Number 02 – RDWorks Learning Lab 215 – AT LAST!!! I understand How Laser Lenses Cut.

The Top Ten RDWorks Learning Lab Videos

These are the top ten RDWorks Learning Lab Videos ranked by average number of views per month. In this session, Russ explains his new theory on how laser lenses cut. Backed up with test results of course.

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How Laser Lenses Cut - Pictorial Representation
How Laser Lenses Cut – Pictorial Representation

Transcript for How Laser Lenses Cut

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0:00welcome to another rdworks learning lab today we’re going to carry on with our

0:07session about gallium arsenide benzes now in the last session we saw that

0:12a one and a half inch gallium arsenide lens had some really strange properties it looked as though

0:18there was a focal point buried underneath the existing focal point maybe five millimeters down

0:25below the focal point are we going to see the same sort of characteristic or properties when we test a two inch

0:32and a two and a half inch now as i was editing the last video it did cross my mind that i was using

0:38mdf as a means of finding the focal point for this lens i wondered whether

0:46in fact i should be using acrylic because hey that was the material i was

0:52testing so what i’ve done is to very quickly run the same sorts of tests again with

0:59acrylic and let’s just have a quick look last session we were using this 38 millimeter lens

1:05and we set it up like this so it had a 28 millimeter gap

1:10i changed it to 24 millimeters so that we can carry out this focus test and sure enough 24 25

1:1726 27 28 is the thinnest line so the focus came out absolutely spot-on

1:23but when we did the penetration tests maybe if i’d have done my focus test on acrylic

1:29it would come out differently did i get it wrong with the focus we’re going to carry out this focus test

1:35at various speeds and powers to see if we can

1:41get an idea of what’s happening to the focus the first test is what we were using

1:47originally which is 15 power this time we’re going to speed up to 200 millimeters a second which is

1:53engraving speed and i’m purposely overlapping

Transcript for How Laser Lenses Cut (Cont…)

2:01the test with the edge so that i can see the depth of cut as it runs onto the

2:08edge we’ll look at these under the microscope in a minute 15

2:14200 millimeters a second

2:22so that’s 15 [Music] at 20 millimeters a second

2:32so that’s 99 at 20 millimeters a second

2:38right so we’re going to go really slow now with 100 power i don’t think we should burn right

2:44through but we might get pretty close

2:53[Applause]

3:01we need to go and have a look at those results under the microscope to see if we can work out what on earth is going

3:07on perhaps this will give us some clues as to where the focal point is on this lens

3:13so here’s our standard test at 15 power and 20 millimeters a second so if

3:18you remember we set it up to 24 24 25 26

3:2327 plus the 10 that was buried inside the lens which is 37 millimeter focus

3:30and then it start going out of focus again okay now let’s see what happens when we

3:35run at 200 millimeters a second you can hardly see it here but there is a very faint line here

3:43because the beam is four millimeters out of focus into the material so that’s 24 25

3:5026 we’ve just got a little bit of a hump there and then 27 looks as though we’ve got

3:56more and let’s see what 28 is like [Music]

Transcript for How Laser Lenses Cut (Cont…)

4:0628 looks as though it’s getting shallower again and 29 has nearly disappeared so again at 200 millimeters a second

4:14at 27 millimeters we’ve still got the same intensity focus point i mean the actual

4:20intensity is a lot less because we’ve run faster and the power has been spread out more

4:26this is 20 millimeters a second but a hundred percent power maximum power now you can see here

4:33we’ve now achieved the best focus we first of all we’ve got a very narrow

4:40cut and secondly we’ve got quite a deep cut you can see that that cut is deeper than

4:45this one 24 25 26 27 the actual position

4:53where the intensity of the focus has not changed it’s remained at 27 millimeters

5:00and then it starts disappearing again and getting bigger and bigger and bigger as we go

5:08past the focal point and start drawing the beam out this is what’s happening after the focal

5:15point you can see it’s growing pretty quickly so now we’ve slowed the beam right down

5:20there’s no obvious focus detectable by the depth of cut

5:25we’re running very very slowly now at five millimeters a second with a hundred percent power and we’re seeing the same sort of effect

5:33that we saw with our spike test look we’ve got powerful beams coming in here

5:40which are passing through what looks like a focal point here at maybe two or three millimeters into the job

5:46now as we raise the focal point up we can see that we’re also raising this focal point up as well to

5:53probably close to one millimeter and we’re getting a wider neck

5:58we’re getting slightly less erosion on the inside here and then here we

Transcript for How Laser Lenses Cut (Cont…)

6:05nearly got the focus right out to the surface again and as we put the focus out to the

6:12surface we’ve got a little bit less erosion on the inside minimum intensity at the surface is

