Number 01 – RDWorks Learning Lab 202 – More Laser Cutting Investigations

0:02 no welcome to another rdworks learning lab now in the last session we tried to

0:07 analyze how and why a laser beam cuts it all boiled down

0:12 to two basic factors one of them was light intensity and the other one was

0:19 linear speed the more intense the light the faster you can cut

0:26 now i always remember some very wise words that somebody mentioned to me when i was

0:32 in my youth can i remember back that far i remember these words size is not that

0:38 important it’s what you do with it that’s important and i’ve only got 70 watts on this machine

0:44 and we’ve got to try and make the most of what we’ve got so today we’re going

0:51 to try and analyze the best way in which we can try and

0:57 get the maximum intensity of light out of the 70 watts the 70 watts is relatively unimportant

1:04 as i said it’s what we do with those 70 watts that’s important so let me involve you with some of my sketches and drawings again

1:11i tried to make them a little bit more professional this time by preparing them beforehand i still don’t know where i’m going in

1:17this session at all but i’ve got a starting point this is the light intensity graph

1:25of what’s inside a typical beam so there’s our beam width raw straight

1:31out the laser machine and let’s assume it’s typically anything from eight to twelve millimeters diameter

1:38now when we run at 100 power let’s just call that light intensity what we get at

1:43let’s just keep things nice and round 60 watts now i know it doesn’t work this way but let’s assume that 50 power is 30

1:50watts and 20 power is actually 12 watts the point i want to make with this

1:56diagram is that when you decrease the power you are basically blunting the beam

Transcript for More Laser Cutting Investigations (Cont…)

2:04it’s no longer a sharp beam like this okay so don’t expect to get the same

2:11sort of performance cutting performance out of 20 power you might think you’re getting just less

2:18power but you’re changing not only the power you’re changing the characteristic of the beam itself

2:23the beam doesn’t change in diameter just because you change the power and mathematically the area under this

2:31under this graph here is 60 watts and this area under this graph is 30

2:38watts and under this graph here is 12 watts the area is proportional to the power that’s a

2:45crude mathematical relationship that’s a nice model to work with now in the last session we

2:51established that the intensity of the light was the most important thing because it

2:57is the intensity of the light that causes material damage and if you

3:03remember we we talked about the fact that the light intensity was going to cause the molecules to vibrate

3:09and if we had enough light intensity we could vibrate them to their breaking point

3:14and the more the light intensity the quicker we could get to that breaking or destruction

3:20threshold for a material so light intensity is most important and

3:25if you remember the lot the results that we got from our compound lens tests were very confusing to me

3:32yeah they were interesting but you know the fact that regardless of the type of lens that i’m using

3:38and the spacing between the lenses look 18 17 18 18 18 17

3:4518 16 18 19 17 18 18 there is a huge

3:51number of 18 millimeters a second what were the variables here well we had

3:57the same top lens but we were varying the bottom lens they were all two and a half inch but they were different materials and

Transcript for More Laser Cutting Investigations (Cont…)

4:04they were different directions in other words we turned them flat side down and flat side up now i mean

4:10they should not be giving the same sort of results we’ve got constant power for every one of these tests

4:16so look that means we’ve got the same shape entering the top lens every time by the

4:22time it gets down to the bottom lens it can probably be half the size of the beam it started with

4:27or maybe a quarter of the size of the beam that it started with okay the power is the same but the

4:34energy distribution across the bottom lens has changed but we still get the same

4:40output result so what that means is the intensity of light coming out of

4:47that bottom lens must be the same because we can do the same amount of damage with every lens

4:55combination how is that possible that’s the puzzle

5:00that we’re going to try and see if we can investigate today the shape of this the

5:07height of this is its intensity but the area underneath this graph is the power

5:15that we’re putting into the system but that graph

5:20is defined by the diameter of the beam that i’m sending into the system so

5:26typically this may well be an anything between 8 and 12 millimeters diameter

5:31right out at the extremes here but if you remember we mentioned in a couple of sessions ago that the middle

5:38third of this beam in other words out of 12 millimeters maybe the central four millimeters two

5:45millimeters either side of center contains 70 percent of the

5:50light intensity the next question i ask is how can we improve the light intensity

5:58well we know that we can improve the light intensity by putting it through a lens

Transcript for More Laser Cutting Investigations (Cont…)

