Number 07 – RDWorks Learning Lab 054 – Acrylic laser cutting: beam drag and flame polishing

The Top Ten RDWorks Learning Lab Videos

These are the top ten RDWorks Learning Lab Videos ranked by average number of views per month. In this session, Russ investigates acrylic laser cutting and the processes of beam drag and flame polishing.


  • Explanation of and demonstration of Through Hole Power
  • What is beam drag?
  • How do I get rid of it?
  • Set focus onto surface and close air assist to almost off   
  • Modify the focus depth to 3mm below surface
  • Great flame burnished edge
  • Increase speed…….cut success
  • Examination of cut characteristic
  • Non-square faces explained.
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Acrylic Laser Cutting - Example of Beam Drag
Acrylic Laser Cutting – Example of Beam Drag

Transcript for Acrylic laser cutting: beam drag and flame polishing

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0:00welcome to our t-works Learning Lab

0:02today we’re going to take a look at two

0:05problems at least two problems that

0:07people have mentioned to me can I

0:09explain why one I can explain why and

0:13the other one I can’t explain why so

0:16we’re going to do a few experiments to

0:18see if we can come up with an answer to

0:19the question question number one is why

0:21does the beam drag if you don’t know

0:24what beam drag is I will demonstrate

0:26that to you in a minute the other

0:28question is I’ve got little rough edges

0:31on my acrylic cuts how can I get them

0:34nice and smooth now maybe some of you

0:36guys already know the answer for this

0:38question but the majority of people me

0:40included doesn’t know the answer at

0:42least one of those questions so what

0:45we’re going to do is just have a bit of

0:46a play here and we’re going to change

0:49one factor at a time so what we’ve got

0:52here is an eight millimeter acrylic now

0:54I’ve already measured the power out of

0:56my nozzle here and I know that I’ve got

0:58approximately 60 watts coming out there

1:01because I’ve got a one and a half inch

1:03lens in here I’ve got a distance away

1:06from the work of only about six or 6.25

1:09I think it is a quarter of an inch so

1:12I’m going to set my focus up first of

1:14all to sit right on the surface of the

1:18job I’ve got a pressure control valve on

1:20my machine but if you listen carefully I

1:22can turn the air pressure

1:28off or certainly right down so we could

1:33have it if I can get it just right there

1:38we go we can just hear a whisper there’s

1:41just enough positive air coming out of

1:42that nozzle to make sure that we don’t

1:44get any fumes back up on the lens so

1:47that’s about as weak as I can possibly

1:49get it so we’ve got several factors that

1:51we can play with it I play with speed we

1:54can play with power but it’s best to

1:56keep the power fixed and just change the

1:58speed we can change the airflow so that

Transcript for Acrylic laser cutting: beam drag and flame polishing (Cont…)

2:01we can see what effect cooling or

2:04clearing of the fumes has and then we

2:08can change one other vital thing as well

2:10what I’m going to do is do a series of

2:12test squares I want to use acrylic so

2:15that you can see clearly what’s

2:16happening to the cut several other

2:18people have asked me what does

2:20through-hole do what’s through-hole

2:22power well I’m going to demonstrate here

2:25what through-hole power is I have set on

2:29my program through-hole power

2:32ticked and just above it there are two

2:34windows the top window is the important

2:37one because that’s when you start the

2:39cut why on earth you’d want one at the

2:40end of the cut I don’t know but when you

2:42start the cut you will see that the cut

2:44stands still for half a second because I

2:48program that to 500 milliseconds and it

2:51will pierce through and then it will

2:54move off so you will ever hold through

2:56before you even start moving and you

2:59will see that happening at the beginning

3:00of this square

3:09turn some extraction on

3:13now you can see the beam dragging there

3:21oh dear and it hasn’t dropped out so I’m

3:24going a bit too fast

3:27No so that was six millimeters a second

3:33now I think you can clearly see on this

3:36here how we’ve got striations it’s not a

3:40very smooth cut

3:42okay I’ve now reduced speed to four

3:45millimeters a second and we see what

3:47effect this has

3:51so it’s PS through and now it’s moving

3:54off it was a bit of a strange it’s still

3:56dragging look can you see that

Transcript for Acrylic laser cutting: beam drag and flame polishing (Cont…)