6:18occurring just here that’s the smallest net beam intensity

6:26and here we’re starting to get bigger again as the beam moves out this is what’s happening below

6:32the focal point it’s getting very messy 24 25 26 27 millimeters

6:40is still the smallest entry point so it appears that speed or power

6:48does not change the effective intensity focus it remains at

6:54approximately the same distance away from the lens well time is passing

7:00and i’m sure i’m getting grayer since the last time i spoke to you a few days have passed i’ve been

7:07puzzling over these little test pieces that we made now since we checked

7:13that the focal point for this material appears to be 37 millimeters and not 38 millimeters

7:21as per the lens focal point i’ve been back and i’ve redone

7:28these little tests setting the focal point on the surface at 37 millimeters

7:35and also five millimeters into the surface as well so that i’ve got some

7:40comparisons because i’ve got tremendous faith in this material acrylic

7:47just like a young child that doesn’t know how to lie this is exactly the same it’s going to

7:53tell me what’s going on although this stuff doesn’t tell lies sometimes it’s difficult to read what

Transcript for How Laser Lenses Cut (Cont…)

8:00it’s actually saying one of the puzzles that i’m trying to solve is what is this ballooning

8:08effect that happens inside here it doesn’t happen inside a four inch

8:14lens but it is definitely happening inside this one and a half inch lens several

8:20possibilities exist i think that it’s most likely

8:25sideways beams coming in and crossing over and scouring out

8:30beams that are very very slightly off axis but on the other hand it’s been

8:36suggested to me that it’s probably scouring

8:41caused by heated gases inside the enclosed tube

8:47if the gas is getting that hot inside there then it’s certainly a viable proposition

8:54that we could be heating the gas up and that gas the longer it exists in

9:00that tube could be scouring away the inside of the tube away from what the effect of the laser

9:07beam actually are and i think i found a way of testing and solving that problem now i’m

9:14not going to bore you with all the details i’ve set all the machine up manually so

9:20that i’ve got a 300 millisecond pulse and it’s been set 37 millimeters above

9:26the surface so each one of these little marks along here has been done at the focal point on the surface okay

9:33and then what we’re trying to do is to analyze what’s happened to each one of these you said but they’re all the same well

9:40no they’re not because what i’ve done i’ve started off

9:46maybe two millimeters inside the edge of the material and i’ve skewed the material so that as

9:53i step across the material i’m going to start scanning down and at some stage and i didn’t know at

Transcript for How Laser Lenses Cut (Cont…)

10:00which stage i’m going to start producing half a hole and that will tell me

10:05whether or not what we’re seeing is a trapped gas situation and we need to go and look at

10:11that under the microscope to get the answer to that question now there’s the view that the laser beam

10:17sees and as you can see we’ve pierced into the top there and we’re gradually creeping towards the

10:23edge of the material because i set the piece of material across at an angle well we can clearly see that all these

10:29are exactly the same and then if i change the light a little bit

10:34what we’ve got the neck just in here is within the material and then it

10:40breaks out of the material just here so there is no possibility of

10:46captive gases in this column here and yet it’s still ballooned and this

10:53last one which is a perfect section still showing all the signs of that

10:58ballooning so it must be rays that are almost parallel to the axes of the beam

11:05that are coming in here and scouring away the edge of this so that’s one problem solved but now

11:10what i’ve got to do is to try and decode what the rest

11:15of these pipes are telling me so i’ve got a lot of thinking and some i think some cad work to do

11:22some ray tracing to see if i can establish what might be happening i’ve done the focus test and on acrylic

11:29it says the best focus is 37 mil we’ve got the gap set to 22

11:35which is equivalent to 32 millimeter focal distance so the focus is actually 5 millimeters into the material at the

11:41start of the test

11:59the flat side down is consistently cutting shallower than the flat side up

Transcript for How Laser Lenses Cut (Cont…)

12:06next test that we’re going to do is something i call a speed test because this is an important test to try and

12:12find out how deep and how fast a lens can cut

12:18at the moment we’ve been interested in mainly depth but here what we’re doing we’re using

12:23two milliseconds four milliseconds six milliseconds etc

12:29to see how quickly we can pulse into this material in other words

12:35this will give us an idea of the cutting speed performance of a lens

12:40[Music] now before i’ve tried to reverse

12:47engineer what’s happening to the rays i’ve done

12:52eight tests two for each of the lenses that i’ve got

12:59one has been with the lens flat side up and the other with the lens flat side

13:06down now while i had the lenses in position i did another test for every lens that

13:13was to try and determine the speed of cutting of each lens

13:19the speed of penetration is in fact the speed of cutting so what we’ve got

13:25here is 2 milliseconds 4 6 8 10 all the way up here we’ve got a little bit of a trace

