Lesson video

Hello there, my name is Mrs. Dhami Thank you for joining me for your design and technology lesson today.

Now the big question for today is what are the options when we come to joining metals? So we're going to explore a whole load of different options together, hard hats on, let's get cracking.

Our outcome for today is we will be able to explain various mechanical and chemical methods to join different materials.

We have three different keywords today.

First of all, mechanical fixings.

They are a part that holds materials together using force.

We then have component, which is a ready-made part used across different products to make manufacturing easier.

And lastly, we have chemical joining, which is where we use adhesives or heat to bond materials through a chemical reaction.

Feel free to come back to this slide at any point.

We have two learning cycles today, mechanical fixings, and then moving on to heat and chemical joining.

So let's get started with mechanical fixings.

Mechanical fixings are a way of holding materials together using physical means like fasteners or shaped joints.

Mechanical fixings tend to use force or shape to hold materials in place.

They are common in metal construction and product assembly.

They can be permanent or sometimes removable depending on which one, and they can be components.

Now components is one of our keywords today, and a component is a ready-made part used across different products to make manufacturing easier.

Time for a quick check in.

Which of the following is a mechanical fixing? Is it A, PVA adhesive, B, epoxy resin, C, a screw, or D, contact adhesive? Have a little think perhaps chat with the person next to you.

Come back to me when you've made your decision.

Well done if you got C, screw is an example of a mechanical fixing.

Screws are mechanical fixings that grip the metal with its thread and if you take a little look at the picture on the right, you can see the thread going all the way up along the shank of the screw.

Now, softer metals, such as aluminum usually use what we call self-tapping screws.

This means they have sharp threads, much sharper threads which cut into the material as it's directly driven in.

and hopefully you can just about see in that bottom right picture that that screw along the shank is slightly sharper than the screw above.

So uses, screws are used to join sheet metal.

Benefits are, they produce a strong hold and you can take them out, so they're removable.

Limitations, you usually require a pre-drilled hole.

Screws used in metals require what we call a pilot hole.

Now a pilot hole is a small hole which is slightly smaller than the diameter of the screw that is pre-drilled, so we drill it before we put the screw in.

This reduces the stress and prevents damage such as stripping threads.

And you can see that in the picture on the right.

Depending on the type of screw, a countersunk hole may be needed.

Now a countersunk hole is a conical recess drilled into the material to let a screw head sit flush.

So take a little look at diagram two and you can see that conical recess there at the top of the material.

Now what I mean by the word flush is that the screw head sits in line with the material and you can see that on image three.

So it means that the screw head does not stick out.

Understanding the parts of a screw makes differentiating between the different types easier.

So let's take a closer look at the diagram on the left.

If we start at the top, we have the head of the screw.

Coming off the head we have the shank, which is the main body of the screw.

Going around the shank we have the thread.

And at the bottom we have the sharp point, which we call the tip.

As we've just said on our previous slide, countersunk heads sit flush with a surface so they do not stick out.

Whereas round head screws sit above the surface.

So you might want to choose which one you have depending on A, aesthetics, how you want it to look, and B, function, you might not want to be able to feel the top of a screw head in the product that you are designing.

Now there are different drive types.

Now this is the shape of the slot in the screw's head.

So we have the cross which we call the Phillips we have the straight line across, the slotted, the pozi, which kind of resembles a star and then the hex, which is obviously short for hexagon.

Thread types and shank diameters can differ depending on the material type and the thickness.

Lots to consider when you're choosing which screw to use.

Machine self-locking screws have what we call nylon inserts or specially deformed threads that stop them coming loose when machinery vibrates.

And if you look carefully at the diagram it looks like a normal screw, doesn't it? But then, all of a sudden, you can see the polymer insert going along the thread and along the shank.

So where do we use them? We use them in machinery, automotive, and then appliances, especially appliances that move or vibrate.

Benefits, they are secure in vibrating environments.

Limitation is, that they are more expensive than a standard screw.

Nuts and bolts are mechanical fixings used to securely join metal parts such as steel beams in construction, metal machinery or bike frames.

They provide strong removable and reusable connections and are often used with washers to spread the load and prevent surface damage.

Let's take a closer look at the diagram on the right.

At the top we have the bolt.

We then have the washer, remember the washer helps to spread the load and protect the surface.

And then obviously at the bottom we have the nut, that attaches onto the bolt.

So where do we use them? In construction and bicycle frames.

Benefit is, they produce a strong hold but they're also removable.

Limitation is, you require access to both sides to be able to attach that nut onto the bolt.

A tap and die set is used to cut internal and external threads in metals, allowing bolts and screws to be fitted securely as a mechanical fixing.

