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

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 continues to with his Laser Cutting Investigations to determine the best lenses and parameters for laser cutting performance.


  • We recap on the previous session
  • We look again at the remarkable results from my compound lens tests where despite the focal length of the bottom lens and the spacing from the top 7.5″ pre-focus lens nearly all the cutting speeds were the same at 17 or 18 mm/s.
  • We have to conclude from this that the light INTENSITY through each of these many different lens types must be exactly the same to produce the same amount of damage. How is this possible?
  • Is it to do with the difference between a normal parallel beam hitting the lens or is it caused by the converging pre-focussing lens?
  • Quick mode burn test followed by a comparative burn with a 190mm Fl lens but set 110mm above its focal point to simulate its use in the compound lens tests.
  • This clearly demonstrates that reducing the footprint of a beam dramatically increases its intensity and damage capability.
  • I wondered if I took two matching lenses placed flat sides to each other and with a special spacing between, could I make a parallel input beam exit as a smaller parallel output. Theory says yes but in practice it failed.
  • I carry out a series of cutting tests to see if I can find any useful patterns that may guide my thinking.
  • Just going round in circles, I conclude I must be trying to look at lenses in the wrong way. Just measuring their cutting capability sounds logical but is very one dimensional.
  • I write a new lens test program based on DOTS and DOT MODE. The aim is to standardize the cutting parameters for lenses. I use the same power for every test and a series of fixed exposure times. Measuring the depths of cut for each exposure time will produce a cutting profile for each lens.
  • With the focus set to the top surface of a special test block I run a test on a 4″ lens. The data in this format is not easily understandable so the penetrations were plotted graphically.
  • When we examine the graph, it becomes obvious that I only need to see a small part of the graph to represent the performance at REAL cutting speeds.
  • I use the same 4″ lens to cut a 25mm square and then measure the block and the hole to calculate the cut width,
  • I use the 4″ graph to find out how long it takes to pierce 10mm deep. I use the kerf width to build a series of touching holes (kerf diameter) to fit into 1mm.
  • This allows me to predict a cutting speed for 10mm acrylic with this lens. The speed is about 5mm/s. In practice 5mm /s was easy and it just about made a cut at 6mm/s. This may prove to be a good cutting speed predictor (for my machine only).
  • I run a whole series of similar tests on all my lenses and then examine the graphical results.
  • I then chose the best performing 1.5″ lens and added the 7.5″ lens above it to see how the combination performed the same test. This clearly shows no difference 
  • A similar no benefit result for compound lenses also existed for the 2″,2.5 and 4″ lenses.
  • I then use the graphs to see which lens focal length I should use to cut 10mm thick acrylic for example. Now comes an interesting observation The longer the focal length the quicker it will cut.
  • This kills the myth that you need more power to use a 4″ lens.
  • We have finally proved no benefit for having a compound lens.
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Laser Cutting Investigations - Laser Cutting Depth Matrix
Laser Cutting Investigations – Laser Cutting Depth Matrix

Transcript for More Laser Cutting Investigations

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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

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Last updated April 25, 2024


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