13:31of how quickly the cutting develops in each one of these lenses whether it be face down or face

13:38up they’re all down the side here flats up flat down again something that we’ve proved before is

13:44that this orange which is the two and a half inch is definitely one of those that’s going

13:51to carry on going up there and is responsible for deep cutting this

13:56pink one here which is a 50.82 inch flat side up may also perform pretty well

Transcript for How Laser Lenses Cut (Cont…)

14:04according to this prediction the other advantage of this two inch lens is if you look here the rate at which it

14:11climbs extremely quickly here at somewhere in the region of about

14:17three milliseconds for a three millimeter thick piece of acrylic

14:23so what that really means is that if we’ve got a 0.2 curve

14:28the diameter of the beam is say point two that means that in

14:35a one millimeter length of cut i’ve assumed that i’ve got these all

14:40joined up and i’ve got one two three four five cuts per millimeter in other words

14:49i’ve got five cuts at four milliseconds a cut which gives me 20 milliseconds to cut

14:57one millimeter now there’s a thousand milliseconds in a second

15:02and 20 milliseconds to cut one millimeter divide those two and what you get is a

15:09cutting speed of 50 millimeters a second can i cut three millimeter acrylic at 50

15:16millimeters a second i don’t know that’s what the facts tell me so

15:21it may will be marginally less than that i think we experimented like this before and we found that maybe we had something like about 0.8 of

15:28that value but it’s still a way of trying to assess how efficient

15:33you can cut with different lenses well the cutting speed test was just an interesting diversion because i had the

15:39lenses in place and it was easy enough to do the test but the real task for this session was to try and see if

15:46there’s any way that i can decode what’s happening in these tubes so we’ve

15:52got five millimeters into the material four three two one that’s supposedly the focal point

Transcript for How Laser Lenses Cut (Cont…)

16:00and then it’s one two three four five six seven eight millimeters below the focal point

16:06and the further we drop below the focal point you see the beam is becoming very blunt and it does not penetrate very much into

16:13the into the material so decoding these is my next task now as i said to you maybe in the last

16:20session the biggest problem i’ve got is trying to decode and decide whether or not these are the result of a focus

16:28which is below the focal point or whether these are the result of a mode burn

16:35i must remind yourself and me of something that i did several months ago and that was this

16:42mode burn test under two different circumstances the first circumstance

16:47was a parallel beam straight out the laser and burning into this material this is

16:54not a focus point this point happens because i’ve got high intensity

17:01at the center of the beam and low intensity at the edge of the beam and if i give it

17:08enough time and in this case it was 10 seconds the center of the beam

17:13burns in faster than the edge of the beam hence i get this conical shape now

17:20this more slender conical shape happened in three seconds

17:27as opposed to 10 seconds because i had interposed a long focus lens

17:35into the beam and reduced the beam diameter before it hit the edge of the block

17:42now that basically increased the i suppose the energy density or what i prefer to call the intensity of

17:49the light before it hit this piece of acrylic and higher intensity

17:54light means faster damage so if you allow intense light to hit this

Transcript for How Laser Lenses Cut (Cont…)

18:02it will penetrate faster than the less intense light

18:08so we finish up with the same result a pointed beam but look this pointed beam has got

18:14nothing to do with the focus of the actual lens path itself so we mustn’t be confused between

18:22these two issues that is not a focus point that is a point caused by very high intensity

18:30and an exposure time the low intensity here is doing very little damage

18:36and the high intensity is doing much more damage what we are now trying to do with the

18:42analysis of these tubes we’re trying to move further and further

18:47down the beam and we’re taking samples at various points across the beam and we’re getting

18:53different more intense mode burns if i go past the focal point

19:00then the mode burn is going to change dramatically the only problem is these lenses do not have a single

19:08focal point and that causes all sorts of problems in trying to

19:14analyze it i feel as though i’m reading tea leaves trying to analyze or reverse engineer

19:21these rays so i need some additional information and i’m doing that with this

19:27piece of card so i’ve got my four inch or 101 millimeter lens in here and here i’ve got a 61

19:36millimeter spacer i’m 40 millimeters below the focal point at the moment

19:42[Music] and what i’m going to do is i’m going to burn some marks

19:48in this paper at five millimeter

19:56[Music]

Transcript for How Laser Lenses Cut (Cont…)

20:04intervals [Music]

20:10now you will see that there are a whole load of holes in that card now every one of those holes because the

20:18card is a piece of constant thickness material and the power is constant

20:26the holes will be created by the same intensity of light

20:31so that will tell me basically what the intensity is at various positions

20:36along the beam and that maybe helped me to determine uh some of the tracks of the rays now

20:44here we’ve got a little fixture that i’ve made for testing the focal lengths of my lenses