4:03I would say we’re cutting through this

4:05time because all the fumes underneath

4:07but it’s still dragging even though I’ve

4:10reduced the speed

4:17and I would say it nearly dropped out

4:20look there we go if we look along the

4:25bottom edge of the cut you can see the

4:27little striations along the bottom of

4:28the cup there it’s not bad for 60% of

4:34the depth 80% of the depth

4:38but you can clearly see something’s

4:41going on on the last two millimeters

4:45okay now we’ve set to two millimeters

4:48the second

5:00beam is still dragging even though we’re

5:02going really slowly

5:07but the gas is coming out underneath

5:12I have got the merest whisper of gas on

5:15there at the moment

5:26I think you see the beam is not dragging

5:28quite as much

5:32but it’s still definitely dragging

5:47okay let’s take a look at this one now

5:51I don’t know when you can see the edge

5:52this time it is not bad at all going

5:56slowly we’ve got a lovely smooth mirror

Transcript for Acrylic laser cutting: beam drag and flame polishing (Cont…)

6:02polished edge on there now and I can

6:04feel that this piece is hot

6:06the edge is hot and that’s because we’ve

6:09going so slowly that the heat was

6:11actually flame polishing the edge as it

6:13goes we’re improving the edge cut and

6:17the depth of the cut the quality but

6:25something that you might notice on there

6:27if we can catch it in the light right

6:30let’s just see if we catch it in the

6:31light guy

6:33and that’s that these edges are not

6:35really upright

6:40and he what I wanted to show you there

6:42really was because we’ve got through

6:44power on to start with nothing to pierce

6:47the hole to start with it doesn’t make a

6:50lot of difference we’re getting beam

6:52drag even though we’re starting off with

6:54a nice clean up right cut now I’m going

6:57to change the program and we’re going to

6:59go for just a straightforward 25

7:03millimeter square

7:05ok so now we’re going to do 4

7:07millimeters a second with no start no

7:10through-hole to start with

7:27you see we’ve got being dragged across

7:29the front there quite significant

7:40and the lacer should start immediately


7:47so why is there a delay there and on

7:49delay and I still got my 500

7:52milliseconds in there

7:54through hole

7:56I reckon I have that’s what the problem

7:59is I’ve still got 500 millisecond delay

Transcript for Acrylic laser cutting: beam drag and flame polishing (Cont…)

8:02in there before I start the cut well

8:06that’s interesting

8:09so they’re going to be careful of that

8:11it could be useful it could be annoying

8:14like I’ve just found out so I’ve got to

8:17go back and edit the program now and

8:18take that 500 millisecond out and make

8:20it zero even though I haven’t got the

8:22through-hole ticked it delays the onset

8:25of the beam now that we’ve taken that

8:29500 millisecond delay out we’ll try a 25

8:33millimeter square

8:36there we go that was quite an

8:39interesting observation

8:42so let’s take a look and see what this

8:43beam is doing as it travels across the

8:45front here

8:49four millimeters a second it’s not quite

8:52this draggy but it’s definitely dragging

9:02and it dropped out let’s hung up on a

9:05corner because the beam is dragging it

9:08didn’t quite cut the bottom corner

9:13and again if we look carefully we shall

9:17find that the edges are not exactly

9:20upright they’re not square

9:25surface is not too bad but we’ve

9:28definitely from two millimeters away

9:30from the bottom we’ve definitely got

9:33some some striations now I just turned

9:38the extraction off and what I’m going to

9:40do now turned it off so you can hear I’m

9:45going to turn the air pressure on four

9:48so that we can see what effect

9:50increasing air pressure has

Transcript for Acrylic laser cutting: beam drag and flame polishing (Cont…)

10:02I would say looking at that across the

10:05back it’s even more dragon

10:08oh yes

10:11it’s very dragging out

10:21dropped out

10:24there’s actually a slight improvement

10:26but the edges seem to be very out of


10:37now I’m going to try something slightly


10:48cool now definitely dragon

10:51you’re seeing a reflection at the bottom

10:53there that’s why it’s a sort of a bitter

10:55v-shape okay are we coming across the

10:58front end

11:08see what I was doing here

11:12I was playing

11:14with the focus

11:17so what I’m now going to do because that

11:19was rather interesting I don’t know if

11:21you observed it but I certainly saw


11:24I do have a piece of 3 millimeter

11:27material here

11:37okay so we know three nine inches off

11:39the surface which means we’re about

11:40three and a half millimeters into the

11:42job with the focus point

11:57being dragged at all

Transcript for Acrylic laser cutting: beam drag and flame polishing (Cont…)