And I'm hoping that some of you might have had the opportunity to use these in your school workshop.

Let's take a closer look.

First of all, we have the die.

Now a die is used to cut external threads on metal rod.

Now you might have used that before where you pop it on and then you start to turn it to start the cutting and you normally do a full turnaround and half a turn back full turnaround, half turn back so that the thread does not become too clogged up and might unfortunately break if you keep going.

So you have to release that little bit of metal that you have been cutting.

We then have a tap.

Now a tap is used to cut internal threads and you can see if you zoom into that picture on the right, you can see the thread that then cuts the internal thread.

So the use, the use is obviously to join.

Benefit is, you can make custom pieces.

You could get a piece of lots and lots of different types of metal rod and easily be able to put a nut onto it by using a die cutter thread on it, it's great.

Limitation, it does require skill and of course specialist equipment.

Time for a quick check in.

True or false? A die can be used to cut external threads on a metal rod to create a mechanical fixing.

Have a think.

Decide on your answer.

Come back to me when you've got a decision.

Well done if you've got true.

And why is that? External threads are die cut and a tap is used to cut an internal thread allowing bolts and screws to be fitted securely as a mechanical fixing.

<v ->Pop rivets are also known as blind rivets</v> and they join thin metal sheets together.

So let's take a little look at how the process works.

First of all, you need two metal sheets with a hole drilled through.

A hollow rivet is then inserted through those two holes.

A rivet gun pulls and deforms the pin expanding the rivet to secure the joint.

Now the reason we often call it a pop rivet is because you're using the pop rivet gun and you're pressing the two handles together and you think this is not working and then all of a sudden, often takes us by surprise, it goes pop and off pops that long bit from the rivet and the two pieces of metal are joined.

How did they look then? The back maybe doesn't look as aesthetically pleasing as the front.

The front tends to lie quite flat with the material and looks quite nice, whereas the back is a little bit more raised.

So where do we use them? We use them in aircraft, automotive and metal panels.

Benefit is, that you can join thin metal quickly without access always to the other side.

Limitation, it's not as strong as solid rivets, which we're about to explore.

Solid metal rivets are mechanical fixings which are inserted and then hammered or pressed to deform the ends.

Now I really enjoy using these there's something satisfying about it.

So you need two sheets of metal, you need a hole drilled through you need to pop the solid metal with it through.

You then put it onto a metal plate or perhaps the back of a metal vice like I am doing here.

And then you get a hammer and you hammer the other end to deform it until it looks a little bit like my picture on the right.

Now you can do this on larger pieces, but you can also do it on teeny tiny pieces and you can actually make your own mini solid metal with it by a little bit of metal rod.

And sometimes we do that when we're making pieces of jewelry.

They're quite good with a little bit of brass rod.

So use, we use them, tend to use them in aircraft and structural steel work.

Benefit, they are extremely strong and reliable.

Limitation is, it does require access to both sides and it is permanent, so you cannot reverse it or remove it.

Metal crimping mechanically holds parts by pressing together or deforming.

Now it's a really, really satisfying process.

There's lots and lots of different metal crimping tools that you can purchase or that you can use.

And if we zoom into that, you can see what it has done.

It has deformed that metal together so that it will not move.

So where do we use it? We use it with electrical connectors.

We can sometimes, rather than using a crimping tool, we can simply use a pair of pliers for that.

We also use it in mental housings and piping.

Benefits, it is fast and simple to assemble.

Limitations, it's usually permanent, it's hard to reverse.

However, it may loosen under a lot of stress.

Time for a quick check in.

I would like you to match the image of the mechanical fixing to its label.

So we have A, pop rivet, B crimping C, solid metal rivet and D machine self-locking screw.

Have a go at matching them up and come back to me when you're happy with your choices.

Well done with all of your efforts with your matching.

Let's take a little look at the answers and see whether you've got them right.

So for A, pop rivet, we match that to image two.

For B, crimping, we match that to image four.

For C, solid metal rivet, we match that to image one.

And lastly, D, machine self-locking screw we match that to image three, well done folks.

Onto task A, part one, I'd like you to describe the difference between a screw and a machine self-locking screw when joining metals.

Part two, I'd like you to explain the use of tap and die in mechanical fixings.

Part three, I'd like you to explain why metal crimping might be appropriate for a metal fixing.

And lastly, I'd like you to compare pop rivets and solid metal rivets when joining metal, giving one benefit and one limitation.

Have a great go at this and come back to me when you've got some lovely answers.

For part one, I asked you to describe the difference between a screw and a machine self-locking screw when joining metals.

You may have said something along the lines of this.

A standard screw for metals requires a pilot hole and can loosen under vibration.

