Transcript for How Laser Cutting Happens
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The Concise RDWorks Learning Lab with Russ Sadler.
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Session 39 How Laser Cutting Happens. Well, we’ll soon be coming to the end of this series.
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You’ve learnt got almost as much as you can learn about using this technology.
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Now I can’t teach you, as I’ve stressed many times before.
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How to use your machine, how to use the software. My aim is to teach you the technology behind that software and behind the machine,
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because understanding how the laser works would enable you to be creative with your machine.
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As I stressed last time, there’s a big difference between laser engraving and cutting.
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We’ve been concentrating on engraving and we’ve done some cutting. OK.
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But what I haven’t explained to you is, is how cutting works.
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Engraving is easy. All you’ve got to do is what you do with a magnifying glass.
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Focus your lens down onto the surface and you produce a burn.
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But you may well remember, that what I’ve already said to you is that every lens that you
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put in this machine is a cutting lens because it will cut deeply into the surface.
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If you want to do engraving, you normally have to lift the lens higher so that you get a much gentler, non focused beam to work with for engraving.
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Today, I’m going to try and explain to how cutting actually works, the mechanics of cutting.
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Because very few people actually understand that, they use it and they play with it,
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but they don’t understand it. Now as I’ve stressed to you many times before, cutting and damage to material is all about the intensity of the beam.
Transcript for How Laser Cutting Happens (Cont…)
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So let’s look at that this way round and assume that this is a graph. I mean, the bigger the graph, the more the intensity.
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This machine says it’s got a five millimetre beam, but that’s an important factor.
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The beam size, and I think we’ve already talked about that in an earlier session, the importance of beam versus intensity.
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But let’s just step through that again because it’s part of this overall procedure, and I need to refresh your memory.
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So here I’ve got a very simple set up.
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Right, so I’ve got a piece of masking tape on here, very thin masking tape, which has got a very low damage threshold.
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Now we’re also going to do, is behind the scenes here.
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I’m going to put an air jet on the back of this because it’s going to catch fire.
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I can assure you, and I’m going to give it just a gentle pulse like that.
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OK, now, that’s already six millimetres diameter.
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Let’s just keep going. And you’ll notice the little… ooh can you see.
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OK. I’m now going to hold on for a long time, and I’ll count and tell you how long I’m holding it on for. Zero one two three four five six seven.
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Now it’s not growing anymore, so that must be the size of the beam.
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I think you can see we’re actually 10 millimetres, so this is a 10 millimetre beam.
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It’s not five millimetres as per the manufacturer’s specification. Now, there was even power, very, very low power outside
Transcript for How Laser Cutting Happens (Cont…)
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what you see here. Because there was just not enough power outside here to exceed the damage threshold of this very thin and flimsy masking tape.
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But it is still there and we need to bear that diameter in mind, 10 millimetres when we start doing the next part of the exercise.
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Okay, well, I’m going to show you how a cut develops. Now you’ve seen me do this before.
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This is basically a mode burn, the raw beam, with no lens in it at all.
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This is just what’s coming out of the laser tube and fired down onto this piece of acrylic after three mirrors.
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The first one is one second, two seconds, three seconds all the way up to 10 seconds, and you’ll see the way in which the burn develops.
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So you can see in which, you can see the way in which the beam is developing.
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It looks as though I might have got the timing wrong on number three. I think this is nine seconds, and it’s nearly made it through at eight.
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It looks as though it’s made it through at nine seconds and this is ten.
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OK, now let’s look at these in slow motion.
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Now, if you look very carefully, you’ll see there’s a great big heat affected zone on the surface before the cut even starts.
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And something strange is also happening, if you look at the left hand side of the cone, you see that it wants to veer off to the left.
Transcript for How Laser Cutting Happens (Cont…)
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But as the cone develops, it straightens itself up.
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Now that veering off to the left is basically hot gases out of control and they’re trying to get away to the left.
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But the parallel rays that are coming straight down from the surface are straightening the cone up, because remember,
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the outer parallel rays are of low intensity and they’re not doing a lot of damage.
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They’re just creating a hole at the top. But those at the center of the beam are the ones that are high intensity,
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and they’re pushing the beam forward because it’s intensity that’s doing the damage.
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Now look very carefully there, you can see the, you can see the hot gases swirling around inside.
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That’s liquid on the surface plus gases, as you can see.
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Now
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the other thing I must point out to you is that sharp point on the burn has got nothing to do with focus because there is no lens in this system.
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It’s high intensity that’s causing that pointed burn.
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Let’s just take a look at the size of this beam.
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Remember, the manufacturer said it was a five millimetre beam, and all of these are very nicely the same size.
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And what size are they?
Transcript for How Laser Cutting Happens (Cont…)
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I would say they’re six millimetres. OK, now I’m going to use a lens now, but it’s a lens that nobody else would probably even consider using.
