21 – RF Laser: PWM And Laser Cutting

0:01happy new year everybody i’m glad you could join me again look over my shoulder and watch me trying to burn some grey

0:09cells as i work out some difficult technical issues with this machine

0:14not mechanical issues technical issues associated with that old favorite of mine pwm

0:22it’s the simplest principle in the world but how it works with this machine

0:28is giving me all sorts of mental puzzles now while you were sitting down eating

0:34your christmas dinner and sleeping in front of the television i was musing on a problem

0:40if you remember we had to try and sort out the the parameters for getting a certain

0:47depth of burn we didn’t want too much and we didn’t want uh too little there was a balance trying

0:52to find exactly the right depth of cut i did a couple of test cuts there

0:58at different frequencies the puzzle occurred when despite the

1:03different frequencies they came out looking exactly the same

1:09depth and color so what effect does frequency have i

1:15don’t suppose many people have really bothered with this but it was a puzzle to me because it didn’t

1:21really make any sense we’re going to try and investigate today to see if we can work out why

1:26changing the frequency did not seriously if at all change the depth of the cut

1:34and we’ll also investigate how the power changes the depth of the cut because

1:39technically twice the power twice the depth of cut very simple same power double the speed

1:48half the depth of cut i mean it’s all to do with one simple thing exposure times as i

1:54always do i’ll invite you to come on in and let’s have a look at some of my some of my scribbles

2:00now we’re going into these sessions with a certain amount of thinking only i like to think about the problem before

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

2:06we tackle the practicality so let’s start off at the beginning here’s a very quick take on pwm

2:13how it applies to this machine the rf tube has got no control of power we can’t switch on

2:2015 watts we switch on 30 watts only or off and so

2:28here’s a control mechanism which operates on a naught to 5 volt system it’s either 5 volts on or nothing

2:35off we’re switching 30 watts on or off on off on off so simple as that first

2:42example i’ve got here look we’re switched on for 99.9 percent of the time

2:48and then we’ve got this little teeny weeny instance here where it’s off if we were 100 on we wouldn’t have a

2:53little time base a little tick which is the frequency of the actual pwm itself

2:58that’s as good as 30 watts so what we’re expecting to see out the end here

3:05is 30 watts of power and here we’ve got 50 on and 50 off

3:12so technically we’re only delivering 15 watts

3:23and down here we’ve got seven and a half watts we’ve got zero with just this little

3:29tick time base as well just to make sure that we can check the time base itself and we’ve basically got zero

3:35what’s coming out of here so that’s in theory what this digital

3:42signal is doing it’s converting a 5 volt signal

3:47into different levels of power sounds wonderful doesn’t it the question

3:53is is 50 on and 50 off really 15 watts

4:01if we were working in electronics and we either had an rc network which integrated this data or maybe

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

4:08something called a a d a converter we would get an answer of 15 watts cleanly

4:15but we do not have electronic components decoding this electronic signal what we have

4:22on the end of our chain is a piece of material

4:29our material is the thing that is actually integrating these signals and deciding

4:36how much power it can see technically it should be the same as

4:42this materials are not all the same we know that we’ve got light in here at something called 10.6

4:50microns wavelength you know what does that mean well we’ve seen what our pwm signal is

4:58here it’s a signal which goes up and down and it that’s exactly what light does light

5:05is in waves as well so light waves go up and down like this

5:13and what we’re talking about here 10.6 microns is this distance here the distance

5:18between any two light waves

5:23now 10.6 microns is something which is a bit difficult for probably people to imagine if i could pull one of my there

5:29we go i have got some hairs left on my head they’re grey so they don’t show up well against this background

5:35um so 10.6 microns is probably about

5:41close to about half a hair thickness this wavelength does not excite the molecules in the back of our eye so

5:47we can’t see it in terms of frequency i wonder what 10.6

5:52microns actually means now hold onto your hat here 28 1 2

6:003 4 5 6 7 8 9 10 11

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

6:0712. now look that’s a thousand that’s a million that’s a billion

6:16and that’s a trillion that’s 28 trillion cycles

6:22a second that is what’s happening in this machine we’ve got light which is vibrating going up and down at 28

