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spatter the gas nozzle is particularly affected. The accumulation of metal
particles can cause turbulence on the shielding gas, which may draw some
air into the shielding, inducing weld bead defects such as porosity and
oxidation. This effect can be minimised by using torch cleaning stations,
usually consisting of a wire brush arrangement to which the robot drives
the torch periodically. A stable process also minimises the need for torch
cleaning.
" Consumables
running out: e. g. shielding gas and welding wire.
" Torch overheating. due to blockage in water line, or water cooling system
failure.
" Faulty lead connections: Increased resistance due to corrosion or loose
connections affects the voltage drop across the torch.
" Robot faults: Generally complete failure is rare but increased backlash in
the gears, servo overheat and instability cause changes in travel speed and
accuracy, juddering etc.
Most of these are mainly technological problems which can be dealt with by
using proper maintenance procedures, carried out periodically, and by utilising some
kind of fault detection system such as the ones reported in references 21,102 and
104.
On the other hand, faults due to workpiece variation and workpiece
positioning are more difficult to deal with, since they depend on each part component.
For example, if the joint is placed in a position which differs from the programmed
weld path, the weld bead will be deposited in the wrong place. This may lead to bead
asymmetry relative to the joint axis, which may cause lack of fusion on one side of the
joint, or the robot may miss the joint completely, hence not welding the parts
together. Conversely, the robot may accurately follow the seam but if a gap exists and
it is wider than the allowable process tolerances, the gap will not be bridged and,
consequently, burn-through will occur or the two parts will not be joined. In some
applications, distortion due to heat during welding can also cause excessive gapping.
These problems can be minimised by: a) tightening the tolerance levels of the
pre-welding operations such as part cutting and joint fit up [refs. 89,105]; b)
modifying the component design approach [ref. 105]; c) improving the weld fixture
design [ref. 104]; and/or d) using some kind of adaptiveness [refs. 87,88].
2.5.1 Tolerance requirements
The tolerance requirements in welding are generally related to the fulfilment of
weld quality demands, which are normally set in the design stages according to
standardised weld quality specifications [ref. 106]. These usually provide the general
minimum quality requirements and limitations for the various types of discontinuities
commonly encountered in welds (e. g. American Welding Society Standard AWS
D8.8-79 [ref. 107]).
For weld quality assurance, it is necessary that the torch and wire electrode
are guided within permissible limits relative to the centreline of the weld joint [ref.
106]. Tolerances are determined by welding position, type and size of weld, selected
process variation, welding power, etc. [refs. 106,108]
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