Joining and assembly

Welcome to Joining and Assembly!

In this chapter, we are looking at how engineers put pieces together to create a finished product. Think of it like building with LEGO – some pieces snap together, while others might need glue. In engineering, we call this Joining and Assembly. It is a vital part of the Engineering manufacturing processes because most products are made of more than one part!

Don't worry if some of these terms sound new. We will break them down step-by-step so you can master them for your exam.

1. Permanent vs. Temporary Joining

Before we look at the tools, we need to understand the two main ways engineers think about joints:

• Temporary Methods: These allow you to take the product apart without breaking it. Think of a battery cover on a remote control.
• Permanent Methods: Once these are joined, you can’t take them apart without cutting, melting, or breaking the material. Think of the frame of a bicycle.

Analogy: A temporary joint is like using a belt (you can take it off), while a permanent joint is like getting a tattoo (it’s there for good!).

Quick Review:

Temporary = Can be undone (e.g., screws).
Permanent = Cannot be undone easily (e.g., welding).

2. Rivets

Rivets are metal pins used to join two or more sheets of metal together. They are usually considered a permanent joining method.

How it works:

1. A hole is drilled through both pieces of metal.
2. The rivet is placed through the hole.
3. The "tail" end of the rivet is smashed or "upset" using a hammer or a special tool called a rivet gun.
4. This creates a second "head," squeezing the two sheets tightly together.

Where do we see them? You’ll find rivets on the wings of airplanes, on metal bridges, and even on the pockets of your denim jeans!

Did you know? Before modern welding was common, massive ships like the Titanic were held together by millions of steel rivets.

3. Threaded Fastenings

This is the most common temporary joining method. It includes nuts, bolts, and screws. They use a screw thread (a spiral groove) to pull parts together.

Key Types:

• Bolts: These usually go all the way through a hole and are secured with a nut on the other side.
• Screws: These usually grip directly into the material (like wood or plastic) or into a pre-cut threaded hole in metal.

Common Mistake: Students often think screws are permanent because they are hard to get out. Remember: If you can use a screwdriver to remove it without breaking the product, it is temporary!

Key Takeaway:

Threaded fastenings are great for products that need maintenance or repair, like a car engine or a laptop case.

4. Soldering (Soft and Hard)

Soldering uses a "filler" metal (called solder) to join two pieces together. A key thing to remember: The main parts being joined do not melt; only the solder melts.

Soft Soldering:

This uses a low-temperature heat source (like a soldering iron). It is used almost exclusively for electronic circuit boards (PCBs) to connect components to tracks.

Hard Soldering (Silver Soldering):

This happens at a much higher temperature using a blowtorch. It is much stronger than soft soldering and is often used by jewelers or for joining copper pipes.

Memory Aid: Soft Soldering is for Small electronics.

5. Brazing

Brazing is very similar to hard soldering, but it uses an even higher temperature (above 450°C). It usually uses a brass filler rod to join different types of metal, like steel to copper.

The Process:

1. The joint is cleaned and a paste called flux is added (this prevents bubbles and dirt).
2. The metal is heated until it is glowing red.
3. The brass rod is touched to the joint, melts, and flows into the gap by capillary action (it "sucks" into the joint).

Why use it? It is stronger than soldering and can join metals that don't like to be welded together.

6. Welding

Welding is the strongest permanent joining method. Unlike soldering or brazing, welding melts the actual edges of the materials being joined.

How it works:

The edges of two metal parts are heated until they melt and run together. Often, a "filler rod" made of the same metal is added to make the joint thicker and stronger. When it cools, the two pieces become one single piece of metal.

Analogy: Imagine two ice cubes. If you melt the edges and push them together, they freeze into one big block. That is exactly what welding does with metal!

Summary Table of Heat Joining:

Soldering: Low heat, filler melts, parts don't. (Electronics)
Brazing: High heat, brass filler melts, parts don't. (Bicycles)
Welding: Extreme heat, parts themselves melt together. (Car frames)

Quick Review Quiz: Which process should I use?

1. Joining a resistor to a circuit board? (Answer: Soft Soldering)
2. Putting a wing on an airplane? (Answer: Rivets)
3. Building a heavy-duty steel skyscraper frame? (Answer: Welding)
4. Attaching a battery door you need to open later? (Answer: Threaded Fastenings/Screws)

Final Tip: When answering exam questions, always ask yourself: "Does this need to come apart later?" If yes, choose a temporary method like a bolt. If it needs to be super strong and stay together forever, choose welding!