Session 32: Laser Cutting Science and Basic Rules

Transcript for Laser Cutting Science and Basic Rules

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The Concise RDWorks Learning Lab with Russ Sadler.

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Section 32, cutting science and basic rules. In this session,

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I know you’re desperate to get creating smoke and cutting things, but hey,

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you can’t dive into deep water if you can’t swim and an understanding of what cutting is,

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we started with last time. The beam, the lens and what the lens does.

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And then this strange relationship between the lens and the beam.

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But I must stress again that this part here the shape of your beam is the most important part of cutting.

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If you have a rubbish beam, you will not get very effective cutting.

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You can amplify the intensity in this beam as much as you like, but you still won’t get efficient cutting. Sharp beam, lens,

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good results cutting. The lens itself has got different characteristics.

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It will shape the intensity that you put into it, and it will produce different shaped intensity profiles with your cutting.

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So you need to have a little bit of understanding about what your lens can do.

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And what I would advise you to do is to make sure you download that PDF document with all my test

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data in it so that you can have a look at your lens and see how it affects material as it cuts material.

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Now we will eventually make it to the workshop later on, because there are some important things that

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I want to show you. Some practical things that I want to show you about your steps into cutting.

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But first of all, I want to go back through some of the physics and chemistry of materials that we discussed in some of the very,

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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very early sessions about laser technology. This is partly a reminder, and some of it will be new.

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Now, some of it may look as though it’s veering a long way away from cutting.

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But as I try and explain to you, these things all eventually come together to make sense.

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The most powerful thing that you have with using your laser machine is your eyes and your brain.

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I mentioned that before observation looking without seeing is not really an option.

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You must look and understand what you are seeing. To help you move forward with this, in an efficient and knowledgeable manner.

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So what I’m going to show you, first of all,

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is a step through at twenty millisecond intervals of a pulse hitting a piece of, in this case, half inch MDF.

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And then we’ll move on to a piece of half-inch poplar plywood. Two different materials,

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the same pulse. In this first image, the pulse is only 10 milliseconds long.

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So let’s take a look at what happens when the laser beam hits the wood. A laser beam is invisible.

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So why are we seeing a white splash? Let’s step on.

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Now you’ll observe because of the lighting, the effect of the smoke coming up here, look,

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it’s coming up like a little volcano, and producing a mushroom cloud well away from the work.

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And look, what’s happening to our whatever it is that’s happening here.

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Now, the pulse is virtually finished and look where the smoke is.

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The smoke is up here. Remember, it went up like this, and it’s ballooning out just up here around the nozzle.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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And then we’ll go to probably the last frame in the burn. And you can see where the smoke is.

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Here and here, it’s like a candle being snuffed out. That was a piece of MDF.

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MDF is not wood. It’s a mixture of wood and a plastic binder.

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Let’s move on to a piece of wood now. As you can see, this is plywood. This is very soft poplar plywood.

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This is just pure wood and no plastic involved. The pulse will actually burn a lot deeper in this material.

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Again, what’s this white stuff here?

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Because the laser beam has got no visible light, it’s an invisible beam.

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And again, we’ll take a look here and we see our smoke billowing up like a little volcano and collecting at the top here.

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Let’s push on and watch the smoke, particularly you see the way that it’s puffed up.

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Look, here is our little jet of smoke,

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after the event. There’s a huge amount going on, in those images. Which most people have never bothered to look at or watch or even understand.

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That’s the basis for our first look at some science. This is something called an electromagnetic spectrum, because these are electromagnetic waves.

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Now they do the same thing as sound waves. They hit things and then make them vibrate.

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We’ve got a range that we can see in this spectrum, it’s this colour range here.

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Look, those are all the colours of the rainbow. That’s because we’ve got sensors in the back of our eye, which are only sensitive to this range of frequencies.

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Outside this range of frequencies,

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we cannot see what’s happening elsewhere. The wavelength that we’re going to be working with is ten point six microns a trillion cycles a second.

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And that’s what our eyes are capable of perceiving.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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There are some very interesting properties that happen in this region here and now we’re going to go to materials.

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I think everybody knows what that is

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a picture of: an atom. Isn’t Google wonderful because all of a sudden we can very quickly show you, without any of my dodgy diagrams,

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something that it’s very easy for me to explain. These are individual atoms.

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In a very diagrammatic format that most people will recognize.

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I mean, they don’t actually look like this at all, but it’s a way of describing them.

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Now look, this is H2O. It’s got two hydrogen atoms attached to an oxygen atom.

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Now there is a bond of some sort here, which holds all these atoms together.

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Now look, hydrogen hydrogen, two hydrogen. They’ve both got the same strength of bond to the oxygen atom that’s in the middle.

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That’s the thing that holds them together. Call it love. Call it whatever you want.

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It’s magnetism. It’s gravity.

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I got no idea what it really is, because I’m not a physicist, but there’s something that keeps this molecule together in that shape.

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Now, oxygen is a very, very strange material. It’s very happy to love-in with anybody.

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It’s almost incestuous. Look, it likes to go around on its own and join to itself.

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So you’ll always find oxygen generally attached to itself in the form of O2.

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There is another form of oxygen called ozone, which you smell when you go down into the London Underground or you go to the seaside,

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and that’s O3. But if you start taking a look at these structures, look at this one here.

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This is very open and wobbly, as I would like to call it.

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This one is pretty stiff because there’s only one bond.

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These are quite stiff bonds here, but as you get more complex, some molecules have got very short,

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strong bonds between them and other molecules have got very weak bonds between them.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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The strength to the bond between the atoms is totally variable, depending on the material that you’re working with.

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Let me give a very silly example. You’re at a birthday party and you’ve got jelly on the table.

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You’ve also got a birthday cake on the table. If you just gently bang on the edge of the table, which one will wobble?

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The answer is obvious it’s the jelly, because the jelly is obviously a very open structure like this.

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It’s not a stiff structure, and it very easily wobbles.

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It’s a little mechanical analogy that I want you to remember when we move on a little bit further.

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Everything around you is made up of molecules and atoms. And you might not believe it, but they are all,

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doing this, vibrating. Something that they didn’t tell you when you’re at school is that that vibration in a molecule or an atom is its temperature.

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Let me just put that a different way; if I can make a molecule vibrate faster.

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It’s going to get hotter. You only imagined things getting hot,

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but what you don’t see is that they’re getting so hot that they’re probably breaking faster and faster and faster.

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And hang on, there is a limit to how much these bonds will stand.

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If you shake something hard enough, it will break. And that’s what happens to these bonds when you shake them hard enough.

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So these two things heat and vibration, all are interchangeable.

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You’ve never imagined molecules vibrating and shaking themselves to pieces.

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You only see things burning if we break these bonds here, for example,

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this oxygen will go off on its own and it will join with something else because it doesn’t like being alone.

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So we get a chemical change when we shake these molecules to death. Such that they self-destruct and they break apart.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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Our laser beam is generating a frequency at around about 28 trillion cycles a second, 28 trillion vibrations a second.

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Now that sounds unbelievable, but it just so happens that that is the frequency at which molecules and atoms can be stimulated to vibrate.

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So if we fire our laser beam at a piece of material, it will stimulate the molecules,

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make them hotter or make them more energetic to the point where they will eventually fly apart and change into a different material.

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So that’s the way in which our laser actually works. Now we’re going to jump around to another subject: Light. I know we’ve just been dealing with light.

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But hang on. Look, I’ve got a handful of light here at the moment. Can you see it?

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What colour is it? Well, look, I’ve got some radio waves in there. I’ve got some X-rays.

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I’ve got all sorts of waves in my hand. They’re all part of the electromagnetic spectrum and I’ve just got a handful of them.

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Where’s the colour? I said to you, this colour in that spectrum.

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And that colour comes from the Sun. Basically, it’s white light.

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It’s frequency of light that comes from the Sun and we see it around us all the time.

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It’s invisible, but it’s there. The problem with light is that it’s energy.

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It’s not anything, really. It’s photons, little packets of nothing that are energy floating in the air.

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Until they hit something. Now look, a stone falling from the sky is not in the least bit dangerous. Until it hits you on the head and then you know that it had energy.

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The same applies to light. What colour is that?

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You’re going to say that’s white. Well, not entirely true.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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That’s all the colours of the rainbow there. Mixed up so that you can’t see them, but that’s what makes white light.

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All those colours. So what colour is that? Yes, it’s a sort of a purply colour.

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But why? Why is that different to this piece of paper?

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There’s every colour of the rainbow hitting both of those pieces of paper with white paper, every colour is reflected back to your eye.

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And you still see it as white light. It wasn’t white until the photons hit it and did something,

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and they made it reflect back to your eye where you see it as all the colours of the rainbow mixed up as white. With this one,

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the only difference is, that all the colours of the rainbow that are not mauve have been absorbed into that

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paper. And the only thing that’s being transmitted back to your eye is the colour combination that makes mauve.

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So, white is total reflection of all the light, all the white light.

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And we only call it white light because that’s what you see.

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In fact, look, there is no colour to this stuff here, which is light.

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Look what colour is that? I know it’s black.

