Session 31 – Laser Beam Cutting: Where Do We Start?

Transcript for Laser Cutting: Where Do We Start?

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The Concise RDWorks Learning Lab with Russ Sadler.

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Session 31 Cutting: where do we start?

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Well, you’ve seen this before. It’s a lens! You’re very familiar with lenses now because you’ve been using them for all your engraving.

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We’re going to change the subject now. Because we’re going to talk about cutting. Now if you

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remember, way back in session 18, when we opened up the discussion about engraving.

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I said there was no such thing as an engraving lens all lenses are basically cutting lenses.

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Well. That is essentially true. But before we get to the lens, we’ve got the laser beam.

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It’s a very common term in computer language. Rubbish in equals rubbish out.

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Well, it equally applies to this. This is only an amplifying device for intensity, light intensity.

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If you put rubbish light intensity in, you’ll get more of it out.

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It’ll intensify it, but it won’t intensify it very much.

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You get out an amplification of what you put in.

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Now everybody knows what lenses do. You’ve seen this picture before back in session eighteen When we introduced you to lenses for engraving.

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And you may remember at that point, I said this picture is total rubbish.

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This is what the lens manufacturers would have you believe happens.

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Well, you’ve already seen that. We don’t need the focal point.

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We don’t need the intensity that they talk about here to do engraving. With engraving,

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we just want to damage the surface of the material. Most of the time.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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And so consequently, we’re not interested in this very, very fine focal point here.

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And the very high intensity that they claim for that focal point. You’ll notice

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the cynicism in my voice, because I’ve spent a long time trying to understand how lenses cut.

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And lenses do not cut the way that you believe.

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Let’s take a look at this one as a classic example look, the rays come in, they hit the focal point,

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which happens to be the surface of your material and you get huge intensity of light at that point to do damage to your material.

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What then happens to that light? It dissipates. Now you remember when we were trying to find the way to get a soft beam,

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that only damaged the surface of the material and did not cut into the surface of the material. We had a lot of trouble.

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This was an inch and a half lens that we were using and we had the power turned right down.

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But we still were penetrating into the surface here with this very deep cut.

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I mean, this is somewhere in the region of about two millimeters thick.

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This top layer of material on this plywood. When we turned the power up to 100 percent.

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That same lens was able to do this amount of damage to the material, it’s gone right through.

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Is that the sort of cut that you’d expect after the focal point?

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Look what’s happening to the beam after it passes through the focal point. Look, here it is, you can clearly see it comes to zero and then it grows again.

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The lens that I used for this job was focused onto the top surface of the material.

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So two questions arise; Number one. Why have we got a cut?

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And Number two; look at this.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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It’s not just a cut that passes through the material because I’ve increased the power, that should not be possible according to that drawing,

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because the intensity is now doing this.

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It’s diverging at an alarming rate and there’s no intensity there to actually cut through that material.

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Now I’ve approached, learned professors of physics.

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I’ve approached senior people in the laser industry that design these machines,

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and I’ve approached the lens manufacturers to ask them, can they explain why and how their lenses do this?

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As you might expect, the people in the lens industry basically tell me to go away.

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What happens after the focal point and how you use and apply the lens is basically your problem.

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People in the laser industry and learned professors come up with all sorts of maybes, things like; Well, you know,

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once you start producing a gas in here, the laser beam finds its way in somehow and it vaporizes or ionizes

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the gas inside there and it gradually finds its way through?

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Did you believe that? It sounded like Bullshit to me, to be honest.

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So the first thing that we must say is, you must throw away all your conceptions

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of how lenses work because they do not work the way that you think they work.

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Crude lens theory was around in the time of the Greeks. The Egyptians used it.

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Then we get as far as the Middle Ages, where people like Galileo started inventing the telescope and Kepler and people like that.

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So this lens theory? Tracing of rays like this.

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Look, we’ve got rays, we’ve got rays. That’s the outline of where people think the beam is. Lasers themselves,

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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have only been around for about 60 years. Serious industrial application for cutting materials probably only about 30, maybe 35 years.

