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and kinematic model mismatch and the use of incorrect inverse kinematic algorithms
in the simulation program.
If the off-line programming method is based on language programming, in
addition to the errors mentioned above, the difficulty in visualising cell components in
space may result in collisions.
2.4.5 Workpiece tolerances and positioning
In an industrial environment, the components and tooling are designed in such
a way that they allow small dimensional errors on their nominal values. When parts
are assembled together for a subsequent welding operation, joint fit-up errors often
appear in the form of joint misalignment and gap variation. These dimensional
variations may increase as a result of inaccurate or worn press tools being used for
shaping the parts to be welded These discrepancies are called manufacturing
tolerances. The combination of parts and tooling tolerances with joint fit-up errors
result in considerable variations in the joint shapes and positioning with relation to
what has been designed. In addition, thermal distortion during the welding process
can cause on-line joint movement. These problems are easily compensated for by a
skilled manual welder. However, in robotic welding, they can cause serious quality
problems, like undercut, poor weld profile, insufficient penetration and burn-through.
[refs. 87,89,99,100,101]. Hence, tighter tolerance levels are required for robotic
welding.
2.5 Common production problems with robotic welding
Although robotic welding produces higher quality welds than the human
welder, this is only true when conditions are right. The welding process is prone to
faults not only in terms of workpiece variations, but also because of the rugged
environment imposed to the equipment. Apart from inadequate welding parameters
and from workpiece variations several problems can occur in robotic welding, among
which the most typical are given bellow. [refs. 21,102,103]
" Wire feed slip: Caused by dirt/grease on the feed rollers, wear on the rollers
(grooves), wear in the contact tip and/or conduit, inadequate pressure on
the rollers, surface contamination on the wire, dynamic effect of rapid robot
motion between welds, etc.
" Contact tip wear: Usually caused by normal friction between the moving
surfaces but consistency of wear is dependent on electrical contact
characteristics and arcing within the tip.
" Contact tip fusion: Sudden burnback of the electrode particularly during
arc ignition causes the electrode wire to fuse to the contact tip.
" Failure to strike the arc: caused mainly by burnback at the start of welding.
" Wire sticking in the weld pool: Inadequate burnback control at the end of a
weld can lead to the wire sticking to the weld pool
" Spatter build-up around the nozzle: Hot spatter is normally ejected during
welding, sticking to most surfaces. Due to its proximity to the source of
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