Miller MSW 41 portable 'resistance' spot welder, $5.

The little Miller portable spot welder is another great example of equipment whose work we see all over the place, but whose appearance leaves us confused when we see the tool in person. Welding equipment is all over the place out there, and your eBay buyers bid on it like crazy.

When  we hear the term 'weld', we usually think of the process where two pieces of metal are fused along a seam, using some sort of welding rod to provide the metal needed to fuse the pieces together. And all the most popular forms of welding done nowadays, like arc welding, TIG welding, MIG welding and torch welding all use some sort of additional metal to form the weld. The pieces to be welded are held together, heat is applied and extra metal is added via a rod or wire, welding the pieces together.

There's another type of welding that doesn't use additional metal, which is called resistance welding, or 'spot' welding. In spot welding, two pieces of metal are laid together on an overlap, and current is run across a tiny 'spot,' whereby heat actually welds the two pieces together. In spot welding, no additional metal is added because the material is simply melted together, one tiny spot at a time.

A limitation of spot welding is that it can only be used to weld thin sheets of metal no thicker than about 1/8", and the sheets must overlap one another in what's known as a lap joint. As you can imagine, one of the most popular uses of spot welding is in the automotive industry to assemble the cars' bodies. Every car has literally thousands of spot welds all over it, and all you have to do to see them is to open the hood or a door and look for the little dimples along the seam where any two pieces of sheet metal meet.

The use of spot welding in the automotive industry is not an economy measure, either. When you think about it, it would be really impractical to use any other form of welding to secure thin sheet metal. But there's another reason spot welding is very preferable to any other method: heat. Heat has the tendency to warp metal, and the thinner the metal is, the more liable it is to warp. A major advantage of spot welding over other forms of welding is that since only a tiny spot of metal gets heated up at any one time, the metal doesn't overheat to the point of warping.

Take a look at the spot welder in the photo, and notice the long copper arms, which are called 'electrodes.' The two pieces of metal to be welded are placed between those electrodes, and the handle on top of the unit is pushed down, closing the electrodes and pressing the pieces of metal tightly together. Then, a button on the welder is pressed, sending electrical current through the point where the two pieces of metal meet. The current passing through the electrodes causes the metal under the point where the electrodes meet to melt, forming a 'nugget' of molten metal that is then allowed to solidify into a solid weld.

You may be wondering why, if those electrodes get hot enough to melt the metal, the whole welder doesn't get red hot as well. To see why only the small area being welded gets hot, check out how the tips of the electrodes come to a point. As the current passes through those thick electrodes, it gets funneled down as it reaches the very small area where the electrode tips come together on the metal. And as the welder's high amperage current is forced through that tiny bottleneck of metal, resistance increases greatly, causing only that point to get hot.

This welder was sitting on a bench in the garage at an estate sale, and since nobody at the sale even knew what the heck it was, I knew they wouldn't be able to tell me whether it worked! So, I did what I usually do whenever I run across one of these welders - I got a couple of coins from my pocket, clamped them in the jaws, and pressed the power button. I waited for the area under the electrodes to get red hot, released the power button, then lifted the handle and dropped the coins on the floor. There's no rocket science in the construction of these machines, and if the welder will turn my coins red hot, I can be pretty certain that it works. These deceptive little devices cost a lot to buy new, and they usually bring very good money on eBay used.

This little Miller MSW 41 prtable spot welder sold on eBay for $215

Photo of Miller MSW 41 portable spot welder