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So this is a very young technology. People that design these laser machines,

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they take their technology and put it through something that’s been existing for hundreds of years.

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It’s a proven technology. Why would you question it?

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Here’s how it works. Well, now I’m afraid it’s your misfortune that

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you’ve encountered somebody that does not believe any of this.

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Essentially, there’s nothing wrong with the lens theory.

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If you had a camera, a telescope, a microscope, a projector.

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This stuff works a 100%. It’s well proven, but when you throw a laser beam at a lens, that’s not the same as throwing an image,

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which has got fairly uniform light across the image.

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No, we’re interested in only one thing; light intensity. Light intensity is the thing that causes material damage.

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Remember how material damage happens, you’re busy shaking molecules and the more energy you can put to shake those molecules,

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the faster you will be able to create damage. We’ve created damage here, which tells us exactly what the profile of intensity is. Here,

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we’ve got a parallel beam which doesn’t really tell us what the profile of the intensity is, but it tells us where the intensity has passed through.

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And look, if we look just here, you will see down at the bottom here, we’ve got a hint of this taper.

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Yes, we’ve broken through the bottom, but we haven’t broken through with the beam fully.

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We’ve still got part of the tapered beam there.

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So in the instance of this cut, it’s very interesting to see that we’ve got two different types of material.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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We’ve got material which is this type.

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Then we’ve got a softer material, which, if you look, has got a little bit more damage taking place and then we’ve got this harder material again,

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and then we’ve got this softer material again, finishing up with the hard material.

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And you can see the slight difference in damage as the beam has passed through all of those different material structures.

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Now the other observation that I’d like you to make is the whole of this surface here has been cut with the laser beam.

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But you notice this is a different colour to this, but it’s all wood.

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This happens to be end grain. This happens to be long grain.

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So the grains are going in the opposite directions and you can see here, look,

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we have got a certain amount of dark brown in certain parts of this grain structure.

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But other parts are completely white, and uncharred. I said to you, that engraving was a complicated process.

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Cutting is actually simple. But just as complicated in different ways, because then you need to understand much more about the materials themselves,

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the damage that you caused to materials when you’re only trying to mark the surface.

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You’re not worried about penetration and rate of damage. In fact, if anything, we were trying to do exactly the opposite.

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We were trying to minimize the amount of damage that we put onto the surface and just produce this brown scorching.

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We found how difficult that was because look, this little cut here, even though it’s done with high intensity light, is not brown.

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Remember, we looked inside that and we saw that it was absolutely clean.

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What you saw from the top as a black line was, in fact, the occlusion of light, just the fact that it’s a deep groove.

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This has got some sort of charring, some sort of carbon surface that’s left behind.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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You can see from my little torch here that I’m shining on the surface.

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There is no real edge to that beam. It’s very, very bright in the centre and it sort of tapers away to almost nothing on the edge.

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Well, that’s the intensity of the light disappearing towards the edge.

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High intensity in the center. Low intensity at the edge.

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Well, that’s what that picture shows us.

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That’s exactly the situation happening within a laser beam.

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The perfect laser beam will have an intensity distribution. As per this curve.

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It’s a curve called a Gaussian distribution curve.

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Even though the proportions of it might change.

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The same mathematical formula is used to generate this curve, regardless of what the shape actually happens to be.

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Now what I’ll show you here is a beam that is seven millimeters diameter.

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I’ve just chosen some numbers here. They’re just numbers to show the relative differences and what happens within a laser beam.

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This curve represents two properties of the laser beam.

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On the one hand, the area underneath that curve is its power.

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Now, in this instance, we’ve made it 50 watts and the area is 1621 square millimetres under that curve.

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So 621 square millimetres represents 50 watts, and that happens to give us a curve, which is a height of 50mm.

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Now, if we keep the base dimension of this mathematical formula the same and we change to 100 watts.

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Well, if we go from 50 to 100 watch, we expect the area to go up by double.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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And sure enough, here is look 3242. But 3242 area with the same base has caused the curve to change to this shape.

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This is a sharper curve, then that one, let’s call that one blunt and this one sharp.

