Session 14 – Laser Beam Versus Intensity

The Concise RDWorks Learning Lab Series

Welcome to Module 2 of the new Concise RDWorks Learning Lab Series with Russ Sadler. Module 2 will build on the information learned in Module 1 and will provide practical machine set-up sessions as well as more detailed subject material. So let’s try and get an understanding of laser cutting based on beam versus intensity.

In this Session, Russ explains some of the reasons a laser beam is capable of interacting with materials the way it does. He also discusses some of the key parameters and properties that are required in the laser beam. Why B-Grade tubes can be so very poor, even if they have high power outputs. Pre-ionisation zone discussed and how it can be used. Testing the profile of your laser beam.

Release Date: 15th October 2021

Over the last 6 years, Russ has built up a formidable YouTube following for his RDWorks Learning Lab series which currently has over 200 videos.

The original RDWorks Learning Lab series on his “Sarbar Multimedia” YouTube Channel, follows Russ as he tries to make sense of his new Chinese laser machine and to sort out the truths, half truths and outright misleading information that is available on the web.

Six years later with over 3 million YouTube Views under his belt, Russ has become the go to resource for everything related to the Chinese CO2 laser machine user or wannabe user.

Understanding laser cutting through beam power versus intensity
Understanding laser cutting through beam power versus intensity

In this new series, Russ has condensed his knowledge and experience of the last 6 years to provide valuable information and insights into the purchasing, understanding, use, repair and maintenance of the Chinese CO2 laser machines and their key component parts.

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Podcast Session 14 – Laser Beam Versus Intensity

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Laser beam cutting performance: beam mode test
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Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity

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Understanding laser cutting

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The Concise RDWorks Learning Lab with Russ Sadler. Session 14: Laser Beam Versus Intensity.

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Well, just to remind you, I know it’s not long ago, but in the last session, we used the laser machine to actually do some cutting.

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Now, it wasn’t the intention of the session to do the cutting, but we actually used a laser beam to do some damage to material.

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What we’re really trying to do was to show you all the ways in which you could get a program loaded into the machine.

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Today, I’m going to try and explain some of the reasons why the laser beam can do the sort of things that it does,

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only some of them, because we’re going to describe the fundamental properties that you require for a good laser beam.

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Now, I know we talked about the laser beam in a session.

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I think it might have been session three, but those were the just the basic scientific properties of a laser beam.

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They didn’t specifically mention the key properties that you must have in your laser beam to do this sort of damage.

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Everybody seems to think that Watts power is the important thing that you should have when you buy a laser beam.

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Well, all I can caution you is be careful, first of all, two many watts,

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on a glass tube machine is fine for cutting, but it’s not very good if you want to do some fine, particularly photo engraving.

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So Watts of power is an important parameter that you need to be able to determine

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about your laser beam. The manufacturer or the person that sold you the machine will claim that they’ve sold you a 50 or 60 or an 80 watt tube.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity

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Now, I can almost guarantee that that will be an outright lie.

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I think I might have mentioned it when we talked about buying a machine.

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The sellers of these machines will exaggerate the power of your laser.

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And secondly, they will be selling you B-grade tube.

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Because B-tubes are cheap. A-grade tubes are quite expensive, but to do proper cutting with a machine,

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you need an A-grade tube, which will have specific properties.

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Now, some B-grade tubes, don’t get me wrong, some B-grade,

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some B-grade tubes will have just failed final test and they may well have pretty good properties.

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So your B-grade tube is a bit of a lottery. It might be near junk or it might be near perfection.

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What I’m going to try and show you today is what a near perfection laser beam should be like.

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But before we do that, we’re going to talk about how you measure the power of a laser beam.

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Now, measuring the watts of an invisible beam is a bit of an interesting problem.

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There are many instruments out there that have been designed to do just that,

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they’re obviously all based on energy absorption from the light and converted into some other form,

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whether it be electrical energy or whether it be heat energy.

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Those devices can cost you anything from a few thousand dollars down to maybe as little as a hundred dollars for a Chinese something or other.

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But typically you’d expect to pay two to three hundred dollars for a piece of reasonable kit.

