25-01-2013, 02:18 PM
New Rotating Dynamometer for the Analysis of
High Speed Cutting Processes
New Rotating Dynamometer for the Analysis.pdf (Size: 883.49 KB / Downloads: 49)
Abstract
Piezoelectric measuring technology has already been used successfully in
conventional metal-cutting processes in order to investigate and further
develop machining processes, tools and machine tools. In addition to the
stationary small Dynamometer MiniDyn, there is now also a rotating highspeed
Dynamometer available for clamping in machine tool spindles for
research and investigation into applications in the new field of high speed
cutting technology (HSC). This is the subject of the following article, which
deals in depth with the Dynamometer construction, the type testing
undertaken and the safety aspects.
Introduction
High speed cutting (HSC) is a technology
which today is already in widespread use
and increasingly displacing conventional
machining technologies. High metal
removal rates, improved surface quality,
machining of hardened components and
the possibility of producing geometries it
was previously impossible to manufacture
(thin walls) within a short time, are the
reasons for this success. These advantages
are clearly depicted in Fig. 2.
It has been possible to counter to a great
extent the reduction in tool life by
appropriate expenditure on research and
development. The following cutting
materials have thus been developed which
must be carefully matched to the workpiece
material and the machining method:
sintered carbide, extremely fine-grained
sintered carbide, cermet, ceramic, CBN and
CBN metal ceramic. All these tool materials
are coated according to use.
From the processing aspect, HSC is used for
turning, milling, drilling, reaming, thread
cutting (milling and drilling) and grinding.
Fig. 3 gives an overview of the combined
advantages of HSC.
Bild 1: HS-RCD
Fig. 2: Characterization of high-speed machining
Increase of
Quality
- Better surface
- Lower cutting forces
- Vibration poor cutting
Decrease of costs
- Shorter manufact. time
- Shorter process chain
- Dry machining
Shortening of manufacturing
- Higher cutting velocity
- Higher feed velocity
- Higher cutting volume
Combined advantages of HSC
Fig. 3: Advantages of HSC
Figure 4 shows how the machining time can be reduced by reducing the manufacturing time
and by shortening the process chain with HSC.
Fig. 4: Reduction of the manufacturing time with HSC [1]
characteristic of force/vibration and
AE-sensors [4]
In the past, cutting force measurements
made it possible to assess conventional
metal-removing machining methods safely
and accurately, as well as to further develop
them. For this purpose, rotating
Dynamometers (built into machine tool
spindles) were employed in research and
development along with spatially fixed
Dynamometers (screwed onto a machine
table). AE-sensors (acoustic emission) were
used for the acquisition of structure-borne
sound signals – primarily for monitoring
purposes. This measured variable is
described in detail in [8].
The reasons why force measurements are
so successful include the relatively simple
interpretation of the signals. Furthermore,
the signal-to-noise (S/N) ratio of force
sensors for the chip thickness range of >0.1
μm is considerably higher than, for
example, an acoustic emission (AE) signal
Measuring devices for high speed
machining need to have a natural frequency
which is above the frequencies occurring
during machining. Moreover, the measuring
instruments must supply a reproducible
measuring signal even with the smallest
machining forces. The spatially fixed
MiniDyn and the rotating HS-RCD
described below meet these requirements
thanks to their high rigidity and the
piezoelectric measuring principle.
Rotating High Speed Dynamometer
A rotating cutting force Dynamometer is
fitted in a machine tool spindle and
measures the forces acting on the rotating
tool. The data measured are thereby
amplified directly in the Dynamometer and
transferred by wireless transmission to a
stationary receiver (Fig. 6).
Signal Conditioner Data acquisition
PC
DynoWare – Data
acquisition software
Machine spindle
HS-RCD measuring chain
The advantages of the rotating Dynamometer
are:
• Measurement at the rotating cutting
edge, i.e. measurement of the action
force close to the cutting process during
cutter engagement without measuring a
reaction force.
• Constant mass of the measuring
instrument during the cutting process
and thus constant natural frequency, in
contrast to the stationary Dynamometer,
where the mass of the workpiece
constantly reduces during metal removal.
• Low and, during machining, constant
moments of inertia
• The Dynamometer can be positioned
anywhere during machining
As shown in Fig. 2, the cutting forces
during high speed machining reduce with
increasing speed. The new 2-component
Dynamometer (Fz, Mz) can be used to
quantify these forces in different materials
and machining processes.
High speed machining is defined by the
cutting speed vc. At the maximum speed of
25’000 rpm, the HS- RCD equipped with a
D = 10mm tool operates at vc = 785 m/min
and therefore for many materials already
operates within the range of high speed
machining (see Fig. 7). When a tool with
D = 20mm is used, the cutting speed is Vc =
1,570 m/min.