Resistance Seam Welding

Seam Welding ControlSeam welding operations are traditionally faced with such problems as:

  • inconsistent heat from weld to weld
  • inconsistent spot spacing
  • inconsistent wheel velocity
  • inconsistent force over the length of the seam
  • edge variation
  • inconsistent wheel acceleration at the start and end of the seam
  • inconsistent face contact area of the seam wheel rolling on the part
  • wheel-to-brush contact resistance variations
  • Variable loading of the part presented to the machine, motor torque limitation, gear backlash, belt oscillation, motor speed control instabilities and machine mechanical resonances are factors that cause instantaneous wheel velocity fluctuations on seam welding machines. These velocity variations translate into variations causing hot and cold spots that affect the size of the welds produced.

Solution:

  • Adding Adaptive features is the easiest way to minimize these variations by automatically adjusting the heat up and down in response to these instantaneous velocity fluctuations.
  • Conventional seam welding controls set a motor speed and then depend on a fixed programmed time between welds to control the spot spacing. This practice is based on the often-false assumption that the wheels operate at a perfectly uniform speed. In actuality, the same factors that cause wheel velocity fluctuations on the seam are also responsible for causing inconsistent spot spacing.

Solution:

Adaptive features eliminate this problem completely by precisely placing each spot at the desired location, based on actual measured position of the wheel on the part, instead of depending on timing and the hope that the wheels are rolling at a perfectly uniform speed.

  • Weld cylinder and ram friction, air pressure fluctuations or servo anomalies, wheel cyclic variations and machine vibration are factors that cause force variations as the wheels roll along the seam. These force variations translate into irregularities in the size of the welds produced.

Solution:

  • Adaptive features offer the easiest way to minimize these variations, by automatically adjusting the heat up and down in response to the instantaneous force fluctuations.
  • Seam operations that need to maintain edge-to-edge control of a seam are plagued with edge variations, because synchronizing the heat with the dynamics that take place at the beginning and end of the seam is nearly impossible to achieve with a conventional control. Starting the heat too soon makes the front edge too hot and expulses material; starting the heat too late makes the front edge too cold, and an undersized front edge weld. Similar problems occur at the end of the seam; turning off the heat too late makes the back edge too hot and expulses material; turning off the heat too soon makes the back edge too cold, and an undersized back edge weld. These are some of the reasons why manufacturers employing traditional approaches to perform edge to edge welding are plagued with high scrap rates.

Solution:

  • Adaptive features solve this problem by accurately synchronizing the process and optimally sculpting the heat in coordination with the contour of the front and back edge of the part. As the wheel rolls up onto the front of the part, the control initiates the heat. The adaptive weld schedule traces the path of the seam welding wheel, while monitoring instantaneous wheel velocity, force, and other parameters as the part is being welded. It automatically adjusts the heat every millisecond to compensate for the velocity fluctuations, wheel bounces, and other sources of variation. Then it reduces and shuts off the heat as the wheel rolls off the back of the part.

All of these factors together result in:

  • inconsistently sized welds that can have variable spacing on the seam
  • the first few welds being hotter than the rest of the welds in the seam
  • similar problems with the welds at the end of the seam

In edge-to-edge seam welding operations, where the seam wheels have to roll up on the front of the part, roll across the part, and off the back edge of the part, these problems are further compounded by:

  • wheel bouncing
  • vibration and dynamic geometry changes that occur on the front and back edges

Monitoring can detect all of these sources of weld variation, and keep poor-quality and inconsistent welds from leaving the factory. However, manufacturers can still end up with a very high, very expensive reject rate.

Operating with an adaptive seam schedule is the easiest way to prevent bad welds from occurring due to these types of problems. The WeldComputer® adaptive features can automatically detect and correct each of these problems to produce consistent welds, and prevent a bad weld from occurring in the first place. And when the variations are too severe to be corrected, the system can notify the operator instantly about the problem.

Learn more about WeldComputer’s Adaptive Control with seam welding applications in their 2015 The Welding Journal article, “Increasing Resistance Seam Welding Throughput with Adaptive Controls as well as a presentation regarding, Adaptive Controls and Sealed Air/Gas Tight Continuous Seam Welds.

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