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That’s an unreasonable amount of money to pay for something that you’re not going to use very often.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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And so in the early days of my learning journey, I, along with several other guys,

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tried to invent a different and more cost effective way of measuring power.

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Now, we didn’t really succeed,

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but what I finished up doing with decoding the systems that were out there and making my own much more cost effective system for you guys to use.

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When I first got my regional China blue laser machine, it had a very, very poor quality tube in it.

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And I struggled for a long time to prove the power in that tube was not as claimed.

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I spent a long time trying to devise ways of measuring power through calorimeter techniques and in the end I had to give up

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and I searched the Internet for a reasonably priced power meter that enabled me to measure the power coming out of my tube.

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On the surface of it, it looks like a meat thermometer with a blob of aluminum araldited to the stem.

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And in essence, that’s what it is. But it’s a very expensive piece of equipment, relatively speaking, for a hobby use.

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This cost me around about 350 dollars because I couldn’t buy anything like this in the U.K. I had to get it specially imported from the states.

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This is a company called Mackan Instruments. This is the cheap end of their product range.

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Although I invested in this and it did a superb job of telling me everything I wanted to know about the machine,

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it’s still out of the price range of many people. And so consequently, on this site here, RDWorks Learning Lab 53,

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I’ve done an analysis of how this works and I’ve designed my own simple version, cheap version that will do the same task as this.

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This reads out Watts directly. This does not read out Watts directly.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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It’s a compromise. We get the same answer, but we have to put a little bit more effort in.

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So before we can start any testing, we need to put a test program into the machine. We’ve got

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a starting point here, which is the blue.

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And the first thing that happens is at a speed of one millimeter a second, we traverse outwards by about 10 millimeters.

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So that means it’s going to take 10 seconds to run up the blue line.

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And at the end of that period, 10 seconds, we start running around this red spiral, getting closer and closer towards the center.

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Now, that red spiral has been very carefully designed.

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Its length and its speed has been set to 14 millimeters a second because that will give us the correct amount of exposure

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time for the calibration to work.

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We don’t have to panic when we press the start buttons. We’ve got the power set to one percent and one percent will not work the laser.

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Under normal circumstances, you would not run this machine at more than its maximum rated

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milliamps, but for a very, very short period of time for doing these tests,

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it’s not going to cause any major damage to your tube,

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so don’t worry. It really doesn’t matter where you place the laser head, but somewhere into the middle of the machine is a good idea.

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And then press your origin button. So I always fire my beam into a little puddle of water.

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Okay, so that’s the program set up and all the safety precautions. Put the probe into the water bath and we’re going to swill it around,

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turn the meter off and back on and that will be a live temperature,

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the doohickey is measuring. Yeah, give or take 0.2 / 0.3 of a degree C

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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its stable and it’s at around about twenty two point eight to 23 and I’m going to

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walk round to the front of the machine and I’m going to press the start of the program button.

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No immediate panic. We can turn the meter on and we press the max power.

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Twenty three point one is the number that I want to see.

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Here we go. We can see the belt moving. And that tells us we’ve got something going on.

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Twenty three point one was our starting temperature, remember? There we go.

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It’s finished now we can take a look on the meter and we can see that it says fifty five point four.

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OK, that’s two pieces of vital information that we need, the starting temperature and the finishing temperature.

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If you’ve got some help, then maybe somebody else can read this for you.

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But if you haven’t got help, then you’ll need to run the program again, as I will do here.

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And of course, it’s safe to run it because we’re running into a bucket of water.

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And what we shall need to do now is to make a note of the current

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once the program is running. Twenty four point five, probably. OK, so that’s basically what I’m recording,

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power 90 percent, start temperature twenty three point one, maximum temperature that we established,

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fifty five point four and our milliamps twenty four point five.

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Those are the four numbers that we need to record. 10 percent steps is good enough to get a reasonably good calibration graph.

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And now we go through the same procedure again and we do that for seventy, sixty, fifty, 40, 30, all the way down to 10 percent.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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We’re going to put the final temperature into a calculation, fifty seven point five and then we’re going to subtract

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the start temperature from it, which was twenty four point four.