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If you buy a 100 watt tube. To replace the existing 50 watt that’s in your machine, you will get twice as much cutting power from it.

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Well, sorry, that isn’t the case.

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That isn’t the way it works because generally when you buy a more powerful tube, the diameter of the beam increases as well.

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And so now we’ve gone from 50 watts to 100 watts, so we’ve still got 3242 as our area.

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But because the diameter of the beam has increased. For the same amount of area under there, and you would expect the intensity to double.

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It’s only gone from 50 to 77, so it’s only a 50 percent increase in intensity for a doubling of the power.

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Now here you’ll see a block of, a little block of acrylic and I’m going to fire the laser beam at this block of acrylic.

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Now, just down at the bottom here, you’ll see that I’m blowing some air at where I’m going to fire it, because I don’t want it to catch fire.

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But this is about 70 watts of power on a

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probably what the manufacturer claims to be a five or six millimetre beam.

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Now, if you watch, you’ll see the centre of that beam growing at a much faster rate than the outside.

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The Outside is staying virtually the same diameter. And there we go.

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Let’s stop there. So that’s the outside diameter of the beam.

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And this is the centre of the beam, which has done a lot more damage to the material because of its higher intensity.

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This is not a focal point. There is no lens in this system.

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The sharp point on this burn is nothing to do with focus.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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It’s all to do with the intensity distribution within the beam itself.

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This part of the beam has got a much higher intensity, so it’s burning forward at a faster rate.

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This little demonstration is probably the most important demonstration that I can make to try and illustrate how cutting works.

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It took 10 seconds roughly for, say, a six millimeter beam to burn through 25 millimeters of acrylic.

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Now that’s just a gauge for you. But that is the shape that you’d expect from a cutting beam.

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Now I’ve turned the power right down to 15 percent. And here you’ll see what happens for the same 10 seconds of burn.

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So you will notice the bubbling that took place in there as the acrylic was boiling and then it vaporizes and turns to acrylic vapor at 200 degrees C.

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But look how relatively blunt the beam in.

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So there’s the sharp beam at the bottom with its 10 second burn.

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As I’ve illustrated here, 10 seconds of burn produces this sharp form.

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Here we’ve got a blunt beam because we’ve turned the power right the way down.

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OK, so it’s not doing the damage as quickly, so we can get both a sharp beam and a blunt beam out of my tube just by adjusting the power.

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We went all the way through a piece of 10 millimetre thick material with this shape of

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beam and this shape of beam was doing just the smallest amount of damage on the surface.

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The point I want to stress here is that the beam itself is very important to your ability to cut.

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Many of the tubes that you buy online are basically junk.

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They cannot perform this function here of a sharp beam.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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They will only be able to do something like this, fine for engraving.

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No good for cutting. Now this is a gaussian curve, which very conveniently,

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I’ve just made six millimetres diameter because look, we’ve got one millimetre, two millimetres, three millimetres either side of the centre line.

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The first central two millimeters of the beam contains about 70 percent of the power.

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The blue sections around the outside each contain about 10 percent and the remaining five percent each side

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here, is the remnant part of the power, right at the extreme edge of the beam.

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Power is not the same as intensity when we allow that beam to fire into a piece of

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material after one second, two, three, four, five, six, seven, eight, nine, 10 seconds.

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This is the shape that we will get, a nice sharp burn.

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Now it’s important to remember these three coloured zones. This red zone has got virtually no intensity in it.

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The blue zone? Well, it’s got some moderate intensity in it, but the real working zone of your tube is this high intensity part of the tube.

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So let’s move our attention away from the beam itself. Remember rubbish in, rubbish out.

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This is not rubbish. This is a good quality cutting beam, and we’re now going to talk about what happens when we pass it through this amplifier.

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This energy amplifier, which you call a lens.

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The most common lens that you’ll come across is something called a plano convex lens. Flat on one side, curved on the other.

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The way in which these lenses are designed, is for the light to come in, in a parallel manner.

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Parallel to the axis of the lens itself, and pass through the lens to something called the focal point.

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You can see that I’m pointing here at something which appears to be the focal point.

