Session 34 – Let’s Get Laser Cutting – Laser Cutting Parameters

Let’s Get Laser Cutting

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Oh. The Concise RDWORKS Learning Lab with Russ Sadler.

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Session 34: Let’s get cutting- Part one.

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Well, the good news is we are at the machine today. That means we’re going to do some real cutting.

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Doesn’t mean to say we’re going to do a lot of cutting because there’s still a lot to learn about cutting.

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There’s a good reason why I chose cutting as a second option. You thought that engraving was a long, drawn out process.

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There’s, in fact, quite a lot more to learn about cutting.

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The good news is you understand how materials get damaged.

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You also understand now from the recent theory that lenses are not quite what they see.

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You have to be a bit cautious with them because there isn’t such a thing as a focal point.

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You can’t just set the focal point on the surface, set power and a speed and off you go.

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It works, no. Now, one of the first things that new people normally ask me is can you supply me with a set of cutting parameters, please?

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Well, I have to be honest and say, no, there is no such thing as a set of cutting parameters.

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There are a set of cutting parameters for your machine, but those parameters will not necessarily work on anybody else’s machine.

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So if you go to the Thunder Laser website, which is a dot com website, you’ll see that under technical support.

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You’ve got downloads.

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Now if you scroll down this page, you’ll get to a point where it says thunder laser cutting power and speed parameters for different materials.

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And here you’ve got a list of tube powers that are the basic nominal parameters for a tube power.

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Let’s just choose this one because my machine is about 80 watts and you’ll see that for an 80 watt tube.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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They say this is a two inch lens for cutting. And this is a two inch lens for engraving. A four inch lens, for cutting.

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And they don’t recommend it for engraving. That’s not entirely true because a four inch lens can be very good for 3D engraving

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and doing other sorts of engraving because of the soft nature of the burn.

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Hopefully, you’ve already learned that from previous engraving sessions. You know, focus is not that important when it comes to engraving.

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Focus is very important when it comes to cutting.

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The only numbers that you’re going to be able to achieve are the ones that are suitable for your machine.

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So I’m afraid you will have to have a notebook and a pencil, and you will have to make a note of things that work for you.

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Now within RDWorks,

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there is a material library that you can fill in. But of course, you fill that library in and change your tube and it will no longer be any use to you.

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So all I’m saying is be very careful about parameters. So cutting is dependent upon many elements, and the most important is the top one: beam quality.

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If you don’t have a good Gaussian beam, you’re not going to be able to get a sharp burn.

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Now remember, your maximum power might be 70 or 80 percent.

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It depends on the current flow that you’re allowed to pass through your tube.

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And if your maximum sharp beam looks like this, then everything that

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I’m going to tell you today and demonstrate to you today, you will not be able to achieve.

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You still need to learn about it because hopefully it will help you identify that you’ve got

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a rubbish tube and that you really need to replace your tube with a good quality A-Grade tube.

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The next thing that’s most important to you is the choice of lens. I’m very lucky that I have got a wide range of lenses to play with,

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and I have done a lot of work comparing the performance of different types of lens.

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The X’s is on that table indicate the lenses that I’m going to be able to test.

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That basically covers Chinese lenses, American lenses and gallium arsenide lenses, zinc selenide, zinc selenide.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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And we’ve got these four focal lengths and I’ve been testing the lenses both flat side up and flat side down. Meniscus,

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flat side down, meniscus flat side up. So there’s a whole range of tests there that cover virtually everything the you are ever likely to encounter.

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The first part was all about proving that, hey, there is no such thing as a focal point, so we go through all the lenses there and miss that out.

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And then we get to a summary section here now on this table. We asked the question Does the focus vary for different materials?

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And there’s a whole range of materials here that I’ve tested and I’ve tested them at different speeds and powers.

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So we can see at the top here, that I used a thirty eight point one gallium arsenide plano

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convex lens flat side down. The correct way to use a lens and using that lens with these materials.