20:49and i’ve set up a target in here which i’m going to move backwards and forwards and i’m going to get you to look at that

20:55target through this 2-inch zinc selenide lens that i’ve put in here

21:00right so i’ve got the two inch lens roughly in focus now and as i move the target towards the

21:07lens only by a millimeter it’s already gone out of focus

21:12so let’s put it back into focus and take it a millimeter out of focus and this is very much in accordance with

21:19standard lens theory you have a very short focal range over

21:24which a lens works now the other thing that’s interesting about this image

21:30where’s all that red and orangey stuff coming from around the edge of the image

21:35so unlike this camera which has got a very good set of optics which focuses perfectly on the surface

21:42the lens that we’re using here has got something called spherical

21:48aberration and even as i hold it in front of this target you can see how it’s actually

21:54producing this strange orange glow around the outside which is the spherical aberration

21:59the problem is we’re looking at this in white uniform light

Transcript for How Laser Lenses Cut (Cont…)

22:06now standard lens theory works well for cameras as you can see telescopes

22:14microscopes projectors because they all rely on uniform

22:20light now i know there might be different brightnesses and colors in a projected image

22:28but that’s not quite the same as the intensity problem that we face with a laser beam

22:36we are not trying to focus images we are trying to focus light intensity

22:43and that’s a completely different subject which is not really covered by lens theory now in the in the

22:50description text below this video you’ll find some links to some other youtube videos where

22:57people are trying to link standard lens theory into laser cutting technology now one

23:06particular video i would love you to look at is this by a moderately famous company

23:13now you would think that these would be professional enough to explain correctly and technically what’s

23:20going on here what they’re trying to describe is that a four inch lens

23:25is to blame for this loss of text down here but hang about

23:33that implies that a lens has got some intelligence and it knows when to transmit light and

23:40when not to transmit light it’s been perfectly okay transmitting light here

23:45and burning these parts of the text so why is the four inch lens to blame for this missing text

23:52down here in reality it’s nothing to do with the lens at all it’s all to do with the light

23:59signals that are coming from their machine if there are no signals the lens cannot make light happen

Transcript for How Laser Lenses Cut (Cont…)

24:06it’s all to do with the parameters that they have their machine set to they are not transmitting every single

24:12pixel that’s in this bitmap image and these are actually

24:17missing pixels it’s not the fault of the lens that the pixel is not there it’s the

24:23fault of the encoding that the light is not there to be transmitted by the four inch lens

24:31now this four inch lens looks half reasonable it’s not doing a bad job around the edge it’s not blobby and lumpy around the

24:37edge even here look it’s not really blobby and lumpy but the verticals are missing because

24:44they’re not scanning the correct pixels what i’m saying is there’s a great deal

24:49of misinformation out there about lenses how they work people just

24:55don’t understand how lenses and laser systems work together so

25:00go and have a look at the video links that i’ve provided and you’ll see what the current

25:06understanding of lenses is you might want to do that before you push on with the rest of my video

25:13that shows you how lenses actually cut now as you can see i’ve already done

25:18some cad work and that cad work has allowed me to

25:24work out the essence of how a laser beam actually cuts

25:29in other words it’s the interaction between the laser beam and the lens that produces these strange effects that

25:35i’ve been looking for and after two years of searching i think i’ve now

25:40uncovered a model that actually describes how and why a laser beam cuts in the way

25:47that it does my basic principle of a spiky focus below the focal point has not proved to

25:55be wrong it’s just proved to be the wrong way of looking at the problem

Transcript for How Laser Lenses Cut (Cont…)

26:01the first thing we must do is quickly go back over the basic principle of a gaussian

26:06distribution curve now i know that my beam is approximately

26:1212 millimeters diameter you’ve seen me burning tape for longer and longer

26:18periods to try and work out exactly what the maximum diameter of my beam is because obviously

26:24when we get right down here to the bottom tail of this intensity it takes a long time

26:31for that very low intensity to damage the tape okay well this central middle third of

26:36the beam which in this case represents uh four millimeters

26:42so that central four millimeters contains roughly 70 percent of the power of the beam

26:49because the area under this curve represents 100 percent of the power of my beam 70 watts so therefore this

26:57central 70 percent power is roughly equivalent to 50 watts

27:03so in this four millimeters the yellow four millimeter section i’ve got 50

27:08watts of power being used that 50 watts is not being used as watts we are using those watts

27:17to develop intensity and the shape of this curve is very

27:23important and a proper gaussian curve means that at my 50 watt extremity here my 50 watts in the center

27:30here is developing 62

27:35intensity within the beam and as i get closer to the center of the beam my intensity

27:43starts going up and so this red zone here which is actually one-sixth of the diameter of

27:48the beam two millimeters contains roughly 89 of the intensity available

27:58in this gaussian distribution right at the center here we’ve got a hundred percent