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But if we look at this a little bit more carefully, what you’ll see is that all the rays are not actually focusing at one single point.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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There is a range of focal points that’s happening here.

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It’s manufactured with a spherical surface, and that spherical surface has a property called spherical aberration.

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An aberration basically means that it doesn’t all finish up at one fixed focal point.

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If we look carefully, you’ll see that these rays from the outside are actually crossing over here.

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Whereas, the rays from the centre are crossing over here.

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So there’s a difference between the focal points across different diameters of the lens.

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So it is incredibly difficult to get a decent single focal point out of a plano

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convex lens. If we use the lens the wrong way round and we put our parallel rays of light onto the flat surface of the lens,

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aberration becomes absolutely ridiculous.

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This is an exaggerated picture. It demonstrates exactly what happens when you flip a lens over. These rays from the

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outside focus at an even earlier point than they did when it was used the right way round.

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And look at these. Look at the discrepancy between the focal points of all

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these various rays that are coming in from these disparate parts of the lens.

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And let’s have a look what happens to these red rays from this low intensity region of the beam?

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So here are these red rays. And we’ve got another one here.

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So there’s our red zone. Hitting the material, there’s not much intensity there.

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Let’s take a look at these rays that are coming in from the blue zone.

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So there we are. Look, we’ve got our blue zone coming in and it looks as though our blue zone is actually focusing onto the surface.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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It is very small, so we have still only got around about 10 or 15 per cent of the available intensity that we’re applying to that surface there.

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Now this yellow zone is not focusing until we get down to here,

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and it’s only at that point that we get a true central focus onto the surface in the material.

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Most of you guys will be using your lens this way round. You will get moderately good cutting if you use the lens this way round. Because

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there is sufficient aberration in here to still create this filtering effect.

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Now these learned professors and these lens people don’t happen to have one of these machines tucked away in their shed in their back garden.

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So they’ve got no idea of how their lens actually work,

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when we ask it to cut material. But my study of the way in which lenses and laser beams work together start to reveal

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this strange mechanism, the filtering mechanism that’s caused by lens aberration.

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Lens aberration? That’s the last thing that you must talk about when you speak to a lens manufacturer. Aberration is our friend, not our enemy.

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Now, as I’ve said, it’s taken me a long time to decode how cutting works.

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But one of the interesting stages in that progression was this experiment here that I carried out.

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You can see that we’ve got parallel rays of light coming from our laser beam, which has got variable intensity firing at a piece of material.

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Now you’ve seen this experiment, 10 seconds fired into that material produced a point at the end of the acrylic block.

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There is no lens in that system. That point was all to do with the intensity difference between the outside and the inside centre part of the beam.

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Here we’ve got a lens which I’ve put into this system. It was a very, very long, focused lens.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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It started off here at, say, six millimeters diameter,

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but at the time it got down to here it was maybe five millimeters diameter because the lens had already started to decrease the diameter of the beam.

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But look, I produced exactly the same sort of pointed burn,

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but this time it only took three seconds to pass through the 25mm block, not the 10 seconds that it did before.

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And here are those two results. This was the one that took 10 seconds.

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And this is the one that took three seconds.

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Now you may or may not be able to see that there is a significant difference in the diameter of this one compared to this one.

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A much smaller beam and a higher concentration of intensity.

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And that is why we were able to burn through quicker. This is still not focused because the focus is down here.

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It’s just a smaller diameter start point.

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And that really is the whole essence of what cutting is about.

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And then when we change the focus and reduce the diameter of the beam to an even smaller entry wound.

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Look how quickly it runs in. And now we move the focal point even closer.

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Now you can clearly see the difference in the diameter of the entry points of these beams.

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Slow. Faster. Very fast. Okay, now look you can see what we’ve got here.

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We’re getting more of a parallel beam.

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It’s tapered here, very tapered here. But here we’ve got a parallel beam.

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This is beginning to get much closer to what we see with a deep cut, a parallel cut.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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Look what we’ve got at the top here, though. You see, that’s the effect of the blue zone creating a cone at the entry to the cut.

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So we got some of our medium power doing damage.

