machining
Aerospace subcontractor discusses 5-axis versus 3+2-axis machining,
and the importance of volumetric accuracy.
Keeping it simple
Oldland CNC
machines this aircraft
gearbox bracket on five
sides from solid aluminium
in one hit on a Hermle C
600 U 5-axis vertical
machining centre using a
3+2-axis strategy
AN established practitioner of 5-axis machining
for medium-size aerospace parts, in 2006 Oldland CNC
decided to extend its use of the technology to
manufacture smaller components falling within a
nominal half-metre cube. The Bristol subcontractor
chose a Hermle C 600 U vertical-spindle machine with
600 by 450 by 450mm working envelope and 30-tool
magazine. Whereas Oldland extensively uses fully
interpolative 5-axis profiling for producing aircraft parts
that incorporate complex surfaces, full 5-axis machining
on the Hermle accounts for only 10%of production. The
remaining 90% is 3+2-axis work, with two axes clamped
while the others perform the cutting cycle.
Comments managing director John Tucker: ‘Smaller
aluminium brackets on aircraft do not generally
include complicated features needing full 5-axis
machining. However, simpler geometries benefit just as
much from being milled on five sides in one hit using a
5-axis machining centre rather than in several set-ups
on a 3-axis machine. We regularly evaluate components
being produced on our 3-axis machines, looking for
parts that will benefit from being positioned under
CNC in a sequence of compound angles on the Hermle
and milled using the other three axes. This approach
can save one or two re-clampings and subsequent
operations, leading to significant savings in production
costs, particularly for larger batch work.’
Engineering manager Steve Hayes suggests that moving
all five axes simultaneously should be a last resort, as any
5-axis machine runs more productively in 3-axis mode.
Complex calculations that the control has to carry out
prevent the use of fast feed rates in full 5-axis profiling.
Furthermore, the machine is only as quick as its slowest
axis, usually the rotation of the table, so if that is one of the
axes that are clamped, machining speed is increased.
High accuracy is a priority at Oldland, a CMM being
used for inspecting every component before delivery. A
feature of Hermle machines that Tucker particularly likes,
and believes promotes precision machining, is the mineral
casting that forms the bed. It has good dynamic stability
and up to ten times better vibration damping
characteristics than steel or cast iron, especially at high
spindle speeds and feeds. ‘It is not a finely powdered
mineral either - you can see large pieces of granite
embedded in the compressed matrix if you look at a cutaway
section of a bed in the Hermle factory, as we did.’
Another design attribute he appreciates is Hermle’s
patented arrangement of three staggered Y-axis
guideways, one of which is positioned between two
outer ways and set further back. This effectively halves
the span and gives stable traverse due to three-point
52 MWP november 2007
support combined with a single drive in line with the
central carriage on the gantry.
‘The name of the game when producing high accuracy
components, especially large ones, is not repeatability but
volumetric accuracy of the machine tool’ continues
Tucker; ‘manufacturers are often reluctant to disclose it,
content to quote positional accuracy and repeatability in
their catalogues. Interestingly, Hermle does not appear to
publish any accuracy figures at all; perhaps they do not
need to. In any case, for all of our machine tools we
measure volumetric accuracy ourselves using a calibration
ball and probe in the spindle. It is the only way to
determine how accurate features in a component will be,
relative to one another, if they are machined in
diametrically opposite corners of the working area. When
we checked the C 600 U, it had a total volumetric
accuracy of 50 microns over the entire machining
envelope, which I regard as excellent for a machine of this
size moving in five axes.’
Most work going through Oldland’s machine shop
(working round-the-clock from 6.00 am on Monday
until 4.00 pm on Friday) is for the Airbus family
(A300/310, A320, A330/340 and A380). The Hermle’s
maximum 16,000revs/min spindle rotation is
frequently used, although 12,000 to 14,000revs/min is
more usual, the elevated speeds enabling cutting feed
rates between 5 and 10m/min, leading to high metal
removal rates. (A 40,000revs/min spindle is an option).
Productivity is further enhanced by acceleration of
5m/s 2 to 35m/min rapid traverse in all linear axes, and
5.5 seconds chip-to-chip time.
Tucker cites three main reasons for choosing this
machine. First, the ±115 o swivelling trunnion carrying
the 280mm diameter rotary table is an integral part of
the machine construction, with the trunnion bearings
set directly into the sides of the bed casting. Some other
machines offered a less robust solution involving a 2-axis
compound table bolted to the bed. Secondly, all 24
machining centres on the shop floor have Heidenhain
controls, so to facilitate operators swapping between
machines, the new VMC had to be fitted with a control
from that supplier - in this case the Ethernet-enabled
Heidenhain iTNC 530 (although the Siemens S840D is
available as an option). CAE software in use at the Bristol
site for off-line program generation includes Catia V5
CAD, MasterCam and CG Tech’s Vericut for simulation
and tool path verification. The third reason given for
choosing Hermle was Geo Kingsbury’s good reputation
for after-sales service, which has been confirmed in the
first nine months of C 600 U ownership.
www.gkholdings.com
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