6:04so with our compound lens strategy what we were trying to do was to take this raw beam and pass it

6:10through a lens so that we could reduce the footprint i.e this diameter here before it hit

6:18the second lens now what happens when you decrease the diameter

6:25of the beam we’re not going to change the power of the beam and remember what i said to you the

6:31power of the beam 60 watts equals this area under this curve

6:37when i decrease the diameter of the beam here’s what happens the footprint

6:44gets smaller but the area under the curve remains exactly the same as it was here so the more i can reduce

6:53this footprint the greater the light intensity that i should get

6:58in my beam i’ve still got 60 watts there but it’ll be 60 watts of incredible

7:04light intensity if this happens to be a six millimeter beam and this happens to be 0.06

7:09then the amplification factor is 100 and that means we’ve got an incredibly

7:15intense spike of light at this focal point

7:20and intensity equals damage we reduce the footprint from this size

7:27[Music] to this size if the beam had been smaller the green line would have be a parallel

7:34beam hitting the lens and that would have come out at the same focal point i’m wondering whether my

7:40non-parallel reduced footprint is actually giving me

7:47the effect that we are seeing in other words we’re getting a consistent result because this beam here

7:54is focusing our energy down to a specific point almost regardless of whether or not what

Transcript for More Laser Cutting Investigations (Cont…)

8:00type of lens we’ve got so let me just carry out a very simple test to see if there is a big difference

8:06between these two conditions so first off we’re going to do a mode

8:12burn test to see what sort of beam we’ve got coming down here hitting our

8:19lens just pop that down there to make sure i

8:24sit there on the right spot 0 1

8:292 3 4 5 6 7

8:368 9 10 but you can also see that some of the

8:41beam was hanging outside here look it’s actually scorching the pace seven inch lens

8:48we had it set to around about 70 or 80 millimeter spacing if you remember so i’ll just

8:56drop this table down that’s 80. anything between 75 and 90.

9:03so 80 is a reasonable number to test it out so just do a pulse mark on there we’re

9:09going to have a beam that is about half the size of what it was

9:17before if we reduce the beam to 50 percent we would expect

9:23the intensity of the beam to be doubled and whereas it took me 10 seconds

9:28to burn almost through there with one size beam i would expect it

9:35probably to take half as long with half the size of beam in other

9:40words the intensity is twice as great therefore it will damage the material

9:46twice as fast 0 one two three oops

9:53it only took three seconds to get through the bottom there and it has actually gone right at the

9:59bottom i think the point i’m trying to make has been proved

Transcript for More Laser Cutting Investigations (Cont…)

10:05intensity means more damage this beam here is not

10:12at its focus point i’m intersecting this beam what 80 millimeters below the lens and yet

10:18this is something like 190 millimeter focus point lens

10:23so what we have here is an intensity which is roughly twice as much as this

10:30one so this one here is a four millimeter beam

10:37and this one here is about seven millimeters increasing the intensity which is what we’ve done

10:44has dramatically increased the rate at which we can do damage i hate using this word energy

10:51density because problem is there isn’t a specific energy density in that beam

10:57that beam is a continuum of different energy densities

11:03because if it was a fixed energy density you would have something like a drilled hole that would be parallel with a flat

11:09bottom we haven’t got a parallel hole with a flat bottom so that clearly shows you that the energy

11:15density is not uniform and in fact i hate using the word energy density because really it’s

11:23light intensity that’s the thing that does the damage it’s the intensity of the light that it

11:29vibrates the molecules faster and faster and faster the more the intensity the faster you can vibrate

11:35molecules and i think we’ve seen that clearly here because at the time taken

11:41to damage that deep was roughly a third of what that one was

11:47just a quick diversion for a minute look i’ve recently acquired this valve

11:53off of ebay and [Applause]

Transcript for More Laser Cutting Investigations (Cont…)

12:00look this is a proper ball valve

12:08it’s absolutely superb and it was only about five five pounds i think it was so we’ve

12:13proved the point here with this little test that if we leave the beam raw we get a

12:19certain intensity and if we put that raw beam through and a lens a preparatory lens

12:27at the top of the system we can focus it down to a smaller footprint

12:32and make it do more damage we’ve reduced the footprint from seven to four

12:38and we’ve changed the intensity from a 10 second burn to about a three second burn to do the

12:44same amount of damage so intensity is a very important fact

12:53increase the intensity and we could do the same amount of damage in a much shorter time that’s exactly

