Seam Welding
Questions and Answers
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?
To address this question we will assume that we are making a liquid tight seam weld. In this process the welds overlap as the weld wheels roll forward. In a seam weld the process current is developed to allow for shunting current through the trailing welds which have already been made.
This part has tack/spot welds occasionally to hold the two surfaces in place. When the weld wheels approach one of these tack/spot welds the weld conditions change. As the wheel approaches, increasing shunting current can now travel forward through the parent material and the tack/spot weld. This forward shunting path is not part of the normal current allowance. This portion of the current is no longer passing through the intended seam weld. Our seam weld will be cooler because less current went through the desired seam weld. This might lead to the seam weld decreasing in size and possible failure to meet specification.
The best document for troubleshooting a seam welder would be:
AWS J1.2:2016 “GUIDE TO INSTALLATION AND MAINTENANCE of RESISTANCE WELDING MACHINES”
This document explains the importance of the various aspects of a proper installation. These installation guides give you many points to troubleshoot on an operating machine.
It also lists many undesirable weld results and what may be causing them.
It has a trouble shooting section. What to look for and where to go to correct the problem.
A seam welder is a resistance welder with circular rotating electrodes and flood cooling. All other factors are the same. The weld head is different in that it acts as a bearing and conductor. Wear of the bearings and sliding contacts is common over time. The conductive grease in the weld head normally needs to be replaced at the same time the weld head is overhauled.
Maintenance of the weld head should be left to seam welder manufacturers.
See: Article “CAN A WORN SEAM WELDING SHAFT BE REWORKED?”
Assuming the two workpieces being seam welded are the same, then two different diameter wheels will make contact with the part over two different surface areas. If the surface areas are different, the side with the larger area could run cooler than the other and the nugget will tend to form in the other half of the part with the smaller surface area. Therefore the smaller wheel with the same face geometry will run hotter than the larger diameter wheel. Depending upon the part being run and the process this diameter differential may or may not be enough that action needs to be taken. There are many factors going on during a seam weld that may minimize this variation.
If the diameters should be addressed to correct the issue, then this becomes a simple case of heat balance. The large wheel side is too cold or the small wheel side is too hot. One can address this from either side. Remember if you are making changes in weld faces, it is best to change just one at a time and measure results before making additional changes.
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