6:32trillion cycles a second remember what i’ve told you many times before the way that light

6:39causes damage to this product is because this product and everything

6:45else around us is made up of molecules and atoms and atoms and molecules are always

6:54doing this they’re vibrating and the more they vibrate the hotter

7:00their temperature so if you can stimulate molecules to vibrate faster

7:06the material gets hotter now i guarantee that when you’re at school you did this [Music]

7:12with your ruler over the edge of the desk and then you move it in a little bit

7:18the note gets higher but the note getting higher means the frequency has increased

7:23and we can take it again and there we go it’s getting faster and

7:29faster and faster the shorter this distance becomes now this distance

7:35is going to be infinitesimally small when we get down to the molecular an

7:42atomic level so the amount of vibration that’s happening at the atomic level

7:47is incredibly fast and it just so happens that at this sort of frequency we can

7:54make the molecules do exactly

8:00that we can make them vibrate that’s the basis upon which this machine

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

8:06works and this frequency has an effect on a very wide range of

8:14materials to a greater or lesser extent some materials have got structures which

8:19respond very easily to this and other structures like this we can excite this

8:27but we can’t excite it as quickly or as easily as we can that some of the molecules

8:33that are in this material

8:39so that’s one of the properties of material that we have to think about its ability to be excited at 10.6 micron

8:48wavelength light 28 trillion cycles a second now there

8:54are two other important properties of material that come into play one of them is thermal conductivity

9:00which are the green numbers and thermal conductivity

9:06is exactly what it says we’ve got aluminium here for example which we know conducts heat very very easily it

9:14doesn’t take you know if you if you put a piece of aluminium in a flame and hold it like this it wouldn’t take

9:20many seconds before that heat traveled back up to here and your hand would get burnt do the

9:25same thing with a piece of wood and you can hold a piece of wood in a flame for a long time and it just won’t get hot until the

9:32flames get backwards when we’re firing energy at the surface of a material if the molecules are

9:39easily excited and they start exciting the molecules beside them and the molecules beside

9:45them and so etc etc we get like a little chain reaction it means that we’re putting energy into

9:51the surface of the material and we’re not really doing a lot of damage what we’re doing we’re causing

9:56energy to dissipate into the material what we really want is a material that doesn’t conduct

10:02so that the excitement that we create on the surface of the material remains on the surface of the material

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

10:08and only heats those molecules up there that becomes an efficient form of heating

10:13with the amount of power that we’ve got here and at 10.6 microns we find it very difficult to work with anything other than

10:20non-metallic materials now look at this for example diamond you wouldn’t believe this the diamond is

10:26the most conductive heat conductive material in the universe look at it relative to

10:35other products aluminium 200 wood 0.23

10:40diamond 1000 water 0.6 is there anything else that comes

10:47anywhere near well look actually glass is actually better than water at conducting heat and lots

10:55of other materials like aluminium copper silver gold they have got very high conduction rates

11:02so that’s why we can’t engrave or mark them on this machine because they reflect the energy because of their crystal structure

11:10so that’s one property of materials that can influence how efficiently we can cut

11:17once the heat starts to travel away from the actual cut zone itself where we’re applying the

11:23light energy and stimulating the molecules if stimulating the molecules doesn’t remain in that little local area where we can

11:30heat the molecules up more and more and more but as we heat the molecules up more and more and more some of that heat is

11:36disappearing which limits the amount of temperature that we can actually achieve with our light so here’s the problem

11:46we’ve got two frequencies we’ve got the same amount of power

11:5130 watts and 50 percent power means we’re spending 50 percent of the

11:56time on and 50 percent of the time off 50 on 50 off we’re going to finish up with a net

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

12:04effect of 15 watts worth of damage well i’ve just popped in to um get some

12:12brain fertilizer and it’s given me a chance to think for

12:18a few seconds and i’ve actually had to ask myself a very silly

12:25question after five years or more i’m losing track of using these machines

12:31do i understand what cutting is i know it’s a fundamentally stupid thing

12:38to say but if i put my hand underneath a piece of material

12:44it’s completely safe because until i actually break through

12:52there’s no damage can take place to my hand the damage for a cut does not

13:00just happen it happens in very small increments but incredibly fast

13:08we fire the laser beam at the surface of the material and we excite the molecules just here

13:16and those molecules heat up and in the case of acrylic for example they get to 160 degrees c

13:23and they turn into liquid just like water appears when you heat ice above zero

13:30degrees c there is no plastic water it just turns to water

13:36well that’s exactly what happens with acrylic it just turns the liquid at 160 degrees c

13:44so we carry on putting energy into this surface here and we hope that the energy is not going

13:49to dissipate as we talked about earlier we can put more light in and excite the molecules to even higher

13:56vibration levels we raise the temperature to 200 degrees c at 200 degrees c this stuff evaporates

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

14:04just like water boils at 100 degrees c this boils at 200 degrees c and turns

14:11into steam or acrylic vapor once the vapor has disappeared then we’ve got solid material underneath

14:18again which we can heat up to 160 degrees c turn it into liquid and then evaporate it and it’s a

14:25continuous process that takes place as it works its way

14:30through the material and eventually it’ll get out the bottom

14:36of the material and if my hand is under the material at that point in time yes it’s dangerous cutting takes place

14:42in little teeny weeny increments by surface damage and that’s the important

14:48thing that i think we need to understand the light does not cut the material

14:54it damages the material and heats it up in some way and when it heats the material up

14:59different materials act in different ways acrylic evaporates

15:04paper burns one’s a chemical reaction the paper burning where the molecules vibrate so fast they

15:11can’t hang together in a paper form they want to break down

15:17and combine with the oxygen that’s in the air and form different chemicals so we’ve got all these different types of damage

15:24that are caused when you heat molecules up if we’ve got enough energy in this beam and we keep the beam in this place long

15:31enough we can cut through the material in this process of nibble nibble nibble all the way through

15:37but of course this is happening very very quickly when the laser beam comes in from the machine it is about let’s just say six

15:44millimeters wide we pass it through a lens

15:50and all of a sudden it becomes only about say 0.1 diameter

15:56so we’ve got a huge amplification factor here okay what’s happening is the light

16:02energy at this point is very low and it cannot do much damage it’s like a candle flame

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

16:09and yet we amplify it with the lens and we compress the energy down into a very very small area

16:15and it then becomes like a blowtorch so we get much more energy here

16:20than we had up here we’ve got the same amount of power but the energy density here is much

16:26greater energy density an interesting term

16:31in reality what we’re talking about here is light intensity because it is light that is doing the

16:38damage so and the more intense the light the faster we can do the damage

16:46so if we have a lens which only focuses down to that size spot we can’t do as much

16:54damage as if we have a lens that focuses down to that size spot if we start moving the

17:00beam at the same time as we are burning well hang on we’re not going to be able

17:06to go through in the same amount of time and then we’re moving along we’re working with this bit here which has to

17:12cut a little bit deeper and we keep moving along and we keep joining up the cuts

17:18and eventually we might break through when we sum up what we’ve just said

17:23there there are only really two factors that affect

17:28a cutting result one of them is light intensity and light intensity

17:36basically means the power that we put in and the way in which we focus that power

17:44we can either focus it gently through a long focus lens or we can focus it sharply through a short focus

17:51lens the power does not change but the intensity of the light

17:57changes so that’s one factor key factor the light intensity that

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

18:04we’re aiming at the surface because the more intense the light the faster we can do

18:09damage to the surface and then we can limit the damage that we

18:15can do to the surface by changing the velocity the greater the velocity the less

18:22exposure time we shall allow so those are the two key

18:28factors that are really govern cutting we’re now going to carry out some experiments

