A look at spot welding

Spot welds are used in many day-to-day items, but how is it performed? And what are some of the considerations that come with spot welding? The post A look at spot welding appeared first on EDN.

Electronics Jun 17, 2024 0 7 Add to Reading List

Joining one flat piece of metal to another flat piece of metal is a common requirement, but sometimes the choice of method lies open. For a case in point, please consider these two kitchen spatulas in Figure 1 and Figure 2.

Figure 1 One side of a pair of kitchen spatulas, one with two rivets and one with two spot welds. Source: John Dunn

Figure 2 The back side of the pair of kitchen spatulas. Source: John Dunn

Handle attachments to the spatula blade are made with two rivets in the tool on the left while the attachments are made in the other tool using spot welds. The fixture used for doing spot welding can be roughly sketched as follows in Figure 3.

Figure 3 A diagram of the fixture used to spot weld where a large current is passed through the junction of two pieces of metal, creating a high enough DC resistance between the electrodes to melt some of the metal. This then cools off and solidifies, fusing the flat pieces of metal to each other in a specific “spot”. Source: John Dunn

The welding process passes a very large current, AC or DC, through the junction of two pieces of metal being joined so that the DC resistance in between the two electrodes gets hot enough to melt some of the metal which then cools off and solidifies to fuse the two pieces of metal to each other at that “spot”, hence the name “spot welding”.

This process can be scaled for very small pieces of work like the two welds on this flashlight D-cell (Figure 4):

Figure 4 Two Very small spot welds on a flashlight D-cell. Source: John Dunn

to very large pieces as in automotive spot welding like Figure 5:

Figure 5 Two spot welds on an automobile door. Source: John Dunn

The more detailed scenario in spot welding involves:

When to apply the physical force
When to turn on the welding current
When to turn it off
How long to let the work pieces cool before releasing the physical force
Whether the two contacting electrodes need to be given extra cooling measures such as water flow within

The technology is quite sophisticated.

There are also personal cautions to bear in mind. One is that this procedure makes some very strong magnetic fields. If/when the work pieces melt, molten metal can be sprayed out.

“Danger, Will Robinson!”

The other thing is that magnetic fields can do a destructive number on some wristwatches as well as on credit card strips and the like, so if you are operating such a fixture, pay attention to what you may be wearing or carrying on your person.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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