17 – Laser Beam Settings To FIX The 4th Corner Problem

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. This session, Russ tackles a viewers problem with their laser beam settings being out of alignment in the 4th corner. This fix will work on most laser machines, so watch and learn.

Laser Beam Settings to Fix the 4th Corner Problem
Laser Beam Settings to Fix the 4th Corner Problem

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!


The eternal problem with beam settings has been the “4th corner” There is a well tried and tested Laser beam settings procedure that works very well in principle. In practice there are many instances where after setting 3 corners perfectly, when you check to see where the beam is landing on mirror 3 at the 4th corner, you discover that there may be a significant error of several millimetres.

THIS IS NOT A MECHANICAL PROBLEM, but an issue where the beam has been deflected to a different path, For several years I have pointed to the flimsy mounting bracket at mirror 2 as the root cause of this problem and when people fix that problem (usually be adding an extra fixing screw) or re-fixing the cable chain, the problem can be cured. Recent design changes I created for the mirror 2 mounting bracket have made it so stiff that I MUST have fixed this issue for ever.

Wrong. I have had to go back to absolute basics and try to understand just how it is possible for the beam to be deflected so much after what appears to be perfect setting. It turns out that there is a vital setting step that is missing from this long established procedure. Add this step and it will fix your 4th corner problem.

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Transcript for Laser Beam Settings To FIX The 4th Corner Problem

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0:00welcome to this rather extra special


0:03of my tangerine tiger as you can see

0:06this has already got the mirror 2

0:09and stiff motor bracket

0:12but somebody who’s fitted this up

0:15was saying that they’ve still got a

0:18fourth corner problem

0:20well hang on you can’t have a fourth

0:22corner problem because

0:23the fourth corner problem always seems

0:25to stem from the

0:27flexibility of that mirror we proved

0:30that many times before

0:32i mean many people have fixed the

0:35problem by

0:36sticking up their mirror bracket but in

0:38this particular instance

0:41it hasn’t fixed the problem so

0:44i have to admit that on one previous

0:47occasion i came across a machine which i

0:49could not fix and i still didn’t

0:52understand it

0:54now this second time round about a year

0:56and a half later

0:59makes me ask the question what is it


1:02actually happening because i thought i

1:05understood it

1:06but obviously i don’t

1:09because if i did the problem wouldn’t


1:12my solution would be the solution here

1:16we are today

1:17and i’m going to try and get to the

1:18bottom of this because there are all

1:20sorts of myths

1:21about how you fix this fourth corner

1:23problem so at this point here

1:25on the fourth corner the head either

1:27appears to be up

1:29or down sometimes a little bit left and

1:32right but generally

1:33it’s up and down and that makes people

1:36believe that really the solution to the


1:38is to put packing shims under the rail

1:41here or something like that to fix the


1:44no this is a perfectly flat plane anyway

1:48because it’s been designed on a frame

1:50like this okay it’s not absolutely


1:53thousandths of an inch or 0.01 of a


1:57perfect but it’s certainly not two or


2:00millimeters out as this would indicate

Transcript for Laser Beam Settings To FIX The 4th Corner Problem (Cont

2:04by putting a spot on here and then doing

2:06the same thing at the front

2:07this is definitely not a mechanical

2:09problem this is a beam

2:11problem of some sort where we’re getting

2:13a reflected beam

2:14which is not hitting the same place here

2:17as it is when it’s at the front of the


2:20so something is happening between the

2:21front and the back of the machine

2:23and up to now the general thing that

2:25seems to be happening

2:26now look you can see how this cable

2:28chain is raised up here it doesn’t want

2:30to go

2:31flat so if it does go flat then the

2:34curvature of this is exerting a slight

2:36upward twist

2:37on this bracket but hey this bracket now

2:40is so stiff

2:43you could probably stand on the machine

2:44and hardly move

2:46this mirror so we do not have this

2:49same problem that has existed on the

2:52naked and brand new

2:53raw machines although having said that i

2:56noticed the new machines are now

2:58now no longer using 16 gauge material

3:01here for this bracket

3:02but they’re using two and a half

3:03millimeter material which is why i’ve

3:06made this out of two and a half

3:07millimeter material

3:08and even stiffer by making a continuous

3:11l-shaped bracket people have suggested

3:13that it’s all to do with the

3:14squareness or levelness of your machine

3:17now people

3:18people interpret level as being

3:21water level so they get a spirit level

3:23out and they use a spirit level and they

3:24get their machine

3:25absolutely perfect no

3:28i think what’s implied by that is if

3:30your machine is somehow

3:32twisted in other words you could jack

3:34one corner of the machine up and get rid

3:36of the problem

3:37no you can’t i’ve already proved that in

3:40another session you can twist this

3:42machine as much as you like

3:43and it does not get rid of the problem


3:47it’s not a mechanical issue it must be

3:49coming back to

3:50after we fix this number two mirror

3:52amount it must come back to

3:54something to do with the beam itself


3:58you don’t get huge motions on here

4:01because of mechanical movement you get

Transcript for Laser Beam Settings To FIX The 4th Corner Problem (Cont…)


