Tube Power Decline

[glenboy]

Hi All,

Been giving the LS3020 a hammering recently (in terms of time more than power): 10 - 12 hours per day cutting card at about 15% power and 25mm/s.

Late last night the beam stopped cutting all the way through the card so I upped the power to 25%, and then moments later it stopped cutting through so I upped it to 35%, and then again moments later to 45%. Thought something must be up and hit stop.

I checked alignment - all was fine (I'll come back to that in a moment). I have a chiller and the temp was around 18 DEG C. The tube is a year old - but only recently been getting a lot of use - the first 6 - 7 months of it's life it maybe had 7 - 8 hours per week.

I cleaned the mirrors, cleaned the lens and checked the focus height (it was good). I started another cut and all seemed fine (cutting through the card at 15%). Finished the cut and went to bed (It was late...).

My questions are as follows:

1) When a tube is on the out how quickly can it's decline be? I've seen a very slow decline over the year (I used to cut card at about 7% power) but when it's about to die does it take minutes, hours, days, weeks? i guess it probably varies from tube to tube?

2) Could sooty lens/mirror cause this apparent drop off in power? Cleaning the lens/mirrors seemed to cure the diminishing power, but I only had time for a quick test because it was 2 am! Also, the cotton bud didn't look dirty after I cleaned so not sure how much soot was actually on the mirrors/lens.

3) I have a small crack in the lens. It's been like that ever since I first cleaned the lens (month 1 or 2). The crack is a straight line from the 1 O'Clock position to the 4 O'Clock position. The lens is still held together even though the crack appears to go all the way through?? Also, due to it's position it doesn't appear to influence the beam. Is it worth replacing?

4) Alignment. I said earlier that the machine alignment is fine - should have said nearly fine! I test the alignment in 9 carriage locations - imagine splitting the bed up like a naughts and crosses board - I check the alignment in every box (the centre position, all the extreme positions and half way between each). The alignment is spot on in all of the "boxes" except for "middle row, right hand side". The beam strikes about 1 mm vertically below mark. How do I correct this without throwing everything else out of alignment? Could this be caused by a warped track or something loose coming loose? I checked the belts and runner connections and all seemend fine.

Lots of info there! and lots of questions!

As always, any help/words of wisdom will be much apreciated.

Thanks in advance,
Glen

[SimonMallas]

Hi Glen,
Wow big question! try and help with some of it...... yes lenses and mirrors do need to be clean, they can/will affect the power of the laser. A laser tube normally gives you around 6 to 8 week notice, the power will start to drop. Alignment.... picture will help with this one. I'll post some friday Glen.
Cheers Simon

[Matrix]

Hi Glen,
It's quite amazing how much of a difference lens/mirror cleaning can make - even a small speck in the wrong place (especially on the lens) can really throw off the tightness of the beam.
Another factor is that muck on the optics will cause them to absorb energy from the laser - i.e. Heat! - and this can lead to slow destruction of the coatings and mirror silvering, and in extreme cases even cracking due to thermal expansion of the glass.
We had a slow drop off of power in our machine (used very heavily) that we thought at first to be the tube deteriorating - down to about 50% after 18months or so. This was completely fixed by installing a new set of mirrors and lens. It was only when comparing new and old side-by-side that it became obvious just how much the silvering/lens coatings had become "clouded" - we just hadn't taken care of the optics enough.
Cheers,
Steve.

[glenboy]

Hi Simon/Steve,

Thanks for the info/advice.

I tried replacing the lens with a one I have from China (it has a smaller diameter and the collar doesn't allow air assist) just to see if that would make a difference. Seems to have solved the issues. So I have purchased a new lens from HPC - hopefully that will get the performance back. I do clean the optics regularly (perhaps not regularly enough) but will learn from this and ensure I clean the mirros,lens and end of tube every 2 - 3 days, especially as I have increased production.

Thanks again,
Glen