Whilst a machine self-locking screw has a special feature such as a nylon insert or deformed thread that prevents it from loosening.

Part two, I asked you to explain the use of tap and die in mechanical fixings.

You might have said, a tap and die set is used to create threads in metal.

A tap cuts internal threads in a hole so a bolt can be screwed in and a die cuts external threads on a rod or a bolt.

This allows secure mechanical fixings between metal parts.

Part three, I asked you to explain why metal crimping might be appropriate for a metal fixing.

You might have said metal crimping is appropriate for a fixing because it creates a secure, permanent joint without heat or additional fasteners, making it quick, reliable and suitable for joining thin metal parts or wires.

Part four, I asked you to compare pop rivets and solid metal rivets when joining metal giving one benefit and one limitation.

You might have said, pop, also known as blind rivets are easy to install from one side, but generally not as strong as solid rivets.

Whereas solid metal rivets are very strong and durable but require access to both sides of the joint and more complex installation.

However, that's the one I enjoy using the most.

Well done with all of your answers.

Onto learning cycle two, heat and chemical joining.

Some joining processes use heat and/or chemicals.

So heat processes include welding.

Chemical processes include adhesives.

And then processes that include both heat and chemicals include soldering and brazing.

And we're going to explore a few of these over the next few slides.

So welding, welding involves very high temperatures to melt the edges of metals so that they fuse together to form a strong, permanent bond.

There are a range of different welding techniques depending on the use of the product.

MIG stands for Metal Inert Gas and TIG stands for Tungsten Inert gas.

So MIG and TIG are both electric arc welding methods.

An electric arc is a bright hot spot between the metal and welding tip.

The heat generated by these sparks melts metals so that they can fuse together.

A shielding gas is used to protect the molten metal by keeping it clean until the weld solidifies.

Let's zoom in.

You can see the green part that is the shielding gas and the reddy-orange part that is the electric arc.

MIG uses an electric arc to generate intense heat which melts the edges of the metals, usually steel and the filler metal fusing them together into a strong joint.

The filler material acts as the electrode and is machine-fed.

So where do we use this? We use this in structural steelwork, automotive repair work.

Benefits, it's fast and it works on quite thick materials.

However, the limitation is, it needs cleaning up so that it looks neat.

TIG uses an electric arc to generate intense heat, which melts the edges of the metals and the filler metal fusing them together into a strong joint.

Now the big difference is the filler rod is introduced separately and can join a wide range of materials.

For example, aluminum, titanium and steel.

Take note of that filler rod in the diagram.

So where do we use it? We use it in aerospace and sculptural uses.

Benefit, it is more precise and it provides a neater finish.

However, limitation is, it requires skill and coordination.

Oxy-fuel welding is a portable joining process where a flame from burning oxygen and fuel gas, just like acetylene melts and fuses metals together alongside a filler metal.

So where do we use this? It's often used in car repairs, pipelines and metal art.

The benefit is, it does not need electricity, so it is portable.

Limitation is, it's less precise and much slower than arc welding.

Spot welding uses pressure and heat from an electric current passed through electrodes, which melts the contacting surfaces of the sheet metal and fuses them together at small spots.

Now this one, take note, it does not require a filler metal.

So where do we use it? We use it in car body panels, appliances and sheet metal products.

The benefit is, it's very fast for sheet metals.

However the limitations, it's limited to very thin sheets, which makes it not quite so versatile.

Time for a quick check in.

Which welding process does not require electricity? Is it A, arc welding, B oxy-fuel welding, or C, spot welding? Have a think.

Make a decision.

Come back to me when you have chosen.

Well done if you got B, oxy-fuel welding does not require electricity but the other two do.

Chemical Joining is the process of bonding materials using substances otherwise known as adhesives that create a chemical reaction to hold parts together.

Chemical joining is often invisible and used for clean finishes, not always invisible though.

It's dependent on material compatibility and it is not easily reversible.

Epoxy resin chemically joins by mixing two parts, the resin and hardener, which chemically react to form a very strong and rigid bond.

Epoxy resin has a long curing time in comparison to other adhesives.

However, it is useful for bonding dissimilar metals when welding or brazing is not suitable.

So where do we use it? We use it to bond dissimilar materials.

Benefit, it's very strong and it's also water and chemical resistant.

However, limitations are, that it requires precise mixing.

Now the example that I've got on the right hand side, you can see it's sold as a double syringe so that when you press that orange syringe up it dispenses it at exactly the right quantity of both to create that precise mixing.

Another limitation is obviously long curing time it takes a long time to set.

What makes chemical joining different from mechanical fixings? Is it A, it's always weaker B, it uses force to bond materials C, it requires bolts to join materials or D, it involves a chemical reaction to bond materials? Have a think.

