It is the norm for the welding wheels to be the same diameter. This is necessary if they are driven by a common drive. If they are driven by the same drive and they were not the same diameters one wheel would turn faster than the other and would be slipping on the workpiece constantly. If they are the same diameter and geometry the weld should be well controlled and consistent.
The answer to this depends upon the material being seam welded. Highly conductive softer materials (Aluminum) will be seam welded with Class 1. Stronger more resistive material (low carbon steels) would use Class 2. High strength resistive material (nickel base and stainless steel) would use Class 3. This is the same as for spot welding electrode selection and can be found in various industry charts in AWS and other publication.
The person making this inquiry was not sure what wheel material he had in this stainless cloth to stainless sheet application. He was experiencing excessive pickup on the wheel face. This could be due to the unknown alloy wheel or it could be normal. Wheels just like spot welding electrodes will experience pick up and mushrooming during normal use.
In many seam welding applications a knurling wheel is installed on the wheel periphery which continually trims the edges and roughens/knurls the face. This makes sure that the weld wheel is always in the same condition without build up. This is a continuous dressing operation. Sometimes the knurler functions as the wheel drive mechanism. In other designs it rides on the wheel and merely dresses.
A seam welder is a spot welder using a wheel for an electrode. The shunting current rules apply the same for seam welding just as they do for spot welding. In the case of a traveling head seam welder the first issue is – How many wheels are mounted? Sometimes there are two. Their spacing/separation will be an issue to include. The next of course is the spacing of the spots. If this is a liquid tight seam weld then shunting currents will be present and must be allowed for. If this is a roll spot seam, the distance can be adjusted/increased to prevent shunting currents.
A stitch weld is a series of spot welds made in a row down two pieces of material. It is similar to a seam weld but is a weld gun or machine cylinder applying the individual welds rather than a rolling seam welding wheel.
VARY SPOT SPACING TO MAKE A STITCH WELD
The answer to the question is maybe. It depends upon the coating and how the schedule and wheel geometry are set up to handle the change in the material surface. If this is a galvanized coating which will melt at a low temperature and could squeeze out of the weld joint area. Yes, one might expect a little more material pushed out of the seam area. This may or may not be noticeably more than with the bare material. This would need to be addressed in how the schedule and wheel are applied. However this could be a nonconductive paint or other coating which is another issue completely.
The answer is one would address the coated or non-coated material in a similar fashion as described in article:
HOW CAN YOU PREVENT SEAM WELDING BURRS WHEN WELDING 0.8 mm MATERIAL?
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