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MECHANIK NR …/201… …
Adoption of the Omative system in Inconel 718 turbine
blade machining
Zastosowanie systemu Omative w obróbce łopatki turbiny ze stopu Inco-
nel 718
JAN BUREK
PAWEŁ SUŁKOWICZ
MICHAŁ GDULA
JAROSŁAW BUK
MARCIN SAŁATA * DOI:https://doi.org/10.17814/mechanik.2017.1.3
This paper presents a research focusing on adopting
adapting control system Omative for tool condition
monitoring during milling of Inconel 718 turbine
blade.
KEYWORDS: milling, turbine blade, adaptive control,
Inconel 718
The modern aviation industry is increasingly demand-
ing its components, while striving for maximum efficiency
and maximum profit. New construction solutions make it
necessary to use special materials such as heat-
resistant super alloys. These are alloys based on nickel,
cobalt or iron. Of these, one of the best properties is
Inconel 718 [1, 2].
The advantages of this alloy are particularly important
in those engine points that are subjected to the largest
loads. The turbine blades work in the most difficult tem-
perature and load conditions. The blade end speed
reaches 390 m/s, the gas temperature is even 1200 °C
and their speed is 600 m/s. The turbine blade material, in
addition to high strength, must be characterized by high
heat resistance, high temperature creep resistance, cor-
rosion and oxidation resistance and high hardness. Den-
sity of the alloy, which affects the weight of the engine, is
also important in generating centrifugal forces [3, 4].
Inconel 718 is one of the hardest materials to work on.
Blade locks are currently being successfully developed in
the Creep-Feed Grinding (CFG) process. This method
allows for efficient machining of elements made of super
alloys and other hard-working materials. It allows the
parts to be polished after heat treatment and ensures
high surface quality [5]. On the other hand, in the case of
free surfaces of the blades, the method of their execution
is simultaneous process of five-axes milling. Due to the
complexity of this machining, the presence of high milling
strength components and the high cost of the workpiece
and tooling, it is legitimate to use a system that monitors
the correctness of the cutting process. They are based
on the measurement of selected physical quantities,
such as: cutting force, vibration, power and engine
torque, sound emission or coolant flow. Measured sig-
nals (after processing) serve to obtain process measures
* Dr hab. Inż. Jan Burek prof. PRz (jburek@prz.edu.pl), mgr inż.
Paweł Sułkowicz (sulkowicz@prz.edu.pl), mgr inż. Michał Gdula
(gdulam@prz.edu.pl), mgr inż. Jarosław Buk (jbuk@prz.edu.pl),
mgr inż. Marcin Sałata (msalata@prz.edu.pl) – Katedra Technik
Wytwarzania i Automatyzacji, Wydział Budowy Maszyn i Lotnictwa
Politechniki Rzeszowskiej
that enable application of adaptive AC control and detec-
tion of tool contact with workpiece, alarm condition or tool
wear [6, 7].
Test conditions
The purpose of the study was to use Omative system
to monitor tool status while milling the Inconel 718 alloy.
This system is based on recording the Iw current signal or
the linear axis motors of the machine - on this basis level
of the tool wear in real time can be determined.
Experiments were carried out at a research station
based on the DMU 100 monoblock machining center by
DMG (Fig. 1).
Fig. 1. Test stand: 1 - toroidal cutter, 2 - workpiece, 3 - divider, 4 -
control system with Omative system
The research was carried out during the milling of the
turbine blade made of Inconel 718 alloy using toroid 5-
blade cutter of d = 50 mm from Sandvik Coromant. The
cutting parameters were: cutting speed vc = 40 m/min,
cutting depth ap = 1.15 mm, cutting width ae = 30 mm,
feedrate per blade fz = 0.15 mm/blade. The workpiece
was fastened in a divider using pre-tensioning. The
machining program was done in the NX10 system.
Test results
The test was started from milling of the workpiece in
the teaching mode of the Omative system. For this
purpose, a number of parameters should be determined,
including: tool and workpiece parameters, load limits,
type of monitoring strategy and machining conditions.
Then, the tool was milled with a new tool until it was
+ MECHANIK NR .../20...
worn. As a result, the so-called load-learning curve was
recorded, which is the basis for using the system to
monitor the tool wear. Next, the type of strategy should be
determined – Maximum Load Monitoring or Load Band
Monitoring. In the case of the former strategy during
machining, the system compares the currently measured
spindle load with the maximum load recorded in the learning
mode (every 100 ms). On the other hand, the latter strategy
enables the real-time comparisons of the measured load
with that recorded in the learning curve (Fig. 2).
Fig. 2. Comparison of tool status monitoring strategies: 1 - alarm
level, 2 - warning level, 3 - maximum recorded load, 4 - learning
curve
After the learning mode execution, a monitoring
strategy in the learning curve was adopted to the study.
After entering the appropriate commands of Omative
system into the NC software and specifying the learning
curve bandwidth, that must be greater than the spindle
load at the preset speed without machining, milling was
started with the active tool status monitoring mode (Fig. 3).
a) b)
Figure 3. Omative monitoring window during milling (a) and new
toroid milling view (b)
Tool status monitoring with the Omative system allows to
detect: tool wear, excessive milling load (caused by e.g.
oversize or local material hardness), tool failure or fracture.
In addition, there is the possibility of early detection of
programming errors or workpiece setting and, consequently,
damage to the tool or machine. During coarse milling of the
blade, 60 cutting edges were used. The warning level
has been reached ten times, which means that the
system has detected tool wear, but allowed the program
to complete the machining course - stopping to change
the milling cutter occurred when it was outside the
material (Fig. 4). The alarm level was reached twice,
which resulted in the immediate machine stopping and a
tool change message was displayed. The average
working time of the cutter was similar to that declared by
the manufacturer and amounted to approximately 42
min.
a) b)
Fig. 4. Omative monitoring window after detecting tool wear (a) and
view of worn toroid milling cutter (b)
Conclusions
The use of the Omative system can improve the
cutting performance by adjusting the tool running time to
actual machining conditions and thus prolonging its
service life as compared to the manufacturer's
recommended fixed working time. In addition, this
system, by constantly monitoring the spindle load, in
some cases is able to protect the tool and machine from
damage. Due to the use of the spindle current signal for
load monitoring, it is not necessary to install additional
sensors in the working space of the machine, which are
often expensive and inconvenient for assembly.
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