QUALITY & METROLOGY A NOVEL APPROACH TO AN ESTABLISHED PROCESS
very simple. And like any optimisation, you
just keep tweaking things until you minimise
errors to get everything within tolerance.”
The key thing to understand here is that
all important dimensions that will be the
subject of offset update are measured after
the full machining cycle, be that carried out
on one machine or across several machines,
because the operation number, tool and
related dimensions are simply entered into a
table and the optimisation process set
running to produce all offsets for all tools
under consideration. “So, we’ve now got a
system that I think is unique, which does
allow us to optimise and bring everything
close to nominal,” Aberlink’s technical
director underlines.
The key point about offset optimisation
across all important dimensions/tools versus
the one dimension-one-offset-update
approach is that with the historical approach,
updating one tool/dimension at a time can
create errors where dimensions are related.
For example, take a block that has to be
milled around its pro le but which also has,
say, three bores created by the same cutter.
With Aberlink’s approach, all can be brought
as close to nominal as possible, or the most
demanding one made the lead in the
optimisation. So even in this simple one tool
case, bene ts of global optimisation over
trying to de ne equations are clear.
In another example, Eales suggests a twooperation
set-up where in the second
operation a cutter is producing the width of a
block but where an edge is related to a bore
machined in the rst operation. “Now I want
to make two changes. I want to change the
Left: Tool offset and workpiece offset optimisation together
Above: Tool offset and workpiece offset optimisation involving multiple features
overall width, but as I do that, the distance,
say, from one of the edges to the bore centre
is also going to vary. It might be wrong
because the overall width is wrong, or it might
also be wrong because I haven’t set the
offset. So, immediately I want to do two
corrections that are related,” he explains,
continuing: “So, if I add a millimetre onto the
face to make the component a millimetre
wider – half a millimetre each side – then the
distance from one of the edges to the bore
centre has changed by half a millimetre, too.
If the bore position was wrong by half a
millimetre in the same direction, it might be
that I don’t have to change the offset at all,
even though when I measured that bore it
looked like it was half a millimetre out of
position. Alternatively, the bore centre could
be wrong in the opposite direction. So, the
truth is, those two things are obviously
related and need to be considered together.”
MULTIPLE RELATIONSHIPS
In another example, we go back to our block
and consider a single operation where the
left-hand edge of the block is machined with
the same tool that also interpolates a bore in
the block and where a separate tool
machines the right-hand edge. “So again, I’m
in a situation where I want to update tool one
and tool two. I can’t work out what to do with
tool two until I work out what to do with tool
one, and I can’t work out what to do with tool
one independent of the bore.
“We structured this as a single
optimisation, so we don’t try and create all
these equations internally, we simply say,
‘okay, keep tweaking tool offset one and tool
offset two until you get no errors’. And, of
course, that process takes a fraction of a
second for the PC.”
The fundamental reason why Aberlink is
able to apply this optimisation process is
because its measuring machine understands
direction and so it knows what is air and what
is material. “We know what direction that
surface is going to move in after an offset
update. We know from measuring a feature
that it is, say, a left-hand edge and where it
is. If we then know that it has been cut with a
12 mm carbide cutter and make an
adjustment for that cutter based on that
measured edge, we can re-analyse all the
features on the job machined with that cutter
– and similarly any other cutter/dimension/
feature under consideration – understanding
that they have shifted due to tool offset or
workpiece offset changes. And, having reanalysed
all measured features based on
those changes, we can then see the effect on
all measured dimensions and whether they
are better or worse. This process is repeated
until all dimensions conform to the selected
user preference.”
It’s all clearly very clever and a major
advance on established practice, but there is
one simple bene t that the technical director
emphasises: “The one thing that it does that
is probably more important than anything else
is that it gets the sign right – whether it’s a
plus or a minus offset change. It’s often easy
to see an offset needs to move by 10
microns but very easy to move it in the wrong
direction. You can’t imagine how useful that
is for very many people, including those on
our own shop oor.”
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