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Here’s the variation in focal distance that I got: 36 to 38, 36 to 39, 37 to 39.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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So the lens focal distance changes with material. Does the focus change with speed?

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Well, there’s an answer to the question. Now I’ll let you read that.

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And then there’s a third question here does the focus change with power?

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There’s another set of data there that you can read.

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OK, so this table basically tells you that the intensity focal point, which is not the same as the manufacturer’s focal point,

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was defined as that position that achieves the deepest cut the red number.

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OK, now my line method and the ramp method that people use for determining the focal point.

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It basically finds a completely different intensity focus. The green numbers, which are basically the thinnest line, so minus numbers,

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means the focal distance has gone short and plus numbers means the focal distance has gone long.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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Now there’s also another set of data that I derived, which is basically when you set the best depth intensity, i.e. the red numbers.

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Here is an idea of the cutting performance by time in milliseconds to reach four or six millimetres. Basically,

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it determines the speed of penetration for your lens, for a given, for a given amount of power, and the power that I was using was maximum power.

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OK, so all these lenses have got different performances.

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And so what we’re really looking here, the best performing lens will be the lens that produces the smallest numbers.

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Well, look, most of the time you can see that with tens. eights, eight milliseconds, eight milliseconds.

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Some are a lot longer. And here we get eight milliseconds again, 8 8 8.

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But look at this one. Not only is it good to six millimetres, to four millimetres, look how it gets to six millimetres.

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It gets to six millimetres very quickly in relation to all these others.

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Look at them. All right. So this one is obviously the fastest penetrating lens,

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and that is the lens that I personally use; a two and a half inch gallium arsenide plano convex flat side down.

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And so here’s the test I’ve just been talking about. So look, there’s two millimetres in the background there four, six, eight, 10 millimetres.

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So this this particular one, which is a four inch CVD, used

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flat side up. Took two four, six, eight, 10, 12, 14, 16, 18, so it took 18 seconds to reach two four six mm.

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These nine cuts on here are four millimetres into the material three two one zero and then we’ve got one two three four.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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In other words, I’ve now drawn the focal point out above the manufacturer’s focal point.

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We’ve basically got our thinnest line. So if you look here, we’ve got a very thick, going thick line.

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We’ve got all this curvature on the top of these. All right. And then look, that one might be the thinnest.

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It’s difficult to say, but it’s that one or that one.

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One of those two represents the thinnest, and I suspect I must have chosen that one when I looked at it under the microscope because look,

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there is a little bit of curvature on the top there, whereas this one has got a nice, crisp corner to it.

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But look at the difference between the penetration, the cut penetration here and here.

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I mean, these, as it happens, are all about the same. This line drawn across here is five mm depth, just as a reference.

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So the important thing is to set the focal point correctly.

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What is correct? Well, if you want the deepest cut, it may well be in this instance that you’ve got to set it on the surface.

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Zero. Fifty point eight. But if you set it to fifty point eight, you get this effect

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on the surface. You get a wide cut mouth, you get a curvature at the input.

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Okay, you get a deep cut, but the compromise is you’ve got to suffer a wider line.

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If you want the thinnest possible line, then obviously you’re going to have to go maybe one, two,

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three or four millimetres above the focal point to get the thinnest possible line for engraving. Or the thinnest possible line

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for instance, if you’re doing marquetry where you want the thinnest possible line. You don’t want anything like this if you’re trying to produce very,

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very crisp, sharp edges. So you need to go through and study these pictures for your particular lens.

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Hang on. This may well be your lens a 50.8 CVD, but you wouldn’t generally use it curve side down.

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You’d use it flat side down. So let’s have a look at what flat side down which is your lens.

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Let’s just have a look. It must be here. There it is, fifty point eight CVD.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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That’s your lens. Almost certainly. I’ve got a PVD meniscus lens, the correct way and the correct way.

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Well, the deepest cut appears to be here, but look at the difference in the line entry width.