4:03movements on your burn spot because the


4:07is not following the same path there is

4:09a problem with the beam alignment so

4:13i spent a long time thinking about this

4:15and the only thing that i can come

4:17to a conclusion on is that we the

4:20conventional beam settings method

4:24is not fully error-proof

4:27and that there are other steps that you

4:29need to take in that procedure

4:32to fix the fourth corner problem and

4:34what i’m going to try and do today

4:37is to first of all illustrate the


4:40and then how we’re going to fix it now

4:42this machine is already

4:43roughly set up it’s well it’s pretty

4:45well set up to be on

4:46the first thing we’re going to do is to


4:50the y-axis and we’ve done that by

4:52putting a target on the mirror

4:54it’s not easy to see what’s going on at

4:56this first target here so

4:57i’m looking at a reflection in a mirror

5:02so there we go we’ll put our mark on

5:04there so let’s just try

5:06see what it’s like at the front here and

5:10it looks as though we’re not lined up we

5:13need to move the mirror

5:15so that the burn comes closer to this


5:21that looks better let’s repeat that test

5:23again because that’s the first

5:25of our several iterations we probably

5:27need to do this at least

5:28probably three times if you’re setting

5:32up from scratch

5:33if you’re doing it from a position where

5:35you’re pretty close to perfect then you

5:36might only need to do it twice

5:46it’s not far off center let’s go down

5:50here again and check it see what’s

5:51happening at the front

5:57i would say that that’s good enough to

5:59move on with

6:00you need to make sure that your mirror

Transcript for Laser Beam Settings To FIX The 4th Corner Problem (Cont…)

6:02is clean because every time you burn the

6:04target on there you get a little bit of


6:06condensation or halo of fumes and we

6:08need a different sort of

6:10target now and these two holes here the

6:12funny shaped holes

6:13press onto the screws that project out

6:15from the springs

6:17so you just pop them on there like that

6:19so there we are with with the target in


6:21i’m not going to bother to move the

6:22camera so i’m going to move the target

6:24close to mirror 2 and i’m going to put

6:26a reference burn on there a target burn

6:29now you’ll see what i mean about it not

6:30being perfect

6:31and spot on but don’t let that worry you

6:34we’re now interested in getting a dot to

6:36match that dot

6:38so let’s have a quick pulse and see if

6:40we can find out where we are

6:45okay that’s probably not far out we’re

6:48very lucky because this machine is more

6:49or less set up

6:50what we’re going to do is we’re going to

6:52move this to the back position

6:54and now we’re going to see what sort of

6:57burn that we’ve got on the fourth corner

7:03well it’s not bad it’s probably about

7:06a millimeter or so forward now that this

7:09is the problem

7:10i’ve got a slight fourth corner issue on

7:13this machine but i think that it all

7:17comes down to

7:18the accuracy with which you manage to

7:21spot your x

7:22and your y settings

7:25now i’m not criticizing the way that you

7:27set the machine what i’m saying to you


7:29that on a small machine like this you

7:31don’t get

7:32much distance much leverage if you like

7:36to do your setting i’m going to now

7:38do something that i’ve never ever

7:41recommended before and that is that we

7:43go back

7:44to the beginning i’m just going to turn

7:46over now because you can use these

7:48twice first of all we’re going to

7:51go across to the left hand side

7:55and right at the back of the machine

7:57we’ve got a burn reference mark in

7:59a new target mark and there we go

Transcript for Laser Beam Settings To FIX The 4th Corner Problem (Cont…)

8:03so now we’ll check where it is at that

8:07end of the stroke not bad

8:15so it’s difficult for me to get a fourth

8:17corner problem on this machine

8:20but if i come to the front now and try

8:22the same thing i have got a small

8:26problem look

8:27you can see that it’s not lined up bear

8:30in mind we made the target mark

8:32right at the back left hand corner

8:35okay so we’re now just about as far away

8:38with this mirror as we could be

8:42from where we made the original target


8:45we don’t normally make a target mark on

8:48mirror 3

8:49at the back left hand corner

8:52but this is what i think probably should

8:55be the final stage

8:56of setting and that’s to do a diagonal

9:00setting across the machine from the back

9:03corner right close to the mirror with

9:06mirror three

9:08and then we go back to mirror one

9:12and we adjust this error out

9:15diagonally across the machine not x and

9:17y but diagonally across the machine

9:20so what we’re going to do now we’re

9:22going to change mirror

9:24one and try and push this mark further


9:28me at the front of the machine here

9:32now this is quite complicated in terms


9:36imagineering because we’ve got

9:39reflections going all over the place

9:41and it isn’t quite what it seems

9:44if you want to move the burn across

9:46towards the front of the machine

9:48you would tip your mirror that way

9:51so i’m going to put a small amount of


9:55on mirror one now

9:59and i want it to go towards the front of

Transcript for Laser Beam Settings To FIX The 4th Corner Problem (Cont…)

10:01the machine

10:03which means basically i’ve got to move

10:05it across

10:08mirror 2 towards the front of the


10:12which means i’ve got to

10:15remove a little teeny weeny bit of


10:18off of mirror one very very small amount


10:22adjustment on mirror one and now we’ll

10:25see what effect

10:26that has had

10:32well it’s brought it more or less in

10:34line the only thing that we have to do

10:35now is

10:36raise it very slightly so to raise it

10:38i’ve got to lift

10:39the mirror up very slightly


10:43just i don’t know a tenth of a turn

10:46or less okay so what we do now

10:51we’ll put a new target on so we’ve got a

10:54target burn

10:55at the on mirror three at the back left

10:57hand corner of the machine

11:07and we’ll see how close we are now with

11:10our target

11:14still a little bit low

11:20and there we are i think we’re now just

11:22about spot on

11:24what we’re going to do we’re going to do

11:25this one here we’re going to make a mark


11:29we’re going to go to the back of the

11:30machine and see how close

11:33we are with our mark

11:36and there we go we’re just about spot on


11:39and that’s at the back corner so it

11:42looks as though we no longer have a

11:44fourth corner problem

11:46we’ve tuned it out by adjusting very

11:48slightly adjusting

11:49and i do mean very slightly adjusting

11:51mirror one

11:54but because we’re adjusting mirror one

11:56over a much much

11:57longer distance and referencing it to

11:59mirror three

Transcript for Laser Beam Settings To FIX The 4th Corner Problem (Cont…)

12:01we’re actually making the setting much

12:04more accurate

12:05so if we now go back and check what

12:07we’ve done

12:08to the x and the y settings just clean

12:10that mirror again because as you can see

12:13it’s got some crap on it

12:18you can see i’m not doing anything

12:19special with these mirrors these are

12:21molybdenum mirrors

12:22you could be quite abusive with them i

12:24would suggest you normally use toilet

12:26tissue or something to clean them

12:28um not normal kitchen roll or anything

12:31like that

12:32because kitchen roll has got an abrasive

12:34kale in it that stiffens it

12:36um but ordinary toilet tissue is just

12:40basically wood pulp and it’s very soft

12:44if you want to be really fussy you use

12:46lens tissues

12:48so i’ll make a target burn on our y-axis

12:51right close to mirror one

12:58it’s a bit of a weird burn doesn’t it

13:03you see so it’s not hitting the hole in

13:05any way at all see what’s happened

13:07to our y-axis adjustment have we messed


13:11with it in any significant way

13:20and the answer is no

13:24so has it affected our x-axis alignment

13:32and the answer is we haven’t

13:35so what we’ve done we’ve put an extra


13:39of accuracy into the setting procedure

13:42by doing one final extra

13:46move we’ve seen that we’ve not affected

13:48x and y

13:51so we go back to the far back corner


13:55see how good our burn is

Transcript for Laser Beam Settings To FIX The 4th Corner Problem (Cont…)

14:05pretty good and now we’ll go to the back

14:08corner of the machine

14:10and we do a fourth corner check at the


14:16and there we go so

14:19just one extra procedure at the end of

14:23setting x and y

14:25before you go to setting z will


14:30and reduce the errors that you produce

14:33by doing

14:34x and y separately now we can go to z

14:37and for z we need to drop the nozzle

14:39down so that it’s below

14:41this step here

14:46because we’re going to use this third


14:53which is this one i’m going to put it so

14:55that the

14:56hole sits in the corner we either move

14:59the mirror

14:59in and out for y or the head up and down

15:03for x to try and get our

15:06theme right in the center of the cross

15:09check where our beam is in relation to

15:11the central axis of the lens tube

15:16what can i say you must do two other


15:20one you must make sure that mechanically


15:23is sitting square to your table in both

15:26this axis

15:27and that axis and we put a burn on here

15:33like that drop the table down maybe four


15:37we then tweak number three mirror to

15:39make sure that we get the

15:41beam running true perpendicular to the


15:46so it’s a repeat of the x and the y axis

15:48but we’re doing it in the z

15:49axis and then finally

15:53once you’ve done that you do the check

15:55that i’ve just done

15:56which is to make sure that the beam is

15:58actually trued up to the

15:59axis of the lens tube purpose of today’s

Transcript for Laser Beam Settings To FIX The 4th Corner Problem (Cont…)

16:03session is all about trying to find out

16:05and solve

16:06what the fourth corner problem is and i


16:09at last we’ve solved it

16:13because i don’t think it’s anything any

16:15longer to do

16:16with mechanical things i think we’ve

16:18fixed all the mechanical issues on this


16:20go forth experiment and play and if it

16:22doesn’t work for you

16:24well keep it to yourself thanks for your

16:26time and patience

16:28and i hope i never hear from you! bye for


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