12:59what we’re trying to achieve for cutting if we can increase the

13:04intensity we will be able to cut the same material with the same power in a much faster period of time

13:13so i hope you’re beginning to see the disconnect between power and intensity people think

13:18that if you just chuck more power at cutting it will cut deeper and better no

13:24it comes back to this point that i mentioned earlier it’s not what you’ve got it’s what you do with it i’ve got friends all around the world i’ve got

13:30people that speak to me with their problems i’m basically a laser agony aunt one server’s engineer

13:36phoned me up one day and said he got a bit of a problem that he couldn’t sort out he’d been called to a customer

13:42who’d got a recce resi w6 tube 130 watts

13:48and he couldn’t make it cut three millimeter plywood 130 watts

13:55so i said to the guy i said have you done a mode burn test to check what your beam is like [Music] he didn’t understand what a mode burn

Transcript for More Laser Cutting Investigations (Cont…)

14:02test was so i explained it to him and he found some material and when he

14:07tested this w6 tube it didn’t look anything like this at all he sent me a

14:14sent me the results that he got and they weren’t even as good as this it was almost

14:20just a little flat pancake but when he tested it with his power

14:25meter he basically said it was delivering roughly 130 watts as he’d expected so this again is

14:32proof that power is not the important thing you can have the power spread over a large footprint

14:39and it’s useless to you you want the sharpest beam you can possibly get to get maximum cutting efficiency now

14:45i’ve already got a very good beam here that’s excellent at cutting but if i can improve it with a compound

14:52lens as well well my goodness me i should be able to cut my way to australia without a visa

14:59this is not the same as that because the beam is coming in at an

15:06angle and it’s causing the beam to focus

15:11and i wondered whether that was the problem so what i was looking to do is to see whether maybe this approach

15:19is a better approach if i take two lenses that are exactly the same if i put a parallel beam in to that lens

15:27it focuses down to a point now provided i go past the focal point

15:34then the angle of attack onto the face of the matching lens will be exactly the same

15:40as that coming out of the first lens and therefore it should finish up passing through that

15:46lens and coming out parallel now whether or not that is the case we should have to find out

15:53but if that is possible then it means i’ve found a very simple way of converting a parallel six millimeter

Transcript for More Laser Cutting Investigations (Cont…)

16:00beam down for instance to a parallel three millimeter beam which i reason should give me a much

16:05better footprint smaller footprint onto the second lens i’ve got two meniscus lenses which gives

16:12me a much better focal point so if i put that one in curved side down

16:18and then we’ve got a spacer here which is about 30 millimeters that should give me what

16:23i’m looking for 0 1 2 3 4 5

16:306 7 8 9 ten

16:36[Music] [Applause] well that’s rubbish isn’t it my two one

16:42inch lenses put back to back didn’t work it was producing an

16:47increasing beam rather than d rather than a parallel beam so i changed to a couple of

16:54one and a half inch lenses and set them 60 millimeters apart when i got when i

17:00got quite close with the lens it was it was reasonably good like this

17:05so i thought well i’ll tell you what i’ll do i’ll put a one and a half inch sorry i’ll put a two and a half inch lens in front of it

17:11and just see what happens and hey here’s what happened i’ve got a parallel

17:17beam out of three lenses and the beam is um three millimeters diameter roughly so

17:26i’ve used three lenses to get a parallel beam now let’s just see what effect i have

17:32when i do a mode it’s got to be better than that 10 second burn

17:39there on the right hand side otherwise it’s useless 0 1

17:45two three wow well that one compares very favorably

17:53with that one there’s not a lot to choose between these if i put a lens in front can i

17:59amplify that even more hopefully it should punch through that see what 11 looks like

Transcript for More Laser Cutting Investigations (Cont…)

18:11well just beginning to show look but this is only 14 millimeters a second so

18:17again we’re we’re really wasting our time 10. let’s try nine and let’s put it up to

18:2616. 16 well it’s making it through

18:35we’ve bounced off a wall at 16 and we’ve got four lenses in there so we’ve automatically lost roughly four

18:41percent per lens so we’ve lost 16 percent just in there

18:49so if i take that one and a half inch lens remember what i’ve got in here is a two

18:54and a half inch lens and this two and a half inch lens is actually [Music]

19:00straightening the beam up so if i put a one and a half inch lens

19:06in here [Music] will it focus it [Applause]

