QUALITY & METROLOGY ABERLINK’S NEW MEASUREMENT MARVELS
gives us a good head start in the market,
because it definitely isn’t a trivial exercise.”
Of course, the technical director stops
with the detail here, simply adding that the
company has worked with a non-UK third
party to develop a camera-based solution to
capture the data, which is not only a huge
amount but must also be captured in a
suitable timespan, so as to avoid seeing the
introduction of errors due to temperatureinduced
movement over time.
Returning to the impressively high
accuracy of Extol, Eales underscores this
with an example. A 150 mm diameter ring
gauge was measured using a Renishaw TP20
probe and saw roundness of under 3
microns, around 2.8, in fact, with a trilobal
shape visible. “Normally, you’d say that
lobing was the probe, but
because the machine itself has
got three axes, we thought it
could be the machine. So we
indexed the probe 60° and, sure
enough, the lobing also moved.
We have developed software to
map such lobing, which requires
64 points, and that will go
across all our machines. In this
example, the probe was
responsible for 1.8 microns
of the lobbing, with the rest
noise. If you are measuring
down to very low values,
compensating for lobing
is likely to be of interest.”
Of course, this sort of accuracy has to be
maintained in a varying shopfloor
environment. The Xtreme did this by using
waste heat from its linear motors to heat the
struts to a constant 40 °C. Extol adopts
another approach, employing five sensors
within the structure to monitor temperature
change. If there are any faster than
acceptable changes in temperature,
a warning message is displayed saying that
measurements could be unreliable. In fact,
in the X-Y planes, Extol is very temperature
stable, due to the carbon fibre arms. In Z, the
steel frame expands as steel does, so any
movement can be easily compensated. Extol
provides measurement data as if it had been
taken in a 20 °C environment.
SUITS THE SHOPFLOOR JUST FINE
As to basic ruggedness to cope with a
shopfloor environment, the machine weighs
in at 180 kg, with Aberlink saying it is
engineered to a level in excess of what is
theoretically necessary. But should any
damage occur to the carbon fibre arms/
platform assembly, onsite replacement and
error map update are possible (the carriages
and slideways cannot be replaced onsite).
This is important, because Aberlink exports
some 70% of its production, selling through
agents around the world. Aberlink managing
director Gavin Bailey emphasises Extol’s
robustness by comparing it to the Xtreme,
which gives the impression of
being more so. “Each of those six
struts in Xtreme is telescopic with
a linear motor and scale inside,
with rollers and mechanical
bearings; it’s a complex assembly on each
strut. On the Extol, the struts that you see
are just fixed carbon fibre struts; the only
moving parts are the mechanical guides,
which are proven in the machine tool
industry. The integrity in a nasty environment
is much better than for the Xtreme.”
Moving on to the seemingly traditional
cartesian-style bridge-type CMM, Horizon, and
the innovation here is the application of
linear motors. Eales again: “People will quote
speed and various other advantages, like low
friction speed, no maintenance, as there are
no moving parts and low noise, but the single
most important thing, from my point of view,
is that you get a very pure thrust on the
moving axis. In driving a CMM, what we are
trying to do more than anything else is to not
put any loads in a direction other than the
direction you want to push the machine in.
And what you get with linear motors is 100%
thrust in the direction you want, with no side
loads at all. So, from a metrology point of
view, linear motors are great, and to me this
is their single biggest win.” Fast, accurate
measurement is also supported by the linear
motor set-up, too, with the typical 20 mm/
min scanning speed of its other CMMs
becoming even as high as 100 mm/min,
although there will likely be some drop-off in
accuracy at that speed.
But linear motors come with an issue;
heat generation. To overcome this, the linear
motors driving the X and Y axes are
positioned a long way from the structure.
Having eradicated one issue, the solution
delivers another challenge, that of now
having to drive the structure a long way from
the centre of gravity of metrology, which
means introducing twisting. To deliver
thrust to that centre of gravity in X and
Y, it is transmitted via carbon fibre
structures that are completely
independent of the CMM structure.
Finally, should the CMM need to be
housed in an uncontrolled environment,
then Aberlink’s automatic temperature
compensation ensures that
measurement results are reported as if
parts are measured at 20°C. ■
The Horizon is the most accurate CMM ever produced by Aberlink.
When fitted with the Renishaw SP25M scanning probe, the firstterm
accuracy statement is just 1.75 micron. It employs linear
motors and a novel force transmission system to deliver their drive
force to the centre of gravity of metrology
18 November 2020 | www.machinery.co.uk | MachineryMagazine | @MachineryTweets
/www.machinery.co.uk