Reducing beam width and dot size

[Jonathan Bowen]

So I've just upgraded to a 60watt tube. However, I have noticed that it's a lot trickier to focus because the beam is much larger (like 25%) from the tube. Maybe my mirrors and stuff are undersized but now it's taking up close to 50% of the mirror area and the end result at the cone on the head is a larger dot then before. It seems to be focusing to about the same size as before but it is a little larger. However the beam width at the end of the cone is really close to the size of the hole.

Is it possible to reduce that or do I need to deal with the tight focus needed and/or upgrade to a larger set of mirrors and head? Does the distance of the head to the 1st mirror matter? I'd assume not because... it's a laser so it should be expanding much over that short a distance. However, I figured I'd check and see if anyone had any suggestions.

[Jerome Stanek]

I have a 60watt tube in mine and the beam going to the first mirror is around 3mm and the mirrors are 25 mm

[Dave Sheldrake]

Hi Jon,

Bigger tubes produce wider beams, the mirrors should be 25mm for that size of tube really but you cab get away with 20mm (18mm) is needed, The final spot should be smaller not larger as the incident beam diameter is bigger (the bigger the incoming beam the smaller the final focussed spot) A 60 watt tube (even a bad one) should have an incident beam to the optical train of around 3 to 6mm in diameter.

cheers

Dave

[Jonathan Bowen]

My mirrors are 18mm. How do you normally measure the incident beam? I'm just going by how big the burn mark is on the tape when focusing. The focused dot is about the same maybe smaller then the 40 watt was but beam width at the tip of the cone is like 1mm in diameter and the hole isn't much larger. I am noticing a slightly wider kerf on MDF then with the old tube. I'm also having trouble getting enough air into the system without running my compressor harder. I'm about 75% duty cycle right now on a 1.5 HP oil less compressor. It's already being budgeted to upgrade that in a few months once I recover from the repairs.

[Richard Rumancik]

Jonathan, your assumption that the larger incident beam needs to be smaller is (as Dave noted) not quite correct . . . the larger beam diameter should be creating a smaller spot size for you. Of course you do need to make sure everything is well-aligned, that your beam is not missing a mirror or catching an edge of a nose cone etc. You have lost some depth of field as a result of larger beam diameter. But the solution is not to make the incident beam smaller as that will increase spot size and reduce energy density. It may seem counter-intuitive but you actually need a beam expander. Now, your laser construction might not permit it, and it costs money.

Take a look at this page and see if you can make some sense of it all. It is all a matter of compromises.

http://www.parallax-tech.com/faq.htm

[Jonathan Bowen]

Well I figured out part of my problem with the laser. My mirrors were loss. Must have shaken loss during the move or something. The caps on mirror 1 and 2 needed tightening. I would focus it but then a few cuts later it would shift slightly and cause alignment issues. I think I was also clipping the bottom corner of mirror 2 a bit so I shaved the hole a bit larger to catch more light.

Do I need to calibrate my table depth? Does the focal length change with the increased in beam width?

[Richard Rumancik]

Glad you found the source of the problem. I am not sure what you mean by calibrating the table depth. If you mean levelling it, that may be possible - how to do it depends on the laser construction. As far as focal length, it is only dependent on the lens so it is constant regardless of the incident beam diameter. So your focal length probe or whatever tool you used before should work the same. It is advisable to double check the focus probe though because there are tolerances on the carriage/mirror assembly and you may find that the factory probe is slightly in error. Normally people check focal length doing test lines on anodized aluminum or painted metal. You can search for the procedure on this forum.

If you use auto-focus, I suggest that you don't . . . a lot of members seem to agree that manual focus is better and less likely to cause a machine crash.