18:34with one of my favorite materials which is acrylic now this might not be the fastest cutting material because it’s

18:41quite a difficult material to cut but it does allow us to see what’s going on very clearly we’re running at a

18:46hundred percent power 30 watts a four inch lens has got quite a large

18:52spot size and that means that we’re not focusing the lens down to its possible

18:58highest intensity of light and we’re going to fire one seconds worth of 30 watts

19:06at this piece of material what do we think is going to happen

19:1130 watts with a 4-inch focal length lens has pierced

19:18a virtually parallel hole right the way through that block of acrylic how is that possible instead of one

19:24second static we’re now going to move at five millimeters per second with exactly the

19:30same amount of power lens everything else we’ve just added motion to it now

19:42moving up to 10 millimeters a second

19:48doubled the speed which means technically we should have halved the depth if we double the speed again

19:55to 20 millimeters a second we should halve the depth yet again

20:02we will run at 50 power so the first one is at five millimeters

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

20:07a second with 50 power

20:14same thing at 10 millimeters a second we’ve got the frequency running at 25 000

20:22right now we’re going to change the plan we’re doing 5 millimeters a second but

20:28this time the frequency is no longer 25 000 it’s 100.

20:53here’s the one second pulse through 25 millimeters of

21:00acrylic with a four inch lens 30 watts and this is how deep five millimeters a

21:07second went

21:13that’s about half as deep then we change to five millimeters a second at fifty percent power

21:23twenty millimeters a second compared to ten millimeters a second should be half the power looked at like

21:29that there is a big difference isn’t that because the one on the left was done

21:35at 100 hertz and the one on the right was down at 25

21:42000 hertz so there definitely appears to be more depth

21:48at 25 000 the one on the left is five millimeters a second and the one on

21:53the right is five millimeters a second the difference is that on the left

21:59we have got 10 hertz 10 cycles a second as opposed to 25 000

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

22:05i mean you can see the actual pulses there so the one on the left and the one on the right are exactly the same power settings and

22:13frequency settings the difference is speed the one on the

22:18left is done at 20 millimeters a second and the one on the right is done at five millimeters a second well here we

22:25are in the office trying to get warm new cup of rocket fill what i’ve done i’ve looked at all my

22:31data here and i’ve actually physically measured the depth of cut and i’ve transferred

22:38that information onto this chart here so here in blue we’ve got the depths

22:43of each one of these cuts to try and analyze them what i’ve done i’ve put

22:49some little pencil marks in each one of these which we’ll go through in a second but let’s start off look five millimeters a second i

22:55established a seven millimeter deep cut and that was using a hundred percent power what we talked about before was

23:02the relationship between speed and power if i double the speed i’m actually halving the exposure time

23:09i.e i should expect to get half the depth of cut well

23:14i’m expecting 3.5 but i’m getting 3.7 so now i double the

23:21speed again from 10 to 20 millimeters a second and again i would expect to get half

23:27that depth of cut based on our seven to start with seven three and a half now we’re down to 1.75

23:35and the actual result that i get was 1.8 so in general it’s sort of following the

23:43rules i mean this is just probably a little measurement problem

23:49so now what i did was to change to 50 power and obviously

23:5650 power means i’m going to cut three and a half millimeters deep not

24:02seven millimeters deep because i’ve got half as much power for the same speed and if i follow that

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

24:09through i shall find that at doubling that to 10 millimeters a second i get down to 1.75

24:16and then to 0.085 hmm now these are not quite following

24:22the pattern as well look at this one eight eight point eight five i mean that’s fifty percent adrift

24:27this one appears to be one seven five two that’s almost fifty percent adrift as well this one is not quite well i don’t

24:34know that’s uh three and a half one point seven yeah that’s probably about thirty percent of drift

24:40so now we come down to this one here where again we’re fifty percent power but we’re using different frequencies

24:46now these were all done at 25 000 hertz 25 kilohertz

24:52that’s what the pink indicates and now we’re going to move down to the blue which is 10 and 10 and this one which is 100 hertz

