Which is Best for Projection Welding of Nuts and Studs—AC, MFDC, OR CD?
July 26, 2016
If you have issues with your resistance welding (RW) of projection nuts and studs, you are not alone. While some believe alternating current (AC) is better than medium frequency direct current (MFDC) for this process, others believe the use of capacitive discharge (CD) RW process is best for improving quality. As you read this blog we will share what we know about AC vs. MFDC vs. CD for projection welding of nuts and studs.
First, let’s start with the basics since in most cases this is where the problems lie in the RW process. We have found that most issues with projection welding start with bad welding machine mechanics; secondly, bad part design/mechanics and the third most common issue is inadequate secondary current/energy.
Projection welding is most sensitive to force follow-up. This means the ability of the moveable welding head to follow the collapse of the projections. To help keep the welding force constant and avoid a dip or loss of full force during the welding process, most machines should incorporate some type of mechanical “fast follow-up” device. There are many ways to provide “fast follow-up” on your projection welding machines, so make sure some type of fast follow-up is specified on your projection welding equipment
AC vs. MFDC
Assuming proper mechanics on the machine, most “common” projections can be welded with either AC or MFDC, with little or no difference in the weld itself. In recent years, there has been talk about AC being a better choice than MFDC. After reliable evaluations, and side-by-side comparisons, AC and MFDC made the same basic welds on the same parts, with all else being equal.
MFDC vs. CD
The CD process has been part of the industry for a long time. The process utilizes large capacitors that are charged up and then discharged into a transformer to apply a large, single-current/energy spike at the right time during the welding process. Until recently, MFDC control and transformer technology could not generate similar current spikes as compared to CD.
MFDC Fast Rise
New developments to the design of specific MFDC transformers and controllers, have improved rise times, resulting in short rise times and high current pulses similar to that of CD welding machines. Systems using these newer MFDC units and controls are less expensive than CD equipment, and unlike CD equipment, MFDC equipment can be repurposed for use on other spot welding tasks. In addition, MFDC equipment can also be easier to retrofit to existing projection welders than CD equipment, which normally requires purchasing a complete new machine.
To download a complete article including diagrams and tables on AC, MFDC, OR CD—Which is Best? click on the link below.