Come back to me when you've made your choice.

Well done if you got D, chemical joining is different from mechanical fixings because it involves a chemical reaction to bond materials.

Soldering is a chemical joining process requiring heat where a filler metal with a low melting point, for example, solder with electronics is melted and flows between parts without melting the base materials.

So you can see in the picture we have a picture of a soldering iron and holder.

And then in the GIF you can see an electronic joint being soldered so that those electronic components are being soldered onto the PCB board.

And we can zoom in and have a little look at what a good solder joint looks like.

So where do we use it? It tends to be electronics, circuit boards joining wires and plumbing.

Benefit, its low temperature safe for delicate components.

However, you do have to be careful with some of the electronic components not to hold the soldering iron on it for too long, otherwise you will damage some of the components.

It is easy to undo if you do need to take a component out.

Let's say you've soldered an LED the wrong way round the legs, the wrong way round, we can undo that.

Limitations are, it does produce quite a weak joint and we are limited in the size of things that we can join together.

Silver soldering is similar to regular soldering, but uses a silver based filler that melts at a higher temperature producing stronger joints.

And you can see that joint in the picture of a ring.

That's a ring that I've made out of copper.

You can see the silver little join between the two copper sides.

So it is a little bit more noticeable, but if it's the other side of the ring, it doesn't matter too much.

Flux is required because it prevents oxidation and helps the solder flow smoothly.

Use, it tends to be used in jewelry, fine metal work, plumbing and instruments.

Benefit, it's much stronger, produces much stronger joints than normal soldering.

Limitations are, it requires a higher heat source, so you need a jewelers torch.

We do ours on the brazing (indistinct) at school.

And you need greater skill too, especially because quite often it's such a small joint that you are joining.

Brazing is a chemical joining process requiring heat where a filler metal is melted at a lower temperature than the melting point of the base metals.

The filler metal flows between them to form a strong bond without melting the base metals themselves.

Flux is used to prevent oxidation and help the filler metal flow.

So where do we use this? This doesn't tend to be in jewelry.

This tends to be on much bigger pieces.

So it's used in pipe work and bike frames.

Benefit is, it can join dissimilar metals cleanly.

Limitation is, it's weaker than welding and it's less heat resistant.

But it's a great process and a great process that is often used in schools.

My students in our school, we make metal legs, metal hairpin legs out of metal rod and metal bar and we braze them together.

Time for a quick check in.

I would like you to match the image of the joining process to its label.

So the labels are the following, A, welding, B, adhesive, C, silver soldering, and D soldering.

Have a go at matching them up and come back to me when you've made your decisions.

Well done with your hard work on that.

Let's have a little look to see if your answers match mine.

So A, welding, matches to image two.

B, adhesive, matches to image one.

C, silver soldering, matches to image four.

And D, soldering, matches to image three.

Well done with your hard work on that.

Onto task B, part one, I'd like you to compare the use of soldering and silver soldering when joining metals and give one advantage of each.

Part two, a designer is creating metal playground equipment.

Explain why welding may be more suitable than brazing for this project.

And lastly, part three, I'd like you to explain why epoxy resin is often used when joining metals in high strength applications.

Give it your best shot.

Come back to me when you've got some wonderful answers.

Part one, I asked you to compare the use of soldering and silver soldering when joining metals giving one advantage of each.

You might have said, soldering uses a low melting point filler making it suitable for delicate components such as electronics, so to not cause heat damage.

Silver soldering uses a silver based filler at higher temperatures, creating much stronger joints, which are durable for jewelry, which may be worn every day.

Part two, a designer is creating metal playground equipment.

Explain why welding may be more suitable than brazing for this project.

You may have said, welding fuses the base metals together, producing a permanent, very strong joint, able to withstand heavy loads and stress.

This makes it more appropriate for playground equipment where safety and structural strength are essential.

Part three, I asked you to explain why epoxy resin is often used when joining metals in high strength applications.

You might have said, epoxy resin adhesives create a strong durable bond that can resist a vibration, impact and environmental conditions.

They distribute stress evenly across the joint and can bond dissimilar metals, making them ideal where welding or brazing is not suitable.

Well done with all your hard work on task B.

This brings us to the end of our lesson today.

Let's summarize what we have found out.

Materials can be joined using mechanical fixings or components.

Mechanical fixings for metals can include rivets, nuts and bolts and screws.

Some joining processes require heat and/or chemicals.

Chemical joining without heat include epoxy resin.

Chemical joining with heat includes brazing and soldering.

Joining by welding requires high temperatures.

Well done with all of your hard work today.

I hope you have enjoyed this lesson and I hope to see you in another lesson soon.

Take good care.

Bye-bye, bye.