machining
If you suggest to a turned parts
manufacturer that it should invest in
a CNC multi-spindle automatic
(multi), the most likely response
would be ‘too expensive’. Not so,
says Ian Sheldon, managing director
of subcontractor Witon Engineering
Pay back time in the West
Witon’s ‘multis’ stay
busy - the key to a
viable business case for
this type of machine
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ALONGSIDE the more familiar line up of around
thirty single-spindle, fixed- and sliding-head lathes,
Witon Engineering’s Barnstaple factory houses four
‘multis’. Specifically these are Index machines: two
MS32C six-spindle models which arrived in 2002/2003;
a 32mm capacity multi added in 2006; and an MS52C,
capable of highly productive turn-milling of parts up to
52mm diameter acquired last year.
‘The business case for these machines is straightforward,
provided that you can feed them with enough work,’
explains Ian Sheldon. ‘People calculate payback in different
ways, but however you do it, I see no difference in the
amortisation period of a multi and that of a single-spindle
auto - or any other machine tool, for that matter.’ He says
that there are several factors to take into account when
considering buying a CNC multi. The most significant
benefit is the high production output for the footprint.
Factory space in the UK is expensive and in any case, many
companies do not have room to fit in the four, five or six
single-spindle lathes and their full-length bar magazines
that would be needed to match the output of one multi.
Machine price is important and there is no doubting that
a multi is an expensive item of plant, albeit the price has
been coming down in real terms over the years. However,
Sheldon points out that his Index MS machines are only
between two and three times the price of a good quality,
twin-spindle mill-turning centre of equivalent capacity.
‘When you consider that a turned and prismatically
machined component coming off a single-spindle millturn
lathe can be produced four times faster on a multi
with driven tool capability, the latter, more expensive
66 MWP march 2008
machine starts to look good value’ he suggests.
‘Moreover, there is only one machine to operate, not
several, so labour costs are lower. One setter and two
operators look after all four of our multis during the
days and at night we often have only one operator,
unless we are running very close-tolerance work. The
efficiency of our MS machines is high too, as reliability
is good and set-up times rapid. All of our CNC multis
produce parts for over 90% of available hours.’
One of Witon Engineering’s specialisms is turn-milling
of mainly ferrous components up to 60mm diameter, for
which purpose the plant list includes four Index G200
twin-opposed, single-spindle turning machines. Three are
fitted with high-pressure coolant delivery to increase
efficiency when machining stainless steel and when
cutting cycles include deep hole drilling. The MS52C was
bought to introduce the productivity benefits of the multi
to these larger parts. One example is a 45mm diameter,
stainless steel component for the automotive industry, that
Witon had been machining since 2005 in a 50-second cycle
on the G200s at the rate of 40,000 per month. The job,
which does not use driven tooling, entails removing about
75% of the bar volume by boring and turning a stepped
bush, so high power is important. Now that the job has
been moved to the MS52C, cycle time is 16 seconds - over
three times faster than on a single-spindle G200.
Second-generation director, Howard Sheldon, has been
impressed at how quickly the machine converts bar into
high precision components. ‘We operate 120 hours a week,
during which time the MS52C can work its way through
12 tonnes of 41mm diameter stock,’ he says. ‘When the
machine is running stainless steel, that material accounts
for more than half of our throughput.’ A feature of the
MS52C is the synchro sub-spindle mounted on a robot arm
which picks up a parted-off component and presents it to
static and live endworking tools. This modern system is
more flexible than the endworking on the older MS32Cs at
Witon as it allows the component to visit extra tools for
more comprehensive in-cycle reverse-end machining.
According to Howard Sheldon, there is a perception that
it is cam-type multi users that are migrating their work
onto the more capable CNC versions, but this is not
necessarily the case, even though a CNC multi can be
between two and three times more productive than a cam
multi. The latter machines in firms all around the UK
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