12:08we have a lovely polished edge

12:17now what I’m now going to do is try

12:20something else

12:24I’m now going to increase the speed back

12:27up to six millimeters a second

12:30remember all the tests we’ve done so far

12:32six millimeters per second has not

12:35dropped out

12:38Oh being dragged six millimeters a


12:49just about did it

12:52we’re getting a small hint of striations

12:54across the bottom again

12:58if you go slower

13:01you will flame polish the edge better

13:03and if you push the focus point into the

13:06material you will get more depth of cut

13:09but you also get a smoother finish all

13:12the way through your cut

13:14okay now finally I’m going to do

13:15something very strange first of all I

13:18put the speed back to four millimeters a

13:20second and I still got the nozzle set

13:22three millimeters above the surface but

13:24I’m going to start the cut off right in

13:26the middle of that previous square when

13:29we started looking at these and saying

13:31well the the edge looks as though it’s

13:32you know not cutting square it’s

13:34completely off angle I realized that

13:36there was a problem that I needed to

13:37show you

13:45quite a nice cleanup right cut

13:53now I’m going to do something strange

13:55I’m going to set the speed on my machine


Transcript for Acrylic laser cutting: beam drag and flame polishing (Cont…)

14:00sorry I’m going to send it to four

14:02millimeters a second

14:04I’m going to change the power max power



14:12five percent which is what I was cutting

14:14the programmer

14:16okay so I’ve now set my manual speed


14:21and what I’m going to do I’m going to

14:22hold the pulse button down

14:25and I’m going to drive the head manually

14:31and I’m doing a manual cut now

14:35now I’m going to take it down this way

14:41cutting across my

14:43workers underneath there

14:47I woke up back

14:59and I’ll go up

15:03so I’ve done a manual cut there without

15:05going anywhere near the programming


15:09so there we are we’ve cut our

15:14we’ll cut our piece of material like a

15:16jigsaw puzzle

15:17let’s have a look at this carefully if I

15:20push those parts together like that

15:22that’s obviously not the cut because if

15:25I pull it one way that’s twice the

15:28thickness of the cut and if I push it

15:31together that’s not the cut but if you

15:33look at the way in which the shapes

15:35don’t match up both of them are actually

15:40going the opposite way to the shape of

15:43the beam the beam is necked but the

15:48center of that cut halfway through the

15:50material has actually got bigger and

15:54then if you look carefully at the bottom

15:56of the cut you’ll see the bottom that

15:58cut is touching but the top of the cut

Transcript for Acrylic laser cutting: beam drag and flame polishing (Cont…)

16:01there’s actually a gap at the top

16:04there’s a very short focus of only

16:06something less than a millimeter I think

16:09on this machine with my 38 mil lens the

16:15focal distance is very very short and

16:18after that the beam starts diverging and

16:21getting out of focus and as it gets out

16:24of focus obviously the beam gets bigger

16:27in diameter and it’s effective density

16:29power density gets less which means it

16:32has less and less cutting power the

16:35further we get away from the focus point

16:37but that then accounts for why we

16:39haven’t got such a cut at the bottom

16:41there look the power is disappearing the

16:43cutting power is disappearing at the

16:44bottom of the cut and so consequently

16:49that’s why the bottom of the cut is

16:51actually narrower than the top of the

16:53cut just because the edge is not square

16:57doesn’t necessarily mean to say that the

17:01beam isn’t square it’s all to do with

17:04the heat radiation and the shape of the

17:07beam and the power of the beam at a

17:08specific point through the cut so those

17:12of you would 2-inch lenses for example

17:15will have less of this problem you will

17:17be able to focus up better and probably

17:19have a straighter cut when you’re

17:21cutting thicker materials but then again

17:23you won’t be able to cut such thick

17:25materials because your power density is

17:26lower so we’re into a bit of a circular

17:29situation here

17:31you know but if you’ve got a lot of

17:32power 60 80 watt machine then you should

17:35be able to cut 8 millimeter with no

17:36problem at all and probably get

17:38reasonably straight edges better edges

17:39than I can get it cook because we cut it

17:42very slowly

17:43it’s a superb quality edge all I’m

17:47saying to you is although the edge isn’t

17:48up right

17:49don’t run away with the idea that

17:51there’s something wrong with your

17:52machine because there probably isn’t

17:54it’s all to do with the way in which the

17:56energy density in the beam is affecting

17:59the material that you’re cutting

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