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It’s a colour, but it’s only a colour because the light is hitting it.

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How much of that light is being reflected to your eye? How many colours of the rainbow can you see?

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The answer is none, because all of the light has been totally absorbed by the surface.

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There’s none of it being reflected. It’s a colour black.

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But in fact, it just means there’s no light being reflected from it.

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Colour is a very interesting subject on its own, light.

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And how colour appears to us in our eyes. We’ve only got sensitivity to that range of colours that I showed you on the electromagnetic spectrum.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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So you’ve got a little bit of a flavour now to why you’re seeing black, white and colour.

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We only see the colour in light because of the non-absorption element of it.

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Now here’s an interesting question. What colour is that? What’s happening to the light?

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Basically, in this instance, there is no absorption of the light.

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It’s not being absorbed is going right through. Look, you can see what’s happening in the background.

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The light is passing right the way through this and it’s not taking any notice of it at all.

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Light is very strange. This is just water. I have to admit, I have put one drop, one drop of washing up liquid in there.

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Now. Watch this. What colour is it now and why?

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OK, so it’s turned white. And this is an analogue for what you’re going to see in this video.

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I’m going to say that that is smoke in a bottle. Slightly the wrong way round.

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What you’ve got in here are air bubbles, which are playing with the light and reflecting it and making it white.

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Remember, total reflection. When we see smoke in air, and

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it’s white, like steam, is a good example.

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You know, steam is still water, but it’s micro droplets of water that are so small that they can actually float around in the air.

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But they are still micro raindrops, solid water droplets, very, very small.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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Now the important word there is solid. Now you remember what happens when light hits a solid surface, it vibrates the molecules.

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Let’s go back to our pictures that we started the session off with and just take a look again to see if we can have a different

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perspective on what we saw in those pictures with these little bits of information that I’ve given you these little science facts.

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The one thing I must state is that laser light can pass through air, gases without any problems at all.

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It only wants to hit solid things. Things are molecules that make up solid items.

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You can clearly see that our laser beam here is passing through air.

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It’s invisible here, but it must be doing something to something else here.

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Not only have we got our heating and our reheating of this crud that we’re producing down here.

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We’re squirting the material out from being solid into something very, very much more volumetric and gaseous.

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And then along the way, we’re also producing, look, this plume of white stuff. Hang on!

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This is more white stuff. These are solid particles of whatever this is down here, but it’s suspended in the air.

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That’s why we can see them as white, just like fog or steam.

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OK, so this is not just gas, this is solid particles suspended in the air, and that’s partially what we are probably reheating and reburning here.

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But the key thing that I’d like you to see in this picture, is the way in which this white plume of smoke is going upwards.

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Then we move on to the next frame. It’s starting to explode up here big, like an atomic cloud.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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The next frame, we’ve virtually finished. Yes, we have.

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So, look, the energy has ceased here. It’s just a little teeny weeny reaction at the surface

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there. A continuation of the burning action that was stimulated by the laser beam.

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But the laser beam is no longer there. But what we’ve got look, is our tell-tale puff of smoke that went straight up.

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So this is only 10 milliseconds of pulse and in 10 milliseconds, we have not converted a huge amount of material here.

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Some of this material has been burned, and some of this material has been left as smoke.

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Now I’m going to go all the way up to 500 milliseconds now for a Pulse. In half a second,

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we shall probably burn through this material completely. We’re converting, so much material,

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it can’t come up here quick enough. It’s having to fly out sideways.

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Look at it. Oh, my goodness me, look what we’ve done up here.

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So there are several things I would like you to take note of in this image.

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First of all, the actual nozzle is about 15 mm away from the work surface.

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That’s quite a long distance from a cutting point of view. But the moment, what I want you to observe is the fact that this very powerful

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beam and I’ve got probably fully 70 watts firing straight down at the material.

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Those 70 watts of energy are concentrated into a very small area,

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but that small area is instantly being converted into a different chemical because the light is shaking the molecules,

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making them self-destruct, turning them into smoke, vapor, gases.

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Tars. all sorts of things are happening at that surface there, but it’s happening at such a high rate that the smoke is exploding sideways.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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It’s no longer drifting upwards, as we saw in the first images.

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So that’s one thing I want you to note; the way in which the smoke is actually exploding sideways.

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It’s no longer volcanic. The second thing is, look at the diameter of that flame.

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That’s not the laser beam, that’s the laser beam that has ignited some of the fumes.

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Remember what the white is? The white is solid particles suspended in air.

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Remember, the laser beam has no effect on gas or air.

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It only has an effect on solid particles.

