QUALITY & METROLOGY ABERLINK’S NEW MEASUREMENT MARVELS
under inspection
Horizon is, in fact, the most accurate
CMM ever produced by Aberlink. The
company’s smaller Axiom Too range with
640 mm in X has equivalent figures of TP20
(2.4 + 0.4L/100)/TP200 (2.3 + 0.4L/100)/
SP25M (2.1 + 0.4L/100) micron. Horizon’s
better performance is reflected in the price:
Axiom Too starts at £22,350 ex. VAT versus
Horizon’s from-£32,860 ex. VAT figure,
although this is less than the Zenith 3.
The second CMM, Extol, follows in the
mould of the company’s Xtreme, a hexapodstyle
unit and the world’s first such
commercially available CMM. Getting its big
UK launch at MACH 2016, it has sold 500+
units. Xtreme’s measurement volume is not
defined by three linear axes, and therefore is
not a cube but a somewhat more complex
bounded volume. Extol replaces Xtreme,
building on the hexapod, leg-style approach,
but adopts a delta axis drive arrangement
that does look superficially similar. Again,
the measurement volume is not a cube, it’s
sort of a tapered cone, in fact. But sticking
with the top line stuff to start with, its
measurement volume is X-Y diameter of
370 mm and Z of 270 mm; the accuracy
statement is (2.6 + 0.4L/100) micron
versus Xtreme’s (3 + 0.4L/100) micron. It is
a more compact unit than is Xtreme for the
envelope, too. Price for Extol is £19,950 ex
VAT versus Xtreme’s £16,850 ex VAT figure;
Extol will progressively replace Xtreme.
That’s the data in brief, but the
technology on which these machines’
capabilities rest is the real story, with Extol
the most novel of the two. This unit looks
similar to Xtreme, but the previous non-
Cartesian axis approach utilising linear
motors and mechanical bearings to extend/
contract its six struts that join the frame and
the probe-holding platform is replaced by a
delta system – Xtreme’s six arms are in fact
constrained to move in just three axes by
software (easier to error map).
The new delta approach for Extol sees six
arms once again, but with these in three
pairs of parallel carbon fibre fixed-length
struts. These three pairs connect the probeholding
platform to three belt-driven sliding
carriages running on metrology grade
machine tool-type linear recirculating ball
guides. These guides are positioned in the
vertical, on the inside of the machine’s three
vertical columns connecting CMM base to
the top of machine structure.
Moving from the hexapod to the delta
system was not achieved in one leap,
however, as technical director Marcus Eales
explains: “Although intended for general
application, Xtreme found a ready home in
shopfloor measurement applications. It has
been an eye-opener as to how important the
shopfloor market is, so we started to
consider what would be the ultimate
shopfloor machine. For a long time, we were
designing a machine in box, which was going
to be machine tool-type technology with
similar robustness, employing linear
recirculating ball guides. We made a
prototype, and it was okay. But we started
discussing the idea of using these linear
guides in a Delta mechanism, because there
would be fewer unknowns.
“Essentially, the problem you’ve got with
any machine is that you have a set of
bearings, but you need to move away from
these bearing locations when you extend in
the Y-axis or a Z-axis quill, for example. And
as you as you move away from the bearings,
you get twisting loads, bending loads and
distortions. The nice thing about a delta
arrangement is that all the loads go into the
carriages running on the guideways right in
the middle; there is no twisting or bending
load. And that did prove to be the case in
that almost straight away the first prototype
was extremely repeatable.”
All looked good, then, but it was error
mapping that would present the big
challenge – error mapping is an Aberlink
speciality, it should be remembered, the
company’s established use of aluminium for
its CMM frames is founded on that.
Explains Eales: “It is actually very
problematic on a delta machine, because
you have all the same degrees of freedom
on each of the columns that you get on the
X- axis, Y- axis and Z-axis of a Cartesian
machine pitch, roll, yaw; X, Y and Z=6 per
axis, but the relationship between each has
more unknowns, because it is isn’t simply
that 18 for a 3-axis system plus three
squares between the three axes.
“A Cartesian machine has 21 degrees of
freedom (3x6 per axis)+3 squares. But in
order to work out where the probe tip is, we
need to know the position of the six balls
joints on the platform, the separation of
the two balls on the guides as they go up,
and the length of the six arms. This all ends
up with 42 variables, as opposed to 21.
So, mapping it did prove to be a significant
amount of work, and that is the cleverest bit
of the whole machine. It’s the thing that
Offset update software innovation
In true Aberlink fashion, the machine tool offset update software developed for Extol, but
also available for all the firm’s CMMs, is a step up from the accepted norm. It results
from an analysis of available standard packages that took the ‘one offset-one-dimension’
approach, apart from demonstrating poor user friendliness. Dimensions are often
related, such that any changes in one impacts another, explains Eales. “What we’ve
done is a complete optimisation of all the dimensions and all the things that you want
to control. So that, in theory, we could take a part being machined in several stages on
several different machines, with different offsets for different sets of tools, and at the
end of it come up with a complete update for all the changes, based on optimising all the
selected dimensions.” The system is easy and fast to use, so that it is useful to firms
running small batches, Aberlink’s main market. (Full story next issue)
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