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The higher the intensity, the faster the damage can be done. And here is a classic example of that. The speed at which damage is being done,

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is all to do with the concentration of that energy by the lens. Decreasing the diameter of the beam through a lens towards

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its focal point, gradually reduces the diameter of the beam and increases the intensity at the centre of the beam.

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We started off before the lens with this nice Gaussian distribution, we start putting these various intensities of light through the lens,

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and all of a sudden we get some sort of strange filtering effect down here, as you can see as we get closer to the focal point.

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Now, this is not a focal point of image.

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This is a focal point of intensity because we’ve got lots of different intensity focal points sitting in this beam. When we sample across

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there, is no longer a nice Gaussian distribution.

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Effectively, what we’ve done there, we’ve pushed the yellow up here like this because it’s intensified.

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The red beam is out here somewhere.

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Virtually useless and the blue beam, we’ve will actually change the shape of that

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gaussian intensity to something completely different than what it was when it started out.

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We’ve actually intensified the centre part of the beam itself, and we have filtered out or decreased the useless part of the beam.

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That’s what this lens has actually done with the aberration effect.

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So let’s take a look at what’s actually happening just here.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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Because remember,

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the yellow zone is not not just a zone. It is an area where the intensity is gradually going up and up and up towards the centre of the beam.

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Like that, so the center of the laser beam is still much, much higher than the edge of the yellow zone.

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This is the entry point. This may be only 0.2 of a millimetre diameter.

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The beam is coming in here, but this is effectively the intensity zone.

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The yellow intensity zone of that beam, which has been grossly amplified.

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Now outside this zone, we’ve got our blue zone.

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And outside that. We’ve got our red zone. Now our

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red zone is going to have no effect on our material because it’s such a low power to start with and it’s diverging and becoming even lower.

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So it’s intensity is unimaginably small. It’s never going to exceed the damage threshold of the material. The blue zone,

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well, given enough time. It may well have an effect.

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But the first effect that’s going to happen. Remember back to the pictures a few moments ago, the smaller we make the entry wound, the faster

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the damage will occur. So we’ve decreased this damage zone to very, very small proportions.

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So any intensity that happens to exist at that point is going to have a very, very rapid effect at damaging the material.

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And so consequently, what we’re going to get, we’re going to get the beam,

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which remember has still got a Gaussian distribution because we’ve not messed around with the yellow part of the beam specifically.

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That is still going to have a very high intensity path to the centre of that beam.

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Which is immediately going to rapidly grow down into and through a piece of material.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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So the only part of the laser beam that is going to have an effect is the bit that can get through the hole that’s made in the material.

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So the material is actually going to be a filter for all this other crap that you normally imagine is going to have an effect on your hole.

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No, it’s never going to get through the hole. It’s going to be filtered out by the surface of the material itself.

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OK, so this beam that you imagine happening below the focal point is never going to get there.

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The only thing that’s going to get below the focal point is what gets through the damage hole that you create on the surface.

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And the smaller the damage hole, the faster the material will erode.

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So this damage, technically this damage could only ever be as big as this beam.

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So the front of the beam might be pointed. But actually, if you give it enough time.

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The whole of that beam will produce a parallel hole.

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And if you stop the beam just before the material has finished, you will see that the front of the beam will still have a small taper on it.

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And that’s what we saw when we looked at our piece of, when we looked at the cut passing through our 10 millimetre material.

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Just at the bottom of the cut, we could see evidence where the beam had not quite passed through the material.

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We needed to give it a few more parts of a second to allow it to pass through cleanly.

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Now, by the time, you give it enough time to pass through the material,

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what will have happened is, some of this blue will have had an effect at the top here.

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Okay. And what will happen is this blue will effectively be doing this.

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It will erode. The top of the cut.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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So there’s your cut profile now. It’s a mixture of a little bit of blue beam with a lot of yellow beam,

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which has produced a parallel hole as it’s wandered through the material.

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So that’s how cutting, that’s what the cutting mechanism is. And your imagination about what’s happening below the focal point…

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It’s just totally misleading you.