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OK, the damage that’s caused by the low energy at the outer part of the beam.

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Okay, now if you want to use it for engraving, then the sharpest corners are here maybe the thinnest line might be here.

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So that’s what I mean about lens choice.

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There’s a huge range of cutting effects you get from different lenses and the focus settings, as we’ve just demonstrated.

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So it is probably fairly important that you download this document and keep it as a reference.

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OK, now speed and power, they both affect the cutting performance,

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so we’ll go further into that later on. Because speed and power both affect the focal point.

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Not the not the physical focal point that the manufacturer tells you about,

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but the focal point of the intensity, which is the bit that we need for cutting.

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So Air Assist is a subject that people don’t really understand, but they use.

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So we’ll treat Air Assist as a completely separate session, but we shall be using it today.

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But I’m not going to go into great detail about air assist. There are materials that cut.

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And they generally tend to be organic materials, things like leather, wood card, MDF, plywood and any other natural woods.

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Cherry ash, maple, those sorts of things.

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You can’t cut glass. You can’t cut slate.

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You can’t cut marble or any mineral materials and metals.

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You can’t cut any metals. We will be able to mark metals.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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We haven’t tackled that yet, but we will do that as a separate subject. At the end, you could mark metals.

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But you have to be very careful about how you mark. Now that big table of test data that you’ve seen, was all performed with this wonderful material.

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Acrylic, acrylic is a great material, it’s my best friend when it comes to CO2 laser cutting, because it tells me what’s going on.

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I can actually watch exactly what’s going on inside a cut.

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I can’t do that with wood. But those results may be just for acrylic.

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But acrylic is quite slow at cutting. It’s a difficult material to cut, at a one hundred and sixty degrees C it turns to liquid.

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And at 200 degrees C, it turns to steam acrylic steam, basically vapor, acrylic vapour.

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That’s how the cutting action happens. You’re evaporating away the material.

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Now it’s only happening at 200 degrees C, but when it comes to wood, well,

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wood has to evaporate away carbon and carbon doesn’t disappear till over 3000 degrees C.

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So you think to yourself, well, hang on 3000 degrees C. It must take a lot longer.

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Well, you’re coming back to some of the things that we talked about earlier.

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Remember the jelly in the cake? Carbon is like the jelly.

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It shakes very easily and accepts energy extremely quickly.

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And so it can get up to those sort of high temperatures very, very quickly.

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Whereas it technically this stuff is like the cake.

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It’s very difficult to shake it to make it do something.

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And so consequently, this is quite a difficult material to cut. When you look at that chart.

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All the figures in those charts are for acrylic.

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But if you want an approximation of what it means for other materials, there are basically two approximations. One, if you’re using MDF,

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which has got plastic in it as a binder for wood, then typically it will be about the same sort of speed cutting as acrylic.

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Slow. OK, but if you want to cut wood, whether it be plywood or whether it be normal wood, you can assume that about twice as fast.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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But they’re numbers that I have experimented with and found out to be true. When you want to select parameters,

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you can use that chart, because you can either use the same numbers for acrylic or you can try and interpret them for wood or MDF.

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An Acrylic cut can be fascinating because it teaches so much about what you can’t normally see inside a cut.

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Now I’ve got this set to full power, and I’m going to run it at a fairly slow speed, five millimetres a second.

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What I want you to watch carefully is the way in which the laser beam goes straight down, except for the last little bit.

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OK, the last little bit will be what I call dragging.

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It’ll be curved away from the line of the cut. Can you see how the cat is lying backwards?

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Now, the piece didn’t fall off and it didn’t fall off because it didn’t cut through.

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But let’s just see if we can break it off, which we can. Now there’s the a bottom face of our cut.

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This is the material that’s left inside, so you’re going to ask the question, why on earth have we got all this spiky stuff inside?

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Well, what you’re seeing there is the power.

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It’s absolutely rock solid, steady, but the stepper motor is not.

