11 – RF Engraving: Making Dots, How Hard Can It Be?

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. RF Laser machines are generally considered to be the best solution for high speed engraving. Russ investigates whether there is any truth to the superiority of RF Engraving.

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!


Dots are the basis of dithered photo engraving. The simple rule is 1 dot = 1 pixel. This machine has pulses at its operating heart so therefore must be fast responding, Fast response should give the capability of producing crisp clean dots…….that’s what I have always imagined of RF lasers, It turns out the opposite is true. That fast response prevents crisp ROUND dots.

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Transcript for RF Engraving: Making Dots, How Hard Can It Be?

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0:00welcome i’m glad you could join me for another session with my little tangerine tiger

0:05um i’ve been topping up with coffee because i’ve already got a headache

0:10and we haven’t even started today we’re going to look at the response time it’s one of the most

0:16interesting aspects of the machine for me because i’m really quite interested about

0:22how it can make dots dots are the basis of photo engraving

0:30and it’s very difficult on a glass tube machine to get a fast

0:36enough response out of the hv power supply to put down a single dot cleanly the hv power supply

0:43is the limiting factor on a glass tube machine it won’t be a limiting factor on this

0:49machine because this machine has got very fast switching capabilities now we’re throwing away one of the most

0:57basic parts and one of the most important parts of a glass tube machine

1:02and that’s pre-ionization now the pre-ionization phase of a glass tube

1:07machine is extremely useful because it’s a bit like this machine but

1:13totally random it produces very very high power very very high frequency pulses of energy

1:20so you look at your ammeter and you’ll say yeah but it’s only it’s only outputting one milliamp well that’s only because

1:28the amp meter cannot do anything about the high frequency it doesn’t know what’s going on all it looks at the

1:34average and the average might be one milliamp this machine does not have access to the

1:40pre-ionization zone it has got a pre-ionization zone but

1:46they operate something called a tickle current or tickle frequency even which keeps

1:52the tube operating at just below the switch on point so the moment you

1:58ask for something to happen it does not have to go through the pre-ionization phase

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

2:04it turns on instantly so we’ve got a great advantage here in one respect we’ve got a fast switching machine and

2:11we’ve got a disadvantage in another respect that we haven’t got access to the pre-ionisation zone because it’s being controlled for us and taken away

2:17from us let’s go and look to see if we can find any real positives with this machine potentially where i’m looking today we

2:24should have real positives but trust me it’s a headache

2:30and i can begin to understand why the big companies that make and use these machines

2:36hide everything behind menus i think at the end of this session you’ll probably understand why

2:41this stuff is not for the faint-hearted should we say come on in and let’s have a look at some

2:46of the problems i perceive here’s the test program that we’re going to use today because

2:51one of the most important things that i want to find out is how well the machine can produce

2:58single dots we need single dots for photo engraving yes i know i could produce ordinary scans

3:05backwards and forwards and i could switch on and off very quickly but that’s not going to tell me much about what’s going on with this machine

3:12a single dot is the most powerful thing that tells me how this machine works and how i can

3:18control it we’re really interested in what’s happening at this bottom line if you’ll notice i’ve got my settings so that it

3:24will scan from the bottom up and when i look at oscilloscope pictures which we shall do shortly

3:29we shall be looking at these dots here this is an image which is done at 254 pixels per

3:36inch and that means each one of those pixels is 0.1 of a millimeter square

3:43i know that i can get point one diameter dots and the idea of photo engraving is very

3:50simple one pixel one dot there’s nothing more complicated than that about photo engraving

3:56 it sounds like a very simple task for a pulsing machine like this to be able to get single dots the more i dive into

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

4:03how this machine could work the more complicated it might be this machine as we spoke

4:10about last time is controlled by something called a pwm signal now i’ve got a little section here that i’ve

4:17drawn pre-drawn which allows me to scale a pwm signal if i want but i’m going to do this freehand

4:23at the top here and here’s a pwm signal which is

4:32let’s just call that 10 kilohertz and it’s 50 on and 50

4:39off this is zero watts and this is 30 watts okay now there is

4:47nothing in between we can only switch this machine on to have 30 watts or nothing

4:54that’s the way that this mode of operation works how much damage can we do

5:00with a pulse the answer is very difficult to define for you because we are not