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So the fact that we can see a flame, which is going right the way up to the nozzle means that that laser beam is using its energy

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to ignite those particles.

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Now, if it’s using its energy to ignite those particles, it’s not using its energy to convert molecules down in the pit that it’s making.

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The first thing is, what is the cutting efficiency here? Pretty low.

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Because we’re allowing the smoke to get in the way. Now this is what happens every time you start a cut.

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The ideal situation is to force this smoke out the bottom of the material.

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If you force the smoke out the bottom of the material, it doesn’t get in the way of the laser beam and absorb energy that could be used for cutting.

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But the very first part of cutting is drilling a hole through the material.

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Only when you drilled a hole through the material have you got somewhere that you can send the smoke out from,

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then your cutting becomes more efficient, if you can make the smoke go out the bottom of your cut.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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I haven’t got any air passing through that nozzle. Now at this moment in time,

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there’s a pretty fair chance that that smoke is going right the way back up inside the nozzle and contaminating the face of the lens.

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Now, if you take a look at the top of the picture there, you’ll see that there is a pipe going into the nozzle.

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And that pipe is in there purposely to send air into the nozzle. Because provided we can get a flow of air through the nozzle,

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then it will suppress that flame,

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push it down, keep the fumes away from the lens itself. So we can protect the lens with air flow. Right, now just above my nozzle here.

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I’ve got a little tap. It’s a ball valve which is controlling my air assist airflow out of this nozzle.

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Now this nozzle has got a big hole in it, so there’s not a great deal of directed airflow.

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It is a strange airflow coming out of here.

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It’ll be totally chaotic. There we go,

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you can hear it. Now that is my pulsing,

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silly little Chinese air pump that they supplied with the laser machine six years ago. We’ll to do the same test again with a 500 millisecond pulse. Immediately,

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one thing I think you can see. Is this shape, it’s a smoke cone.

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Look, it’s no longer emanating up this way as a volcano would do, explosively upwards.

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Look, we’ve got smoke, which is being blown down.

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This smoke that would normally be wanting to go up is being forced down onto the work by the air

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assist that’s coming out of this nozzle. And look, it’s blowing out the side here right across the work.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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The actual fumes themselves this time are not going as far up towards the nozzle, because we’ve got the air that is driving this,

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let’s call it gas, before it burns downwards.

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But you can clearly see the path of the laser beam itself. It’s straight down, and it’s through the centre of the nozzle.

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Okay, so now I’m going to do a cut with the same power, but away from the camera so that we can see what happens.

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And again, you’ll notice we’re getting air blown everywhere sideways.

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But you look carefully, you’ll see that there is much happening out this side, but a lot of the air is blowing out this way.

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We’ve got some sort of bias happening.

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Maybe it’s because of this air assist direction here, but we’ve definitely got a bias to the left with this smoke if you watch.

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Can you see that puff of smoke going out left virtually nothing coming out here to the right?

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We’re going to run the same line away from the camera, but this time with no air assist.

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And wow, I’m no good this way.

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Now you’ll notice with no air assist how all the smoke is going upwards.

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There’s nothing going down anywhere near the work, so stop there because it’s enough.

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Now this is the poplar plywood. Neither of these cuts are going right the way through.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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So they’re both blind cuts, which means that all the fumes that are produced have to come upwards.

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Now, in this case, the fumes coming up, which have been blowing back down with a left biased air assist, and that’s why we’ve got this here.

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It’s not on the right hand side, it’s here on the left.

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When we had no air assist, we’ve got a nice clean cut because as I pointed out to you, all the fumes were drifting upwards.

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They weren’t going anywhere near the work and we had exactly the same results of the MDF.

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Look at this super clean cut here. Haven’t even got a start point because all the fumes were being burnt out there.

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Put the work onto spices. 10 No spices to get it off the deck so that we don’t get those marks on the back and we’re going to attempt through cuts.

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So we’ve got plenty of power there. We’re going through very nicely. So the cut was through.

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Something is happening here because we’re not getting air flowing through the cut, which keep still getting some crud here on the left hand side.

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So we must be getting some hot wash a Fuze when we move along this cut.

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Some of the cut is deep, but there will always be a piece of cut that is right on the surface because remember,

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we’re cutting this slot molecule by molecule.

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And the first molecules that are cut and disturbed are those on the surface, and there’s nowhere for the fumes to go on the surface.

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And so unless we get some well directed airflow to suck the fumes down into the cut and the bottom of the cut,

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we’re going to get some debris on the surface here. Now again, we’ve got this left biased debris because of the airflow from our nose.

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So look, here’s our nose. We’ve got about a six millimetre hole in the end.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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It’s a huge hole. So here we have what I would call a cutting nozzle.