Transcript for How Laser Lenses Cut (Cont…)

28:03okay so you mustn’t lose sight of the fact that it’s intensity that does damage

28:11the intensity of the naked beam is vital because if you’ve got a substandard

28:20gaussian distribution here and let’s just say that it’s a very flat distribution as they say in

28:27computing terms rubbish in rubbish out okay so don’t expect the performance

28:33that you you see me getting from your laser beam if it doesn’t look like a good gaussian

28:40distribution beam i’m very lucky that i’ve got a superb quality beam my rays if you like are colored

28:48to represent intensity boundaries within the beam itself so this represents zero percent

28:55intensity right at the extreme here because there is no power there there is a boundary sort of

29:01but then here 13 intensity boundary then we’ve got our 62 intensity boundary

29:07and here we’ve got our 89 or 90 intensity boundary so most of the

29:13intensity is passing right through this red zone at the center here this gaussian

29:20distribution exists here in the naked beam but it doesn’t last for very long

29:29i always thought that that was going to be compressed down to a very small version

29:35of it just here at the focal point that’s what i’ve believed all the way along

29:40in other words if you turn the telescope round you see a small version of the moon when you look at the moon

29:46you don’t see some distorted version of the moon or a white blob that’s not the case with this shape here

29:54when we start amplifying it down here it’s no longer gaussian at this point

29:59here and it’s totally destroyed when we get here but we’ll talk further about that in a

Transcript for How Laser Lenses Cut (Cont…)

30:06few moments when we’ve looked at more detail in this diagram so when we did this test if you remember

30:12we set the table at this height here okay and we did this first burn

30:19now this first burn the focal point was buried five millimeters into the material

30:26and then we dropped the table by a millimeter and now it’s four millimeters into the material

30:31we dropped it dropped it dropped it and eventually by the time we got to line number five here

30:37we were at supposedly the focal point which is what this blue line is and what

30:44i’ve then done is to measure the diameter the entry hole

30:49damage on each one of these test spikes and that’s what these

30:56red circles are the damage at the entrance to each one of these

31:01holes we can’t get damage unless there’s light intensity there to excite the molecules

31:07up above their vaporization point and make them disappear so these diameters here are very exact

31:15markers of light intensity that’s what i placed into my diagram first

31:20i then realized i was having quite a bit of difficulty trying to work out how my rays were actually passing

31:28through these various points couldn’t make a lot of sense of them

31:33and it was at that point that i decided i had to go back and use something that i’d used several

31:39years ago to plot what i call what i then called energy density within the beam

31:47and that’s what some people call it but i would much prefer to use the term light intensity not energy density

31:55because it is light intensity which is causing the damage to the

Transcript for How Laser Lenses Cut (Cont…)

32:01material it’s the thing that’s exciting the molecules making the molecules vibrate faster and

32:09faster vibrating molecules get hotter and the hotter they get we see it as

32:15burning but in fact what it’s doing it’s getting the molecules vibrating so fast that the molecular

32:22bonds start breaking down and we get chemical changes and that’s what you’re witnessing here

32:28you’re witnessing chemical changes to the material as it heats up

32:33as it vibrates faster and we’re getting a combination of oxygen in the air which is beginning

32:40to oxidize some of the material away and we’re leaving some

32:45carbon behind which is the black stuff we’ve got two materials involved here

32:51the red the red circles are these spikes which are disappearing

32:58and being created at 200 degrees c at 200 degrees c acrylic

33:05evaporates and what we’re left with is nothing we’re left with an edge so what’s inside

33:12that edge must have evaporated and gone above 200 degrees c to disappear so we’ve got basically

33:21a an intensity boundary that we’re setting with these red circles

33:26light intensity boundary now we’re doing exactly the same thing here and what in this instance we’ve got

33:33we’ve got two intensity boundaries that we can look at first of all we’ve got a hole in the

33:40center and i hope you might be able to see those see the light through those holes there but we’ve got holes

33:46through the center of these black dots now those holes can only exist because

33:53we have made the material evaporate we’ve burnt

33:59it we’ve made the molecules shake so hard that they’ve destroyed themselves and disappeared somewhere

Transcript for How Laser Lenses Cut (Cont…)

34:04this card is basically wood pulp and wood has got some very strange

34:10properties when it comes to let’s call it self-destruction you would like to see it as burning

34:16but i like to think of it as self-destruction because of stimulation by light this

34:21self-destruction happens in three basic stages number one

34:27you’ve got cellulose and the cellulose disappears at about 250

34:35to 300 degrees c what’s then left is the cell structure of the cellulose

34:42the cellular walls a material called lignin which shakes itself to pieces and disappears at about