5:07holding this 50 power in one place actually not 50 as i said

5:13it is 30 watts and we’re applying 30 watts over that distance because

5:20while the power is on the head is moving so we’re spreading 30 watts

5:28over a distance that means we do not get 30 watts in one place

5:33we start sharing the 30 watts out over this distance x now i tried to

5:39explain this last time with the candle flame analogy whereas if i put my hand over a candle flame

5:45there’s 30 watts coming from there and hey it’s gonna hurt i will feel it’s warm if i do that and

5:52if i hold it still it’ll be hot because i’m allowing the energy that’s available from that candle flame

5:58to build up in one spot and do damage so the faster i move my hand over the candle flame the less it hurts

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

6:06or the other way around the faster i move my laser head the less damage i’m going to do to the

6:12material now that is a very very important concept as far as this machine is concerned

6:18because speed is a vital part of power control this system is

6:24controlled by this pwm and we’ve got various frequencies that we could use changing the frequency of the pwm signal

6:3250 on and 50 off my pulse time now the time that the power is on let’s just

6:38call that y now i’ve still got 30 watts being applied to the work that’s one way

6:45of controlling the duration of the power on this one has 50 on and 50 off this might be for

6:53example 25 kilohertz so the frequency is a lot higher here

6:59we’ve got 10 kilohertz and the frequency is lower so now we’ve got a much longer duration

7:05for our pulse we could get back to y

7:10by doing this we could change the power to 10 percent so now we’ve got

7:17a signal there a pulse which is the same length as y even though it’s a

7:2410 kilohertz signal there is no difference between this y and that y having understood the concept

7:32that we can get the same pulse out of different frequencies and speed has an effect so if the power is

7:40at 25 kilohertz 25 000 and i’ve got 50 on and 50 off the 50 percent on represents

7:4820 microseconds and if i’m running at a speed of a

7:54thousand millimeters a second then during that 20 micro seconds

8:00the head is going to move by .02 millimeters that’s what this chart is

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

8:07all about it tells me how far the head is going to move for each one of these combinations can i

8:15get to a pulse of 0.1 well first of all a thousand millimeters a

8:21second is where i’d like to do my engraving with 0.1 whether or not this system will handle it i can’t

8:26tell you but let’s just change that number there for example to 500 what effect does it have it’s gone from

8:330.02 to 0.01 now logical because we’re slowing down therefore our

8:41head is not traveling as far now we’ve still got 25 000 pulses but each pulse

8:4820 microseconds travels 0.01 let’s change the percent power from say

8:5550 to 90 now we’ve gone back up to nearly 0.02

9:04because we’re on for 36 micro seconds now and not 20 and we’re off

9:11for four because we’ve got the frequency fixed at 25 000 we’ve still got the same overall pulse

9:16length it’s just that we’ve changed the ratio of on to off to 36

9:22microseconds on and four microseconds off you think ah that means we’ve got more power because

9:28we’ve got more time on no come back to what i just talked about earlier

9:34we’re on for a longer period of time but we’re still traveling at the same speed

9:39we’re still putting down the power into the surface at the same rate if i keep the power on my dot is going

9:46to get longer that’s what that means why don’t we change the frequency let’s drop the frequency down to 10 so

9:53now we’ve got 50 milliseconds on and 50 milliseconds off have we changed

9:58anything if we run at 500 millimeters a second we’re running slower so we should put down twice as much

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

10:04power per millimeter this hurts your head a little bit to think about all these possible combinations here

10:10now look pulse distance 0.025 and you might say hang on 0.025 so in a

10:18point one pixel i’m going to get four pulses no one two

10:25three four each one of those pulses is point zero two five so i’ve got

10:32.025 high point zero two five low point zero two five high and point

10:39zero two five low i’m only going to get two of those pulses because i’ve got my remember my percent power is

10:46set to 50 so it’s 50 high 50 low so i’m going to get two pulses of 30

10:54watts into that pixel now if your head isn’t already hurting

11:00i’m going to just add another little interesting part to the equation for you ideally i’m looking for a point one

11:08pulse or am i let’s just see what happens when i put down a 0.1 pulse how do we

11:15get to a 0.1 pulse we could set the frequency to less let’s set the frequency to 5000

11:23and that should double it 5000 enter and it has we could reduce the

11:30speed to 250 millimeters a second

11:35hang on this is supposed to be a high speed engraving machine

11:40now i can’t do that i can’t compromise that that dream and go down to 250.

