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 Sadler explains his new theory on how laser lenses cut. Backed up with test results of course.
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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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