34:50350 degrees c and then what we’re left with is this

34:56black stuff and everybody knows what that black stuff is it’s carbon

35:02but what is going to surprise some people is that that black stuff doesn’t shake itself to pieces

35:08until 5 000 degrees c it boils

35:15at about three and a half thousand degrees c but it disappears to gas at about 5000

35:22degrees c now on the way up to 5000 degrees c it may well combine with other elements

35:29mainly oxygen in the air and it will produce carbon dioxide gas and carbon monoxide gas but any carbon

35:36that’s left has to exceed about 5000 degrees c before it will

35:42disappear so there’s a very good marker of high intensity

35:49in this card but there’s also another marker in this

35:54card as well which is the outer brown mark what i want to do now is to

Transcript for How Laser Lenses Cut (Cont…)

36:01take a look at this particular focal point here because

36:06that is a vital point that helped me to construct these diagrams and i say these diagrams because i’ve

36:13done one of these diagrams for each of the four lenses that i have

36:18we’ll just take a look at the uh the first hole on here which you’ve got a a burn around the outside of the hole

36:25now you can clearly see the hole itself here but you would ask yourself the question

36:32why is this not black i mean look you can clearly see the

36:38clear card and the paper underneath and yet we’ve got lots of carbon around the outside

36:44and in the center here and we’ve got lots of scorching around here well this black carbonaceous material combined

36:50with the white gives the impression when you mix it in your eye that this is a brown

36:56material a scorched material and you get various shades of scorching as you move out from

37:02the center of this hole now this card is not quite what it seems it’s not

37:07simple wood pulp about 20 of this card is something called

37:14kaolin china clay and china clay is used widely in the paper industry in the card

37:20industry to a stiffen the card b to make it a slightly glossier

37:27harder finish so that you can write on it and it doesn’t soak the ink into the

37:32paper so kaolin is in itself

37:37a mineral material which does not burn when i say it doesn’t burn it melts at

37:44around about 1700 degrees c and probably by the time it gets to two and

37:51a half or three thousand degrees c it will vaporize

37:57but it’s nowhere near the vaporization temperature of this stuff which is the carbon so we’ve got a huge

Transcript for How Laser Lenses Cut (Cont…)

38:04range of temperatures going on as this material destroys itself this

38:10brown which is not evaporating and it’s not turning to carbon but it’s just scorching so we’ve got two

38:19light intensity boundaries when we look at these pictures so as the beam gets more condensed

38:26we’re getting a different pattern of burning and then we get to this rather strange one here which i said

38:33is very strange because you can see not only have we got our total evaporation of material in the center here

38:40we’ve also got a slightly brown halo around here and then we’ve got no damage at all and

38:47then we’ve got a rear currents of our brown halo around here we’ve got equal boundaries of intense

38:54light intensity at these points so again these are very important

38:59markers that tell us what is going on within the beam this one is getting close to the focal

39:06point we’ve got total destruction in the middle here where the material is shaking itself to pieces we’ve got partial destruction here

39:12around this boundary and then right around the outside we’ve got virtually no destruction

39:18you can just see a hint of brown around here where there’s obviously some very light energy around the outside but

39:23basically we’ve got a an intensity boundary here which is very obvious now the focal point is very interesting

39:31so we’ve got a very heavy layer of carbon just around here indicating we’ve had very high temperature there but not

39:37enough to make it disappear and then we’ve got our thin layer around the outside here

39:43of brown but you’ll notice that the brown and this destruction are very close

39:50together now intensity beams are all converging on this one point that people

39:56recognize as the focal point it’s not a focal point because if it was a focal point

Transcript for How Laser Lenses Cut (Cont…)

40:04we would not have any of this brown around the outside we would just have total destruction because all the rays

40:11would be passing through that hole most of the rains are passing through that hole

40:16but some of the rays are still not passing through the hole so this represents effectively the size

40:23of the focal point not this so the next part of this picture that i’d like you to understand

40:29is these green circles which represent the brown extremity of the scorch layer and the

40:35white circles which represent the total internal destruction of the material

40:42i.e a whole now this card pattern of burns was crucial

40:48to helping me define where the focal point for the lens really is because remember

40:55what i said to you a focal point is the point through which

41:01all the rays pass and so the smallest hole that i could find and

41:07let’s just zoom in you’ll see that what i’ve been able to do i’ve been able to concentrate my

41:12yellow rays my red rays my blue rays onto this hole here because this is

41:18where all the power is hitting to cause that amount of damage

41:23that must be the point of maximum intensity that white circle okay now there are some other things

41:29happening around the outside here which are going to cause scorching okay now remember this white beam

41:35although it’s got virtually no power in it at all because it’s the extremity of zero

41:40there is still some small amount of power there to do damage so combine the white with the blue

41:48and the yellow and even the red and we’re going to get this very very

41:53small scorch boundary around the focal point but this focal point basically defines

Transcript for How Laser Lenses Cut (Cont…)

42:01where i’ve got to make all my beams pass very close to it and

42:06buy it and it was this key feature the focal point on card which has allowed me to if you like

42:13project and guess where the various intensity of rays are passing through

42:22this focal point so now we can start looking at them in a slightly different way now what i’ve done here is to strip away

42:29all the useless data that was very useful for constructing this diagram so that we can

42:35see what’s actually happening and if we look carefully you’ll see that the outer rays

42:43are crossing over here the next row the next ones in which are

42:49the blue rays are crossing over here beyond the focal point

42:54and the yellow rays are crossing over even further beyond the focal point and eventually

43:01the red rays are crossing over beyond the focal point so there is

43:08no single focal point if there is any sort of focal point

43:14this has got the maximum amount of power the reds so that should technically be

43:19the focal point but it turns out not to be because there is a strange

43:27set of circumstances that exists just here and you think well hang on

43:33there must be power down here because this is the focal point that isn’t the

43:39way it works i’m afraid that’s why i said my focal point idea

43:44was both correct and incorrect there is a spike of focus way beyond the focal point but that’s

43:52not that spike of focus which is causing the damage let me try and explain that because this

43:57is a very difficult concept and i’m going to work back in a very simple way to start with

Transcript for How Laser Lenses Cut (Cont…)

44:03remember what i said to you the area under the curve equals 70 watts they’re all

44:09gaussian distribution curves but they have got different baselines as i shrink the baseline

44:16so the intensity within the beam goes up so you remember back to my

44:22picture of the two mode burns that i did one with parallel beam and one with a focusing beam well here

44:29we’ve got a focusing beam okay and if i take this shape here

44:37so let’s just assume that this is a section through the beam and it’s obviously above the focal point

44:43but what’s going to happen then we’re going to get a mode burn and the mode burn will be intensity

44:50times time so we should be able to burn quite a deep hole quite quickly so as you can see this one

44:57has got substantially more intensity than that one because i’ve got a slightly smaller

45:03baseline as you can see i’ve only moved a very small amount down the beam but now what i’ve got

45:10i’ve got a tremendously increased intensity

45:15and if we take a look you’ll see that that intensity is not spreading out here that intensity

45:22starts here at the focal point which is say 0.2 and very quickly bores its way in

45:30leaving a 0.2 diameter hole behind because that’s where it’s eventually going to grow to

45:35now you’ll remember when we do a normal mode burn we’ve got parallel rays of light hitting the surface here

45:41and it is the high intensity rays that are in the center of the beam that are actually working

45:47their way in quicker and quicker to make this point whereas the low in lower intensity rays at the outside are

45:54doing less damage but the high energy in the center doesn’t change because the rays

Transcript for How Laser Lenses Cut (Cont…)

46:00are parallel that isn’t the case here because as we start nibbling away

46:07into here with our erosion look what’s happening we’ve got yellow rays which are adding

46:13into the equation we’ve got the red rays which are actually still compressing down

46:19and they are actually increasing the intensity of the light even more

46:24and then we’ve got the blue rays which effect to a certain extent and then they stop working so we’ve got

46:31a very complex rate of erosion taking place within this

46:36orange spike of potential intensity that intensity is not

46:43constant that’s really what i’m saying so we will actually be not working with a proper

46:50gaussian spike of energy here as i’ve shown it this

46:57will be a really strange distorted spike of energy i do not have the mental capability to be

47:05able to analyze all these variables and tell you exactly what’s going to happen i can tell you in essence what’s going

47:12to happen and i’ve broken down the elements to show you the essence of what’s going to happen now i’ve done exactly the same

47:18drawing for all four of the lenses and what i’ve done i’ve put the focal points

47:24all in line so that you can clearly see that as the focal distance of a lens changes

47:33the red high intensity focal point moves further and further beyond the

47:39lenses natural focal point which is why we can cut thick material with the longer

47:47focal length so although this one a half inch has got a much smaller

47:54focal point it may well have a much higher intensity but

47:59its core its red core if you like of power does not project as far

Transcript for How Laser Lenses Cut (Cont…)

48:07as the four inch one the four inch one intensity beam is lower

48:14because it’s a bigger diameter but if you put more time into it in other words you cut

48:20slower you will be able to cut deeper you will never be able to cut deep

48:26with a one and a half inch lens although it has got a lot of intensity it does not have the potential to cut

48:34deep because its focal point disappears in a much shorter distance

48:40[Applause] so all this energy density at the focal

48:46point is meaningless first of all as i’ve indicated before there isn’t such a