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We’re only running this 5mm a second, and at five millimetres a second.

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The stepper motor is going jump, jump, jump, jump, jump like that.

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So as it goes fast, we get less cut, and as it stops in between it’s steps.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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We get a deeper cut. So what we’re seeing here is the stepper motor,

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creating this sawtooth, not the power. The power is constant, but of course, if we leave the power on for longer, it cuts deeper.

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And when we leave it on for less time, it cuts less. But this is the direction of cutting across this way.

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And look, all of these are leaning in that direction. We’re getting all sorts of bubbling on the surface here.

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Can you see that? That’s the boiling effect that I talk about, that’s the liquid phase.

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Because it’s boiling and not fully evaporating, and it’s frozen before it’s had a chance to evaporate.

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Now when we get right to the other end of the cut. Here’s our bottom surface and here’s the edge of the material.

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Look what’s happened? If we put that up there?

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It it’s going to cut onto the surface of the material right at the top edge, but it hasn’t cut right to the bottom.

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It hasn’t come out to the bottom. This is what I call drag, you know, we’ve got this much drag in the beam.

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Many people will interpret this slope here. As a reflection of the laser beam off the acrylic surface.

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I’ve had this argument with people many, many times. So hopefully I’m going to be able to prove to you now,

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that there is no reflection off this surface and what you’re seeing here is a strange effect of hot gases scouring this.

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You can see the damage from our first cut here, the sawtooth from our first cut.

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Well, I’m going to put that on the top. So the next cut is on the bottom and it’s easy for us to see.

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So I’m going to stop this mid-cut. But what I want you to do is to watch the drag and then when I stop.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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Watch what happens. Now, sometimes high magnification is good and sometimes it’s a bit of a pain.

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In this instance, it’s a little bit of a pain because it’s difficult to show you the angle completely.

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So what I’ve done, I’ve set the bottom surface of the material true to the bottom edge of the screen.

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This edge here is basically vertical, and there’s the stop edge of the beam.

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This is the draggy edge of the beam. So let’s pan up the beam.

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So that’s the beam diameter there. And as we go down, look, you can see the beam beginning to drag on the right on the left hand side there.

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See the angle? Now, the point I’m really making there is, if the beam was actually reflecting off that curved surface,

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that angled surface. When I stopped the beam, it would continue to reflect around that surface.

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But it doesn’t. It catches up and does what you expect of a light beam, it travels in a straight line straight down,

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which is the way that we’re firing the beam into the material. So that drag cannot be anything to do with reflection.

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So the point is, if you cut too fast, you’ll be cutting with hot gases and not necessarily,

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with the light beam itself. And if the surface happened to be here like this,

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then what happened is you’ll get perforations through the bottom surface of your cut and you’ll think. Hello. what’s going on there?

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Well, it’s this effect here. It’s your stepper motor that’s allowing the power to come through the bottom of the cut

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and then go back into the cut through the bottom of the cut and back up into the cut.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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So if the cut line is across there, you’ll produce a perforated cut across the bottom.

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And the only way that you can overcome that is to slow down, let your beam go straight through.

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If you allow enough time, the beam will pierce through all the time, straight.

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Previously, I was running this at five millimetres a second.

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I’m now going to run it at three millimetres a second. Yes, it’s nearly upright, but not quite. As I said to you,

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this is acrylic. It melts at 160 degrees C and it evaporates at 200 degrees C.

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Now what I want you to do this time is to watch the cut as it traverses along and look just behind the cut itself,

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and you’ll see a rather strange effect taking place.

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Specifically at the bottom there, can you see that little bit of what looks like drag. It’s not drag.

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That’s actually the liquid phase of the material. Bearing in mind,

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this stepper motor is pulsing. It’s stepping across and producing all sorts of weird pulses.

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Why don’t we have?, those pulses

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on the surface of our cut? Look, it’s completely smooth.

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And that’s because the liquid phase of the material has settled on the wall and smoothed all the pulses out.

