Laser focusing above expected focal height

Hello all,
I have a cloud ray 50w fiber with a f=420mm lens for a 300mmx300mm working area. When I first purchased the laser I adjust the focus by burning steel and adjusting the height until optimum burning based on a YouTube video I watched. I’ve recently noticed after 3d printing a 420mm focal stick that my current focal height is higher than the 420mm focal stick but if I try to burn at the focal stick height it is clearly out of focus. Any help with this or just telling me I have no idea what I’m talking about would be greatly appreciated.

The photo is the difference between currently burning focal height and the bottom of the focal stick.

After you got it into focus, what distance did you end up with… ?

I have a 254mm lens and the measuring stick is 297mm in length…

I used this video…


Thanks for the reply i dont have a large enough micrometer to measure exactly but my inches to mm conversion shows roughly 445mm.

Also a newbie here, After viewing a hundred YT videos on fiber lasers, I have found that the “recommended” focusing method is to listen for the loudest burn noise and view the brightest burn… BUNK TO THAT… MY summary follows:
ALUMINUM business card.
205.5 mm loudest burn noise - some material removed
206 mm loud burn noise - a bit more material removed - loud burn noise
207 mm almost silent - the most material removed - to bare aluminum
207.5 mm loud burn - noise - a bit more material removed
208 mm loudest burn noise - some material removed

I made a test file with “TEST 20X.x MM” and made separate layers for each focus test distance. I may be thinking wrongly, but, that is my method - most material removed is THE “best” focus distance.
Additionally, I started to engrave a brass coin and set the focus to the “best” distance for aluminum… WRONG… It seems that after a coin burn fail at the 207 mm distance, I made another test file starting from 200 mm to 206.5 mm. Come to discover the “best” focus for my brass coins is 203 mm…??..?? I don’t know why, but, as having two whole weeks under my belt with my Commarker B4/30, I’ve learned to make damn sure of the best" focus distance for every material I plan to engrave… I still don’t understand the different focus distances between aluminum and brass… Any ideas?? Thanks.

Without knowing what lens you have… it’s tough to make suggestions.


Thanks for the reply. It is not so much that I need any suggestions. just the fact that the focus distance for the black aluminum card (207mm), and a brass coin (203mm), are different. My Commarker B4/30 came with a 110mm and a 200mm lens and after receiving my unit, I did also install the 200mm lens to get it calibrated and made sure is was basically OK. I used the LB 9 point galvo lens calibration tool and it provided great results. I initially did use the “loudest sound/brightest light” method on the black aluminum card and the finish was a lighter color and no aluminum could be seen. After using a

It is my understanding that the total burn to bare metal requires the best focus, both in the aluminum card example and the brass coin for engraving depth. I set the focus for the brass coin at 202.75mm which, for a .5mm burn depth, would finish the burn at the effective height of 203.25mm, with the mid focal point of 203mm. This "mystery started when I set the focus to 207mm and started the brass coin engraving. The laser beam was barely visible and after realizing the best focus for brass was 203mm, only then did the “sparks fly” and the “noise began”… :slight_smile: