Page 73 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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force can be variable during the cycle. Metals which have high shrinkage during
solidification may need an increase of force to forge nugget after current passage.
Modern systems allow control of the clamping force during all stages of the spot
welding process. New portable gun units, incorporating the power transformer and
the actuator into a common platform, facilitate the fitting of RSW to robotic
systems.

2.4.2.2 Electrodes

Electrodes should have high electrical and thermal conductivities and must develop
low electric contact resistance in order to prevent deterioration of the work-piece
and electrodes. In addition they must have good strength to resist to deformation
and wear at high temperature. They are made from copper containing alloy
elements such as chromium (0.6-0.9%), cobalt (1-2%), beryllium (0.5%) and
zirconium (0.08%). Electrodes are composed of three parts: the electrode cap or
tip, the body of the electrode and the cooling system. Most of the electrodes are
cylindrical with the tip machined to a truncated cone with an angle of 30º, though a
variety of tip shapes (pointed, flat, dome and radius) is used to obtain access with
complex joints. When welding thin sheets the electrode tip diameter (d) can be
estimated by Equation 2.6, where t is the thickness of the sheet in contact with the
electrode. For thicknesses above 1 mm electrode tip diameter is estimated by
Equation 2.7:

d t
(a) 2.6
d 5 t
(b) 2.7

When welding sheets of dissimilar thicknesses the electrode diameter should be
specified for the thinner material. The weld diameter (D) should be similar to the
electrode diameter. Repeated heating and cooling of the electrode and pick-up of
metal particles causes electrode tip deterioration. Maximum tip diameter allowed is
1.3 times the initial diameter. When the electrode tip reaches this diameter it
should be replaced or redressed to the original diameter.

Electrodes should be water cooled at a flow rate above 4 l/min and separate water
circuits must be used for both top and bottom electrodes.

2.4.3 Process Parameters

According to Joule’s law the welding parameters are current, time and electric
resistance. Electric resistance is a function of several parameters such as electric
resistivity of materials, surface quality of sheet metal and clamping force. Electric
resistivity is a characteristic of the materials and varies with temperature during
welding. Material surface quality depends on roughness and cleanliness of the
surfaces of sheet materials. Parameters that can be programmed in the welding