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Just like the surface of a piece of water. So the liquid phase is the thing that causes that lovely, smooth finish.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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This is extruded acrylic. It’s very easy to get that effect on extruded acrylic and more difficult to get it on cast acrylic.

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So there we go. We’ve shown you that we can cut through a piece of material with an upright cut,

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if we do it slow enough. Here we’ve got a piece of 10 millimetre thick plywood.

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I should be able to cut this at twice the speed that I can

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acrylic. It’s the same thickness, same lens, same settings.

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So I’m going to run this at six millimetres a second, and we’ll see what happens.

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I’m going to have to turn some extraction on. And I know that it’s cutting through because the smoke is coming out the bottom.

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Well, it’s cut through, but it looks a bit of a mess on the bottom, doesn’t it?

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Well, that’s because of the reflection off of this steel surface.

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This is metal. Remember, it’s a mirror. And the problem is, I’ve got that mirror too close to the underneath of that material.

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So first of all, let’s demonstrate this to you. The same settings.

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But we now lift it off the deck by 25mm as opposed to 10mm.

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You can see lots of smoke coming out at the bottom, which is good. That means we’re blowing air out through the bottom of the cut.

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And this time. Pretty clean, just a little hint of something there.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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OK. And. It’s come out.

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Look, we’ll run it slightly faster. Instead of,

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instead of six millimetres a second, we’ll run it at 8mm a second.

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Yeah, there’s some smoke coming out the bottom. We’ve got a lot more debris out the top here.

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The cut is black. The other thing to note is that cut at the bottom looks like a good cut, it’s through.

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So why didn’t it fall out? Well, the reason it didn’t fall out is, if we look just here.

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You’ll see that there’s a bit missing. The cut didn’t complete. That’s beam drag, because the top of the cut is complete all the way round.

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This is the start point. So it started here, went round and finished up here.

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But when it got to there, the bottom of the cut is dragging. Sometimes you can use that to your advantage.

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It’s almost like an automatic bridge, if you want to stop the pieces from falling out.

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I mean, it will fall out very easily. All right.

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But the reason I’m not going to push out for the moment is because I want you to look at something else.

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And that’s the difference in the cut width at the bottom here.

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And the cut width the top. OK, I haven’t necessarily got this set to the correct focus. We’ll reset this focus

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in a minute and we’ll change the relationship between the top and the bottom.

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We’re relying on a through cut to allow the smoke to come out of the bottom of the cut.

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You may well remember a couple of sessions ago, how I said to you that the very first part of a cut,

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is like a drill hole. It’s piercing through, and that is what you’re seeing here,

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you’re seeing the fumes coming upwards and spreading across your material because there is nowhere for those fumes to go.

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Only when we get the cut established along here do we have a gap out the bottom of the cut for the fumes to escape.

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I can improve things slightly by increasing

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the air assist. Now at the moment, my air assist is turned off nearly, so I’ll run the same program again with full air assist on.

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And you can see now the smoke pouring out the bottom. ok, because I’m blowing it through the cut.

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And now, two things you’ll notice.

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Number one, it’s a lot cleaner around the edge of the cat because I’m not allowing these fumes to settle on the top of the cut.

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They’re being blown out the bottom of the cut. Let’s just turn over.

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Look how the air assist has affected cut drag. We’ve improved the cutting efficiency with the air assist to the point where it’s nearly disappeared.

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There’s still not what I call a proper cut, because the bottom is not the same as the top.

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We’re looking for a parallel cut to be the ideal cut.

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Now, the reason why it’s not parallel at the moment is because I haven’t got my focus set. But these pieces, they’re dropping out nicely.

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One of the ways you can tell whether or not your cut is good. Is to have a piece of kitchen towel handy.

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And look, if you can do this with your kitchen towel. And you get virtually no charring on there.

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Then you got you’ve got a pretty good cut. I’m now going to carry out a little focus test on this machine using an automatic program that I’ve developed.