19:14i should be able to just put that straight on the front there

19:20you have a lot of power there but

19:29that’s a pretty weird burn wow

19:36i mean this is a this is this is a one and a half inch lens look um i’m still hunting for the focal point

19:43down here so i’m four inches away right so we’re we’re in the range now

19:57but it’s not so here we’ve got a one inch meniscus lens

Transcript for More Laser Cutting Investigations (Cont…)

20:03a one inch meniscus lens

20:11at the moment we’ve run out of options i’m gonna have to go and have a sleep on it and see what else i can think of

20:16because what basically it says here is i have got the same result

20:21with three lenses that i can get with one lens

20:28this one is a parallel beam and this one is a non-parallel beam and

20:34with that non-parallel beam we know that there are certain combinations that will get me up to 18 millimeters a second one observation

20:42that i’ve made is this when we talk when we take a look

20:47at the entrance damage on each of these this is a normal tube this is the raw

20:54beam now you can clearly see right around the outside here there is an area of heating

21:01influence which is not the main beam this is obviously the tail end at the

21:06at the end of the distribution curve where there’s not much power but because we’re keeping it on for a

21:12long time 10 seconds as we burn down here that low power has a chance to disturb the

21:19material around the outside and do some burning [Music] now when we focus the beam down with a

21:27seven and a half inch lens which is what we did here you can see there’s a lot less

21:33disturbance around the outside we’ve got a much cleaner beam but with

21:39this one more or less the same result look at the serious amount of damage around the outside there’s something going on

21:45around here which is not very good this one although it looks the same as that one is not there’s something significantly

21:52different about the performance of this one so we’re going to go back and now look at our previous compound lens approach

21:59which is one single seven and a half inch lens on top of some other lenses because we

Transcript for More Laser Cutting Investigations (Cont…)

22:05know that they produce typically 18 millimeters a second cutting

22:10and what we’re going to try and do is establish what an 18 millimeter a second cut looks like in terms of

22:17beam penetration so here we’ve got our seven and a half inch lens meniscus lens

22:22which has got curved side up my engraving lens what i’ve got i’ve got a pvd

22:29one and a half inch meniscus lens with the curved side up now virtually

22:35any combination of any lens was going to give me 17 or 18 millimeters a second

22:41so we just go back and check this to make sure that we can get that result

22:50it’s a nice thin cut as you can see a look at the back just about come through at 17.

22:59you can just about push that out very nice clean cut no burning on that at all just scorching so what we’ll do we’ll

23:06emulate that 17 millimeters and i’ve now set the

23:12beam to stay on for 900 milliseconds 0.9 of a second and that’s what we use

23:18as our standard for determining how deep we can pierce

23:27it’s the same depth just to prove there’s a degree of consistency there but we get less of these strange marks

23:34in between because we’re not allowing the gas to escape out through these slots

23:41so that’s what a one and a half inch lens looks like doing 18 millimeters a second

23:47now swap the lens over and turn it from flat side down curved

23:52side down 19.

23:58[Music] there’s that 18 millimeter burn which

Transcript for More Laser Cutting Investigations (Cont…)

24:04will push out quite nicely what we’re trying to prove that we have consistency here

24:10another nice clean burn so that’s two millimeters difference let’s have a look to see what the

24:17difference in the burn is now technically because we’re doing the same amount of damage

24:24in the same amount of time we should have exactly the same

24:30depth of burn and shape of burn [Music]

24:35yeah i think you could probably say that they’re pretty much the same but let’s go to the other extreme now

24:41where we found that we could get good results as well there’s a four inch plano convex flat side down

24:46quite make it through at 17 at 18 millimeters a second so i’ve just dropped it to 17

24:53millimeters a second

25:00and now yeah look it pushes out lovely we’ll just try it back at 18 millimeters

25:07a second now 18 millimeters a second

25:15not really so 17 millimeters a second with a four inch lens

25:20is what we’ve been able to achieve so let’s see what that looks like basically almost the same cutting power

25:28so we might expect it to be very slightly less depth

25:36just repeat that

25:42which it does now that is a serious puzzle isn’t it

25:48it won’t cut at 18 millimeters a second

25:53but in the same amount of time it has a greater penetration in acrylic i’ve had to walk away from

Transcript for More Laser Cutting Investigations (Cont…)

