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Table 2.1: Joint positioning tolerances for robotic arc welding [ref 108]
Weld Size I Positioning tolerance
Up to 3.18 mm (0.125 in) ±0.38 mm
(±0.015 in)
3.2 to 6.35 mm (0.126 to 0.250 in) ±0.64 nun (±0.025 in)
Above 6.35 mm (0.250 in) ±1.14 mm (±0.045 in)
Kurkin and Drikker [ref. 109] investigated the effect of several geometrical
factors on the permissible weld path deviation for 4 mm sheet steel in T joints, when
using CO2 and other gas mixtures. From the experimental results, the authors reached
the following conclusions:
" the allowable joint deviation relative to the robot path (see Figure 2.16) is
increased by increasing the weld leg length, as a result of reducing the
welding speed;
" the positioning tolerances increase when using downhill welding;
" transverse oscillations increase positioning tolerances, but reduce the
allowable gap. They may also lead to undercutting and lack of fusion
defects in sheet metal welding.
Wadsworth [ref. 110] investigated the effects of welding electrode
misalignment relative to the joint centreline on the weld quality, for different transfer
modes and welding speeds. The author observed that, for globular mode of metal
transfer, the weld quality deteriorates with increasing welding speeds when the
electrode is one wire diameter off joint centreline. The reduction in weld strength
between a fillet weld made with zero wire offset and one made with the welding wire
positioned one wire diameter off the joint was used to evaluate weld quality. For
spray transfer, the author observed that the loss in strength was almost constant for
the range of speeds studied, 305 to 610 mm/min (12 to 24 in. /min)
Middle [ref. 105] suggests that designers should design welded parts to allow
the greatest possible use of flat position welding, which has larger tolerances and the
highest productivity rates. The same author proposed the adoption of design
procedures such that increased joint fit-up and positioning tolerances could be
obtained.
2.5.2 Fixturing
In section 2.5.1, fixturing tolerances have been included in the components'
tolerance group. This, however, does not imply that fixturing tolerances are less
important than the other contributing factors. Consistent welds require reproducible
weld placement and welding conditions. The correct placement of a weld depends on
the adequate joint positioning relative to the welding torch path. Consistent joint
positioning, in its turn, depends on adequate fixturing, further to joint fit up.
Widfeldt [ref. 104] suggests that for off-line programming, the design of
fixtures should be made utilising the same CAD system as used for designing the
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