After several hours of engraving, my laser seemed weaker than originally. The lens had not been cleaned a single time, nor was there a fume exhaust system for the machine. Both these caused the lens to accumulate fumes, which eventually got burned on the lens. A black circle could be seen on the lens, and despite cleaning, it never got perfectly clear.
Basic Info on the Lens Assembly
The laser lens assembly is contained into a small threaded cylinder, 8 mm in diameter. It sits below the laser assembly, and can be adjusted via a knob. The lens itself can be removed when unscrewing it all the way. Be careful, there is a small spring behind the lens assembly, and is used to put pressure on the threaded lens to prevent it from unscrewing by itself from the vibration of the machine. The lens is extremely important, as it concentrates the light into a fine spot, and increases its burning power. A damaged or mis-adjusted lens assembly will significantly affect the outcome of a project.
Replacing the Lens
The original lens was apparently made of plastic, and easily damageable. A good replacement piece was the following: https://www.banggood.com/Triple-Glazing-Focusing-Lens-Collimating-Coated-Glass-Lens-Blue-Laser-Diode-405nm-p-1035471.html?rmmds=detail-left-hotproducts__1&cur_warehouse=CN . The threaded part of the assembly was longer than the original lens, but fitted nice into the laser assembly. It did not come with the black adjustment knob, but it could easily be moved from the original lens to the replacement one. The knob only screws onto the lens assembly. The replacement lens assembly has one side with grooves on the inner part, and one side with grooves on the outer part. The side with the grooves on the outer part goes down, away from the laser assembly. This is where the adjustment knob would fit. Ensure you put Teflon tape on the threat before screwing it in the laser assembly, as it will make it tighter and will prevent it from moving when engraving.
Cleaning the lens
Lens should be cleaned regularly depending on the amount of fume generated from the burning. Having a fume extractor helps a lot, and reduce the amount of cleaning needed. To clean the lens, unscrew it from the laser assembly, and clean it carefully with a Q-tip and Alcohol. 90% or more ethanol should be used, as it will evaporate quickly and doesn’t lean stain. A good practice is to roll the Q-tip instead of rubbing it, which will prevent any debris from scratching the surface.
Focusing the Beam
After either cleaning the lens, or changing it, the laser beam needs to be focused. This is done by screwing or unscrewing the lens assembly, until the laser dot is as small as possible. To do this, wear the safety goggles, and activate the weak laser via the button on the top part. Put the material that will be engraved, or something with same thickness, and focus it on it. Use a dark sheet of paper and focus as much as possible by looking at the dot. Once you are satisfied, but masking tapes on the wood and engrave a small 10x10mm square at 25%. Scrap wood can be used in case the laser burn through the paper, but it should not. Adjust the knob and repeat to get the darkest engraving possible.