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Now you can go and have a look at a lot of these tests that I’ve carried out, a lot of these experiments on my RDWorks learning lab channel.

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Now I carried out lots and lots of tests with lenses to try and establish what was going on.

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And one of the tests that I carried out was here. It was, it was basically a, what I call a speed test.

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It’s characterizing the beam itself. What we did, we put a thirty eight point one lens correctly focused onto the surface.

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And here we’ve got a two millisecond burst of power at 100 percent. Four, six, eight, 10 milliseconds. 12, 14, all the way up to about 30 milliseconds.

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As the exposure time was increased, you could see the rate at which the beam was growing.

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But if you look carefully, you’ll see that across here. There seems to be almost like a neck.

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But we’ve also got this strange ballooning effect which gets greater, more ballooning as time goes on.

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This is not uniform intensity, even at 0.2 diameter, and we’ve still got rays that are coming in.

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Like this? And like this.

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OK, they’re still not absolutely parallel to the axis.

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There are rays that are very slightly off axis, and they’re still passing through this 0.2mm diameter entry hole.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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OK, so the bore that’s been left behind by the major part of the yellow beam is being scoured

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by some more rays that are able to get into this hole through the entry diameter.

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These are rays that are very, very close to the centre and will come down here like this and they will produce a sort of a scouring effect.

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Which the more time you allow, the more effect they will have.

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Who said cutting was simple? Did I say that earlier on?

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That is the way that cutting takes place. That has got nothing to do with your imagination,

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which is dragged around in all directions by these pictures that have been out there for many, many years.

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These do not take into account the intensity of the beam.

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These just are rays and as I have shown you, these rays are totally ineffective.

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They don’t do anything at all. The only bit that works is the central part of the beam.

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Now it took me a long time to work out how lenses work,

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and I spent a great deal of time doing tests with lenses to try and reverse engineer what was happening to the beam.

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Acrylic is a fantastic material that leaves footprints in the sand.

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Can I read those footprints in the sand and find out what’s actually happening?

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Well, the answer is finally, yes. And I’ve tried to describe that process in this session.

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All the test data that I’ve collected about various types of lenses are included in this very, very long PDF document.

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It’s 109 pages long. Now, that isn’t 109 pages of reading, because although there’s a little bit of text there, we’ve got summary graphs.

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So we’ve got a whole range of lenses, a whole range of different set up configurations that I’ve used for the following tests.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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Now, although, as you can see from the crosses, it’s a pretty comprehensive spread of testing for lenses.

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Not every lens is in there. Now, as I pointed out to you earlier on, there is no such thing as a focal point.

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The focal point depends on the speed. It depends on the power and one of the things that surprises people.

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It also depends on material. So we’ve got a whole range of material here and a whole range of different focal

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points that happen to have the best focal points for different materials.

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OK, now it clearly says in this that these are tests using acrylic. Now acrylic cuts fairly slowly.

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If you want to cut MDF or HDF, the speeds are about the same as the speeds that acrylic cuts at.

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The only other materials that you can really cut are organic materials, which are the woods, the real woods, not the plastics.

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So real wood will cut at about twice the speed of acrylic.

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This one, which is a gallium arsenide meniscus lens.

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It takes two four six eight, maybe 10.

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And yet when we turn it the wrong way round. Two, four, six eight.

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It’s faster. So that’s the sort of information that you can read in this, immense set of data that I’ve produced.

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So there’s a huge amount of data that you can find out from this document. Look, here we’ve got a thirty eight point one CVD.

Transcript for Laser Cutting: Where Do We Start? (Cont…)

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Look how thin the entry is, it’s deep as well.

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So this is a very good lens for cutting deep and producing thin lines.

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Because if we take a look here, look, the lines on the surface are bigger, even though the cut is smaller.

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All right, so it looks like a thirty eight point one CVD lens has really got the best cut, the deepest cut and the thinnest entry.

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So the thinnest line, so you will be able to download this from the website.

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Well, that’s as much as I can tell you about cutting at the moment. We’ll get on and do some real cutting in the next session.

Transcript for Laser Cutting: Where Do We Start?