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Now you can do the same sort of thing by using a gauge like this and doing it manually.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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So I’m going to set this lens specifically,

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low, and at the moment, that’s sat on three millimetres.

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OK, so remember, I set the first line up to three millimetres, three, four,

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I really ought to be setting this up to five millimetres for the best focus.

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5mm. So there’s a maximum allowable milliamps that you can use on your machine. Set that to the maximum that you can when you’re doing cutting.

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We’ll run this at 10 millimetres a second and see what we get.

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We’re getting a pretty clean cut on top here, except where we enter. As I said, you’ll always tend to get that splash there because of the volcano effect.

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We’ve got a slightly thinner, cleaner line now. Let’s take a look at what we’ve got underneath. Well underneath,

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we’ve got a thinner line than we had before probably. Look, it’s just that little teeny weeny corner just there, that’s stopping

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it from popping out. We can hardly tell the difference between top and bottom. So we’ve got no scorching on the bottom.

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We can see where the bottom is because look, there’s our there’s a little teeny weeny pit there that was holding it in.

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So that’s the bottom. And that’s the top. That’s a nice, clean cut. Something that we’ve got to keep an eye on, is the squareness of our cut.

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OK. To make sure that we’ve got our beam set correctly. As I said to you,

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many people will tell you that you’ll get an angle cut because of the way in which the laser beam tends to come out

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters (Cont…)

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in a cone underneath the focal point. I’ve demonstrated that to you a couple of sessions ago when we spoke about lenses.

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Any angle that you get on your cut is nothing to do with the lens itself, it’s all to do with the direction of the beam of light.

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If the light is not passing through the axis of the lens, it will veer off at an angle and produce an angle cut.

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Now we’ve also shown you that this surface here, this metal surface at the back, which is steel, can reflect.

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It’s about 60 percent reflective even in this fairly cruddy state, and it will have an effect on the back surface if your work if you’re not careful.

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Raise it up. Allow it to dissipate more so that there’s less intense energy.

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And here you can see we’ve got hardly any effect of, well we’ve got zero effect of reflection here for two reasons.

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Number one, we’ve lifted it up and number two, we’re running faster. Your parameters,

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will be the ones for your machine. I can’t give you any of those parameters.

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All I can give you is basic rules that you should follow to obtain your parameters.

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Run the machine as fast as you can. Run the machine until your material drops out.

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Use something like this a square test 25 mm square.

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Run it too fast and the material won’t fall out. Run it a little bit slower and slower and slower until the material falls out.

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Make sure that just before the material falls out,

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you’ve got a nice cut at the bottom and that your smoke is passing right through the material and coming out the bottom.

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Look at the condensed tar that came out at the bottom of my wood there.

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A little bit of acetone. And it wipes off. Don’t leave it on because if you leave it on.

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Look, this is some stuff that I happened to leave on, and it sort of somehow corrodes into the surface.

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The only way I’m going to get this off is with something like a 3M pad or a rotary sanding disk.

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I can remove this, we got a solid bar bed.

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And I’ve got one of these horrible things that I use occasionally.

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Piece of honeycomb. Most people on their machines will have honeycomb, but hey, look at this, for example,

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can you see that piece of, can you see that little teeny tiny piece of acrylic wedged in there?

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Hmm, that’s pretty dangerous.

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It’s called honeycomb for good reason, because that’s what it looks like. In the same way that if you open up a bees nest, you’re going to get stung.

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This is almost as dangerous.

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This is a very convenient point to stop this session because the beginning of the next session, we’re going to have to talk about this stuff.

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Now, look, I’ve had this for six years.

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Yeah, it’s a little bit worn, but it’s not terrible in relation to what some people’s honeycomb looks like after a few months.

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That’s because I have a love hate relationship with this stuff.

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And so on that cliff-hanger, we’ll save that story till the next session.

Transcript for Let’s Get Laser Cutting – Laser Cutting Parameters