Weak laser power during auto-calibration / Understanding laser calibration

[SabinSnorlax]

Dear all,
I’m starting this thread for 2 reasons.

Reason #1 is to understand what’s going wrong with my laser auto focus calibration.
After I start the calibration, the laser barely leaves any marks on the birch wood I’m working with, see attached images.
Because of this, the calibration obviously fails.
And here’s what’s strange:

• When I set the power manually to 100%, the laser leaves a clear, strong mark on the wood, again see images
• When I manually position the laser to an “OK height” and I start an engraving work, the results turn out fine; no clear differences to older engravings
• This issue suddenly appeared about 2 weeks ago; and I’ve been working with the same type of birch wood, same settings, etc.
• I can rule out any issues with the enclosure

So why then would the laser be so weak during calibration???

Reason #2 is to understand what is exactly going on during laser focus calibration.
I understand the basics, so why you’d want to focus the laser in the first place.
But here’s what I still don’t understand, even after re-reading the manuals and several threads on this topic:

• What is the physical meaning of the “laser height”? (formerly denoted as “laser focus”)
• What is the connection between laser height, thickness of the material, the Z-coordinate after calibration, and the actual distance between lens hood and material?
• Has anyone made a simple diagram showing this connection? Or could someone please make one quickly to explain?
• Why do I keep seeing different values of the laser height tossed around, say 21mm or 25mm etc.?
• Shouldn’t there be a “universally correct laser height”, assuming the lasers are identical and they function in the same way?
• Or to put it another way, shouldn’t there be a “universally correct distance between the lens hood and the material”?
• What’s up with the lens hood, does it serve any role beyond added safety?
• I saw this hood being mentioned as not strictly necessary, but is it “better” or “worse” to keep it on? (again, talking not just about safety, but also lasering quality etc.)

I apologize if some of these questions are plain dumb, but I simply don’t get it, even after going through several threads on this forum.
So I hope to understand things “once and for all” with your kind support.

[brent113]

@SabinSnorlax

Reason #1 is to understand what’s going wrong with my laser auto focus calibration.

Only thing I can think of is the power is too low or the speed is too fast for the material you’re trying to calibrate on. You could investigate further with M1028 S6

@SabinSnorlax

What is the physical meaning of the “laser height”

It is the Z coordinate where the laser is in focus with a material thickness of 0.

@SabinSnorlax

• What is the connection between laser height, thickness of the material, the Z-coordinate after calibration, and the actual distance between lens hood and material?

Z coordinate = laser height + thickness of material. Lens hood will have to be adjusted by you to have a few mm clearance above the material, enough that with Z steps down into the material it won’t hit. Or just remove it like I did, it’s only purpose is as a ‘shield’ against stray laser light - just wear your goggles.

@SabinSnorlax

• Why do I keep seeing different values of the laser height tossed around, say 21mm or 25mm etc.?

There is a slotted lens holder up in the toolhead - by changing that you can change the laser focal length. Not every laser module comes with it assembled the exact same way.

@SabinSnorlax

Shouldn’t there be a “universally correct laser height”

Yes, and it’s when the beam waist is in the middle of the kerf you are cutting. There is not a fixed distance, however, it’s a functional definition.

@SabinSnorlax

Or to put it another way, shouldn’t there be a “universally correct distance between the lens hood and the material”?

Since this is something the instructions (at least the online ones) instruct you to adjust to a few mm, I’d say yes, and it needs to be adjusted manually (if you’re using a lens hood at all).

@SabinSnorlax

• What’s up with the lens hood, does it serve any role beyond added safety?

No, other than additionally possibly funnel dust and soot up into the lens area.

@SabinSnorlax

I saw this hood being mentioned as not strictly necessary, but is it “better” or “worse” to keep it on? (again, talking not just about safety, but also lasering quality etc.)

No functional improvement to keeping or removing that I’m aware of.

Section 6.5 from here: https://support.snapmaker.com/hc/en-us/articles/360041254874-Snapmaker-2-0-Laser-V1-0-0

6.5 Adjust Lens Hood

During the laser engraving / cutting process, if you feel that the lens hood is too far from the surface of the material to have the best light-blocking effect, you can follow the steps below to adjust the settings for the lens hood:

Sounds pretty subjective to me.

I’ll also add I changed my laser focus length to 37mm (by adjusting the slotted lens holder). Has some advantages and disadvantages, but it’s better for me. Advantages include more clearance to the material, less smoke gets drafted up into the lens, deeper depth of focus (which means less variability when engraving surfaces that aren’t perfectly flat). The main disadvantage is the longer focal length will reduce peak beam intensity, making cutting harder. I only use it for engraving though, so that’s not an issue for me.

