process development
Sample superabrasive
grinding points -
footnote reference [1]
design and manufacturing strategies, process parameter
optimisation and workpiece quality/integrity
validation. The role played by Advantage West
Midlands (AWM) has been critical in providing support
for appropriate capital equipment for the latest phase of
research and the contribution of ancillary suppliers
such as System3R and Agie-Charmilles for workpiece
fixturing/damping and workpiece preparation has
been invaluable.
By itself, point grinding provides only part of the
solution to slot/root manufacture. Pre-slotting either
using creep feed grinding to rapidly form a truncated
V-shaped channel, HSM of the same or to produce a
preform slot (using a full form end mill), or wire EDM
to cut and undersize full form slot, are all feasible
options. Over the past 15 years, EDM generator
technology combined with developments in coated
wires has seen cutting speeds increase five fold such
that the Charmilles Technologies Robofil 240cc or Agie
Progress are capable of cutting rates of 500mm 2/min.
Furthermore, they are able to work unattended round
the clock.
CBN outperforming diamond grit - so far
Equipment currently used in the point grinding
evaluation to machine slots in Udimet 720 nickel based
superalloy includes two Matsuura high speed
machining centres: an FX-5 and LX-1. The first of these
has been retrofit with two Faemat spindles, an FA80H
rated at 3kW operating at up to 60k revs/min and an
FAI102C rated at 4.5kW able to operate at up to 90k
revs/min, together with their ancillary cooling,
lubrication and control equipment. Work on the LX-1
linear motor machine has utilised its own
60,000revs/min spindle (3kW). The unit is also fitted
with Renishaw NC3 laser tool setting equipment.
Modifications were made to both machines to
incorporate high pressure, high flow rate fluid supply,
mist extraction and CO2 fire suppression systems.
Although water based cutting fluid has been evaluated,
all mainstream tests have utilised Oel-Held Sintogrind
polyalphaoleofin (PAO) synthetic mineral oil supplied
via two ‘Cool-Grind’ laminar flow nozzles from a twin
filter pump system. Grinding points made by Saint-
Gobain Abrasives have incorporated both diamond and
CBN grit which varying from 46 to 151μm. Producing
repeat batches of grinding points to the required
form/geometrical accuracy has involved significant
technical development and finance at SG, where the
task of co-ordinating technical input and
38 MWP january 2008
manufacturing resources across the UK/Europe and the
USA in order to meet the demanding requirements, has
been the job of the European Superabrasives Technical
Director, Andreas Grimm.
Performance wise, best results to date have been
obtained with CBN as opposed to diamond grit,
although for other workpiece materials this may
change. As one would expect, grinding point wear
progress has been characterised by an initial ‘wearing
in’ phase followed by a relatively flat response over an
extended period equivalent to the finish machining of a
significant number of slots. In production, preconditioning
of the grinding points would in all
likelihood be necessary in order to achieve the required
accuracy and workpiece surface quality from the start.
Tests involving the 90,000revs/min spindle produced
lower wear than for 60,000revs/min, most probably as a
result of the smaller undeformed chip thickness with
knock-on effects relating to forces and power.
There are however practical considerations relating to
the use of such high spindle speeds not least spindle life
and reliability, particularly in an industrial setting.
Quoting workpiece surface roughness results in the
absence of detailed operating parameter values, where
variations in grit type, size, morphology, feed rate etc.
are critical, would be an arbitrary exercise, however
suffice it to say that values approaching Ra 1.0μm were
obtained. On the workpiece integrity front, sample
finished root slots produced on the FX5 exhibited little
change in microhardness towards the workpiece surface
compared with the bulk and even rough ground slots
produced with a mix of up and down grinding have
shown minimal hardness variation.
The research work is still on-going with more indepth
studies planned relating to workpiece residual
stress and fatigue performance. At this moment in
time, the process shows promise and may in future
supplant broaching for disc slot production, however as
anyone working with the aerospace industry knows,
process change is generally and understandably
measured, even when the cost case may be attractive.
For further reading see;
[1] D.K. Aspinwall, S.L. Soo, D.T. Curtis, A.L. Mantle,
2007, Profiled superabrasive grinding wheels for the
machining of a nickel based superalloy, Annals of the
CIRP-Manufacturing Technology, Vol. 56/1, pp. 335-
338.
[2] D.T. Curtis, D.K. Aspinwall, S.L. Soo, C. Davis, A.L.
Mantle, Performance of electroplated CBN grinding
points machining Udimet 720, Proceedings of the
International Symposium on Advances in Abrasive
Technology X (ISAAT 2007), September 2007,
Dearborn, USA, pp. 383-388.
www.matsuura.co.uk
www.agie-charmilles.com
www.saint-gobain.com
www.wendt.3i.com
www.haefeli-info.ch
www.ncmt.co.uk
www.jones.shipman.com
www.advantagewm.co.uk
www.system3r.com
www.oelheld.de
To view a Ceros magazine you need to have JavaScript enabled in your browser and have the flash player 8 or higher installed. To install the latest flash player, click the icon above, install the flash plugin and then click here to refresh this page.