11:47so let’s change the power what do we need to do to the power to get up to 0.1 well the answer i think

11:55is to make it a hundred percent there we go so we’re running a frequency of five thousand hundred

12:01percent power 500 millimetres a second and we get a 0.1 pulse would that work we should be

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

12:08getting a signal that looks like this 200 milliseconds on and 200 milliseconds off

12:14and we should have 30 watts applied over that pixel for that period of time

12:23that’s exactly what we want isn’t it remember we’re starting off with the dot which is diameter 0.1 of a millimeter

12:31i don’t think this system has enough intelligence to say ah that’s a pixel and i want you to

12:36apply the power at the center of that pixel i think that what will happen is this the controller comes along and it

12:44sees the edge of that pixel and says right we need some power and then it will turn the power off

12:50when it gets to the other end of the pixel so at the moment we shall have exactly

12:55this situation here where we switch the power on have 30 watts over the duration of that

13:00pixel how much of that 30 watts is spread over that pixel depends on the speed that

13:05we’re running at are we going to finish up with the dot look we switch on here

13:11and we switch off here but here is our dot so we haven’t produced a dot

13:21we’ve produced a sausage so i’m losing the will to live almost

13:28the more we dig down into this the bigger problems we are finding we’re getting the same amount of power

13:36all the way across that scan let’s call it a scan i know it’s only a an incredibly small

13:42amount of distance but it still represents a scan we’re looking at this at such a micro level that people don’t

13:49normally look at they just look at a picture and say oh i think this picture at 600 dpi looks

13:54better than this picture at 300 dpi that’s not understanding what’s going on

14:00we’re trying to establish now can we produce a dot that somehow

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

14:06matches the frequency of these pixels how can we achieve that goal hopefully

14:13the controller is going to come along and it’s going to say right we’ve reached this edge here switch on 30 watts and so the pwm

14:21will switch on 30 watts for whatever frequency we’ve chosen i’ve chosen a frequency here which purposely matches

14:27the width of my pixel so we’ve got one two and a half power spikes

14:34per pixel then we’ve got one two and a half power spikes with

14:41nothing in it and then we’ve got two and a half power spikes again pixel two and a half with nothing in it

14:47the first question i don’t know or understand because i’m not an electronics person

14:52is whether or not the signal that i’m seeing here is the

15:00result of a background signal which looks like this being switched on

15:06and off in other words if this signal has switched on i shall see that much of the signal so

15:13it’ll switch on and then it will switch off and then it

15:19will switch on and it will switch off

15:26and on and off but in the background we’ve got this call it a time base the

15:32pwm signal is running continuously in the background and what i’m doing i’m switching the signal on and off

15:38now if that is the case then fine i’ve got matching frequency that

15:45matches perfectly my pixels because every time i reach a

15:50pixel i’ve reached a switch on point a switch on point now if i choose a frequency that doesn’t

15:57match the pitch of my pixels like this example here

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

16:02[Applause] i’ve stretched the frequency and now the first one

16:08starts off the same and so on if i’ve got a switching system where i switch on and then i switch off

16:17i’ve got my two and a half pixels i’ve got my two and a half pulses but ish but now because the

16:26signal is running in the background when it comes to switching on to the next pixel i’m not going to

16:31switch on at a rising point i’m going to switch on part way through and this one switches

16:38on at part way off and this one switches on nearly the whole distance so if i’m doing a

16:46switching on and a switching off against a background time base i will get this sort of signal

16:54like this where i will not get a repetitive signal for every pulse

17:02or one question i’ve got to ask i don’t know how this system works or

17:08does it work this way where every time i encounter a black edge

17:16here for my black pixel it switches the pwm on

17:23for nothing white it switches the pwm off and for black edge it starts the pwm

17:31offer game from scratch so i get complete signals every one of

17:36these being two and a half pulses as i’ve shown here i don’t know

17:41how this machine works if i set the frequency to 25 kilohertz 25 000

17:47speed to a thousand and the percent power to 50 percent um i should get a pulse distance

17:55in other words i should get a distance traveled by each pulse of point zero two of a millimeter