26:01this problem for several days now it’s not frustration

26:07maybe it’s my age lack of ideas you would think after x number of years playing with these machines and cutting

26:14all sorts of materials and doing all sorts of experiments i don’t have some sort of knowledge

26:20about how cutting works these magical 18 17 16 millimeters a

26:27second numbers that keep cropping up with compound lenses it tells me i really don’t understand

26:33what’s going on and i can’t tell you how annoying that is because i feel i should

26:40be able to decode it i’m not using the right approach because

26:45even playing with all these 0.9 millisecond if you like pulse trails

26:52it’s not pointing me to it it’s not pointing out anything to me yes they’re a bit different but

27:02what are they saying to me they’re probably laughing at my face saying you’re an idiot

27:08i started going over some of the things that we talked about

27:13in the previous session how cutting works then i realized that i was not tying

27:20together some vital pieces of information that i’d already discussed in that session

27:26maybe it’s something to do with the gap between my two remaining gray cells but you know something fell between that gap

27:32but i think now with a bit of luck earlier today i had a bit of a light bulb moment

27:37when i started to put some of these facts together exposure

27:42intensity and material damage you only get material damage because of

27:49intensity and exposure is the key that links those two things together but exposure in

27:55itself means intensity and time there’s an element of exposure which

Transcript for More Laser Cutting Investigations (Cont…)

28:03ties the two things together and that’s what i was missing now i might be talking in riddles to you but

28:09it’s very very clear in my mind that there is a possible way in which we

28:15can use those facts to objectively

28:20look at the performance of each of the lens types that we’ve tested okay so i’ve got to go back and do all

28:26my test work again but i think at the end of the day it will be

28:32more meaningful than those confusing trails that have got in there

28:38i found a great use for a feature that i’ve hardly ever used

28:44just occasionally played with on rd works and it’s this feature over here which

28:51basically is dot and here we are i’ve got a line of dots now each one of

28:58these dots has been set on a different layer

29:05i’ve got 15 layers each one of these is set differently so how are they set

29:12let’s just have a look first of all we’ve got it set to dot mode

29:18and then i’ve chosen a dot time and this is great because i can define

29:25the amount of duration of time that the beam is firing

29:30and so basically what i’ve done i’ve set these layers here two times between

29:3820 milliseconds 40 60 80 100 all the way up to 300 milliseconds

29:45so every one of these dots will be a different depth of cut

29:52so what i hear you say what good is that well i think it might be very useful to me because one of the

29:58things that i’ve noticed as i’ve been doing these um spot trails is that

Transcript for More Laser Cutting Investigations (Cont…)

30:08the trail slows down as it gets further down into the acrylic so what we’ve got we’ve

30:14got a fixed power and i’m running these all at max power we’ve got a fixed lens for each one of

30:21these tests but what we can do is we can vary the exposure time

30:28for each one of these test dots and what that will tell me is the

30:33progression of the dot into the material there’s lots of valuable information i think

30:39that i can glean from this so what i’ve done is in addition to this

30:44program i’ve written another program which has created some test pieces

30:50zero at the top and you’ll see the slightly thicker lines well that’s 10 20 and 30 millimeters

30:56depth and each one of those lines is a two millimeter gap so i should be able to assess to the

31:04nearest millimeter the depth of cut that each pulse length produces i’ve got to set this to about a

31:10two millimeter gap for the correct focus we set that to the

31:16origin so here is my automated test run [Music]

31:24it’s as simple as that and we can clearly see a very nice graph there almost of the penetration

31:33for different pulse lengths well it means very little just as we look at it like

31:39this but if i take those dimensions and convert them into a graphical format

31:44i get a graph that looks like that here we’ve got our cut duration in milliseconds

31:51up to 300 milliseconds which was the last cut and here we’ve got the cut depth which i’ve been able to assess from just

31:58reading the bars so you know these are these are a combination of me not reading them right

Transcript for More Laser Cutting Investigations (Cont…)

32:03and maybe a certain amount of inconsistency who knows but you get the general trend of what these results are

32:10i hear you ask well what value is this well actually most of the value

32:16is somewhere down here now let me try and explain what i mean

32:24i’ve redrawn these graphs and thrown away most of the information look i’ve only

32:31looked at the first hundred milliseconds of cut and this happens to be

32:37a four inch meniscus lens flat side down now we compare that with the meniscus