25:00okay so 50 50 power should still give us 3.5 and this one should be 3.5

25:09and then when we look down here at this blue one this again 50 power 20 millimeters a second that

25:16should give us 0.085 well this is way way off the scale

25:22so is that one this one is well so we’re getting something strange happening here as we reduce the frequency

25:29or are we the question comes back or are we

25:35because when we look at the calibration chart

25:41for the tube itself 50 power is 25 watts

25:50100 power is 38 watts

25:56okay well half of 38 watts is 19 not 25. so

26:02we haven’t got 50 power equals 50 power

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

26:09fifty percent power is probably more like sixty or seventy percent power okay so if i take seven millimeters

26:17and i divide it by 38 watts that tells me how much depth i get per

26:24watt and now if i multiply that by 25

26:32the answer is 4.6

26:38so that 4.6 will also apply to this one as well

26:44and this one so that’s what i should be expecting

26:49for 25 watts and using the same calculation

26:56this one should be 2.3 half of that 2.3 half of that again

27:05one point one five so this one should be 1.15

27:12parts are pretty beginning to make sense i mean this one is now closer to 4.6 this one’s close to 2.3 this one’s close

27:20to 1.15 so hey it’s it’s roughly following the rules of

27:25speed to power when we start lowering the frequency things start

27:31changing quite dramatically now there’s something really weird going on with these low frequency

27:38now we’ve already talked about this and agreed that you know if you half the speed you double the depth of cut if you

27:45double the power you double the depth of cut and it remains that basic proportion that we’ve explained

27:52and seen in action just here so just because i’m running

27:59at 20 millimetres a second here doesn’t mean to say that I’m going to

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

28:05cut any shallower or deeper when the power is on

28:1150 power in this instance which is this one 50 power which means we’re actually

28:17using a pwm signal which looks like this 50 on 50 off

28:23okay and here it is here’s the on and here’s the off here’s the on and the

28:29off i mean we’ve got a perfect match there’s no problem with these dimensions

28:35the speed etc everything works out absolutely perfectly except the depth so we should have a

28:42depth here of 1.15

28:47and what have we got we’ve got a depth which is 2.3 that’s twice

28:54what it should be how is that possible if we were running at constant

29:02speed as we are here look we get 1.15 we know that if we’re running at

29:09constant speed we will get a 1.15 depth although i’m switching on

29:14off on off i’m actually running at a constant speed so therefore these should look like

29:22they should look like almost little square bottoms like this 1.15 deep and at the moment i

29:30do not understand it but hey that’s what this whole exercise is about

29:36trying to understand what is actually going on how pwm is controlling this

29:42laser so the interesting question is when we switch the laser on and off like this

29:50does it mean to say we’re getting a an initial pulse which is more powerful in other words

29:57when we switch it on are we actually getting that effect i don’t know we should have

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

30:04to go back and have a little bit of a closer look okay so i’ve been through now and i’ve

30:09examined each one of these to see whether or not the first pulse

30:15is deeper than the rest and the answer is yes there is a power surge when you switch

30:22the pulse on even though this is continuous the very first part

30:30dips down lower than the rest just like this okay now that isn’t the case here it

30:37isn’t the case here it’s um it’s a maybe in these two cases so technically i suppose i could

30:46put these down and say yeah there’s a hint of a pulse on those so in every one of

30:53those we’ve got something that looks like that at the start of the cut

31:00this one which has got the zigzag cuts if you remember

31:06like this the first cut here is shallower than all the others

31:14i mean this one at four millimeters is not as deep as it could be possibly

31:21running at 100 hertz makes the cut shallower for some reason or other

31:28whereas running at 10 hertz makes the cut deeper than what we expect

31:36and again here the cut is deeper than what we expect

31:42in this instance we can really see why it’s happening this one it may well be

31:49the same effect so every one of these pulses is effectively a start-up pulse which is

31:54going deeper than what we would normally expect which is 4.6

32:00it’s never getting a chance to stabilize out okay now i’ve got a very soft four inch lens in here i say soft

