Seam Welding
Questions and Answers
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.
Coated materials have an increased tendency to stick to and react with the surface of the seam welding wheel because the coatings melt at relatively low temperatures. Aluminized and galvanized coated steels are very common in industry and can lead to a buildup on the surface of the seam welding wheels. This buildup can raise the contact resistance between the wheel and the part surface. You cannot change the melting point of the coating so you must dress the wheel to keep it clean and at the desired weld face size. The most common method is to use a knurl drive system. The knurl wheel cleans and reshapes the weld wheel back to near original dimensions. It also drives the rotation of the weld wheel and part movement. The knurl wheel cuts a knurl type pattern on the wheel surface and shaves the sides to maintain the original width. The knurl on the wheel surface ensures good contact with the part and prevents the wheel from slipping on the part. Sometimes a water stream is directed at the knurl area to remove loose weld and knurl product.
There are two types of drives for seam weld wheels. One is driven by the central hub, direct drive. The other is driven by knurled wheels riding on the perimeter of the seam welding wheels, indirect drive. There are pros and cons for both systems.
KNURL DRIVE
The knurl drive, indirect drive system provides a constant linear wheel speed at all wheel diameters. This allows the use of two wheels of different size. Frequently it is advantageous to use a smaller seam welding wheel on one side. The knurl driven system accommodates this easily. The knurl is also used to continuously dress the wheels on every rotation so that fresh wheel material is always being used to make the weld. The knurl imprints a knurl pattern on the wheel which enables the wheel to grip the workpiece and move it without slipping. One negative is it may leave a knurl patter on the part face. Water is frequently sprayed on the knurl drive area to remove loose material and keep the heat down. A knurl driven machine may cost up to 20% more than a direct drive machine.
DIRECT DRIVE
The alternative is a central axis shaft driven machine (direct drive). Either one or both wheels may be driven. If both wheels are driven, they must be the same diameter in order to travel at the same lineal speed. If only one wheel is driven, the size can vary. As the wheels are dressed or wear the lineal speed will change and must be compensated for to maintain weld spacing. To dress the wheels they are frequently removed and dressed off line on a lathe. The option is to design an automated dresser. At the proper time the automated dressing tool would move into position and dress each or both wheels as desired. Every time you dress the wheels, their diameter is changing and therefor the rotational speed of direct driven wheels must be adjusted to maintain the desired lineal speed and weld spacing.
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