LIBRARY ı6ıul 0) - Cranfield University

As a general rule, the welding process tolerances must be greater than or equal
to the combined errors associated with positioning and orientation of the arc and the
variability in joint fit up [ref. 105]. The main factors which have an influence on the
positioning of the wire relative to the joint centreline are [ref. 106]: a) torch guidance;
b) part positioning ; c) the straightness of the wire stickout. According to Stenke [ref.
106], the joining tolerance, Ti , can be determined by applying equation (2.7).
TJ = T, ' +Tr +T" 5TG
where:
To. is the wire positioning tolerance;
TT is the torch guidance tolerance;
(2.7)
Tc is the component tolerance (assembly and positioning).
The resulting allowable joining tolerance must be less than the ability of the process to
bridge gap tolerances (TG).
The first two factors have already been discussed
in the former sections. The
third factor, the potential misalignment of the wire electrode caused by varying radii
of spooling and by torch curvature, is an important element when considering robotic
welding since, depending on the relation between the wire spooling plane and torch
plane, the resulting wire ring diameters will differ in size. The varying radii of wire
bend and variations in the contact tube-to-work distance affect the wire outlet
deviations (see Figure 2.15). In order to reduce the effect of wire bend, two
recommendations must be followed [ref. 106]:
" to use wires that form the largest possible uniform rings after passing
through the torch tip;
" to use the shortest possible contact tube-to-work distance with as little
changes as possible.
Kuk [ref. 108] has presented a study on the acceptable joint mislocation (see
Figure 2.16) in robotic systems without adaptive control. In his study he has analysed
the case of on-line programmed robots, for which case, he concluded that the largest
sources of joint mislocation could be found in piece part manufacturing tolerances and
in the ability of the weld fixtures to locate and hold parts accurately. It is worth
mentioning that, in the situation analysed, the robot absolute accuracy did not play an
important role, since the robot was taught on-line. Hence, only the effects of robot
repeatability, part tolerances and part positioning accuracy were taken into account.
The same author came out with a guideline table suggesting the positioning accuracy
required for different weld sizes relative to the welding torch. The data from that table
is repeated here for reference (see Table 2.1).
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