Thickness capability for 40 laser

Hello all.
I bought myself a 40 watt laser from a local supplier.
can someone please tell me if this machine should be able to cut 40mm perspex and if so at what speed?
My machine seems to battle with 3 mm perspex and I suspect that the tube is faulty

fred evans

From K40 initial setup and regular maintenance please see this graphic:

The typical lens in a K40 is a 50mm lens; that is, it’s 50mm from the bottom of the lens to the focal point. The beam is a few mm wide when it enters the lens. Note that most of the height between the lens and the focus point is hidden inside the nozzle. Therefore, if you were cutting 40mm perspex, the beam when it exited the perspex would be almost as wide as before the lens focuses it. The cut would have a substantial angle, except that as it widens it loses effective power so that substantial angle wouldn’t actually show up for 40mm of depth.

@HalfNormal recently linked to a very useful resource on understanding focal depth in a laser:

Ultimately, I would not expect 40mm perspex to be a success. (I wouldn’t expect it to be a success on the 100W laser I’m building even with a 4" lens.)

Now, 3mm perspex should work fine. Please post pictures and describe your problem in detail, and there are lots of folks here with experience to share. No guarantees, but more eyes with more experience can’t hurt! :relaxed:

Have you added air assist?


At that thickness wouldn’t it be better to use a table saw or bandsaw? You could flame polish it then, I think.

Unless of course you are doing complex cuts…

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40mm not a chance, maybe 10mm on a couple of passes but never 40

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Yeah, I agree. Even with a long focal length lens 40mm is just too much material for a K40 wattage level laser to handle.

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I have with a standard lens cut 15 mm and 16 mm acrylic. It takes 100 years and is not the nicest thing I have made. It takes more than ordinary professional equipment to cut 40 mm, in my opinion.


O sorry sorry sorry - a typeing mistake in my question
i am wanting to cut 4 mm not 40 mm !!
please forgive me
but thank you for the replies-- i look forward to your reviewed answered

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… ahhh, that makes the task much easier to work with ;-). In these corona times, I have made quite a few stands for screens for public institutions and offices. I have made them in 6 mm, it also takes some time with a K40 but I have received a lot of praise for the finished product.
Ackyl is a fantastic material for testing and fine-tuning the laser. And of course also to play with.

This is a safe with candy and chocolate I made as a gift for a very candy-sick friend. If I remember correctly it is made of 4 mm and there are 3 working number locks and a large gear for decoration.


Don’t feel bad about the typo! I had no idea that a K40 could (though barely) cut 15mm acrylic until @bernd_dk answered! :relaxed:

I suggest starting a new topic in #k40:help with lots of pictures of the problem you have had with 3mm perspex, so that the conversation isn’t lost at the bottom of some advice about 40mm. :relaxed:

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I would suggest you do a test run on some material scraps to find the precise focus point of your laser and then insert the focus point 2mm into your 4mm acrylic. This is important because then you can cut properly through the material and to keep the cut as straight as possible. By experimenting, you will find a power / speed combination that best suits your machine. Unfortunately, you can not just take over some finished values from other K40 users, there are no two lasers that are identical. But you can use, for example, my values to start from. Once I have found the right values that are reproducible, I make a test item and hang it on my test wall. It’s nice to just be able to see what it looked like and you can also keep an eye on your laser tube and find out when the tube is about to retire.

Picture no.2 is my updated version of my test subjects, with mA

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I forgot to write that speed is in mm / min and the% values are from laser software - LightBurn, which corresponds to the specified mA, I have made a list of these values which I keep an eye on.

thank you very much for the advice and this will become standard practice for me.
From what you say my laser is probably faulty- ( I dont have any figures to work from now because i am busy modifying the table and the machine is in pieces at the moment.)
When you look down the tube while curring - it appears that the laser beam is not aiming straight down the tube - rather it veers to the left.
I have a new tube which I will be installing once the machine is up and running again. I will be using k40 whisperer and inkscape as the soft ware.
My experience with this machine and setting up may be of interest to other new users . Where should i post the details?
best regards


Hi Fred, It would be a good idea and useful to many if you could document your remodeling project with a K40.
Rebuilding the K40 is part of it all, it’s just like getting your own, safer, more efficient and personalized laser machine. AND it’s fun.
I assume you got the new laser tube as a replacement after a complaint? By my laser tube, the exit lens itself had gone off, during transport and because it is not the best quality tube. But in addition and after my modifications, it is a fine stable machine which has given me many good hours the last 2 years and earned itself home many times.
Fred, when the laser beam dances or sticks to the side of the cathode, it does not have to be dead. My first tube, which I repaired myself with epoxy, has been doing the same thing at certain frequencies for some months. But because I was not experienced enough, I did not document the settings properly. I just remember that by changing power or turning it off and on, I could change the behavior of the beam. Use your laser tube as long as possible, you can also run it a little higher as usual if you still want to replace it. (min max limit is 16mA)
I look forward to following your project, if you have any questions you are most welcome.