32:44lens which is flat side up i think you can clearly see that there is a difference a where this

32:52gets to after 100 milliseconds is about 12 millimeters deep and this one after 100 milliseconds

33:00gets to 16 millimeters deep 100 milliseconds is like a lifetime in cutting we’re

33:06never going to use a 100 millisecond cut now i think i probably need to explain that a little

33:11bit as well here i’ve got some 10 millimeter acrylic and i’ve just cut this

33:1725 millimeter square out of that hole

33:22there with a four inch lens what we’re going to do is quickly just measure we find that it’s about 24.6 we’ve got

33:30about 25.3 25.32 actually

33:36it’s now time to phone a friend so we’ll just do that and we’ll ask him what uh

33:4325.32 minus 24.6 is and the answer is 0.72

33:50now if we divide that by two that tells me what the curf is so that

33:56means that gap there is point three six millimeters

Transcript for More Laser Cutting Investigations (Cont…)

34:02and that’s quite a wide curve but that’s roughly what you’d expect for a four inch lens that’s an important fact in what i’m

34:08just about to demonstrate to you when i say demonstrate that’s that’s a bit of a stretch of the imagination

34:15what i hope i’m going to demonstrate to you if i burn a hole through this perspex and it’s 0.36

34:22diameter and i burn another hole through the perspex another hole through the perspex

34:28that will take up approximately a millimeter now i know that’s not exactly a cut but for

34:34approximation purposes that gives us an idea what we’re trying to do now here’s my graph

34:40of the lens that i’ve got in here it’s a 4 inch cvd with a flat down and it tells me that i should be able to

34:47cut through 10 millimeters in about 70 milliseconds and 70

34:52milliseconds and 70 milliseconds to burn one millimeter so it’s taken roughly 210 milliseconds

35:01to burn one millimeter there’s a thousand milliseconds in a second

35:08so if i divide the thousand milliseconds by 210 now i need to find a friend again um

35:16a thousand no no no i don’t want to phone anybody i want a calculator thank you all right

35:22so 1000 milliseconds [Music] divide that by 210 milliseconds

35:30equals 4.76 this predicts that with that lens

35:38i should be able to cut 10 millimeter thick material at 4.76 millimeters

35:45a second so somewhere between four and five millimeters a second

35:53i should be able to cut 10 millimeters with that lens just set the focus

Transcript for More Laser Cutting Investigations (Cont…)

36:00remove it out of the way so you can see what’s going on now i’ve got my honeycomb table on here at the moment so

36:06if the burn is going through the material i should see reflection flashes as it

36:11passes across these metallic edges

36:16that’s good news and let’s try and look at the cut as it

36:21comes across the front let’s see how upright the cut is

36:28it’s looking pretty good actually very very slightly bent away at the

36:34bottom but only slightly so that’s four

36:43now i don’t know if you could see it but if you look right across the bottom edge

36:49of the cut you’ll see it’s a little bit um what can i say serrated it’s just on the

36:56verge of failing to cut even though we can see the flashes

37:02this is quite good news another good cut so let’s push it up to six where

37:10i’d like it to cut at six but on the other hand to stop me looking stupid it really ought to fail

37:17oh we can hardly see any flashes now can you notice that

37:33might just make me look stupid

37:41yes only just that was hard work so

37:48it’s only just making it through on the bottom there it’s uh

37:55i think i probably had to push that out and break it out just there so we go up to seven

Transcript for More Laser Cutting Investigations (Cont…)

38:04and now we’ve got no flashes at all and we may well see can you see the way

38:10the beam is now dragging backwards

38:15and the smoke is coming out the top i can’t i can’t even begin to push that out there’s definite failure i know it’s

38:22only a very quick test but it’s enough promise to tell me that we’re on the edge of something interesting now

38:29i’m going to spend quite a bit of time running lots of tests on many of the

38:34lenses that i’ve got in my collection because i want to see what the difference in performance of these lenses

38:40is here we are a couple of days later now i’ve been a very busy boy

38:47that’s not the bit that takes the time the bit that takes the time is to sort the results out and turn them

38:52into graphical results that are meaningful [Music] i’m going to go back and we’ll look at

38:58these results in a bit more detail maybe on another occasion but

39:04i demonstrated to you the reason why these results might be useful to you

39:09last time when i was able to plot the relationship between these times along the bottom here and