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And that gives us an answer of thirty three point one, now that’s thirty three point one degrees C.

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The speed that I’m running this at, there is a multiplication factor of two.

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So if I put two times two into there, the answer that comes out is sixty six point two.

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Now that’s sixty six point two watts, there is a calibration chart that you can work to.

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But to be honest, if you’re running at 14 mm a second, then just taking the difference and multiplying it by two gives you the Watts.

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It’s a nice, simple, almost mental calculation. We’ve drawn this characterization for the tube.

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Now this blue line here is power Watts and this red line here is current milliamps. As you can see it is pretty well approximately linear.

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So that bit checks out. And what you can see is that the power is not linear.

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So the power that comes out of this tube is not directly related to the current that you’re using to drive the tube

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with. People get rather confused and they think that I’ve got a 60 watt tube.

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Therefore, if I put in 50 percent power, I’m going to get 30 watts out.

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Well, look, 50 percent power is in fact going to generate something like about 55 watts.

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OK, the other vital piece of information that we must get from this graph now,

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you would normally expect to be able to drive a 60 Watt tube to a maximum drive current of 22 milliamps, 22 milliamps, OK, now 22 milliamps.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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If we draw that up there like that, we get two bits of information, 78 percent.

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And the other piece of information is that at 78 percent, we’re getting so this tube, a 60 Watt tube is delivering 64 watts at 78 percent power.

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And provided we don’t program more than seventy eight percent into this machine,

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then we shall not be exceeding the 22 milliamp limit for a 60 watt tube.

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Now, those are crucial pieces of information that you need to know and you would not be able to get this information without the use of a power meter.

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Now, the first time that I measured the power of the laser beam, as we’ve just seen,

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I thought I knew everything there was to know about the laser tube itself.

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Apart from the science that we talked about in the beginning. No, that turns out not to be the case.

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As I get older and grayer and by the way, every one of these white hairs is a pound (£) that my wife has spent,

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it turns out that there’s a lot more to know and understand about the laser tube.

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And I’m now going to try and explain some of the finer points of the laser tubes performance.

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This tube, every time it starts up, goes through a very strange set of conditions.

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Now for a 50 Watt tube, 20 milliamps is approximately the maximum operating current that you should use, but up to about six milliamps,

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the actual beam itself is totally unstable and only becomes a stable,

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constant current laser at between six and 20 million amps, which is where we get a lovely, smooth pink beam.

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Although your ammeter might be showing you six milliamps. In fact, what’s happening is this.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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And I don’t exaggerate that in any way at all, very, very high frequency, very,

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very high current flows are happening in that tube. Six milliamps is, if you like the average current flow for all this rubbish.

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Now, this happens in a few microseconds or less every time you switch the tube on,

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unless you hold the tube at these very low current values and then you can see

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and experience the benefits of this very strange operating zone in your tube.

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In fact, some manufacturers actually claim this as a real advantage of their tube.

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They call it high frequency impact engraving. There’s nothing special about this for that particular manufacturer’s tube.

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Every tube in the universe has this property, but not too many people know and understand,

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A: that this is here, and, B: that it’s a very useful property of the tube.

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It may well be six to 10 percent. It depends on your tube, what percentage power this strange characteristic occurs on your tube.

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So I’m going to show you how to find that particular characteristic.

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And bear it in mind, when you’re using your tube, it is a unique feature that has got some great properties.

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Here we go, oh, we’ve got something happening there at nine per cent power,

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you can just about see right at this end here a very faint glow at this end of the tube here.

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Now, if we start looking along the tube, we shall find that we have got no, no real discernible beam along the middle of the tube just at the end here.

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So here’s what eleven percent looks like. And now we’re starting to get more along the tube, but it still isn’t all the way along the tube.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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And you can see how the beam is jumping around at the end there.

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We’re now going up to 13 percent, which is where we think the beam is nearly going to make it along the tube.

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Still jumping around a lot at this end, as you can see, and there is just the faintest glow that goes more or less all the way along the tube.