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Look, it’s 190 millimetres focal length, seven and a half inches.
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We’ll put it in there and we’ll protect it with a nozzle and proper air
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assist. This nozzle, has got a big hole in it, so it shouldn’t interfere with the beam.
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But I do need to know how far that lens is inside there.
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Give or take a little bit, it’s 26 mm. We’re going to run a one second test again, and we’re going to set this focal distance here.
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It’s not a focal distance, really, it’s just a distance away from the lens. Now we know the lens is 26mm up inside.
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I’m going to set this to roughly 60 millimeters below the lens, roughly a third of the way to the focal point.
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We’re not too fussed. That’s about 34 or 35. So that’s what a one second burn looks like a third of the way to the focal point.
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And we now change the distance to two thirds of the way.
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So we want to add another 60mm to our 35.
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So that’s around about 95 millimetres. OK, so here we’ve got a one second burn, at two thirds of the way down the beam. We’ll see what we get.
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As you can see, we burnt a lot deeper in the same amount of time.
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Let’s go to the focal point itself. Roughly 190 mm.
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OK, now the flame didn’t come out,
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until it went through the bottom of the cut. That was an indication of how long it took to get through the cut. Less than probably three quarters of a second.
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So what you’re seeing here is the development of a cut. Here we’ve got one second with no focusing at all.
Transcript for How Laser Cutting Happens (Cont…)
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Here we’ve got one second with no focusing at all.
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And here we’ve got one second with no focusing at all.
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I’m just going to repeat that last one again.
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And I’m going to reduce the cycle time to half a second. So now we can see what’s really going on. One second –
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no focus, because we were nowhere near the focal point. Remember this point that we’ve got here.
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It’s got nothing to do with the focus, it’s all to do with intensity.
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So now we’ve moved down, two thirds of the way to the focus, and we still haven’t got a focal point, focusing this beam. What we’ve done.
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We’ve just moved down the beam to a smaller part of the beam.
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And then we’ve moved to an even smaller part of the beam here, at the focal point.
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And this is exactly the same thing, as that one. This was one second.
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This was half a second. Now the only difference between these two is if you look carefully, is the size of the burn.
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This has still not got a focal point down in here at all.
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What’s happening down inside the cut has got nothing to do with the focal point.
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The only thing that the focal point is doing is controlling the size of the beam at entry.
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Every one of those cuts was done with a 70 watt beam.
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The only difference was the time that it took to penetrate.
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Just go and have a quick look at why that is the case, just in case you’ve forgotten some of the stuff that we spoke about earlier.
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Now you will remember back several sessions ago when we spoke about this beam and its relationship with intensity.
Transcript for How Laser Cutting Happens (Cont…)
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And we said that this is always a Gaussian shaped beam. But this is the beam size and what the area is under this graph is the power.
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So if that was 10 per cent power, then when we increase the power, we’re not increasing the beam size.
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So if this area here is moving from 10 to 20 as it is here,
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the only way that it can move from 10 to 20 is if it stretches up and this one stretches up to 30.
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It’s still a Gaussian distribution graph,
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but its shape is changing in that it is getting more pointed as you put more power into the beam, for the same size of beam.
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Now what are we looking at here is our beam.
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And you can clearly see. The shape of that beam in these cuts. Now, you can clearly see the shape that that beam in this damage here.
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Now, it’s not so obvious in these, although it’s a little bit obvious here.
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It’s not obvious in these, but that’s because something else has happened and that’s something else that’s happened.
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Is this, we’ve got Gaussian distribution and we’ve got a beam, which we know to be 10 mm diameter.
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So in this example here, what I’ve done, I’ve got a six millimetre beam, which I’ve squashed to three millimetres.
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I haven’t changed the power. All I’ve done is change the beam size and as I change the beam size.
Transcript for How Laser Cutting Happens (Cont…)
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Look what happens to the intensity. We can damage material faster if we’ve got more intensity.
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And that’s exactly what we’re seeing here. Look, six millimetre beam and here we’ve got about three millimetres, two millimetres and one millimetre.
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You can see that that beam is smaller because it’s not had a chance for the, if you
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like, the power down here to open up the beam. When we let it run for one second,
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the diameter of the beam higher up was allowed to be exposed to this higher intensity for a little bit longer.
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But this one stopped short after half a second, and it only had a half a second worth of exposure time in there.
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OK, so now we’re going to go to the other extreme, we’re going to put a one inch, plano convex lens in that nozzle.
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And now I’ve put a twenty five point four mm focal length lens in there. And set it onto the surface at exactly twenty five point four,
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And we’ll run a half second burn. Now a twenty five point four lens should have a smaller focal point.
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And cramming 70 watts into a smaller focal point should make it project further into the work.
