JEC COMPOSITES MAGAZINE - Issue #144 - January/February 2022 - 22

MANUFACTURlNG equipment
bre composite replacements. At the same
time, the company has been actively
talking to machine tool companies, flying
the additive processing and composites
flag, educating machine designers on the
potential that CFRP can offer the machining
performance, as well as offering
guidance to the best practises to design
and integrate the material that can be
used for optimal investment payback.
Over this time, CompoTech has progressively
perfected composite structural
designs and constantly developed its
own automated production process
to meet the specific needs of these
requirements, making improvements for
composite application in machine tools.
Today, there are some very successful
examples of composite machine tools
and acceptance from the industry of the
use of composites.
Where are the benefits
The basic principle of using composite
materials for machine tool structures is
stiffness driven, bringing their high stiffness-to-mass
ratio and added damping
properties compared to steel. The table
of unidirectional properties highlights
why the " ultra-high " modulus pitch fibre,
despite the low relative strength to PAN
fibre, is key to replacing steel components
in a machine tool.
CompoTech's automated AFL process is
designed around the optimal processing
of these delicate fibres. Converting the
pitch fibre into a composite beam structure
makes it possible to replace a same
section steel beam with potentially up to
twice the axial stiffness at a quarter the
weight.
Super-stiff structures
Naturally, if you want to cut faster you
need to accelerate your machine tool
faster. To achieve this with the same
accuracy of cut, then the stiffness of the
manipulating structure needs to increase.
However, to improve the overall performance
of a machining system, there
is a more complex balance of natural
frequency and damping to consider.
22
jec composites magazine / N°
Natural frequency, a function
of stiffness and weight
Using the low mass and high stiffness
properties of pitch fibre results in high
natural frequencies of a part. In machine
tool design, it is often the harmonic
resonance that limits the working speeds.
Therefore, this point is vital to bringing
the properties required to make a step
change in performance. Vibration behaviour
can be optimised in several ways.
The dimensions, internal structure and
wall thickness of the part can be adjusted
to tune its natural frequencies, but the
main influence in first frequencies is the
fibre choice and lay-up design.
Damping during machining
One area of particular focus in machine
tool design is the optimization of
vibration damping properties. Carbon
composite parts already provide good
vibration and vibration-damping characteristics
compared with steel or aluminium
alternatives. Vibration is so important
in many machining applications that it
requires special attention during design.
Damping materials,
including
rubber
and cork fillers, can be incorporated
into the structure. Internal high-density
foam reinforcements may be included to
improve the vibration stability of thinwalled
parts. As an added benefit, the
foam can dampen audible frequencies,
with health and safety benefits for the
machine operator.
Light weighting machine tools
Light weighting machine tools is
considered today, however,
it has not
always been high on the list of priorities.
Reducing the weight of large moving
components allows designers to select
smaller ancillary components. And once
a machine tool goes into service, the
reduction in energy costs or increased
life span of such components provided
by this weight reduction can pay back
any additional material costs well within
the lifetime of the machine. Certain
components, especially tools, can offer
new functions or increase a machine's
performance when machining structures
that would normally be outside of its
payload capacity.
Thermal expansion
As a function of the fibre thermal expansion
properties, a shaft or beam can be
designed to have zero thermal expansion
in one direction. For high-speed milling,
the design of a hybrid steel-composite
spindle to reduce thermal expansion
considers the combination of steel parts
and specific placement and angles of
the fibre to be close to zero. Testing has
shown 75% less displacement on the
shaft alone and a 30% decrease within
the spindle motor. This difference can
be significant for milling accuracy and
Thermal displacement of a steel and composite spindle measured over time
144 January - February 2022
JEC COMPOSITES MAGAZINE - Issue #144 - January/February 2022

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #144 - January/February 2022