39:16this material thickness here to predict the speed at which we might

39:21be able to cut various thicknesses of material with various lenses now here we’ve got a summary of what i

39:28was able to find for a one and a half inch lens now this is not a one and a half inch lens but

39:34by measuring the depth of the penetration we’re able to convert these into this graph here so we’ve got eight

39:40different lens combinations here four lenses different types of material

39:45all one and a half inch focal length in general you can see that they’re all very similar in performance now acrylic

39:53is a very difficult material to cut i know it ought to be simple but it is

39:58actually quite a difficult material to cut and it actually cuts as slow as ndf and probably

Transcript for More Laser Cutting Investigations (Cont…)

40:05when we get onto wood and my soft poplar plywood we’ll probably find that

40:11we’ll be able to cut twice as fast as this this set of shapes here is determined by the

40:17power of my tube if you’ve got a more powerful tube then these shapes would be higher up the

40:23graph because you’d be able to cut deeper with the same lenses so there are all sorts of caveats to this

40:28set of results here these are results for my lenses on my machine but they should give me a relative set

40:36of data to work with now when i take a 1.5 inch lens

40:42and this is the bit that’s interesting people and i add a seven and a half inch lens at the

40:48top of the stack in other words this is what we were setting out to do two sessions ago we

40:53were trying to improve the performance of a lens by focusing the beam down onto the bottom lens with

41:01a very long focal length lens at the top of the stack so here we are we’ve got a 1.5 inch

41:09pvd meniscus flat face up that was the strongest con contender

41:16in this group of one and a half inch lenses the dark blue there is the original lens and adding the

41:24seven and a half inch lens made a minor improvement when we got to

41:30those longer deeper cuts but down at the bottom end here which is

41:35where we’re really interested in these high speed cuts which is what which is what we’re cutting is going to

41:41take place i mean very little cutting is going to take place here 60 70 milliseconds is only three or four millimeters a

41:47second well most of the time you’re going to be cutting at 8 10 12 maybe even 20 millimeters a second

41:53some materials which means we need to be working and analyzing what the lens is doing right down at this bottom here in the 5

41:59and 10 millisecond region and then when we look down here there’s no difference at all

Transcript for More Laser Cutting Investigations (Cont…)

42:06when we add a seven and a half inch lens to it so let’s move on now to the two inch lenses i didn’t have as

42:13many two inch lenses but there was enough of a selection here to again prove the point that in general all

42:18two-inch lenses look roughly the same these were cvds and pvds

42:24up and down now the strongest one of those was really i think this red one so we

42:31paired that up the original best lens as this black line the two seven and a half inches meniscus

42:38side down and meniscus side up if anything it was worse

42:44it was certainly no better as you can see here with this yellow line okay meniscus up gave virtually the same

42:52result as the single two inch plano convex up so there was no benefit to putting

42:59that second lens into the system the second lens remember is going to lose you

43:05another four percent of energy i just come back to that so it loses us four percent of energy

43:10but if we use it the right way round we gain it back and we run exactly the same as a single

43:17lens so there appears to be no benefit to having the

43:23compound system now we go onto the two and a half inch lenses and i had a much larger selection of

43:29those to play with but again you look they’re all bunched together in roughly the same region

43:34but the best one of those was probably this yellow one i mean you can’t see it

43:40down here but it does run up here it’s quite one of the best ones at the bottom end here as also it stays high

43:47all the way up so it’s only marginally more advantageous but that was the one that i chose to

43:53pair up with a seven and a half inch lens the yellow one here is our straightforward gallium arsenide

Transcript for More Laser Cutting Investigations (Cont…)

44:00plano convex face down and this is a lens i use all the time i

44:07love it and i can see why i love it now look it beats the hell out of a compound lens

44:14when i stack when i start stacking the seven and a half inch lens above it there’s no competition in fact it makes

44:20it worse as you can see so that’s rather an interesting observation and now we look at the four inch lenses

44:27now the four inch lenses are showing a much bigger variation so the best performing lens

44:32there which is this red one is a four inch cvd meniscus lens with the meniscus face upwards

44:39okay now it’s not the sharpest tool in the box down at the very bottom end here you

44:44wouldn’t use that for cutting one millimeter plywood for example it starts to gain about two millimeters

44:50and it starts to beat everything else up here once we get into the slightly thicker materials we’ll compare

44:58that four inch lens which we had high hopes for when we going to put a seven and a half inch lens above it