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Look. But it’s still not what I would call a continuous beam, because, look,

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we’ve still got it jumping around at the end there and that jumping around is a characteristic of this pre-ionization phase.

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Now, this is 14 percent. Yes. You can see we’ve still got a jumpy beam there and we’re getting a much stronger beam.

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And it’s more or less well, it is all the way along the tube.

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Now, we’ve still got this jumpy characteristic there on that cathode.

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Well we’re now at 15 percent, it’s still a bit jumpy there.

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And we’re getting quite a good solid beam all the way along. But you can see how it’s the whole beam is a little bit jittery.

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And if you listen carefully, you can hear it actually jumping around, OK, now this is 16 percent.

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And look, we’re switching pretty good now.

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We’ve got a steady beam. Can you see that? It’s flipping, but it’s basically a nice, steady beam.

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It’s no longer jittery. And just to verify that, here we are driving at 20 percent and as you can see,

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we’ve got a nice steady pink beam all the way along here and a little bit of movement on the cathode.

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But it’s not jittery movement. It is just like a little teeny weeny pulsing movement.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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Now, we should be able to go backwards and we should be able to find that at 15 percent,

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that different characteristic, 15 percent was the upper threshold of this pre-ionization.

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Now you can see that jittery behavior on the cathode.

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And if you listen carefully, you may be able to hear it hissing.

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Now, look, it’s gone to steady mode now, so it’s on a on a cusp of being stable and unstable at 15 percent.

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So therefore, I think the limit probably is 14 percent.

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So let’s just drop back to 14 percent. And here we are at 14 percent.

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As you can see, it’s it is a weak looking beam. It isn’t a lovely solid pink beam it’s a bit wispy.

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But it’s this jumping around characteristic that is the thing that gives away this pre-ionization.

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Anywhere between nine and 14 percent for this particular tube is the pre-ionization zone.

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Now, normally, your bean operates silently. But in this pre-ionization phase,

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the beam is very noisy because of the intermittent nature of the cut that it’s generating because of these high frequency pulses.

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It’s very hissy. That’s the best description I can give you of it.

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Now, in this next little clip, you see me using the pre-ionization zone to cut card,

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thin hard and it cuts thin card extremely well without doing any burning around the edges.

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Now, when you listen to the last part of this clip, you’ll see that when we go around the outside shape.

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It’s mainly hissy, but on a few occasions you’ll hear it going quiet and then back to hissy and then quiet and back to hissy,

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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and that’s because I’ve set the power right onto the top edge of this operating zone.

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And so it’s moving from mainly hissy mode. Just occasionally it flips into stable mode and then back to hissy mode.

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It’s also a zone I use for photo engraving, for engraving on glass, as I said, or engraving on slate and tile.

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So it’s got some fantastic properties, if you know that it’s there and you go and play with it and find out exactly what it can do for you.

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Now, earlier on in this session, I mentioned that, you know,

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when I found out how to measure the power of the laser tube, I felt like king of the world.

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It wasn’t till later that I realized that, you know, actually power isn’t the most important thing.

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And I’m going to show you something that is the most important thing ever about your laser beam.

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I’m not going to dumb this down too much. Just a little bit.

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Doesn’t matter where on the body? But if I was to poke you with that pencil, would you prefer I poked you with that end or the end?

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I know the answer’s, not at all. I think it’s pretty obvious which end would hurt the most.

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Well, that’s exactly the same as your laser beam. You can get blunt laser beams and you can get sharp laser beams.

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I’m going to now demonstrate what a sharp tube looks like and how you can controllably

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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make it blunt. If that’s what your machine was supplied with. I’m afraid you know you’re not going to be able to do too much with it.

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That’s good for engraving, but it’s no good for cutting. Now, to aid me in this quest, I’m going to use this material here.

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It’s one of the best materials ever. For experimenting and testing and finding out what’s going on with a CO2 laser, it’s acrylic.

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This is the only material which will actually tell you what your laser beam is doing.

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And what we’re trying to find out now is the light intensity distribution within the laser beam.

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Now, I’ve produced a little one inch block, of perspex from 10 millimeter thick material.

