Spot welding, (or Resistance Welding), is a fast and economical way to permanently join metal assemblies. Just about any metal can be joined through spot welding. Two parts or several layers of parts can be welded together at the same time. Spot welding requires force and heat. Force is applied by pneumatic cylinders, hydraulic cylinders, springs or servo motors and is applied to both sides of the material. Heat is applied using high electric current and very low voltage. Typical voltage at the materials is one to two volts, yielding a very safe and effective process. Current is passed from spot welding transformers through copper buss bars to electrodes that contact the material to be welded. A typical spot welding system consists of a weld control, spot welding transformers, and a welding head – a device that applies clamping force and a current path. Spot welding components are simple and easy to maintain. Once tuned for the application, a spot welding device is capable of maintaining a repeatable process over the life of the equipment.

Several types of spot welding exist for different applications. General spot welding is a process in which metallic bonds are created in one or several areas of a manufactured part. Spot welding devices can also be used for brazing. In a brazing process, a metal with a low melting temperature is placed between materials requiring a bond. Current and force are applied and the brazing material bonds the two materials. This spot welding process is typically used for highly conductive materials, like copper, that are otherwise difficult to spot weld. Seam welding is a process in which weld current and force are applied with wheels that ride over the material. Using this process, air tight or fluid tight seals can be created. Seam welding heads with welding wheels can also be used to roll spot – a process in which a welder performs a spot weld, moves the material with weld wheels, does another spot and repeats the process.

Spot welding is readily applicable to prototype shops and hi-speed production. Simple bench-top spot welding devices with foot pedal actuators can be used for applications where throughput isn’t critical. Spot welding systems may be applied to machinery that yields 500 welds a minute or more. Such efficiency can be realized due to short weld times. A typical spot weld takes 10 to 200 milliseconds to complete. 

To achieve a repeatable spot welding process, two parameters are critical; heat and force. Many operators mistakenly discount the importance of force. Accurate and repeatable application of force are key factors in a spot welding process. Devices used to create force are pneumatic cylinders, hydraulic cylinders and servo motors. The most popular force-applying device is the pneumatic cylinder. Pneumatic cylinders are inexpensive, light, compact and are easy to maintain. The primary control components in a pneumatic system are an air valve, (used to apply air pressure to the top or bottom of a piston in a pneumatic cylinder), and a pressure regulator to adjust force via the amount of pressure that is allowed into the cylinder.

Using the right amount of force in a spot welding application is critical. Too little force creates too much welding heat. Too much welding heat can cause expulsion, (molten metal spits out of materials being welded). Low force spot welding can also cause brittle welds and material discoloration. Too much force decreases welding heat. Parts can deform as too much force is applied. Extra welding current is needlessly applied when force is too high.

What to Look for in a Weld Head:
The heart of a pneumatic weld head is the air system. This includes the pneumatic cylinder, the air valve and a pressure regulator. Air system components should be of the highest quality so applied force will be the same weld after weld. Quality components also assure minimal down-time.

During the spot welding process, materials being welded collapse. This process happens at a very high rate of speed. The weld head must be capable of accelerating its electrodes at the same high rate of speed to maintain welding force. In the spot welding world, this is known as weld head “follow-up force”. If a weld head is incapable of accelerating its electrode, extra heat is created. Extra heat causes weak welds and expulsion. A typical weld head has very few moving parts. The piston in a pneumatic cylinder has a pushrod that pushes on an assembly typically known as a “Ram”. The ram is used to guide the spot welding electrode below it. The ram assembly must have the least amount of mass possible. A lower-mass ram can accelerate faster. Manufacturers of spot welding equipment also use springs to help accelerate the welding electrode. Springs may be conventional coil type, Belleville or air diaphragm.

In a weld process, the upper electrode must move up and down only. Side to side or rotational movements create poor welds. Bearings in weld heads must have very tight tolerances to minimize unwanted movement. They should also be robust enough to withstand the high forces necessary for the spot welding process. Good weld heads have dedicated anti-rotation devices that keep electrodes from rotating after they contact parts. The frame and electrode holders should be built to have no deflection when maximum force is applied. Frame/electrode holder deflection cause a phenomenon called “wiping”. Wiping is a rotational movement of the spot welding electrodes as the weld-head components distort under load. Wiping creates poor fit-up between the electrodes and parts to be welded. Poor fit-up creates inconsistency in the process.

A good weld head has optimum copper in the secondary to conduct current necessary for welding. All of the copper in the weld head is referred to as the “secondary circuit”. If the secondary is not robust enough for the process, the secondary will overheat and degrade quickly leading to more down-time. A bad secondary also wastes weld energy.