45:05the red one is our four inch cvd and the blue one is the compound it wins

45:12it loses it wins it loses on average i think we could say it makes no difference at all i think we’re

45:18beginning to prove here that there is no advantage to putting a compound

45:23in the system we can’t make the machine work any harder than it works with one

45:29lens but there is something that i have noticed which i’m going to investigate next time you remember that i went back

45:36and i checked the um the cut width for the four inch lens

45:42and it was about 0.35.4 a very wide curve something i have

45:48noticed is that when i put compound lenses in conjunction with something like the four inch lens i can

45:55get a narrower cut so maybe there is a good reason

Transcript for More Laser Cutting Investigations (Cont…)

46:01why you’d want to use a compound it’s not that it’s any better performing

46:08but it does mean possibly and i say possibly because we’ve got to check this next time that we should get a narrower curve

46:16out of a four inch lens and a better overall cutting performance i’ve pulled out the summary for each of

46:23these lenses because i want to just show you something if we want to choose

46:28a lens to do a specific job for example we might have some let’s just say we had some six

46:34millimeter acrylic because these are acrylic numbers now i do believe that the ratio of these

46:40numbers will read across to other materials i’m fairly confident that these numbers

46:45will be very similar for for instance mdf because mdf has got much similar

46:50cutting speed properties to acrylic but if we go to something like my

46:55popular plywood then i should be able to cut at twice the speed almost that we’re showing here

47:03but that’s what we’ll get on to in the next session so let’s just take a look at for instance eight millimeters because

47:09here we can see that a four inch lens will cut eight millimeters in 30 milliseconds

47:16a two and a half inch lens we’ll do it in 25 milliseconds a two inch lens will do in 40

47:24milliseconds and a one and a half inch lens takes 45

47:30milliseconds on balance if you want to cut eight millimeter material

47:35probably the best and fastest thing to use would be a two and a half inch lens which i have to be honest is one of the

47:42lenses that i’m using at the moment most of the time a two and a half inch lens for cutting but you know that was prior to all these

47:48data that’s just something that i have found out from past experience now all of a sudden you

47:53want to go up to 10 millimeters when you want to cut thicker materials

47:59the 4-inch lens does it in 40 milliseconds the 2 and a half inch lens does it in 45

Transcript for More Laser Cutting Investigations (Cont…)

48:05milliseconds the two inch lens does it in nearly 70 milliseconds

48:11and the one and a half inch lens does it in something over 70 milliseconds so as the material gets

48:18thicker it’s more efficient to use the longer focal lengths lens

48:26now the old myth is very simple if you want to use a long focal length lens you’ve got to have lots of power

48:33well all i can say is crap this shows a completely different picture these are all the same tube

48:40cutting all the same material there is no proof here that we need more power

48:46to use a four inch lens we’re getting higher performance if we cut thicker

48:51material with a longer focal length lens you just get more efficient cutting with

48:58a longer focal length four inch lens provided you choose the right four inch focal length lens

49:04and the right four inch focal length lens is a cvd with meniscus up

49:11and as you can see there’s quite a significant difference between the performance of some four-inch lenses this is getting

49:18very interesting we’ve now got to do so a lot more physical testing now that we’ve got this

49:23data to verify what we’re finding and whether or not these numbers are meaningful and

49:30transferable to other materials can we use these graphs to predict cutting performance

49:36on other materials and it may be possible for me to predict cutting performance on other materials

49:42with my machine and my tube that doesn’t mean to say these numbers will necessarily apply to you

49:48but i would expect you to get similar patterns of results maybe not physically the same

49:55absolute results well this has turned out to be quite an interesting session

Transcript for More Laser Cutting Investigations (Cont…)

50:00and like pandora’s box we started this off knowing not knowing

50:05what we were going to find with compound lenses and we still haven’t found out a real answer yet other than we’ve got some

50:12evidence here which says we’re not going to get any major advantage power advantage from using a compound

50:19lens we may get some other advantages which as i said possibly a curve advantage

50:25but we have to investigate that and prove or disprove that in a future session so i think we’re

50:31going to leave it there for the time being and i’m going to catch up with you in the next session when we shall do some

50:36more physical cutting work based on some of this data that we’ve got here and see whether

50:42we can do any predictive work from this information thanks very much for your time and your patience and i’ll

50:47catch up with you in the next session