LIBRARY ı6ıul 0) - Cranfield University

experimental trials, Ogunbiyi proposed the model shown in equations (2.22) and
(2.23) for estimating variations in contact tip-to-workpiece distance.
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OSOi ý2: (Ii-Il-1ý
r-1
(2.22)
SOj = SOS + dSOi (2.23)
where ASOO is the estimate of the stand-off variation at the j-th sampling interval,
SO, ej is the reference stand-off, I, is the average welding current at the i-th sampling
interval and 0 is a variable which depends on the welding voltage (V) and the wire
feed speed (WFS), according to equation (2.24). 0 was obtained by differentiating
both sides of equation (2.21) with relation to SO while keeping the other parameters
constant.
ý-
1 (2.24)
K3V + K4WFS
where K3= 0.1223 and Ka= -0.7396 are the regression analysis coefficients, which
were obtained for 1mm (BS 2901 A18) mild steel wire and BOC Argonshield 5 (Ar +
5%CO2 + 2%O2).
Through-the-arc seam tracking is made possible by oscillating the torch over
the joint and by calculating the torch-to-workpiece distance at the right and left
oscillation extremes. The most common oscillation method is characterised by a
weaving movement provided by the mechanism that holds the welding torch. Due to
mechanical restrictions, the maximum movement frequency is normally about 10 Hz.
This limitation results in low welding speed, which affects the process productivity.
Furthermore, when welding thin sheet fillet joints the fast change of direction at the
oscillation extremes may cause instability and undercut [refs. 21,109]. Therefore, its
application to high speed welding or lap welding for thin sheets is restricted
According to Nomura et al. [ref. 134], the sensitivity of the arc sensor is
greatly affected by the oscillation frequency. The higher the oscillation frequency, the
greater the available sensitivity is (for spray transfer). In order to overcome the low
welding speed problem and to increase the sensor sensitivity, Nomura et al. [ref. 134]
developed the high speed rotating arc welding process, in which the rotation
movement is provided by feeding the wire through a rotating electrode nozzle with an
eccentric hole (resulting in an offset applied to the tip of the wire). This system can
easily achieve oscillation frequencies of 100 Hz or even more, consequently
increasing
the sensitivity to torch-to-workpiece distance changes. Hence, this process could be
applied to lap welding of thin sheets, besides fillet, V-groove and narrow-gap
welding. The conventional welding process (non-rotating arc) normally produces a
convex bead with penetration concentrated at the centre. On the other hand, the
rotating arc produces a flat bead with smooth surface and less penetration at the
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