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Tooling is becoming simpler and smarter,
and joining methods are as well. Simon Bickerstaffe reports
Vehicle platforms deliver more derivatives than ever before and multi-model
lines are commonplace. Shorter lifecycles are a given. OEMs and suppliers
need flexible manufacturing solutions to cope.
Tooling and joining methods have evolved. Adjustable axes allow jigs to
locate different components. Vision systems can bring intelligence to
the assembly process, improving accuracy and quality. Laser welding, bonding
and riveting offer alternatives to spot welding and can simplify fixtures.
 “The
only thing that governs the amount of tooling you need is the joining
process you’re using,” says Paul Meeson, manufacturing engineering
director, Stadco. “High or low volume, it takes the same time to
do a spot weld.”
Stadco is trialling two methods to cut down on both. It is designing features
into parts to make them self-locating, meaning no fixturing is needed
at all. And the firm is moving away from spot welding toward laser welding.
“If you’re spot welding, you still need fixtures,”
says Meeson. “The quantity required depends on how long the spot
welding process is. Laser welding takes a third of the time compared to
spot welding.”
Combining self-locating parts and laser welding can reduce manufacturing
times and tooling requirements. Making the joints from one side only can
simplify tooling still further.
Warwick Manufacturing Group is also researching these areas. It finds
that a punched locating feature in pressed panels can replace tooling
pins and holes.
“We’ve got parts that have gone through production trials
on door inners, such as side impact bars,” says Dr Ken Young, director
of WMG’s manufacturing research centre.
The application of such self-locating features depends on the tolerances
required. Pressings tend to have little deviation in form so if the set-up
is done correctly, accuracy can be very good.
“In certain areas, the feature we’re putting into the bottom
panel doesn’t go all the way through so you don’t have to
plug holes in trim and final assembly,” says Young. “You’re
performing component elimination, reducing the cost and complexity of
tooling and improving access for joining.”
WMG sees potential for remote laser welding: the laser is away from the
tooling and the beam guided along the workpiece with mirrors. Switching
from CO2 to fibre lasers offers even more flexibility for tooling design
because beam density is better.
“We get a more parallel beam so we can move further away from the
job and still achieve the same spot size,” says Young. “A
typical gap using a CO2 laser is 1.5m but with the fibre laser we’re
hoping for 3m.”
Most OEMs still rely on spot welding and model-specific jigs to complete
the body in white. Honda manufactures the Civic and CR-V at its Swindon
plant in this way. The jigs change on an automatic transfer system to
match production.
The firm also uses servo jigs to manufacture assemblies made from different
panels on one tool. “They have a flexible axis so we can move templates
and location pins to suit the different body panels on different models,”
says Grant McPherson, director of manufacturing for weld, paint and engine.
The tooling isn’t intelligent. Jig movements are pre-programmed
but steps are taken to ensure correct operation: sensors detect if all
parts are correctly loaded, the welding robots can detect collisions and
know the material thickness they should be welding.
But they are, effectively, blind. A vision system would be a logical progression
but Honda doesn’t use these systems for welding. “It’s
not popular with us yet,” says Tony Foster, senior weld division
staff engineer. “The reliability of vision systems hasn’t
been so great: the technology is still developing.”
Manufacturing solutions provider EDAG has a vision system for doors and
hang-on parts called BestFit. A robot places the door in the nominal position.
Five laser beams measure the gaps. Software calculates the best fit position,
based on the OEM’s panel gap strategy, in around four seconds. This
best fit position allows for the weight of the final assembly plus settling.
 “We
get easily get results of ±0.25mm. The goal of OEMs such as BMW
or Audi is to be ±0.5mm,” says Christian Koerbel, manager
of sales and cost engineering, EDAG System Technologies. “The gaps
themselves are typically 4mm. With more time we can do better.”
The system is costly but can produce savings over time because manual
inputs are reduced. This automated process replaces the fitters who adjust
closures at the end of final assembly. And because it measures the gaps
after fitment, this quality data can replace a downstream measuring station,
used to record gap/flush conditions.
“The quality measurement calculation is self-learning,” says
Koerbel. ”When you bolt or weld a door to the body there’s
always settling. The system knows this and over a number of measurements
determines it’s a statistically-stable process and can correct it.”
The data can be fed back to quality departments, immediately showing any
problems in sub-assembly manufacture. Koerbel says that OEMs are doing
this.
The system is used by Opel at the Eisenach and Zaragoza plants which make
the Corsa. DaimlerChrysler uses it on the S-Class line at Sindelfingen
for doors, bonnet, bootlid and fenders, which Koerbel says is a unique
capability.
Honda uses fixed jigs to position its closures, preferring to control
the geometry of the BIW and the closures rather than adopt a best-fit
approach. “Our aim is to keep the whole of the body accuracy within
a very tight tolerance, therefore there is no need to use cameras to give
a best fit,” says Foster.
Vision systems will become more prolific in the bodyshop simply because
the number of bodystyles built in any one plant will increase. Joining
methods will also require more accuracy and control: laser welding and
structural bonding require precise gaps between panels.
“By 2020-2025, I think we’ll have lots of different models
based on one type so cost and flexibility will be vital,” says Koerbel.
“I think development will go towards intelligent tooling, or intelligent
robots able to determine what component is currently in the process. Best
fit would be part of this system.”
It won’t matter which model is being assembled. The robots will
have vision systems and know exactly how to position the parts.”
Honda agrees that vision systems will become more common but that tooling
won’t always favour the most flexible solution.
“We’re trialling more vision systems: in the future we will
be able to rely on them to improve accuracy and quality,” says McPherson.
“We will see more servo systems but they’ll need to be reliable
and maintainable. The more complexity you add the more risk there is of
breakdown so there is a balancing act.”
There is the cost too. Honda takes a pragmatic view of tooling methods
of the future, with fixed jigs remaining. “We have to install models
very fast – no stoppages on the production line to introduce new
models,” says McPherson. “If it doesn’t require lots
of automation or a high technology then we won’t do it. I can’t
see a completely flexible solution – not 100 per cent.”
There is also the question of materials and processes. The replacement
of the welded steel monocoque as the default choice has been is discussion
for some time. With OEMs under increasing pressure to cut weight and CO2,
tooling design might follow a different route altogether.
© Automotive Engineer, 2007 |
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