The timing for a spot weld should start after an arc is established by having the welding machine start the timing sequence only when voltage and amperage are detected. If more strength is needed, for example on heavier top sheet materials, welds can be made in an elongated slot.
In Chapter 1, I mentioned oxyacetylene welding as an ideal process to learn and practice in order to gain the fundamental knowledge of fusion welding. Melting of the base material occurs slowly and is easy to observe. Filler metal is added separately and the two-hand technique used is similar to what is employed for TIG welding. Since it is a slower process than TIG, the manual skills are easier to practice. Maintaining a fixed distance from the welding tip to the work is not as critical as with TIG, which also helps develop manual skills.
In addition, the equipment needed is relatively inexpensive and can be used for welding, brazing, cutting, and heating. With the proper attachments, it is ideal for cutting steel of any thickness or heating metal for bending.
Therefore, although it may not be widely used in automotive work for general welding, it is good to review the basics before discussing TIG welding (see Chapter 4). It may be the proper tool to weld a very lightweight chassis made from small-diameter 4130 chrome-moly tubing (see Chapter 7). In addition, there are some unique joining applications, such as repairing a crack in cast iron, when braze welding with oxyacetylene is often preferred. The simplicity and relative low cost of oxyacetylene welding is another reason to consider its use for a number of applications.
Fig. 3.1. Oxyacetylene welding is an ideal process to learn and practice to gain the fundamental knowledge of fusion welding. The equipment is inexpensive and flexible to use. With the proper attachment, it is ideal for cutting steel of any thickness or heating metal for bending.
Fig. 3.2. Two cylinders, one containing oxygen and the other acetylene, supply these gases through long hoses to the torch. The gases are mixed before exiting through a small hole in the torch tip and ignited. For welding, acetylene is the only practical gas to use. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
An oxyacetylene welding rig consists of two cylinders with regulators and hoses attached to a torch.
One cylinder contains oxygen and the other acetylene; they supply these gases to the torch through long hoses. Valves in the torch control the flow of gases. The gases mix before exiting through a small hole in the tip and are ignited. Different-size tips are available for welding various thicknesses of materials. For welding, acetylene is the only practical gas to use. It has the hottest inner cone temperature of any fuel gas (5,720 degrees F). Other fuel gases are acceptable for cutting because the hottest flame is not necessary.
Oxygen Regulator—Avoiding Explosions
Oxygen is usually supplied in a high-pressure cylinder with a pressure of 2,000 to 2,600 psi (pounds per square inch). This high pressure must be reduced to less than 15 psi for welding the typical material thicknesses used in automotive applications.
Connecting an oxygen regulator and adjusting the pressure must be done following the manufacturer’s instructions. Be sure to read and understand the operating and safety precautions. When installing the regulator on a cylinder and opening the cylinder valve, very high oxygen pressure enters the regulator passages. This sudden high-pressure surge can cause a high temperature similar to what occurs in a Diesel engine combustion chamber when air is compressed. Most materials, including stainless steel and copper, burn and melt in the presence of pure oxygen. If not handled properly, the regulator could explode causing serious injury.
The internal design of an oxygen regulator must consider the possibilities of an oxygen fire or explosion. For these regulators to function efficiently, a valvespring is located on the high-pressure side of a diaphragm to regulate the pressure. This valvespring attaches to a valvestem and valvestem guide, which smoothes out the movement of the valvestem, so the regulator does not chatter due to rapid opening and closing of the valve.
Fig. 3.3. An oxygen regulator reduces the approximate 2,600 psi in a high-pressure cylinder to 5 to 15 psi, so it can be safely used for welding. Proper precautions must be followed when installing an oxygen regulator to avoid the possibility of a fire or explosion. Always follow the manufacturer’s instructions.
Here are the steps to avoid an explosion:
1. Before installing a regulator on a cylinder, visually check the cylinder outlet to make sure there is no debris.
2. Crack open the cylinder contents valve to clear any potential contaminants from the opening.
3. A slight opening is all that is needed. Quickly close the valve.
4. The oxygen regulator must have an inlet filter, so carefully check to see that it’s in place. If it is missing or not working, do not use the regulator. Take it to your gas supplier and have one installed or have the existing one repaired.
5. With an inlet filter in place, make sure it is clean, and free of all oil, grease, or contamination. This is particularly important in a location where grease or oil may be in the area where the regulator was placed when changing cylinders. Locate a clean location to place the regulator while swapping cylinders. The workbench needs to be free of grease or oil so you do not contaminate the regulator, and never put the regulator on the floor. Remember that even a small amount of oil or grease located on the regulator inlet can cause a regulator explosion.
6. After the regulator is connected, tighten the nut with no more than a 12-inch-long wrench. The sealing seats on the regulator inlet and cylinder valve are carefully machined metal-to-metal surfaces. Never use thread sealer on any regulator threads because regulator threads are not sealing threads, rather they are only for pulling the seats together. If the seats leak, the threads do not block the flow of gas. With oxygen service, not using thread sealant is even more important because contamination of the inlet occurs.
7. If the seats are clean, they do not need high torque to properly seat. If they do need high torque, either the cylinder or regulator may have a defective seat.
8. If a leak is found after testing, remove the regulator, clean the seat with a clean cloth, and reinstall. If the leak is still present, replace the cylinder or the regulator.
The way a cylinder valve is opened is very important. A valve on any high-pressure cylinder should always be opened very slowly. One reason is the pressure gauge has a small, sealed, curved tube that bends when subjected to pressure. When the tube bends, it actuates levers that move the gauge pointer, and rapidly opening the cylinder stresses the tube.
An explosion is possible if the valve on the oxygen sensors is opened too quickly (I have witnessed many tests of oxygen regulator explosions in a