best. ‘The cutter is changed after 120 workpieces.
Although this is before the end of its
service life, we wanted to be on the safe side,’
says Jörg Eisenberg, tool planner at the
Mercedes-Benz Kassel plant.
This apparently simple solution at first
glance was only made possible by the
knowledge gained of the best way to use the
shoulder milling cutter, coupled with its
capabilities for this particular special
machining task. This knowledge has since been
confirmed by an independent body; a
comparison of milling cutter designs from
well-known manufacturers by the Institut für
Fertigungstechnik und Werkzeugmaschinen
(Institute of Production Engineering and
Machine Tools) at the University of Hannover
certified that the Sandvik shoulder milling
cutter had the lowest feed, normal feed and
axial forces. www.coromant.sandvik.com
Top: the friction
seam weld
before milling.
Bottom: sectional
view of a completed
axle tube.
read more about
automotive sector
at www.mwponline.com
Case Studies
■ Fiat tools up for
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(Keyword site search
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■ High speed testing
(Keyword site search
on Anthony Best
Dynamics)
MWP
23
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