I assume Michael will find the right place for this thread.


yes I will do just that-
WHen I purchased the machine i expected to plug and play !!
To my surprise this is not the case. Alignment was the first issue and to my surprise i found that the laser tube was resting on a pack of folder insulation tape !!
this got me going-- so I will do as you suggest as soon as i am satisfied that the machine is working well. at the time of purchase i fortunenately took the precaution of purchaseing a spare tube.
the biggest problem was that the cabinet, gantry and table are all out of square.( I would have thought that the cabinet at least would be square but nothing is true).
i will keep you informed
By the way the local supplier was not of much assistance either


I moved this thread here because the topic title didn’t help someone other than @ntambomvu understand what would be in it.

I’d suggest that @ntambomvu start his own new topic for diagnosis and rebuild, in #k40:help . An “attention” topic doesn’t really help others find and learn from it.

@ntambomvu, I’d strongly encourage you to post lots of pictures and write a factual description of what you’ve found in the machine. By this point, there isn’t much in the way of silly errors as shipped/delivered that would surprise the experienced members of this site; if you read through the history here you’ll find that almost anything that can go wrong already has gone wrong, probably for multiple poeple.

It’s worth going through everything in detail; even if you are right that the first tube is dead, you want to understand why and avoid having two dead tubes.

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thanks mcdanij -
I will post a comprehensive letter when all the mods are working-
When I post this I will post to #k4:help ? is that right?
\best regards


@ntambomvu, ah, possibly two new threads would eventually make sense. Let me clarify.

  1. I originally was suggesting that you start a new thread now for diagnosing your weak cutting action. Even if the current problem is a bad tube, if it’s bad because of some other problem on the machine you’d like to know that before putting a second tube in place. :relaxed:

    We want to be able to help people fix machines, but we also want to make the conversation help the next person with a similar problem find the answer. That’s why I suggested a separate thread for diagnosing the problem with weak cutting. A “Please help me confirm my diagnosis of a bad tube in my K40” topic will be something someone else can easily find, read, and learn from.

    You don’t need to create a new thread for this. I was just saying that a thread that started in the middle with @bernd_dk responding to you made me realize I hadn’t been clear about the point of a new thread — It was to put your question first, and then the answer, into a place where it would be easier for the next person to find. Fortunately discourse makes it easy for me to fix the confusion I created.

  2. Beyond that, also, in #k40:mods creating a thread documenting all your mods would be great! Either one thread with all your mods, or one thread per mod, or something in between. Feel free to write about them as you go along — I have several build logs. I did the “one long post” version for a few 3D printers, and right now I’m building a large-format laser and I’m trying to do one post per major topic for it.

But for now, let’s just keep going here. :slight_smile:

Here’s something important to understand: What you can see in the tube is not the laser beam. It’s plasma, like in a florescent light bulb, that excites the laser beam, and it goes between a cathode and an anode that are next to but not part of the mirrors at either end of the tube, so at the ends that plasma basically has to bend. The laser beam itself is completely invisible. The wavelengths of the infrared laser are more 10 times longer than the human eye can see. It’s primarily 10,400 nm, with some at 9,400 nm. We can see light in the range of about 400 to 700 nm. This is related to the laser being dangerous. You can’t see anything about the laser except the effect it has.

Are you aware of how laser tube high voltage wiring is different from normal wiring?

The thing that matters most is that the X and Y axes are square to each other. Are they now?


Very nice explanation. Purely “psychological” it looks best with a straight, not nervous dancing plasma beam in the tube, but as you nicely describe, it is not the laser beam itself.
If the color is purple, violet and not pale white and if (only) a single laser beam comes out, there should be hope ahead.

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It will amaze me a lot if you find 2 angles that fit together on a standard K40. The frame itself was relatively ok in my machine but the level at the bottom is a bit of a challenge. But if you can still adjust the mirrors and the nozzle in relation to the laser beam, it is only a cosmetic problem. You must also consider the effort in relation to what the machine costs and is intended for. It is easily possible to upgrade a K40 with new mirrors and lens holder, air nozzle, bed, control board, power supplies, cooling machine and new laser tube … In the end you have spent money equivalent to what a 60Watt laser with 400x600 costs, but you have still only a 35-40 Watt laser machine with a 320x230mm working area.
What you can not price is the pleasure of the good time you spend on such a project - if you like to tinker :wink: and the new contacts to friendly like-minded people.


Right. How about “square enough?” — (I just yesterday watched the latest ToT video avoid buying a cheap vice, which is mostly about precisely measuring aspects of not-quite-squareness of his new vice. Cheap Milling Machine Vise is CHEAP! - YouTube if you are curious… :relaxed:)

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