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I’m not going to set the power on the max power button to 95 percent in my case, because I know that my machine will do it.

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I would suggest you set yours to about 80 percent. I’ve got a small piece of tube here, which I can remove my air assist.

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And just plug air into here and now I’ve got a stream of air, it’s coming out of there. Now,

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we shall eed that stream of air because at high power it will burst into flame. We’re trying to get the mark roughly in the center of the block.

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Hold the pulse button on and count to ten seconds.

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I’m blowing air at the block. Zero, one, two, three, four, five, six, seven, eight, nine, ten.

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Look, ten seconds has driven my beam right the way through that block.

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And this time I’m going to change the power to 15 percent.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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Zero, one, two, three, four, five, six, seven, eight, nine, 10.

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Now, let’s take a quick look at this in relation to my pencil analogy. So here’s the sharp side of the beam and here’s the blunt end of the beam.

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Hang on, this is the same beam. All I’ve done is change the power from ninety five percent down to 15 percent.

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Now, just as you would anticipate, I’ve done less damage with 15 percent power than I did with 95 percent power.

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But the other thing that’s changed is the shape of the beam.

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This is not a small, sharp beam. This is now a short blunt beam.

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If you look carefully, you’ll see that basically the diameter of the beam at the top here.

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They’re both approximately the same. Now here I’ve got a simple little LED key FOB torch.

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Wehen I shine that down onto that surface.

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I think you can clearly see the very bright light in the middle and how intensity gets less and less as we get further away from the center.

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That’s exactly how a laser beam should look. An ideal laser beam should follow this pattern.

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Exactly, which is a Gaussian distribution, in other words there’s more light intensity at the center of the beam,

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than there is at the edge. Exactly like that little torch image that I’ve just shown you.

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So if I allow this laser beam to damage material in one spot, what’ll happen is the most intense part,

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which is the center will damage material faster than the outer edge.

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And what we shall finish up with after one, two, three, four, five, six, seven, eight, nine, ten seconds is the shape that you see there.

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So when we reduce the power, it’s effectively the same as doing this.

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But it’s a blunter Gaussian form. We can interpret this picture two ways.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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If we’ve got a Gaussian intensity distribution within our beam,

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then if we allow it to last for certain periods of time, then this is the way in which damage will occur.

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Or we can look at it the other way and say that this is effectively the shape of our sharp beam when we’ve got it working at 100 per cent.

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But when we back the power off to, say, 10 per cent, here’s what it’s going to look like and there is what it looks like.

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You could very quickly check whether your beam is a Gaussian intensity distribution using this method.

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If you start off life with a beam that’s blunt like this, it’s just a flat pancake.

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You can make the exposure time. One, two, three, four, five, six, seven, eight, nine, ten.

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And after ten seconds, it won’t look like this. It will still look like this.

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A pretty horrible blunt beam. So this beam is naturally blunt, but this beam is controllability, sharp or blunt.

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You will never be able to cut anything deep and fast with a beam like this.

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It’ll work for engraving,

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but this one will allow you to cut and engrave because you can cut with a beam like this or you can blunt it down accordingly and engrave with it.

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It’s taken me probably four or five years to fully understand and appreciate just how important the shape of the laser beam is.

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Now, I think you can probably see from the difference between these two here that the power of the laser beam is not that important.

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The power of the laser beam is used to develop the shape.

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You may well have a bad shape to start with. But if you’ve got a powerful beam with a bad shape to start with, it’s still of no use to you.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity (Cont…)

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So power is not the most important thing. You need the correct intensity distribution within the beam and the power will then create the correct shape.

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So power is only a subsidiary part of this exercise.

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The important bit which you have no control over is the shape of the beam itself.

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Now the shape of the beam is determined by certain parts of the manufacture of the tube itself.

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So just because you buy a tube that says it’s 80 watts does not mean to say it’s a good quality tube, it could be an 80 watt poor quality tube.

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The only way you’ll ever find out is by doing this.

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Then, you’ll know, whether you bought a pup or not.

Transcript for Understanding Laser Cutting: Laser Beam Versus Intensity

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Last updated August 26, 2021

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