not cutting though on large bed

Discussions on optics for laser cutter/engravers

not cutting though on large bed

Postby educa » Wed Dec 05, 2012 9:15 am

Hi,

I have 80watt reci tube and a 1200x600mm table.

I see that in the second half of my cutting table, the laser can't cut though stuff where it could in first half. This is most probably because the laser beam gets too wide and with my 2" lens I have too little focal depth at these locations.

Would a 100mm lens solve this problem ? What would be the downside of switching from 50.8mm to 100mm lens ? Bigger spot size, worse engraving quality ?

Kind regards,

Bart
educa
 
Posts: 239
Joined: Thu Dec 22, 2011 9:13 pm

Re: not cutting though on large bed

Postby bdring » Wed Dec 05, 2012 1:20 pm

In general a wider beam is better for focusing, so is a shorter focal length. Are you sure all the power is getting to the lens?
Bart
"If you didn't build it, you will never own it."
bdring
Site Admin
 
Posts: 2966
Joined: Sun Nov 22, 2009 7:33 pm
Location: Chicago, IL, USA

Re: not cutting though on large bed

Postby cvoinescu » Wed Dec 05, 2012 3:05 pm

The beam from the laser spreads out with distance. When X is all the way to the left and Y all the way to the rear, you get the shortest beam path, thus the smallest beam size, so it's fairly easy to align your optics so that all of that beam makes it to the lens. When you move further apart, the beam makes an ever wider spot, so some of it may no longer hit the mirror on the gantry or the lens. That power is lost (and can damage the mirror and lens holders). If the beam is still narrower than the mirror and lens, with careful adjustment you can still get it all to the workpiece, but if it spreads out more than that, there's nothing you can do except get a larger mirror and a larger lens, or somehow improve the collimation of the beam (I'm not sure whether collimators exist or are affordable for this wavelength). It's not the focal length of the lens that matters, but the diameter; and if some energy misses the last mirror, a different lens certainly won't help. Things are also made worse by the fact that higher-power lasers have wider beams to begin with, so they are more difficult to work with or require larger mirrors and lenses.
cvoinescu
 
Posts: 501
Joined: Thu Aug 09, 2012 9:12 am
Location: Camberley, Surrey, UK

Re: not cutting though on large bed

Postby bdring » Wed Dec 05, 2012 3:25 pm

Long tubes tend to have a pretty low divergence. Do you know what the divergence value is?
Bart
"If you didn't build it, you will never own it."
bdring
Site Admin
 
Posts: 2966
Joined: Sun Nov 22, 2009 7:33 pm
Location: Chicago, IL, USA

Re: not cutting though on large bed

Postby lasersafe1 » Thu Dec 06, 2012 1:24 am

The best way to handle a large system would be to build or buy a beam expander and place it at the exit of the tube. You could then better control the collimation of this larger beam (5X expansion would be good). Even at that expansion, I wouldn't think you would exceed the capabilities of 1" mirrors around the table. You will then be able to hit your final lens and focus to a nice tight spot.
lasersafe1
 
Posts: 599
Joined: Fri Nov 27, 2009 8:23 pm

Re: not cutting though on large bed

Postby educa » Thu Dec 06, 2012 1:13 pm

The beam is allready around 6 to 6.9mm when it exits a 80Watt industrial quality reci tube. Divergence is 2.3mRad.
The fact is these lasers do have less divergence, but at the aperture of the laser tube the laser is allready quite wide.

So I do understand that you need to get it aligned perfectly. In fact I'm 100% sure its aligned compeletely ok, but I think the problem is something else then misalignment. I get the beam perfectly aligned at any point on my table.

If the beam is shortest path (laser head upper left) then I guess the beam diameter is about 8mm at the lens. 8mm at the lens gives me a DOV of 1.069mm

But at the other end (with laser beam longest) the diameter is around 12mm, so I only have a DOF anymore of 0.475mm. That means that at upper left position in my machine I can cut very nice vertical lines, but at the right (with longer beam) you can clearly see that the angle gets bigger. the cut is not straight down but you see that it is angled a little.

Thats why I think the lens might be a factor too. The laser does get its full power at any time straight to the lens, but it just doesn't focus it enough to get a clean cut through. I'm pretty sure if I would half my speed and double my PPI I will probably also cut nicely there, but its just something I saw when cutting at same speed and same ppi settings. It simply doesn't seem to have the same power, but because it certainly has the same optical power I guess the problem is the focal depth.


If I then try to enter the details (in the calculator on buildlog homepage) with a 100mm focal lens, then with a beam of 8mm diameter I would get 4.141mm focal depth and with a 12mm diameter beam it would still be 1.840mm, so a lot better focal depth then with the 50.8mm lens.


The only thing which I am afraid of is that my smallest spot size might increase considerably.

Kind regards,
Bart
educa
 
Posts: 239
Joined: Thu Dec 22, 2011 9:13 pm

Re: not cutting though on large bed

Postby lasersafe1 » Thu Dec 06, 2012 3:53 pm

I would recommend you ask these guys over at CNC zone if they think your situation is unusual. They also discuss the beam expander.

http://www.cnczone.com/forums/general_laser_engraving_cutting_machine/155884-do_i_need_beam_expander.html
lasersafe1
 
Posts: 599
Joined: Fri Nov 27, 2009 8:23 pm

Re: not cutting though on large bed

Postby cvoinescu » Thu Dec 06, 2012 5:23 pm

If this is the case, and the full beam gets delivered to the lens even at the farthest corner, then yes, a lens with a longer focal distance will have a larger depth of field; in other words, the focused beam will be delivered more vertically -- in a narrower cone -- at the far end of travel. It will be even closer to vertical at the near end, though! I'm not sure what the longer length does to the minimum spot size, so I can't comment on that.
cvoinescu
 
Posts: 501
Joined: Thu Aug 09, 2012 9:12 am
Location: Camberley, Surrey, UK

Re: not cutting though on large bed

Postby evokanivo » Sat Jan 05, 2013 7:05 pm

Hi educa, Did you ever resolve this issue? I'm currently designing a laser with about the same size cutting area and am worried about having alignment difficulty.
evokanivo
 
Posts: 61
Joined: Mon May 31, 2010 1:18 am
Location: Renton, WA

Re: not cutting though on large bed

Postby educa » Sat Jan 05, 2013 7:16 pm

I tested with 100mm focal and indeed it cuts better, but with larger kerf.

Until now I didn't do other tests, but I did discover that with proper power settings everything is possible. It might need a reduction in speed too, but thats not a big problem, so I plan on doing some tests when all is ready on the machine and then I can let the machine adapt its cut parameters like speed, ppi and power automatically based on where the head is on the cutting table. That should solve it for me.

A beam expander is not really what I am looking for since that will automatically allready get my beam very wide even at the upper left table position where I now get much better cutting capabilities. So no expander for me.
educa
 
Posts: 239
Joined: Thu Dec 22, 2011 9:13 pm


Return to Optics and Lasers

Who is online

Users browsing this forum: No registered users and 4 guests

cron