An Introduction to Cutting and Joining Metal

Three families of joints

Nearly every way of joining metal falls into one of three families, and the difference between them comes down to what happens to the metal.

• Mechanical joints hold parts together with fasteners or by deforming the metal. No heat is needed, and the parts can often be taken apart again.
• Soldered and brazed joints use a filler metal that melts at a lower temperature than the parts being joined. The parts themselves do not melt.
• Welded joints melt the edges of the parts, usually with a filler metal, so they fuse into one piece when they cool.

Adhesives and epoxies are also used on metal, but this guide sticks to the three families above because they cover most shop and garage work.

Mechanical joints

Screws, bolts, nuts and rivets are the simplest way to join metal. A bolted joint needs only holes, a pair of wrenches and some care. Choosing among the many fastener types comes down to the thickness of the material, whether you can reach both sides, and how much load the joint carries. Washers matter more than beginners expect, because they spread the load and protect softer metal, which is why bolts and washers are usually discussed together.

Rivets are a heat-free alternative that suit sheet metal and thin sections. A blind rivet can be set from one side only, which makes it handy where you cannot reach the back of the work. Riveted joints are generally permanent and have to be drilled out to be undone.

Strengths of mechanical joints:

• No special skills beyond accurate drilling.
• Easy to inspect and, with bolts, to repair or replace.
• No heat distortion or changes to the metal near the joint.

Weaknesses:

• Holes remove material and can concentrate stress.
• Fasteners can loosen under vibration unless they are locked or tightened properly.
• Mixing metals, such as steel fasteners in aluminum, can invite corrosion, so fastener corrosion is worth a thought at the planning stage.

Soldering and brazing

Soldering joins metal with a filler that typically melts below roughly 450 degrees Celsius (about 840 degrees Fahrenheit). That is a common dividing line in the trade. The solder flows into the narrow gap between the parts by capillary action, helped by flux, which cleans the surface and keeps it from oxidizing while hot. This is how electrical connections are made and how copper plumbing joints are sealed. A basic soldering iron is enough for small electrical work, while plumbing joints usually need a torch. The details are covered in the soldering article in this cluster.

Brazing follows the same idea with a filler that melts above that dividing line but still below the melting point of the parts. Brazed joints are generally stronger than soft-soldered ones and need more heat.

Soldered joints are not usually used where heavy structural loads are expected, and the joint will weaken if it gets hot enough to soften the filler.

Welding

Welding is what most people picture when they think of strong metal joints. Because the base metal is melted, a good weld can be about as strong as the parts themselves, though results depend heavily on technique, preparation and the metals involved. Common processes include:

• Stick welding, also called arc welding, which uses a flux-coated rod and a power supply.
• MIG welding, which feeds a wire continuously through a hand-held gun.
• TIG welding, which uses a non-consumable tungsten electrode and a separately fed filler rod.

Each has its own balance of speed, control, cost and skill level. Welding also asks the most of your safety gear: eye and skin protection from intense light, protection from sparks and spatter, and attention to fumes. A welding helmet and flame-resistant clothing are basic requirements rather than extras.

Choosing a method

Treat this table as a starting point only. Structural, pressure-bearing and vehicle parts can fail in ways that injure people, so those jobs call for the relevant codes, manufacturer instructions and often a qualified professional.

Cutting and preparation come first

Joints are only as good as the parts that go into them. Metal is cut to size with hacksaws, snips, cutoff wheels and similar tools, and the cut edges are then deburred and filed so parts fit together cleanly. Holes for bolts and rivets need accurate drilling with sharp bits, slow speeds and good clamping. Welding and soldering both depend on clean surfaces, since paint, oil and rust get in the way.

A grinder is involved at several stages, from cutting to cleaning up welds, and it is one of the tools that causes the most injuries in home shops. Holding the work firmly in a vise or with workholding systems also prevents a surprising number of accidents and poor joints.

Safety across all methods

• Wear safety glasses at all times, with a face shield when grinding or cutting.
• Use hearing protection around grinders and cutoff saws.
• Keep a clear area free of flammable materials, since sparks travel farther than most people expect.
• Wear snug, non-synthetic clothing and sturdy footwear around hot or sparking work.
• Ventilate the space. Some coatings, such as galvanizing, can release harmful fumes when heated.
• Read the manual for every tool and follow it.

A good set of personal protective equipment costs far less than one injury, and the basics are the same whichever method you choose.

Where to start

For most beginners, the sensible path is to begin with mechanical joints and accurate cutting and drilling. Then try soldering a wire splice, which teaches heat control and cleanliness at low risk. Welding is best approached with a class or an experienced person, because early habits around safety and setup are hard to unlearn. A set of basic metalworking tools will cover most of the cutting, filing and drilling along the way.

This article is general information and not professional trade advice.