[SabinSnorlax]

Many thanks @brent113, very clear and very helpful!
Still, 3 questions left from my side:

@brent113

Only thing I can think of is the power is too low or the speed is too fast for the material you’re trying to calibrate on. You could investigate further with M1028 S6

Fair enough, though I still find this very strange, since there were no issues 2 weeks ago with the exact same material.
The speed should be fine, to me it looks identical as before, though of course this is a subjective view.
So I’d rather suspect the laser power;
Could a lower power setting have been caused by an improper firmware update or some other fault in the system?

@SabinSnorlax

• Why do I keep seeing different values of the laser height tossed around, say 21mm or 25mm etc.?

@brent113

There is a slotted nut up in the toolhead that holds the laser lens - by changing that you can change the laser focal length. Not every laser module comes with it assembled the exact same way.

Very interesting, I was not aware of that…
Nonetheless, can this assembly reasonably vary by that much?
I could understand something in the range of say 1-2mm, but I’m getting suspicious when I see all kinds of laser focus values being cquoted.

@SabinSnorlax

• What’s up with the lens hood, does it serve any role beyond added safety?

@brent113

No, other than additionally possibly funnel dust and soot up into the lens area.

@SabinSnorlax

I saw this hood being mentioned as not strictly necessary, but is it “better” or “worse” to keep it on? (again, talking not just about safety, but also lasering quality etc.)

@brent113

No functional improvement to keeping or removing that I’m aware of.

So overall, it sounds like it doesn’t matter much if the lens is on or off, right?
In that case I’ll probably remove it to perhaps reduce dust and soot within the lens area.
I’m not overly concerned with safety issues; as you mentioned, the goggles are a good back-up.

[brent113]

@SabinSnorlax

Could a lower power setting have been caused by an improper firmware update or some other fault in the system?

I have seen the latest update has had some unintended side effects for some people - very possible the default calibration power could have changed. I haven’t updated to the latest firmware so I don’t have many other thoughts.

@SabinSnorlax

Nonetheless, can this assembly reasonably vary by that much?
I could understand something in the range of say 1-2mm, but I’m getting suspicious when I see all kinds of laser focus values being cquoted.

To change my laser focus length from 25mm as it came to 37mm I think I rotated the slot approximately 1/8th of a turn, definitely less than 1/4 turn. It’s pretty sensitive.

I agree with your last paragraphs also, my thoughts as well.

[SabinSnorlax]

Many thanks again for you clear answers;
I wish the official Snapmaker documentation was this clear and detailed.
Hopefully this thread (albeit short) will be useful for others with the same questions in mind.

[sdj544]

@brent113 did a great job of answering most of this.
Just wanted to help clarify a couple things that he said.

The “laser height” or the “focus height” or “laser focus” is the distance between the work surface and the lens that gives the optimal results, i.e., smallest, most concentrated, most powerful beam.
This number, which is generally somewhere between 21-25mm, confuses people because there isn’t an obvious way to measure this. So you get some people hearing this and measuring the distance between the hood, or without the hood, and the work piece and ending up being way off.

So why isn’t this number the same (‘universally correct’) on every SM 2.0? Well, it could be if they decided to focus and calibrate each lens unit at the factory. This would require more expense, so they’ve decided to set it fairly close and then have the end user adjust for it. It’s a perfectly acceptable way of doing it. As @brent113 has done, you can adjust the physical focus distance yourself.

The other confusion is that (I believe) the original SM actually had a way to focus by turning the collar, and they mistakenly read those instructions and think that turning the SM 2.0 lens hood does something. It’s just for extra shielding/protection. I like to leave it on, but there is an argument that having it off will allow better air flow. Not sure if it makes it better or worse as far as keeping soot from lens.

-S

[SabinSnorlax]

Many thanks for explaining why Snapmaker chose to proceed like this, @sdj544!
I hadn’t considered that a calibration at the factory would make a difference in costs, but it makes sense now.

One further benefit of leaving the lens hood off is that it’s easier to clearly see the laser point and to ensure the work origin is exactly where I want it to be.
Sounds like a no-brainer, but I hadn’t thought about it much before removing the hood.

Sorry to bother again, but one more question @brent113 - is there any other thread with an explanation of how to adjust the slotted lens holder?
I’m not planning to do this asap, but might also consider it in the future…

[brent113]

@SabinSnorlax

is there any other thread with an explanation of how to adjust the slotted lens holder?

I haven’t seen one. You’ll need a very wide screwdriver or equivalent. I just turned it a bit on the machine, checked it by eye, and adjusted until it was about where I wanted. I don’t remember if clockwise is in or out.

[Lunamaria]

Hi,
any tips to complete the first calibration when using the laser ???
it’s doing this for me and I’ve to power off/on my A350 each time I try

can’t touch any buttons to change the screen