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Interesting. I’m going to change that to one second. We can’t cut as deep, and we’re cutting bigger. If we look closely just here.
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You’ll see, there is a sort of a secondary neck. It’s almost
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as though, we’re about one or maybe one and a half millimetres too deep with our focus.
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So let’s change that from twenty five to twenty six point four. Doesn’t seem to have improved it.
Transcript for How Laser Cutting Happens (Cont…)
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Let’s go the other way shall we? Twenty four point four. Ah-ah!
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Not only have we changed the focal point at the top there.
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Look how small it is. It’s also penetrated deeper.
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Look what it’s done, it’s got this really weird shape in here.
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What I call ballooning, you can see this separate spike here.
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Remember, I talked about hot gases running adrift.
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Well, that’s what we’re seeing here.
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We’re seeing hot gases in the bottom here, which are causing the beam, not to run away, because, look, the beam is actually straight.
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Once the light passes through it and corrects it, it goes straight,
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but the hot gas is trying to make it go off to the side. In each one of these cases, as you can see it.
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So let’s just do something else rather strange. I’m going to take the lens out and
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I’m going to turn it over. So that it is flat side up, curved side down, which is totally the wrong way to run a lens.
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So I’m reliably informed. Now, although it won’t be right, we’ll set this to twenty five point four.
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So isn’t it strange? Look, that’s a, that’s a seven and half inch lens, which is producing a smaller cut than that one and a half inch lens.
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This was half a second, this was half a second. This was one second and it didn’t make a lot of difference.
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This was one second, with the focus pushed, pulled out of the work by one millimetre relative to that.
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And that was with the focus pushed into the work by one millimetre.
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And now we’ve flipped the lens over. Let’s see what half a second with the lens the wrong way round
Transcript for How Laser Cutting Happens (Cont…)
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looks like. It cuts deeper than a half second the right way round, and it cuts thinner than a half second
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the right way round.
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There is a small amount of ballooning on here, very small amount of ballooning on here, but you’ll notice it’s got a very, very small entry hole.
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This seven and a half inch lens with 70 watts still beats the pants off of this one and a half inch lens at 70 watts.
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So as we’ve seen with all these tests, if we get a smaller and smaller entry point,
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it means we’re focusing the beam and making it more intense and more intense means faster rate of damage.
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So this should actually get me to Australia without a visa.
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Did it? No, this was the most efficient, a seven and a half inch lens,
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which when you compare that seven and a half inch lens to this one and a half inch lens, it’s illogical. This one should cut much better.
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But what I see is this. We’ve got a very small entry point,
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with this short focus lens, the one inch focus, and this is a seven and a half inch focus lens.
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So the difference in the way in which the angles approach the surface.
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It’s completely different. This has got a very shallow approach angle and a very shallow expansion angle.
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But as I’ve pointed out to you before, what happens below the focal point,
Transcript for How Laser Cutting Happens (Cont…)
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has got nothing to do with these beams here, really. And it’s all to do with what happens at this point here.
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So if we can get a small entry point, we are looking at an energy distribution, which looks something like this.
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It’s a huge, rapid, very spiky, very high intensity, let’s just call it a Gaussian distribution at that focal point.
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And as such., it’s just a mode burn.
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There is no focus involved, the focus is involved in getting the concentration of energy, not in projecting energy down into the hole.
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So basically what happens at that point there is this, and what happens at this point here may well be something a little bit less than that.
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Because it’s got a bigger entry point than this one.
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But once I’ve got my hole established here, what’s happened is, some of this energy here coming in just here and just here is still very high energy.
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And it’s able to pass through that hole and start eroding away at the side of the hole producing this,
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ballooning shape. So we’ve got two effects going on there, we’ve got the penetration caused by the entry hole, which is this bit here,
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and then it’s being followed up by some of these very intense beams, which are close to the centre line.
Transcript for How Laser Cutting Happens (Cont…)
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But because they are at such an acute angle, relatively speaking,
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they will pass through the hole and still do damage and produce ballooning. Whereas in this case,
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we’ve got a slightly larger entry hole. And because these rays here are so close to the axis, just like these are.
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But of course, look at the angle of them, they’re so shallow they’re not really going to do very much ballooning.
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And so consequently, we’re not going to see much of the effect that we see here.
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We’re just going to see penetration. It’s weird, isn’t it, that the shorter the focal length, the less damage you can do,
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even though technically from a, what people call an energy density point of view at the entry point. We should be doing more damage.
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Well, I think that’s enough to keep you interested in lenses for quite a while.
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There’s a lot more to lenses and cutting than there is to lenses and looking at the Moon or the stars.
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So I’ll leave you with an interesting point.
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And you can do your own experiments with lenses, now that you know things are not quite what everybody’s told you.
Transcript for How Laser Cutting Happens
