21 – RF Laser: PWM And Cutting

The Tangerine Tiger Series with Russ Sadler

In this Series, Russ has purchased a new 500 x 300mm, 50W laser machine from eBay with a view to modifying and upgrading it. In fact, he rips out the glass laser tube and high voltage power supply and replaces them with an RF laser source and PSU from Cloudray. Find out how the expensive RF laser source compares to a glass CO2 laser tube and prepare to get your hands dirty!

If you are considering purchasing a CO2 laser machine with an RF laser source from one of the big boy suppliers, I would suggest you check out this series before making a decision!

RF Laser - PWM and Cutting
RF Laser – PWM and Cutting

Contents

In reading operating manuals for some of the “expensive” RF machines I find instructions for using different PWM frequencies for different materials. This session investigates just how/if PWM duty cycle and frequency affect cutting I cannot see how frequency is related to specific materials because as I explain in the video the frequency to excite the various molecules is dramatically different to the PWM range of frequency. Just looking at most of these expensive machines, tells me the designers had scant understanding of just how important efficient air assist is to cutting. Is messing with frequency an attempt to compensate?

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This video is the last of the Tangerine Tiger Series. Why not try out one of our other video series such as:

The Lightblade Learning Lab is a series of videos that Russ did for Thinklaser Limited based on using the Lightblade 4060 Laser Cutting and Engraving Machine. The Lightblade 4060 has a 400 x 600mm bed size and was supplied with a 60W EFR laser tube.

The Fiber Laser Series is all about Russ’s adventures and investigations with a 30 watt fibre laser, loaned to him by Dean at Lotus Laser Systems. If you are looking to engrave metal, without the fuss of using coatings such as Thermark, Cermark or Molybdenum Disulphide spray, then this is the series for you.

The Lightburn Software Series is probably the best generic laser cutting software available (I use it for almost all of my laser work). This series has tutorials, hints and tips as well as the occasional speed comparisons between it and RDWorks. My thanks to the team at Lightburn for allowing me to embed their videos.

The Top 10 Ranked RDWorks Learning Lab Videos

The Concise RDWorks Learning Lab Series that condenses all of Russ’s work into an easy to follow encyclopedia of laser knowledge.


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Transcript for RF Laser – PWM and Cutting

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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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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

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

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

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

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 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 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

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

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

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