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

32:07because it’s going to have quite a large beam width 0.15 maybe

32:140.2 but i’m going to overlap the beams the cuts but that doesn’t matter because

32:22it’s going to be the same set of conditions for all the tests that i’m going to carry out here’s our little test square and we’re

32:29going to run this at 200 millimeters a second we’re going to scan and we’re going to set the power to

32:3750 that’s what we’re trying to look at on our diagram 50 power 50 power

32:44and frequency here we are advanced tick we’re going to set the frequency to

32:5025 which is the maximum frequency the fastest we can run this system

32:58and we’ve got a step interval here of 0.1 the noise that you can hear in here at the moment is not it’s not the extract

33:05this is the cooling system for the rf tube it makes a lot of noise it’s basically

33:13it’s transmitting high frequency that you can hear through this steel panel here which is where the unit

33:20is mounted you’ll see and hear the extraction coming on separately in a few moments as soon as we start the cycle

33:26remember i’ve got the extract system automatic so that when i start the cycle the

33:32extract will start and it will remain on for about 15 seconds 10 to 15 seconds after the cycle stops to clear the smoke

33:40okay there we go the extraction has now started

33:49change the parameters from 25 kilohertz

33:56to one kilohertz using 50 power and 200 millimeters a second

34:02nothing else has changed at 1 000 hertz you should hear

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

34:07the beam switching on and off

34:20there’s our end result they look exactly the same 0.95

34:280.94 to all intents and purposes they’re exactly the same depth i’m using

34:34this poplar plywood because it’s soft and it’s easy to cut as far as the laser is

34:41concerned in other words for a small amount of power we get quite a deep

34:46cut and yet if i’d tried to do the same thing with a piece of acrylic

34:53in fact we will just try copying that one with a piece of acrylic and we can see

34:59the difference in the damage that we can do between these two different materials

35:12in the that’s the depth that we cut in the wood and that’s the depth that we cut in the

35:18acrylic half the depth so this material is twice as difficult

35:23to damage as this one and that’s the reason why i’m using this because i get a bigger

35:30difference to see we’re going to run exactly the same test again we’re going to change the parameter back to

35:3525k 25 kilohertz this time we’re going to change the power from 50 to

35:4199 so we’re just going to double the power what should we expect we’re going to get

35:48twice the depth of cut

35:53let’s move that across and we do exactly the same thing for one kilohertz [Music]

36:031.47 and this one

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

36:091.434 virtually no difference again we haven’t got twice as much power

36:16so we haven’t got quite as much depth of cut have we got the correct proportionate

36:22depth of cut well we’ll have to do a quick calculation so if we take 0.94

36:29which is our depth and divide that by 25 watts

36:36that tells us how much depth per watt we’re getting and now if we multiply that by 38 watts

36:44that tells us how much depth of cut we should have

36:51wow so we’ve got exactly the correct ratio of depth of cut to power

36:58i’ve just come back to the warm office to do a bit of video editing because i’m trying to find any excuse to

37:04not be out in the cold workshop so a little bit of video a little bit of editing a little bit of video

37:11cup of coffee hey it’s a nice combination and i thought i’d take time to reflect

37:18on what we found so far have i found anything outstanding

37:24on the surface of it no what we’ve proved clearly and demonstrated as well is that if you

37:32double the speed you halve the depth of your cut or vice versa

37:37if you double the power you double the depth of your cut those are direct linear relationships

37:46the other thing that i’ve shown you clearly is something that i’ve known for a long time and maybe you’ve discovered for yourself

37:52different materials cut differently you’ve seen that acrylic is

37:58twice as difficult to cut as my soft popular plywood that really confirms all the sort of

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

38:03things that i’ve been telling you about the molecular structure and the difficulty of vibrating the molecular

38:09structure and creating heat within the molecular structure i mean these are all scientific facts

38:15which not many people really thought about or considered i’m sure before i started looking at them in

