Page 143 - Welding of Aluminium and its Alloys

128 The welding of aluminium and its alloys

distance from the reel to the contact tip, enabling wires as small as 0.4mm
in diameter to be used. The length of the umbilical cable is limited only by
the voltage drop in the power delivery and return leads and perhaps the
need to provide water cooling to the torch.
All of these systems require that the wire is driven at a constant, con-
trolled rate unaffected by continuous operation,variations in supply voltage
or fluctuations in temperature. They must also be able to reach the desired
wire feed speed as rapidly as possible in order to give good and stable arc
starting.The control for feed speed may be mounted on the torch or on the
wire feeder.
While manual welding may use any of the systems mentioned, push–pull
systems are becoming the standard method of wire feeding in robotic appli-
cations because of the need for highly consistent feed speeds and defect-
free arc starting.

7.2.2.3 Wire drive rolls
Aluminium wire is very much softer than steel and this can result in feeding
difficulties, the wire being easy to deform by excessive roll pressure, causing
the wire to jam in the feed conduit or in the contact tip. With push wire
feeders any impediment to the wire feed, such as metal shavings or wire
drawing soap compacted in the contact tip, kinks in the wire feed liner or
spatter on the contact tip, may cause the wire to buckle within the wire feed
conduit.Wire feed rolls must not be knurled but should be smooth, grooved
rolls or, better still, one flat roll and one with a 60° V-groove. Wire feeding
systems for aluminium also employ four drive rolls (Fig. 7.12) rather than
the two rolls that conventionally are used to feed steel wires. It is impor-
tant that the roll pressure is adjusted such that the wire is not grooved or
flattened by the rolls since this will also lead to wire feeding problems. The
wire should be kept as clean as possible. Covers to protect the reel from
dust and heated cabinets are available and it is recommended that these
are used where the highest quality is required.Also available are wire clean-
ing devices comprising a cloth or felt pad clamped around the wire and
soaked in a cleaning fluid such as alcohol or acetone. This can be used to
remove grease, drawing soap and loose particles of swarf or oxide at the
point at which the wire enters the conduit.
A relatively recent innovation in wire drive rolls is finding increasing use.
This is the orbital welding system in which the wire passes through the
hollow centre of the drive motor and is driven by a set of rolls set at an
angle to and orbiting around the wire. This method of driving the wire has
the advantages of both straightening and vibrating the wire, aiding in
feeding the wire through the conduit.