18:02where we’ve got point two high point two low point two high point two point two so we

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

18:08should have three pulses high and then nothing three pulses high

18:13nothing so that’s a pattern that i hope i’m going to be able to see fifty

18:18percent power fifty percent a thousand millimeters a second

18:25[Music] the interval doesn’t matter but the frequency does

18:3025 kilohertz there we go so we’ve got everything set up correctly

18:36now we’ll go on to the oscilloscope and i’ve got this already set up and

18:41running waiting for it to be triggered right well let’s see what we get

18:51nearly but not quite good news is the pwm is switching on for every pixel

18:59there it is the first pulse seems to be short

19:04now the time base across the bottom here is 0.1 of a millisecond 1000 millimeters a

19:11second basically a second divided by a thousand is one

19:16millisecond equals one millimeter but we’re not talking about a millimeter

19:23distance we’re talking about 0.1 of a millimeter for a pixel therefore one pixel should represent

19:310.1 milliseconds and so that is the time base that we’ve got set up on the

19:37scope so that is point one of a millisecond and should represent one pixel it looks

19:43as though we’ve lost a little bit on the first pulse the time base is switched on perfectly

19:49but it stopped early the very first pulse has got some sort of electronic delay on it

19:55this is only the electronics this is not what’s happening down in the real world and so we’ve got to

20:01relate this to that later on we’ve still got this fundamental problem of how are we going to get a pulse

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…) down

20:09that’s the right size instead of 50 power i’m putting down 100 power in other

20:14words i should have a straight line will the pwm still switch on and off

20:20so 100 on 100 off thousand millimeters a second 25 kilohertz so hopefully we’re gonna

20:27see point one on point one off point one on point one off

20:34we’ve got point one on absolutely with no time delay we’ve got an exact point one

20:40and off point one but we’ve still got our 25 kilohertz switching in the background

20:47look so we found a nice clean way of getting 0.1 pixels but it’s also a nice clean way of

20:54producing sausages so yeah we’re making headway we’re understanding how the system works but

21:00we still haven’t found a way to get a 0.1 pixel what happens if i run that at a thousand

21:06one thousand enter oh i get 0.5

21:14so if i change that to 5000

21:22i’ll get my 0.1 pulse i want a short time on and a long time off

21:28so let’s change this power here to 10 percent so why don’t i go for

21:35something like about 5 000 now i’ve got a 0.02

21:40pulse distance 0.02 so now my sausage will only be

21:49that long let’s go and have a look what signal is when we program that in

21:57that’s about twice what i expected

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

22:02i was expecting one two one two something a bit strange is going

22:10on because at five thousand render settings look what we’ve got here

22:15pre-ignition frequency 5k i’m wondering whether that’s actually a lower limit

22:20for sensible control doesn’t mean to say we might not be able to control it below that but

22:26who knows whether it goes out of control see what we can do with a slightly different value see whether i can just get away with

22:32something like about five thousand and fifty five thousand and fifty okay so now we’ll see whether or not

22:38that’s changed things ah-ha yes it was something to do with

22:44the pre-ignition frequency now it’s rather interesting that when we look at this

22:51we can see that it gets short longer longer longer and then it’s about stable

22:58so it’s taken four pulses to get longer the first pulse didn’t really make it to 30 watts okay now i’ve added another

23:07factor into my uh formula here because i want to make sure that my

23:14dots are pitched at 0.2 because i’ve got a dot then a gap then a dot then a gap so

23:24this is probably more important to me than this dimension here i mean this dimension here is now very

23:30very small and it’s virtually a circle i mean we’ve only got point zero

23:35two sausage if you like we’ll change the

23:43parameter for frequency to five

23:50point zero zero one that’s one hertz above the critical

23:56value now it works one hertz

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

24:04the difference between working and not working 0.1.2 milliseconds 200

24:09microseconds that looks like about 20 microseconds and here it is look

24:1420 microseconds so it’s doing exactly what we’ve asked it to do now but hey this is a major fiddle to get

24:21the correct parameters for doing 0.1 dots now the quick next question is

24:27what sort of dots am i going to get and i’ve got 30 watts for 20 microseconds we’re gonna have to just

24:35give it a try right now last time i was using some white card we’ve got two other things

24:40that we could try one of them i thought i might try is slate because it’s a nice hard material and