48:53thing as an energy density at the focal point there can only be that can only be a

49:01nominal energy density and that doesn’t describe the model that we see here

49:06we don’t have an average density across the focal point

49:12it’s a gaussian distribution of some sort but a distorted gaussian distribution

49:19because we do not have all the beams perfectly lined up if we had the perfect lens would we be

49:27able to cut with it that’s an interesting question and i would think it would be very

49:32poor at cutting the only reason these lenses cut

49:38is because we’ve got this variation this aberration because it’s a property

49:44of a spherical shaped lens so my point really is no matter how

49:49good you make a spherical lens it can be as perfect as possible

49:54but it will still have this weakness of aberration which is the one thing

Transcript for How Laser Lenses Cut (Cont…)

50:00that we need for cutting i’m absolutely sure that there’ll be many people on the other side of this

50:07camera lens that will not accept the fact that hey we need this

50:12aberration to make a lens cut i’ve even doubted it myself all these

50:18diagrams are actually wrong technically because this parallel beam

50:23will refract at an angle inside this material and then have another refractive

50:29path when it comes out of the material so this is a very simplistic diagram but it

50:35describes what actually happens which is this confusion of focal points as i have

50:41described you in the previous parts of the video now we can get rid of this confusion

50:48and we can try and get a perfect focal point with something called an espheric lens

50:54which you can see has got a weird shape and again the paths through that lens are not technically correct

51:00but the end result is more or less correct in other words we shall be able to produce with some very

51:06special distortions a perfect focal point

51:11now i believe that that perfect focal point will not allow us to do anything other

51:17than engrave secondly we cannot get hold of any of these lenses

51:23to prove the point they’re very expensive and they’re not typically available for our little chinese machines so we have to

51:31suffer with either this type of lens which always has spherical aberration and is great for

51:37cutting or there is an attempt to try and reduce this problem

51:43with something called a meniscus lens a meniscus lens has got a spherical surface on the

51:49outside and it’s got a secondary spherical surface underneath the lens to try and correct

51:56for this problem here although you can’t see in great detail

Transcript for How Laser Lenses Cut (Cont…)

52:02this is not a perfect focal point this still has some aberration on it because we’ve got two spherical surfaces

52:10the second spherical surface attempts to remove the major part of the top surface spherical aberration but it

52:17doesn’t succeed completely so this lens will cut because it’s not

52:22perfect but it won’t cut as well as this lens

52:28you don’t believe me let me show you something now what we’ve got here is my dropping

52:35table test where the focal point is approximately the fourth or the fifth

52:40one in i can’t remember which but basically we’ve got very deep penetration

52:46because we’ve got very high intensity coming through the center of the lens where the spherical

52:52aberration is worst or in this case best because it produces the deepest

52:58cuts what i’m now going to show you and i’m going to slide it into the background is

53:05exactly the same shape size of lens except this is a gallium arsenide

53:11two and a half inch meniscus lens

53:21i think you can clearly see the cutting ability of that lens is compromised

53:29crap use whatever adjective you want but it’s not the sort of lens that you

53:34would use for deep cutting i think you can see that very clearly all i can do is show you the proof

53:41you can choose to believe it or not go ahead and do the tests yourselves

53:47let me just step back a second and say what about engraving well engraving is not any different

53:54to cutting we need to set the power of the machine lower and as we reduce the power of the

Transcript for How Laser Lenses Cut (Cont…)

54:00machine what we should be doing we should be changing the shape of the beam and making it blunter and if we make the

54:06beam blunt it means that we won’t be using the full extent of this red projection we should be pulling

54:13everything backwards to make it a much softer beam and so we

54:18won’t get as much cutting during our engraving process

54:25so i think i’ve reached a point where i don’t need to destroy any more lenses i’ve already proved that if you take the

54:31center away from a lens you can destroy its cutting ability and

54:37when you look at my diagrams you can see why because that’s where the power of cutting exists it isn’t necessarily a

54:43focus below the focal point that defines the cutting ability of a lens although it is a contributor

54:50it is the the strange non-gaussian mode burn that happens below the focal

54:55point which is actually responsible for the cut that we produce in materials

55:01so the longer the focal length the deeper you can cut but you either have to go slower

55:09or have a slightly more powerful laser tube to achieve it faster but again

55:16there is a physical limitation even on a four inch lens as to how deep you can go

55:21i should be limited by my 70 watts of power so in the next session i think what i

55:27should be doing is looking at that four inch lens to see whether or not i can make it cut

55:32deeper than the two and a half inch lens where i was able to achieve 26 millimeters at

55:38three millimeters a second can i achieve 50 millimeters at one millimeter a second with a four inch

55:45lens with my 70 watts interesting so thank you very much for your patience

55:51and I’ll see you in the next session

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