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It’s got about a two or two and a half millimeter diameter hole in the end of the nozzle.

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So this won’t produce a waft of air. It will produce a jet of air.

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Do we want a jet of you? Listen to the sound difference. First of all, because it’s restricted, you can hardly hear the policy.

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It’s another piece of physics which I’m not going to go into. Let’s just see what difference this house.

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Remember that smoke that is busy absorbing your energy because it’s coming up?

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We’re pushing the smoke down as fast as it’s being produced.

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We’re not allowing it to go near the rule.

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Lovely new naked laser beam is being blown away from the laser beam.

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And there we go. A lovely clean cut. And the bottom, not all your classes are cut.

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Now is it cut it would come through, but basically if we take a look at the difference between the top.

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And the bottom. Effectively, what we’ve got, we got a v cut right at the top, very narrow at the bottom there.

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That’s going to do is going to choke the air flow through the cut.

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It’s not going to allow very much air flow through the cut. So we’re basically although we’ve got what many people regard as a good through cut.

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I don’t regard that as a proper good quality, cut some and take that down to about seven.

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We run another beside it so that we can compare them. And I know we’ve got a cut, which is much the same on the top as it is on the bottom.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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And that means we’ve basically got a parallel cut through which the air can flow and it does not get choked.

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I’ve got my solid table on now, which is a piece of nice cold steel and I’ve got some 3mm spaces and some three millimeter hgf.

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Now I’m going to do the same thing that I’ve just been doing. We’re going to run a line away from the camera.

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OK, so two things aren’t going to make Kit note.

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Number one, if you catch in the light, you can just see a shiny edge to it.

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So there’s some sort of override, there’s some sort of painting that’s taking place along the edge of the cut near the back of the cut.

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He’s not bad. But look what we’ve got here.

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Look at this. That’s what would normally go onto the surface of your work.

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That’s the tar that’s being generated in those fumes that I talk about.

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That’s the liquid content of that white smoke. I’ve just condensed it onto this very cold surface.

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So if you’ve got any doubt as to what that brown stuff is on the surface, if you have a tube, it’s not burning.

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It’s very condensed to just clean off a little teeny weeny bit of acetone,

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put on 10 mm spices to keep it away from the surface because although I got a reasonable cut through,

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I got terrible back reflection off of this base plate.

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So we’re still getting some reflection off the back plate, so I’ve got to go even further away.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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On the back of got rid of all the reflection on the front, I got rid of all but just the merest hint of that marking,

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though it was talking about because most of the air is going through.

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But I say most of the air because look again, we’ve got a much smaller cut on the back that we have on the top.

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And that means I’ve got a tapered cut on choking the airflow through the cut.

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So I’m running too fast, but it’s getting better.

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As you can see, we’ve got a much wider cut at the bottom now.

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Then again, I’ve got a lovely, clean mark, free cut on the top.

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OK, so here I’ve got a piece of 3mm birch plywood.

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I’m going to use the same settings at the moment, the same speed, but I know I should be able to go a lot faster.

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But let’s just run this very slow speed to start with. And we’ve got a very nice clean cut on top.

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And the same thickness of cut underneath. So nice change to a quite a fast speed that 30mm a second.

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Two things, you know, first of all, the thickness of the line is just about through its adjust cut.

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But look, what’s happened to the surface?

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There is a hint on here the throwing around the edge because I’m not allowing the air to pass cleanly through the cut.

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I’m getting some backwash from the cut itself, so I’m running too fast.

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OK, so we know getting a better cut on the back, but it’s still not for which cut, but we virtually got rid of all the marking on the surface.

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We’ve got airflow through the cut a little bit slower.

Transcript for Laser Cutting Science and Basic Rules (Cont…)

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Oh, I got a lovely clean cut on the top. And it’s becoming a half decent cut underneath here.

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And that looks like a very nice cut.

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No marks on the top one little burn mark here on the outside, where we always get a burn mark because that’s where you start,

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you’re blowing fumes upwards because there is no hole through which to blow the fumes.

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My recent work has made me understand exactly how lenses work and what you see here is me just about to cut a piece of hardwood 26 millimeters thick.

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Using only 60 watts and a two and a half inch gallium arsenide lens used the wrong way round flat side up.

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This is a quick hit, the video it did a couple of months ago.

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I set my lens a millimeter off the surface, and I’m only using my silly or Chinese pulsing pump.

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And look, it’s cutting right through it. He’s just hanging on by that last little bit at the end, which may have been a not.

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And looking square and clean, the cut is.

Transcript for Laser Cutting Science and Basic Rules