24:48it might take quite a nice fine dot

24:55well i saw some sparks which is good news well after all that faffing around

25:01we’ve got pretty rubbish results really yes we’ve got some dots nothing spectacular

25:07at all in fact particularly they’re particularly weak i’ll tell you what i’m going to do

25:14i’m going to grab some numbers out of the air we’re going to have 50 power

25:21700 millimeters a second and because 7 is a lucky number we can

25:26have seven kilohertz

25:32now out of nowhere

25:37that looked pretty pumped

25:43let’s just chuck another random number in shall we

25:49we’ll put the power on to continuous 100 percent and we’ll slow it down

25:57see if we can make it a bit darker and we’ve got a fairly good they’re not not brilliant i mean the

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

26:03black spots in the middle you can see look but we’ve also got a halo around the outside now that halo could be because i’ve not got

26:11it in focus properly so let me try and focus it up a little bit we’ve got sausage shaped dashes

26:18but to be honest they are pretty thin those lines probably closer to 0.07

26:25the sausages are a bit more than 0.1 and the gaps are a little bit less than 0.1

26:30so it’s not perfect but it’s not bad that would give quite a nice


26:42they’re so pathetically faint that they don’t work really so my random numbers work better than

26:49the calculated numbers 500 millimeters a second

26:5810 000 hertz 100 power

27:04is working go figure now that’s not the cleanest set of dots

27:10i’ve ever seen but the black fit in the middle as opposed to the halo around the

27:16outside that’s the bit that you’re going to see so i suspect

27:22if i was to do a picture with that now i would get a pretty good picture at 254

27:28i’ve just done my scanning offset there and fixed that and that looks pretty perfect if you look there’s virtually

27:35zero over travel on it

27:41so even though it’s only going at 500 millimeters a second

27:50so that’s a picture that was prepared for a um for a glass tube so

27:59it’s doing a lot better than i thought it might be i suppose overall that was a bit of a failure because hey we’ve got a half

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

28:06decent picture it’s not a good one but it’s half decent picture much better than i ever thought it’s very dark which is good news

28:12because i was expecting it to be very light and pathetic so

28:18i think we better do a bit of investigation further and find out how this machine works because obviously

28:23i have got no idea at the moment well i think when we look at those in

28:29this light um i mean when i look up

28:35when i look at the very close this one is probably the best in terms of detail

28:41and definition it’s still not a particularly good picture in terms of the

28:46the shading and the the definition but it’s the best picture of the lot and

28:52that came out basically 100 power 10 kilohertz 800 millimeters a

29:00second these are all done at 800 millimeters a second because i can’t see the point of doing

29:05any engraving at less than that it’s you know there’s no benefit but the

29:10great thing is that surprisingly enough there is quite a lot of burn depth

29:16in these pictures and i can only come to one conclusion

29:21on the basis of what we’ve seen and that is the fact that the

29:26below 20 kilohertz is not bad 10 kilohertz seems to be a balance between the number of dots

29:34per pixel because obviously the more kilohertz we have

29:40i just have to look at that and see if we can analyze what’s going on 10 kilohertz is there

29:46anything stark and obvious about that set of numbers

29:5210 000. ninety-five eight hundred

30:00there’s nothing stand out about those numbers that said they match up with anything that i’m attempting to do

Transcript for RF Engraving: Making Dots, How Hard Can It Be? (Cont…)

30:05there’s no point one or 0.2 in there let’s turn that off well it wasn’t quite

30:13the disappointment that i was expecting this hope yet i mean on

30:19paper with a very crisp lens we’ve been able to get quite a lot of

30:25color now whether or not i should be able to change lenses and get different results

30:32something we should be testing in the future i’ve got a long way to go yet but it’s showing more promise than i

30:38anticipated plus the fact that most of the test work that i’ve been doing has been 800 millimeters a second so yeah that’s

30:45substantially faster than i could get on the glass tube machine um i don’t want to push it too far we

30:51still might get up to a thousand or more for engraving it looks possible um but

30:57i’ve got to do a lot more detailed investigation yet so there’s some good stuff to come in the next few sessions i think so

31:04thanks for your time and patience today and i’m going to go in and have a a warm top

31:10up and catch up with you in the next session bye for now

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Last updated April 25, 2024


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