Resistance welding electrodes and holders
Requisites For Electrodes: The resistance welding electrodes are meant for the following functions:
- Conducting electricity
- Conducting (away) the heat
- Locating
- Forming
- Heat balancing functions
From the above, the requirements for the electrodes can be derived as:
a. Mechanical strength - This is measured by the hardness.
b. Adequate current carrying capacity This is given by the electrical conductivity.
c. Provisions for cooling.
d. Resistance to deformation and wear under heat and pressure.
Electrode Materials: Resistance Welder Manufacturers Association (RWMA) has categorised the materials used for resistance welding electrodes.
Group A - Copper base D alloys
- Class 1 Cu - Cd (1 % Cd) - high conductivity
- Class 2 Cu - Cr (0.8 % Cr) - high mechanical properties.
- Class 3 Cu - Be (0.5 % Be, 1 % Ni, 1 % Ca) - high conductivity and mechanical properties.
- Class 4 Cu - Be (1.8 % Be, 0.3 % Cu - do -)
- Class 5 Al - Bronze (12 % Al) Flash or butt dies, Cu - Zr - Plated materials (galvanised iron or steels).
Group B - Refractory
- Class 10, 11, 12 - Cu & W
- Class 13 - W
- Class 14 - Mo - For welding of Cu or brass.
Spot and Projection Welding Electrodes: Four main parts of the electrodes are:
- Face
- Shank or body
- Means of attachment to the electrode holder
- Provision for cooling
Spot welding electrodes can have pointed, dome, flat, eccentric, truncated or radius face. The shank may be tapered or it may be straight having threads for attachment. Internal cooling may be provided or immersion cooling, flooding or a wrap around copper tube cooling may be adopted. The holders may be of ejector or non ejector type and straight, universal or offset models. The electrodes should be used with the prescribed holders and adopters in order that a good weld is obtained. Electrode material dressing, cooling, cleanliness of the engagement section, proper alignment and proper machine settings are also important. Projection welding electrodes usually make contact with the work on flat surfaces that must always be larger than the base diameter of the projection. Generally, flat face spot welding electrodes are used. In cross wire welding, rectangular bar stock is used. The projection welding electrodes have internal water cooling and replaceable inserts is used.
Seam Welding Electrodes: These are shaped like wheels or discs. The basic considerations of these electrodes are
- Face contour
- Width
- Diameter
- Cooling and
- Attachment
The above factors depend on the thickness, size and shape of the work, current density, pressure requirements and the type of drive mechanism. Floor cooling of top and bottom electrodes are employed. Care should be taken to preserve the shape and cleanliness of the electrodes. Special types of elect rodes are also manufactured depending on the requirements.