38:21detail i’ve had a serious distrust of pwm

38:27because i can’t see that it makes sense

38:32i think this session more than anything has partially cleared those doubts because

38:40it turns out that the material is the integrator of

38:46the square waves the pulses now remembering back to my list that i showed you earlier most of the materials

38:52that we are going to want to cut or work with have got a very very low

38:57thermal conduction rate so they don’t conduct the heat away from the work surface very easily which means that we

39:04can concentrate our energy in one place and know that that’s where it’s going to stay

39:09that makes cutting very efficient when it comes to pwm

39:15i’m not interested in the areas that most people are working with i’ve been told i shouldn’t go below five

39:22kilohertz well yeah that’s like a red rack to a ball isn’t it even though they’re colorblind no that

39:29doesn’t stop me digging into dark corners running my acrylic tests at

39:35stupid hertz numbers cycles per second like 10.

39:43well i’m sure that’s somewhere that people have never been before with these

39:49rf tubes i suspect that if you have one of the more expensive machines

39:55the software will prevent you from going anywhere near the sort of frequency numbers that i’ve

40:00been using but because i’ve got a completely uncontrolled system that tells me that i can go

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

40:08anything from 0 to 25 kilohertz i will the reason i’m saying that is because

40:14when i look at this set of data it looks pretty confusing

40:21on the one hand it confirms that yes there is a good relationship between speed and power and depth of cut

40:29that’s confirmed but then we poke around the edges of low frequency and there are some rather strange anomalies pop up

40:36although i didn’t understand it at the time a little bit of reflection as a few hours have passed by has forced

40:44me back to look at this again because

40:49there’s an observation that we could turn to our advantage

40:55the possibility that we could cut twice as deep with a 30 watt tube than

41:03we would normally expect to cut how can i claim that come in and let me

41:11show you it’s this number here and this pattern here which is creating a certain amount of

41:18excitement in my brain have i discovered something that other people haven’t seen i’ve

41:24already explained to you that this is probably caused by that

41:29there is an initial pulse of energy when you switch the laser on which is more powerful than

41:36the standard settled energy level the standard cut depth for 25 watts

41:43is here look it’s 1.2 millimeters and yet we’ve got 2.3 millimeters here

41:49for our nominal cut depth can i close these gaps up

41:57like this and find a frequency

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42:06i don’t know maybe it’s 20 maybe it’s 40 maybe it’s 60.

42:11where at 20 millimeters a second and think what 20 millimeters a second

42:17is it’s a typical cutting speed for three four two millimeter material

42:24if you’ve only got 30 watts hey does that mean to say i can have 60

42:31watts worth of cutting power out of a 30 watt machine if i choose

42:37the right frequency so now i have a

42:42dilemma i finish this session now and go off and do some experimental work

42:49or do i keep you looking over my shoulder for another half an hour while i go out

42:55and explore this i could sit in here and talk about it in the wall or i could go in the cold

43:01workshop and see if i could discover if there is a magic number now here’s another interesting thought

43:07whenever you fire up a glass tube it goes through this region called pre

43:14ionization where you have to break down the nitrogen gas

43:20with high voltage to make it into a plasma it ionizes

43:26and when it ionizes and has free electrons floating around in it those three electrons carry current

43:33which stimulate the gas and make it hotter and a hotter gas is vibrating faster

43:39and the vibrating faster means that we produce more collisions with carbon dioxide molecules and hey that’s where we get

43:47our co2 laser from these collisions the more collisions the more co2 power we get that is a very

43:55very powerful phase of operating for a glass tube

44:02if you go and look on places like the recce or reci you say tomatoes i say tomatoes

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44:07um website you will probably find them boasting about something called

44:13high frequency impact engraving and i believe you’ll find it on the efr website as well

44:19basically they’re talking about this pre-ionization zone if we stay below 4 milliamps

44:26then first of all at very very low currents we’re getting virtually no

44:34watts out of it at all because if you’ve watched your tube with very very low currents in it you’ll

44:40see that it is very wispy and fluffy the beam it’s not a it’s not a bright pink beam but

44:47you get to a point where maybe maybe two or three milliamps you’ll find that the beam is pinkish

44:56nearly all the way along the tube but still very wispy and fluffy and you can see the

45:02beam actually shaking and jumping and what the what’s actually happening

45:08at that point before the steady current starts coming in

45:14you have got a region where the current is doing

45:19this we have got huge current flows huge spikes of power

45:27for very very very short instances of time so you can do quite a lot of damage this

45:33is why it’s called high imp high frequency impact engraving because you’ve got very high frequencies

45:39and you’ve got very high spikes of power which your ammeter cannot pick up okay and then once you

45:46get to four milliamps all of a sudden the power starts

45:51becoming nice and stable now the whole point about me mentioning this is that

45:58an rf tube supposedly has got a pre-ignition system on it

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

46:05which holds the the power somewhere about here every time you

46:11switch the machine on it passes through this zone almost instantaneously

46:16but the great thing about the glass tube is we can control the current and we can hold it in this very useful

46:24zone of operating we don’t have that possible opportunity here

46:30on the rf tube because it has been preset to some value as soon as we ask for the

46:36beam to switch on it doesn’t have to go through this phase it immediately jumps to here

46:42and switches on steady power or does it do we have this little

46:49high energy spike just after switch on yes it switches on quickly instantly but

46:56we’re still into this possible pre-ionization zone here and we may actually find that we have

47:03got some high current that we can play with so if we switch it on

47:09and switch it off and then switch it back on again and switch it off and switch it back on again

47:14we might be able to double the depth of our cut by playing with this potential

47:22pre-ionization loophole that’s in an rf system this is a

47:28fascinating prospect isn’t it okay now i’ve just been out into my workshop to do some quick tests

47:34to see whether or not there’s any merit and reason for my excitement

47:39um i have to report not a lot

47:45let’s have a look at the results and i’ll show you what i mean now i’ve just done some little test blocks here to see what the possibilities are and here’s a

47:52summary of the results that i got i did some tests at 50 60 and

47:5890 i also did 10 20 40 and 80 hertz yes

Transcript for RF Laser – PWM and Laser Cutting (Cont…)

48:05i did get some interesting results here i’ve got

48:10a number 1.2 millimeters which compares with the 1.2 in this chart which i was

48:18looking to beat and sure enough i was able to beat it 2.2 1.9

48:23and as the frequency creeps up we get to 1.4 which is

48:30not a lot better than this 1.2 and it’s only at 1.4

48:36that the cuts join together and i can truly call it a cut these are still dotty and

48:45not what you would class as a cut i did a quick test at sixty percent yeah and then i did ninety percent power

48:52which is a close comparison for the hundred percent power where i’m looking to have

48:57roughly 1.8 millimeter as my target and here is 1.8 millimeters

49:03yes i did beat the target one put 2.2 2 millimeters and then as the frequency

49:09increased to 40 and 80 hertz i finished up with a 1.9

49:14cut which is nothing to shout about in relation to the 1.8 that i’m begin that i was getting

49:20previously so despite my temporary excitement um there wasn’t anything interesting

49:26there to discover after all well where do we finish up the whole point of this session was to investigate

49:32pwm and cutting now pwm breaks down into two completely

49:38separate sections on the one hand you’ve got frequency and on the other hand you’ve got duty

49:44cycle which basically is power level now we’ve established that the power level the duty cycle

49:50gets integrated by the material and 50 power 50

49:57duty cycle actually turns out to be what it says provided you use

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50:04your calibration power chart and don’t rely on the percent power that you see on your keyboard because

50:12the power is not linear and therefore the percent power is not linear so don’t fall into that

50:18trap so i suppose the only thing that came out of this session is that my distrust if you like of what pwm is

50:25doing has been quelled to a certain extent with cutting there isn’t anything magical to be found

50:31here well thanks very much for your time and i’ll catch up with you in another session

50:36bye for now